Treybal Cooling Tower
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Transcript of Treybal Cooling Tower
Cooling Tower Application according Treibal
Index
1 Data
2 Tower height
3 NTU and HTU
4 Tower area
5 Compensation water
6 Operaqting diagram
7 Cooling tower schematic
Cooling Tower Application according Treibal
Rev cjc 30012014
Index
Data for cooling tower application
Main equations and results
Cooling Tower height NTU and HTU
Free-cross sectional surface of tower
Compensation elimination evaporation and entrainment fow rates
Equilibrium curve and operation lines
Schema
Cooling Tower Application Data
This application will be realized with following numerical data (Note 1)
Data for numerial exampleWater flow rate entering the tower L1 = 15 kg aguas
Temperature of water entering the tower at the top (2) 45 ordmC
Dry bulb temperature of air entering the tower 30 ordmC
Wet bulb temperature of air entering the tower 24 ordmC
Local height above sea level H = 0 mMaacuteximum cooling temperature will be defined with
5 K
Air to water flow rates ratio shall be r times its
minimum possible value r = 15
temperature 10 degC
and will have a hardness 500 ppm
hardness 2000 ppm
Mass transfer coefficient in the air 62E-05
Air molecular mass 2896 kgkmol
Tower effective heat or mass transfer surface a = 500 msup2msup3
Liquid unit mass flow rate 27
Air unit mass flow rate 20
Note 1Basic data has been taken from [1] pages 278-281
tL2
=
tdbG1 =
twbG1
a differential temperature ∆t above air wet bulb temp ∆t =
The compensation water entering the system wil havetcomp =
da_c
=
The in system circulating water sould have a maximum da
_M =
ky_kmol = kmol ( m2s)
Mair =
Lu = kg(sm2)
Gu = kg(sm2)
Help Variables Cooling Tower Schema
State L1
Water leaving the tower
L2 =
24
5 K
29 ordmCCompensation water
State G1 10 ordmC
Ambient air entering the tower 2000 ppm
30 ordmC
24 ordmC
H = 00 m
h = Psychro_Enthalpy_tdb_twb_H Qh = VALUE kJkg
x = Psychro_AbsoluteHumidity_tdb_twb_H
x = VALUE kglg
Mass transfer coefficient
Mass transfer coefficient in the air
62E-05Air molecular mass
2896 kgkmol
Mas transfer per kilogram
62E-05
2896 kgkmol
00018
Product Kya
Kya
00018a = 500 msup2msup3
090
Q = 270 W
tL1
= twbG1
+ ∆t
tbhG1 tL2 =
∆t =
tL1
=
tacomp
=
dac =
tbsG1
=
tbhG1
ky_kmol
= kmol ( m2s)
Mair
=
daM =
ky = k
y_kmol M
air
ky_kmol
= kmol ( m2s)
Mair
=
ky = kg ( m2s)
Kya =
ky = kg ( m2s)
Kya = kg ( m3s)
W
Blowdown water B
Rev cjc 30012014
Cooling Tower Schema
15 kg s
45 ordmC Air
Water
30 ordmC
24 ordmC
Water
Air
2000 ppm
Rev cjc 30012014
tdbG1 =
twbG1
L1G1
Torreenfriadora
G22
Blowdown water B
G1
L2
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Cooling Tower Application according Treibal
Rev cjc 30012014
Index
Data for cooling tower application
Main equations and results
Cooling Tower height NTU and HTU
Free-cross sectional surface of tower
Compensation elimination evaporation and entrainment fow rates
Equilibrium curve and operation lines
Schema
Cooling Tower Application Data
This application will be realized with following numerical data (Note 1)
Data for numerial exampleWater flow rate entering the tower L1 = 15 kg aguas
Temperature of water entering the tower at the top (2) 45 ordmC
Dry bulb temperature of air entering the tower 30 ordmC
Wet bulb temperature of air entering the tower 24 ordmC
Local height above sea level H = 0 mMaacuteximum cooling temperature will be defined with
5 K
Air to water flow rates ratio shall be r times its
minimum possible value r = 15
temperature 10 degC
and will have a hardness 500 ppm
hardness 2000 ppm
Mass transfer coefficient in the air 62E-05
Air molecular mass 2896 kgkmol
Tower effective heat or mass transfer surface a = 500 msup2msup3
Liquid unit mass flow rate 27
Air unit mass flow rate 20
Note 1Basic data has been taken from [1] pages 278-281
tL2
=
tdbG1 =
twbG1
a differential temperature ∆t above air wet bulb temp ∆t =
The compensation water entering the system wil havetcomp =
da_c
=
The in system circulating water sould have a maximum da
_M =
ky_kmol = kmol ( m2s)
Mair =
Lu = kg(sm2)
Gu = kg(sm2)
Help Variables Cooling Tower Schema
State L1
Water leaving the tower
L2 =
24
5 K
29 ordmCCompensation water
State G1 10 ordmC
Ambient air entering the tower 2000 ppm
30 ordmC
24 ordmC
H = 00 m
h = Psychro_Enthalpy_tdb_twb_H Qh = VALUE kJkg
x = Psychro_AbsoluteHumidity_tdb_twb_H
x = VALUE kglg
Mass transfer coefficient
Mass transfer coefficient in the air
62E-05Air molecular mass
2896 kgkmol
Mas transfer per kilogram
62E-05
2896 kgkmol
00018
Product Kya
Kya
00018a = 500 msup2msup3
090
Q = 270 W
tL1
= twbG1
+ ∆t
tbhG1 tL2 =
∆t =
tL1
=
tacomp
=
dac =
tbsG1
=
tbhG1
ky_kmol
= kmol ( m2s)
Mair
=
daM =
ky = k
y_kmol M
air
ky_kmol
= kmol ( m2s)
Mair
=
ky = kg ( m2s)
Kya =
ky = kg ( m2s)
Kya = kg ( m3s)
W
Blowdown water B
Rev cjc 30012014
Cooling Tower Schema
15 kg s
45 ordmC Air
Water
30 ordmC
24 ordmC
Water
Air
2000 ppm
Rev cjc 30012014
tdbG1 =
twbG1
L1G1
Torreenfriadora
G22
Blowdown water B
G1
L2
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Cooling Tower Application Data
This application will be realized with following numerical data (Note 1)
Data for numerial exampleWater flow rate entering the tower L1 = 15 kg aguas
Temperature of water entering the tower at the top (2) 45 ordmC
Dry bulb temperature of air entering the tower 30 ordmC
Wet bulb temperature of air entering the tower 24 ordmC
Local height above sea level H = 0 mMaacuteximum cooling temperature will be defined with
5 K
Air to water flow rates ratio shall be r times its
minimum possible value r = 15
temperature 10 degC
and will have a hardness 500 ppm
hardness 2000 ppm
Mass transfer coefficient in the air 62E-05
Air molecular mass 2896 kgkmol
Tower effective heat or mass transfer surface a = 500 msup2msup3
Liquid unit mass flow rate 27
Air unit mass flow rate 20
Note 1Basic data has been taken from [1] pages 278-281
tL2
=
tdbG1 =
twbG1
a differential temperature ∆t above air wet bulb temp ∆t =
The compensation water entering the system wil havetcomp =
da_c
=
The in system circulating water sould have a maximum da
_M =
ky_kmol = kmol ( m2s)
Mair =
Lu = kg(sm2)
Gu = kg(sm2)
Help Variables Cooling Tower Schema
State L1
Water leaving the tower
L2 =
24
5 K
29 ordmCCompensation water
State G1 10 ordmC
Ambient air entering the tower 2000 ppm
30 ordmC
24 ordmC
H = 00 m
h = Psychro_Enthalpy_tdb_twb_H Qh = VALUE kJkg
x = Psychro_AbsoluteHumidity_tdb_twb_H
x = VALUE kglg
Mass transfer coefficient
Mass transfer coefficient in the air
62E-05Air molecular mass
2896 kgkmol
Mas transfer per kilogram
62E-05
2896 kgkmol
00018
Product Kya
Kya
00018a = 500 msup2msup3
090
Q = 270 W
tL1
= twbG1
+ ∆t
tbhG1 tL2 =
∆t =
tL1
=
tacomp
=
dac =
tbsG1
=
tbhG1
ky_kmol
= kmol ( m2s)
Mair
=
daM =
ky = k
y_kmol M
air
ky_kmol
= kmol ( m2s)
Mair
=
ky = kg ( m2s)
Kya =
ky = kg ( m2s)
Kya = kg ( m3s)
W
Blowdown water B
Rev cjc 30012014
Cooling Tower Schema
15 kg s
45 ordmC Air
Water
30 ordmC
24 ordmC
Water
Air
2000 ppm
Rev cjc 30012014
tdbG1 =
twbG1
L1G1
Torreenfriadora
G22
Blowdown water B
G1
L2
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Help Variables Cooling Tower Schema
State L1
Water leaving the tower
L2 =
24
5 K
29 ordmCCompensation water
State G1 10 ordmC
Ambient air entering the tower 2000 ppm
30 ordmC
24 ordmC
H = 00 m
h = Psychro_Enthalpy_tdb_twb_H Qh = VALUE kJkg
x = Psychro_AbsoluteHumidity_tdb_twb_H
x = VALUE kglg
Mass transfer coefficient
Mass transfer coefficient in the air
62E-05Air molecular mass
2896 kgkmol
Mas transfer per kilogram
62E-05
2896 kgkmol
00018
Product Kya
Kya
00018a = 500 msup2msup3
090
Q = 270 W
tL1
= twbG1
+ ∆t
tbhG1 tL2 =
∆t =
tL1
=
tacomp
=
dac =
tbsG1
=
tbhG1
ky_kmol
= kmol ( m2s)
Mair
=
daM =
ky = k
y_kmol M
air
ky_kmol
= kmol ( m2s)
Mair
=
ky = kg ( m2s)
Kya =
ky = kg ( m2s)
Kya = kg ( m3s)
W
Blowdown water B
Rev cjc 30012014
Cooling Tower Schema
15 kg s
45 ordmC Air
Water
30 ordmC
24 ordmC
Water
Air
2000 ppm
Rev cjc 30012014
tdbG1 =
twbG1
L1G1
Torreenfriadora
G22
Blowdown water B
G1
L2
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
Cooling Tower Schema
15 kg s
45 ordmC Air
Water
30 ordmC
24 ordmC
Water
Air
2000 ppm
Rev cjc 30012014
tdbG1 =
twbG1
L1G1
Torreenfriadora
G22
Blowdown water B
G1
L2
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Cooling Tower height
Tower packing height [2] Number of Transfer Units
The packing height (l) of a tower can be calculated as The number of transfer units (NTU)is calculated by numerical integtation
[1] eq (753) page 276 Sheet 2- NTU presents a calculationexample of the NTU
withResult of NTU example (sheet NTU)
NTU =NTU =
and [1] eq (754) page 277 Height of Transfer Unit HTU
Z Tower packing height [m]
flow rate of dry air (is a constat) [kgs] HTU =molar mass of air [kgkmol] HTU =mass transfer coefficient in the air [kmol(msup2s)]
a effective heat or mass transfer surface [msup2msup3] Tower packing heightA free cross-sectional surface of the tower [msup2] Z =
enthalpy in the air phase = enthalpy of humid air [Jkg] HTU =(in the bulk phase) NTU =enthalpy in the air phase (i at ther boundary [Jkg] Z =that is in saturated condition)
HTU Height of Transfer Unit
NTU Number of Transfer Units
Subscripts
B dry air
y air phase = humid air
a top of the tower
b bottom of the tower
i corresponds to the boundary (ie saturated state)
QS =V
MB
ky
Iy
Iyi
NTUHTUZ sdot=
AakMQ
HTUyB
S
sdotsdotsdot=
( )int sdotminus
=ay
by
I
I
yyiy
dIII
1NTU
AakMQ
HTUyB
S
sdotsdotsdot=
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
Number of Transfer Units
The number of transfer units (NTU)is calculated by numerical integtationSheet 2- NTU presents a calculationexample of the NTU
Result of NTU example (sheet NTU)
VALUE -
Height of Transfer Unit HTU
VALUE m
Tower packing heightHTU NTU
VALUE mVALUE -VALUE m
(hL_a - hL_b) N Σf(x)
GS (M
B k
y a A)
AakMQ
HTUyB
S
sdotsdotsdot=
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
NTU and HTU calculations column 1 column 2
Equilibrium curve for saturated airWater temperature at inlet of tower This curve is
45 ordmC f(t)
Water temperature at tower outlet where the function
29 ordmC hairsat = Psychro_Enthalpy_tdb_HumRel_H
Range has been used
The curve strats at point
Number of sections 3 (29964) The range will be divided in a number Nand ends at point
of sections 4 (452182)N = 6
column 3
Between both temperatures N-1 Operation line for r = 1
temperatures are inserted to define where r is the ratio between the actual
the N sections All section are defined mass flow rate and the minimum flow
with the same temperature differential rate
The line strts at a point defined by the
Temperature differential inlet air properties (point 1 in operating
ordmC diagram) also called state G1
45 ordmC
29 ordmC State G1 (Point 1)
16 K 29 ordmC
24 ordmCSection temperature increment H = 0 m
Psychro_AbsoluteHumidity_tdb_twb_H
16 K VALUE kgkgN = 6 -
267 K VALUE kJkg
Temperature at point i+1
Table 1 Tower packing height calculation1 2 3 3a 4 5
Equilibrium Operation Operation curve for line for line for
saturated air r = 1 r = 15
tL2
= hairsat
=
tL1 =
tL2 - tL1
Column 1 starts with temperature tL2
and ends with temperature tL1
∆tL = t
L2 - t
L1
tL2
=
tL1 =
∆tL = tdbG1 =
twbG2
=
∆tL_Sect
= ∆tL N x
G1 =
∆tL = xG1 =
∆tL_Sect
= hG1
=
ti+1
= ti + ∆t
L_sect
∆h = ∆h =
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
kJkg kJkg kJkg kJkg kJkg25 VALUE
255 VALUE
Top(2) 45 VALUE VALUE VALUE VALUE VALUE
4233 VALUE VALUE VALUE VALUE VALUE3967 VALUE VALUE VALUE VALUE VALUE3700 VALUE VALUE VALUE VALUE VALUE3433 VALUE VALUE VALUE VALUE VALUE3167 VALUE VALUE VALUE VALUE VALUE
Bottom (1) 29 VALUE VALUE VALUE VALUE VALUE
In Operating diagram (sheet 6)t H
degC kJkgPoint 1 29 VALUEPoint 2 45 VALUE Line 1-2 Operation line for r = 1Point 2 45 VALUE Line 1-2 Operation line for r = 15Point 3 29 VALUEPoint 4 45 VALUE Line 3-4 Equilibrium (saturation) line
From column 3a is possible to see that the differences between the Equilibriumcurve and the operation line for r = 1 are very small (053 kJkg) at a certain temperature (3967 degC) Thus this operation line is near enough a minimumflow rate line In the operating diagram figure (sheet 6) the operating line for r = 1 appearsto be tangent with the Equilibrium curve This fact visualizes that this line is close enough to the real minimum flow rateThe condition of tangency between these two curves is due to the fact that ifthe operating line would cross the equilibrium line it would not be possible to have mass transfer in this region
Height of tower packing Number of Transfer Units NTU
From Treibal Equation (7-51)
(751a)
(751b)
The numerial integration of NTU is
performed by means of the trapezoidal
integration method
the air-water mixture for the actual case According this method the integration
where H2 and H1 are the enthalpies of
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
int minus=
2
1
H
H i HHdH
NTU
[ ]mHH
dHak
GZ
H
H iy
S int minussdot
sdot=
2
1
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
that is in this case for r = 15 is realized as it is shown in the columns6 7 and 8
The final evaluation is done according(751c) following equation
(751d)
NTU =Also where
VALUE
VALUEN = 6
VALUE
Numerical results shown are from next NTU = VALUEcalculation sheets
Treybal [2] result isNTU = 325
The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integrationmethod is not indicated
Comparison between the example calculation table and the tabvle from Treibal
Calculation table using psychrometric functions1 2 3 4 5 6
Curva de Liacutenea de Liacutenea de equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE
29 VALUE 72 72 VALUE VALUE3167 VALUE 96 877 VALUE VALUE
3433 VALUE 119 1034 VALUE VALUE
3700 VALUE 143 1191 VALUE VALUE3967 VALUE 166 1349 VALUE VALUE4233 VALUE 190 1506 VALUE VALUE4500 VALUE 213 16627 VALUE VALUE
Table from Treybal [1] page 2801 2 3 4 5 6
Curva de Liacutenea de Liacutenea de
(hL_a
- hL_b
) (2 N)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
Z=HTUsdotNTU [m ]
HTU=GS
k ysdota[m ]
HTU=GS
M Bsdotk ysdotasdotA
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
tkJkg kJkg kJkg kJkg 1(kJkg)
25 VALUE255 VALUE29 1000 72 72 280 00357
3167 1140 96 920 220 00455
3433 1298 119 1065 233 004293700 1470 143 1210 260 003853967 1668 166 1355 313 003194233 1910 190 1495 415 002414500 2160 213 16350 525 00190
∆h = 1∆h
hairsat
hoper_r=1
hoper_r=15
hairsat
-hop_r=15
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
The line ends in a point defined by the Slope of line with r = 15 Air flow rateproperties of the leaving satutared air S = m = L Cpw Gs (754)Point 2 L = 15 kgs From heat balance
Estado G2 Cpw = 41868 kJ(kgK)
45 ordmC Gs = VALUE kg ass
f = 100 S = VALUE (kJkg)K
H = 0 m Exit enthalpy
S =
VALUE kJkgThe operation linefor r = 1 is the
VALUE JkgSlope of operation line witrh r = 1 S = VALUE (kJkg)K
45 ordmC
VALUE kJkg 29 ordmC [1] Eq (754) page 277
VALUE kJkg VALUE Jkg
45 ordmC
29 ordmC column 5
VALUE (kJkg)K Driving enthalpy difference at a point i
(754)
m = L liquid flow rate [kg s] (assumed constant)
VALUE (kJkg)K column 6
m = VALUE Reciproc of driving enthalpy difference
L = 15 kg aguas
Cpw = 41868 kJ(kgK) column 7
VALUE kg ass Coefficients for numerical integration
1 at both endscolumn 4 2 in the other elements
Operation line for r = 15
The line starts from the same point 1 column 8the properties at the inlet defined as the Numerical integration elements
state G1
Gs =r = 15
VALUE kg assGs = VALUE kg ass
Table 1 Tower packing height calculation6 7 8
Numerical Air conditions in the towerfor r = 1integration Conditions at the bottom of the tower (point 1 in diagram)coefficient Point 1
f(x) 290 ordmC (Column 1)
tbsG2
=
hG2
= Psychro_Enthalpy_tdb_HumRel_H (hG2
- hG1
) (tbsG2
- tbsG1
)
hG2
= hG2
= hG1
+ S (tbsG2
- tbsG1
)
straight line 12 where r = G Gcon r = G Gmin hG1 =
Sr=1
= (hG2
- hG1
) (tL2
- tL1
) tbsG2
=
hG2
= tbsG1
=
hG1
= hG2
=
tL2
=
tL1
= GS gas flow rate [kg as s]
Sr=1
= hG2
exit air enthalpy (top) [kJkg]
GS = G
r=1 = 1m L c
pw ∆hi = h
airsat_i -h
op_r=15_i h
G1 inlet air enthalpya (bottom) [kJkg]
Sr=1
Sr=1 = cpw liquid specific heat [kJ(kgK)]
tL2
inlet water temperature (top) [degC]
tL1
exir water temperature (bottom) [degC]
Gr=1 =
Ci =
f(xi) = C
i (1∆h
i)
r Gr=1
Gr=1
=
1∆h
Ci tbs1 =
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
1(kJkg) VALUE kJkg (Column 3)Conditions at the top of the tower (point 2 in diagram)Point 2
VALUE 1 VALUE 45 ordmC (Column 1)
VALUE 2 VALUE VALUE kJkg (Column 3)VALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 2 VALUEVALUE 1 VALUE
VALUE
Number of Transfer Units NTU Height of Transfer Unit HTU Height of Transfer Unit HTU
From Treibal Equation (7-51)
HTU =withGs = VALUE
The numerial integration of NTU is 2896
performed by means of the trapezoidal HTU = 62E-05
20 kg(msup2s) a = 500
According this method the integration 09 A = VALUE
hG1
=
tbs2 =
h2 =
Σf(x) =
GS (M
B k
y a A)
MB =
GS ( ky a) ky_kmol =
GS
kya kg ( m3s)
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
HTU=GS
k ysdotaHTU=
GS
M Bsdotk ysdotasdotA
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
is realized as it is shown in the columns HTU = 22 m HTU = VALUE Treybal [2] result is
The final evaluation is done according HTU = 22 m flow rate of dry air (is a constat)
molar mass of air
Height of packing tower mass transfer coefficient in the air
a effective heat or mass transfer surface
A free cross-sectional surface of the tower
Z = HTU NTU
HTU = 22 mNTU = VALUE
kJkg Z = VALUE m
kJkgTreybal [2] result is
Z = 722 m
-
-The difference comes from the values ofthe psychrometric properties Treybal takes its values from graphics and inthe example are taken form psychrometricfunctios Also the numerical integration
Comparison between the example calculation table and the tabvle from Treibal Trapezoidal numerical integration rule
Calculation table using psychrometric functions7 8
Numericalintegrationcoefficient
f(x)
1 VALUE NTU =2 VALUE where
2 VALUE 1663 kJkg
2 VALUE 720 kJkg2 VALUE N = 62 VALUE VALUE1 VALUE
VALUE NTU = VALUE -
Table from Treybal [1] page 2807 8
Numerical
QS
MB
ky
L_a - h
L_b) (2 N) Σf(x)
Ci
NTU=(hL2
- hL1)
2N (f(x1) + 2f(x
2) + 2f(x
3) + hellip+ 2f(x
N-1) + f(x
N) )
(hL_a
- hL_b
) (2 N) Σf(x)
hL_in_r=15
=
hL_out=r=15
=
Σf(x) =
Σf(x) =
sumsdotsdotminus
= )(2
__xf
N
hhNTU
bLaL
)1(22
11
2)(
1
minus====
sdotsdotminusasympsdot sumint
=
Niparag
Nyiparag
fgN
abdxxf
i
i
k
N
ki
b
a
Z=HTUsdotNTU
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
integrationcoefficient
f(x) Treybal table differs from the calculationtable in the values of the psichrometricpropertiesAdditionaly Treibal uses a different
1 003575 numerical integration method where 1 004545 the numerical integration coefficienst are
1 0042921 003846 The numerical integration used is not1 003195 indicated and Treybal gives as a1 002410 final result a NTU value1 001905
023767 NTU = 325 -
Ci
not required (or Ci = 1)
Σf(x) =
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
Page 1 of 4
L liquid flow rate [kg s] (assumed constant)
Rev cjc 30012014
Page 2 of 4
Table 1 Tower packing height calculation
Conditions at the bottom of the tower (point 1 in diagram)
gas flow rate [kg as s]
exit air enthalpy (top) [kJkg]
inlet air enthalpya (bottom) [kJkg]
liquid specific heat [kJ(kgK)]
inlet water temperature (top) [degC]
exir water temperature (bottom) [degC]
( ) ( )( )
( )
( )
m
cLG
tt
hhm
hh
ttcLG
ttcLhhG
bottom
top
HH
pwS
LL
GG
GG
LLpwS
LLpwGGS
Liquidair
sdot=
minusminus=
minusminussdotsdot=
minussdotsdot=minussdot
∆=∆
12
12
12
12
1212
1
2
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Page 3 of 4
Height of Transfer Unit HTU
kg ass
kgkmol
msup2msup3
msup2
S (M
B k
y a A)
kmol ( m2s)
HTU=GS
M Bsdotk ysdotasdotA
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
m
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]
effective heat or mass transfer surface [msup2msup3]
free cross-sectional surface of the tower [msup2]
Page 4 of 4
Microsoft Editor de ecuaciones 30
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Microsoft Editor de ecuaciones 30
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
HTU TreibalEquation (7-51)
HTU = V (MB ky a A)withV = 5684 kg ass
MB = 2896 kgkmol
ky_kmol = 62E-05 kmol ( m2s)
a = 500 msup2msup3
A = 533 msup2
AakM
VHTU
yB sdotsdotsdot=
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
HTU = 119 m
Tower height l = NTU HTU
NTU = 425 -HTU = 119 m
l = 504 mV = G AG = 1066 kg as(smsup2)
533 msup2
5684 kg ass
GS ( ky a)2 kg(msup2s)
Microsoft Editor de ecuaciones 30
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Free-cross sectional surface of tower
Area of cross sectional surface Seleted area
From borh results the smallest value
L = should be selected to ensure that the
L liquid flow rate [kgs]
the indicated value ofsectional surface) [kg ( msup2s)]
A area of cross section 090So
A = L Lu A = VALUEL = 15 kgs
27 kg(msup2s)A = 556 msup2
Using the gas flow rate
A =G gas rate [kgs]
sectional surface) [kg ( msup2s)]A area of cross section
Gs = VALUE kg ass
2 kg(msup2s)A = VALUE msup2
Lu A
value of the product kya has at least
Lu unit flow rate (for unit of cross
kya =
Lu =
GS G
Su
Gu unit flow rate (for unit of cross
GSu
=
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
From borh results the smallest value
should be selected to ensure that the
msup2
value of the product kya has at least
kg ( m3s)
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Compensation water
Cosidering a compensation and a Entrainment loss rate W water that is Applicationcontinuous elimination the mass being transported with the exit airbalance is leaving from de top of the tower as a Evaporation rate E
(a) loss of water1- Absolute humidity of exit air
Evaporation rate E water that is Assuming that the leaving air is basicallyA water hardness balance is evapotated in the air flow producing saturated at Point O
de cooling of the water flow(b) From equation (d) The enthalpy at this point from the
calculation Table 1 is
and thereforeAssuming initially a temperature value
(c) the corresponding enthalpy for thisassumed temperature is with
Eliminating M from (a) and (c)H =h =
(d) Now using Solver find a value of the
With calculated exit air temperature
M compensation rate [kgh] the corresponding absolute humidity
B elimination rate [kgh] can be be calculatedE evaporation rate [kgh] t =
W entrainment loss rate [kgh]
H =
circulating water [kgkg] or [ppm]
compensation water [kgkg] or [ppm] 2- Absolute humidity of inlet air
From sheet 2
Elimination rate B required to replace
water with a maximum allowable salts
content with fresh water with the in this 3- Humidity change
water existing salt content This is
called the compensation rate
(e)
φO =
hO =
tO =
tO =
φO =
temperature t
φO =
daC
hardness weight fraction of
xa2
=
daM
hardness weight fraction of
xa1
=
∆x2-1
=
xa2
=
xa1 =
∆x2-1
=
WEBM ++=
( ) CM daWBdaM sdot+=sdot
( )M
C
da
daWBM
sdot+=
( )M
C
da
daWBWEB
sdot+=++
Wdada
daEB
MC
M minusminus
sdot=
Wdada
daEB
dadada
EWB
da
dadaE
WB
dadaE
WB
Eda
daW
da
daB
Eda
daW
da
daB
Eda
daW
da
daW
da
daB
dada
WEdada
Wdada
B
WEdada
Wdada
BB
dada
Wdada
BWEB
MC
M
M
MC
M
CM
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
M
C
minusminus
sdot=
minus+minus=
minusminusminus=
minusminusminus=
minus
minussdotminus=
minussdot
minus
minussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=
minussdot
minusminussdot=sdotminus
sdot+sdot=++
1
11
11
1
1
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Entrainment water
E =Dry air flow rate (sheet 2)
Evaporation rate E Gs = VALUE kg ass Water
VALUE kgkg1- Absolute humidity of exit air E = VALUE kgs Assuming that the leaving air is basically saturated at Point O Entrainment loss W
100 To estimate the entrainment lossesThe enthalpy at this point from the one assumes that these losses are calculation Table 1 is a percentage ampW of the water flow rate
VALUE kJkg ampL = 02 Assuming initially a temperature value The water flow rate is
30 degC L = 15 kgsthe corresponding enthalpy for this W = L ampL Compensation waterassumed temperature is with L = 15 kgs
400 degC ampL = 0002 -
100 W = 0030 kgs 0 masl
VALUE kJkg Elimination rate BNow using Solver find a value of the B =
E = VALUE kgsWith calculated exit air temperature W = 0030 kgs
the corresponding absolute humidity 500 ppm
can be be calculated 2000 ppm400 degC B = VALUE kgs
100
0 masl Compensation rate M
VALUE kgkg M =
B = VALUE kgs2- Absolute humidity of inlet air W 0030 kgs
From sheet 2 2000 ppm
VALUE kgkg 500 ppm
M = VALUE kgs3- Humidity change
kgkg
VALUE kgkg
VALUE kgkg
VALUE kgkg
GS ∆x
2-1
∆x2-1 =
E ( daM (daC - daM) ) - W
temperature tO to obtain that h = hO
da_M
=
da_c
=
(B + W) daC da
M
da_c
=
da_M
=
xa2
- xa1
M daM
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
Entrainment water
Air
Elimination water
Microsoft Editor de ecuaciones 30
L1 G1
Cooling tower
G2
E
W daC
B daC
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Operation Diagram
1 2 3 3a 4 5 6Curva de Liacutenea de Liacutenea de
equilibrio para operacioacuten operacioacutenaire saturado para r = 1 para r = 15
kJkg kJkg kJkg kJkg kJkg 1(kJkg)250 VALUE255 VALUE
290 VALUE VALUE VALUE VALUE VALUE VALUE317 VALUE VALUE VALUE VALUE VALUE VALUE343 VALUE VALUE VALUE VALUE VALUE VALUE370 VALUE VALUE VALUE VALUE VALUE VALUE397 VALUE VALUE VALUE VALUE VALUE VALUE423 VALUE VALUE VALUE VALUE VALUE VALUE
450 VALUE VALUE VALUE VALUE VALUE VALUE
∆h = ∆h = 1∆h
tL
hairsat
hoper_r=1
hairsat
-hop_r=1
hoper_r=15
hairsat
-hop_r=15
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
2
1
2
3
4
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
7 8Numerical Hintegration (gas-vapor mixture)coefficient kJkg as
f(x)
4
1 VALUE2 VALUE2 VALUE2 VALUE2 VALUE2 VALUE
1 VALUE 3
VALUE
1
Ci
H2
H2
H1
Σf(x) =
H1
Liquid temperature t
tL1
tL2
250 300 350 400 450 50000
20
40
60
80
100
120
Operation Diagram of Cooling Tower
Equilibrium curveColumn EOp L r = 15
Liquid temperature [degC]
En
tha
lpy
air
-va
po
r [
kJk
g d
a]
Equilibrium curve for saturated air
Operating line with r = 1
Operating line with r = 15
R
ST
U( )HtL
( )acuteHtL
( ) ii Ht
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Rev cjc 30012014
2
Liquid temperature tL degC
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Cooling tower schematic [3]
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill2003
Page 274 Enfriamiento de agua con aire
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Microsoft Editor de ecuaciones 30
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
[1] Operaciones de transferencia de masa 2eRobert E TreybalMcGraw Hill 2003
[2]
[3]
httplibrarykfupmedusaISI20065-2006pdf
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
1- Packing heightThe packing height of a tower can be calculated according [2] equation (65)
Z = I Tower packing height
flow rate of dry air (is a constat)
molar mass of air
Naming the first term as Height of Transfer mass transfer coefficient in the air Unit (HTU) a effective heat or mass transfer surface
A free cross-sectional surface of the tower
enthalpy in the air phase = enthalpy of humid air (in the bulk phase)
enthalpy in the air phase (i at ther boundaryand the second term as the Numbert of Transfer that is in saturated condition)Units (NTU)
HTU Height of Transfer UnitNTU Number of Transfer Units
Subscripts
B dry airthe packinh height becomes y air phase = humid air
a top of the towerb bottom of the toweri corresponds to the boundary (ie saturated state)
Packing height and free-cross sectional surface of a tower [2]
QS =V
MB
ky
Iy
Iyi
Z=(QS
M Bsdotk ysdotasdotA )sdotintI y b
I y a1
( I y iminusI y )iquest dI y
AakM
VHTU
yB sdotsdotsdot=
( ) yyiy
I
I
dIII
NTU
ay
by
sdotminus
= int
1
Z=HTUsdotNTU
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
Tower packing height [m]
flow rate of dry air (is a constat) [kgs]
molar mass of air [kgkmol]
mass transfer coefficient in the air [kmol(msup2s)]effective heat or mass transfer surface [msup2msup3]free cross-sectional surface of the tower [msup2]
enthalpy in the air phase = enthalpy of humid air [Jkg](in the bulk phase)
enthalpy in the air phase (i at ther boundarythat is in saturated condition)
Height of Transfer UnitNumber of Transfer Units
air phase = humid airtop of the towerbottom of the towercorresponds to the boundary (ie saturated state)
Microsoft Editor de ecuaciones 30
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-
- Index
- 1- Data
- 2- Tower height
- 3- NTU amp HTU
- 4- Tower area
- 5- Compensation water
- 6- Operating diagram
- 7- Cooling tower schematic
- Ref 1
- Ref
- Ref 2
-