DESIGN OF COOLING TOWER.docx

8/14/2019 DESIGN OF COOLING TOWER.docx

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DESIGN OF COOLING TOWER

DESIGN CONDITIONS:

Cooling Tower Type COUNTER FLOW INDUCED DRAFT

Water Temperature

Leaving C25

Entering C30

Air Condition

Leaving WBCDBC 98,38

Entering RHDBC %65,26

Make-up Water

Temperature C26

Barometric Pressure kPa325.101

Note:

(1)The practical cooling range 21 tt is C 7.166.5 (Morse, 1990) p 122(2)For cooling towers the rating conditions are C35 entering water, C4.29

leaving water, and C9.23 wet bulb of the outdoor air (Wang, 2001) p 10.50

(3) In most cases, the temperature of the water leaving the tower will be Fto 107

above wet bulb temperature of the entering air (Dossat, 1978)p 333


2/17

Calculation Procedure:

Amount of water to be handled by the cooling tower, mcw

sec74.13

sec78.1096.2

kgm

kgm

mmm

cw

cw

condensercw

compressorcwcw

Air Properties Using Psychometrics Chart

At RHandC %6526

airdryofkgmoistureofkg

HRkg

KJh 0136.0;61 44

At RHandC %9832


3/17

airdryofkg

moistureofkgHR

kgKJh

0297.0;108 55

Mass of Air, ,am

Energy Balance on the cooling tower

3421

4321 &

hhmtCpmttCpm

mmmmmm

amwwmwcwcw

acw

mww

cwcwamw

tCp

ttCpmhhmm 2134

1.642.243174.0

26187.4

2530187.4sec

74.1361108

00

00

eqmm

CCkg

KJ

CCkg

KJkgkg

KJm

m

amw

a

mw

Mass Balance

Mass entering = Mass leaving

2.0161.0

)0136.00297.0()..(

..

34

43

eqmm

mRHRHmm

RHmmRHmmm

amw

aamw

awawmw


4/17

Substituting eq.1 to eq.2

642.243174.00161.0 aa mm

aa mm 0161.04317.0642.2

sec36.6 kg

ma

Mass of Make-up water, 5wm

From Equation 2

waterofkgkg

m

mm

w

aw

sec1023.0

sec36.60161.0

0161.0

5

5

Volume flow rate of air, aV

KPaKPaPsatRHPsKPaPsatRHandCAt

18595.2363.365.0

363.3,%6526

MRTPV

3

0

0

18595.2325.101

273262871.0sec

36.6

mKNorKPa

KKkg

KJkg

PP

TRmV

SB

aaaa

sec507.5 3m

Va

Where: R.H= relative humidity

vaporwaterofpressurepartialP

CatpressuresaturationP

V

Sat

26


5/17

Volume flow rate of air, wV

From Refrigeration Engineering by Mc Intire P. 495. cross-sectional area of

induced draft fan is found by allowing gpm/ft3.0 2

t =2

3025= 27.5 C @ 27.5 C

3512.996

m

kgw

Therefore,

Volume flow rate ( w ) =3

512.996

sec74.13

m

kg

kgM

w

w

=min1

sec60

003785.0

1

sec0138.0

3

3

xm

galx

m

= 218.57 gpm

Cooling tower range, C.T.R.

C.T.R. = Ctt o521

Cooling tower approach, C.T.A.

At 26 Co and 65% RH ; Ct owb 75.18

C.T.A. = Ctt owb 25.675.18251


6/17

Cooling tower efficiency, C.T.E.

C.T.R. = %10075.1830

2530%100

1

21 xxtt

tt

wb

= 44.4 %


7/17

PIPING SYSTEM FOR COOLING TOWER

Design Condition:

Mass flow rate cooling water

Compressor sec96.2 kg

Condenser sec78.10 kg

water = 996.512 kg/ 3m ; f @ Ct

o

ave 5.27

Water Velocity, v = 1.83 m/sec (range: 1.5 to 2.1m/sec)

Solving for the total amount of cooling water, TcwQ

sec/1097.2/512.996

sec/96.2 333

mxmkg

kgQcompressor

sec/1082.10/512.996sec/78.10 333 mxmkg

kgQcondenser

condensercompressorTcw QQQ

sec/)1082.101097.2( 333 mxxQTcw

sec/01379.0 3mQTcw


8/17

PIPE SIZE FOR THE SUPPLY AND RETURN PIPES

From cooling tower pool to tee-run

vDQTcw2

4

sec83.1

4sec01379.0 2

3 mD

m

D= 0.0979 m = 97.9 mm

USE: 100 mm NPS schedule 40 (from Table 7-2 Dimensions of steel pipe by

Wilbert F. Stoeker, P 136)

OD = 114.3 mm ID = 102.3 mm

From tee-run to condenser inlet

vDQcondenser2

4

sec83.1

4sec1082.10 2

33 mDm

x

D= 0.08676 m = 86.76 mm

USE: 100 mm NPS schedule 40 (from Table 7-2 Dimensions of steel pipe by

Wilbert F. Stoeker, P 136)

OD = 114.3 mm ID = 102.3 mm


9/17

From tee-run to compressor inlet

vDQcompressor2

4

sec83.1

4sec1097.2 2

33 mDm

x

D= 0.04545 m = 45.45 mm

USE: 50 mm NPS schedule 40 (from M.E.T.C. Steel Pipe Dimensions, P 114.)

OD = 73.03 mm ID = 62.65 mm


10/17

COOLING WATER PUMP

Design Data:

Type Centrifugal

Capacity, QP 0.01379 m3/sec

Average water temperature, tave 27.50C

Water density, 996.4129 kg/m3

Main pipe 100 mm NPS

Capacity, QT 0.01379 m3/sec

Condenser pipe 100 mm NPS

Capacity, Qcond

0.01082 m

3

/sec

Compressor pipe 50 mm NPS

Capacity, Qcomp 0.00297 m3/sec

Static head, hS 2 m


11/17

Friction Heads,hf

kpLfactorcorrectioneTemperaturdropessurelengthequivalentTotalh ef Pr

For the main line, 100 mm NPS, Schedule 40, from cooling tower to Tee of

condenser

Straight Pipe ------------------------------------------------------------ 6.42 m

2 900elbow ------------------------------------------------------------ 6 m

1 Standard tee ------------------------------------------------------------ 4.16 m

1 Gate valve ------------------------------------------------------------ 0.52 m

1 Check valve ------------------------------------------------------------ 9.15 m

__________

Total length 26.25 m

For the condenser line, 100 mm NPS, Schedule 40, from condenser tee to

condenser inlet

Straight Pipe ------------------------------------------------------------ 2.23 m

1 Standard tee ------------------------------------------------------------ 5.18 m

1 gate valve (open) --------------------------------------------------- 0.52 m


12/17

1 900elbow ------------------------------------------------------------ 3.0 m

____________


For the condenser tubes, 25 mm NPS, Schedule 40

Straight Pipe ------------------------------------------------------ 279 m

3 Close return bend --------------------------------------------- 49.41 m

____________


For the compressor line, 50 mm NPS, Schedule 40, from condenser tee to

compressor tee

Straight Pipe ------------------------------------------------------------ 3.8 m

1 Standard tee ------------------------------------------------------------ 0.9 m

1 gate valve (open) --------------------------------------------------- 0.09 m

1 900elbow ------------------------------------------------------------ 0.6 m


13/17

____________

Total length 5.39 m

From Compressor Tee to compressor inlet, 50 mm NPS, Schedule 40

Straight Pipe ------------------------------------------------------------ 3.0 m

1 Standard tee ------------------------------------------------------------ 0.9 m

1 gate valve (open) --------------------------------------------------- 0.09 m

____________

Total length 3.99 m

For the compressor jacket water line, assume equivalent length of 2 m inside

the compressor cooling water system.

From Figure 76, Pressure drop for water flowing in schedule 40 steel pipes,

Refr igeration and Ai r-conditioning by Stoecker & Jones, page 138.

For 100 mm pipe & 7.89 L/sec, p= 220 Pa / m


14/17




From Figure 7 7, Multiplying factors for pressure drops to correct for

temperature, Refri gerati on and Air -conditioning by Stoecker & Jones, page 139.

For taveof 27.50C & 1.52 m/sec water velocity, k= 0.98

Therefore the friction heads hf, are equal to,

ofmkPa

OHofmxkPaPax

m

Paxmhf

21 5756.01017.06595.55.659,598.0

22025.26

omkPa

OHofmxkPaPax

m

Paxmhf

22 403.01017.0963.3218.963,398.0

37093.10

mkPa

OHofmxkPaPax

m

Paxmhf

23 55.421017.039.4184.394,41898.0

130041.328

ofmkPa

OHofmxkPaPax

m

Paxmhf

24 2148.01017.0112.288.211298.0

40039.5


15/17

HofmkPa

OHofmxkPaPax

m

Paxmhf

25 159.01017.0564.108.156498.0

40099.3

OHofmkPa

OHofmxkPaPax

m

Paxmhf 2

26 3.01017.098.2298998.0

15252

Also the total friction head hfT,is equal to,

hhhhhhh ffffffSf 654321 443.0159.02148.055.42403.05756.0

Total friction head,hfT

Assuming the supply line equal to the discharge line,

SfTf hh 2

Therefore,

OHofmOHofmh Tf 22 4.882.442

Total Pumping Head,HT

OHofmhhH SfTT 24.9024.88


16/17

Water Horsepower,Whp

hp

hpkW

mmkNmHQW TwaterPhp 4.16

746.0

4.9081.9sec01379.0

746.0

33

Motor Power,Pm

p

hpm

m

hpp

WPPW

;

Using 75 % pump efficiency,

hphp

Pm 86.2175.0

4.16

For standard motor sizes, use 25 hp electric motor for the cooling water pump.


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DESIGN OF COOLING TOWER.docx

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