Kek Final Lab Thermo

7/31/2019 Kek Final Lab Thermo

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Spiral Heat Exchanger

1) Objective: The objective of this experiment is to investigate heat transfer in a spiral heat

exchanger and to compute and compare the heat losses, log mean temperature

and overall heat transfer coefficient for both co-current and counter-current

modes of operation.

2) Introduction: Heat exchanger is a device which is used for transferring energy in the form

of heat from one fluid to another. In some cases, a solid wall may separate

the fluids and prevent them from mixing. In other designs, the fluids may be in

direct contact with each other. In the most efficient heat exchangers, the

surface area of the wall between the fluids is maximized while simultaneously

minimizing the fluid flow resistance. Fins or corrugations are sometimes used

with the wall in order to increase the surface area and to induce turbulence.

Heat exchangers are widely used in the process industries so their design

has been highly developed. Most exchangers are liquid-to-liquid, but gas

and no condensing vapours can also be treated in them.A spiral heat exchanger may refer to a helical (coiled) tube configuration.

A spiral heat exchanger is more compact than many other types of heat

exchangers. It has two concentric spiral channels, one for the hot fluid and

the other for the cold fluid. The main advantages of a spiral heat exchanger

are its high overall heat transfer coefficient, compact size for a given heat

exchange area, operational flexibility, relatively low pressure drop, and ease

of cleaning. Good access for cleaning is available when needed by

removing one or both of the ends of the heat exchanger, exposing the spiral

channels from the side. It is, in fact, self-cleaning for many applications

because the fluid turbulence created by the spacer studs and curved pathway

for the fluids tends to flush away deposits as they form. An important

feature of spiral plate exchangers is its capacity to handle high viscosity and

highly suspended liquids, exhibiting lower tendency to fouling. Because of

the well defined flow path through the spiral channels for both fluids and

the fluid turbulence generated by the spacer studs and the curved fluid path,

the overall heat transfer coefficient is typically higher for a spiral heat

exchanger than for other heat exchanger types. Spiral heat exchanger flow
http://www.wisegeek.com/what-is-turbulence.htmhttp://en.wikipedia.org/wiki/Spiralhttp://en.wikipedia.org/wiki/Helixhttp://en.wikipedia.org/wiki/Helixhttp://en.wikipedia.org/wiki/Spiralhttp://www.wisegeek.com/what-is-turbulence.htm


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may be counter-current flow, co-current flow, or cross flow.In the counter-flow heat exchanger, the fluids enter the exchanger from opposite sides.

This is the most efficient design because it transfers the greatest amount of

heat. In the parallel-flow heat exchanger, the fluids come in from the same

end and move parallel to each other as they flow to the other side. The

cross-flow heat exchanger moves the fluids in a perpendicular fashion.

Figure 1: Concurrent and countercurrent flow

3) Theoretical Background:

The Heat Exchanger Design Equation

Heat exchanger theory leads to the basic heat exchanger design equation:

Q = U A Tlm , where

Q is the rate of heat transfer between the two fluids in the heat exchanger in W,

U is the overall heat transfer coefficient in W/m2.k,

A is the heat transfer surface area in m2,

and Tlm is the log mean temperature difference in K, calculated from the inlet and outlet

temperatures of both fluids.


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The basic heat exchanger design equation can be used to calculate the overall heat transfer

coefficient for known or estimated values of the other three parameters, Q, A, and Tlm. Each

of those parameters will now be discussed briefly.

Heat Transfer Rate, Q

Heat transfer rate, Q can be calculated from the known flow rate of one of the fluids, its heat

capacity, and the required temperature change. Following is the equation to be used:

Qhot = mt Cpt (THin - THout) = Ws Cps (TCout - TCin) , where

mt = mass flow rate of hot fluid, kg/s,

Cpt = heat capacity of the hot fluid, J/s,

Ws = mass flow rate of cold fluid, kg/s,

Cps = heat capacity of the cold fluid, J/s,

The required heat transfer rate can be determined from known flow rate, heat capacity and

temperature change for either the hot fluid or the cold fluid. Then either the flow rate of the

other fluid for a specified temperature change, or the outlet temperature for known flow rate

and inlet temperature can be calculated.

Log Mean Temperature Difference

The driving force for any heat transfer process is a temperature difference. For heat

exchangers, there are two fluids involved, with the temperatures of both changing as they

pass through the heat exchanger, so some type of average temperature difference is needed.

Log mean temperature is defined in terms of the temperature differences as shown in the

equation at below. Th,inand Th,out are the inlet and outlet temperatures of the hot fluid and

Tc,in and Tc,out are the inlet and outlet temperatures of the cold fluid.


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Overall Heat Transfer Coefficient

The overall heat transfer coefficient, U, depends on the conductivity through the heat transfer

wall separating the two fluids.

Overall heat transfer coefficient, U = Q /A Tlm , where

Total exchange area, A : X Length of tube X Tube OD (m)

Tube OD : coil tubing outer diameter (m)

Tlm : log mean temperature difference in K

Heat Transfer Coefficient At Tube Side

Area of the tube, At = di2)/4

Mass velocity, Gt = mt/At

Linear velocity, ut = Gt/

Ronolds No, Re = (Gt.de)/

Prandtl No, Pr = (.Cp)/k

Tube side coefficient, hi = (0.023R0.8

.Pr0.33

.k)/di for laminar flow, where

di: coil tubing inner diameter (m)

mt : mass flow rate (kg/s)

: fluid density (kg/m3)

: fluid viscosity (Pa.s)

k : thermal conductivity (W/m.K)

Cp : heat capacity (J/kg.K)

de : equivalent diameter (m)

Heat Transfer Coefficient At Shell Side

Cross flow area, As = (D32D2

2+ D1

2)./4

Mass velocity, Gs = Ws/As


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Linear velocity, us = Gs/

Equivalent diameter, de = (D32D2

2+ D1

2)/(D1 + D2 + D3)

Reynolds number, Re = (Gs.de)/

Prandtl number, Pr = (.Cp)/k

Nuselt number, Nu = 0.023Re0.8

.Pr0.33

Stanton number, St = Nu/(Re.Pr)

Heat transfer factor, jh = St.Pr0.67

Shell side coefficient, hs = (jh.Re.Pr0.33

.k)/de, where

d1 : coil outside diameter

d2 : coil inside diameter

d3 : shell inside diameter

Ws : mass flow rate (kg/s)

4) Experiment Setup:

Equipment

The apparatus used in this experiment are a Spiral heat exchanger, a cold water circuit

consists of a 50L tank and centrifugal pump, a hot water circuit consists of a 50L tank and

centrifugal pump, temperature and flow rate indicators from SOLTEQ, model HE158E.

Figure 2:SOLTEQ, model HE158E


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Figure 3: Spiral Heat Exchanger

Experimental Setup

General Start-up Procedures

A quick inspection was done to make sure the equipment is in a proper working condition.

All the valves are made sure to be initially closed except V1 and V11. The hot water tank was

filled up via a water supply hose connected to V25. The valve was closed once the tank is full.

The cold water tank was filled up by opening valve V26 and the valve was left opened for

continues water supply. A drain hose was connected to the cold water drain point. Then, the

main power and the heater for the hot water were switched on. The temperature controller

was also set pointed to 50oC. The water temperature in the hot water tank was allowed to

reach the set point. After that, the equipment is ready to be run.

General Shut-down Procedures

The heater was switched off and the hot water temperature was waited until it drops below

50o

C . Then, pump P1 and P2 were switch off. After that, the main power was switched off

and all water in the process lines were drained off. The water in the hot and cold water tanks

was retained. Finally, all the valves were closed.

Experimental Procedure (Counter-current)

A first, general start-up procedure was performed before the experiment begins. The

arrangement of the valve of Spiral heat exchanger was switch to counter-current. Pump P1

and P2 were also switched on. Then, valves V3 for hot water while valve V13 were openedand adjusted to obtain the desired flow rates for hot water and cold water stream respectively.


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The system was allowed to reach steadystate for 10 minutes. Data for Fl1/Fl2, Fl3/Fl4, TT1,

TT2, TT3, and TT4 were recorded. These steps were repeated for different combinations of

flow rate FT1/FT2 and Fl3/Fl4 as recorded in the result tables. Pump P1 and P2 were switch

off when the experiment complete. All the results are tabulated under the tables below. After

that, proceed to the co-current experiment.

Experimental Procedure (Co-current)

A first, general start-up procedure was performed before the experiment begins. The

arrangement of the valve of Spiral heat exchanger was switch to co-current as the experiment

begins. Pump P1 and P2 were also switched on. Then, valves V3 for hot water while valve

V13 were opened and adjusted to obtain the desired flow rates for hot water and cold water

stream respectively. The system was allowed to reach steadystate for 10 minutes. Data for

Fl1/Fl2, Fl3/Fl4, TT1, TT2, TT3, and TT4 were recorded. These steps were repeated for

different combinations of flow rate FT1/FT2 and Fl3/Fl4 as recorded in the result tables.

Pump P1 and P2 were switch off when the experiment complete. All the results are tabulated

under the tables below. Finally, the equipment was shut-down.

5) RESULT:Counter-Current Spiral Heat Exchanger

`Table 1: Result Table For Counter-Current Spiral Heat Exchanger

FL1/FL2

Hot Water

(LPM)

FL3/FL4

Cold Water

(LPM)

TT1

Hot Inlet

(C)

TT2

Hot Outlet

(C)

TT3

Cold Outlet

(C)

TT4

Cold Inlet

(C)

5.0 2.0 50.9 47.8 37.5 31.0

5.0 3.0 51.2 47.9 36.2 31.4

5.0 4.0 50.8 47.0 35.3 31.2

5.0 5.0 51.0 46.5 34.4 31.2

2.0 5.0 50.7 43.7 33.5 31.1

3.0 5.0 50.9 46.1 33.7 30.9

4.0 5.0 50.7 47.0 34.0 31.2

5.0 5.0 51.1 47.8 34.3 31.2


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Co-current spiral heat exchanger

Table 2: Result Table For Co-Current Spiral Heat Exchanger

6) Data Analysis:

Typical Data

Hot Water

Density,t 988.18kg/m3

Heat Capacity, Cpt 4175 J/kg.K

Thermal conductivity, Kt 0.6436 W/m.K

Viscosity, t 0.0005494 Pa.s

Cold Water

Density,s 995.67kg/m3

Heat Capacity, Cps 4183 J/kg.K

Thermal conductivity, Ks 0.6155 W/m.K

Viscosity, s 0.0008007 Pa.s

Shell & Tube Heat Exchanger

Tube O.D. (do) : 9.53 mm

FL1/FL2

Hot Water

(LPM)

FL3/FL4

Cold Water

(LPM)

TT1

Hot Inlet

(C)

TT2

Hot Outlet

(C)

TT3

Cold Inlet

(C)

TT4

Cold Outlet

(C)

5.0 2.0 50.8 47.7 31.1 37.0

5.0 3.0 50.9 47.4 32.6 36.1

5.0 4.0 51.1 47.2 31.8 35.3

5.0 5.0 50.9 47.1 31.5 34.5

2.0 5.0 50.7 39.6 33.1 31.3

3.0 5.0 50.8 44.0 34.4 31.5

4.0 5.0 50.8 45.7 35.8 32.4

5.0 5.0 51.1 46.8 35.0 31.8


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Tube I.D. (di) : 7.05 mm

Tube Length (L) : 5 m

Shell diameter (d3) : 85 mm

Coil Surface Area : 0.15m2

Coil I.D. (d2) : 34mm

Coil O.D. (d1) : 44mm

Sample Calculation:

Sample calculation is based on data from test 1 of fixed hot water flow rate at 5 LPM.

a) Heat transfer rate for

i) Hot water

Qhot = mh Cp (T1-T2)

=[(5.0 L/min).(1/1000 m3/ L) .(1/60 min/s).(988.18 kg/m

3)] X (4175 J/kg.C)

X (50.9-47.8)

= 1065.17 W

ii) Cold water

Qs = WsCps (t1-t2)

=[(2.0 L/min).(1/1000 m3

/ L) .(1/60 min/s).(995.67 kg/m3

)] X (4183 J/kg.C)X (37.5-31.0)

= 902.39 W

b) Heat lost rate = QhotQs

= 1065.17W - 902.39W

=162.78 W

c) Efficiency = (Qs/ Qhot) x 100%

= (902.39W / 1065.17W) X 100%

= 84.72%

d) Log mean temperature difference

Tlm = [ (T1-t2) - (T2-t1) ] / ln [ (T1-t2) / (T2-t1) ]

= -3.4 / ln (13.4/16.8)

= 15.04 C


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e) Heat transfer coefficient at tube side

Cross flow area, At= di2/4

= 3.142 X 0.007052m

2

= 3.90 X 10-5

m2

Mass Velocity, Gt = mt/At

= 0.0823 kg/s/ 3.90 X 10

-5m

2

= 2108.03 kg/m2.s

Linear velocity, ut = Gt/t

= 2108.03 kg/m2.s / 988.18 kgm

-3

= 2.133 ms-1

Reynolds No, Re = (Gt X di)/t

= [2108.03 kg/m2.s X (7.05/1000)m]/0.0005494 Pa.s

= 27050.62 (turbulent flow)

Prandtl No, Pr = X Cp/ k

= (0.0005494 Pa.s X 4175 J/kg.K)/0.6436 W/m.K

= 3.56

Tube side coefficient, hi = 0.023Re0.8

Pr0.33

k/di

= 11221.13 W/m2K

f) Heat transfer coefficient at shell side

Cross flow area, As= /4 [d32d2

2+ d1

2]

= 0.00629 m2

Mass velocity, Gs = Ws/As

= 0.033kg/s/0.00629 m2

= 5.25 kg/m2.s

Linear velocity, us = Gs/s

= 5.25 kg/m2.s/995.67kg/m

3

= 0.00527 m/s


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Equivalent diameter, de = (d32d2

2+ d1

2)/(d1 + d2 + d3)

= (852

- 342

+ 442)mm

2/ (85+34+44)mm

= 49.11mm

Reynolds No, Re = Gs X de/

= 5.25 kg/m2.s X (49.11/1000)m /0.0008007 Pa.s

= 322.00 (Laminar flow)

Prandtl no, Pr = X Cp/ k

= 0.0008007 Pa.s X 4183J/kg.K / 0.6155W/m.K

= 5.44

Nuselt no, Nu = 0.023 X Re0.8 X Pr0.33

= 0.023 X 3220.8

X 5.440.3

= 4.08

Stanton no, St = Nu / (Re x Pr)

= 4.08 / (322 X 5.44)

= 0.00233

Heat transfer factor, jh = St X Pr0.67

= 0.00230 X 5.440.67

= 0.00715

Shell side coefficient, hs = (jh X Re X Pr0.33

X ks)/de

= 0.00715 X 322 X 5.440.33

X 0.6155 W/m.K/0.04911m

= 50.46 W/m2.K

g) Overall heat transfer

Total exchange area, A = X 0.00953 m X 5m

= 0.15 m2

Overall heat transfer coefficient, U = Qhot / (A.Tlm)

= 1065.17W / 0.15m2 X 15.04K

= 472.46 W/m2.K


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Table

Experiment 1: Counter-current spiral heat exchanger

Fixed Hot Water Flow Rate at 5LPM

Parameter Unit Test 1 Test 2 Test 3 Test 4

Hot Fluid(Tube): Water

Volumetric flowrate L/min 5.00 5.00 5.00 5.00

Inlet temperature, T1 C 50.9 51.2 50.8 51.0Outlet temperature, T2 C 47.8 47.9 47.0 46.9

Mass flow, mt kg/s 0.0823 0.0823 0.0823 0.0823

Heat transfer rate, Qhot J/s 1065.17 1133.89 1305.69 1409.60

Cold Fluid(Tube): Water


Inlet temperature, t1 C 31.0 31.4 31.2 31.2Outlet temperature, t2 C 37.5 36.2 35.3 34.4

Mass flow, mt kg/s 0.033 0.050 0.066 0.083

Heat transfer rate, Qs J/s 902.39 999.57 1138.40 1110.64

Temperature difference

T log mean C 15.04 15.74 15.65 15.94

Heat loss W 162.78 134.32 167.29 298.96

Efficiency % 84.72 88.15 87.19 78.79

Heat Transfer Coefficient

Total exchange area, A m2 0.15 0.15 0.15 0.15

Tube coefficient, hi W/m.K 11221.13 11221.13 11221.13 11221.13

Shell coefficient, hs W/m.K 50.46 71.50 88.98 107.00

Overall heat transfer

coefficient

W/m .K 472.46 480.26 556.20 589.54

Table 3: Calculations for counter-current spiral heat exchanger ( fixed hot water at 5 LPM)


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Fixed Cold Water Flow Rate at 5LPM





Mass flow, mt kg/s 0.033 0.049 0.067 0.082





Mass flow, mt kg/s 0.083 0.083 0.083 0.083



T log mean C 14.78 16.18 16.35 16.70

Heat loss W 129.67 18.35 45.85 58.65Efficiency % 86.53 98.15 95.49 94.83






coefficient

W/m.K 434.21 407.97 414.95 452.91

Table 4: Calculations for counter-current spiral heat exchanger ( fixed cold water at 5 LPM)


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Experiment 2: Co-current spiral heat exchanger

Fixed Hot Water Flow Rate at 5LPM





Mass flow, mt kg/s 0.0823 0.0823 0.0823 0.0823





Mass flow, mt kg/s 0.033 0.050 0.066 0.083



T log mean C 15.16 14.50 15.60 16.00

Heat loss W 246.71 349.52 369.03 265.24

Efficiency % 76.85 70.95 72.48 79.70






coefficient

W/m .K 468.69 553.25 573.00 544.36

Table 5: Calculations for co-current spiral heat exchanger ( fixed hot water at 5 LPM)


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Fixed Cold Water Flow Rate at 5LPM





Mass flow, mt kg/s 0.033 0.049 0.067 0.082





Mass flow, mt kg/s 0.083 0.083 0.083 0.083



T log mean C 12.73 14.36 14.13 15.54

Heat loss W 901.76 396.21 222.67 367.72

Efficiency % 40.93 71.75 84.13 75.51






coefficient

W/m.K 799.42 651.22 661.82 634.22

Table 6: Calculations for co-current spiral heat exchanger ( fixed cold water at 5 LPM)


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Graph

1)

Figure 4: Temperature Profile For Counter Current Spiral Heat Exchanger

(fixed hot water at 5 LPM)

2)

Figure 5: Relationship between Heat Transfer Coefficient and Cold Water Flowrate for

Counter Current Spiral Heat Exchanger (fixed hot water at 5 LPM)

50.98 47.4

35.85

31.2

0

10

20

30

40

50

60

Temperature

Temperature Profile for counter current Spiral Heat Exchanger

hot cold

0

2000

4000

6000

8000

10000

12000

0 1 2 3 4 5 6

Hea

tTransferCoefficient

Cold Water Flowrate (LPM)

Relationship between Heat Transfer Coefficient and Cold Water

Flowrate

Tube

coefficient, hi

Shell

coefficient, hs


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3)

Figure 6: Overall Heat Transfer Coefficient versus Cold Water Flowrate for Counter Current

Spiral Heat Exchanger (fixed hot water at 5 LPM)

Discussion:

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6

OverallHeattransferCoefficient

Cold Water Flowrate (LPM)

Overall Heat Transfer Coefficient Vs Cold Water Flowrate

Overall heat

transfer

coefficient

(counter-

current

flowrate)Overall heat

transfer

coefficient

(co-current

flowrate)

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