Concentric Tube Heat Exchanger

ABSTRACT / SUMMARY

This experiment is conducted based o n the heat transfer at a different temperature gradient. In order to control and indicate the te mperatures of T H i n , T H o u t , T H m i d , T C m i d , T C i n , and T C o u t as well as the hot and cold water flow rate, we used the concentric tube heat exchanger which was aided with ther mo meters an d flow rate meters. The e xperi ment is separated into two parts; Part A and Part B. Part A is conducted with varying temperatures at constant flo w rate whereas Part B is conducted with varying flow rates at constant te mperature. In Part A, counter flow is more efficient than parallel flow. For parallel flow, the average efficiency is 32.33%, whereas the o verall heat transfer coefficient, U, is 0.6962 W /m 2 K a t 40°C, 1.0885 W /m 2 K a t 50°C, and 1.0098 W /m 2 K at 60°C. For counter flow, the average efficiency is 36.88%, whereas the overall heat transfer coefficient, U, is 1.2144 W /m 2 K at 40°C, 1.0885 W /m 2 K at 50°C, and 1.0429 W /m 2 K at 60 °C. In part B, counter flow is also more efficient than parallel flow. For parallel flow, the average efficiency is 33.41%, whereas the overall heat transfer coefficient, U, is 0.8101 W /m 2 K at 2000 c m 3 / min, 1.1139 W /m 2 K at 3000 cm 3 / min, and 1.0818 W /m 2 K at 4000 c m 3 / min. For counter flow, the average efficiency is 34.42%, whereas the overall heat transfer coefficient, U, is 0.9572 W /m 2 K at 2000 cm 3 / min, 1.1139 W /m 2 K at 3000 cm 3 / min, and 1. 0818 W /m 2 K at 4000 cm 3 / min. The experi ment is co mpleted and successf ully conducted.

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INTRODUCTION

The heat exchange process bet ween t he fluids that are at distinct te mperatures with a separation of solid wall occurs in many engineering applications. Heat exchanger is a device used to imple ment this exchange process. A fe w applications may include space heati ng and airconditioning, waste heat recovery and che mical processing.

Heat exchangers can be divided into two classifications, which are flow arrange ment accordance and con struction type. The heat exchanger applied in this experiment is the si mplest one, with the hot as well as the cold fluids move in the sa me or opposi te directions in a concentric tube construction. In the parallel flow arrange ment, both hold and cold fluids enter at the sa me end, flo w in the sa me direction, and leave at the same end. In the counter flow arrange ment, t he fluids enter at different ends, flow in different directions, and leave at different ends. The two configurations are differentiated by an idealisation that controls the fluid motion over the tubes as being unmi xe d or mixed.

The heats were transferred bet ween t wo fluids via convention mode, which refers to the hot fluid to the wall and also by conduction which occur within the wall itself and back to the convection process fro m wall to the cold fluid.

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AIMS / OBJECTIVES

The objectives of this experiment is to demonstrate the working principles of

concentric flow heat exchanger under parallel as well as

counter flow conditions, to demonstrate the effect of heat water inlet te mperature variation on the performa nce characteri stics of a concentric tube heat exchanger, to demonstrate the effect of flow rate variation on the performance of a concentric tube heat exchanger and also to deter mine the mo st efficient of concen tric tube heat e xchanger whether it is the parallel flow or counter-current flow.

THEORY

Concentric tube heat exchanger is one of the most co mmon conductive-convective types of heat exchanger. Parallel flow is defined as when both fluids enter the concentric tube heat exchanger fro m the sa me sides and flow through the sa me directions whereas the counter flow is defined as when both fluids enter from the opposite sides and flow through the opposite directions. It is co mmonly claimed that the counter flow is more efficient than the parallel flow.

Consider a double-pipe heat exchanger. The heat transfer rate at any distance 2 along the tubes bet wee n the hot and cold fluids is given by q x = UA(T H ± T C ) ........... ................(1) þ
  

where A

: surface area for heat transfer consistent with definition of U

TH

: hot fluid temperature

TC

: cold fluid temperature

U

: the overall heat transfer coefficient based on either the inside or outside area of the tube.

As a matter of fact, the te mperature of the hot and cold fluids changes along the tube. Therefore, in order to calculate the heat transfer between the t wo fluids, equation (1) should be integrated between the inlet and outlet conditions, giving that

q = UA¨T l m ................ .........(2) where ¨T l m is the mean te mperature difference across the heat exchanger and it can be given as

¨T l m = ¨T i n - ¨T o u t / ln (¨T i n / ¨T o u t ) ..... .......................(3)

This temperature difference is called the log mean te mperature difference (LMTD) and is valid for both flow conditions. The derivation shown above is made according to two significant assumptions: first, the fluid specific heats do not vary with te mperature and second, the heat convection heat transfer coefficients are constant throughout the exchanger. The second assu mptions a re influenced by entrance effects, fluid viscosity and thermal conductivity changes.

The heat loss from the hot fluid flowing in the inner tube can be deter mined from (
  

qH =

H Cp H

(T H i n ± T H o u t ) ......... ........... ..........(4)

where

H

= hot water ma ss flow rate

Cp H

= hot water specific heat

THin

= hot fluid temperature at entrance

T H ou t = hot fluid temperature at e xit Si milarly, the heat gained by the cold fluid flowing in the space between the inner and outer pipes can be calculated fro m

qC =

C Cp C

(T C i n ± T C o u t ) ......... ........... ......(5)

where

C

= cold water mass flo w rate

Cp C

= cold water specific heat

TCin

= cold fluid temperature at entrance

T C ou t = cold fluid temperature at e xit

Suppose that q C is less than the q H , so me heat is lost through the insulating material to the surrounding air, abide the outer surface of the concentric tube is insulated. Thus, the efficiency can be obtained from

= q C ................... ...........(6) qH

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The effectiveness of a heat exchanger is defined as

=

actual heat transfer

........................(7)

Maximum Possible Heat Transf er

The value of the actual heat transfer may be obtained f rom calculating the energy lost by the hot f luid f rom equation (4) or the energy gained by the cold f luid f rom equation (5). Since the energy gained by the cold f luid is lost through the insulating material to the surrounding air, it is pref erable to substitute the value of energy lost by the hot f luid as the actual heat transf er in equation (7). In order to determine the maximum possible heat transf er for the heat exchanger, one of the f luids is logically required to undergo a temperature change which represents the maximum temperature diff erence present in the heat exchanger, which is the diff erence in the temperatures for the hot and cold f luids entering the heat exchanger. Likewise, the f luid is the one having the minimum value of

Cp. Thus, the ma xi mu m possible heat transfer then can

be expressed as

qmax = (

Cp) m i n (T H i n ± T C i n ) ........ ....... ........(8)

The minimu m fluid may be either the hot or cold fluid, depending on the mass flo w rates and specific heats, and so the efficiency

=

qH

, is

x 100 % .............. ........(9)

q max

K
  

APPARATUS

- Concentric tube heat exchanger - W ater tank - Thermo meters - Volumetric flow meters

EXPERIMENTAL PROCEDURE Part A: Constant flow rate, varies te mp eratures 1. The main switch is switched on. 2. The te mperature and pu mp switches are switched on. 3. The valve is set to parallel flow. 4. The hot water flo w rate is set at 3000 cm 3 / min and the cold water flow rate at 2000 cm 3 / min. 5. The te mperature is set at 40°C. 6. The syste m is let stable until T H i n is 40°C and the values of the te mperature at T H o u t , T H m i d , T C m i d , T C i n , and T C ou t are taken. 7. Step 4 until 6 are repeated by varying the te mperature at 50°C and 60°C. 8. Step 4 until 7 are repeated with counter flow.

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Part B: Constant te mperature, varies flow rates 1. The valve for parallel flow is set. 2. The te mperature is set at 60°C. 3. The hot and cold water flow rates ar e set at 2000 cm 3 / min. 4. The syste m is let stable and the temperature at T H o u t , T H m i d , T C m i d , T C i n , and T C ou t are taken. 5. Step 3 and 4 are repeated by varying the hot water flo w rate to 3000 c m 3 / min and 4000 cm 3 / min. 6. Step 2 until 5 are repeated with counter flow.

RESULTS Part A: Constant flow rate, varies te mp eratures Heat

Temperature (°C)

H 3

Exchanger

TH

in

TH

mid

TH

out

TC

in

TC

mid

TC

C 3

( cm /m i n)

( cm /m i n)

3000

2000

3000

2000

out

40

39

38

29

30

31

Parallel

50

48

45

29

30

36

Flow

60

56

53

29

33

39

40

39

37

29

30

32

Counter

50

48

45

29

31

36

Flow

60

57

53

29

33

40


  

Part B: Constant te mperature, varies flow rates Heat

Temperature (°C)

Exchanger

Parallel

TH

in

TH

60

Flow

Counter

60

Flow

mid

TH

out

TC

H

TC

in

mid

TC

( cm 3 /m i n)

C

( cm 3 /m i n)

out

54

51

27

31

36

2000

56

52

27

32

38

3000

57

54

27

33

39

4000

55

50

27

30

37

2000

56

52

27

31

38

3000

58

54

27

32

39

4000

2000

2000

SAMPLE CALCULATIONS

Part A: Constant flow rate, varies te mp eratures Parallel flow at 60°C : Take that the density of saturated wat er, ȡ = 988 kg/ ù , the specific heat capacity of hot and cold water as Cp H @ 6 0 ° C = 4.185 kJ/kg. K and Cp C @ 2 9 ° C = 4.179 kJ/kg. K respectively and the heat transmission area, A = 0.067 ù
. H

= 3000 cm 3 /min = 5 x 10 - 5 m 3 /s

C

= 2000 cm 3 /min = 3.33 x 10 - 5 m 3 /s

H

=

C

=

H

ȡ

= 5 x 10 - 5 m 3 /s x 988 kg/ ù = 0.0494 kg/s

C

ȡ

= 3.33 x 10 - 5 m 3 /min x 988 kg/ ù = 0.0329 kg/s

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
          












  

= 0.0329 kg/s x 4.179 kJ/ kg.K



= 0.1375 kJ/s.K   
 q H

=



= 0.0494 kg/s x 4.185 kJ/ kg.K x (333 ± 326) K



H Cp H

(T H i n ± T H o u t ) 

= 1.4472 W Ma xi mu m heat transferred, q m a x

= 




(T H i n ± T C i n )

= 0.1375 kJ/s.K (333 ± 302) K = 4.2625 W

Efficiency,

=

qH

x 100 %

q max = 1.4472 W x 100% = 4.2625 W

= 33.95 %

¨T i n - ¨T o u t

Log Mean Te mperature Difference, ¨T l m =

ln (¨T i n / ¨T o u t )  (333-302)K ± (326-312)K

ln ((333-302)K/(326-312)K) = 21.39 K

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Overall Heat Transfer Coefficient, U =

q A x ¨T l m

=

1.4448 W 0.067 m 2 x 21.39 K

= 1.0098 W /m 2 K

Heat Exchanger

T (°C )

q H (W )

qmax

(%)

(W )

Parallel Flow

Counter Flow

Average (%)

U (W /m 2 K)

40

0.4128

1.5125

27.29

0.6962

50

1.0327

2.8875

35.76

60

1.4472

4.2625

33.95

1.0098

40

0.6192

1.5125

40.94

1.2144

50

1.0327

2.8875

35.76

60

1.4471

4.2625

33.95

32.33

36.88

1.0885

1.0885 1.0429

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Part B: Constant te mperature, varies flow rates

Counter flow at 4000 cm 3 / min: The te mperature is fixed at 60°C. Take the density of saturated water, ȡ = 988 kg/ù , the specific heat capacity of hot and cold water as CpH @ 6 0 ° C = 4.185 kJ/kg. K and CpC @ 2 7 ° C = 4.179 kJ/kg. K respectively and the hea t trans mission area, A = 0.067 ù
.

H

= 4000 cm 3 /mi n = 6. 67 x 10 - 5 m 3 / s

H

=

H

= 2000 cm 3 /mi n = 3. 33 x 10 - 5 m 3 / s

ȡ

C

=

C

ȡ

= 6. 67 x 10 - 5 m 3 / s x 988 kg/ ù

= 3. 33x 10 - 5 m 3 /mi nx 988 kg/ ù

= 6. 59 x 10 - 2 kg/ s

= 0. 0329 kg/ s


          

C












  

= 0.0329 kg/s x 4.179 kJ/ kg.K



= 0.1375 kJ/s.K   
 q H = 



H Cp H

(T H i n ± T H o u t ) 

= 6.59 x 10 - 2 kg/s x 4.185 kJ/kg. K x (333 ± 327) K = 1.6547 W

Ma xi mu m heat transferred, q m a x = 




(T H i n ± T C i n )

= 0.1375 kJ/s. K (333 ± 300) K = 4.5375 W

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Efficiency,

=

qH

x 100 %

q max = 1.6547 W x 100% 4.5375 W = 36.47 %

Log Mean Te mperature Difference, ¨T l m =

¨T i n - ¨T o u t ln (¨T i n / ¨T o u t )

 (333-300)K ± (327-312)K

ln ((333-300)K/(327-312)K) = 22.83 K

Overall Heat Transfer Coefficient, U =

q A x ¨T l m

=

1.6547 W 0.067 m 2 x 22.83 K

= 1.0818 W /m 2 K

Heat

Flow

Exchanger

Rate

q H (W )

q m a x (W )

(%)

Average (%)

U (W /m 2 K)

(cm 3 /m in)

2000

1.2392

4.5375

27.31

0.8101

Parallel

3000

1.6539

4.5375

36.45

Flow

4000

1.6547

4.5375

36.47

1.0818

2000

1.3769

4.5373

30.34

0.9572

Counter

3000

1.6539

4.5375

36.45

Flow

4000

1.6547

4.5375

36.47

33.41

34.42

1.139

1.1139 1.0818

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SAMPLE ERROR CALCULATIONS

Part A: Constant flow rate, varies te mp eratures Parallel flow : Percentage of error = 100% ± %calculated value 100 = 100% ± 32.33 % 100 % = 0.6767%

Part B: Constant te mperature, varies flow rates Counter Flow : Percentage of error = 100% ± %calculated value 100 = 100% ± 34.42 % 100 % = 0.6558%

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DISCUSSION

There are a few ob jectives which are to be achieved in this experi ment; to de mon strate the workin g principles of concentric flow heat exchanger

under

parallel

as

well

as

counter

flow

conditions,

to

de monstrate the effect of heat water inlet temperature variation on the performan ce characteristics of a co ncentric tube heat exchanger, to de monstrate the effect of flo w rate variation on the perfor mance of a concentric tube heat exchanger and the most important part of the objectives is to determine the most ef ficient flow of concentric tube heat exchanger whether it is the parallel flow or counter-current flow.

A

concentric

tube

heat

exchanger

is

used

to

archive

these

objectives. The heat e xchanger itself is co mbined with ther mo meters and flow rate meters. Thus, the control of t he hot fluids temperatures and both hot and cold fluids flow rates are made easier. W e can observe the values of T H i n , T H o u t , T H m i d , T C m i d , T C i n , and T C ou t . This e xperiment is conducted with two parts of separated conditions, which are by varying the flow rates at constant te mperature and by varying the te mperatures at constant flow rate.

Part A is conducted by varying the temperatures fro m 40°C, 50° and 60°C at 3000 cm 3 / min of the hot fluids flow rate and 2000 cm 3 / min for th e cold fluids flow rate. The efficiency of parallel flow calculated is 32.33% and values of the overall heat transfer coefficients are 0.6962 W /m 2 K at 40°C¸ 1.0885 W /m 2 K at 50° C and 1.0098 W /m 2 K at 60° C. In contrary, the calculated results for counter flow is 36.88 % of efficiency and the overall

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heat transfer coefficients are 1.2144 W /m 2 K at 40°C, 1.0885 W /m 2 K a t 50°C and 1.0429 W /m 2 K at 60°C.

Part B is conducted with constant temperature at 27°C yet varying fluid flow rates. However, the cold fluid flow rate is maintained constant at 2000 c m 3 / min for both parallel and counter flow. The calculated efficiency for parallel flow is 33.41% whereas th e overall heat transfer coefficient is 0.8101 W /m 2 K at 2000 cm 3 / min of hot fluid flow rates, 0.9572 W /m 2 K at 3000 cm 3 / min and 1.0818 W /m 2 K at 4000 cm 3 / min. For counter flow, the efficiency is 34.42% mean while the heat transfer coefficients are 0.9572 W /m 2 K at 2000 cm 3 / min, 1.1139 W /m 2 K at 3000 cm 3 / min and 1.0818 W /m 2 K a t 4000 cm 3 / min.

Notice that for both e xperi ments in part A and Part B, the counter flow produce greater efficiency than parallel flow. This result follows the theoretical conclusion where counter flow heat exchanger is more efficient than parallel flow. Ho wever, there are a lot of errors and mistakes that may have affected the results obtained. The very co mmon error occurs during conducting the experiments are careless way of reading the ther mo meters when taking the te mp eratures of fluids. The eye of an observer must be parallel to the thermo meter meniscus to avoid parallax error. Another mistake that may have been co mmitted is not pressing the enter button after setting the tempe ratures. This has caused a minor problem when the te mperature always manipulate even after setting it to the desired temperature. Besides that, the flow rates always change easily during the experi ments. Moreo ver, the reading of T H

in

fro m the

typical laboratory thermo meter is mere ly different from the reading on the digital thermo meter.

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CONCLUSION

In Part A, counter flo w is more efficient than parallel flow. For parallel flow, the average efficiency is 32.33%, whereas the overall heat transfer coefficient, U, is 0.6962 W /m 2 K at 40°C, 1.0885 W /m 2 K at 50°C, and 1.0098 W /m 2 K at 60 °C. For counte r flow, the average efficiency is 36.88%, whereas the overall heat transfer coefficient, U, is 1.2144 W /m 2 K at 40°C, 1.0885 W /m 2 K at 50°C, and 1. 0429 W /m 2 K at 60 °C.

In part B, counter flow is also more efficient than parallel flow. For parallel flow, the average efficiency is 33.41%, whereas the overall heat transfer coefficient, U, is 0.8101 W /m 2 K at 2000 cm 3 / min, 1.1139 W /m 2 K at 3000 cm 3 / min, and 1.0818 W /m 2 K at 4000 cm 3 / min. For counter flow, the average efficiency is 34.42%, wher eas the overall heat transfer coefficient, U, is 0.9572 W /m 2 K at 200 0 cm 3 / min, 1.1139 W /m 2 K at 3000 c m 3 / min, and 1.0818 W /m 2 K at 4000 cm 3 / min.

RECOMMENDATIONS

There are a few reco mmendations and precautions that have to be considered when conducting this experiment so that better results can be obtained with fewer errors.

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First and foremost, the eye of an obser ver must be parallel to the meniscus when reading the te mperatures. This is to assure that no parallax error is commit ted.

Secondly, the experi ment should at least be repeated 3 times in order to get average values. Thus, comparisons can be made and the results are more convincing and precise.

Thirdly, the flow rates as well as the temperatures must be monitored thoroughly during the experiment so that they re main constant. This is to avoid such erroneous results or else, the objectives of the experi ment may not be achieved succe ssfully.

Besides that, any leakage of the instruments involved should be avoided and they should be assured to work properly. In addition, any direct contact with the water or the instru ments should as well be avoided as this experi ment involves hot fluids which can cause burn to skin.

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REFERENCES

1. Fundamental of Heat and Mass Tra msf er ( 6th Edition,) John W iley & sons (Asia) Pte Ltd 2. Saunders, E. A. (1988). Heat Exchang es: Selection, Design and Construction. New York: Long man Scientific and Technical.

APPENDICES ð

Refer to the attach ment provided on the next page.

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