Concentric Tube Heat Exchanger

ABSTRACT / SUMMARY

This experiment is conducted based on the heat transfer at a

different temperature gradient. In order to control and indicate the

temperatures of THi n , THou t , THmi d, TCmi d, TCi n , and TCou t as well as the hot

and cold water f low rate, we used the concentric tube heat exchanger

which was aided with thermometers and flow rate meters. The experiment

is separated into two parts; Part A and Part B. Part A is conducted with

varying temperatures at constant f low rate whereas Part B is conducted

with varying flow rates at constant temperature. In Part A, counter f low is

more eff icient than parallel f low. For parallel f low, the average eff iciency

is 32.33%, whereas the overall heat transfer coefficient, U, is 0.6962

W/m2K at 40°C, 1.0885 W/m 2K at 50°C, and 1.0098 W/m 2K at 60°C. For

counter f low, the average eff iciency is 36.88%, whereas the overall heat

transfer coefficient, U, is 1.2144 W/m 2K at 40°C, 1.0885 W/m 2K at 50°C,

and 1.0429 W/m2K at 60°C. In part B, counter f low is also more eff icient

than parallel f low. For parallel f low, the average eff iciency is 33.41%,

whereas the overall heat transfer coefficient, U, is 0.8101 W/m 2K at 2000

cm3/min, 1.1139 W/m2K at 3000 cm3/min, and 1.0818 W/m 2K at 4000

cm3/min. For counter f low, the average eff iciency is 34.42%, whereas the

overall heat transfer coefficient, U, is 0.9572 W/m 2K at 2000 cm3/min,

1.1139 W/m2K at 3000 cm3/min, and 1.0818 W/m 2K at 4000 cm3/min. The

experiment is completed and successfully conducted.

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INTRODUCTION

The heat exchange process between the fluids that are at distinct

temperatures with a separation of solid wall occurs in many engineering

applications. Heat exchanger is a device used to implement this exchange

process. A few applications may include space heating and air-

condit ioning, waste heat recovery and chemical processing.

Heat exchangers can be divided into two classif ications, which are

f low arrangement accordance and construction type. The heat exchanger

applied in this experiment is the simplest one, with the hot as well as the

cold f luids move in the same or opposite directions in a concentric tube

construction. In the parallel f low arrangement, both hold and cold f luids

enter at the same end, f low in the same direction, and leave at the same

end. In the counter f low arrangement, the f luids enter at different ends,

f low 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 unmixed or mixed.

The heats were transferred between two fluids via convention mode,

which refers to the hot f luid to the wall and also by conduction which

occur within the wall i tself and back to the convection process from wall to

the cold f luid.

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AIMS / OBJECTIVES

The objectives of this experiment is to demonstrate the working

principles of concentric f low heat exchanger under parallel as well as

counter f low condit ions, to demonstrate the effect of heat water inlet

temperature variation on the performance characteristics of a concentric

tube heat exchanger, to demonstrate the effect of f low rate variation on

the performance of a concentric tube heat exchanger and also to

determine the most eff icient of concentric tube heat exchanger whether it

is the parallel f low or counter-current f low.

THEORY

Concentric tube heat exchanger is one of the most common

conductive-convective types of heat exchanger. Parallel f low is defined as

when both f luids enter the concentric tube heat exchanger from the same

sides and flow through the same directions whereas the counter f low is

defined as when both f luids enter from the opposite sides and flow

through the opposite directions. It is commonly claimed that the counter

f low is more eff icient than the parallel f low.

Consider a double-pipe heat exchanger. The heat transfer rate at

any distance x along the tubes between the hot and cold f luids is given by

qx = UA(TH – TC ) ...........................(1)

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where

A : surface area for heat transfer consistent with definit ion of U

TH : hot f luid temperature

TC : cold f luid temperature

U : the overall heat transfer coefficient based on either the

inside or outside area of the tube.

As a matter of fact, the temperature of the hot and cold f luids

changes along the tube. Therefore, in order to calculate the heat transfer

between the two fluids, equation (1) should be integrated between the

inlet and outlet condit ions, giving that

q = UA∆T l m .........................(2)

where ∆T l m is the mean temperature difference across the heat exchanger

and it can be given as

∆T l m = ∆T i n - ∆Tou t / ln (∆T i n / ∆Tou t) ............................(3)

This temperature difference is called the log mean temperature

difference (LMTD) and is valid for both f low condit ions. The derivation

shown above is made according to two signif icant assumptions: f irst, the

f luid specif ic heats do not vary with temperature and second, the heat

convection heat transfer coefficients are constant throughout the

exchanger. The second assumptions are influenced by entrance effects,

f luid viscosity and thermal conductivity changes.

The heat loss from the hot f luid f lowing in the inner tube can be

determined from

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qH = ṁHCpH (THi n – THou t) ..............................(4)

where

ṁH = hot water mass flow rate

CpH = hot water specif ic heat

THi n = hot f luid temperature at entrance

THou t = hot f luid temperature at exit

Similarly, the heat gained by the cold f luid f lowing in the space between

the inner and outer pipes can be calculated from

qC = ṁCCpC (TCi n – TCou t) ..........................(5)

where

ṁC = cold water mass flow rate

CpC = cold water specif ic heat

TCi n = cold f luid temperature at entrance

TCou t = cold f luid temperature at exit

Suppose that qC is less than the qH, some heat is lost through the

insulating material to the surrounding air, abide the outer surface of the

concentric tube is insulated. Thus, the eff iciency can be obtained from

Ƞ = q C ..............................(6)

q H

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The effectiveness of a heat exchanger is defined as

ɛ = actual heat transfer ........................(7)

Maximum Possible Heat Transfer

The value of the actual heat transfer may be obtained from calculat ing the

energy lost by the hot f luid from equation (4) or the energy gained by the cold

f luid from equation (5). Since the energy gained by the cold f luid is lost through

the insulat ing material to the surrounding air, i t is preferable to substi tute the

value of energy lost by the hot f luid as the actual heat transfer in equation (7).

In order to determine the maximum possible heat transfer for the heat

exchanger, one of the f luids is logical ly required to undergo a temperature

change which represents the maximum temperature dif ference present in the

heat exchanger, which is the dif ference 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 ṁCp. Thus, the maximum possible heat transfer then can

be expressed as

qmax = (ṁ Cp)mi n (THi n – TCi n) .......................(8)

The minimum fluid may be either the hot or cold f luid, depending on the

mass flow rates and specif ic heats, and so the eff iciency ɛ, is

ɛ = q H x 100% ......................(9)

qmax

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APPARATUS

- Concentric tube heat exchanger

- Water tank

- Thermometers

- Volumetric f low meters

EXPERIMENTAL PROCEDURE

Part A: Constant f low rate, varies temperatures

1. The main switch is switched on.

2. The temperature and pump switches are switched on.

3. The valve is set to parallel f low.

4. The hot water f low rate is set at 3000 cm 3/min and the cold water f low

rate at 2000 cm3/min.

5. The temperature is set at 40°C.

6. The system is let stable unti l T Hi n is 40°C and the values of the

temperature at THou t , THmi d, TCmi d, TCi n , and TCou t are taken.

7. Step 4 unti l 6 are repeated by varying the temperature at 50°C and

60°C.

8. Step 4 unti l 7 are repeated with counter f low.

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Part B: Constant temperature, varies f low rates

1. The valve for parallel f low is set.

2. The temperature is set at 60°C.

3. The hot and cold water f low rates are set at 2000 cm 3/min.

4. The system is let stable and the temperature at T Hou t , THmi d, TCmi d, TCi n ,

and TCou t are taken.

5. Step 3 and 4 are repeated by varying the hot water f low rate to 3000

cm3/min and 4000 cm3/min.

6. Step 2 unti l 5 are repeated with counter f low.

RESULTS


Heat

Exchanger

Temperature (°C) Ѵ H

(cm 3 /min)

Ѵ C

(cm 3 /min)TH i n TH m i d TH ou t TC i n TC m i d TC ou t

Parallel

Flow

40 39 38 29 30 31

3000 200050 48 45 29 30 36

60 56 53 29 33 39

Counter

Flow

40 39 37 29 30 32

3000 200050 48 45 29 31 36

60 57 53 29 33 40

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Heat

Exchanger

Temperature (°C) Ѵ H

(cm 3 /min)

Ѵ C

(cm 3 /min)TH i n TH m i d TH ou t TC i n TC m i d TC ou t

Parallel

Flow

60

54 51 27 31 36 2000

200056 52 27 32 38 3000

57 54 27 33 39 4000

Counter

Flow

60

55 50 27 30 37 2000

200056 52 27 31 38 3000

58 54 27 32 39 4000

SAMPLE CALCULATIONS


Parallel f low at 60°C :

Take that the density of saturated water, ρ = 988 kg/m3, the specif ic heat

capacity of hot and cold water as Cp H@ 60 ° C = 4.185 kJ/kg.K and CpC@ 29 ° C =

4.179 kJ/kg.K respectively and the heat transmission area, A = 0.067 m2.

ѴH = 3000 cm 3 /min = 5 x 10 - 5 m3 /s ѴC = 2000 cm 3 /min = 3.33 x 10 - 5 m3 /s

ṁH = ѴH ρ ṁC = ѴC ρ

= 5 x 10 - 5 m3 /s x 988 kg/m3 = 3.33 x 10 - 5 m3 /min x 988 kg/m3

= 0.0494 kg/s = 0.0329 kg/s

Minimum heat capacity, C m i n = ṁC x CpC

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

= 0.1375 kJ/s.K

Heat transferred, qH = ṁHCpH (THi n – THou t)

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

= 1.4472 W

Maximum heat transferred, qmax = Cm i n (THi n – TCi n)

= 0.1375 kJ/s.K (333 – 302) K

= 4.2625 W

Efficiency, ɛ = q H x 100%

q max

= 1.4472 W x 100%

= 4.2625 W

= 33.95 %

Log Mean Temperature Difference, ∆T l m = ∆T i n - ∆T ou t

ln (∆T i n / ∆Tou t)

= (333-302)K – (326-312)K

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

= 21.39 K

Overall Heat Transfer Coefficient, U = q

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A x ∆T l m

= 1.4448 W

0.067 m 2 x 21.39 K

= 1.0098 W/m 2K

Heat Exchanger T (°C) qH (W) qm a x (W) ɛ (%) Average

ɛ (%)

U

(W/m2K)

Paral lel Flow

40 0.4128 1.5125 27.29

32.33

0.6962

50 1.0327 2.8875 35.76 1.0885

60 1.4472 4.2625 33.95 1.0098

Counter Flow

40 0.6192 1.5125 40.94

36.88

1.2144

50 1.0327 2.8875 35.76 1.0885

60 1.4471 4.2625 33.95 1.0429


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Counter f low at 4000 cm 3/min:

The temperature is f ixed at 60°C. Take the density of saturated water, ρ =

988 kg/m3, the specif ic heat capacity of hot and cold water as CpH @60 ° C =

4.185 kJ/kg.K and CpC @27 ° C = 4.179 kJ/kg.K respectively and the heat

transmission area, A = 0.067 m2.

ѴH = 4000 cm 3 /min = 6.67 x 10 - 5 m 3 /s ѴC = 2000 cm 3 /min = 3.33 x 10 - 5 m 3 /s

ṁH = ѴH ρ ṁC = ѴC ρ

= 6.67 x 10 - 5 m 3 /s x 988 kg/m3 = 3.33x10 - 5m 3 /minx988 kg/m3

= 6.59 x 10 - 2 kg/s = 0.0329 kg/s

Minimum heat capacity, C m i n = ṁC x CpC

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

= 0.1375 kJ/s.K

Heat transferred, qH = ṁHCpH (THi n – THou t)

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

= 1.6547 W

Maximum heat transferred, qmax = Cm i n (THi n – TCi n)

= 0.1375 kJ/s.K (333 – 300) K

= 4.5375 W

Efficiency, ɛ = q H x 100%

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qmax

= 1.6547 W x 100%

4.5375 W

= 36.47 %

Log Mean Temperature Difference, ∆T l m = ∆T i n - ∆T ou t

ln (∆T i n / ∆Tou 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 2K

Heat

Exchanger

Flow

Rate

(cm3 /min)

qH (W) qmax (W) ɛ (%) Average

ɛ (%)

U

(W/m2K)

Parallel

Flow

2000 1.2392 4.5375 27.31

33.41

0.8101

3000 1.6539 4.5375 36.45 1.139

4000 1.6547 4.5375 36.47 1.0818

Counter

Flow

2000 1.3769 4.5373 30.34

34.42

0.9572

3000 1.6539 4.5375 36.45 1.1139

4000 1.6547 4.5375 36.47 1.0818

SAMPLE ERROR CALCULATIONS

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Parallel f low :

Percentage of error = 100% – %calculated value

100

= 100% – 32.33 %

100 %

= 0.6767%


Counter Flow :

Percentage of error = 100% – %calculated value

100

= 100% – 34.42 %

100 %

= 0.6558%

DISCUSSION

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There are a few objectives which are to be achieved in this

experiment; to demonstrate the working principles of concentric f low heat

exchanger under parallel as well as counter f low condit ions, to

demonstrate the effect of heat water inlet temperature variation on the

performance characteristics of a concentric tube heat exchanger, to

demonstrate the effect of f low rate variation on the performance of a

concentric tube heat exchanger and the most important part of the

objectives is to determine the most eff icient f low of concentric tube heat

exchanger whether it is the parallel f low or counter-current f low.

A concentric tube heat exchanger is used to archive these

objectives. The heat exchanger itself is combined with thermometers and

flow rate meters. Thus, the control of the hot f luids temperatures and both

hot and cold f luids f low rates are made easier. We can observe the values

of THi n , THou t , THmi d, TCmi d, TCi n , and TCou t . This experiment is conducted with

two parts of separated condit ions, which are by varying the flow rates at

constant temperature and by varying the temperatures at constant f low

rate.

Part A is conducted by varying the temperatures from 40°C, 50° and

60°C at 3000 cm 3/min of the hot f luids f low rate and 2000 cm 3/min for the

cold f luids f low rate. The eff iciency of parallel f low calculated is 32.33%

and values of the overall heat transfer coefficients are 0.6962 W/m 2K at

40°C¸ 1.0885 W/m2K at 50°C and 1.0098 W/m 2K at 60° C. In contrary, the

calculated results for counter f low is 36.88 % of eff iciency and the overall

heat transfer coefficients are 1.2144 W/m 2K at 40°C, 1.0885 W/m 2K at

50°C and 1.0429 W/m 2K at 60°C.

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Part B is conducted with constant temperature at 27°C yet varying

fluid f low rates. However, the cold f luid f low rate is maintained constant at

2000 cm3/min for both parallel and counter f low. The calculated eff iciency

for parallel f low is 33.41% whereas the overall heat transfer coefficient is

0.8101 W/m2K at 2000 cm 3/min of hot f luid f low rates, 0.9572 W/m 2K at

3000 cm3/min and 1.0818 W/m2K at 4000 cm3/min. For counter f low, the

eff iciency is 34.42% meanwhile the heat transfer coefficients are 0.9572

W/m2K at 2000 cm 3/min, 1.1139 W/m 2K at 3000 cm 3/min and 1.0818

W/m2K at 4000 cm3/min.

Notice that for both experiments in part A and Part B, the counter

f low produce greater eff iciency than parallel f low. This result fol lows the

theoretical conclusion where counter f low heat exchanger is more eff icient

than parallel f low. However, there are a lot of errors and mistakes that

may have affected the results obtained. The very common error occurs

during conducting the experiments are careless way of reading the

thermometers when taking the temperatures of f luids. The eye of an

observer must be parallel to the thermometer meniscus to avoid parallax

error. Another mistake that may have been committed is not pressing the

enter button after sett ing the temperatures. This has caused a minor

problem when the temperature always manipulate even after sett ing it to

the desired temperature. Besides that, the f low rates always change easily

during the experiments. Moreover, the reading of T H i n from the typical

laboratory thermometer is merely different from the reading on the digital

thermometer.

CONCLUSION

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In Part A, counter f low is more eff icient than parallel f low. For

parallel f low, the average eff iciency is 32.33%, whereas the overall heat

transfer coefficient, U, is 0.6962 W/m 2K at 40°C, 1.0885 W/m 2K at 50°C,

and 1.0098 W/m2K at 60°C. For counter f low, the average eff iciency is

36.88%, whereas the overall heat transfer coefficient, U, is 1.2144 W/m 2K

at 40°C, 1.0885 W/m 2K at 50°C, and 1.0429 W/m 2K at 60°C.

In part B, counter f low is also more eff icient than parallel f low. For

parallel f low, the average eff iciency is 33.41%, whereas the overall heat

transfer coefficient, U, is 0.8101 W/m 2K at 2000 cm3/min, 1.1139 W/m2K at

3000 cm3/min, and 1.0818 W/m 2K at 4000 cm3/min. For counter f low, the

average eff iciency is 34.42%, whereas the overall heat transfer

coefficient, U, is 0.9572 W/m 2K at 2000 cm3/min, 1.1139 W/m2K at 3000

cm3/min, and 1.0818 W/m 2K at 4000 cm3/min.

RECOMMENDATIONS

There are a few recommendations 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 observer must be parallel to the

meniscus when reading the temperatures. This is to assure that no

parallax error is committed.

Secondly, the experiment should at least be repeated 3 t imes in

order to get average values. Thus, comparisons can be made and the

results are more convincing and precise.

Thirdly, the f low rates as well as the temperatures must be

monitored thoroughly during the experiment so that they remain constant.

This is to avoid such erroneous results or else, the objectives of the

experiment may not be achieved successfully.

Besides that, any leakage of the instruments involved should be

avoided and they should be assured to work properly. In addit ion, any

direct contact with the water or the instruments should as well be avoided

as this experiment involves hot f luids which can cause burn to skin.

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REFERENCES

1. Fundamental of Heat and Mass Tramsfer ( 6th Edit ion,) John

Wiley & sons (Asia) Pte Ltd

2. Saunders, E. A. (1988). Heat Exchanges: Selection, Design and Construction. New York: Longman Scientif ic and Technical.

APPENDICES

Refer to the attachment provided on the next page.

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Concentric Tube Heat Exchanger

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