concentric tube heat exchanger
TRANSCRIPT
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
Part A: Constant f low rate, varies temperatures
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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Part B: Constant temperature, varies f low rates
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
Part A: Constant f low rate, varies temperatures
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
Part B: Constant temperature, varies f low rates
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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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Part A: Constant f low rate, varies temperatures
Parallel f low :
Percentage of error = 100% – %calculated value
100
= 100% – 32.33 %
100 %
= 0.6767%
Part B: Constant temperature, varies f low rates
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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