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1.0 ABSTRACT
The objective of conducting this experiment is to demonstrate the effect of flow rate variation
on the performance characteristics of a counter-flow concentric tube heat exchanger. There are
two types of flow which are parallel flow and counter flow. For every flow, the procedure is the
same but the arrangements of valves are different in order to change the direction of flow. Thevariable that needs to change is the volumetric flow rate of the hot fluid and all six readings of
the temperature are recorded for every changing. Using the data, the heat exchanger performance
factors such as power emitted, power absorbed, power lost, efficiency, logarithmic mean
temperature difference and overall heat transfer coefficient are calculated. The effect of changing
the volumetric flow rate of the hot fluid on each of these heat exchanger performance factors are
discussed.
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2.0 INTRODUCTION
Heat is the transfer of energy from one system to surrounding when they have different
temperature. Based on 2nd
law of thermodynamics, heat is transferred in the direction of
decreasing temperature. This law gives information that heat flows from high temperature to low
temperature. Heat is basically exchanged by two method which mainly convection and
conduction processes. Conduction is when there are contacts between the bodies and convection
is when there is no contact between them and heat transfer through movement of air. In this
experiment, student will conduct an experiment of heat exchange between two fluids that has
different initial temperature. At the end of the experiment, the results between counter flow and
parallel flow that flows in the concentric tube heat exchanger machine will be differentiated.
A heat exchanger is a system which thermal energy is transferred from one fluid toanother. The types of heat exchangers that are to be tested in this experiment are parallel flow
heat exchanger and counter flow heat exchanger. Heat exchanger is built for efficient heat
transfer form one medium to another. A metal wall separate the fluid flows so that they will not
mix or may be in direct contact. Heat exchangers are widely used in space heating, refrigeration,
air conditioning, power plants and many more. One of the most common heat exchanger is the
radiator in a car where it transfers heat to air flowing through the radiator. The variable that
affect the performance of a heat exchanger are the fluid physical properties, fluid mass flow rate,
inlet temperature of fluid, physical properties of heat exchanger materials, the area of heat
transfer surfaces and the ambient conditions. The way that a heat exchanger works is when the
cold water entering the heat exchanger inlet gaining heat and the hot water losing heat before
both of this water exit the exchanger.
The primary advantage of a concentric configuration, as opposed to a plate or shell and
tube heat exchanger, is the simplicity of their design. As such, the insides of both surfaces are
easy to clean and maintain, making it ideal for fluids that cause fouling. Additionally, the heat
exchanger robust build means that they can withstand high pressure operations. Common heat
exchanger works under turbulent conditions which at low flow rates in order to increase the heat
transfer coefficient, and hence increase the rate of heat transfers.
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3.0 THEORY
A heat exchanger is equipment where heat exchange takes place between two fluids that enter and
exit at different temperature. The primary design objective of the equipment may be either to remove heatfrom hot fluid or to add heat to cold fluid. In parallel flow or concurrent flow, hot and cold fluids flow in
the same direction, thus entering and exiting the heat exchanger on the same end. Meanwhile in counter
flow or counter current flow, hot and cold fluids flow in the opposite directions, thus entering and exiting
the heat exchanger from opposite ends.
Figure 1: Parallel flow and Counter flow configurations
In a heat exchanger, the temperature difference between the hot fluid and cold fluid may vary
along the length of the heat exchanger as shown in the Figure 3 below. This is due to the fact that the hotfluid temperature decreases as it transfers heat to the cold fluid, while the cold fluid temperature
increases. As shown in the Figure below, for parallel or co-current flow arrangement, the temperature
difference is maximum at the inlet and decreases slowly towards the outlet. Accordingly, the heat transfer
rate is maximum at the inlet and minimum at the outlet.
For counter flow arrangement, the difference between temperatures of hot and cold fluid, and
consequently the heat transfer rate at any location usually maximum at hot fluid inlet end, point 2. The
temperature difference decreases dramatically compared to parallel flow arrangement as we move
towards the hot fluid exit. The mean temperature difference is not simply taken as the difference between
average bulk temperature of hot fluid and cold fluid but being calculated based on the formula given.
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Figure 2: Temperature distribution for counter flow heat exchanger
Figure 3: Temperature distribution for parallel flow heat exchanger
The overall heat transfer coefficient, although very important in heat exchanger analysis, can also
be difficult to obtain experimentally. This coefficient depends primarily on fluid convection and wall
conduction resistances as well as resistances caused by deposits and chemical reactions known as fouling
which take place on the surface of the heat exchanger during normal operation. It may also depend on
whether or not fins are used; as we have seen in an earlier experiment, fins will decrease the overall
resistance by increasing the total area available for heat transfer.
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The equations for calculating the performance characteristics: power emitted, power absorbed, power lost,
efficiency (), logarithmic mean temperature difference (Tm), and overall heat transfer coefficient (U).
The efficiencyfor the cold medium is:
100,,
,,
incinh
incoutc
c
TT
TT
The efficiencyfor the hot medium is:
100,,
,,
incinh
outhinh
h
TT
TT
The mean temperature efficiencyis:
2
hc
mean
The power emittedis given below (where hV is the volumetric flow rate of the hot fluid):
outhinhphhh TTCVEmittedPower ,,
The power absorbedis given below (where cV is the volumetric flow rate of the cold fluid):
incoutcpccc TTCVAbsorbedPower ,,
The power lostis therefore:
AbsorbedPowerEmittedPowerlostPower
The overall efficiency() is:
100EmittedPower
AbsorbedPower
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The logarithmic mean temperature difference(Tm) is:
incouth
outcinh
incouthoutcinh
m
TT
TT
TTTT
T
T
TTT
,,
,,
,,,,
2
1
21
lnln
Theoverall heat transfer coefficient (U) is:-
Where the surface area () for this heat exchanger is 0.067m2
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4.0 OBJECTIVES
1. To demonstrate the effect of flow rate variation on the performance characteristic of heatexchanger.
2. To study the working principle of parallel flow and counter flow heat exchangers.3. To study the effect of fluid temperature on counter flow heat exchanger performance
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5.0EQUIPMENT
Figure 4:Concentric Tube Heat Exchanger Figure 5:Schematic diagram of heat exchanger
Figure 6:Valve and heat insulator Figure 7:Volumetric flow rate meter
Hot temperature
Cold temperature
Valve
Insulation
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Figure 8:Power supply switch Figure 9:Temperature control/thermostat
Hot water inlet
temperature
Hot water outlettemperature
Cold water inlet
temperature
Cold water outlet
temperature
Hot water middle
temperature Cold water middle
temperature
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6.0 PROCEDURES
Counter Flow Heat Exchanger
1. The valve was set up according to the schematic diagram of counter flow heatexchanger.
2. The hot water inlet temperature, Th,inis set to 60oC with the decade switch.3. The cold water volumetric flow rate, Vcis set to run at a constant 2000 cm3/min.4. Then, the hot fluid volumetric flow rate, Vhis set to 1000 cm3/min.5. The temperature readings of hot water inlet, Th,in, hot water middle, Th,mid, hot water
outlet, Th,out, cold water inlet, Tc,in, cold water middle, Tc,midand cold water outlet, Tc,out
are recorded after 5 minutes.
6. Step 4 and 5 are repeated by changing the value to 2000 cm 3/min, 3000 cm3/min and4000 cm
3/min.
Parallel Flow Heat Exchanger
1. Set up the valve according to the schematic diagram of parallel flow heat exchanger2. Repeat the whole experiment from step 2-6.3. All of the data are recorded in proper table.
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7.0 RESULTS
1. Counter Flow Heat Exchanger:
Vh Th,in
(C)
Th,mid
(C)
Th,out
(C)
Tc,in
(C)
Tc,mid
(C)
Tc,out
(C)(cm
3/min) (m3/s)
1000 1.6667 x 10-5 62 51 46 27 30 34
2000 3.3333 x 10-5 60 53 50 27 32 37
3000 5 x 10-5 59 54 51 27 33 38
4000 6.6667 x 10-5 58 53 51 27 33 38
Table 1.1:Temperatures for counter-flow heat exchanger
Vh Power
Emitted
(W)
Power
Absorbed
(W)
Power
Lost
(W)
Efficiency
()
(%)
T1
(C)
T2
(C)
Tm
(C)
U
W/(m2.C)
(cm3/min) (m3/s)
1000 1.6667 x 10-5 1096.2997 971.8214 124.4783 88.65 28 19 23.21 624.94
2000 3.3333 x 10-5
1371.6898 1388.3164 -16.6266 101.21 23 23 0.00 0.00
3000 5 x 10-5
1646.5229 1527.1480 119.3749 92.75 21 24 22.47 1014.54
4000 6.6667 x 10-5
1921.5114 1527.1480 394.3634 79.48 20 24 21.94 1038.93
Table 1.2:for counter-flow heat exchanger
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2. Parallel Flow Heat Exchanger:
Vh Th,in
(C)
Th,mid
(C)
Th,out
(C)
Tc,in
(C)
Tc,mid
(C)
Tc,out
(C)(cm
3/min) (m3/s)
1000 1.6667 x 10-5 60 50 47 28 32 33
2000 3.3333 x 10-5 60 52 50 28 33 36
3000 5 x 10-5 59 53 51 28 34 37
4000 6.6667 x 10-5 58 53 52 28 34 38
Table 2.1:Temperatures for parallel-flow heat exchanger
Vh Power
Emitted
(W)
Power
Absorbed
(W)
Power
Lost
(W)
Efficiency
()
(%)
T1
(C)
T2
(C)
Tm
(C)
U
W/(m2.C)
(cm3/min) (m3/s)
1000 1.6667 x 10-5 891.6251 694.1582 197.4669 77.85 32 14 21.77 475.83
2000 3.3333 x 10-5 1371.6898 1110.6531 261.0367 80.97 32 14 21.77 761.32
3000 5 x 10-5 1646.5229 1249.4847 397.0382 75.89 31 14 21.39 872.04
4000 6.6667 x 10-5 1647.0098 1388.3164 258.6934 84.29 30 14 20.99 987.03
Table 2.2:for parallel-flow heat exchanger
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8.0 SAMPLE CALCULATION
From table A-9 (Properties of saturated water):
AtTc, in=27 C
Vc=2000 cm/min
= 2000 cm/min 1 min/60 s 1 m/100 cm
= 3.3333 x 10-5m
3/s
c=997 + 2(996-997)/(30-25)
=996.6 kg / m
Cpc =4180 + 2(4178 - 4180)/(30 - 25)
=41792 J/kg.K.
AtTh, in= 62 Ch=983.3 + 2(980.4 - 983.3) / (65 - 60)= 982.14 kg / m
Cph= 4185 + 2(4187 - 4185 ) / (65 - 60)
=4185.8 J/kg.K
As = 0.067 m2
a) Power Emitted = VhhCph(Th, in- Th, out)= (1.6667 x 10
-5)(982.14)(4185.8)(62 - 46)
= 1096.2997 W
b) Power Absorbed = VccCpc(Tc, out- Tc, in)= (3.3333 x 10
-5)(996.6)(4179.2)(34 - 27)
= 971.8214 W
c) Power Loss = Power Emitted Power Absorbed= 1096.2997 - 971.8214
= 124.4783 W
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d) Overall Efficiency () = (Power Absorbed / Power Emitted) x 100= (971.8214 / 1096.2997) x 100
= 88.65 %
e) Logarithmic Mean Temperature Difference, Tm = ( )
i) For Counter Flow Heat Exchanger:T1 =Th, inTc, out
= 62 - 34
= 28 C
T2= Th, outTc, in
= 46- 27
= 19C
Tm =
= 23.21C
ii) For Parallel Flow Heat Exchanger:T1 =Th, inTc, in
= 60 - 28
= 32 C
T2= Th, outTc, out
= 47-33
= 14C
Tm =
= 21.77C
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iii) Overall Heat Transfer Coefficient, U = Power Absorbed / (As. Tm)
U =971.8214 / (0.067 x 23.21)
= 624.94W/(m2.C)
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9.0DISCUSSION
[Refer next page]
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10.0CONCLUSION
[Refer next page]
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11.0 REFERENCES
1. MEC 551 Thermal Engineering, McGraw-Hill,2013, ISBN 978-112-130510-62. Thermodynamics an engineering approach,Yunus A.Cengel,Michael A.Boles
,McGraw-Hill,2011,ISBN 978-007-131111-3
3. Kays, William Morrow, Michael E. Crawford, and Bernhard Weigand.Convectiveheat and mass transfer. Vol. 3. New York: McGraw-Hill, 1993.
4. Bejan, A. "Concept of irreversibility in heat exchanger design: Counterflow heatexchangers for gas-to-gas applications."J. Heat Transfer;(United States)99.3
(1977).
5. Mozley, J. M. "Predicting dynamics of concentric pipe heatexchangers."Industrial & Engineering Chemistry48.6 (1956): 1035-1041.
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12.0APPENDICES
[Refer next page]