Counter flow and parrallel flow exchanger
Short Description
heat exchanger characteristic...
Description
TABLE OF CONTENT
1.0
ABSTRACT..…………………………………………………………….……………………….2
2.0
INTRODUCTION..……………………………………………………………………………….3
3.0 THEORY……..………………………………………………………………………………...…4 4.0
OBJECTIVES..……………………………………………………………………………….…...8
5.0
EQUIPMENT…..………………………………………………………………………….………9
6.0
PROCEDURES………………………………………..…………………………………………..11
7.0
RESULTS……………………………………………..…………………………………………...12
8.0
SAMPLE CALCULATION…………………………..……………………………………………14
9.0
DISCUSSION………………………………………..……………………………………..………17
10.0 CONCLUSION……………………………………..………………………………………………18 11.0 REFERENCE ………………………………………………..………………………………..……19 12.0 APPENDIX……………………………………………..………………………………………..…20
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. The variable 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 to another. 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 heat from 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 hot fluid 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 efficiency for the cold medium is:
c
Tc ,out Tc ,in 100 Th,in Tc ,in
The efficiency for the hot medium is:
h
Th,in Th,out Th,in Tc ,in
100
The mean temperature efficiency is:
mean
c h 2
The power emitted is given below (where Vh is the volumetric flow rate of the hot fluid):
Power Emitted Vh h C ph Th,in Th,out
The power absorbed is given below (where Vc is the volumetric flow rate of the cold fluid):
Power Absorbed Vc c C pc Tc,out Tc,in
The power lost is therefore:
Power lost Power Emitted Power Absorbed
The overall efficiency () is:
Power Absorbed 100 Power Emitted 6
The logarithmic mean temperature difference (Tm) is:
Tm
T1 T2 Th ,in Tc ,out Th ,out Tc ,in T1 Th ,in Tc ,out ln ln T2 Th ,out Tc ,in
The overall 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 heat exchanger. 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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5.0 EQUIPMENT
Figure 4: Concentric Tube Heat Exchanger Figure 5: Schematic diagram of heat exchanger
Valve Hot temperature
Insulation
Figure 6:Valve and heat insulator
Cold temperature
Figure 7: Volumetric flow rate meter
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Figure 8: Power supply switch
Figure 9: Temperature control/thermostat
Cold water inlet temperature Hot water inlet temperature
Hot water outlet temperature
Hot water middle temperature
Cold water outlet 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 heat exchanger. 2. The hot water inlet temperature, Th,in is set to 60oC with the decade switch. 3. The cold water volumetric flow rate, Vc is set to run at a constant 2000 cm3/min. 4. Then, the hot fluid volumetric flow rate, Vh is 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,mid and cold water outlet, Tc,out are recorded after 5 minutes. 6. Step 4 and 5 are repeated by changing the value to 2000 cm3/min, 3000 cm3/min and 4000 cm3/min.
Parallel Flow Heat Exchanger 1. Set up the valve according to the schematic diagram of parallel flow heat exchanger 2. 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
Th,mid
Th,out
Tc,in
Tc,mid
Tc,out
(°C)
(°C)
(°C)
(°C)
(°C)
(°C)
(cm3/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
58
53
51
27
33
38
6.6667 x 10
4000
-5
Table 1.1: Temperatures for counter-flow heat exchanger
Vh
Power Emitted
Power Absorbed
Power Lost
Efficiency (ƞ)
ΔT1
ΔT2
ΔTm
(°C)
(°C)
(°C)
U W/(m2.°C)
(cm3/min)
(m3/s)
(W)
(W)
(W)
(%)
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
Th,mid
Th,out
Tc,in
Tc,mid
Tc,out
(°C)
(°C)
(°C)
(°C)
(°C)
(°C)
(cm3/min)
(m3/s)
1000
1.6667 x 10-5
60
50
47
28
32
33
2000
-5
60
52
50
28
33
36
59
53
51
28
34
37
58
53
52
28
34
38
3.3333 x 10 5 x 10
3000
-5
6.6667 x 10
4000
-5
Table 2.1: Temperatures for parallel-flow heat exchanger
Vh
Power Emitted
Power Absorbed
Power Lost
Efficiency (ƞ)
ΔT1
ΔT2
ΔTm
(°C)
(°C)
(°C)
U W/(m2.°C)
(cm3/min)
(m3/s)
(W)
(W)
(W)
(%)
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
1646.5229
1249.4847
397.0382
75.89
31
14
21.39
872.04
1647.0098
1388.3164
258.6934
84.29
30
14
20.99
987.03
-5
3000
5 x 10
4000
6.6667 x 10-5
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 ºC Vc=2000 cm³/min = 2000 cm³/min × 1 min/60 s × 1 m³/100³ cm = 3.3333 x 10-5 m3/s ρc=997 + 2(996-997)/(30-25) = 996.6 kg / m³ Cpc =4180 + 2(4178 - 4180)/(30 - 25) = 41792 J/kg.K.
AtTh, in= 62 ºC ρh=983.3 + 2(980.4 - 983.3) / (65 - 60) = 982.14 kg / m³ Cph= 4185 + 2(4187 - 4185 ) / (65 - 60) = 4185.8 J/kg.K As = 0.067 m2
a) Power Emitted = Vh ρh Cph (Th, in - Th, out) = (1.6667 x 10-5)(982.14)(4185.8)(62 - 46) = 1096.2997 W b) Power Absorbed = Vc ρc Cpc (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 (ƞ) = (Power Absorbed / Power Emitted) x 100 = (971.8214 / 1096.2997) x 100 = 88.65 % e) Logarithmic Mean Temperature Difference, ΔTm =
i)
(
– (
) )
For Counter – Flow Heat Exchanger: ΔT1 =Th, in – Tc, out = 62 - 34 = 28 °C ΔT2= Th, out – Tc, in = 46- 27 = 19°C ΔTm = = 23.21°C
ii)
For Parallel – Flow Heat Exchanger: ΔT1 =Th, in – Tc, in = 60 - 28 = 32 °C ΔT2= Th, out – Tc, out = 47-33 = 14°C ΔTm = = 21.77°C 15
iii)
Overall Heat Transfer Coefficient, U = Power Absorbed / (As . ΔTm ) U =971.8214 / (0.067 x 23.21) = 624.94W/(m2.°C)
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9.0 DISCUSSION [Refer next page]
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10.0 CONCLUSION [Refer next page]
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11.0 REFERENCES
1. MEC 551 Thermal Engineering, McGraw-Hill,2013, ISBN 978-112-130510-6 2. 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.Convective heat and mass transfer. Vol. 3. New York: McGraw-Hill, 1993. 4. Bejan, A. "Concept of irreversibility in heat exchanger design: Counterflow heat exchangers for gas-to-gas applications." J. Heat Transfer;(United States) 99.3 (1977). 5. Mozley, J. M. "Predicting dynamics of concentric pipe heat exchangers."Industrial & Engineering Chemistry 48.6 (1956): 1035-1041.
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12.0 APPENDICES [Refer next page]
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