Experiment no. 4 Concentric Tube Heat Exchanger Objective: 1. To study the working principle of parallel flow and counter flow heat exchangers. 2. To study effect of fluid temperature on counter flow heat exchanger performance 3. To study effect of fluid flow rates on heat exchanger performance
Theory: A heat exchanger is a piece of process equipment in which heat exchange takes place between two fluids that enter and exit at different temperatures. The primary design objective of the equipment may be either to remove heat from a hot fluid or to add heat to a cold fluid. Depending upon the relative direction of fluid motion, shell-and-tube heat exchangers are classified as parallel flow, counter flow, cross flow. In parallel flow, the hot and cold fluids flow in the same direction and therefore enter the exchanger on the same end and exit the exchanger on the same end. In counter flow, the two fluids flow in opposite directions and thus enter the exchanger and exit the exchanger from opposite ends. Cross flow heat exchangers will not be analyzed as a part of this laboratory experiment.
Figure 1 - Diagram of Parallel and Counter Flow Configurations
Observations:
Hot Water Parallel HX Qc = 1 lpm Qh = 2 lpm Counter HX Qc = 1 lpm Qh = 2 lpm
Cold Water
thi
thmid
tho
tci
tcmid
tco
Temperature ( C)
51
48
46
29
35
39
Corres. Length (m)
0
0.75
1.5
0
0.75
1.5
Temperature ( C)
51
48
46
31
36
41
Corres. Length (m)
0
0.75
1.5
1.5
0.75
0
o
o
Table 1 - Raw Data for Parallel Flow and Co unter Flow Effectiveness Calculation
r e t g n a a e h H c x E
Parallel Flow Counter Flow
m p l c Q
m p l
1 1
c
h
c h m
) c m (
2
69.75 139.5
2
69.75
h
Q
c c m
n i m
139.5
i c
t i h t
c t
h t
∆ c
∆
r / o h i n i c t m ) c t t ) = ∆ ∆ h i c є c c h c m t ( c h ( m m
c t
h t
∆
∆
69.75
10
5
22
697.5
697.5
0.45455
69.75
10
5
20
697.5
697.5
0.5
c c m
h
c h m
Table 2 - Calculation for Parallel Fl ow and Counter Flow Effectiveness Calculation
Figure 2 - Parallel Flow Graph
Figure 3 - Counter Flow Graph
Hot Water
Cold Water
Qcold (lpm)
Qho(lpm)
thin oC
thmid oC
tho oC
tcin oC
tcmid oC
tco oC
1
2
66.5
61
59
29
39
46
1
2
61
56
54
29
37
44
1
2
56
52
50
29
36
41
1
2
51
48
46
29
35
39
0
0.75
1.5
0
0.75
1.5
Corr. Length for temperature (m)
Table 3 - Raw Data for Water Temperature Variation in Paral lel Flow
Hot Water
Cold Water
Qcold (lpm)
Qho(lpm)
thin C
thmid C
tho C
tcin C
tcmid C
tco C
1
2
51
48
46
31
36
41
1
2
56
54
49
31
38
44
1
2
61
58
53
30
38.5
46
1
2
66
62
56
29
39
49
0
0.75
1.5
1.5
0.75
0
o
Corr. Length for temperature (m)
o
o
o
o
o
Table 4 - Raw Data for Water Temperature Variation in Counter Flow
COUNTER FLOW HEAT EXCHANGER Q cold cold (lpm)
Hot Water Q hot hot
(lpm)
o
o
Cold Water o
o
o
o
thin C
thmid C
tho C
tcin C
tcmid C
tco C
2
1
66
57
49
29
32
38
2
2
66
60
55
29
34
41
2
3
67
62
57
29
35
43
67
63
59
29
36
44
0
0.75
1.5
1.5
0.75
0
2 4 Corr. Length for temperature (m)
PARALLEL FLOW HEAT EXCHANGER Hot Water
Cold Water
Q cold cold (lpm)
Q hot hot (lpm)
thin C
thmid C
tho C
tcin C
tcmid C
tco C
2
1
51
46
43
29.5
31.5
33.5
2
2
51
47
45
29.5
32.5
35
2
3
51
48
46.5
29.5
33
37
50.5
48.5
47
30
33.5
37.5
0
0.75
1.5
0
0.75
1.5
2 4 Corr. Length for temperature (m)
o
o
o
o
o
Table 5 - Raw Data for Flow R ate Variation in Counter & Parallel Flow
o
Figure 4 & Figure 5 - Temperature Graphs for Varying Flow Rates of Q hot hot with Constant Q cold cold
Figure 6 & Figure 7 - Temperature Graphs for Varying Flow Rates of Q hot hot with Constant Q cold cold
Analysis: From the data in Table 1, the general characteristics of parallel flow and counter flow heat exchangers can be observed. In the parallel flow configuration, the exit temperature of the hot fluid must be higher than the exit temperature of the cold fluid. This is supported by the data taken. In the counter flow configuration, the exit temperature of the hot fluid must be higher than the entrance temperature of the cold fluid, but it does not necessarily need to be higher than the exit temperature of the cold fluid. This is also supported by the data, even though in this case
the exit temperature of the hot fluid is still hotter than the exit temperature of the cold fluid. From the calculations resulting in overall effectiveness, it is shown that the counter flow heat exchanger is more effective than the parallel parallel flow heat exchanger. This supports generally held knowledge and experimental data concerning the two types of heat exchanger, governed by the Clausius Statement. Additionally, Additional ly, in the counter flow heat exchanger, had the exit temperature of the cold fluid been hotter than the exit temperature of the hot fluid, the effectiveness would have been even higher, reflecting common data in many textbooks. From the data in Table 3 and Table 4, the temperature differences under constant flow rates are shown. Under constant flow rate conditions, the ratio between temperature differences is also constant. If there is a rise in the temperature difference of the hot fluid, there will also be a rise in the temperature difference in the cold fluid. This is governed by a special case of the First Law of Thermodynamics. In this case, the energy is transferred from hot to cold fluids with constant mass flow rates. Therefore the ratio between temperature differences does not change even though the numerical values of the temperature differences may change. From the data in Table 5, the temperature differences under different flow rates are shown. In this case, the ratio between temperature difference in the hot fluid and temperature difference in the cold fluid changes changes with respect to the flow rates. This is governed by the First Law of Thermodynamics. In this case, the energy removed from the hot fluid is the energy added to the cold fluid. The higher the flow rate of a fluid, the lower the temperature change in that fluid will be. The opposite is also true, the lower the flow rate of the fluid, the higher the temperature change in the fluid will be.
Conclusions: The heat exchanger apparatus follows the basic laws of thermodynamics and this can be shown experimentally. From all of the parallel flow configurations, the exit temperature of the hot fluid is always hotter hotter than the exit temperature of the cold fluid. This supports the Clausius Statement in which heat may not spontaneously transfer from a colder body to a hotter body. From the other experiments that hold flow rates constant or vary the flow rates, it is clear that the First Law of Thermodynamics and conservation of energy applies to the heat exchanger apparatus. In practical application, the counter flow configuration is preferred for its higher effectiveness. This experiment did show that this configuration does in fact have a higher effectiveness than the parallel flow configuration. configuration . Additionally, Additionall y, the counter flow configuration is also capable of have a cold fluid exit temperature that is higher than the hot fluid exit temperature. This was not shown experimentally, however from the data collected it is clear that the flow rates were too high to achieve this desired result. If the experiment were repeated with lower flow rates, it would be possible to demonstrate a situation where the exit temperature of the cold fluid is hotter than the exit temperature of the hot fluid.
