This experiment was meant to investigate heat transfer between two fluids of different temperatures in a double pipe heat exchanger in the counter or co-current direction. The hot water ran through the inner tube at three distinct flow rates, while the cold water flowed through the annulus at four different flow rates that were studied. The experiment contributed to a better knowledge of how heat exchangers work via indirect cooling and heating via considerable transfer from hot to cold water in the annulus that was separated by a solid wall (Hussein 2015, p.13). The heat exchanger was operated at different flow rates of both the hot and cold water with different surface areas that were determined by the number of tubes. This was essential at investigating the effect of the different flow rates as well as surface areas on the outlet temperatures. It was observed that with increased flow rates, the change in temperature for the cold fluid decreased, and there was a significant increase in the outlet temperature as the surface area increased. Mass and energy balances as well as efficiency of heat transfer were carried out by combining the product of the flow rate of mass as well as the fluids’ specific heat capacities. In theory the rate of heat transfer from the hot fluid should equal the rate of heat transfer to the cold water, and this experiment failed was efficient in demonstrating that as evidenced by the proportional changes in temperature for the hot and the cold water as demonstrated in the graphs.
Introduction
Temperature is the amount of energy contained in a substance. Exchangers of heat are utilized in transferring temperature between substances. In process units, it is essential to ensure control of temperature of both streams that are incoming and outgoing (Thulukkanam 2013, p.27). The exchangers of heat either raise or lower the stream’s temperature by moving heat to or from them. Heat exchangers interchange heat between two fluids that have different temperatures, and which a solid wall separates them. The difference in temperature or temperature gradient facilitates the heat transfer through radiation. Conduction occurs when heat from the fluid with higher temperature passes through the solid wall. In heat exchanger, the major contribution to heat transfer is usually contributed by convection (Kreith, Manglik and Bohn 2012, p.47).
Double-pipe heat exchanger is among the simplest forms of heat exchangers. Usually, it entails a pipe inside a bigger pipe, where one fluid flows in the inner pipe, while the other fluid flows in the annulus between the two pipes, and the walls of the inner pipe serves as the surface of heat transfer (Sulzberger and Tui Industries 1989, p.3). The flow in a double-pipe heat exchanger is either co-current or counter-current, which means that there are two configurations of flow, where the flow of the two streams is in the same direction and where the flow of the streams in in opposing directions (McCabe, Smith and Harriott 2001, 43). The pattern of the flow as well as the needed duty of heat exchange permits the calculations of the log mean temperature difference. If the overall heat transfer coefficient is estimated, then together with the log mean temperature difference can be used in calculating the heat transfer surface area that is required. These determinations are very essential in determining the sizes of pipes, lengths as well as bend numbers (Kumar, Saini, Sharma and Nigam 2006, p.4403).
In many industrial processes, some form of heat transfer is involved, which entails the absorption or emission of heat. Therefore, there are many applications in which fluids have to be heated or/ and cooled for safe and efficient operations of processes. Therefore, it is very important to understand the optimal design of heat exchangers for the operations of industrial plants to achieve the greatest possible efficiency level (Wu, Wang and Sundén 2013, p.266). This experiment was aimed at investigating the transfer of heat between two water streams of varying temperatures flowing either in co-current or counter-current direction in a double-pipe heat exchanger. Specifically, the experiment objectives were firstly, to have a general understanding of the manner in which heat exchanger works through a heating or cooling that is indirectly done by heat transfer from a fluid stream to another separated by a solid wall. Secondly, the specific objective was to operate a heat exchanger running at varying hot and cold water flow rates at either co-current or counter-current direction with dissimilar surface areas (tube numbers). This objective was also important in aiding the investigation of the effect of rates of flow of water and heat exchange surface area on the outlet temperature. The third and fourth specific objectives of the experiment were to do mass and energy balances as well as overall efficiency, and to compute efficiencies of heat transfer at the flow rates that were investigated.
Methods
The apparatus that were needed for this experiment were the heat exchanger rig that consisted of a concentric stainless steel pipe put inside a bigger pipe so that a fluid could be passed in the innermost pipe and a different one in the annulus with no contact made. In addition, there was a service unit HT30XC, in which the heat exchanger was mounted, which provided the needed services like sensor output displays and heated reservoir.
Figure 1: Representation Diagram HT30XC
Figure 2: HT30XC
Figure3: Heat exchanger consisting of four double pipes that was placed on top of HT30XC
The heated reservoir pumped hot water at 50°C through the inner tube and cold water was passed through the annulus. Heat was transferred from the hot water to the cold water lead to a change in both streams.
The HT36 tubular heat exchanger and the base unit (HT30XC) were appropriately assembled. The supply of cold water was connected to the pressure’s inlet that regulated the valve. The vessel with hot water was filled on the HT30XC before connecting it to the electricity. After switching on the computer, the computer’s USB port was connected to the HT30XC’s USB. The counter current alternative was chosen from the start-up screen after engaging the HT36’s Armfield Armsoft software. The unit was switched on and the circuit for hot water primed. The circuit for cold water was also primed, and the flow of cold and hot water controlled. The temperature for hot water was set and heat exchanger configured for counter current flow. In addition, the number of active tubes for heat exchanger was configured. The flow for hot water was set to 1 L/min. Data was recorded upon stabilization of the temperature. The control valve for cold water was adjusted to 0.5 L/min and the process repeated. All data was saved on excel sheets. After the experiment, the heater, pump and cold water flow valve was set to zero before setting the HT30XC to standby.
Results
Results for One Tube
Table 1: Derived Results for one tube
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Reynolds number, Hot water (inner tube)
Reynolds number, cold water (outer tube)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
1
1
0.25
8209
3480
50.7
47.3
20.8
48.2
3.4
27.4
2
0.5
8217
6973
49.7
43.7
20.8
33.8
6.0
13.0
3
1
8215
13969
49.7
42.5
20.8
24.9
7.2
4.1
4
1.5
8222
20962
49.8
41.0
20.8
21.8
8.8
1
5
2
0.25
16418
3480
50.9
49.6
20.8
47.2
1.3
26.4
6
0.5
16434
6976
50.6
49.5
20.8
47.5
1.1
26.7
7
1
16430
13969
49.6
44.3
20.7
25.6
5.3
4.9
8
1.5
16448
20962
49.6
43.9
20.8
23.6
5.7
2.8
9
3
0.25
24627
3478
51.3
50.4
20.8
48.0
0.9
27.2
10
0.5
24651
6978
51.0
50.2
20.8
48.0
0.8
27.2
11
1
24645
13970
49.1
45.4
20.9
27.6
3.7
6.7
12
1.5
24666
20962
49.4
45.7
20.8
27.4
3.7
6.6
Figure 4: Change in temperature when flow rate change (hot water flow rate = 1)
Figure 5: Change in temperature when flow rate change (hot water flow rate = 2)
Figure 6: Change in temperature when flow rate change (hot water flow rate = 3)
Results for Two Tubes
Table 2: Derived Results for Two Tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Reynolds number, Hot water (inner tube)
Reynolds number, cold water (outer tube)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
1
1
0.25
8169
3480
50.7
48.1
20.7
48.7
2.6
28
2
0.5
8222
6970
49.7
41.4
20.9
40.6
8.3
19.7
3
1
8222
13962
49.6
37.5
20.9
27.0
12.1
6.1
4
1.5
8229
20954
49.8
36.5
20.8
24.5
13.3
3.7
5
2
0.25
16338
3482
49.8
45.2
20.7
44.2
4.6
23.5
6
0.5
16444
6970
50.9
48.5
20.7
49.6
2.4
28.9
7
1
16442
13964
49.7
41.5
20.8
32.1
8.2
11.3
8
1.5
16458
20954
49.8
40.0
20.7
28.2
9.8
7.5
9
3
0.25
24507
3478
49.9
46.4
20.7
45.0
3.5
24.3
10
0.5
24666
6970
50.9
49.7
20.7
49.6
1.2
28.9
11
1
24678
13960
49.7
43.7
20.7
34.3
6.0
13.6
12
1.5
24687
20958
49.9
42.6
20.7
30.7
7.3
10.0
Figure 7: Change in temperature when flow rate changes (hot water flow rate = 1)
Figure 8: Change in temperature when flow rate changes (hot water flow rate = 2)
Figure 9: Change in temperature when flow rate changes (hot water flow rate = 3)
Results for Three Tubes
Table 3: Derived Results for Three Tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Reynolds number, Hot water (inner tube)
Reynolds number, cold water (outer tube)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
1
1
0.25
8208
3480
51.8
46.1
21.0
49.5
5.7
28.5
2
0.5
8222
6970
49.5
38.8
21.1
38.5
10.7
17.4
3
1
8229
13955
49.6
34.7
21.1
30.4
14.9
9.3
4
1.5
8229
20943
49.7
33.3
21.1
27.1
16.4
6
5
2
0.25
16416
3478
51.7
49.0
21.1
49.4
2.7
28.3
6
0.5
16444
6970
49.6
44.9
21.2
47.3
4.7
26.1
7
1
16458
13960
48.9
39.1
21.2
35.9
9.8
14.7
8
1.5
15452
20948
49.0
37.7
21.2
31.4
11.3
10.2
9
3
0.25
24624
3480
51.3
47.7
20.1
49.0
3.6
28.9
10
0.5
24666
6970
50.8
47.4
20.3
49.1
3.4
28.8
11
1
24689
13950
47.9
40.8
20.8
38.0
7.1
17.2
12
1.5
24678
20944
48.7
39.7
20.9
34.8
9.0
13.9
Figure 10: Change in temperature when flow rate changes (hot water flow rate = 1)
Figure 11: Change in temperature when flow rate changes (hot water flow rate = 2)
Figure 12: Change in temperature when flow rate changes (hot water flow rate = 3)
Results for Four Tubes
Table 4: Derived Results for Four tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Reynolds number, Hot water (inner tube)
Reynolds number, cold water (outer tube)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
1
1
0.25
8215
3480
49.9
43.8
19.5
48.6
6.1
29.1
2
0.5
8222
6970
49.6
37.7
19.3
42.5
11.9
23.2
3
1
8235
13955
49.5
31.9
18.7
32.6
17.6
13.9
4
1.5
8235
20943
49.5
30.2
18.5
29.2
19.3
10.7
5
2
0.25
16430
3478
50.0
47.4
19.5
49.3
2.6
29.8
6
0.5
16444
6970
50.4
46.2
19.2
49.5
4.2
30.3
7
1
16470
13955
49.2
39.0
18.5
42.1
10.2
23.6
8
1.5
16468
20943
49.4
36.1
18.3
36.4
13.3
18.1
9
3
0.25
24645
3482
50.2
48.6
19.4
49.6
1.6
30.2
10
0.5
24666
6969
50.9
46.4
18.6
49.8
4.5
31.2
11
1
24707
13960
49.1
40.8
18.4
42.4
8.3
24.0
12
1.5
24710
20943
48.6
38.8
18.3
39.0
9.8
20.7
Figure 13: Change in temperature when flow rate changes (hot water flow rate = 1)
Figure 14: Change in temperature when flow rate changes (hot water flow rate = 2)
Figure 15: Change in temperature when flow rate changes (hot water flow rate = 3)
While the temperature change in the hot water inner tube increases with an increase in flow rate of the cold water, the temperature of the cold water in the annular decreases with increasing flow rate. Therefore, generally, in all the set-ups including one tube, two tubes, three tubes and four tubes, it was observed that as the flow rate of the cool water was increased, the outlet temperature of the cool water decreased (Yilmaz 2003, p.1169). In addition, as the number of the pipes increased, which represented an increase in distance or surface area, the temperature of the hot water tended to decrease, while the temperature of the cold water tended to increase.
The arithmetic mean diameter of the tube (dm) = (do + di)/2
= (0.0095+0.0083)/2
=0.0089
Reynold’s number for the inner tube that contains the hot water was calculated as Re = pt ut di/µt, while the Reynold’s number for the annular containing the cold water was calculated using the formula Re = ps us de/µs as outlined in appendix 1.
Mass and Energy Balances and Efficiencies of Heat Transfer at the Flow Rates That Were Investigated
Table 5: One Tube
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Temperature °C
mhot
mcold
Qe
Qa
η
(%)
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
1
1
0.25
50.7
47.3
20.8
48.2
3.4
27.4
988.9
249
14121
28655
203
2
0.5
49.7
43.7
20.8
33.8
6.0
13.0
989.8
489
24943
26699
107
3
1
49.7
42.5
20.8
24.9
7.2
4.1
989.8
998
29932
17186
57
4
1.5
49.8
41.0
20.8
21.8
8.8
1
990.6
1497
36613
6287
17
5
2
0.25
50.9
49.6
20.8
47.2
1.3
26.4
1976
250
10789
27720
257
6
0.5
50.6
49.5
20.8
47.5
1.1
26.7
1976
490
9129
54949
602
7
1
49.6
44.3
20.7
25.6
5.3
4.9
1980
998
44075
20539
47
8
1.5
49.6
43.9
20.8
23.6
5.7
2.8
1980
1496
47401
17593
37
9
3
0.25
51.3
50.4
20.8
48.0
0.9
27.2
2964
249
11204
28560
255
10
0.5
51.0
50.2
20.8
48.0
0.8
27.2
2964
489
9959
55863
561
11
1
49.1
45.4
20.9
27.6
3.7
6.7
2969
999
4638
27693
597
12
1.5
49.4
45.7
20.8
27.4
3.7
6.6
2969
1496
46138
41469
90
Table 6: Two tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
mhot
mcold
Qe
Qa
η
(%)
1
1
0.25
50.7
48.1
20.7
48.7
2.6
28
988.9
249
10799
29282
271
2
0.5
49.7
41.4
20.9
40.6
8.3
19.7
989.8
489
34504
40460
117
3
1
49.6
37.5
20.9
27.0
12.1
6.1
989.8
999
50302
25595
51
4
1.5
49.8
36.5
20.8
24.5
13.3
3.7
990.6
1497
55335
23263
42
5
2
0.25
49.8
45.2
20.7
44.2
4.6
23.5
1978
248
38215
24478
64
6
0.5
50.9
48.5
20.7
49.6
2.4
28.9
1980
490
19958
59476
298
7
1
49.7
41.5
20.8
32.1
8.2
11.3
1980
998
68191
47365
69
8
1.5
49.8
40.0
20.7
28.2
9.8
7.5
1981
1498
81538
47187
58
9
3
0.25
49.9
46.4
20.7
45.0
3.5
24.3
2967
250
14215
25515
179
10
0.5
50.9
49.7
20.7
49.6
1.2
28.9
2969
489
14964
59355
397
11
1
49.7
43.7
20.7
34.3
6.0
13.6
2969
999
74819
57063
76
12
1.5
49.9
42.6
20.7
30.7
7.3
10.0
2972
1499
91122
62958
69
Table 7: Three Tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
mhot
mcold
Qe
Qa
η
(%)
1
1
0.25
51.8
46.1
21.0
49.5
5.7
28.5
988.9
250
23674
29925
126
2
0.5
49.5
38.8
21.1
38.5
10.7
17.4
989.8
490
44482
35809
81
3
1
49.6
34.7
21.1
30.4
14.9
9.3
991.4
999
62042
39021
63
4
1.5
49.7
33.3
21.1
27.1
16.4
6
991.4
1498
68288
37750
55
5
2
0.25
51.7
49.0
21.1
49.4
2.7
28.3
1979
249
22442
29596
132
6
0.5
49.6
44.9
21.2
47.3
4.7
26.1
1980
489
39085
53604
137
7
1
48.9
39.1
21.2
35.9
9.8
14.7
1983
998
81620
61678
76
8
1.5
49.0
37.7
21.2
31.4
11.3
10.2
1983
1499
94113
64217
68
9
3
0.25
51.3
47.7
20.1
49.0
3.6
28.9
2967
250
44861
30345
68
10
0.5
50.8
47.4
20.3
49.1
3.4
28.8
2970
499
42412
60359
142
11
1
47.9
40.8
20.8
38.0
7.1
17.2
2974
999
88685
72168
81
12
1.5
48.7
39.7
20.9
34.8
9.0
13.9
2974
1498
112417
87453
78
Table 8: Four tubes
Experiment
Hot water flow rate (L/min)
Cold water flow rate (L/min)
Temperature °C
Thot-in
Thot-out
Tcold-in
Tcold-out
Thot= T1-T5 (°C)
Tcold= T10-T6 (°C)
m hot
mcold
Qe
Qa
η
(%)
1
1
0.25
49.9
43.8
19.5
48.6
6.1
29.1
989.8
249
25359
30433
120
2
0.5
49.6
37.7
19.3
42.5
11.9
23.2
990.4
497
49500
48428
98
3
1
49.5
31.9
18.7
32.6
17.6
13.9
990.4
997
73210
58205
80
4
1.5
49.5
30.2
18.5
29.2
19.3
10.7
990.6
1496
80298
67230
84
5
2
0.25
50.0
47.4
19.5
49.3
2.6
29.8
1980
249
21622
31165
144
6
0.5
50.4
46.2
19.2
49.5
4.2
30.3
1982
496
34962
63121
181
7
1
49.2
39.0
18.5
42.1
10.2
23.6
1986
996
34962
98724
282
8
1.5
49.4
36.1
18.3
36.4
13.3
18.1
1989
1496
111106
113726
102
9
3
0.25
50.2
48.6
19.4
49.6
1.6
30.2
2967
249
19938
31583
158
10
0.5
50.9
46.4
18.6
49.8
4.5
31.2
2970
496
56133
64996
116
11
1
49.1
40.8
18.4
42.4
8.3
24.0
2974
997
103674
100498
97
12
1.5
48.6
38.8
18.3
39.0
9.8
20.7
2976
1497
122492
130149
106
When the flow rates were increased for both the cold and the hot water, the heat power that was emitted from the hot fluid (Qe) increased while the heat power that was absorbed by the old fluid (Qa decreased). The mean temperature efficiency was to be computed in order to determine both the Qa and Qe. The logarithmic mean temperature difference (LMTD) was calculated as the average temperature difference between the two fluids including the cold and the hot fluids using the formula Where, = ; as indicated in the table. Heat transfer area was derived using (m2), where dm was found to be 0.0089, and L was given as 0.330 per tube, which was 0.0092m2 for one tube, 0.018 m2 two tubes, 0.028 m2 for three tubes and 0.037 m2 for four tubes
Analysis of Results and the Discussion
According to (Eiamsa-ard, Thianpong and Promvonge (2004, p.26), generally, in heat exchangers, the heat gained by the cold water should be equal to the heat that is lost by the hot water (Cai and Kim 2005, p.050). This is referred to as the first law of thermodynamics where the heat transfer rate from the hot water is expected to equal the heat transfer rate to the cold water (Foanene and Diaconu 2016, p.2). The results portrayed that with time, as the temperature of the cold water increased, that of hot water decreased. It is evident from the findings that as the conditions in the pipes are altered the mount of transferred heat is also changed. As a result of this transient behavior, the process temperatures change to a point in which the distribution of temperature is steady (Karwa, Maheshwari and Karwa 2005, p.275). At the beginning of heat transfer, the fluids’ temperatures change, and until a steady state is reached, the behavior depends on time. Though in reality the temperatures cannot be entirely stable, with changes in the flow rates, a steady state was observed experimentally (Williams, Walters, Han and Williams 2002, p. 2-3).
Determining the coefficient of the overall heat transfer is very important to allow the determination of heat transferred from hot water in the inner pipe to the cold water in the outer tube. The coefficient accounts for conductive as well as convective resistances between fluids that are separated and the thermal resistances that fouling may cause on inner pipe sides (Sabeeh and Sabeeh 2014, p.189; Yakut, Sahin and Canbazoglu 2004, p.25).
In the current experiment, the cold water was circulated through the annulus. The efficiency would have reduced if the hot water was circulated through the annulus as there would be heat losses to the environment although they are dependent on the temperature gradient (EL-SHAMY, 2006, p.8). It is for this reason that it should be recognized that there is minimized loss of heat when the lower temperature is close to the heat exchanger enclosure (Sahiti, Durst and Dewan 2005, p. 4738; Shah and Sekulic, 2003, p.47).
From the tables, under constant rates of flow, the temperature differences ratio is constant. When the temperature difference of the hot water rises, there is also an increase in the temperature difference of the cold water. Therefore, although the numerical values of the differences in temperature changed, there was no change in the ratio between temperature differences, which agrees with the theory (Patro and Malviya 2012, p17; Mohammed, Hasan, and Wahid 2013, p37).
Conclusions
The four specific objectives of this experiment were demonstrated very well in this experiment. It showed how double heat exchanger pipe is designed and the manner in which it works. It also demonstrated the effects of different flow rates as well as surface areas in the rates of heat transfer as well as the Reynolds number (Afify and Abd-Elghany, 1997, p.6). The higher the rate of flow of a fluid, the lower change in temperature in that fluid was observed, and the opposite was also held to be true. It was evident that higher flow rates had reduced heat transfer compared to lower flow rates, which lead to the first recommendation that reduced flow rates are more effective (Lin 2006,p.986). Secondly the surface are also has an effect on efficiency, and it can be recommended that for efficient heat transfer, designs with increased surface area are more beneficial (Fuskele and Sarviya, 2012, p.5; Naphon and Suchana 2011, p.237; Sundar and Sharma 2010, p.1410).
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Appendices
Appendices 1: Reynold’s Number
