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Shell and Tube Heat Exchanger MECH595 – Introduction to Heat Transfer Professor M. Zenouzi
Prepared by: Andrew Demedeiros, Ryan Ferguson, Bradford Powers November 19, 2009
1
Abstract
2
Contents Discussion of Theory: .................................................................................................................................... 4 Experimental Apparatus and Procedure ....................................................................................................... 7 Experimental Data ........................................................................................................................................ 8 Results ......................................................................................................................................................... 10 Discussion of Results ................................................................................................................................... 14
3
Discussion of Theory: A heat exchanger is a device that is used to transfer energy in the form of heat from one fluid to another. They take two input fluids of different temperatures and as the two fluids run near each other the fluids transfer heat between each other. The heat exchanger looks like a large pipe that consists of 37 small tubes. They are used in various configurations for all sorts of applications such as space heating, refrigeration, air conditioning, power plants, chemical plants, and petrochemical plants. Heat exchangers can be used in two different configurations parallel flow, Figure 1, or counter flow, Figure 2.
Figure 1 ~ Cocurrent flow
Figure 2 ~ Counter current flow 4
Each configuration refers to how the fluid moves through their respective flow ages relative to each other. If each fluid is flowing in the same direction such as in figure 1 it is termed a parallel flow. On the other hand if the fluids flow in opposite directions as in figure 2 it is termed counter flow. Parallel flow in heat exchangers happens when both fluids enter the heat exchanger at their largest temperature difference. The temperature difference becomes less over the length of the heat exchanger. In the counter flow heat exchanger, the fluids enter at opposite ends and therefore at different ends of the temperature scale Figure 2. As the fluids move through the exchanger, they both warm up or cool down at roughly the same rate. The temperature differential between the two fluids is relatively constant over the length of the exchanger.
The heat transfer process which occurs in any basic heat exchanger can be summarized by the following equations.
Qh = m h c ph ∆Th Qc = m c c pc ∆Tc Q=
F (LMTD ) = F (UA)(LMTD ) RT
Where in the last equation F is the correction factor which equals 1 for this experiment. LMTD is the Log Mean Temperature Difference which is described latter in this section. Q is the heat transferred between the hot water and cold water.
The overall resistances can be calculated using:
RT = Rhf + Rw + Rcf
5
Where
Rhf =
1 A1 hh
Rw =
ln[D2 D1 ] 2πLk w
Rcf =
1 A2 hc
hc and hh from the above equations can be found using the appropriate Nusselt number for hot and cold water. For the hot water (fluid inner tubes)
K hh = Nu h h D h Nu h = 0.023 Re 0h.8 Prh0.3 For Cooling
For the cold water
K hc = Nu c c D c Nu c = 0.36 Re 0c .55 Prc0.33 For Heating
The log-mean temperature difference is given by the following equation where a and b represent the ends of the heat exchanger. The LMTD is used because the heat must through four resistances the hot tube to the cold water.
∆Tm =
(∆Ta − ∆Tb ) ln (∆Ta ∆Tb )
6
Heat exchanger effectiveness is defined as the ratio of the actual heat transfer rate of the praticlar heat exchanger to the maximum possible heat transfer rate for the same unit.
ε = Q Qmax Qmax = C min (Thi − Tci ) Where Cmin is equal to either Cc or Ch, whichever is smaller and are defined as, Ch = mh h and Cc = mcc.
ε=
C h (Thi − Tho ) C min (Thi − Tci )
Experimental Apparatus and Procedure For this experiment a HT30X Heat exchanger services unit was used along with an HT33 shell and tube heat exchanger. This device included four K-type thermocouples at the hot and cold inlet and outlets. The exchanger consisted of seven stainless steel tubes 6.35 mm in diameter with a 0.6 mm wall thickness. The outer annulus was constructed from clear acrylic tubing 39.0 mm inner diameter with a 3.0 mm wall thickness. The length of the tube bundle is 144 mm giving a total heat transfer area of 20,000 m2. The procedure for the laboratory is listed below. 1. Set the cold water pressure regulator. Adjust the knob until a flow rate of 3.00 liters per minute is established. Lock down this setting. 2. Prime the hot water circuit. Switch on the hot water circulating pump and expel any air bubbles. Do not let the water level fall below the height of the priming vessel to prevent air from entering the system.
7
3. Set the computer software to countercurrent flow and maintain a hot water temperature of 60 °F.
Experimental Data The results of the experiment are displayed in the tables below. m_cw l/s
RUN
m-ht l/s
Th in ˚C
Th out ˚C
Tc in ˚C
Tc out ˚C
1
1
3
60.7
56.5
15.9
29.7
2
1.5
3
60.5
55.2
15.1
25.1
3 4
2 2.5
3 3
60.2 60.5
54.5 53.7
14.5 14.8
22.9 21.3
5
3 3 60.5 53.8 14.3 Table 1 ~ Parallel flow temperature data
20.5
m_cw l/s
RUN
m-ht l/s
Th in ˚C
Th out ˚C
Tc in ˚C
Tc out ˚C
1
1
3
61.3
56.6
15.1
28.3
2
1.5
3
60.4
55
15
24.8
3 4
2 2.5
3 3
60.5 60.5
54.5 54
16 15.6
23.5 22
3 3 60.8 53.9 15.7 Table 2 ~ Counter current flow temperature data
21.2
5
Sample Calculations: Calculating hot water heat rate:
Calculating cold water heat rate:
8
Calculating Reynolds Number:
Calculating Nusselt Number:
Calculating heat transfer coefficient:
Calculating overall heat transfer coefficient:
Calculating Log-Mean Temperature Difference:
9
Calculating heat transfer rate:
Calculating maximum heat transfer:
Calculating Efficiency:
Results Hot Water Heat Rate Run
Cold Water Heat Rate
Qh
Run
Qc
1 2 3
52.03833 W 65.66742 W 70.62345 W
1 2 3
4 5
84.25254 W 4 83.01353 W 5 Table 3 ~ Cocurrent Heat Rates
10
56.99436 W 61.9504 W 69.38444 W 67.11293 W 76.81849 W
Hot Water Heat Rate Run
Cold Water Heat Rate
Qh
Run
Qc
1 2 3
58.23337 W 66.90643 W 74.34048 W
1 2 3
54.51635 W 60.71139 W 61.9504 W
4 5
80.53551 W 4 85.49155 W 5 Table 4 ~ Counter Current Heat Rates
66.08042 W 68.14544 W
1.8 1.6
Total Thermal Resistance (K/m)
1.4 1.2 1 0.8
Total Resistance
0.6 0.4 0.2 0 0
0.5
1
1.5
2
2.5
Cold Water Flow Rate (kg/s)
Figure 3 ~ Total Thermal Resistance
11
3
3.5
90 80 70
Heat Rate (W)
60 50 Hot Water Heat Rate(Cocurrent)
40
Hot Water Heat Rate (Concurrent)
30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 4 ~ Hot Water Heat Rate 90 80 70
Heat Rate (W)
60 50 Heat Rate Cold Water (Cocurrent)
40
Heat Rate Cold Water (Concurrent)
30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 5 ~ Cold Water Heat Rate 12
50 45 40
Heat Rate (W)
35 30 25 Total Heat Rate (Cocurrent)
20
Total Heat Rate (Concurrent)
15 10 5 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 6 ~ Total Heat Rate 8 7 6
Efficiency
5 4 Cocurrent Efficiency 3
Concurrent Efficiency
2 1 0 0
0.5
1
1.5
2
2.5
3
Cold Water Flow Rate (kg/s)
Figure 7 ~ Exchanger Efficiency 13
3.5
Discussion of Results The results of this laboratory show that the effectiveness of the heat exchanger is related to the cold-water flow rate. This is due to the decrease of thermal resistance decreases with increased cold water flow. Each trend in the Figures 4 through 7 above increases with flow rate. There was no noticeable advantage to using counter current versus concurrent flow in the data. For each run the data collected for heat transfer rate did not vary greatly.
Conclusions The data presented in this report shows that heat exchanger performance increase linearly with increasing cold water flow rate. This follows logically since more cold water is delivered to carry away heat per unit of time. Additionally increased flow rate results in more turbulent flow. This also increases the heat transfer rate. Contrary to heat exchanger theory however, there was no noticeable difference in the heat transfer rate between parallel flow and counter current flow. The counter current flow should show enhanced heat transfer ability.
14
Prepared by: Andrew Demedeiros, Ryan Ferguson, Bradford Powers November 19, 2009
1
Abstract
2
Contents Discussion of Theory: .................................................................................................................................... 4 Experimental Apparatus and Procedure ....................................................................................................... 7 Experimental Data ........................................................................................................................................ 8 Results ......................................................................................................................................................... 10 Discussion of Results ................................................................................................................................... 14
3
Discussion of Theory: A heat exchanger is a device that is used to transfer energy in the form of heat from one fluid to another. They take two input fluids of different temperatures and as the two fluids run near each other the fluids transfer heat between each other. The heat exchanger looks like a large pipe that consists of 37 small tubes. They are used in various configurations for all sorts of applications such as space heating, refrigeration, air conditioning, power plants, chemical plants, and petrochemical plants. Heat exchangers can be used in two different configurations parallel flow, Figure 1, or counter flow, Figure 2.
Figure 1 ~ Cocurrent flow
Figure 2 ~ Counter current flow 4
Each configuration refers to how the fluid moves through their respective flow ages relative to each other. If each fluid is flowing in the same direction such as in figure 1 it is termed a parallel flow. On the other hand if the fluids flow in opposite directions as in figure 2 it is termed counter flow. Parallel flow in heat exchangers happens when both fluids enter the heat exchanger at their largest temperature difference. The temperature difference becomes less over the length of the heat exchanger. In the counter flow heat exchanger, the fluids enter at opposite ends and therefore at different ends of the temperature scale Figure 2. As the fluids move through the exchanger, they both warm up or cool down at roughly the same rate. The temperature differential between the two fluids is relatively constant over the length of the exchanger.
The heat transfer process which occurs in any basic heat exchanger can be summarized by the following equations.
Qh = m h c ph ∆Th Qc = m c c pc ∆Tc Q=
F (LMTD ) = F (UA)(LMTD ) RT
Where in the last equation F is the correction factor which equals 1 for this experiment. LMTD is the Log Mean Temperature Difference which is described latter in this section. Q is the heat transferred between the hot water and cold water.
The overall resistances can be calculated using:
RT = Rhf + Rw + Rcf
5
Where
Rhf =
1 A1 hh
Rw =
ln[D2 D1 ] 2πLk w
Rcf =
1 A2 hc
hc and hh from the above equations can be found using the appropriate Nusselt number for hot and cold water. For the hot water (fluid inner tubes)
K hh = Nu h h D h Nu h = 0.023 Re 0h.8 Prh0.3 For Cooling
For the cold water
K hc = Nu c c D c Nu c = 0.36 Re 0c .55 Prc0.33 For Heating
The log-mean temperature difference is given by the following equation where a and b represent the ends of the heat exchanger. The LMTD is used because the heat must through four resistances the hot tube to the cold water.
∆Tm =
(∆Ta − ∆Tb ) ln (∆Ta ∆Tb )
6
Heat exchanger effectiveness is defined as the ratio of the actual heat transfer rate of the praticlar heat exchanger to the maximum possible heat transfer rate for the same unit.
ε = Q Qmax Qmax = C min (Thi − Tci ) Where Cmin is equal to either Cc or Ch, whichever is smaller and are defined as, Ch = mh h and Cc = mcc.
ε=
C h (Thi − Tho ) C min (Thi − Tci )
Experimental Apparatus and Procedure For this experiment a HT30X Heat exchanger services unit was used along with an HT33 shell and tube heat exchanger. This device included four K-type thermocouples at the hot and cold inlet and outlets. The exchanger consisted of seven stainless steel tubes 6.35 mm in diameter with a 0.6 mm wall thickness. The outer annulus was constructed from clear acrylic tubing 39.0 mm inner diameter with a 3.0 mm wall thickness. The length of the tube bundle is 144 mm giving a total heat transfer area of 20,000 m2. The procedure for the laboratory is listed below. 1. Set the cold water pressure regulator. Adjust the knob until a flow rate of 3.00 liters per minute is established. Lock down this setting. 2. Prime the hot water circuit. Switch on the hot water circulating pump and expel any air bubbles. Do not let the water level fall below the height of the priming vessel to prevent air from entering the system.
7
3. Set the computer software to countercurrent flow and maintain a hot water temperature of 60 °F.
Experimental Data The results of the experiment are displayed in the tables below. m_cw l/s
RUN
m-ht l/s
Th in ˚C
Th out ˚C
Tc in ˚C
Tc out ˚C
1
1
3
60.7
56.5
15.9
29.7
2
1.5
3
60.5
55.2
15.1
25.1
3 4
2 2.5
3 3
60.2 60.5
54.5 53.7
14.5 14.8
22.9 21.3
5
3 3 60.5 53.8 14.3 Table 1 ~ Parallel flow temperature data
20.5
m_cw l/s
RUN
m-ht l/s
Th in ˚C
Th out ˚C
Tc in ˚C
Tc out ˚C
1
1
3
61.3
56.6
15.1
28.3
2
1.5
3
60.4
55
15
24.8
3 4
2 2.5
3 3
60.5 60.5
54.5 54
16 15.6
23.5 22
3 3 60.8 53.9 15.7 Table 2 ~ Counter current flow temperature data
21.2
5
Sample Calculations: Calculating hot water heat rate:
Calculating cold water heat rate:
8
Calculating Reynolds Number:
Calculating Nusselt Number:
Calculating heat transfer coefficient:
Calculating overall heat transfer coefficient:
Calculating Log-Mean Temperature Difference:
9
Calculating heat transfer rate:
Calculating maximum heat transfer:
Calculating Efficiency:
Results Hot Water Heat Rate Run
Cold Water Heat Rate
Qh
Run
Qc
1 2 3
52.03833 W 65.66742 W 70.62345 W
1 2 3
4 5
84.25254 W 4 83.01353 W 5 Table 3 ~ Cocurrent Heat Rates
10
56.99436 W 61.9504 W 69.38444 W 67.11293 W 76.81849 W
Hot Water Heat Rate Run
Cold Water Heat Rate
Qh
Run
Qc
1 2 3
58.23337 W 66.90643 W 74.34048 W
1 2 3
54.51635 W 60.71139 W 61.9504 W
4 5
80.53551 W 4 85.49155 W 5 Table 4 ~ Counter Current Heat Rates
66.08042 W 68.14544 W
1.8 1.6
Total Thermal Resistance (K/m)
1.4 1.2 1 0.8
Total Resistance
0.6 0.4 0.2 0 0
0.5
1
1.5
2
2.5
Cold Water Flow Rate (kg/s)
Figure 3 ~ Total Thermal Resistance
11
3
3.5
90 80 70
Heat Rate (W)
60 50 Hot Water Heat Rate(Cocurrent)
40
Hot Water Heat Rate (Concurrent)
30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 4 ~ Hot Water Heat Rate 90 80 70
Heat Rate (W)
60 50 Heat Rate Cold Water (Cocurrent)
40
Heat Rate Cold Water (Concurrent)
30 20 10 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 5 ~ Cold Water Heat Rate 12
50 45 40
Heat Rate (W)
35 30 25 Total Heat Rate (Cocurrent)
20
Total Heat Rate (Concurrent)
15 10 5 0 0
0.5
1
1.5
2
2.5
3
3.5
Cold Water Flow Rate (kg/s)
Figure 6 ~ Total Heat Rate 8 7 6
Efficiency
5 4 Cocurrent Efficiency 3
Concurrent Efficiency
2 1 0 0
0.5
1
1.5
2
2.5
3
Cold Water Flow Rate (kg/s)
Figure 7 ~ Exchanger Efficiency 13
3.5
Discussion of Results The results of this laboratory show that the effectiveness of the heat exchanger is related to the cold-water flow rate. This is due to the decrease of thermal resistance decreases with increased cold water flow. Each trend in the Figures 4 through 7 above increases with flow rate. There was no noticeable advantage to using counter current versus concurrent flow in the data. For each run the data collected for heat transfer rate did not vary greatly.
Conclusions The data presented in this report shows that heat exchanger performance increase linearly with increasing cold water flow rate. This follows logically since more cold water is delivered to carry away heat per unit of time. Additionally increased flow rate results in more turbulent flow. This also increases the heat transfer rate. Contrary to heat exchanger theory however, there was no noticeable difference in the heat transfer rate between parallel flow and counter current flow. The counter current flow should show enhanced heat transfer ability.
14
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