Introduction
The main objectives of this report include; designing the operational principles of parallel and counter flow of heat exchangers. Improving the understanding of fluid temperature and flow rate heat exchangers and lastly to calculate and determine the overall heat transfer coefficient of a heat exchanger ("Superplastic duplex stainless steel heads for the heat exchanger market", 2010).
The parallel flow of fluid is characterized by the flow of cold and hot fluids in the same direction; it, therefore, means that the entrance for the two fluids are in the same direction likewise their exit are also normally in the same direction. Counter flow is characterized by opposite flow of cold and warm fluids this, therefore, means that the fluids enter from opposite direction and leave in the opposite directions too (Jin, Qiwu, & Minshan, 2008).
Parallel flow
The shell and tube heat exchanger was connected to the parallel movement of fluid and the tank temperature set at 600C; the local ambient air temperature was therefore recorded as a reference temperature. The flow rate of water was set at 3 liters/min. The flow of cold water was started, and by the hand operated flow control valve the flow rate was set to test Run 1 as in the case presented in the Datalog table (Jin, Qiwu, & Minshan, 2008). The system was left to obtain a steady state condition that is the point where there are no more temperature differences this took 5 minutes. After that, the control valve was adjusted slightly to ensure a constant flow rate. The hot and cold fluids circuit’s temperatures were recorded in the data log. The procedure was repeated for test runs 2,3 and 4 and the results recorded as shown in the results section.
Parallel flow
The shell and tube heat exchanger was reconnected to counter flow and the procedure for the parallel flow above repeated. The results were recorded in the data log that was provided.
Calculations
The volumetric flow rate of the system was given by 3 liters/second. To convert it into mass flow rate, it should be multiplied by the value of the density of the fluid thus proceeding as shown below;Vn×ρn=mn
The density of water is given as 1000 kg/m3
Converting the volumetric flow from liters/min to m3/s we have;
Volumetric flow =3×0.00160 m3s=5×10-5 m3/s
The mass flow rate Mn=5×10-5m3s×1000Kgm2
The mass flow rate = 0.05 Kg/s
The heat transfer rate across the heat exchanger walls were calculated from the expressions below;
For the cold water;
Qc=mcCpc∆Tc
Qc=kxA∆T
The Counter flow
Where k is the thermal conductivity, x the thickness of the wall of the pipes, A is the area of constant and ∆T the temperature difference. Since the pipe that carries the cold water is one big pipe then the following expression is used.
The difference between the internal diameter and the outer diameter gives us the double the value of the thickness of the material that is to mean.
The thickness of the wall = (60 -50)/2 mm = 5 mm = 0.005 m
Since the overall length of the system is 260 mm = 0.26 m
3 baffles mean heat transfer rate was given as 0.02 m
Qc=kxA∆T
Qc1=0.60.005×0.023 ×5.7=4.56
Qc2=0.60.005×0.023 ×7.6=6.08
Qc3=0.60.005×0.023 ×12.1=9.68
Qc4=0.60.005×0.023 ×19.1=15.28
For the Hot water;
The pipes thickness is given by;
Thickness = (6 – 4)/2 mm = 1 mm = 0.001 m
Qh1=0.60.001×0.023 ×5.3=21.2
Qh2=0.60.001×0.023 ×4.9=19.6
Qh3=0.60.001×0.023 ×3.8=15.2
Qh4=0.60.001×0.023 ×3.1=9.3
Parallel flow
Cold water
The calculations are generally the same as for the counter flow; the only difference may be in the values and the heating effect on the fluids.
Qc=kxA∆T
Qc1=0.60.005×0.023 ×7.6=6.08
Qc2=0.60.005×0.023 ×7.7=6.16
Qc3=0.60.005×0.023 ×12.2=9.76
Qc4=0.60.005×0.023 ×18.9=15.12
For the hot water pipes
Qh1=0.60.001×0.023 ×5.0=20
Qh2=0.60.001×0.023 ×4.5=18
Qh3=0.60.001×0.023 ×3.9=15.6
Qh4=0.60.001×0.023 ×2.8=11.2
The logarithmic temperature difference calculation can be done as below
LMTD = (dto - dti) / ln(dto / dti )
Where dti is the difference in temperature between the inlet primary and secondary fluid temperatures and dto is the difference between the outlet primary and secondary fluid temperatures.
For test run 1;
dti =58.6-8.3=50.3
dto =53.6-15.9=37.7
LMTD =dto - dtilndto dti
LMTD = - 12.6/ln37.750.3 = 43.70
Test run 2
dti =58.3-8.3=50.0
dto =53.8-16.0=37.8
LMTD =dto - dtilndto dti
LMTD = - 12.2/ln37.850.0 = 43.62
Test run 3
dti =58.8-8.5=50.3
dto =54.9-20.7=34.2
LMTD =dto - dtilndto dti
LMTD = - 16.1/ln34.250.3 = 41.73
Test run 4
dti =59.2-9.1=50.1
dto =56.4-28.0=28.4
LMTD =dto - dtilndto dti
LMTD = - 20.7/ln28.450.1 = 36.47
For counter flow;
LMTD = (dto - dti) / ln(dto / dti )
Where dti is the difference in temperature between the inlet primary and secondary fluid temperatures and dto is the difference between the outlet primary and secondary fluid temperatures.
dti = is the difference in fluid temperature between the outlet primary and the inlet of the secondary.
dto = Is the difference in fluid temperatures of the outlet primary and the inlet secondary.
For test run 1;
dti =57.8-14.6=43.3
dto =53.6-8.9=44.9
LMTD =dto - dtilndto dti
LMTD = 1.6/ln44.943.3 = 44.10
For test run 2;
dti =58.7-16.7=42
dto =53.8-9.1=44.7
LMTD =dto - dtilndto dti
LMTD = 2.7/ln44.742 = 43.34
For test run 3;
dti =59.0-21.4=37.6
dto =55.2-9.3=45.9
LMTD =dto - dtilndto dti
LMTD = 8.3/ln45.937.6 = 41.61
For test run 4;
dti =58.4-28.8=29.6
dto =55.3-9.7=45.6
LMTD =dto - dtilndto dti
LMTD = 16/ln45.629.6 = 37.03
Determination of the Energy coefficient against cold flow.
QC=ρFCP(Tout-Tin)
QC=1oookgm3×5×10-5m3s×4.17 KJKg-0K 60-200C
QC=8.34kJseconds=8.34 k watts
The Effectiveness of a heat exchanger can be calculated from the expression;
E = -exp (-NTU)
Analysis
Parallel flow
Counter current flow
In the parallel flow configuration, the cold water out (Tc (out)) is always lower compared to the hot stream that is also let out. This results in the restriction of heat transfer by the outlet of the cold stream.
In the counter-current flow design, there is no restriction as the temperature of the cold water out may exceed the one of the hot water out. In this design, the cold water temperature out is a function of the restriction of the hot water inlet. For a better heat recovery to be achieved the counter current flow is therefore preferred over the parallel flow (Webb, 2007). It should, however, be noted that there are cases where the identical design of heat exchangers may be used in some cases of design. However, there are some special cases where a concurrent exchanger design might still be adopted, for example where it is desirable that the two fluids in question be brought to the same temperature levels or readings.
Three main advantages are presented when the counter-current flow is applied to the parallel flow they include. Because of the minimal difference in the temperature minimal thermal stress is experienced throughout the heat exchanger. It is also possible for the output temperature of the cold fluid to attain the highest temperature of the hot fluid. There is uniform temperature difference that results in a uniform rate of heat transfer that is expected throughout the system. Heat transfer in the heat exchanger involves conduction and convection (Webb, 2007). That is to say, the hot fluid conveys the heat through convection to the cold fluid, conduction of heat across the pipes wall result in heat transfer also to the fluids within the pipes. Through convection again there is a transfer of heat from one fluid to the next. These processes take place throughout the pipes length, therefore, resulting in the difference in temperature s at different sections of the pipe. Heat transfer rate across the system varies across the length of the exchanger because and is dependent on the difference in temperature between the hot and the cold fluids.
Conclusion
In summary, the objectives of the lab were met, the various aspects of the design of heat exchangers were calculated and results tabulated as shown in the discussion section. Parallel design of heat exchangers is seldom used because of it less efficiency regarding the maximization of the uptake of heat from the hot fluid by the cold fluid (Webb, 2007). Countercurrent flow is preferred in cases where cooling or heating through the fluids needs to be very efficient.
References
Webb, R. (2011). HEAT EXCHANGER DESIGN METHODOLOGY FOR ELECTRONIC HEAT SINKS. Frontiers In Heat And Mass Transfer, 2(2). http://dx.doi.org/10.5098/hmt.v2.2.3001
Webb, R. (2007). Heat Exchanger Design Methodology for Electronic Heat Sinks. J. Heat Transfer, 129(7), 899. http://dx.doi.org/10.1115/1.2717249
Jin, Z., Qiwu, D., & Minshan, L. (2008). Heat Exchanger Network synthesis with Detailed Heat Exchanger Design. Chemical Engineering & Technology, 31(7), 1046-1050. http://dx.doi.org/10.1002/ceat.200800108
Pignotti, A. (2008). Linear Matrix Operator Formalism for Basic Heat Exchanger Thermal Design. J. Heat Transfer, 110(2), 297. http://dx.doi.org/10.1115/1.3250483
Superplastic duplex stainless steel heads for the heat exchanger market. (2010). Materials & Design, 11(2), 107. http://dx.doi.org/10.1016/0261-3069(90)90076-v
