Heat Exchanger Analysis By David Vaughn And Tyler Ware September 19, 2000 UTC Engr 435 DR.’s Henry, Cunningham, Jones Introduction For this analysis a tube and shell heat exchanger was used in a variety of different ways to determine the convection coefficient different parts of the exchanger. The convection coefficient was found for the tube side of the exchanger for varying flow rates, and also for the shell side the exchanger for varying flow rates. This was done by measuring the flow rates for each test and recording the temperatures for the inlet and outlet of the hot and cold water flow. Theory and Background A tube and shell exchanger means that one fluid flows though a series of tubes while the other fluid flows through a shell that surrounds the tubes. This exchanger had a total of 55 tubes, which amounted to a total area of transfer of 2.93ft2 . The total volume of the shell within this exchanger is 16.0 in3 and the tubes with have a volume of 8.7in3 . This exchanger in particular was set up so the hot water is capable of flowing through the tubes or the shell, and also the hot and cold water can be set to flow co-current or counter current. This means that the two streams are flowing the same direction or opposite directions respectively. In Figure 1 is a schematic of the heat exchanger and its system. A, B, C, and D are all 4-way valves, meaning that they have two inputs and two exits. 1 and 4 are the tube entrances into the heat exchanger, and 2 and 3 are the shell entrances. For example, if the hot water was set to flow in the tubes and counter current with the cold water, then in A the hot water in would take the path to C and then to 1, while the cold water in would flow from B to D and then to 3.
Figure 1 – Heat Exchanger Diagram On this exchanger the input function for the hot water is a motor percentage which pumps water through a hot water heater and then into the exchanger, changing the motor input changes the output of hot water flow rate. The temperature is considered a constant coming out of the water heater. For the cold water, the input function is a proportional valve percentage that varies the amount of cold water aloud to flow through the system from the faucet. The output of the cold water is also varying a flow rate, which depends upon some valve percentage. The way that a heat exchanger works is hot water and cold water enter the exchanger, where the process of cold water gaining some heat and the hot water losing some takes place, before they both exit the exchanger. What is actually happening is, the hot water is heating either the inside or the outside of the tubes in the exchanger, depending on where it is flowing, by what is known as convection. Then the heat is conducted through the tubes to the other side, either the outside or the inside, where it is then convected back into the cold water raising its temperature. Convection is a mode of heat transfer that involves motion of some fluid that either absorbs heat from a source or gives heat to some surrounding. Conduction is a mode of heat transfer in which the heat is moving through a stationary object or fluid. For a heat exchanger that flows parallel or counter current then the coefficient of heat transfer is called the over all coefficient of heat transfer. It is calculated using the log mean temperature difference, which is found two different ways, depending of whether the flow is parallel or counter. Modeling There were two main equations used to analyze the heat exchanger. They were: Q = Cp (DT) Where Q is the heat exchanged, Cp is the heat capacity, and DT is the temperature difference between the inlet and the outlet streams of the heat exchanger. This equation can be used for both the cold and hot water streams. The other primary equation was as follows: Q = U*A* DTlm Where the Q is the heat exchanged, U is the overall heat transfer coefficient, A is the surface area for the heat to be exchanged, and DTlm is a log mean temperature difference and is defined below. DTlm = (DT2 - DT1 ) / ( ln (DT2 / DT1 )) For parallel flow : DT1 = Thi –Tci and DT2 = Tho - Tco For counter flow : DT1 = Thi –Tco and DT2 = Tho - Tci
Figure 2 – Tube and Shell Co-Current Flow Procedure In conducting an analysis on the heat exchanger, the first thing to do is to understand the exchanger and to then decide the procedure that will be used in order to take data that will be useful later for analysis. For this experiment we decide to divide the actual procedure in to four different groups: first using the hot water in the shell and the flows counter-current, second hot water in the shell and the flows are co-current, third we would leave the flow co-current and switch the flow of hot water from the shell to the tubes, and for the last experiments we would leave the hot water in the tubes, but the flow would be switched to counter current. Then for each of the partial procedures the flow rate should be varied for the hot water and the cold water. With this procedure in mind we started the labview Temperature Manual Remote program. After selecting the settings we were going to use first, the run button was pushed on the interface, and the experiment was started. The temperatures were then recorded off the computer for the hot water input and output and the cold water input and output. After the data was recorded we stopped the program and left the computer as it was when we arrived. Results The results from the spreadsheet were as follows: HW Tube - Counter Current HW flow CW flow D T1 *C D T2 *C DT Log Mean U (kW/m^2 K) 4.4 4.5 13.8 15.1 14.4 1.29 4.0 4.5 14.4 14.6 14.5 1.20 3.6 4.5 15.2 13.7 14.4 1.14 3.3 4.5 15.9 12.5 14.1 1.11 2.7 4.5 16.5 11.0 13.6 1.04 2.2 4.5 17.2 9.3 12.8 0.99 1.8 4.5 18.0 7.0 11.6 0.98 1.4 4.5 18.9 3.8 9.4 0.97 0.6 4.5 19.6 0.3 4.6 0.90 4.4 4.0 13.2 14.8 14.0 1.21 4.4 3.8 12.8 15.0 13.9 1.19 4.4 3.6 11.6 14.9 13.2 1.17 4.4 3.1 8.6 15.0 11.5 1.14 4.8 3.9 11.1 14.1 12.5 1.29 4.8 4.3 12.0 13.3 12.6 1.29 HW shell Counter Current HW flow CW flow D T1 *C D T2 *C 4.50 3.80 10.50 12.30 3.40 3.80 11.00 9.40 1.70 3.80 12.00 4.50
Table 1 – Collected Data The data was plotted and the results follow:
Figure 2 – HW Tube Counter Current
Figure 3 – HW Shell Counter Current
Figure 4– CW Tube Counter Current The Reynold’s number was calculated for the tubes of the heat exchanger and was found to be less than 1000 for all flow rates that were possible. Therefore the flow through the tube side of the heat exchanger was assumed laminar.
Note! – Insert Reynold # vs. U Here Discussion When the cold water is left at a constant flow rate and the hot water flow is increased, the average U value increases. The response of the average U value with a varying flow rate is different for the hot water in the tubes and the hot water in the shell. When the hot water is in the shell, the U increase until the flow rate is around 3.65 L/min, and which time the U value decreases with increased flow rates. The reason for this is believed to be caused from the baffles within the shell in the heat exchanger, when the flow increases so much the baffles start to make the flow turn more turbulent than laminar, so the U value changes. When the hot water flow is in the tubes and is increased, the average U value increases constantly with flow increase. This is because the water flow through the tubes is staying laminar therefore making the U value more constant. When the hot water was left constant and the cold water varied while flowing through the shell, the results seemed to be completely opposite. The average U value decreased with increasing flow rates until the flow was about 3.7 L/min at which time the U value started to increase with higher flow rates. Conclusions The heat exchanger performed as it was expected to perform. The U calculated for the hot stream was only slightly different then the U calculated for the cold stream. This indicates that there is heat lost to the surrounding areas. The heat exchanger was most effective when the hot water was directed into the tubes and the flow was counter-current. Also for the flow rates examined the heat transfer rate increases as the flow rate increases. It was seen that when the flow in the shell reached about 3.7 L/min the flow began to be disturbed. It is believed that the baffles cause the flow to become turbulent. Recommendations In an effort to reduce the heat loss to the surroundings, it is recommended that the heat exchanger be well insulated. Presently the heat exchanger has no insulation and the ambient room temperature has a large effect on the results. It is also recommended that during the process of data collection that the user adjusts the flow rate of only one stream per setup. If this is not done the graphs of the data becomes very difficult to read and understand. Another recommendation is to ensure that the flow rates obtained are measured accurately. There are two ways that this may be done. One way is to purchase new flow meters, and the other is to manually measure each flow rate with great care. This is extremely important because without accurate flow rates the temperature data is worthless. David Vaughn Wrote for report: Modeling Results Conclusions Recommendations Also: Collected data for two days of lab Tyler Ware Wrote for the report: Introduction Theory and Background Procedure Discussion Also: Gave both oral presentations Collected data for all three days of lab Appendices Physical properties for water: Cp = 4.18 kJ / kg k Equations: Q = Cp (DT) Q = U*A* DTlm DTlm = (DT2 - DT1 ) / ( ln (DT2 / DT1 )) For parallel flow : DT1 = Thi –Tci and DT2 = Tho - Tco For counter flow : DT1 = Thi –Tco and DT2 = Tho - Tci Data curves and calculations: *see the following sheets
Discussion When looking at the results from that data taken it is seen that the heat exchanger does perform as it should. When the cold water is left at a constant flow rate and the hot water flow is increased, the average U value increases. The response of the average U value with a varing flow rate is different for the hot water in the tubes and the hot water in the shell. When the hot
water is in the shell, the U increase until the flow rate is around 3.65 L/min, and which time the U value decreases with increased flow rates. The reason for this is believed to be caused from the baffles within the shell in the heat exchanger, when the flow increases so much the baffles start to make the flow turn more turbulent then laminar, so the U value changes. When the hot water flow is in the tubes and is increased, the average U value increases constantly with flow increase. This is because the water flow through the tubes is staying laminar therefore making the U value more constant. When the hot water was left constant and the cold water varied while flowing through the shell, the results seemed to be completely opposite. The average U value decreased with increasing flow rates until the flow was about 3.7 L/min at which time the U value started to increase with higher flow rates.