Basic Thermal Energy Transfer with a Heat Exchanger

Transcription

1 Exercise 4-1 Basic Thermal Energy Transfer with a Heat Exchanger EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the basic principles of operation of a typical heat exchanger. You will be able to put to use this knowledge to perform measurements of the output temperature of the fluids circulating in the heat exchanger and to compare performance in different modes of operation. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Heat exchangers Brazed plate heat exchanger. DISCUSSION Heat exchangers A heat exchanger uses the heat-transfer mechanisms discussed previously to transfer energy from a warmer fluid to a colder one. The role of the heat exchanger is to optimize the transfer of heat by providing a space where the two fluids will interact thermally as much as possible without any actual mixing. This is done by providing a surface area where the heat of the hot fluid will transfer by conduction to the cold fluid through the thin, heat-conductive walls separating the fluids. Fluid 2 Fluid 1 (a) Parallel flow (b) Counter-flow Figure A simple heat exchanger. The heat exchanger shown above is made of two concentric cylinders in which two different fluids are meant to travel. The wall of the tube in which fluid 2 travels is the surface area where the heat exchange between the two fluids will take place. The outer wall of the tube in which fluid 1 travels is assumed to be thermally isolated from the environment. This is of course an idealization. Festo Didactic

2 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Discussion Temperature Temperature Position (a) Parallel flow Position (b) Counter-flow Figure Temperature distributions in the heat exchanger. In Figure 4-10, two fluids enter the heat exchanger at input temperatures and. It is assumed that fluid 1 is warmer than fluid 2: > (see Figure 4-11). Heat transfer occurs from the warmer fluid to the colder one. Consequently, the warmer fluid 1 will lose energy which will be transferred to fluid 2. The temperatures of each fluid will thus change as they progress in the exchanger to reflect the heat transfer mechanism. The output temperature of fluid 1 will be smaller than its initial temperature ( ) while the output temperature of fluid 2 will be higher than its initial value as fluid 2 gains energy ( ). In Figure 4-10a and Figure 4-11a, the fluids travel in the same direction, i.e., in a mode named parallel flow. At the other end of the heat exchanger (at position L), the output temperatures will have values and. The temperature difference between the two fluids is initially large but diminishes rapidly as the fluids progress in the exchanger. If the exchanger is large enough (or the initial temperature difference small enough), the temperature difference eventually approaches zero asymptotically. Contrast this with the case presented in Figure 4-10b and Figure 4-11b where the fluids flow in opposite directions. This mode of operation is known as counter-flow. Note how the final temperature of fluid 1,, is colder in a counter-flow configuration than in the parallel flow configuration. Likewise, the output temperature of liquid 2,, is warmer in counter-flow. The immediate observation is thus that the heat exchange process is more efficient in a counterflow configuration. Even if the initial temperature difference is smaller than in the parallel-flow case, the temperature gradient diminishes in a much slower fashion, resulting in an overall larger heat transfer (Recall that the magnitude of conduction heat transfer is based on the temperature gradient See Equation (4-1)). This is why most heat exchangers are usually operated in counter-flow configuration. Brazed plate heat exchanger A brazed plate heat exchanger is a compact type of exchanger made of a series of thin, usually corrugated plates which, once joined, are designed to create cavities in which the process fluids will flow. Each plate has four corner ports (one inlet and one outlet for each of the two fluids). When the plates are assembled, the ports line up to give access to the cavities. For each cavity, a pair 68 Festo Didactic

3 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Discussion of gaskets blocks the access to one of the two fluids and lets the other one flow from the inlet to the outlet. By alternating the fluid admitted in each cavity between fluids 1 and 2, one obtains a disposition similar to the one in Figure 4-12b. where the heat exchanger is in the counter-flow configuration. The alternating layers of cold and warm fluids separated by a thin and thermally conductive wall allows for a very good heat transfer between the fluids. By inverting the input and output hoses of one of the two fluids, a parallel-flow configuration (Figure 4-12a) is obtained. This configuration is not as efficient at transferring heat and is consequently not as often used. (a) Parallel flow Figure A brazed plate heat exchanger. (b) Counter-flow Festo Didactic

4 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Heat exchange balance Counter-flow configuration Heat exchange balance Parallel-flow configuration PROCEDURE Set up and connections 1. Verify that the emergency push-button is wired so as to be able to cut the power in case of emergency. 2. Make sure the 3531 system is properly set up to use the Heating/Cooling unit. The system should also be in its basic setup configuration (Refer to the Familiarization manual for details). Power up the electrical unit and start the drives 3 and 4 (pumps P3 and P4). These pumps make the water of the two tanks flow in the Heating/Cooling unit. Ensure the process fluid from each tank is circulating correctly, then power up the heating/cooling unit. Make sure valve HV7 is closed. Continue with the next steps while the water in each tank is respectively heating and cooling towards their temperature set points. 3. Connect the equipment as the piping and instrumentation diagram of Figure 4-13 shows and use Figure 4-14 to position the equipment correctly on the frame of the training system. Table 4-4 lists the equipment that must be added to the basic setup to perform this exercise. Note that drives and pumps 3 and 4 must be connected to the setup as explained in the familiarization manual even though they are not shown explicitly in Figure Festo Didactic

5 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure Table 4-4. Devices required for this exercise. Name Model Identification Brazed plate heat exchanger RTD probe TE 11 Thermocouple probe (type J) TE 12 Temperature transmitter TIT 1 Strainers x2 (at the inputs of the heat exchanger) Tank A hot water Tank B cold water Figure P&ID Brazed plate heat exchanger experimentation. Festo Didactic

6 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure a Figure Setup Brazed plate heat exchanger experimentation. The three-way control valve is not required in this experiment. You can however leave the valve on the workstation as long as it is not in your way and if it is properly secured to the station. 4. Use the temperature transmitter to display the temperature measured by the temperature probes. As only one measurement can be displayed, you will need to connect a single probe at a time to the transmitter to perform the measurements at each output of the heat exchanger. The RTD connects to port A while the thermocouple connects to port B. The measured temperatures should both be close to the room temperature at this time. 5. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements to the proper completion of this exercise. This checklist is not 72 Festo Didactic

7 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure exhaustive, so be sure to follow the instructions in the Familiarization with the Instrumentation and Process Control Training System manual as well. f Every piece of equipment used is secured to the station with the appropriate bolt-and-nut mechanism. A strainer is installed at each of the two inputs of the heat exchanger. The ball valves are in the positions shown in the P&ID and listed below: Open valves: HV1A, HV1B, HV2A, HV2B, HV4B, HV5A, HV8A, and HV8B. Closed valves: HV3A, HV3B, HV4A, HV5B, HV6A, HV6B, and HV7. 6. Test your system for leaks. Use drives 1 and 2 to make pumps P1 and P2 run at low speed to produce a small flow rate. Progressively increase the frequency output of drives 1 and 2 up to 30 Hz. Repair any leak. 7. The temperature in the two tanks should be stable and at their respective set points by now. If it is not the case, identify the problem or wait until the temperature of each tank stabilizes. Heat exchange balance Counter-flow configuration The temperatures must be measured at four points in the system. The output temperature of fluid 1 (the hot fluid) is measured by the RTD (TE 11) while the output temperature of fluid 2 (the cold fluid) is measured by the thermocouple (TE 12). The input temperatures are assumed to be the ones of the fluids in their respective tanks. Consequently, the temperatures measured by the thermostats of the Heating/Cooling unit for each tank correspond to the input temperatures of each of the two fluids. Note that you can instead use extra temperature probes at the inputs of the heat exchanger if you have them. 8. Adjust the speed of drive 1 to 25 Hz and the speed of drive 2 to 20 Hz. Let the temperatures stabilize and write down the measurements in Table 4-5. Increase the speed of drive 2 by increments of 5 Hz. Calculate the change in temperature for each fluid ( ). Festo Didactic

8 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure Table 4-5. Measurements for the heat exchanger in counterflow - Drive 1 at 25 Hz. Speed of drive 2 (Hz) What are your observations? How does the output temperature of fluid 1 compare to the output temperature of fluid 2? 9. Adjust and keep the speed of drive 2 at 30 Hz. Set the speed of drive 1 at 15 Hz and perform the required measurements to fill table. Increase the speed of drive 1 by increments of 5 Hz. Table 4-6. Measurements for the heat exchanger in counterflow - Drive 2 at 30 Hz. Speed of drive 1 (Hz) What are your observations? How do the output temperatures react to an increase in the flow of fluid 1 (hot water)? Heat exchange balance Parallel-flow configuration A heat exchanger is typically used in the counterflow configuration to optimize the overall heat transfer. The upcoming manipulations will allow you to test the performance of a heat exchanger configured to operate in parallel flow. 74 Festo Didactic

9 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Procedure 10. Stop drives 1 and 2. Disconnect the cold water circuit at port 1 of the heat exchanger. Let the water drain and then disconnect the circuit at port 2 of the heat exchanger. Connect the segment which was on port 2 to port 1 of the heat exchanger. Connect the other segment to port 2. The setup now looks as shown in Figure The heat exchanger is now in a parallel-flow configuration Tank A hot water Tank B cold water Figure P&ID Brazed plate heat exchanger experimentation Parallel-flow setup. 11. Slowly increase the speed of drives 1 and 2 in order to test the system for leaks. Repair any leak. Adjust the speed of drive 1 to Festo Didactic

10 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Conclusion 25 Hz and the speed of drive 2 to 20 Hz. Let the temperatures stabilize and write down the measurements in Table 4-7. Increase the speed of drive 2 by increments of 5 Hz. Calculate the change in temperature for each fluid ( ). Table 4-7. Measurements for the heat exchanger in parallel flow - Drive 1 at 25 Hz. Speed of drive 2 (Hz) What are your observations? How does the output temperature of fluid 1 compare to the output temperature of fluid 2? How do the results compare to the counterflow configuration (refer to Table 4-5)? 12. Stop drives 1 and 2. Let the water drain out of the pipes, then close valves HV2A, HV2B, HV4B, and HV5A. 13. Turn off the heating/cooling unit and stop drives 3 and 4. Turn off the pneumatic unit and the electrical unit. 14. Store the equipment adequately, clean up your workspace, and leave the station ready for the next team. CONCLUSION This experiment explored the heat exchange taking place in a brazed plate heat exchanger in which two fluids at different temperatures were flowing. Changes in the flow rate of one of the fluids were made to observe the impact on the output temperatures. The counterflow and the parallel-flow modes of operation were tested and compared. A basic understanding of heat exchanges and its associated instrumentation was gained in the course of the previous experiments. This knowledge forms the basis on which skill in the control of temperature processes will be added in the next manual of the series (Process Control, manual 86010). 76 Festo Didactic

11 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Review Questions REVIEW QUESTIONS 1. In a parallel-flow heat exchanger, can the output temperature of the cold fluid (Fluid 2) be higher than the output temperature of the warm fluid (Fluid 1)? Why? 2. Which of the parallel flow or counter-flow mode is more efficient? Why? 3. On the graph below, illustrate qualitatively the variation of the temperature gradient for the parallel flow and the counter-flow modes. Gradient of Temperature Position Variation of the temperature gradient for the parallel flow and counter-flow modes. 4. Describe a brazed plate heat exchanger. Festo Didactic

12 Ex. 4-1 Basic Thermal Energy Transfer with a Heat Exchanger Review Questions 5. Explain why the temperature gradient is important for heat exchangers? 78 Festo Didactic