Experiment 7: Temperature Control of Heat Exchanger ( TBC 4/4/2009 )

Transcription

1 Experiment 7: Temperature Control of Heat Exchanger ( TBC 4/4/2009 ) Objective: To implement a feedback temperature control of the hot water exiting a doublepipe heat exchanger. I. Introduction: Feedback Control Configuration The control configuration is similar to the configuration given in experiment 6. The signal is a voltage ranging from 0 to 5 volts. The actuator block includes both the voltage to pressure (E/P) transducer, compressed air supply and the steam control valve. The signal represents the amount of steam entering the heat exchanger. The signal is the temperature measured using an RTD sensor, while is the setpoint temperature. Figure 1. Feedback loop. For this experiment, we will implement a simple proportional-integral control law: PI Control: 1 (1) Because of the possibility of saturation in the control valves, integral windup need to be addressed if PI control were to be used, i.e. the integral term of the PI controller continues to unnecessarily grow during saturation. Strategies that try to solve the integral windup problem are known collectively as anti-reset windup approaches. Thus, instead of (1), we could implement an additional logic that shuts off the integral windup during saturation conditions such as: (2) (3) 0 if 0 or 1 otherwise (4) 1

2 II. Procedure/ Tasks: A. Build the data-aquistion/control setup (Figure 1) Figure 1. Control Setup. B. Create the RTD sub-vi ( refer to lab 6, modify to accept 5V from USB ) C. Create the Saturation Check sub-vi ( Figure 2. ). The Greater than or Equal block and the Or block can be found in the Functions pallete, [Programming] [Comparison] menu. Figure 2. Saturation Check sub-vi. D. Create the PI Controller sub-vi (See Figure 3). The Subtraction and Division block can be found in [Programming] [Numeric] menu. The Select block can be found in [Programming] [Comparison] menu. The Integral block can be found in [Mathematics] [Integ & Diff] menu, then select Time Domain Math. A properties window should pop-up, then select [Integral] and [Continuous] buttons then click [OK]. 2

3 Figure 3. PI Control Block. E. Create the Actuator sub-vi by renaming the Dimmer subvi of lab 6 and setting the minimum voltage to 0 volts and maximum to 5 volts. F. Combine the parts and create the Control System VI (see Figure 4). Note that the Feedback Node block will appear automatically after connect the Saturation Check subvi to the PI Controller subvi. Also, for the Write block, choose the [no header] and [one column only] buttons. Figure 4. Control System VI. 3

4 Figure 5. The Front Panel for Control System VI. G. Startup the heat exchanger system (See Appendix A) and select 20% mark on the rotameter. H. Connect the transducer to the controller unit and begin tests below. (For the items below, use the setpoint of 40.) I. Perform a step test and obtain an FOPTD model. (See Appendix C) J. Use the Cohen-Coon tuning rules and apply them to the system. Note your observations. K. Shut down the heat exchanger system (See Appendix B). 4

5 III. Observation: Cohen-Coon Tuning Method: 1. FOPTD model found: 2. Cohen Coon Tuning: Calculations: Kc : : 3. Response observation: Temperature Response: Control Signal Response: 5

6 Appendix A. Heat Exchanger Startup ( based on procedure for heat transfer coefficient experiment found in CM3215 developed by F. Morrison, revised for CM3310 by T. Co) Experimental Procedure CAUTION: Always wear insulated gloves when adjusting the steam valve, when touching uninsulated piping or when handling condensate. The equipment may be hot at the beginning of lab. If the TA has not done it yet, connect the transducer to the compressed air supply and to the steam valve. (see Figure A1.) + E/P Transducer - Figure A1. Connecting the transducer to the air supply and steam valve. Begin water flow in the loop as follows: 1. Start water flow through the ½ pipe as follows. Make sure that tank T-02 is empty and clean. Close the drain valve (DV-2). 2. Open water valve WV-10 and fill T-02 with water. Once the tank is filled, the water control float valve will shut-off water flow. The water control float valve keeps water level constant at a set point. 3. Make sure valves WV-1, WV-2, and WV-3 as well as the needle valve WV-5 are all closed. Turn on pump P Direct the water through the ½ pipe by opening valve WV-1. Turn three-way valve WV-4 knob to direct the water flow through rotameter FI-01. Make sure that valves WV- 6 and WV-7 are closed. Adjust three-way valve WV-8 to direct water to the heat exchanger E-01. Turn three-way valve WV-9 knob to direct water to T Fix the water flow rate to 20% on FI-01 by using needle valve WV-5. Begin flow of steam through the outer jacket of the heat exchanger as follows: 6. Position three-way valve SV-3 to direct flow through the Swagelok steam trap. 7. Open the main steam valve SV-1. Drain condensate from the line as follows: 6

7 8. Check with the TA to see if this step has been done; if not, using insulated gloves, position the black rubber drain hose located downstream of SV-2 in a Styrofoam bucket. One team member should stabilize the bucket and hose. 9. Using insulated gloves, open SV-2 slowly to drain all of the line condensate into the bucket. Close SV-2 when steam begins to come out of the hose. Dispose of the condensate in a floor drain do not use the sink drain. Clean up any water spills with paper towels. 10. Return the rubber hose to common drain. Start steam flow 11. Open DV-1. Turn three-way valve WV-9 knob to direct water to T-01. Verify that the pump is on and that water is circulating through the inside of the heat exchanger. Do not recirculate water back to tank T Open main air valve AV-1 and set the control valve FV-06 to 18 psig. Appendix B. Heat Exchanger ShutdownProcedure ( based on procedure for heat transfer coefficient experiment found in CM3215 developed by F. Morrison, revised for CM3310 by T. Co) 1. Set the air pressure regulator to 0 psig using the pressure regulator. 2. Close the main air valve AV Close main steam valve SV Return all drain hoses to the common drain. 5. Allow the cold water to circulate through the loop to cool the heat exchanger for at least 3 minutes. Stop when the piping at the cold-water exit of the heat exchanger is cool to the touch. 6. Close valves WV-1 and needle valve WV Turn off pump P Close main water valve WV Position three-way valve WV-9 to direct flow to Tank T Drain all tanks. 11. Dry off any wet surfaces with paper towels. Turn off all the electronic devices and properly store them. 12. (If you are in the last session of the day, detach the transducer from the compressed air line and the steam valve then reconnect air line directly to steam valve). 7

8 Appendix C. Obtaining an Empirical FOPTD Model 1. If it is not yet implemented in the Control System VI, include a [Write Meas File] block (which can found in [Express] [Output] menu. 2. Make sure that the temperature is initially at steady state. 3. Introduce a step change in controller output, then collect the data up to the point where the temperature settles to a new steady state. ( One simple way to manually introduce step change is to set 0 and then use to input a value between 0 and 1.) The FOPTD model has the solution given by the following: if (5) exp if 4. You can use Excel solver to find the model (see Figure C1) by minimizing RMS through changing of tdelay, tau, T_init and T_final. Note: you can find t0 from the point where had a step change. =SQRT(AVERAGE(F8:F2008)) =IF=(E8-C8)^2 =IF( B9<$B$1+$B$2, $B$4, $B$5 - ($B$5-$B$4)* EXP( -(B9-$B$1-$B$2) /$B$3) ) Figure C1. Spreadsheet for obtaining FOPTD. Note: process gain can be found as (6) 8