The Chemical Engineering Department at CSULB is interested in improving the performance of a bench-scale double-pipe heat exchanger located in the unit operations laboratory by applying advanced control strategies, such as cascade control and feedforward control. Originally, the double-pipe heat exchanger operates as a single-loop feedback control, in which the cold water outlet temperature is controlled by manipulating the control valve opening in the incoming cold water line. A closed-loop single input single output (SISO) feedback control of the original design was tested to examine its capability in setpoint tracking and disturbance rejection. The conditions of the fluids (saturated steam and cold water) flowing into the heat exchanger were monitored to identify the possible disturbances acting on the system and estimate their impacts on the performance of the heat exchanger. We found that the possible disturbances include, but are not limited to, variations in the source pressure of cold fluid, inlet temperature of the cold fluid, and the source pressure of the incoming steam. To further improve the system, a cascade control structure was designed in this work by inserting an inner loop of flow rate control. In this manner, the outer loop (control of exiting-temperature of cold water) would decide the flow rate setpoint of the inner loop. Furthermore, a feedforward controller is added to the cascade feedback loop to reject possible disturbances. We have found that the double-pipe heat exchanger can achieve the highest level of setpoint tracking and disturbance rejection when the combined system is used. In order to obtain controller parameters for each strategy, open-loop step input tests were performed and the process reaction curve was fit to a desired process model. The controller parameters were determined according to internal model control (IMC) tuning method and other standard tuning methods such as Ziegler-Nichols and Cohen-Coon tuning methods. We also have calculated the overall heat transfer coefficient to be 2.917 kW/m2.°C in region I (tube 1), and 2.652 kW/m2.°C in region II (tube 2), and the total heat transfer rate by the double-pipe heat exchanger to be 18.34 kW.
|Commitee:||Mendez, Sergio, Sciortino, Antonella, Smith, Gregory|
|School:||California State University, Long Beach|
|School Location:||United States -- California|
|Source:||MAI 54/01M(E), Masters Abstracts International|
|Keywords:||Cascade control, Feedforward control, Heat exchanger, Process control|
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