Graduate Thesis Or Dissertation
 

Proportional integral derivative control of an oil-heated fractal-like branching microchannel desorber

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  • A PID controller was designed to control the performance of an oil-heated fractal-like branching microchannel desorber for use in an ammonia-absorption refrigeration system. Both the amount and concentration of ammonia refrigerant generated at the desorber can be controlled by choosing the rectified circulation ratio as the controlled variable in a single-input single-output (SISO) system. The oil pump control voltage was chosen to be the controller input. System identification tests were performed to create a simple model of the dynamical relationship between rectified circulation ratio (f*) and oil pump control voltage for four different operating conditions. The dynamical relationship between f* and oil pump control voltage can be represented as a time delay two pole system with 84% accuracy. The models were used to simulate desorber performance and design PID controllers. The closed loop response of the desorber under various operating conditions and controller combinations was simulated, and two promising controllers were tested on the desorber using an experimental flow loop. Additional tuning of the PID controller on the experimental loop resulted in a closed-loop response that had faster rise time and lower over-shoot than the simulated controllers. The gains of this controller are: kp= 0.3, ki= 0.003, and kd = 2.0. The tuned controller controlled the desorber and regulated to the desired rectified circulation ratio under the four operating conditions studied. The closed-loop response of the desorber varied depending on the operating conditions and the f* value of the desorber. The controller was able to track desired f* signals with a settling time of approximately 10 minutes. Combinations of strong solution flow rate, oil temperature, and manifold pressure disturbances were introduced into the system to test the robustness of the controller. Strong solution flow rate disturbances produced the largest change in f*. All three types of disturbances were controlled with a settling time less than or approximately equal to the settling time observed during the tracking tests.
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