Control of an oil-heated, fractal-like branching microchannel desorber

A PID controller was designed to improve the performance of an oil-heated, fractal-like branching microchannel desorber for use in an ammonia-absorption refrigeration system. System identification techniques were employed to develop single-input, single-output reduced order models relating the oil mass flow rate and the rectified circulation ratio ( f * ) for four different operating conditions. PID controllers were designed for each of these reduced order system models and the closed-loop response of the desorber was simulated for various operating conditions and controller combinations. The preliminary controller designs were refined by examining controlled desorber performance in an experimental flow loop. The resulting PID controller yielded a closed-loop response with a faster rise time and lower overshoot than the simulated controllers. The tuned controller regulated the desired rectified circulation ratio under the operating conditions studied, controlling both the amount and concentration of the ammonia refrigerant generated. Prescribed variations in f * were tracked by the controlled system with a settling time of approximately 10 min. Controlled performance was robust to external disturbances in the strong solution flow rate and the oil temperature, regulating f * about the desired value with a settling time similar to that observed during the tracking tests.