Abstract

Good HVAC control schemes in buildings help reduce energy use and maintain occupant comfort. PID controllers are widely used in commercial buildings to keep variables such as temperature, pressure and humidity at predefined setpoints. These controllers, when they do not include autotuners, must be carefully tuned. Also, one must consider the effect of interactions occurring among the control loops during the fine-tuning stage so one loop response is not improved to the detriment of another. This research emphasizes PI-control of dual duct systems. Three tuning methods (Ziegler and Nichols' sensitivity method, simplified IMC-PID method, and Bekker et al. method) were selected and tested on a small scale dual duct system with four decentralized control loops (cooling valve, fan speed, damper and heater). Each controller was tuned according to the best performing set of tuning parameters and the system was then operated under design conditions to observe the interactions among the control loops. Since interaction was minimum, a better understanding of interaction was obtained by introducing a sustained oscillation in each loop, one loop at a time. An oversensitive valve was found to have a severe effect on damper control loop and an oversensitive damper was found to interact with fan speed control. Considering these interaction effects, two sets of optimizing curves were developed for the dual duct system. The use of these curves improved some loop responses without rendering other loops unstable