A linearised analysis of temperature and relative humidity control using proportional-integral-differential (PID) controls to vary compressor motor speed and evaporator fan speed enables analytical determination of suitable PID coefficients. The PID coefficients are related to refrigeration system performance parameters and the volume of the conditioned space. Analytical determination of coefficients may eliminate or supplement trial-and-error methods. System response characteristics - damping ratios and periods - are related to PID coefficients, sampling intervals, performance parameters of a refrigeration system, and the volume of a conditioned space. Two control strategies are possible in order to control space temperature and relative humidity simultaneously. One strategy controls temperature by varying compressor speed and controls relative humidity by varying evaporator fan speed. The other strategy controls temperature by varying evaporator fan speed and controls relative humidity by varying compressor fan speed. The first strategy was successfully implemented experimentally as well as by simulation. The second strategy was not successfully implemented due to adverse interaction between the two control systems.

KEYWORDS: year 1995, calculating, temperature, humidity, controls, cooling, dehumidifying, compressors, evaporators, fans, speed, variable speed motors, algorithms, relative humidity, refrigeration, performance, experiment, comparing