In air conditioning systems, the cooling load rarely remains constant over long periods, because the internal and external heat and humidity loads are continually changing. Therefore, the demand for cooling capacity varies in terms of time and amount within a broad range.
The refrigeration machine itself is also subject to capacity variations due to changing condensation and vaporization pressures and temperatures. This means that the available refrigeration capacity also varies. The control task is to adjust the machine’s refrigeration capacity as closely as possible to the respective cooling demand. Control normally presents no special difficulties in systems with water chillers: On the consumer side, modulating mixing valves provide exact control of the air coolers. This means that humidity and temperature are continuously controllable. On the machine side, chilled water primary control is, for example, achieved via periodic switching of compressors.