Digital Control of a Refrigerant Evaporator

Abstract Conventional control systems of refrigeration plants realized by thermostatic expansion valves often operate unstable, because the valve parameters can not be adapted sufficiently to the process. Therefore a large value of the setpoint of the super heating temperature must be selected in order to avoid liquid strokes to the compressor and this in turn leads to a lower evaporator efficieney. By better control performance of the superheating respectively the location of the liquid-dry-out-point, the evaporator efficiency can be improved. It is shown in this contribution that the control performance becomes much better with an electrical driven expansion valve and an electronic controller. The controller for the research pilot plant was realized by a micro computer. A special type of control structure is realized, which takes into account the type of disturbances found in the open loop. A comparison of the two control methods demonstrates the improvement of the digitally controlled needle valve.