Patent US 6,138,464

An improved defrost controller is provided for a space cooling system of the type having an evaporator in heat exchange relationship with a space to be cooled, a condenser external to the space, a compressor for circulating heat transfer fluid between the evaporator and condenser, an expansion valve located between an outlet from the condenser and an inlet to the evaporator, and a defroster operatively associated with the evaporator. The expansion valve is positionable in a plurality of positions between a fully open position and a fully closed position to maintain a desired level of superheat across the evaporator. The defrost controller monitors the expansion valve position and activates the defroster in response to the expansion valve being in a more closed position than a predetermined defrost target position. The defrost controller is further operable to prevent defrosting from occurring at times other than predetermined allowed defrost times, even if the position of the expansion valve indicates a demand for defrost. The defrost target position is adjusted in response to changes in selected system operating parameters. Further, at each allowed defrost time, the controller determines whether defrosting can be deferred until a next allowed defrost time, based on the then current rate of degradation in evaporator performance. If the then current rate of degradation indicates that defrosting cannot be deferred to the next allowed defrost time, the controller activates the defroster, even if a demand for defrost is not indicated.