Capacity control systems for screw compressor based water chillers

Claim 1

A control system for a helical rotary screw compressor employed in a refrigeration apparatus having a condenser, an expansion device and an evaporator connected in a closed refrigeration circuit with the screw compressor, the refrigeration apparatus also including capacity varying means for varying the capacity of the screw compressor, comprising in combination: said screw compressor compressing gas from a low pressure at suction to a high pressure at discharge and including a housing with a barrel portion having a pair of parallel intersecting bores, an inlet port and a discharge port, two intermeshed helical rotors disposed in said bores and forming variable volume compression chambers, an axially extending recess disposed within said barrel portion in open communication with said bores, and a slide valve member being axially slidable in said recess between a first position in which the discharge port is fully opened and a second position in which the discharge port is closed; first circuit means connected to provide a control signal to the capacity varying means for regulating the capacity of said screw compressor; a slide positioning motor mechanically coupled to drive said slide valve member; second circuit means connected to provide a signal representing the optimal volume of gas trapped in the compression chamber prior to discharge as a function of the pressure ratio across said screw compressor; and third circuit means, responsive to said second circuit means, connected to provide an error signal to said positioning motor for regulating the position of said slide valve member so as to adjust the actual volume of gas trapped in the compression chamber prior to discharge.