Unloading control system for helical screw compressor refrigeration system

Claim 1

In a closed loop refrigeration system including a compressor, a condenser, and evaporator, conduit means connecting said compressor, condenser and evaporator in series, in that order within a closed loop, said conduit means including an expansion means upstream of said evaporator, and wherein said compressor comprises; a helical screw rotary compressor having a low pressure suction port and a high pressure discharge port, an unloader slide valve movable between compressor full load and full unload position, a linear drive motor including a slidable piston sealably and slidably mounted within a cylinder and forming inboard and outboard chambers on opposite sides of said piston, said inboard chamber being open to the compressor discharge pressure and tending to shift the slide valve to full compressor unload position and wherein said conduit means further includes means for selectively connecting said outboard chamber to the compressor discharge pressure tending to shift said slide valve to full compressor load position, the improvement comprising: a check valve within said conduit means between said evaporator and said compressor suction port, means normally closing off said outboard chamber to said system high side pressure and means for normally opening said outboard chamber to said system low side pressure upstream of said check valve means; whereby, upon compressor shut down, the outboard chamber is vented to the system low side pressure, while the inboard chamber is open to the system high side pressure, such that during the time delay to achieve equalization of system high and low side pressures, said unloader piston and said slide valve are automatically shifted to compressor full unload position.