Abstract: A positive displacement rotary compressor is in circuit with a closable chamber disposed upstream of the compressor gas inlet and including a conduit operable to be placed in communication with the compressor discharge conduit. A control circuit is operable to sense compressor discharge pressure and at a predetermined pressure condition sequentially operate valves to shut off compressor inlet flow thereby evacuating the chamber and then placing the compressor discharge port in communication with the chamber. The compressor thereby runs unloaded or at idle at a greatly reduced inlet and discharge pressure. The operation of unloading the compressor may be controlled by sensing the chamber vacuum or by the use of time delay devices.