Compressor demand control system for long term compressor operation

Claim 1

In a compressor control system including: a. a master-controller operative in one of an inlet valve modulation mode and a bypass valve modulation mode for controlling an inlet valve and a bypass valve of a compressor, respectively; the compressor supplying fluid flow to a load, the master-controller being operative in response to a signal representative of the load pressure and to a setpoint signal; and b. a subcontroller operative during bypass valve modulation and responsive to a present inlet valve position for incrementing the same in relation to a deviation of the present inlet valve position from a predetermined minimum inlet valve position P2 to restore said predetermined minimum inlet valve position as the present inlet valve position; the combination of: first means associated with the master-controller for modulating the inlet valve; first limiter means being associated with said first modulating means for limiting the operation downward thereof to a position P2' larger than said P2 position by a predetermined amount; second means associated with the master-controller for modulating the inlet valve; second limiter means being associated with said second modulating means for limiting the operation downward thereof to said P2 position; and means for selecting one of said first and second modulating means upon the P2 position being reached as the present position in the downward direction for said second modulating means and upon the P2' position being reached as the present position in the upward direction for said first modulating means.