Patent ID: 8241007

Claim:
An oil-injection screw compressor in which oil is mixed into compressed gas, the oil-injection screw compressor comprising: a compressor body for compressing sucked air; a motor for driving said compressor body; an inverter for variably controlling a number of revolutions of said motor; a blow-off valve disposed on a discharge side of said compressor body and capable of releasing a quantity of air beyond the discharge capacity of said compressor body when operating said motor at a minimum number of revolutions; a suction non-return valve which, disposed on a suction side of said compressor body, is configured to be opened when said compressor body is operating under load and under no load, and closed when it is at a stop; and a pressure detecting device for detecting a discharge side pressure of said compressor body, wherein it is controlled that, at the time of operation of said motor at the minimum number of revolutions, said compressor is run under no load to open said blow-off valve in a condition of the operation of said motor at the minimum number of revolutions when the discharge side pressure of said compressor body detected by said pressure detecting device has risen to a prescribed upper limit, and said compressor is run under load by closing said blow-off valve when it has fallen to a prescribed control pressure during the no-load running, and wherein it is controlled that, when said motor is stopped after the operation of said motor at the minimum number of revolutions, said blow-off valve is opened and said suction non-return valve is closed.