Oil-injection screw compressor

Abstract

An oil-injection screw compressor in which oil is mixed into compressed gas, in order to prevent back flow during the no-load running and stoppage of the compressor body while contributing to a cost reduction, comprising: a compressor body; a motor for driving the compressor body; an inverter for variably controlling the number of revolutions of the motor; a blow-off valve disposed on the discharge side of the compressor body and capable of releasing a quantity of air beyond the discharge capacity of the compressor body when operating of the motor at the minimum number of revolutions; a suction non-return valve which, disposed on the suction side of the compressor body, is closed when it is at a stop; and a pressure sensor for detecting the discharge side pressure of the compressor body; and, a controller which, at the time of operation of the motor at the minimum number of revolutions, opens the blow-off valve when the discharge side pressure P detected by the pressure sensor has risen to a prescribed upper limit Pu and closes the blow-off valve when it has fallen to a prescribed control pressure Po.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWINGS

FIG. 1 is a schematic diagram showing an overall configuration of a screw compressor according to one embodiment of the invention.

FIG. 2 is a section view showing a detailed structure of a suction non-return valve which constitutes the embodiment according to the invention.

FIG. 3 is a time chart for describing an operation of the screw compressor according to the embodiment of the invention.

Claims

1. An oil-injection screw compressor in which oil is mixed into compressed gas, the oil-injection screw compressor comprising: a compressor body;a motor for driving said compressor body;an inverter for variably controlling the 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 opened when said compressor body is operating, and is closed when it is at a stop;a pressure detecting device for detecting a discharge side pressure of said compressor body; anda blow-off valve control device which, at the time of operation of said motor at the minimum number of revolutions, opens said blow-off valve when the discharge side pressure of said compressor body detected by said pressure detecting device has risen to a prescribed upper limit, and closes said blow-off valve when it has fallen to a prescribed control pressure.

2. The oil-injection screw compressor as claimed in claim 1, further comprising: a first stop control device which computes a pressure rise time taken by the discharge side pressure detected by said pressure detecting device to rise from said prescribed control pressure to said prescribed upper limit and a pressure fall time taken from said prescribed upper limit to said prescribed control pressure during the operation of said motor at the minimum number of revolutions, and stops said compressor body when the ratio between these pressure rise time and pressure fall time has reached a prescribed set ratio.

3. The oil-injection screw compressor as claimed in claim 1, further comprising: a second stop control device which computes a pressure rise time taken by the discharge side pressure detected by said pressure detecting device to rise from said prescribed control pressure to said prescribed upper limit and a pressure fall time taken from said prescribed upper limit to said prescribed control pressure during the operation of said motor at the minimum number of revolutions, and stops said compressor body when this pressure rise time or pressure fall time has reached a prescribed set time length.

4. The oil-injection screw compressor as claimed in claim 2, further comprising: a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure.

5. The oil-injection screw compressor as claimed in claim 3, further comprising: a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure.

6. The oil-injection screw compressor as claimed in claim 1, wherein said blow-off valve is an electromagnetic valve.