Liquid injecting type oil-free screw compressor

An oil-free screw compressor comprising:

• a liquid injecting passage for injecting a liquid into an inlet port of a compressor main body;
• a flow rate control valve in said liquid injecting passage for controlling the flow rate of the liquid;
• a temperature controller for detecting the gas temperature in a discharge passage of the compressor main body, and controlling the degree of opening of the flow-rate control valve as a function of the detected temperature in such a manner as to control the injected liquid quantity to said inlet port for holding said gas temperature at a saturation temperature;
• a flow-rate controller for controlling the degree of opening of said flow-rate control valve to gradually change the flow rate of the liquid quantity from said injected liquid quantity to a predetermined value;
• and
• a selector switch for switching over the control function for controlling the degree of opening of said flow-rate control valve to said temperature controller in a transient state of said compressor main body and to said flow-rate controller in a normal operating state of said compressor main body.