Screw compressor

1. A screw compressor comprising:

a male rotor;

a female rotor that meshes with the male rotor;

a casing that includes a bore for housing the male rotor and the female rotor and in which a suction chamber is disposed on a suction side and a discharge chamber is disposed on a discharge side;

a slide valve forming a part of the bore and that is movable in an axial direction of the male rotor and the female rotor, the slide valve having a main body and a discharge side end face;

a plurality of foot sections protruding in the axial direction from the discharge side end face of the slide valve that supports the slide valve in the casing; and

a discharge port disposed on the discharge side of the slide valve to discharge, to the discharge chamber, compressed gas taken into a compression operation chamber formed by the male rotor, the female rotor, and the casing from the suction chamber and compressed,

wherein the slide valve comprises:

at the discharge side thereof, a first discharge channel to lead the compressed gas discharged from the discharge port and lead the compressed gas to the discharge chamber, and a second discharge channel open to the first discharge channel and the discharge chamber that includes a portion of the discharge side end face that is at an outer periphery of the main body, the second discharge channel to lead a part of the compressed gas flowing in the first discharge channel and feed the part of the compressed gas to the discharge chamber, and

wherein the portion of the discharge side end face at the outer periphery of the main body of the second discharge channel and end surfaces of the foot sections are in different planes.

2. The screw compressor according to claim 1 , further comprising:

a stopper structure protruding from the discharge side end face in the axial direction and having a portion thereof at the outer periphery of the main body,

wherein the foot sections are closer to both the male rotor and the female rotor than the stopper structure.

3. The screw compressor according to claim 2 , wherein

the first discharge channel is disposed between the foot sections,

wherein the stopper section is disposed within the second channel, and

wherein the first discharge channel is closer to both the male rotor and the female rotor than the second discharge channel in a radial direction.

4. The screw compressor according to claim 2 , wherein the second discharge channel includes a portion of the discharge side end face that is a flat surface and is perpendicular to a main axis of the main body of the slide valve.

5. The screw compressor according to claim 1 , wherein respective outer surfaces of the foot sections are at the outer periphery and the first discharge channel is disposed between the foot sections, and

wherein the end surfaces of the foot sections are configured to come into contact with a part of the casing to limit axial direction movement of the slide valve.

6. The screw compressor according to claim 5 , wherein the second discharge channel includes a portion of the discharge side end face that is a flat surface and is perpendicular to a main axis of the main body of the slide valve.

7. The screw compressor according to claim 1 , wherein

the slide valve is a volume ratio valve configured to change a volume ratio of the compressor, and the screw compressor includes a valve-body driving device for driving the slide valve, the valve-body driving device including:

a piston connected to the slide valve;

a cylinder for housing the piston to be capable of reciprocatingly moving in the axial direction;

a continuous path for leading oil in a high-pressure space to a cylinder chamber on a counter rotor side of the piston;

a first continuous path for connecting an inside of the cylinder chamber on the counter rotor side of the piston and a low-pressure space of the compressor;

a second continuous path for connecting the inside of the cylinder chamber on the counter rotor side of the piston and the low-pressure space of the compressor and opened to the cylinder chamber between the continuous path for leading the oil in the high-pressure space and the first continuous path; and

a plurality of valves provided in the respective first and second continuous paths and for opening and closing the respective continuous paths, and

when over-compression or insufficient compression occurs in the compression operation chamber, the valve-body driving device opens and closes the valves provided in the respective first and second continuous paths to thereby move the slide valve via the piston to change a volume ratio in the compression operation chamber and reduce a state of the over-compression or the insufficient compression.

8. The screw compressor according to claim 7 , further comprising:

a continuous path for connecting an inside of a cylinder chamber on a rotor side of the piston and the discharge side of the compressor.

9. The screw compressor according to claim 7 , further comprising:

a discharge pressure sensor for detecting a discharge side pressure of the compressor;

a suction pressure sensor for detecting a suction side pressure of the compressor; and

a controller configured to:

calculate a pressure ratio during operation on the basis of detection values in the discharge pressure sensor and the suction pressure sensor,

compare the pressure ratio with a set pressure ratio stored in advance,

determine whether over-compression or insufficient compression occurs in the compression operation chamber, and

control the valves respectively provided in the first and second continuous paths.

10. The screw compressor according to claim 9 , wherein the controller controls the valves provided in the first and second continuous paths to move the slide valve to a low-pressure side when determining that the over-compression occurs and move the slide valve to a high-pressure side when determining that the insufficient compression occurs.