Screw compressor

Abstract

A screw compressor includes a screw rotor, a rotor casing accommodating the screw rotor, and an internal-volume-ratio-adjusting valve capable of changing internal volume ratio. The internal-volume-ratio-adjusting valve includes a valve body movable along an axial direction of the screw rotor and defining a gas-discharge position, and a cylinder containing at least one pressurizing chamber with a cylinder rod and a piston. The cylinder rod is coupled to the valve body and disposed along the axial direction. The piston is fixed inside the pressurizing chamber to the cylinder rod and divides the pressurizing chamber into two pressurizing spaces along the axial direction. The screw compressor further includes a first communication passage connecting one of the two pressurizing spaces with the discharge gas passage, and a second communication passage connecting the other with a just-before-discharge tooth space immediately before a tooth space that communicates with the discharge port.

1 . A screw compressor comprising:
 a screw rotor including a pair of a male rotor and a female rotor;   a rotor casing accommodating the screw rotor, the rotor casing having a suction gas passage and a suction port through which a gas is sucked and a discharge port and a discharge gas passage through which the gas is discharged; and   an internal-volume-ratio adjusting valve capable of changing an internal volume ratio,   wherein the internal-volume-ratio adjusting valve includes:
 a valve body disposed inside the rotor casing, the valve body being movable along an axial direction of the screw rotor and defining a discharge position at which the gas is discharged from tooth spaces formed between the screw rotor and the rotor casing to the discharge port; and 
 a cylinder containing at least one pressurizing chamber and having a cylinder rod and a piston, the cylinder rod being coupled to the valve body and being disposed along the axial direction of the screw rotor, the piston being fixed inside the or each pressurizing chamber to the cylinder rod and dividing the or each pressurizing chamber into two pressurizing spaces along the axial direction of the screw rotor, 
   wherein the screw compressor further comprises:
 a first communication passage connecting a first one of the two pressurizing spaces with the discharge gas passage; and 
 a second communication passage connecting a second one of the two pressurizing spaces with a just-before-discharge tooth space immediately before a tooth space that communicates with the discharge port. 
   
     
     
         2 . The screw compressor according to  claim 1 ,
 wherein the first one of the two pressurizing spaces is located on a suction port side with respect to the piston, and   wherein the second one of the two pressurizing spaces is located on a discharge port side with respect to the piston.   
     
     
         3 . The screw compressor according to  claim 1 ,
 wherein the valve body has a first end surface facing a suction gas space communicating with the suction gas passage and a second end surface facing the discharge gas passage,   wherein the cylinder contains a first pressurizing chamber and a second pressurizing chamber which are arranged along the axial direction of the screw rotor,   wherein, in the first pressurizing chamber, the first one of the two pressurizing spaces separated by the piston communicates with the first communication passage, and the second one of the two pressurizing spaces communicates with the second communication passage,   wherein the screw compressor further comprises:
 a third communication passage connecting the suction gas passage with a forward pressurizing space of the two pressurizing spaces separated by the piston in the second pressurizing chamber, wherein, in the forward pressurizing space, the piston is pressurized in a forward direction from the discharge port toward the suction port; and 
 a fourth communication passage connecting the discharge gas passage with a backward pressurizing space of the two pressurizing spaces formed in the second pressurizing chamber, wherein, in the backward pressurizing space, the piston is pressurized in a backward direction from the suction port toward the discharge port, 
   wherein an area of a pressure-receiving surface of the first end surface pressurized by the gas in the suction gas space is equal to an area of a pressure-receiving surface of the piston in the forward pressurizing space, and an area of a pressure-receiving surface of the second end surface pressurized by the gas in the discharge gas passage is equal to an area of a pressure-receiving surface of the piston in the backward pressurizing space.   
     
     
         4 . The screw compressor according to  claim 1 ,
 wherein the second communication passage includes:
 a hollow portion formed in the valve body and in the cylinder rod; 
 a first communication hole formed in the valve body and connecting the just-before-discharge tooth space and the hollow space; and 
 a second communication hole formed in the cylinder rod and connecting the hollow portion and the second one of the two pressurizing spaces. 
   
     
     
         5 . The screw compressor according to  claim 1 ,
 wherein the second communication passage includes a channel formed in the rotor casing facing a discharge-side end of the screw rotor, and the channel has an opening open to the just-before-discharge tooth space from a wall surface of the rotor casing.   
     
     
         6 . The screw compressor according to  claim 1 ,
 wherein the cylinder contains a first pressurizing chamber and a second pressurizing chamber which are arranged along the axial direction of the screw rotor,   wherein the first pressurizing chamber is located on a discharge port side of the screw rotor with respect to the second pressurizing chamber, and   wherein, among the two pressurizing spaces formed in the first pressurizing chamber, the second one, communicating with the second communication passage, of the two pressurizing spaces is located on the discharge port side with respect to the first one of the two pressurizing spaces.