The present invention relates to a screw compressor that is preferably applicable to an air conditioner, a chiller unit, a refrigerator, and other machines forming a refrigeration cycle. A screw compressor with the features of the preamble portion of patent claim 1 has been known, e.g., from WO 2010/035592 A1.

When a screw compressor is used for an air conditioner or a chiller unit, wide ranges of suction pressure and discharge pressure are used. Therefore, the pressure within a screw rotor tooth groove may be higher than the discharge pressure depending on operating conditions (this phenomenon is hereinafter referred to as over-compression). Consequently, a screw compressor for reducing the degree of over-compression is proposed (refer, for instance, to JP 1986-79886 A).

The screw compressor described in JP 1986-79886 A includes a male rotor (main rotor), a female rotor (auxiliary rotor), a bore, a main casing (housing), and a discharge casing (housing wall). The male rotor and female rotor rotate while meshing with each other. The rotation axes of these rotors are substantially parallel to each other. The bore houses the teeth of these rotors. The main casing has an end face to which the rotor axis direction discharge side of the bore is open. The discharge casing is connected to the rotor axis direction discharge side of the main casing. The discharge casing includes a discharge side end face, a discharge port (discharge window), a discharge chamber, a valve hole (hole), and a bypass flow path. The discharge side end face abuts on the end face of the main casing to cover the opening of the bore. The discharge port is formed on the discharge side end face. The discharge chamber is configured so that a compression operation chamber formed on tooth grooves in the male and female rotors discharges a compressed gas through the discharge port. The valve hole is disposed near the discharge port on the discharge side end face and open at a position opposite the direction of rotor rotation toward at least either the male rotor and the female rotor. The bypass flow path establishes communication between the valve hole and the discharge chamber. A valve device (overflow valve) is mounted on the discharge casing to open and close the valve hole.

The valve device includes a valve disc and a spring (pressing spring). The valve disc is disposed inside the valve hole. The spring presses the valve disc toward the main casing. When, for instance, the valve hole is closed with the valve disc moved toward main casing, the compression operation chamber discharges the compressed gas to the discharge chamber through the discharge port. When, on the other hand, the valve hole is opened with the valve disc moved away from the main casing, the compressed gas is discharged to the discharge chamber not only through the discharge port but also through the valve hole and bypass flow path. This reduces the degree of over-compression.

As a valve disc stopper, a stepped portion is provided for the valve disc and valve hole. Therefore, when, for instance, the valve disc is moved toward the main casing, the leading end face of the valve disc is flush with the end face of the discharge casing. This prevents the valve disc from coming into contact with the end face of a rotor tooth.

WO 2010/035592 A1 and WO 89/10489 A1 both disclose a screw compressor having a male rotor and a female rotor that have rotation axes substantially parallel to each other and rotate while meshing with each other; a main casing that has a bore for housing the male rotor and the female rotor; a discharge casing that is connected to the rotor axis direction discharge side of the main casing and provided with a discharge side end face which abuts on the end face of the main casing to cover the opening of the bore; a discharge chamber or a discharge flow path that discharges a compressed gas from a compression operation chamber formed by the male rotor and the female rotor through a discharge port formed in at least either the main casing or the discharge casing; a valve hole that is disposed on the female rotor side of the discharge port, formed in the discharge side end face of the discharge casing toward at least either the male rotor or the female rotor, and open to the compression operation chamber; a bypass flow path that establishes communication between the valve hole and the discharge chamber or the discharge flow path; and a valve disc that is disposed in the valve hole; the screw compressor comprising: a valve disc drive device that opens and closes the valve disc; and a control device that detects whether the compression operation chamber is over-compressed, and if the compression operation chamber is over-compressed, controls the valve disc drive device so as to open the valve disc.

However, the conventional technology described above has the following problem.

When the above-described conventional technology is used, the pressure from the compression operation chamber is exerted on the valve disc. Therefore, the compression operation chamber is over-compressed (compression operation chamber pressure > discharge chamber pressure (discharge pressure)). Consequently, when the pressure exerted on the valve disc overcomes the pressing force of the spring, the valve disc opens. However, when the valve disc opens, the compression operation chamber side pressure on the valve disc is immediately equal to the pressure on the discharge chamber side. Meanwhile, the back pressure on the valve disc is constantly equal to the discharge chamber pressure. Therefore, the pressure exerted on the valve disc is immediately brought into equilibrium. Consequently, the valve disc immediately closes due to the action of the spring, which presses the valve disc toward the main casing. As a result, when the compression operation chamber is over-compressed, the valve disc repeatedly opens and closes each time the compression operation chamber passes through the valve disc due to rotor rotation. The valve disc then hits the stopper to generate a hammering sound and vibrates.

The present invention has been made in view of the above circumstances and it is an object of the present invention to provide a screw compressor that is capable of reducing the hammering sound and vibration of the valve disc, which reduces the degree of over-compression.

According to the present invention, this object is accomplished with a screw compressor having the features of claim 1.

Dependent claims are directed on features of preferred embodiments of the invention.

The present invention provides a screw compressor that is capable of reducing the hammering sound and vibration of the valve disc, which reduces the degree of over-compression.

An embodiment of the present invention will be described in detail based on the following figures, in which:

• FIG. 1 is a longitudinal cross-sectional view illustrating a screw compressor according to a first embodiment of the present invention;
• FIG. 2 is a right side view of FIG. 1;
• FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1;
• FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 1;
• FIG. 5 is a diagram illustrating the positional relationship between a compression operation chamber, a discharge port, a valve hole, and a bypass flow path in the first embodiment of the present invention;
• FIG. 6 is a cross-sectional view taken along the line VI-VI of FIG. 2 to illustrate a closed valve disc;
• FIG. 7 is a cross-sectional view taken along the line VI-VI of FIG. 2 to illustrate an open valve device;
• FIG. 8 is a cross-sectional view taken along the line VIII-VIII of FIG. 6;
• FIG. 9 shows a discharge side end face of a discharge casing to illustrate a first modification of the first embodiment;
• FIG. 10 corresponds to FIG. 9 and illustrates a second modification of the first embodiment;
• FIG. 11 corresponds to FIG. 9 and illustrates a third modification of the first embodiment; and
• FIG. 12 is a refrigeration cycle configuration diagram illustrating a chiller unit having the screw compressor according to the first embodiment.

An embodiment of the present invention will now be described with reference to the accompanying drawings.

A screw compressor according to a first embodiment of the present invention will be described below with reference to FIGS. 1 to 8.

FIG. 1 is a longitudinal cross-sectional view illustrating the screw compressor according to the first embodiment of the present invention. FIG. 2 is a right side view of FIG. 1. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1 (FIG. 3 shows a discharge side end face of a discharge casing and the position of a bore in an end face of a main casing is indicated by a two-dot chain line). FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 1 (FIG. 4 shows the end face of the main casing and the position of a valve hole in the discharge side end face of the discharge casing is indicated by a two-dot chain line). FIG. 5 is a diagram illustrating the positional relationship between a compression operation chamber, a discharge port, a valve hole, and a bypass flow path in the first embodiment of the present invention.

Referring to FIG. 1, the screw compressor includes a compressor main body 1, a motor (electric motor) 2 for driving the compressor main body 1, and a motor casing 13 for housing the motor 2. The motor casing 13 forms a suction chamber (low-pressure chamber) 5 on the side away from the compressor main body in such a manner that an inlet 6 allows a gas to flow into the suction chamber 5 through a strainer 7. The motor 2 includes a rotor 11, which is mounted on a rotation shaft 10, and a stator 12, which is disposed on the outer circumferential side of the rotor 11. The stator 12 is secured to the inner surface of the motor casing 13.

The compressor main body 1 is connected to the motor casing 13, and includes a main casing and a discharge casing 16. The main casing 13 incorporates a screw rotor 14. The discharge casing 16 is connected to the discharge side of the main casing 15.

A cylindrical bore 20 is formed in the main casing 15 to house the tooth portion of the screw rotor 14. The rotor axis direction discharge side of the bore 20 is open. A radially-oriented discharge port 23 is formed toward an end face of the main casing 15, which forms the above opening. Further, a discharge flow path 90 is formed and connected to the discharge port 23.

As shown in FIG. 4, the screw rotor 14 includes a male rotor 14A and a female rotor 14B, which have rotation axes parallel to each other and rotate while meshing with each other. The bore 20 includes a bore 20A and a bore 20B. The bore 20A houses the male rotor, whereas the bore 20B houses the female rotor. The discharge port 23 includes a discharge port 23A, which is positioned toward the male rotor, and a discharge port 23B, which is positioned toward the female rotor.

The rotor axis direction suction side (the left-hand side of FIG. 1) of the main casing 15 is connected to the motor casing 13. A space, such as a gap, between the rotor 11 and stator 12 within the motor casing 13 is used as a suction pass that establishes communication between the suction chamber 5 and the compressor main body 1.

As shown in FIG. 4, compression operation chambers 36A, 36B are formed on tooth grooves in the male rotor 14A and female rotor 14B. The compression operation chambers sequentially change their function in accordance with screw rotor rotation. More specifically, the compression operation chambers operate as a compression operation chamber for an intake stroke that communicates with a suction port 22 formed on the suction side (motor casing 13 side) of the main casing 15, as a compression operation chamber for a compression stroke that compresses a gas taken in, or as a compression operation chamber for a discharge stroke that communicates with discharge ports 23, 25 and discharges a compressed gas. The discharge ports 23A, 23B are formed on the radially outer side (upper side of FIG. 1) of the male or female rotor relative to the compression operation chamber for the discharge stroke.

As shown in FIGS. 1 and 3, a discharge port 25 and a discharge chamber 26, which are both axially-oriented, are formed on a discharge side end face 24 of the discharge casing 16. In other words, the discharge casing 16 includes the discharge side end face 24, which abuts on an end face 21 of the main casing 15 and covers the opening of the bores 20A, 20B; a male rotor side discharge port 25A and a female rotor side discharge port 25B, which are formed on the discharge side end face 24; and the discharge chamber 26 into which the compressed gas discharged from the compression operation chambers through the discharge ports 23A, 23B, 25A, 25B flows.

As shown in FIG. 1, the suction side shaft portion of the male rotor 14A is supported by a roller bearing 17, which is provided for the main casing 15, and by a ball bearing 91, which is provided for the motor casing 13. The discharge side shaft portion of the male rotor 14A is supported by a roller bearing 18 and a ball bearing 19, which are provided for the discharge casing 16. The suction side shaft portion of the female rotor 14B is supported by a roller bearing (not shown) provided for the main casing 15. The discharge side shaft portion of the female rotor 14B is supported by a roller bearing (not shown) and a ball bearing (not shown), which are provided for the discharge casing 16. The suction side shaft portion of the male rotor 14A is directly coupled to the rotation shaft 10 of the motor 2 so that the male rotor 14A rotates when the motor 2 is driven. When the male rotor 14A rotates, the female rotor 14B rotates while meshing with the male rotor 14A.

The gas compressed by the screw rotor 14 flows into the discharge chamber 26 or the discharge flow path 90 through the discharge ports 23, 25, is forwarded to an outlet 9 provided for the main casing 15 through the discharge flow path 90, and is delivered to an oil separator 92 through a discharge pipe 94 connected to the outlet 9. The oil separator 92 separates oil from the gas compressed in the compressor main body 1. The oil separated by the oil separator 92 returns to an oil tank 95, which is positioned below the compressor main body 1, through an oil return pipe 93. After being retained in the oil tank 95, the oil is supplied again as a lubricant to the bearings 17, 18, 19, 91, which support the shaft portion of the screw rotor 14 and the rotation shaft 10 of the motor 2. Meanwhile, the compressed gas from which the oil is separated by the oil separator 92 is supplied to the outside (e.g., a condenser that is a part of the refrigeration cycle) through a pipe 96.

The gas taken into the suction chamber 5 through the inlet 6 cools the rotor 11 and the stator 12 when it passes through the inside of the motor casing 13. Subsequently, the gas flows into the compression operation chambers 36A, 36B, which are formed by the screw rotor 14, through the suction port 22 of the compressor main body 1. As the male rotor 14A and the female rotor 14B rotate, the compression operation chambers 36A, 36B decrease their volume during their movement in the direction of the rotor axis to compress the gas. The gas compressed in the compression operation chambers flows into the discharge flow path 90 through the discharge ports 23A, 23B, 25A, 25B and the discharge chamber 26, and then moves into the discharge pipe 94 through the outlet 9.

As shown in FIG. 3, a valve hole (cylinder) 28 is formed near the discharge port 25B on the female rotor 14B side of the discharge side end face 24 of the discharge casing 16, and open at a position opposite the direction of rotation of the female rotor 14B (the right-hand side of FIG. 3). A substantial center of the valve hole 28 is positioned at the opening border of the bore 20B that is positioned toward the female rotor 14B on the end face 21 of the main casing 15. Further, a bypass groove 29 is formed on the discharge casing 16. The bypass groove 29 is positioned between the radially outer circumference of the female rotor 14B and the opening border of the bore 20B positioned on the female rotor 14B side of the end face 21 of the main casing 15 to establish communication between the valve hole 28 and the discharge chamber 26. A bypass flow path is formed by the bypass groove 29 and the end face 21 of the main casing 15 that covers the bypass groove 29. The valve hole 28 is provided with a valve disc 31 that opens and closes the valve hole 28.

A valve disc drive device for driving the valve disc 31 will now be described with reference to FIGS. 6 to 8.

FIGS. 6 and 7 are cross-sectional views taken along the line VI-VI of FIG. 2 to illustrate the structure of the valve disc drive device for driving the valve disc 31. FIG. 6 shows the valve disc 31 in a closed state. FIG. 7 shows the valve disc 31 in an open state. FIG. 8 is a cross-sectional view taken along the line VIII-VIII of FIG. 6.

Referring to FIGS. 6 and 7, the valve disc drive device 30 includes a rod 53, a piston 51, and a cylinder 35. One end of the rod 53 is connected to the rear of the valve disc 31 (the right-hand side of FIG. 6), which is slidably disposed in the valve hole 28. The piston 41 is connected to the other end of the rod 53 through a bolt 52. The cylinder 35 houses the piston 51 in such a manner as to permit the piston 51 to slide. The cylinder 35 is formed on the discharge casing 16. The discharge casing 16 is provided with a rod hole 101 that slidably supports the rod 53. The rod hole 101 is provided with a seal ring 50 that provides sealing between an inner chamber of the cylinder 35 and a back pressure chamber 28a of the valve disc 31. The pressure on the compressor discharge side is introduced into the back pressure chamber 28a through a continuous hole 102 formed in the discharge casing 16. More specifically, one end of the continuous hole 102 is open to the back pressure chamber 28a and the other end of the continuous hole 102 is open to the discharge chamber 26 (see FIG. 3), as shown in FIGS. 6 and 8.

A seal ring 54 is mounted on the outer circumference of the piston 51 to prevent leakage between cylinder chambers 35A, 35B, which are formed on both sides of the piston 51. In the cylinder chamber 35A (in the cylinder 35 positioned on the side away from the valve disc), one end of a continuous hole 32 is open to a region outside the movement range of the piston 51 (open to the right-hand end of the cylinder chamber 35A). The other end of the continuous hole 32 is open to the discharge chamber 26 as shown in FIG. 8. In other words, the cylinder chamber 35A communicates with the discharge chamber 26 (see FIG. 3) through the continuous hole 32 so that the pressure on the compressor discharge side is constantly introduced into the cylinder chamber 35A.

In the cylinder chamber 35B (in the cylinder 35 positioned on the side toward the valve disc), one end of a continuous hole 34 is open to a region outside the movement range of the piston 51 (open to the left-hand end of the cylinder chamber 35B). The other end of the continuous hole 34 communicates with the oil tank 95 through a capillary tube 120, as shown in FIG. 2, to form an oil pressure supply path. Further, the continuous hole 34 also communicates with a low-pressure space (the suction port 22 in FIG. 6) through a continuous path (oil pressure relief path) 80. A solenoid valve 42 is positioned in the middle of the continuous path 80 to open and close the continuous path 80. As the above-described configuration is employed, opening and closing the solenoid valve 42 makes it possible to introduce high-pressure oil in the oil tank 95 into the cylinder chamber 35B and discharge oil in the cylinder chamber 35B toward the suction port 22 through the continuous path 80 and solenoid valve 42. Further, the cylinder chamber 35B is provided with a spring 33 that presses the piston 51 toward an end cover 60 (which is positioned on the side away from the valve disc 31 and on the right-hand side of FIG. 6).

When the compression operation chambers 36A, 36B are not over-compressed, control is exercised so that the valve disc 31 is closed. The solenoid valve 42 opens to close the valve disc 31. The cylinder chamber 35B is then placed under a low pressure as it communicates with the suction port 22 through the continuous hole 34 and the continuous path 80. Meanwhile, the gas pressure on the compressor discharge side is constantly exerted in the cylinder chamber 35A. Therefore, as shown in FIG. 6, the piston 51 overcomes the pressing force of the spring 33 and moves toward the main casing 15. The valve disc 31 is then pressed against the end face 21 of the main casing 15 to close the valve hole 28.

The continuous hole 34 side of the capillary tube 120 also communicates with the suction port 22. However, as the flow of oil is restricted by the capillary tube 120, the amount of oil discharged from the oil tank 95 to the suction port 22 can be adequately decreased. This reduces the amount of oil that may overheat the gas (e.g., refrigerant gas) suctioned into the compressor. Consequently, a decrease in volumetric efficiency is inhibited. Further, the present embodiment is configured so that the oil is discharged to the suction port 22. This makes it possible to minimize the period of time during which the refrigerant gas suctioned into the compressor is overheated by the oil. In this respect, too, it is possible to reduce the degree of refrigerant gas heating by the oil. Thus, the decrease in volumetric efficiency can be inhibited.

When the compression operation chambers 36A, 36B are over-compressed, control is exercised to open the valve disc 31. In this instance, closing the solenoid valve 42 introduces the high-pressure oil in the oil tank 95 into the cylinder chamber 35B. More specifically, when the solenoid valve 42 closes, the high-pressure oil in the oil tank 95 is introduced into the cylinder chamber 35B through the capillary tube 120 so that the pressure in the cylinder chamber 35B is substantially equal to discharge pressure. Therefore, the pressure exerted on the piston 51 remains substantially unchanged no matter whether it is exerted relative to the cylinder chamber 35A or the cylinder chamber 35B. Therefore, the force of pressing the piston 51 to the side away from the valve disc (toward the end cover 60) is greater by the pressing force generated by the spring 33 provided in the cylinder chamber 35B. Consequently, the piston 51 moves toward the end cover 60 as shown in FIG. 7. The valve disc 31 then leaves the main casing 15 to open the valve hole 28.

The valve disc drive device 30 for opening and closing the valve disc 31 is configured as described above. However, the present embodiment further includes a control device that detects whether the compression operation chambers 36A, 36B are over-compressed, and if over-compression is detected, controls the valve disc drive device 30 so as to open the valve disc 31. The control device will now be described with reference to FIG. 1.

Referring to FIG. 1, the reference numeral 110 denotes a suction pressure sensor that detects the pressure of gas suctioned from the inlet 6, whereas the reference numeral 111 denotes a discharge pressure sensor that detects the pressure of compressed gas discharged from the compressor main body 1. Signals from these pressure sensors are transmitted to the control device 112. In accordance with the signals from the pressure sensors 110, 111, the control device 112 calculates a pressure ratio (discharge pressure/suction pressure) prevailing during a current operation. Further, the control device 112 stores a predetermined pressure ratio and compares the calculated pressure ratio against the predetermined pressure ratio.

If the result of comparison indicates that the calculated pressure ratio, which prevails during the current operation, is equal to or higher than the predetermined pressure ratio, the control device 112 concludes that there is no over-compression, and opens the solenoid valve 42. The valve disc 31 then moves toward the main casing 15 and becomes depressed to close the valve hole 28.

If, on the other hand, the result of comparison indicates that the calculated pressure ratio, which prevails during the current operation, is lower than the predetermined pressure ratio, the control device 112 concludes that the compression operation chambers 36A, 36B are over-compressed, and closes the solenoid valve 42. The valve disc 31 then moves away from the main casing 15 (moves toward the right-hand side of FIG. 6) to open the valve hole 28. This ensures that compressed gas is discharged from the compression operation chambers 36A, 36B to the discharge chamber 26 through the valve hole 28 and the bypass flow path (bypass groove 29). Consequently, the pressure in the compression operation chambers is reduced until it is substantially equal to the pressure in the discharge chamber 26. This makes it possible to reduce the degree of over-compression and suppress the unnecessary consumption of motive power.

The present embodiment is configured so that a set volume ratio Vs/Vd, which is the ratio between a compression operation chamber volume Vs prevailing during suction confinement and a compression operation chamber volume Vd prevailing at the beginning of discharge, is within the range between 1.5 and 3.0.

Further, the present embodiment is configured so that a substantial center of the valve hole 28 in the discharge side end face 24 of the discharge casing 16 is positioned at the opening border of the bore 20B in the end face 21 of the main casing 15. More specifically, the inner portion of the valve hole 28, which is positioned between the radially inside of the rotor section and the opening border of the bore 20B, is open to the compression operation chamber 36B as shown in FIG. 3. Therefore, a large opening area is obtained while the outer portion, which is positioned between the radially outside of the rotor section and the opening border of the bore 20B, is covered with the end face 21 of the main casing 15. This ensures that the end face 21 of the main casing 15, which covers the outer portion of the valve hole 28, functions as a stopper for the valve disc 31 (that is, the valve disc 31 does not tilt because it comes into contact with the end face 21). In a conventional case, a stepped portion is provided for the valve disc and valve hole and used as a stopper for positioning the valve disc. In contrast to such a conventional case, the present embodiment is configured to use a simplified stopper for positioning the valve disc. Thus, the present embodiment does not require high-precision machining unlike in the conventional case and makes it possible to provide increased productivity.

Furthermore, the valve disc drive device 30 can be positioned toward the radially inside of the rotor section in contrast to a case where a substantial center of the valve hole 28 is positioned between the radially inside of the rotor section and the opening border of the bore 20B. This ensures that the valve disc drive device 30 does not interfere with the roller bearing 18 and the ball bearing 19, which are provided for the discharge casing 16 to support the discharge side shaft portion of the female rotor 14B. As this eliminates the necessity of lengthening the discharge side shaft portion of the screw rotor 14, it is possible to suppress an increase in the size of the compressor.

Moreover, the present embodiment is configured so that the bypass flow path is formed by the bypass groove 29, which is formed in the discharge side end face 24 of the discharge casing 16, and by the end face 21 of the main casing 15, which covers the bypass groove 29. This makes it possible to form the bypass groove 29 at the stage of casting. The number of machining steps can be decreased as compared to a case where, for example, a bypass hole is formed as the bypass flow path.

Modifications of the first embodiment, which has been described above, will now be described. In the first embodiment, it is assumed that one valve hole 28 is provided on the female rotor 14B side of the discharge side end face 24 of the discharge casing 16 as shown in FIG. 3. However, the number of valve holes and their positions are not limited to those described in connection of the first embodiment. For example, the configuration may be modified as described below in connection with three different modifications shown in FIGS. 9 to 11.

FIG. 9 shows a first modification. In the first modification, one valve hole 37 is provided on the male rotor 14A side of the discharge side end face 24 of the discharge casing 16. More specifically, the valve hole 37 is disposed near the male rotor 14A side discharge port 25A on the discharge side end face 24 of the discharge casing 16 and open at a position opposite the rotation direction of the male rotor 14A. The reference numeral 38 denotes a bypass groove that permits the valve hole 37 to communicate with the discharge chamber 26. In the same manner as indicated in FIGS. 6 to 8, the valve hole 37 is provided with the valve disc 31 and the valve disc drive device 30, which opens and closes the valve disc 31. Further, the set volume ratio Vs/Vd, which is the ratio between the compression operation chamber volume Vs prevailing during suction confinement and the compression operation chamber volume Vd prevailing at the beginning of discharge, is within the range between 1.5 and 3.0, as is the case with the first embodiment, which has been described earlier. In addition, a substantial center of the valve hole 37 in the discharge side end face 24 of the discharge casing 16 is positioned at the opening border of the bore 20B in the end face 21 of the main casing 15, as is the case with the first embodiment, which has been described earlier. Consequently, the first modification, which has been described with reference to FIG. 9, provides virtually the same advantages as the first embodiment.

FIG. 10 shows a second modification. In the second modification, the male rotor 14A side and female rotor 14B side of the discharge side end face 24 of the discharge casing 16 are provided respectively with a valve hole 28 and a valve hole 37. More specifically, the female rotor 14B side of the discharge casing 16 is provided, for instance, with the valve hole 28, the bypass groove 29, and the valve disc drive device 30 in the same manner as indicated in FIG. 3, whereas the male rotor 14A side of the discharge casing 16 is provided, for instance, with the valve hole 37, the bypass groove 38, and the valve disc drive device in the same manner as indicated in FIG. 9. The second modification may be configured so that the valve hole 28 and the valve hole 37 may be equal to each other or different from each other in the set volume ratio Vs/Vd, which is the ratio between the compression operation chamber volume Vs prevailing during suction confinement and the compression operation chamber volume Vd prevailing at the beginning of discharge through each valve hole.

The second modification, which has been described above, provides the same advantages as the first embodiment. In addition, as the male rotor 14A side and female rotor 14B side are provided respectively with the valve hole 28 and the valve hole 37, an over-compressed gas can be discharged from the compression operation chambers to the discharge side with increased promptness in the event of over-compression. This makes it possible to virtually avoid over-compression and further suppress the unnecessary consumption of motive power.

FIG. 11 shows a third modification. In the foregoing examples, either the female rotor 14B side or the male rotor 14A side is provided with a valve hole 28 or a valve hole 38, or the female rotor 14B side and the male rotor 14A side are respectively provided with a valve hole 28 or a valve hole 38. Meanwhile, the third modification is configured so that either the female rotor 14B side or the male rotor 14A side is provided with a plurality of valve holes or the female rotor 14B side and the male rotor 14A side are both provided with a plurality of valve holes. For example, as shown in FIG. 11, the discharge casing 16 is configured so that the female rotor 14B side is provided with two valve holes 28A, 28B, and that a bypass groove 29A is formed to let the valve holes 28A, 28B communicate with the discharge chamber 26. As is the case with the present embodiment, valve discs are provided respectively for the valve holes 28A, 28B while a valve disc drive device is provided to open and close the valve discs.

In the third modification, the set volume ratio Vs/Vd, which is the ratio between the compression operation chamber volume Vs prevailing during suction confinement and the compression operation chamber volume Vd prevailing at the beginning of discharge through the valve holes 28A, 28B, is within the range between 1.5 and 3.0 for both the valve hole 28A side and the valve hole 28B side. However, as the valve hole 28A side and the valve hole 28B side are disposed differently relative to the direction of female rotor rotation, they differ in the set volume ratio Vs/Vd. In the third modification, too, substantial centers of the valve holes 28A, 28B in the discharge side end face 24 of the discharge casing 16 are positioned at the opening border of the bore 20B in the end face 21 of the main casing 15.

The third modification, which has been described above, also provides the same advantages as the present embodiment. In addition, a plurality of valve holes is disposed differently relative to the rotor rotation direction. Therefore, the total pass area of the valve holes can be efficiently enlarged without causing interference with the rotors.

FIG. 12 is a refrigeration cycle configuration diagram illustrating a chiller unit having the screw compressor according to the first embodiment of the present invention.

Referring to FIG. 12, the reference numeral 130 denotes the screw compressor according to the first embodiment. The refrigerant gas discharged from the screw compressor 130 enters the oil separator 92 through the discharge pipe 94. After the oil is separated from the refrigerant gas in the oil separator 92, the refrigerant gas is forwarded to a condenser 140 through the pipe (refrigerant pipe) 96. In the condenser 140, the refrigerant gas is cooled by ambient air, condensed, and turned into a liquid refrigerant. The liquid refrigerant is then forwarded to an electronic expansion valve 142 and expanded. The expanded refrigerant is forwarded to an evaporator 141 installed downstream of the electronic expansion valve 142. In the evaporator 141, the expanded refrigerant is evaporated as it draws heat, for instance, from external cooling water. The evaporated refrigerant is then taken back into the screw compressor 130. The cooling water cooled by the evaporator 141 is used, for instance, for cooling purposes.

The suction side of the screw compressor 130 is provided with a suction pressure sensor 110. The discharge side of the screw compressor 130 is provided with a discharge pressure sensor 111. The suction pressure sensor 110 and the discharge pressure sensor 111 detect a refrigerant gas suction pressure and a refrigerant gas discharge pressure, respectively. The reference numeral 42 denotes a solenoid valve that is identical with the solenoid valve 42 shown in FIGS. 6 and 7. This solenoid valve 42 opens and closes in accordance with a command from the control device 112. The control device 112 determines a pressure ratio prevailing during an operation in accordance with the suction pressure relative to the screw compressor 130 and the discharge pressure of the screw compressor 130, and compares the determined pressure ratio against a stored preset pressure ratio. If the pressure ratio prevailing during the operation is smaller than the preset pressure ratio, the control device 112 concludes that over-compression has occurred, and then controls the solenoid valve 42 in such a manner that the valve disc drive device 30 opens the valve disc 31 as shown in FIG. 7.

In the chiller unit, control is usually exercised in such a manner that the temperature of cooling water reaches a target value. Therefore, the cooling water temperature does not cause the suction pressure to significantly vary. However, condensation pressure exerted by the condenser decreases when the temperature of ambient air lowers. Therefore, the discharge side pressure of the condenser, which is detected by the discharge pressure sensor 111, varies. Consequently, over-compression is likely to occur in the screw compressor 130. However, using the screw compressor according to the present embodiment makes it possible to reduce the possibility of over-compression and obtain a chiller unit that does not suffer a significant motive power loss.

If the pressure ratio (discharge pressure/suction pressure) calculated from a measured suction pressure and discharge pressure is higher than the preset pressure ratio, the present embodiment, which has been described above, closes the valve disc by relieving the oil pressure within a cylinder on the valve disc side of the piston to the suction side of the screw compressor. If, on the other hand, the pressure ratio calculated from the measured suction pressure and discharge pressure is lower than the preset pressure ratio, the present embodiment opens the valve disc by confining the oil pressure within the cylinder. Therefore, the valve disc can be opened and closed with certainty to reduce the degree of over-compression. As a result, the unnecessary consumption of motive power can be suppressed to provide improved performance. In contrast to a conventional case where a valve is opened and closed in accordance with the balance between compression operation chamber pressure exerted on the valve disc, discharge side pressure, and spring force, the present embodiment opens and closes the valve disc with increased certainty and prevents the valve disc from being rattled by pressure changes in the compression operation chambers. This makes it possible to obtain a screw compressor that is capable of reducing the hammering sound and vibration of the valve disc.

Particularly, the cylinder on the valve disc side of the piston is provided with a spring that presses the piston to the side away from the valve disc. Therefore, even when the pressure changes in the compression operation chambers, the spring prevents the valve disc from hitting the stopper. As the valve disc does not hit the stopper, the hammering sound and vibration of the valve disc can be eliminated. In addition, the reliability of the valve disc can be enhanced because the spring provided in the cylinder does not repeat its violent expansion and contraction.

Moreover, in a conventional type described, for instance, in JP-A No. 1986-79886, flow restriction occurs to increase fluid friction when the valve disc opens or closes to let a gas pass through a valve section. Therefore, the degree of over-compression cannot be adequately reduced. In the present embodiment, on the other hand, the control device provides control so that the valve disc is either fully open or fully closed. This makes it possible to avoid the restriction of a gas flow from a valve disc section, which may conventionally occur due to changes in the valve disc opening, and prevent an increase in fluid friction. Therefore, the degree of over-compression can be adequately reduced.

It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.