SCREW COMPRESSOR

Abstract

 A screw compressor has a compression chamber formed by a pair of screw rotors which is provided with a male rotor (2) and a female rotor (2A) and by a casing which contains the pair of screw rotors. The casing has a discharge port (10) out of which a compressed gas flows, and also has a discharge chamber (12) into which the compressed gas discharged from the discharge port flows: Bypass paths for interconnecting the compression chamber and the discharge chamber are respectively provided to both a male rotor side portion and a female rotor side portion which are located near the discharge port formed in the casing. The compressor also has a valve (110) for opening and closing the bypass paths.

Description

TECHNICAL FIELD

[0001] The present invention relates to a screw compressor which is used for refrigeration, air-conditioning and the like, and is particularly for a screw compressor which is capacity-controlled.

BACKGROUND OF ART

[0002] As a conventional screw compressor, there is the one described in Patent Literature 1. In this screw compressor, when the discharge pressure abnormally rises, a relief valve provided in a slide valve is operated to bypass the compressed gas to a discharge side, and thereby an abnormal load acting on a screw rotor and a bearing member which supports the screw rotor is reduced.

CITATION LIST

PATENT LITERATURE

[0003] 

Patent Literature 1: JP-A-4-43883

SUMMARY OF INVENTION

TECHNICAL PROBLEM

[0004] In the screw compressor of the above described Patent Literature 1, the abnormal load is reduced by providing the relief valve in the slide valve, and therefore, the relief valve needs to form a part of a bore section for a pair of screw rotors, and high machining accuracy is required. Further, the relief valve forms a part of a compression chamber bore section formed by a casing, and therefore there arises the disadvantage of increasing the size of the compressor.

[0005] Further, improvement of the integrated part load value (IPLV) has been required recently, and improvement in performance of screw compressors in a low load range has been required.

[0006] An object of the present invention is to provide a screw compressor which can prevent excessive compression with simple structure.

[0007] Another object of the present invention is to provide a screw compressor which can improve the integrated part load value by enhancing the efficiency in a low load operation range.

SOLUTION TO PROBLEM

[0008] In order to achieve the above described objects, the present invention provides a screw compressor in which a compression chamber is formed by a pair of screw rotors including a male rotor and a female rotor, and a casing which accommodates the pair of screw rotors, and a discharge port for allowing a compressed gas to flow out, and a discharge chamber into which the compressed gas discharged from the discharge port flows are formed in the casing, characterized in that a bypass vent which interconnects the compression chamber and the discharge chamber and a valve for opening and closing the bypass vent are provided near the discharge port in the casing on each of a male rotor side and a female rotor side thereof.

[0009] Here, the valve for opening and closing the bypass vent is preferably configured to be opened when the pressure in the compression chamber which communicates with the bypass vent becomes higher than the pressure in the discharge chamber.

[0010] Further, the bypass vent is preferably formed at a position where the bypass vent communicates with the compression chamber in a set volume ratio range of 1.5 to 3.0, preferably 1.5 to 2.7.

[0011] It is more effective that a plurality of bypass vents are provided on the male rotor side or the female rotor side at positions where the bypass vents communicate with the compression chamber with different set volume ratios.

[0012] The present invention is especially effective when it is applied to the screw compressor configured to drive the screw rotor by an electric motor of which rotational frequency is controllable by an inverter.

[0013] Another characteristic of the present invention is to provide a screw compressor in which a compression chamber is formed by a pair of screw rotors including a male rotor and a female rotor, and a casing which accommodates the pair of screw rotors, and a discharge port for allowing a compressed gas to flow out, and a discharge chamber into which the compressed gas discharged from the discharge port flows are formed in the casing, wherein a bypass vent which interconnects the compression chamber and the discharge chamber, and a valve for opening and closing the bypass vent are provided on each side with respect to the discharge port in the casing.

[0014] Still another characteristic of the present invention is to provide a screw compressor including a pair of screw rotors comprising a male rotor and a female rotor, a main casing which accommodates the pair of screw rotors, a discharge casing provided on a discharge side of the main casing, a motor casing which accommodates an electric motor for driving the screw rotors, a discharge port provided in at least any one of the main casing and the discharge casing, a compression chamber formed by the pair of screw rotors and the main casing, and a discharge chamber formed in the discharge casing, into which discharge chamber a compressed gas discharged from the discharge port flows, wherein the screw compressor further includes a bypass vent provided in the discharge casing near the discharge port, which bypass vent interconnects the compression chamber and the discharge chamber, and a valve which is configured to be closed when the pressure in the compression chamber communicating with the bypass vent is lower than the pressure in the discharge chamber, and to be opened when the pressure in the compression chamber becomes higher than the pressure in the discharge chamber for opening and closing the bypass vent.

[0015] Here, the bypass vent is preferably provided on both of a male rotor side and a female rotor side of the discharge port formed in the casing.

ADVANTAGEOUS EFFECTS OF INVENTION

[0016] According to the screw compressor of the present invention, by the configuration in which the bypass vents which interconnects the compression chamber and the discharge chamber is provided on both male rotor side and female rotor side of the casing near the discharge port, and the valve for opening and closing the bypass vent is provided, it is possible to obtain the screw compressor which can prevent excessive compression with simple structure. As a result, an abnormal load acting on the screw rotor and the bearing member which supports the rotor is reduced, and the screw compressor with high reliability which can prevent deformation of the rotor and damage of the bearing can be obtained.

[0017] Further, the valve for opening and closing the bypass vent is configured to be opened when the pressure in the compression chamber communicating with the bypass vent becomes higher than the pressure in the discharge chamber, whereby excessive compression can be prevented, and the efficiency especially in the low load operation range can be enhanced. Therefore, the integrated part load value can be improved.

[0018] Furthermore, a plurality of bypass vents are provided so as to communicate with the compression chamber with different set volume ratios, and thereby, excessive compression can be prevented continuously in a wide operation range.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0019] 

Fig. 1 is a vertical sectional view of a screw compressor showing example 1 of the present invention;

Fig. 2 is a diagram showing a relationship between volume V and pressure P in an arbitrary compression chamber of a screw rotor section shown in Fig. 1;

Fig. 3A is a sectional view taken along line A-A of the screw compressor shown in Fig. 1, and shows a screw rotor rotation position at a time of opening or immediately after opening of a bypass vent provided in a discharge casing;

Fig. 3B is a sectional view taken along line A-A of the screw compressor shown in Fig. 1, and shows a screw rotor rotation position at a time when the bypass vent is totally opened, or immediately after that time;

Fig. 3C is a sectional view taken along line A-A of the screw compressor shown in Fig. 1, and shows a screw rotor rotation position when a compressed gas in the compression chamber starts to be discharged to a discharge chamber from a male side discharge port and a female side discharge port provided in a discharge casing;

Fig. 4 is a sectional view in a portion of a valve provided in the bypass vent on a male side in the screw compressor shown in Fig. 1;

Fig. 5 is a sectional view taken along line B-B of Fig. 4;

Fig. 6A is a view showing a modified example of the example shown in Figs. 3A to 3C, and corresponding to Figs. 3A to 3C;

Fig. 6B is a view showing another modified example than the example shown in Figs. 3A to 3C, and corresponding to Figs. 3A to 3C;

Fig. 7A is a view showing still another modified example than the example shown in Figs. 3A to 3C, and showing a vicinity of a discharge port in Figs. 3A to 3C by enlarging it;

Fig. 7B is a view showing still another modified example than the example shown in Figs. 3A to 3C, and showing the vicinity of the discharge port in Figs. 3A to 3C by enlarging it;

Fig. 8A is a view showing still another modified example than the example shown in Figs. 3A to 3C, and showing the vicinity of the discharge port in Figs. 3A to 3C by enlarging it;

Fig. 8B is a view showing still another modified example than the example shown in Figs. 3A to 3C, and showing the vicinity of the discharge port in Figs. 3A to 3C by enlarging it;

Fig. 9 is a view in which a leaf spring type valve and a valve presser are respectively provided in the bypass vent provided in the vicinity of the discharge port shown in Fig. 8B, and corresponding to a section taken along line F-F of Fig. 1;

Fig. 10 is a sectional view taken along line D-D of a valve section shown in Fig. 9;

Fig. 11 is a view showing another embodiment than the example shown in Fig. 4, and corresponding to Fig. 4;

Fig. 12 is a sectional view of a vicinity of the valve section seen in the arrow direction taken along line C-C of Fig. 11;

Fig. 13 is a view showing another example than the example shown in Figs. 11 and 12, and is a sectional view of the portion corresponding to Fig. 12;

Fig. 14 is a view showing still another example than the example shown in Figs. 11 and 12, and is a sectional view of the portion corresponding to Fig. 12;

Fig. 15 is a sectional view of the vicinity of the valve section seen in the arrow direction taken along line E-E of Fig. 14; and

Fig. 16 is a view showing still another example than the example shown in Figs. 11 and 12, and is a sectional view of the portion corresponding to Fig. 12 or Fig. 14.

DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, a specific example of a screw compressor of the present invention will be described with use of the drawings. The parts assigned with the same reference signs show the same or corresponding parts in the respective drawings.

EXAMPLE 1

[0021] Fig. 1 is a vertical sectional view of a screw compressor showing example 1 of the present invention. The screw compressor shown in Fig. 1 is composed mainly of a compressor section 17 and a motor section 18. A gas to be compressed (for example, a refrigerant flowing in a refrigeration cycle) is taken in from an inlet opening 20 formed in a motor casing 16 at a side of the motor section 18, passes trough portions of a stator 3 and a rotor 4 constituting a motor (electric motor for drive) 22, and is compressed by the compressor section 17 which is composed of a pair of screw rotors (a male rotor 2, and a female rotor 2A) from an intake port 9. Thereafter, the compressed gas is discharged into a discharge chamber 12 from a discharge port 10 and a discharge radial port 44. Thereafter, the compressed gas flows into an oil separator 80, where oil is separated from the compressed gas, and the compressed gas is discharged outside the compressor from a discharge opening 19.

[0022] The compressor section 17 is composed of a main casing 1 which contains the screw rotors 2 and 2A and houses a roller bearing 6, a discharge casing 21 which forms the discharge chamber 12 and houses a roller bearing 7 and a ball bearing 8, and the like. The intake port 9, the discharge port 10 and the discharge radial port 44 are also formed in the aforesaid main casing 1. The aforesaid intake opening 20 and the intake port 9 form an intake channel to the screw rotors 2 and 2A. The aforesaid discharge port 10, the discharge radial port 44 and the discharge chamber 12 form a discharge path from the screw rotors 2 and 2A. The screw rotor 2 is composed of a pair of the male rotor 2 and the female rotor 2A (see Figs. 3A to 3C) which mesh with each other, is housed in a pair of cylindrical bores (a male side casing bore 40a and a female side casing bore 40b shown in Figs. 3A to 3C), and forms a compression chamber by the cylindrical bores and a meshing portion of the male rotor 2 and the female rotor 2A. Shaft portions provided on both sides of the male rotor 2 are supported by the roller bearing 6 provided in the main casing 1 and the roller bearing 7 and the ball bearing 8 provided in the discharge casing 21.

[0023] The motor section 18 is composed of the motor casing 16, the stator 3, the rotor 4 and the like. The motor section 18 is configured to transmit the drive force thereof to the male rotor 2 in the compressor section 17. The stator 3 is attached to the motor casing 16, and the rotor 4 is fixed to the shaft portion which is at the inner peripheral side of the aforesaid stator 3 and is provided on the motor section side of the male rotor 2. By this configuration, the drive force of the motor 22 is transmitted to the male rotor 2, and the female rotor 2A is driven by the male rotor 2.

[0024] In the above described screw compressor, load capacity regulation is performed by regulating the discharge amount by a signal from an intake pressure sensor (not illustrated) and a signal from a discharge pressure sensor (not illustrated) being inputted into a control device (not illustrated), and the rotational frequency of the motor 22 being controlled by an inverter (not illustrated). When the load becomes small, and the discharge side pressure reduces, the compressed gas pressure in the compression chamber which is composed of the male rotor 2 and the female rotor 2A becomes higher than the discharge side pressure to cause excessive compression. In order to prevent the excessive compression, in the present example, the discharge casing 21 which forms the compression chamber is provided with a bypass vent which interconnects the compression chamber and the discharge chamber (see a bypass vent 50 on a male side and a bypass vent 51 on a female side shown in Figs. 3A to 3C), and a valve 110 which opens and closes the bypass vent, so that pressure regulation in the compression chamber is performed by these bypass vents 50 and 51 and the valve 110.

[0025] Fig. 2 is a diagram showing the relationship between volume V and pressure P in an arbitrary compression chamber of the screw rotors 2 and 2A section shown in Fig. 1. In the diagram, LP represents an intake pressure, HP2 represents a discharge pressure at a time of full load operation, and HP1 represents a discharge pressure at a time of unload operation. In the case of the full load operation with the intake pressure LP and the discharge pressure HP2, the operation cycle becomes a1-b1-c1-d1. Further, in the case of the unload operation time with the intake pressure LP and the discharge pressure HP1, the operation cycle in the case without having the bypass vents 50 and 51 which interconnect the compression chamber and the discharge chamber and the valve 110 becomes a1-b1-g3-f1-d1, and e1-b1-g3 represents an excessive compression region which is wastefully compressed. In the present example, the bypass vents 50 and 51 and the valve 110 are provided, whereby the operation cycle can be made a1-e1-f1-d1, and therefore, the wasteful excessive compression can be prevented.

[0026] The positions where the bypass vents 50 and 51 are provided are determined as follows. More specifically, the bypass vents 50 and 51 are formed at the positions where the compression chamber and the discharge chamber 12 are interconnected with each other when the pressure P of the compression chamber formed by the male rotor and the female rotor meshing with each other becomes the discharge pressure HP1 at the time of the unload operation. For this purpose, the compression chamber volume VD1, where

is firstly obtained from a set volume ratio Vi at the time when the pressure becomes the arbitrary discharge pressure HP 1 at the time of the unload operation from the intake pressure LP, where

and the aforesaid bypass vents 50 and 51 are provided at the rotational angle positions of the screw rotor at which the compression chamber volume VD1 is achieved.

[0027] In the above described expression,
n: politropic exponent set for each refrigerant
VT: intake volume (maximum space volume of the rotor).

[0028] More specifically, if the operation conditions such as the intake pressure LP and the discharge pressure HP1 of the discharge chamber are determined, the set volume ratio Vi is determined, and therefore, the compression chamber volume VD 1 for the arbitrary discharge pressure HP1 at the time of the unload operation is obtained, and the rotational angles of the male rotor 2 and the female rotor 2A corresponding to the compression chamber volume VD1 are determined. Therefore, the aforementioned bypass vents 50 and 51 are provided so as to be interconnected with the compression chamber at the rotational angles.

[0029] Figs. 3A to 3C show sectional views (discharge port section) taken along line A-A of the screw compressor shown in Fig. 1. When the male rotor 2 is rotated by the motor, the female rotor 2A meshing with the male rotor is also rotated, and the compressed gas is confined in the compression chamber. A compression chamber 30a on the male side is formed by the male side casing bore 40a and the male rotor 2, and a compression chamber 30b on the female side is formed by the female side casing bore 40b and the female rotor 2A. Further, the compression chamber 30a on the male side and the compression chamber 30b on the female side are also interconnected with each other.

[0030] Fig. 3A shows a screw rotor rotation position at the time of opening or immediately after opening of the bypass vent 50 provided in the discharge casing 21. The bypass vent 50 on the male side is provided to be in contact with a retreating surface tangential line 120 of the male rotor 2 at a determined rotational angle. Further, the bypass vent 51 on the female side is provided to be in contact with a retreating surface tangential line 123 of the female rotor at a determined rotational angle.

[0031] The sizes of the holes of the aforesaid bypass vents 50 and 51 are set to be the minimum teeth thicknesses of the male rotor and the female rotor or less so that the adjacent compression chambers are not interconnected with each other.

[0032] Fig. 3B shows a screw rotor rotation position at the time when or immediately after the bypass vents 50 and 51 are totally opened. Until the compressed gas starts being discharged from the male side discharge port 42 and the female side discharge port 43, an excessively compressed gas is continuously bypassed to the discharge chamber 12 from the aforesaid bypass vents 50 and 51.

[0033] Fig. 3C shows a screw rotor rotation position at the time when the compressed gas in the compression chambers 30a and 30b starts being discharged to the discharge chamber 12 from the male side discharge port 42 and the female side discharge port 43 provided in the discharge casing 21.

[0034] Fig. 4 shows a sectional view in the section of the valve 110 provided in the bypass vent 50 on the male side in the screw compressor shown in Fig. 1. The pressure of the discharge chamber 12 acts on the inside of a valve path 115 shown in the drawing via a discharge path 100. Therefore, when the pressure in the bypass vent 50 becomes higher than the pressure in the valve path 115, the valve 110 is pushed up by the pressure difference, and the compressed gas in the compression chamber 30a is discharged to the discharge chamber 12 through the valve path 115.

[0035] The female side bypass vent 51 is configured in the same manner.

[0036] Fig. 5 is a sectional view taken along line B-B of Fig. 4. A spring force is always applied to the valve 110 in the direction to close the valve 110 by a spring 112. When the gas pressure from the bypass vent 50 side which acts on a valve section 116 of the valve 110 exceeds the total value of the gas pressure of the discharge chamber 12 which acts on the inside of the valve path 115 and the aforesaid spring force, the valve 110 is opened, and the excessively compressed gas of the compression chamber 30a flows into the discharge chamber 12.

[0037] In Fig. 5, reference sign 111 designates an oil hole, reference sign 113 designates a flange which holds the valve 110, and the flange 113 is attached to the discharge casing 21 via a screw 114.

[0038] Figs. 6A to 6B are views showing a modified example of the example shown in Figs. 3A to 3B, and corresponding to Figs. 3A to 3C. In this example, the bypass vent 50 on the male side opens first to bypass the excessively compressed gas of the compression chamber 30a to discharge the excessively compressed gas to the discharge chamber, and the bypass hole 51 on the female side opens later to discharge the excessively compressed gas of the compression chamber 30b to the discharge chamber similarly.

[0039] In Fig. 6A, the bypass vent 50 on the male side and the bypass vent 51 on the female side are set at the positions at which they open while overlapping with each other for a fixed section, whereby excessive compression can be continuously prevented over a wide range. Further, in the example of Fig. 6B, the opening sections of the bypass vents 50 and 51 are set to be shifted so as not to overlap with each other, and the bypass vents may be set in this manner.

[0040] Figs. 7A to 7B are views showing still other modified examples of the example shown in Figs. 3A to 3C, and showing the vicinity of the discharge port 10 in Figs. 3A to 3C by enlarging it. In the example shown in Fig. 7A, the bypass vent 50 is placed only on the male side, and when the tooth thickness of the female rotor is thin, and the bypass vent on the female side cannot be placed to be large, the bypass vent may be provided only on the male side. In this example, the bypass vent and the valve are not required on the female side, and the cost can be reduced. Also, as in the example shown in Fig. 7B, the bypass vent is not provided on the male side, and the bypass vent 51 may be provided only on the female side. Further, it is effective to provide the bypass vents respectively on the male side and the female side, and to configure the opening area of the bypass vent on the male side to be larger than the opening area of the bypass vent on the female side, though not illustrated.

[0041] Figs. 8A to 8B are views showing still other modified examples of the example shown in Figs. 3A to 3C, and showing the vicinity of the discharge port 10 in Figs. 3A to 3C by enlarging it. In the example shown in Fig. 8A, the bypass vents 50 and 51 provided in the discharge casing are made long holes, and by configuring them like this, sufficiently large bypass channel areas of the bypass vents can be secured, and the channel resistance of the compression gas which is bypassed from the bypass vents to the discharge chamber can be reduced.

[0042] In the example shown in Fig. 8B, a plurality of male side bypass vents 50 and 50a and female side bypass vents 51 and 51 a are respectively provided in positions of arbitrary different set volume ratios. In this example, the bypass vents are provided in two positions of different arbitrary set volume ratios on each of the male rotor side and the female rotor side, but the bypass vents may be provided in three or more positions of different arbitrary set volume ratios. Further, in this example, two holes which form the bypass vent are formed in each position of the same arbitrary set volume ratio, but the bypass vent may be formed by three or more holes. As in this example, the bypass vent is formed by a plurality of holes, whereby as compared with the example shown in Fig. 8A, the bypass vent is easily machined, and the machining time can be shortened.'

[0043] Figs. 9 and 10 are views showing the example in which a leaf spring type valve 70 and a valve presser 71 are provided at each of the bypass vents 50, 50a, 51 and 51a provided in the vicinity of the discharge port section shown in Fig. 8B, and corresponding to a sectional view taken along line F-F in the vicinity of the discharge port section of Fig. 1. Fig. 10 is a sectional view taken along line D-D of the valve section shown in Fig. 9. As shown in this example, the leaf spring type valve 70 and the valve presser 71 are fastened together and attached to the main casing 1 on the discharge chamber side of the bypass vent which interconnects the compression chamber and the discharge chamber, whereby the manufacturing man-hour for the valve mechanism can be reduced, the valve structure can be simplified, and cost reduction of the valve can be realized. Further, the valve which opens and closes the bypass vent is formed by the leaf spring type valve, and thereby a plurality of valves can be provided in the narrow limited space.

[0044] In the examples shown in Figs. 1 to 7, the valves which open and close the bypass vents also can be formed by the leaf spring type valves in this manner.

[0045] Figs. 11 and 12 show another embodiment of the example shown in Figs. 4 and 5. In Fig. 4, the valve 110 is provided in the vertical direction (the direction perpendicular to the axis), but in the example of Figs. 11 and 12, the valve 110 is provided in the lateral direction (the axial direction). Fig. 12 is a sectional view of the vicinity of the valve section seen in the arrow direction taken along line C-C of Fig. 11. Further, in Figs. 11 and 12, the parts assigned with the same reference signs as in Figs. 4 and 5 show the same or corresponding portions. As shown in this example, the valve 110 is laterally parts, whereby the length of the bypass vent 50 which interconnects the compression chamber and the discharge chamber can be configured to be shorter than the example shown in Figs. 4 and 5, and the volume of the bypass vent can be made small. Thereby, the non-compressed volume can be decreased, and reduction in volume efficiency can be suppressed. In Fig. 12, reference sign 117 designates a spacer, which corresponds to the flange 113 in Fig. 5.

[0046] Fig. 13 is a view showing another example than the example shown in Figs. 11 and 12, and is a sectional view of a portion corresponding to Fig. 12. In this example, high-pressure oil is guided to a valve cylinder 143 through piping 141 from an oil tank 25, so that a valve 140 which opens and closes the bypass vent 50 is operated by hydraulic pressure. The valve 140 is pressed against the position where it closes the bypass vent 50 and the discharge path 100 by the high hydraulic pressure. Further, when the pressure of the bypass vent 50 which interconnects the compression chamber and the discharge chamber 12 becomes higher than the hydraulic pressure, the high-pressure oil in the cylinder 143 is pushed back to the oil tank 25 through the piping 141, whereby the valve 140 is operated in the right direction in the drawing, so that the compressed gas is bypassed to the discharge chamber 12 through the bypass vent 50 and the discharge path 100 from the discharge chamber 12.

[0047] Figs. 14 and 15 are views showing still another example than the example shown in Figs. 11 and 12. Fig. 14 is a sectional view of the portion corresponding to Fig. 12. Fig. 15 is a sectional view of the vicinity of the valve section seen in the arrow direction along line E-E of Fig. 14. In this example, a valve 135 with a hole provided in the center of a column is placed halfway in the bypass vent 50 which interconnects the compression chamber and the discharge chamber 12, and the valve 135 is rotated by, for example, 90 degrees as shown in Fig. 15 via a shaft 134 by a step motor 131 to open and close the bypass vent 50. Control of the step motor 131 is performed such that signals from a pressure sensor 133 provided at the bypass vent 50 which interconnects the compression chamber and the discharge chamber and from a pressure sensor 133 placed in the discharge chamber 12 are inputted in a control device 132, and when the pressure of the bypass vent 50 becomes higher than the pressure of the discharge chamber 12, the valve 135 is controlled to be in an open state, whereas when the pressure of the bypass vent 50 becomes lower than the pressure of the discharge chamber 12, the valve 135 is controlled to be in a closed state. According to the example, the valve mechanism with high followability to the pressure change of the compression chamber can be made.

[0048] Fig. 16 is a view showing still another example than the example shown in Figs. 11 and 12, and shows a sectional view of the portion corresponding to Fig. 12 or Fig. 14. In this example, an electromagnetic valve 136 is provided halfway in the bypass vent 50 which interconnects the compression chamber and the discharge chamber 12, and the control device 132 controls the electromagnetic valve 136 while monitoring the pressure of the bypass vent 50 and the pressure of the discharge chamber 12 as in the example shown in Fig. 14. By opening the electromagnetic valve 136, the compressed gas of the compression chamber can be bypassed to the discharge chamber 12 through the bypass vent 50 and the discharge path 100. In this example, the valve mechanism with high followability to the pressure change of the compression chamber as in the example shown in Fig. 14 can be made, without making a complicated valve opening and closing mechanism.

[0049] According to the example described above, by adopting the configuration in which the bypass vent which interconnects the compression chamber and the discharge chamber is provided near the discharge port, and the valve which opens and closes the bypass vent is further provided, the compressed gas in the compression chamber can be held or released to the discharge chamber.. When the compression chamber pressure becomes higher than the discharge chamber pressure, the bypass vent is opened, and thereby, the compressed gas can be suppressed from being excessively compressed in the compression chamber. Especially at the time of low compression ratio operation, the pressure in the compression chamber is easily excessively compressed to an operation pressure on the discharge chamber side or higher, and when the pressure of the compression chamber becomes the pressure on the discharge chamber side or higher, the valve of the bypass vent is opened, whereby the compressed gas in the compression chamber can be discharged to the discharge chamber side through the bypass vent. Accordingly, the excessive compression operation is prevented, the shaft power of the compressor is reduced, and the performance especially in the low compression ratio operation range can be enhanced. As a result, an abnormal load on the bearing member and the screw rotor can be reduced, and deformation of the rotor and bearing damage can be prevented.

[0050] Further, the aforesaid bypass vent is configured to be placed within the range of the set volume ratio of 1.5 to 3.0, preferably 1.5 to 2.7, and the opening and closing valve which opens and closes the bypass vent is provided, whereby optimal operation at the time of unload operation can be performed. Further, the bypass vents which interconnect the compression chamber and the discharge chamber are provided at both male rotor side and female rotor side, whereby the compressed gas of the compression chamber can be efficiently passed to the discharge chamber.

[0051] Further, if the aforesaid bypass vents which interconnect the compression chamber and the discharge chamber are provided at the positions where they are interconnected with the compression chambers with different set volume ratios, it is possible to prevent the excessive compression at the time of unload operation in a wider operation range. Further, by forming the bypass vent with a plurality of holes, the flow resistance of the bypass vent can be reduced, and the volume of the whole bypass vents can be suppressed to be small, whereby the non-compression volume generated by the bypass vents is reduced, and reduction in volume efficiency can be suppressed.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

REFERENCE SIGNS LIST

[0052] 

1:

MAIN CASING

2:

MALE ROTOR

2A:

FEMALE ROTOR

3:

STATOR

4:

ROTOR

6, 7:

ROLLER BEARING

8:

BALL BEARING

9:

INTAKE PORT

10:

DISCHARGE PORT

12:

DISCHARGE CHAMBER

15:

END COVER

16:

MOTOR CASING

17:

COMPRESSOR SECTION

18:

MOTOR SECTION

19:

DISCHARGE OPENING

20:

INTAKE OPENING

21:

DISCHARGE CASING

22:

MOTOR

25:

OIL TANK

26,73:

BOLT

30a, 30b:

COMPRESSION CHAMBER

40a:

MALE SIDE CASING BORE

40b:

FEMALE SIDE CASING BORE

42:

MALE SIDE DISCHARGE PORT

43:

FEMALE SIDE DISCHARGE PORT

44:

DISCHARGE RADIAL PORT

50, 50a, 51, 51a:

BYPASS VENT

70:

VALVE

71:

VALVE PRESSER

80:

OIL SEPARATOR

100:

DISCHARGE PATH

110, 135, 140:

VALVE (OPENING AND CLOSING VALVE)

111:

OIL HOLE

112:

SPRING

113, 142:

FLANGE

114:

SCREW

115:

VALVE PATH

116:

VALVE SECTION

117:

SPACER

120:

MALE ROTOR RETREATING SURFACE TANGENTIAL LINE

123:

FEMALE ROTOR RETREATING SURFACE TANGENTIAL LINE

131:

STEP MOTOR

132:

CONTROL DEVICE

133:

PRESSURE SENSOR

134:

SHAFT

136:

ELECTROMAGNETIC VALVE

141:

OIL PIPING

143:

CYLINDER

Claims

1. A screw compressor comprising:

a pair of screw rotors comprising a male rotor and a female rotor; and

a casing which accommodates the pair of screw rotors to form a compression chamber, wherein a discharge port for allowing a compressed gas to flow out, and a discharge chamber into which the compressed gas discharged from the discharge port flows are formed in the casing, characterized in that

a bypass vent for interconnecting the compression chamber and the discharge chamber and a valve for opening and closing the bypass vent are provided near the discharge port in the casing on each of a male rotor side and a female rotor side thereof.

2. The screw compressor according to claim 1, wherein the valve for opening and closing the bypass vent is configured to open when a pressure in the compression chamber which communicates with the bypass vent becomes higher than a pressure in the discharge chamber.

3. The screw compressor according to claim 1, wherein the bypass vent is formed in a position where the bypass vent communicates with the compression chamber with a set volume ratio in the range of 1.5 to 3.0.

4. The screw compressor according to claim 3, wherein the bypass vent is formed in a position where the bypass vent communicates with the compression chamber with a set volume ratio in the range of 1.5 to 2.7.

5. The screw compressor according to claim 1, wherein the bypass vent provided on the male rotor side or the female rotor side comprises a plurality of bypass vents provided in several positions where the respective bypass vents communicate with the compression chamber with different set volume ratios.

6. The screw compressor according to claim 1, wherein the screw rotors are driven by an electric motor, a rotational frequency of which is controllable by an inverter.

7. A screw compressor comprising:

a pair of screw rotors comprising a male rotor and a female rotor; and

a casing which accommodates the pair of screw rotors to form a compression chamber, wherein a discharge port for allowing a compressed gas to flow out, and a discharge chamber into which the compressed gas discharged from the discharge port flows are formed in the casing, characterized in that

both of a bypass vent which interconnects the compression chamber and the discharge chamber, and a valve for opening and closing the bypass vent are provided on both sides both in the casing to place the discharge port therebetween.

8. A screw compressor comprising:

a pair of screw rotors comprising a male rotor and a female rotor;

a casing which accommodates the pair of screw rotors;

a discharge casing provided on a discharge side of the main casing;

a motor casing which accommodates an electric motor for driving the screw rotors;

a discharge port provided in at least any one of the main casing and the discharge casing;

a compression chamber formed by the pair of screw rotors and the main casing;
and

a discharge chamber is formed in the discharge casing, into which discharge chamber a compressed gas discharged from the discharge port flows, characterized in that

the screw compressor further comprises:

a bypass vent provided near the discharge port in the discharge casing to interconnect the compression chamber and the discharge chamber; and

a valve for opening and closing the bypass vent, which valve is configured to close when a pressure in the compression chamber communicating with the bypass vent is lower than a pressure in the discharge chamber, and to open when the pressure in the compression chamber becomes higher than the pressure in the discharge chamber.

9. The screw compressor according to claim 8, wherein the bypass vent is provided on each of a male rotor side portion and a female rotor side portion of the discharge port formed in the casing.

10. The screw compressor according to claim 8, wherein the bypass vent is formed in a position where the bypass vent communicates with the compression chamber with a set volume ratio in the range of 1.5 to 2.7.

11. The screw compressor according to claim 8, wherein the bypass vent provided on the male rotor side or the female rotor side comprises a plurality of bypass vents provided in several positions where the respective bypass vents communicate with the compression chamber with different set volume ratios.

12. The screw compressor according to claim 8, wherein the screw rotor is driven by the electric motor, a rotational frequency of which is controllable by an inverter.

Drawing