Screw compressor and method for manufacturing screw compressor

Abstract

This screw compressor includes: a pair of screw rotors; a pair of bearings for rotatably supporting each screw rotor of the pair of screw rotors; and a casing including a bore in which the pair of screw rotors are housed. In a cross-section perpendicular to an axial direction, the center of each arc of a pair of arcs, corresponding to the pair of screw rotors, in the contour of the bore is offset, on the opposite side of each bearing from a discharge port, from the center of each bearing.

Description

Screw compressor and method for manufacturing a screw compressor

[0001] This disclosure relates to a screw compressor and a method for manufacturing a screw compressor.

[0002] In a screw compressor, a clearance (gap) is provided radially between the screw rotor and the casing to prevent contact between the rotating screw rotor and the casing.

[0003] Patent Document 1 describes that in a screw fluid machine, clearance design should be carried out considering the thermal expansion of the male rotor and female rotor in order to ensure appropriate clearance during operation.

[0004] Japanese Patent Application Publication No. 04-214980

[0005] Incidentally, during the operation of a screw compressor, the screw rotor deflects in the direction of the applied load. Normally, the radial clearance between the screw rotor and the casing is provided evenly in the circumferential direction with respect to the center of the bearing supporting the screw rotor. However, when the screw rotor deflects, the clearance becomes excessively large in the region opposite to the direction of deflection (direction of the load), making it easier for the compressed gas to leak from the compression space through this clearance. Increased leakage of the compressed gas increases the shaft power and reduces the efficiency of the screw compressor.

[0006] In view of the above circumstances, at least one embodiment of the present invention aims to provide a screw compressor and a method for manufacturing a screw compressor that can effectively reduce leakage of the compressed gas through the clearance between the screw rotor and the casing.

[0007] A screw compressor according to at least one embodiment of the present invention comprises: a pair of screw rotors; a pair of bearings for rotatably supporting the pair of screw rotors; and a casing including a bore in which the pair of screw rotors are housed, wherein, in a cross section perpendicular to the axial direction, the centers of each of a pair of arcs in the contour of the bore corresponding to the pair of screw rotors are offset from the center of the bearing, on the opposite side of the center of the bearing from the discharge port.

[0008] Furthermore, a method for manufacturing a screw compressor according to at least one embodiment of the present invention is a method for manufacturing a screw compressor comprising a pair of screw rotors, a pair of bearings for rotatably supporting the pair of screw rotors, and a casing including a bore in which the pair of screw rotors are housed, the method comprising the step of machining the bore in the casing such that, in a cross section perpendicular to the axial direction, the centers of each of a pair of arcs in the contour of the bore that correspond to the pair of screw rotors are offset from the center of the bearing to the opposite side of the discharge port, with respect to the center of the bearing.

[0009] According to at least one embodiment of the present invention, a screw compressor and a method for manufacturing a screw compressor are provided that can effectively reduce the leakage of the compressed gas through the clearance between the screw rotor and the casing.

[0010] This is a schematic diagram of a gas compression system including a screw compressor according to one embodiment. This is a schematic cross-sectional view of a screw compressor in plan view according to one embodiment. This is a schematic side view of a screw compressor according to one embodiment. This is a schematic diagram showing a cross-section of the casing of a screw compressor according to one embodiment. This is a diagram illustrating how to determine the ratio of the distance between the center of the bearing and the center of the bore to the difference between the bore diameter and the outer diameter of the screw rotor. This is a flowchart of the manufacturing method of a screw compressor according to one embodiment.

[0011] Hereinafter, several embodiments of the present invention will be described with reference to the attached drawings. However, the dimensions, materials, shapes, relative arrangements, etc., of the components described as embodiments or shown in the drawings are not intended to limit the scope of the present invention, but are merely illustrative examples.

[0012] (Configuration of the screw compressor) Figure 1 is a schematic diagram of a gas compression system including a screw compressor according to several embodiments. As shown in the figure, the gas compression system 1 comprises a screw compressor 2, an oil separator 4, a cooler 6, and a pump 8.

[0013] The screw compressor 2 is configured to compress and discharge the inhaled gas. In the figure, the symbol Ps indicates the suction pressure of the screw compressor 2, and the symbol Pd indicates the discharge pressure of the screw compressor 2. Oil is supplied to the screw compressor 2 via the oil supply line 10 for cooling and lubrication. The oil supplied to the screw compressor 2 is discharged together with the compressed gas.

[0014] The oil separator 4 is configured to separate oil from the mixture of compressed gas and oil discharged from the screw compressor 2. The oil separated in the oil separator 4 is supplied back to the screw compressor 2 via the oil supply line 10. The oil separated in the oil separator 4 may be cooled in a cooler 6 before being supplied to the screw compressor 2. The oil separated in the oil separator 4 may be pressurized by a pump 8 before being supplied to the screw compressor 2 via the oil supply line 10. In this case, the pressure of the oil supplied to the screw compressor 2, Poil(Pd+α), is higher than the discharge pressure Pd of the screw compressor 2. Alternatively, the oil separated in the oil separator 4 may be supplied to the screw compressor via the oil supply line 10' without going through the pump 8. In this case, the pressure of the oil supplied to the screw compressor 2, Poil, is approximately equal to the discharge pressure Pd of the screw compressor 2.

[0015] (Screw Compressor Configuration) Figure 2 is a schematic cross-sectional view in plan of a screw compressor according to one embodiment. Figure 3 is a schematic side view of a screw compressor according to one embodiment. Figure 4 is a schematic diagram showing a cross-section of the casing of a screw compressor according to one embodiment, and corresponds to the A-A section in Figure 3.

[0016] As shown in Figure 2, the screw compressor 2 comprises a pair of screw rotors (male rotor 15 and female rotor 17) to which a pair of rotor shafts 14 and 16 are connected, and a casing 12 that houses the pair of screw rotors.

[0017] A pair of rotor shafts 14 and 16 are rotatably supported by radial bearings 18A and 18B and a thrust bearing 20, respectively. Each bearing may be supplied with oil via an oil supply line 10. The radial bearings 18A and 18B and / or the thrust bearing 20 may be sliding bearings or rolling bearings.

[0018] The male rotor 15 and the female rotor 17 have helical teeth that mesh with each other. The meshing of the teeth of the male rotor 15 and the female rotor 17, along with the casing 12, forms multiple tooth groove spaces (compression spaces, chambers) along the axial direction of the rotor shafts 14 and 16.

[0019] The rotor shaft 14 constituting the male rotor 15 is connected to the output shaft of a motor (not shown) and is configured to be rotationally driven by the motor. The female rotor 17, which meshes with the male rotor 15, is rotationally driven by the rotation of the male rotor 15. The female rotor 17 rotates in the opposite direction to the rotation of the male rotor 15. When the male rotor 15 and the female rotor 17 rotate while meshed, the tooth groove space moves axially from the suction side to the discharge side.

[0020] The oil supplied to the bearings and the like is discharged from the casing 12 and returned to the relatively low-pressure space of the screw rotor housing in the casing 12 via the return line 28 (see Figure 2).

[0021] Gas is drawn into the aforementioned tooth groove space from an intake space 50 formed within the casing 12 via an intake port 52. As the male rotor 15 and female rotor 17 rotate, the tooth groove space moves axially from the intake side to the discharge side in accordance with the rotation of these screw rotors. In this process, the volume of the tooth groove space decreases after the intake port 52 is closed, so the gas in the tooth groove space is compressed. When the tooth groove space reaches the discharge port 54 and communicates with the discharge space (not shown) formed in the casing 12, the compressed gas in the tooth groove space is discharged into the discharge space. The discharge port 54 is formed by an opening provided on the end face of the casing 12 and includes an axial discharge port for discharging the compressed gas from the tooth groove space in the axial direction. The discharge port 54 may also include a radial discharge port for discharging the compressed gas from the tooth groove space in the radial direction.

[0022] In some embodiments, the screw compressor 2 may be equipped with a slide valve (not shown) for adjusting the volume. In a screw compressor 2 equipped with a slide valve, the airflow can be adjusted by moving the slide valve, which together with the casing 12 to form a rotor housing chamber, in the axial direction, thereby bypassing the gas drawn into the tooth groove space through the opening formed between the casing 12 and the slide valve to the intake side.

[0023] As shown in Figure 3, the casing 12 includes bearing casings 12A and 12B that house the bearings, and a rotor casing 12C that houses a pair of screw rotors (male rotor 15 and female rotor 17). The bearing casings 12A and 12B are provided on both sides of the rotor casing 12C in the axial direction.

[0024] As shown in Figure 3, the pair of radial bearings 18A and 18B may be housed in bearing casings 12A and 12B, respectively. The thrust bearing 20 may be housed in one of the two bearing casings 12A and 12B (bearing casing 12B in Figure 3). In the illustrated embodiment, an intake port 24 for drawing gas into the screw compressor 2 is connected to the bearing casing 12A.

[0025] The rotor casing 12C is provided with a bore 13 that houses a pair of screw rotors. As shown in Figure 4, in a cross-section perpendicular to the axial direction of the screw compressor 2, the contour of the bore 13 has a shape in which an arc C1 corresponding to the male rotor 15 and an arc C2 corresponding to the female rotor 17 are connected at points P0 and P1, which are the intersections of the circle containing arc C1 and the circle containing arc C2. Point P0 is located on the same side as the discharge port 54 with respect to the center Q1 of arc C1 and the center Q2 of arc C2 in the second direction, which will be described later, and point P1 is located on the opposite side of the discharge port 54 with respect to the center Q1 of arc C1 and the center Q2 of arc C2 in the second direction.

[0026] Here, within a cross-section perpendicular to the axial direction, the direction connecting the center O1 of the bearings supporting the male rotor 15 (radial bearings 18A, 18B) and the center O2 of the bearings supporting the female rotor 17 (radial bearings 18A, 18B) is defined as the first direction, and the direction perpendicular to the first direction is defined as the second direction. Typically, the first direction is the horizontal direction, and the second direction is the vertical direction.

[0027] In Figure 4, the positions of the male rotor 15 and female rotor 17 are shown by dashed lines when their centers coincide with the centers O1 and O2 of their respective bearings.

[0028] The rotor casing 12C may have a pin hole 27 (see Figure 4) into which a positioning pin 26 (see Figure 3) is inserted. The pin 26 may be arranged such that one end is inserted into the pin hole 27 of the rotor casing 12C and the other end is inserted into the pin hole of the bearing casings 12A and 12B. In this way, the position of the rotor casing 12C relative to the bearing casings 12A and 12B in a second direction (typically the vertical direction) may be determined by the pin 26. Alternatively, the position of the rotor casing 12C relative to the bearing casings 12A and 12B in a first direction (typically the horizontal direction) may be determined by the pin 26.

[0029] In some embodiments, as shown in Figure 4, for example, in a cross-section perpendicular to the axial direction, the centers Q1 and Q2 of a pair of circular arcs C1 and C2 corresponding to a pair of screw rotors (male rotor 15 and female rotor 17) in the contour of the bore 13 (hereinafter also referred to as the centers Q1 and Q2 of the bore 13) are offset from the centers O1 and O2 of the bearings corresponding to each screw rotor, on the opposite side from the discharge port 54, with respect to the centers O1 and O2 of the bearings.

[0030] In other words, the centers Q1 and Q2 of the bore 13 are located on the opposite side of the discharge port 54 from the bearing centers O1 and O2 in both the first and second directions. Alternatively, the centers Q1 and Q2 of the bore 13 are located on the opposite side of the aforementioned point P0 from the bearing centers O1 and O2 in both the first and second directions.

[0031] During operation of the screw compressor 2, a load acts on the screw rotor (male rotor 15 and female rotor 17) in a direction from the relatively high-pressure side near the discharge port 54 to the relatively low-pressure side farther from the discharge port 54 (i.e., in the direction from the discharge port 54 (or point P0) toward the bearing centers O1, O2). As a result, the screw rotor deflects in the direction of this load. According to the configuration of the above embodiment, the centers Q1 and Q2 of the bore 13 are offset to the opposite side of the discharge port 54, with the bearing centers O1 and O2 in between. Therefore, during operation, contact between the screw rotor and the casing 12 in the region on the deflection side of the screw rotor (the region on the opposite side of the discharge port 54, with the bearing centers O1 and O2 in between) can be suppressed, while the radial clearance in the region opposite to the deflection direction (the region on the discharge port 54 side) can be reduced. This reduces the leakage of compressed gas from the high-pressure compression chamber (tooth groove space) to the lower-pressure compression chamber through the clearance. Therefore, it is possible to suppress the increase in shaft power and suppress the decrease in efficiency of the screw compressor 2.

[0032] Furthermore, the configuration according to the above embodiment can be realized simply by changing the positions of the bore centers Q1 and Q2 relative to the bearing centers O1 and O2, compared to a conventional screw compressor design where the centers Q1 and Q2 of the bore 13 coincide with the centers O1 and O2 of the bearings. Therefore, it is possible to suppress an increase in shaft power and a decrease in the efficiency of the screw compressor 2 at a low cost.

[0033] In some embodiments, the ratio L1 / (D2-D1) of the distance L1 between the bearing centers O1, O2 and the bore centers Q1, Q2 to the difference (D2-D1) between the diameter D2 (corresponding to the bore diameter) of a pair of circular arcs C1, C2 corresponding to a pair of screw rotors (male rotor 15 and female rotor 17) in the contour of the bore 13 and the outer diameter D1 of the screw rotor is 2% or more and 20% or less.

[0034] In some embodiments, the ratio L1 / (D2-D1) may be 4% or more and 20% or less. In some embodiments, the ratio L1 / (D2-D1) may be 4% or more and 10% or less. In some embodiments, the ratio L1 / (D2-D1) may be 2% or more and 10% or less.

[0035] In the above embodiment, the ratio L1 / (D2-D1) of the distance L1 between the bearing centers O1, O2 and the bore centers Q1, Q2 to the difference (D2-D1) between the diameter D2 of the arcs C1, C2 corresponding to the screw rotor in the contour of the bore 13 and the outer diameter D1 of the screw rotor is 2% or 4% or more. Therefore, during operation, contact between the screw rotor and the casing 12 in the region on the deflection direction side of the screw rotor can be effectively suppressed while reducing the radial clearance in the region opposite to the deflection direction. Furthermore, since the above ratio L1 / (D2-D1) is 20% or less or 10% or less, when stopped and no load is generated due to the pressure difference during operation, clearance between the screw rotor and the casing 12 in the region opposite to the deflection direction (the direction in which the screw rotor deflects during operation) can be secured, thereby suppressing contact between the two. Therefore, contact between the screw rotor and the casing 12 during stopping can be effectively suppressed, while leakage of the compressed gas through the clearance during operation can be effectively reduced.

[0036] The value of the ratio L1 / (D2 - D1) described above can be determined as follows. FIG. 5 is a diagram for explaining how to determine the ratio L1 / (D2 - D1). Here, the amount of expansion of the rotor due to thermal expansion (per radius) assumed during operation is L ＴＥ , the amount of deflection of the screw rotor is L Ｄ , the assembly tolerance of the screw rotor (per radius) is L ＡＴ , the bearing clearance (per radius) is L ＰＢ , and each is defined as such.

[0037] During the operation of the screw compressor 2, the diameter D2' of the arc C0 showing the contour of the bore 13 set in consideration of the thermal expansion, assembly tolerance, and bearing clearance of the screw rotor can be expressed by the following formula (A). D2' = D1 + (L ＴＥ + L ＡＴ + L ＰＢ ) × 2... (A) In the above formula (A), L ＴＥ is the amount of thermal expansion of the screw rotor (length per radius), L ＡＴ is the assembly tolerance (length per radius), and L ＰＢ is the bearing clearance (length per radius). Note that the center of the arc C0 coincides with the centers O1, O2 of the bearings.

[0038] Next, considering the amount of deflection (the moving distance of the screw rotor) L Ｄ at the center of the screw rotor, the arc C0 is shifted by a distance L Ｄ to the opposite side of the discharge port across the centers O1, O2 of the bearings, and the arc C0' is obtained. The center of the arc C0' is the point R in the figure. The circles C1, C2 enclosing the arc C0 and the arc C0' thus obtained can be used as the contour of the bore 13 of the screw compressor 2 according to the above-described embodiment. Here, the diameter D2 of the bore 13 (the diameters of the arcs C1, C2) can be expressed by the following formula (B). D2 = D2' + L Ｄ ... (B)

[0039] Also, since the centers Q1, Q2 of the bore 13 (the centers of the arcs C1, C2) are the midpoints of the centers O1, O2 of the bearings and the point R, the distance L1 between the centers O1, O2 of the bearings and the centers Q1, Q2 of the bore can be expressed by the following formula (C). L1 = L Ｄ / 2 … (C)

[0040] Therefore, the ratio L1 / (D2 - D1) of the distance L1 between the centers O1, O2 of the bearings and the centers Q1, Q2 of the bores to the difference (D2 - D1) between the diameter D2 of the bore 13 (the diameters of C1 and C2) and the outer diameter D1 of the screw rotor can be expressed by the following formula (D) by transforming using the above formulas (A), (B), and (C).

[0041] Therefore, the amount of expansion L of the rotor due to thermal expansion assumed in an actual screw compressor ＴＥ , the amount of deflection L of the screw rotor Ｄ , the assembly tolerance L of the screw rotor ＡＴ , and the bearing clearance (per radius) L ＰＢ From the values of, an appropriate range of the above ratio L1 / (D2 - D1) can be set.

[0042] The outer diameters of the male rotor 15 and the female rotor 17 may be the same or different. Also, the above ratio L1 / (D2 - D1) can be determined for each screw rotor and may be the same or different between the male rotor 15 and the female rotor 17.

[0043] (Method for manufacturing a screw compressor) Next, a method for manufacturing the above-described screw compressor 2 will be described. FIG. 6 is a flowchart of a method for manufacturing a screw compressor according to an embodiment.

[0044] As shown in FIG. 6, in one embodiment, first, a casing 12, here particularly, a rotor casing 12C (a raw material before machining the bore 13, etc.) is created (S2).

[0045] Next, the bore 13 is machined into the rotor casing 12C (material) such that, in a cross section perpendicular to the axial direction, the centers Q1 and Q2 of a pair of circular arcs C1 and C2, which correspond to a pair of screw rotors in the contour of the bore 13, are offset from the bearing centers O1 and O2 to the opposite side of the discharge port 54 (S4). In step S4, the bore 13 is machined into the rotor casing 12C (material) such that, in the first direction (typically the horizontal direction) of each of the centers Q1 and Q2 of the pair of circular arcs C1 and C2, they are offset from the bearing centers O1 and O2 to the opposite side of the discharge port 54 (S4). In step S4, the bore 13 may be machined into the material of the rotor casing 12C by cutting or grinding.

[0046] Next, a pin hole 27 is machined into the rotor casing 12C into which a pin 26 (see Figure 3) for positioning the rotor casing 12C in the vertical direction is inserted (S6). In step S6, the pin hole 27 may be machined into the material of the rotor casing 12C by cutting or grinding. In step S6, the pin hole 27 is machined into the rotor casing 12C such that it is offset from the discharge port 54 in the second direction (typically the vertical direction) of the centers Q1 and Q2 of the pair of arcs C1 and C2, with the bearing centers O1 and O2 in between.

[0047] According to the method described above, since the bore 13 is machined into the rotor casing 12C (casing 12) before the pin hole 27 is machined, the machining position of the relatively small pin hole 27 can be adjusted and determined based on the position of the relatively large bore 13. Therefore, the position of the pin hole 27 is more likely to be as designed, and the vertical positioning accuracy of the casing 12 and bore 13 is improved. Thus, a screw compressor 2 can be obtained at low cost and more reliably, suppressing the increase in shaft power and suppressing the decrease in efficiency.

[0048] In some embodiments, step S6 may be performed before step S4. That is, a pin hole 27 into which the pin 26 described above is inserted is machined into the rotor casing 12C (S6), and then a bore 13 is machined into the rotor casing 12C (material) such that the centers Q1 and Q2 of a pair of circular arcs C1 and C2, which correspond to a pair of screw rotors in the contour of the bore 13, are offset from the bearing centers O1 and O2 to the opposite side of the discharge port 54, with respect to the bearing centers O1 and O2 (S4).

[0049] The contents described in each of the above embodiments can be understood, for example, as follows:

[0050] [1] A screw compressor (2) according to at least one embodiment of the present invention comprises: a pair of screw rotors (male rotor 15 and female rotor 17); a pair of bearings (18A, 18B) for rotatably supporting the pair of screw rotors, and a casing (2) including a bore (13) in which the pair of screw rotors are housed, wherein, in a cross section perpendicular to the axial direction, the centers (Q1, Q2) of a pair of circular arcs (C1, C2) in the contour of the bore that correspond to the pair of screw rotors are offset from the center of the bearing, on the opposite side from the discharge port (54) with respect to the center of the bearing.

[0051] During operation of a screw compressor, a load acts on the screw rotor in a direction from the relatively high-pressure side near the discharge port to the relatively low-pressure side far from the discharge port, causing the screw rotor to deflect in the direction of this load. According to the configuration in [1] above, the center of the arc corresponding to the screw rotor within the bore contour (hereinafter also referred to as the bore center) is offset to the opposite side of the discharge port, straddling the bearing center. Therefore, during operation, contact between the screw rotor and the casing in the region on the deflection side of the screw rotor (the region on the opposite side of the discharge port, straddling the bearing center) can be suppressed, while the radial clearance in the region opposite to the deflection direction (the region on the discharge port side) can be reduced. This reduces the leakage of compressed gas from the high-pressure compression chamber to the lower-pressure compression chamber through the clearance. Thus, an increase in shaft power can be suppressed, and a decrease in the efficiency of the screw compressor can be suppressed. Furthermore, the configuration in [1] above can be realized simply by changing the position of the bore center relative to the bearing center, compared to a conventional screw compressor design where the bore center and the bearing center coincide. Therefore, it is possible to reduce costs, suppress the increase in shaft power, and suppress the decrease in efficiency of the screw compressor.

[0052] [2] In some embodiments, in the configuration of [1] above, the ratio of the distance (L1) between the centers of the pair of bearings and the centers of the pair of arcs to the difference (D2-D1) between the diameter (D2) of the pair of arcs and the outer diameter (D1) of the screw rotor is 2% or more and 20% or less.

[0053] In the configuration of [2] above, the ratio of the distance between the center of the bearing and the center of the bore to the difference between the diameter of the arc corresponding to the screw rotor in the bore contour (hereinafter also referred to as the bore diameter) and the outer diameter of the screw rotor is 2% or more. Therefore, during operation, contact between the screw rotor and the casing in the region on the side of the deflection direction of the screw rotor can be effectively suppressed, while the radial clearance in the region opposite to the deflection direction can be reduced. Furthermore, in the configuration of [2] above, since the above ratio is 20% or less, when stopped, when no load is generated due to the pressure difference during operation, clearance between the screw rotor and the casing in the region opposite to the deflection direction (the direction in which the screw rotor deflects during operation) can be secured, thereby suppressing contact between the two. Thus, according to the configuration of [2] above, contact between the screw rotor and the casing when stopped can be effectively suppressed, while leakage of the compressed gas through the clearance during operation can be effectively reduced.

[0054] [3] In some embodiments, in the configuration of [2] above, the ratio of the distance between the centers of the pair of bearings and the centers of the pair of arcs to the difference between the diameter of the pair of arcs and the outer diameter of the screw rotor is 4% or more and 20% or less.

[0055] In the configuration of [3] above, since the above ratio is 4% or more, during operation, contact between the screw rotor and the casing in the region on the deflection direction side of the screw rotor can be more effectively suppressed while reducing the radial clearance in the region opposite to the deflection direction. Furthermore, in the configuration of [3] above, since the above ratio is 20% or less, at the time of stopping when no load is generated due to the pressure difference during operation, clearance between the screw rotor and the casing in the region opposite to the deflection direction (the direction in which the screw rotor deflects during operation) can be secured, thereby suppressing contact between the two. Therefore, according to the configuration of [3] above, contact between the screw rotor and the casing at the time of stopping can be effectively suppressed while more effectively reducing leakage of the compressed gas through the clearance during operation.

[0056] [4] In some embodiments, in the configuration of [3] above, the ratio of the distance between the centers of the pair of bearings and the centers of the pair of arcs to the difference between the diameter of the pair of arcs and the outer diameter of the screw rotor is 4% or more and 10% or less.

[0057] In the configuration of [4] above, since the above ratio is 4% or more, during operation, contact between the screw rotor and the casing in the region on the deflection direction side of the screw rotor can be more effectively suppressed while reducing the radial clearance in the region opposite to the deflection direction. Also, in the configuration of [4] above, since the above ratio is 10% or less, during stopping when no load is generated due to the pressure difference during operation, clearance between the screw rotor and the casing in the region opposite to the deflection direction (the direction in which the screw rotor deflects during operation) can be secured, thereby more reliably suppressing contact between the two. Therefore, according to the configuration of [4] above, contact between the screw rotor and the casing during stopping can be more effectively suppressed while more effectively reducing leakage of compressed gas through the clearance during operation.

[0058] [5] A method for manufacturing a screw compressor (2) according to at least one embodiment of the present invention is a method for manufacturing a screw compressor comprising: a pair of screw rotors (male rotor 15 and female rotor 17); a pair of bearings (18A, 18B) for rotatably supporting the pair of screw rotors, and a casing (2) including a bore (13) in which the pair of screw rotors are housed, the method comprising the step (S4) of machining the bore in the casing such that, in a cross section perpendicular to the axial direction, the centers (Q1, Q2) of a pair of circular arcs (C1, C2) in the contour of the bore that correspond to the pair of screw rotors are offset from the centers (O1, O2) of the bearings to the opposite side from the discharge port (54) with respect to the centers of the bearings.

[0059] According to the method described in [5] above, a bore having a simple arc-shaped contour in a cross section perpendicular to the axial direction is machined into the casing, making it relatively easy to obtain a screw compressor having the configuration described in [1] above. Therefore, a screw compressor can be obtained at low cost, while suppressing an increase in shaft power and a decrease in efficiency.

[0060] [6] In some embodiments, the method of [5] above includes the step (S6) of machining a pin hole (27) in the casing after machining the bore in the casing, into which a pin (26) for positioning the casing in the vertical direction is inserted.

[0061] According to the method described in [6] above, since the bore is machined into the casing before the pin holes are machined, the machining position of the relatively small pin holes can be adjusted and determined based on the position of the relatively large bore. Therefore, the position of the pin holes is more likely to be as designed, and the positioning accuracy of the casing and bore in the vertical direction is improved. Thus, a screw compressor can be obtained that suppresses the increase in shaft power and the decrease in efficiency at a low cost and with greater reliability.

[0062] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and also includes modified forms of the embodiments described above, as well as forms that combine these forms as appropriate.

[0063] In this specification, expressions describing relative or absolute arrangements such as "in a certain direction," "along a certain direction," "parallel," "orthogonal," "center," "concentric," or "coaxial" shall not only describe such arrangements strictly, but also describe states of relative displacement with tolerances or angles or distances sufficient to achieve the same function. For example, expressions describing things being in an equal state such as "identical," "equal," and "homogeneous" shall not only describe states of being strictly equal, but also describe states where tolerances or differences exist to the extent that the same function is achieved. Furthermore, in this specification, expressions describing shapes such as quadrilaterals or cylindrical shapes shall not only describe geometrically precise quadrilaterals or cylindrical shapes, but also describe shapes including concave and concave parts, chamfered parts, etc., to the extent that the same effect is achieved. In addition, in this specification, expressions such as "equipment," "includes," or "possesses" a component are not exclusive expressions that exclude the existence of other components.

[0064] 1 Gas compression equipment 2 Screw compressor 4 Oil separator 6 Cooler 8 Pump 10 Oil supply line 10' Oil supply line 12 Casing 12A Bearing casing 12B Bearing casing 12C Rotor casing 13 Bore 14 Rotor shaft 15 Male rotor 16 Rotor shaft 17 Female rotor 18A Radial bearing 18B Radial bearing 20 Thrust bearing 24 Inlet 26 Pin 27 Pin hole 28 Return line 50 Inlet space 52 Inlet port 54 Discharge port O1 Bearing center O2 Bearing center Q1 Bore center Q2 Bore center

Claims

1. A screw compressor comprising: a pair of screw rotors; a pair of bearings for rotatably supporting each of the pair of screw rotors; and a casing including a bore in which the pair of screw rotors are housed, wherein, in a cross section perpendicular to the axial direction, the centers of each of a pair of arcs in the contour of the bore corresponding to the pair of screw rotors are offset from the center of the bearing, on the opposite side of the bearing from the discharge port.

2. The screw compressor according to claim 1, wherein the ratio of the distance between the centers of the pair of bearings and the centers of the pair of arcs to the difference between the diameter of the pair of arcs and the outer diameter of the screw rotor is 2% or more and 20% or less.

3. The screw compressor according to claim 2, wherein the ratio of the distance between the centers of the pair of bearings and the centers of the pair of arcs to the difference between the diameter of the pair of arcs and the outer diameter of the screw rotor is 4% or more and 20% or less.

4. The screw compressor according to claim 3, wherein the ratio of the distance between the centers of the pair of bearings and the centers of the pair of arcs to the difference between the diameter of the pair of arcs and the outer diameter of the screw rotor is 4% or more and 10% or less.

5. A method for manufacturing a screw compressor, comprising a pair of screw rotors, a pair of bearings for rotatably supporting the pair of screw rotors, and a casing including a bore in which the pair of screw rotors are housed, the method comprising the step of machining the casing into a bore such that, in a cross section perpendicular to the axial direction, the centers of a pair of arcs in the contour of the bore, corresponding to the pair of screw rotors, are offset from the center of the bearings, on the opposite side of the center of the bearings from the discharge port.

6. The method for manufacturing a screw compressor according to claim 5, further comprising the step of machining the bore in the casing, and then machining a pin hole in the casing into which a pin for positioning the casing in the vertical direction is inserted.

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