CA2626884C

CA2626884C - A liquid injection type screw compressor

Info

Publication number: CA2626884C
Application number: CA2626884A
Authority: CA
Inventors: Yoshifusa Kubota; Yoshiyuki Kobayashi; Shigeru Muta; Yasuaki Endo
Original assignee: Mayekawa Manufacturing Co
Current assignee: Mayekawa Manufacturing Co
Priority date: 2005-10-31
Filing date: 2005-10-31
Publication date: 2012-01-24
Anticipated expiration: 2025-10-31
Also published as: EP1944513A1; WO2007052332A1; JP4702639B2; CA2626884A1; US20080253914A1; JPWO2007052332A1; EP1944513A4

Abstract

A liquid injection type screw compressor in which, in a compression stroke of an working space formed by male and female rotors, liquids such as oil or water is prevented from leaking from the high pressure working space to a gas inlet side, suction resistance of gas sucked from the gas inlet to a rotor casing is reduced to improve volumetric efficiency, and shape forming of the casing is simplified. The liquid injection screw compressor has the male and female rotor pair of screw rotors, the rotor casing (1a, 1b) having a bore for receiving the rotors, a gas suction opening and a gas outlet that are provided in both end sections of the casing and communicate with the bore, and a lip section (4) projected from a bore surface (2) positioned more on the upstream side than a suction seal line (5) of the casing in order to prevent a back flow of the liquid from the bore surface toward the gas inlet side. The lip section (4) is positioned in a region surrounded by the suction seal line (5) and a line separated by a distance of one screw pitch of the rotors from the suction closure line (5) to the suction opening side.

Description

DESCRIPTION
A liquid injection type screw compressor BACKGROUND OF THE INVENTION
Field of the Invention [0001] The present invention relates to a liquid injection type screw compressor comprising a pair of male and female screw rotors that are installed in a space surrounded by a bore face within a casing of the compressor, while a liquid such as oil or water is injected to the bore face; whereby, a lip part is provided so as to prevent the liquid from flowing-back to a working gas inlet side, being placed on the bore face, within a range from a suction seal line (a suction closure line or a suction containment boundary locus) to a line parallel thereto apart from the suction seal line, by a distance equal to one screw pitch (a tooth groove distance in the rotor axis direction) of the rotors.

[0002] In hitherto known liquid injection type screw compressors, a pair of a male screw rotor and a female screw rotor within a casing of the compressor are engaged in each other, so as to form a working/operation space inside which a liquid such as oil or water is injected whereby a working substance of a gas-liquid mixing phase is pressurized. The liquid injection brings the screw compressor a cooling function, a sealing function, and a lubricating function; thus, the compressor of the type obtains high efficiency even during a low speed operation, becoming widespread in the industry.

[0003] The bore faces forming the working space within the casing of the compressor are important elements so as to secure gas/liquid tightness, when the working space is under a compression process where a gas seal across adjacent tooth spaces is required; consequently, it is a prerequisite to keep the clearance between the addendum circles of the rotors (tooth tip surfaces of the rotors) and the bore faces as small as possible. In this specification, the bore faces in the part as mentioned are called main bore faces.
On the other hand, a gas leakage between adjacent tooth spaces does not effect on the performance of the compressor when working spaces are under gas suction process; therefore, the bore faces in the associated part as mentioned are expanded toward outside in comparison with the main bore faces so that a power consumption is reduced by evading useless possible friction between the bore faces and the tooth tips; thus, the bore faces in the part as mentioned are called expanded bore faces.

[0004] In conventional liquid injection type screw compressors as described above, a weir (a lip part) is provided therein so as to prevent oil from scattering through a suction side end face of the rotor casing toward a gas inlet side; thus, it is intended to preserve the compressor volumetric efficiency and reduce the compressor power loss.
Conventional screw compressors with such a weir as mentioned are disclosed, for instance, in a patent literature 1 (JP patent: 1967-10027), a patent literature 2 (JP:
1991-194183) and a patent literature 3 (JP: 1999-13661).
[00051 The figures 1 and 2 in the patent literature 1 disclose that a lip (weir) 44 is provided between a gas inlet 24 and an expanded bore part 40 so as to lessen a heat exchange between a hot back-flow gas from a compression space formed in a rotor tooth space, and a flow-in gas from the gas inlet 24.
On the other hand, the figure 1 in the patent literature 2 discloses a lip (weir) 39 that is provided at a suction side end-face of a casing 3 so that the lip 39 prevents a back-flow oil from flowing from expanded bore parts 7 and 8 back to a gas inlet side, from warming-up an inhaled gas, and also from deteriorating a charging efficiency of the inhaled gas to be charged into a rotor teeth space.

[0006] Moreover, the patent literature 3 discloses that the oil injected into a working space flows back to a gas inhaling space;
thereby, an oil mist generated from the back-flow oil suspends in the gas inhaling space, while the oil mist heats up the inhaled gas under a suction process; namely, a phenomenon, what is called inhaled gas heating, occurs; thus, the phenomenon increases a temperature of the gas to be compressed as well as expands a volume thereof ; as a result, in a displacement type compressor that needs to inhale a gas of a constant specific volume, not only a reduction of mass-throughput but also a deterioration of volumetric efficiency are brought.
In order to evade the above-mentioned difficulties, according to the patent literature 3, as shown in Fig. 2 of the patent literature 3 a lip part 5 is provided at a suction side end-face of a casing 3 that accommodates the rotors, so that the lip part 5 protrudes inside, i.e. toward screw rotors;
further, a heat-up prevention wall (a baffle plate) 8 to close a gap between the screw rotors and the lip part 5 is provided so as to prevent an inhaled gas from leaking toward a gas inlet side.

[0007] Figs. 9a and 9b that are attached to this application shows a casing for conventional screw rotors; for explanatory convenience, Fig. 9a shows a divided upper half and Fig. 9b shows a divided lower half. In Figs. 9a and 9b, a space that accommodates a male rotor and a female rotor is formed inside the casing 01; thereby, the boundary of the space comprises:
a male rotor side main-bore-face 02a that faces a male rotor tooth tip with a slight clearance Al, a female rotor side main-bore-face 02b that faces a female rotor tooth tip with a slight clearance A2, a male rotor side expanded-bore-face 03a that faces a male rotor tooth space during a gas suction process, and a male rotor tooth tip with a clearance Bl greater than the mentioned clearance Al, and a female rotor side expanded-bore-face 03b that faces a female rotor tooth space during a gas suction process, and a female rotor tooth tip with a clearance B2 greater than the mentioned clearance A2.
Further, a lip part 04 is provided along a suction side end-face of a casing 01 so that the lip part 04 of the casing 01 protrudes inside, toward screw rotors; on the other hand, a suction seal line (a suction containment boundary locus) 05 is formed on a boundary between the male rotor side main-bore-face 02a and the male rotor side expanded-bore-face 03a as well as between the female rotor side main-bore-face 02b and the female rotor side expanded-bore-face 03b.

[0008] In the configuration as stated above, a working space is formed with a tooth space of the male rotor and another working space is formed with a tooth space of the female rotor, the pair of tooth spaces being independent; whereby, the tooth spaces are engraved on an outer periphery of rotors along a screw tooth spiral. While the working spaces are communicated with the expanded-bore-faces 03a or 03b, the working spaces are gradually expanded to a maximum volume, inhaling a gas through a gas inlet; then, the working spaces pass through the suction seal line (the suction containment boundary locus) 05, and the working spaces form a closed space the boundary of which includes a male rotor side main-bore-face 02a and a female rotor side main-bore-face 02b. Thus, after the working space becomes a closed space, the volume of the working spaces is gradually reduced and a confined gas within the space is compressed; at a last stage of compression, the gas inside the spaces is discharged through a discharge opening.
During the mentioned compression process, a liquid such as oil or water is injected into the working space, for the purpose of cooling, sealing, and lubricating.

[0009] Figs. l0a and 10b schematically depicts bore faces of a conventional screw rotor casing. Fig. 10a shows a t I

transparently perspective view seen from the top, depicting a suction seal line and a lip part. Fig. 10b is a development of Fig. 10a.
In Figs. 10a and 10b, the reference numeral Ola denotes a male rotor side casing, and the numeral Olb does a femamale rotor side casing Olb; a suction seal line 05 is formed on a boundary between a male rotor side main-bore-face 02a and the male rotor side expanded-bore-face 03a as well as between the female rotor side main-bore-face 02b and the female rotor side expanded-bore-face 03b; a lip part 04 of the casing 01 protrudes inside, toward screw rotors.

[0010]The Theworking (operatispaces formed with the male rotor and the female rotor face the male rotor side expanded-bore-face 03a and the female rotor side expanded-bore-face 03b, while the working spaces gradually increase during a suction process, inhaling a gas through a gas inlet. After the volume of the working spaces reaches a maximum volume and the working spaces cross the suction seal line 05, the spaces form a sealed space, being surrounded by the main-bore-faces 02a and 02b.
Subsequently, as the volume of the working spaces is reduced, the gas confined in the spaces is compressed. And the compressed gas is discharged through a discharge opening at a discharge side end face 07 of the rotor casing.
In the above situation, the liquid such as oil or water injected into the working spaces leaks toward a lower pressure suction side and accumulates in the concaved expanded bore faces 03a/03b. The lip part 04 prevents the liquid from leaking and scattering toward the gas suction end face 06 of the rotor casing.

[0011] Patent literatures:
Patent literature 1, JP 1967-10027;
Patent literature 2, JP 1991-194183;
Patent literature 3, JP 1999-13661.

Disclosure of the invention Problem to be solved by the invention [ 0012 ] In spite of the disclosure according to the configuration such as in the patent literature 3, the conventional technologies are insufficient in sealing a liquid within the working spaces as well as in preventing deterioration as to volumetric efficiency; the insufficiency is caused on the ground that great distances remain between rotor tooth tips and the lip part 04 (04a and 04b in Figs. 10a and lOb) placed at the suction side end face of the rotor casings.

[00131 As just mentioned above, the lip part is provided at the suction side end face of the rotor casings in conventional liquid injection screw compressors; thus, a gas inlet has to be placed outside across the suction side end face; moreover, the lip part lessens a gas inlet passage area (opening area) around the suction side end face of the rotor casings; therefore, in case of manufacturing a mono-block casting of the rotor casings and the gas inlet casing, it becomes difficult to allocate a casting core for rotor casing bores.
Further, in conventional ways, only by means of lengthening rotor space in the axial direction, it is possible to secure a sufficient gas inlet passage for inhaling a gas into the working space; in addition, insufficient inlet passage area enhances suction resistance during a high-speed operation.
Because of the above-mentioned situations, it is conventionally difficult to obtain a mono-block casting of the rotor casings and the gas inlet casing. That is, in casting, it is necessary to manufacture the rotor casing and the gas inlet casing separately; further, it becomes necessary to provide each casing with an essentially useless part such as an additional flange that is needed for assembling the parts. Thus, an increased whole weight and an intricate production process are brought; further, as mentioned above, in a high speed operation, there arise difficulties such as an increased gas suction resistance as well as a lessened volumetric efficiency.

[0014] In view of the mentioned subjects in conventional liquid injection type screw compressors, the goals of the present invention are:
preventing a liquid such as oil or water from leaking outside the working spaces of high compression, which are formed by screw rotors, toward the gas inlet side during a compression process, more effectively than conventional ways;
lessening a gas suction resistance of a gas flow into the rotor casings from an outside gas inlet so as to improve volumetric efficiency of the inhaled gas, as a result; and realizing an liquid injection type screw compressors of a simplified structure so as to bring manufacturing cost reduction.
Further, the invention aims at realizing a liquid injection type screw compressor provided with a variable compression ratio mechanism, that is, an internal volume ratio adjusting valve; wherein, the compressor has a compact structure so as to not prolong a manipulation mechanism of the internal volume ratio adjusting valve, making manufacturing cost be further reduced.

Means to solve the problem [0015] In order to attain the mentioned goals, the present invention proposes a liquid injection type screw compressor comprising of:
a pair of a male rotor and a female rotor, a rotor casing comprising a pair of bores that accommodate the pair of the male rotor and the female rotor, a gas inlet and a gas outlet that are connected to the pair of the bores, the gas inlet being provided at a first end part of the rotor casing, while the gas outlet being provided at a second end part of the rotor casing, and a lip part that is provided on a surface of the bores and protrudes inside so as to prevent the liquid on the surfaces of bores from back-flowing toward the gas inlet, the lip part , ' being located at a gas upstream side of a suction seal line of the rotor casing;
wherein the lip part is placed within a range between the suction seal line and a line that is apart from the suction seal line, by one screw pitch distance of the screw rotors, toward the expanded-bore-face side of the male rotor side casing and/or the female rotor side casing (namely, toward the gas inlet side).

[0016]In a screw compressor according to the present invention, a lip part for preventing liquid from back-flowing toward a gas inlet is provided on a casing bores, within a range between a suction seal line and a line that is apart from the suction seal line, by one screw pitch distance of the screw rotors, toward the expanded-bore-face side of the male rotor side casing and/or the female rotor side casing; hence, the lip part is placed nearer to the suction seal line in comparison with conventional ways ; as a result, a liquid leakage from the compressed working spaces toward the gas inlet side is effectively prevented;
further, the lip part placed nearer to the suction seal line makes it possible to eliminate a part of bore faces that is located at the gas inlet side from the lip part. So can be realized a simplified configuration of rotor casings with a reduced bore surface as well as a reduced suction resistance and a reduced manufacturing cost which are attributable to the simplification.

[ 0017 ] A preferable configuration of the present invention may comprise:
a straight development-line portion of the suction seal line in a development view, lying at right angles to a bore intersection line that is defined as a common generating line of a male rotor bore and a female rotor bore, a lip-entering-edge of the lip part that is placed apart from the suction seal line toward the gas inlet side in a rotor axis direction, whereby the lip-entering-edge in response to the above-mentioned straight-line portion is bent so as to protrudes toward the suction seal line, and a lip ending ( trailing ) edge of the lip part whereby the lip ending edge in response to the straight-line portion is placed parallel thereto so as to form a straight line portion of the lip ending edge in a development view, and a thickened (wide in the rotor axis direction) lip part in response to the straight-line portion.
The above preferable configuration can surely prevent a liquid leakage around a neighborhood along the bore intersection line.

[0018] According to a further preferable aspect of the above configuration, the straight line portion of the suction seal line in a development view lies at right angles to the bore intersection line, and starts from a cross-point of the bore intersection and the suction seal line on the male bore surface as far as a point on the suction seal line on the female bore surface, the lip-entering-edge of the lip part is placed apart from the suction seal line toward the gas inlet side in the rotor axis direction, and the lip-entering-edge in response to the above-mentioned straight-line portion is bent so as to protrudes toward the suction seal line; wherein, a part of the lip-entering-edge in response to the straight line portion starts from a cross-point of the bore intersection line and the lip-entering-edge on the male bore surface, as far as a point on the lip-entering-edge on the female bore surface, the lip ending ( trailing ) edge of the lip part in response to the straight-line portion is placed parallel thereto so as to form the straight line portion of the lip ending edge in a development view, and the thickened (wide in the rotor axis direction) lip part is provided in response to the straight-line portion, whereby a straight line portion of the ending edge starts from a cross-point of the bore intersection and the lip-ending-edge = ' on the male bore surface, as far as a point on the lip-ending-edge on the female bore surface.
The above configuration can surely prevent a liquid leakage around the bore intersection line.

[0019] Another preferable aspect of the invention according to the mentioned configuration is the liquid injection type screw compressor, wherein the rotor casing with the gas inlet casing is formed in one piece or a plurality of divided-pieces from the lip ending edge toward a gas downstream side. Thus, the gas inlet side bore surface of the rotor casing can be omitted in a way that the lip part is located nearer to the suction seal line; as a result, it becomes possible to form a gas inlet casing and a rotor casing in one body.
Consequently, the invention realizes a smaller rotor casing, saving an installation space; in addition, the invention greatly relieves restrictions concerning a position where a gas inlet casing is disposed in a rotor casing; in this regard, the degree of freedom as to the gas inlet casing design can be greatly expanded.

[ 0020 ] According to a further preferable aspect of the invention, a labyrinth structure is embodied on the inner surface which faces rotor tooth tips in the lip part. For example, a pertinent roughness of the surface (e. g. a pertinent casting surface roughness) or an intended uneven surface can realize lesser liquid leakage.
According to another preferable aspect of the invention, different outer diameters are applied to a pair of the male rotor and the female rotor so that the outer diameter of the male rotor is greater than that of the female rotor, and the number of the male rotor teeth is fewer than that of the male rotor teeth in a case when the same outer diameter is applied to a pair of the male rotor and the female rotor.
In this way, a screw pitch distance of the male rotor can be shortened and the lip part can be located nearer to the suction seal line; thus, the liquid leakage can be further surely prevented; in addition, the geometry of the casings can be simplified.

[0021] According to another preferable aspect of the invention, the lip part is provided at the lower side of the casing bore surfaces. In this configuration, the liquid accumulated by gravity at the bottom of the bore surfaces can be easily prevented from scattering toward the gas inlet side; thus, a simple structure can be realized.

[0022] Another preferable aspect of the invention according to the mentioned configuration is a liquid injection type screw compressor comprising:
a slide valve (capacity control valve), and an internal volume ratio (ui) adjusting valve;
whereby, the slide valve has a cut-out part, at a discharge end thereof, which regulates a gas discharge throat between a discharge end part (of gas discharge side) of the slide valve and an end face (of gas discharge side) of the rotor casing, and a valve driving rod (a pushrod) that is prolonged toward the gas inlet casing on a rotor end face so that the valve driving rod protrudes across the gas inlet casing, through a storage space of the internal volume ratio adjusting valve that is provided on a side of the gas inlet casing which comes in contact with the rotor end face, the valve driving rod being connected to a drive source i. e. a hydraulic cylinder in order that the slide valve can move forward and backward along an axis of the driving rod by means of the drive source and the driving rod, while the internal volume ratio ( v i) adjusting valve in the storage space is placed adjacent to a gas inlet side end face of the capacity control valve (a slide valve), with a positioning means that adjusts variably the position of the internal volume ratio ( U 2) adjusting valve, along a direction to/from a gas discharge side, wherein an internal volume ratio ( u i) is adjusted such that the positioning means shifts the internal volume ratio (Ui) adjusting valve to a predetermined position, while an internal gas capacity (that is equivalent to a gas density at a compression commencement) is adjusted by by-passing an inhaled gas back to the gas inlet side through a gap between the internal volume ratio (Ui) adjusting valve, and the capacity control valve (a slide valve) that slides to and fro along the driving rod direction by means of the drive source, through the driving rod.

[0023] The installation of an internal volume ratio ( Ui) adjusting valve makes a whole compressor compact, realizing a compressor of three kinds of compression ratios, namely, of lower/medium/higher compression ratios, without changing a gas inlet casing, only by replacing a rotor casing. Thus, a gas inlet casing can be applied to these kinds of compressors in common.
On the other hand, conventional compressors are apt to be of a large size, as a positioning means to position the internal volume ratio ( U i) adjusting valve is prolonged toward a gas discharge side, penetrating a gas discharging casing so as to be used for setting (positioning) the valve.
In order to solve the difficulty, according to a further preferable aspect of the present invention, the positioning means comprises:
a hollow shaft which is placed concentric to the driving rod, having a screw part on an outer surface of the hollow shaft so that the screw part is engaged into a corresponding screw part inside the internal volume ratio adjusting valve, and a rotation rod that is placed so as to intersect with the hollow shaft in order that the rod can transmit a rotational driving movement of the rod to the hollow shaf t, via a connection part; whereby, the rotational movement transmitted to the hollow shaft is transformed into a to-and-fro movement of the internal volume ratio adjusting valve, through the mentioned screw engagement, so that the internal volume ratio adjusting valve is positioned to a predetermined position.
According to the above aspect, there is no need to prolong a positioning means as in conventional approaches; a driving mechanism to position the internal volume ratio-adjusting valve can be formed as a compact one. The mentioned connection part between the hollow shaft and the rotation rod may be a bevel gear pair or a crossed helical gear pair.

Effect of the invention [0024] In a screw compressor according to the disclosed invention, a lip part for preventing liquid from back-flowing toward a suction inlet is provided on a casing bores, within a range between a suction seal line and a line that is apart from the suction seal line, by one screw pitch distance of the screw rotors, toward the expanded-bore-face side of the male rotor side casing and/or the female rotor side casing.
Therefore, a liquid leakage scattering during a compression process from the compressed working space formed by the screw rotors toward the gas inlet side can be effectively prevented.
Moreover, the lip part placed nearer to the suction seal line makes it possible to eliminate a part of the rotor casing at the gas inlet side from the lip part. So can be realized a simplified configuration of rotor casings with a reduced bore surface as well as a reduced suction resistance of an inhaled gas and an enhanced volumetric efficiency of the compressor.
[0025] Further, the lip part placed nearer to the suction seal line makes it possible to eliminate a part of the rotor bore faces at the gas inlet side from the lip part. Hence, the rotor casing can be formed in one body with a gas inlet casing. As a result, manufacturing processes can be simplified and a manufacturing cost can be reduced. Consequently, the disclosed invention greatly relieves restrictions regarding installation position of a gas inlet casing in a rotor casing.
Further, the degree of freedom as to the gas inlet casing design can be greatly expanded; in addition, a compact casing can be realized and a compressor installation space can be reduced.
[0026] Also as already explained, in a screw compressor according to the disclosed invention, the compressor comprises:
a straight development-line portion of the suction seal line in a development view, lying at right angles to a bore intersection line that is defined as a common generating line of a male rotor bore and a female rotor bore, a lip-entering-edge of the lip part that is placed apart from the suction seal line toward the gas inlet side in a rotor axis direction, whereby the lip-entering-edge in response to the above-mentioned straight-line portion is bent so as to protrudes toward the suction seal line, and a lip ending ( trailing ) edge of the lip part whereby the lip ending edge in response to the straight-line portion is placed parallel thereto so as to form a straight line portion of the lip ending edge in a development view, and a thickened (wide in the rotor axis direction) lip part in response to the straight-line portion.
Thus, the above configuration can surely prevent a liquid leakage around a neighborhood along the bore intersection line.
Further, in a screw compressor according to the disclosed invention, the compressor comprises:
the straight line portion of the suction seal line in a development view lies at right angles to the bore intersection line, and starts from a cross-point of the bore intersection and the suction seal line on the male bore surface, as far as a point on the suction seal line on the female bore surface, the lip-entering-edge of the lip part is placed apart from the suction seal line toward the gas inlet side in the rotor axis direction, and the lip-entering-edge in response to the above-mentioned straight-line portion is bent so as to protrudes toward the suction seal line; wherein, a part of the lip-entering-edge in response to the straight line portion starts from a cross-point of the bore intersection and the lip-entering-edge on the male bore surface, as far as a point on the lip-entering-edge on the female bore surface, the lip ending (trailing) edge of the lip part in response to the straight-line portion is placed parallel thereto so as to form the straight line portion of the lip ending edge in a development, and the thickened (wide in the rotor axis direction) lip part is provided in response to the straight-line portion, whereby a straight line portion of the ending edge starts from a cross-point of the bore intersection and the lip-ending-edge on the male bore surface, as far as a point on the lip-ending-edge on the female bore surface.
Thus, the above configuration can surely prevent a liquid leakage around a neighborhood along the bore intersection line.
[ 0027 ] Further, in a screw compressor according to the disclosed invention, different outer diameters are applied to a pair of the male rotor and the female rotor so that the outer diameter of the male rotor is greater than that of the female rotor, and the number of the male rotor teeth is fewer than that of the male rotor teeth in a case when the same outer diameter is applied to a pair of the male rotor and the female rotor.
In this manner, a screw pitch distance of the male rotor can be shortened and the lip part can be located nearer to the suction seal line; thus, the liquid leakage can be further surely prevented; in addition, the geometry of the casings can be simplified.

[0028]Further,in a screw compressor according to the disclosed invention, the compressor comprises:
a slide valve (capacity control valve), and an internal volume ratio (ui) adjusting valve;
whereby, the slide valve has a slide valve (capacity control valve), and an internal volume ratio (vi) adjusting valve;
whereby, the slide valve has a cut-out part, at a discharge end thereof, which regulates a gas discharge throat between a discharge end part (of gas discharge side) of the slide valve and an end face (of gas discharge side) of the rotor casing, and a valve driving rod (a pushrod) that is prolonged toward the gas inlet casing on a rotor end face so that the valve driving rod protrudes across the gas inlet casing, through a storage space of the internal volume ratio adjusting valve that is provided on a side of the gas inlet casing which comes in contact with the rotor end face, the valve driving rod being connected to a drive source in order that the slide valve can move forward and backward along an axis of the driving rod by means of the drive source and the driving rod, whereas the internal volume ratio adjusting valve in the storage space is placed adjacent to a gas inlet side end face of the capacity control valve (a slide valve), with a positioning means that adjusts variably the position of the internal volume ratio adjusting valve, along a direction to/from a gas discharge side, wherein an internal volume ratio is adjusted such that the positioning means shifts the internal volume ratio adjusting valve to a predetermined position, while an internal gas capacity is adjusted by by-passing an inhaled gas back to the gas inlet side through a gap between the internal volume ratio adjusting valve, and the capacity control valve (a slide valve) that slides to and fro along the driving rod direction by means of the drive source, through the driving rod.
According to the above disclosure, the installation of an internal volume ratio (vi) adjusting valve can make a whole compressor compact, realizing a compressor of three kinds of compression ratios, namely, of lower/medium/higher compression ratios, without changing a gas inlet casing, only by replacing a rotor casing. Thus, a gas inlet casing can be applied to these kinds of compressors in common.

[0029] Still further, in a screw compressor according to the disclosed invention, the compressor comprises a positioning meansfor positioning the internal volume ratio-adjusting valve, the positioning means comprising:
a hollow shaft which is placed concentric to the driving rod, having a screw part on an outer surface of the hollow shaft so that the screw part is engaged into a corresponding screw part inside the internal volume ratio adjusting valve, and a rotation rod that is placed so as to intersect with the hollow shaft in order that the rod can transmit a rotational driving movement of the rod to the hollow shaft , via a connection part; whereby, the rotational movement transmitted to the hollow shaft is transformed into a to-and-fro movement of the internal volume ratio adjusting valve, through the mentioned screw engagement, so that the internal volume ratio adjusting valve is positioned to a predetermined position.
According to the above aspect, there is no need to prolong a positioning means as in conventional approaches; a driving mechanism to position the internal volume ratio-adjusting valve can be formed as a compact one.

BRIEF DESCRIPTION OF THE DRAWINGS
(00301 The present invention will now be described in greater detail with reference to the preferred embodiments of the invention and the accompanying drawings, wherein:
Fig. la shows a transparently perspective view seen from a top as to a first embodiment of the present invention;
Fig. lb is a development view of Fig. la;
Fig. 2a shows a perspective view of an upper side rotor-casing seen from the inside thereof, as to a first embodiment;
Fig. 2b shows a perspective view of a lower side rotor-casing seen from the inside thereof, as to the first embodiment;
Fig. 3 shows a perspective view of a part of a rotor casing as to the first embodiment;
Fig. 4 shows a longitudinal plan view of a second embodiment of the present invention;
Fig. 5 shows a longitudinal section view concerning the second embodiment;
Fig. 6 explains a development view showing a suction seal line (a suction containment boundary locus) as to each of the male/female rotors that have different tip diameters;
Fig. 7 gives an explanation about the male/female rotors that have different tip diameters;
Fig. 8 shows a perspective view as to a variation of the second embodiment;
Fig. 9a shows a perspective view of an upper side rotor-casing seen from the inside thereof, as to a conventional compressor;
Fig. 9b shows a perspective view of a lower side rotor-casing seen from the inside thereof, as to a conventional compressor;
Fig. l0a shows a transparently perspective view seen from the top as to a conventional compressor in consideration of a schematic explanation for bore faces thereof; and Fig. 10b is a development view of Fig. 10a.
Reference Numerals [0031]
Ola, la, and lla a male rotor side casing;
Olb, 1b, and llb a female rotor side casing;
02a and 2a a male rotor side main-bore-face;
02b and 2b a female rotor side main-bore-face;
03a and 3a a male rotor side expanded-bore-face;
03b and 3b a female rotor side expanded-bore-face;
04, 4, and 33 a lip part;
4c and 33c a thickened lip part;
4d and 33d a lip entering edge;
4e and 33e a lip ending (trailing) edge 05, 5, and 32 a suction seal line (a suction containment boundary locus) 5c and 32c a straight line portion of a suction seal line in a development view 06 and 6 a suction side end face 07 and 7 a discharge side end face 08, 8, and 34 a bore intersection line (that is defined as a common generating line of a male rotor bore and a female rotor bore) 9 and 12 a gas inlet (a suction inlet) 11 a rotor casing 13 a gas inlet casing 14 a male rotor shaft 15 a female rotor shaft 16 and 19 a thrust bearing 17, 18, 20, and 21 a radial bearing (a journal bearing) 22 a male rotor 23 a female rotor 24 a mechanical seal 25 a discharge outlet (a gas outlet) 26 a gas outlet casing 27 and 36 a tightening bolt 28 a slide valve (a capacity control valve) 28a a cut-out part 29 a pushrod (a valve driving rod for manipulating a capacity control valve) 30 an oil-hydraulic cylinder (for driving a capacity /displacement-volume control valve) 31 an internal volume ratio (ui) control device of a manual operation type 35 a casing 37 an internal volume ratio ( ui) adjusting valve for adjusting compression ratio vi 38 a hollow shaft 39a and 39b a bevel gear (a movement communicating part) 40 a rotation-rod 41 a discharge opening "a" in Fig. 4 an intersection point of a suction seal line and a bore intersection line "b" in Fig. 4 an intersection point of a lip entering edge and a bore intersection line "c" in Fig. 4 an intersection point of a lip ending (trailing) edge and a bore intersection line DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] Hereafter, the present invention will be described in detail with reference to the embodiments shown in the figures.
However, the dimensions, materials, shape, the relative placement and so on of a component described in these embodiments shall not be construed as limiting the scope of the invention thereto, unless especially specific mention is placed.
Fig. la shows a transparently perspective view seen from the top as to a first embodiment of the present invention; Fig.
lb is a development view of Fig. la; Fig. 2a shows a perspective view of an upper side rotor-casing seen from the inside thereof, as to the first embodiment; Fig. 2b shows a perspective view of a lower side rotor-casing seen from the inside thereof, as to the first embodiment; Fig. 3 shows a perspective view of a part of a rotor casing as to the first embodiment; Fig. 4 shows a longitudinal plan view of a second embodiment of the present invention; Fig. 5 shows a longitudinal section view concerning the second embodiment; Fig. 6 explains a suction seal line (a suction containment boundary locus) as to each of the male/female rotors that have different tip diameters; Fig. 7 gives an explanation about the male/female rotors that have different tip diameters; Fig. 8 shows a perspective view as to a variation of the second embodiment;

[First embodiment]
[0033] Figs. la and lb schematically depict a bore face in a rotor casing of a screw compressor according to the present invention; Fig. la shows a perspective view as to a suction seal line (a suction containment boundary locus) and a lip part on the bore face seen transparently from a top; and, Fig. lb is a development view of Fig. la; in Figs. 2a and 2b, the rotor-casing is divided into an upper side part and a lower side part so that the bore face of the casing is easily explained.
In a male rotor side casing la and a female rotor side casing lb of Figs. la, 1b, 2a, 2b, and 3, a suction seal line (a suction containment boundary locus) 5 is formed on a boundary between main bore faces 2a/2b and expanded bore faces 3a/3b, whereby the main bore faces 2a/2b are located opposite to addendum circles of the a male rotor and a female rotor, with a slight clearance, while a lip part 4 as a protruding part is provided apart from the suction seal line 5, by a screw pitch distance to a suction side end face 6. Here, an example of dimension data is such that a clearance between the main bore faces 2a/2b and the addendum circles of the a male rotor and a female rotor is substantially 0.05 mm to 0.125 mm (a clearance to a diameter c/D = 0.8 to 1. 0/ 1000 ), while a distance between the expanded bore faces 3a/3b and the addendum circles of the rotors is substantially 5 mm (a clearance to a diameter c/D =
0.05 to 0.06). It is noted hereby that c and d denote a clearance and a diameter respectively.

[00341 The suction seal line 5 includes a curved part 5a that is on a main bore face of the male rotor side casing la, a curved part 5b that is on a main bore face of the female rotor side casing 1b, and a curved part 5c that is also on the main bore face of the female rotor side casing lb; whereby, in the development figures of Figs. la and lb, the curved part 5c is seen as a straight line which starts from a point "a" that is a cross point of the curved part 5a and a bore intersection line 8; further, in Figs. la and lb, the straight line lies at right angles to the bore intersection line 8, while the straight line ends a point where the line intersects with the curved part 5b.
On the other hand, in response to the suction seal line 5, the geometry of the lip part 4 comprises:
a lip-entering-edge 4d of the lip part 4c that is placed apart from the suction seal line, within one screw pitch distance, toward the gas inlet side along a rotor axis direction, whereby the lip-entering-edge in response to the above-mentioned straight-line 5c is bent so as to protrudes toward the suction seal line; wherein, a part of the lip-entering-edge in response to the straight line portion starts from a cross-point "b" of the bore intersection line 8 and the lip-entering-edge on the male bore surface, as far as a point on the lip-entering-edge on the female bore surface, a lip ending ( trailing ) edge 4e of the lip part 4c whereby the lip ending edge in response to the straight-line 5c is placed parallel thereto so as to form a straight line portion of the lip ending edge in a development view; wherein, a straight line portion of the ending edge starts from a cross-point "c" of the bore intersection line 8 and the lip-ending-edge on the male bore surface, as far as a point on the lip-ending-edge on the female bore surf ace , and a thickened (wide in the rotor axis direction) lip part 4c in response to the straight-line portion 5c.
Liquid such as oil or water injected into a compression working space is apt to leak toward a lower pressure suction side and accumulates in concaved expanded bore faces 3a/3b. The lip part 4 prevents the liquid from leaking and scattering toward a gas inlet side.

[0035] According to the first embodiment as described above, the distance between the suction seal line 5 and the lip part 4 is substantially within one screw pitch distance; thus, the lip part 4 is provided at a location closer to the suction seal line 5 in comparison with conventional ways. Therefore, in comparison with conventional ways, is effectively prevented a liquid leakage that scatters, during a compression process, from the compressed working space which is formed by the screw rotors toward the gas inlet side. Moreover, the lip part placed nearer to the suction seal line makes it possible to eliminate a part of the rotor casing, located at the gas inlet side from the lip part. So can be realized a simplified configuration of rotor casings with a reduced bore surface as well as a reduced suction resistance of an inhaled gas and an enhanced volumetric efficiency of the compressor.

[0036]Further, in the above embodiment, a straight line portion 5c of the suction seal line 5 is provided in the neighborhood of the bore intersection line, the line lying at right angles with the bore intersection line in a development view. In addition, a thickened lip part 4c is provided, comprising:
a lip-entering-edge 4d of the lip part 4c that is placed apart from the suction seal line, within one screw pitch distance, toward the gas inlet side in a rotor axis direction, wherein the lip-entering-edge in response to the above-mentioned straight-line portion 5c is bent so as to protrudes toward the suction seal line; and a lip ending (trailing) edge 4e of the lip part 4c, being placed parallel to the straight line portion 5c so as to form a straight line portion of the lip ending edge in a development view; wherein, the straight line portion of the ending edge is vertical to the bore intersection line 8 in a development.
In this manner, can be surely prevented a liquid leakage around the neighborhood along the bore intersection line, toward a gas inlet side, from the working (compression) spaces which are formed by the male female rotors.
Moreover, it becomes possible to eliminate a part from the lip part 4 toward a side of the gas inlet 9 in the rotor casing;
in addition, a simplified configuration of rotor casings can be realized. Further, since the gas inlet casing can be placed nearer to the rotor casing, the rotor casing can be formed in one body together with the gas inlet casing. As a result, manufacturing processes can be simplified and a manufacturing cost can be reduced.

[Second embodiment]

[0037] A second embodiment of the present invention is now detailed with reference to Figs. 4 to 8. As shown in Fig. 7, in the second embodiment a male rotor and a female rotor of different rotor sizes, namely different outer diameters, are used; where the outer diameter of the male rotor is larger than that of the female rotor, and the number of teeth as to the male rotor is 5, while that as to the female rotor is 6.
In Figs. 4 and 5, the reference numeral 11 denotes the rotor casing that accommodates both the male rotor and the female rotor, and the rotor casing 11 together with a gas inlet casing 13 that forms a gas inlet 12 is made of mono casting.
The rotor casing 11 accommodates the male rotor 22 and the female rotor 23 shown in Fig. 7, here the detail of the rotors is omitted.
The reference numeral 14 denotes a male rotor shaft that is supported by a thrust bearing 16 and radial bearings 17/18, while the numeral 15 denotes a female rotor shaft that is supported by a thrust bearing 19 and radial bearings 20/21.
[00381 A mechanical seal 24 is provided near a shaft end part 14a of the male rotor shaft 14, the shaft end part 14a being connected to an output shaft of a drive motor (not shown) as a power source.
A gas outlet casing 26 that forms a gas outlet 25 is made of casting; however, the casing 26 is made of different casting from the rotor casing 11, and the casing 26 is fastened thereto with tightening bolts 27. At a lower part of the rotor casing 11, is provided a slide valve (a capacity control valve) 28 that makes it possible to regulate a compressor capacity (an inhaled gas capacity) by means of sliding-manipulation along an axis direction of the rotors; thereby, a pushrod (a driving rod) 29 regulates a length as to the sliding-manipulation of the slide valve 28. In addition, the pushrod 29 is operated through an oil pressure that is supplied to a left cylinder room 30a and a right cylinder room 30b in an oil-hydraulic cylinder 30.
[00391 At the middle part of the pushrod 29, is installed a U

i-control device (an internal volume ratio control device) 31 of a manual operation type; hereupon, the device 31 makes it possible to optimize the internal volume ratio ui. A casing 35 that contains the vi-control device 31 is fastened to the gas inlet casing 13 with tightening bolts 36, while the oil-hydraulic cylinder 30 is fitted to the casing 35. The reference numeral 37 denotes an internal volume ratio (vi) adjusting valve; thereby, the ui-adjusting valve 37 is engaged into a screw part 38a that is provided on an outer face of a hollow shaft 38. Here, the hollow shaft 38 is installed around the pushrod 29 having a round cross-section, so that a round hollow cylinder of the hollow shaft 38 and the round cross-section of the pushrod 29 are concentric, and the hollow shaft 38 can rotate freely around the pushrod 29. Further, the ui-adjusting valve 37 moves along the rotor axes with a rotational movement of the hollow shaft 38.
On the other hand, the reference numeral 39a denotes a bevel gear that is fitted to a suction-side end part of the hollow shaft 38, while the bevel gear 39a is engaged in a corresponding bevel gear 39b that is fitted to an end part of a rotation-rod 40; hereupon, it is noted that the axes of the hollow shaft 38 and the rotation-rod 40 lie at right angles to each other.
[ 0040 ] According to the above-mentioned conf iguration, when the rotation-rod 40 is rotated, either clockwise or counterclockwise, a rotational movement is transmitted to the hollow shaft 38; as a result, the v i-adjusting valve 37 moves back and forth along an rotor axis, through an engagement of the screw part 38a and the uj-adjusting valve 37. A steering wheel (not shown) or the like may be fitted to the rotation-rod 40 so as to enable an operator to turn the wheel by hand in case of manual control.

When the internal volume ratio (vi) is adjusted, the following sequence of manipulations is performed: rotating the rotation-rod 40 under a stop condition of the compressor; making the U i-adjusting valve 37 move along an rotor axis; as a result, thrusting the slide valve 28 toward the gas outlet 25; adjusting an opening level of a discharge opening 41 that is formed between a cut-out part 28a provided at a discharge-front end side of the slide valve 28, and the gas outlet casing 26; thus, initializing the internal volume ratio (vi).

[00411 In addition, when a capacity of the compressor needs to be adjusted, the slide valve 28 is shifted along the axes of the rotors through a movement of the pushrod 29; thereby, a by-passed gas flow toward the gas inlet side from a gap between the slide valve 28 and the pushrod 29 controls the capacity (the inhaled gas flow quantity).
Fig. 6 shows the male rotor 22, the female rotor 23, and the suction seal line 32 in the second embodiment, in which the male rotor 22 and the female rotor 23 of different rotor sizes, namely, different outer diameters, are used, where the outer diameter of the male rotor is larger than that of the female rotor; in addition, the number of teeth as to the male rotor is 5, while the number of teeth spaces as to the female rotor is 6.

[00421 Incidentally, in Fig. 4, are shown the suction seal line 32 and the lip part 33, for explanation use. Similar to Fig.
1 as to the first embodiment, the lip part 33 is provided apart from the suction seal line 5, by a screw pitch distance, toward the gas inlet side.
As shown in Figs. 4 and 6, the suction seal line 32 comprises:
a curved part 32a on the bore in the male rotor casing lla, a curved part 32b on the bore in the female rotor casing llb, and a straight line portion 32c in a development view whereby the straight part lies at right angles to the bore intersection line 34; wherein, the straight line starts from a cross-point "a" of the bore intersection line 34 and the curved part 32a, as far as a point on the curved part 32b on the bore in the female rotor casing lib.

[0043] Further, the lip part 33 comprises a male casing side lip part 33a on the bore in the male rotor casing lla, and a female casing side lip part 33b, while the boundary of the lip part 33 comprises a lip-entering-edge and a lip ending (trailing) edge; hereupon, the lip part 33 is away from the suction seal line, within a screw pitch distance.
Still further, the lip-ending-edge comprises:
a straight line portion 33e, in response to the straight line 32c of the suction seal line 32, and lying at right angles to the bore intersection line 34 in a development view, while the line portion 33e starts a cross-point "c" of the bore intersection line 8 and the lip-ending-edge on the bore in the male rotor casing lla, as far as a point of the lip-ending-edge on the bore in the female rotor casing 11b; in addition, the lip-entering-edge comprises:
a bent curve portion 33d, in response to the straight line portion 32c of the suction seal line 32, whereby the bent curve portion 33d protrudes toward the straight line portion 32c of the suction seal line 32, while the bent curve portion 33d starts a cross-point "b" of the bore intersection line 8 and the lip-entering-edge on the bore in the male rotor casing lla, as far as a point of the lip-entering-edge on the bore in the female rotor casing llb.
In the above-mentioned manner, a thickened lip part 33c is formed with the bent curve portion 33d and the line portion 33e, in response to the straight line portion 32c of the suction seal line 32. So can be surely prevented a liquid leakage around the neighborhood along the bore intersection line, toward a gas inlet side, from the working (compression) spaces in the rotor casing 11.

[0044] According to the second embodiment, in the same way as the first embodiment, the lip part 33 is provided apart from the suction seal line 5, by a screw pitch distance, toward the gas inlet side; thus, can be surely prevented a liquid leakage around the neighborhood along the bore intersection line, toward a gas inlet side, from the working (compression) spaces.
Further, since the lip part is provided around the neighborhood along the bore intersection line as mentioned above, a liquid leakage around the bore intersection line can be surely prevented.
Moreover, can be eliminated a part of the rotor casing on the gas inlet side of the lip part 33. Thus, can be secured a satisfactory space that communicates the bore faces of the rotor casing to the gas inlet 12, the space reducing a suction resistance of the gas inhaled from the gas inlet 12. As a result, a volumetric efficiency of the compressor can be enhanced.
[00451 On the other hand, by means of eliminating a part of the rotor casing the part which is located on the gas inlet side of the lip part, the location of the gas inlet 12 can be shifted toward the rotor casing 11. Thus, the rotor casing 11 together with a gas inlet casing 13 that forms a gas inlet 12 can be made of mono casting. In this way, a compact casing can be realized and a compressor installation space can be reduced.
Consequently, a compact casing can be realized, a compressor installation space can be reduced, a compressor manufacturing cost can be greatly lowered, and the degree of freedom as to the gas inlet casing design about the rotor axis direction can be expanded.
Further, it becomes possible to design a compressor casing of the same kind compressors so that a distance L between an axis "i" of the gas inlet 12 and an axis "o" of the gas outlet 25 can be kept constant. Therefore, a manufacturing line of the compressor casings can be streamlined so as to mechanized and robotized.

[00461 The installation of an internal volume ratio ( ui) adjusting valve makes a whole compressor compact, realizing a compressor with three kinds of compression ratios , namely, of lower/medium/higher compression ratios, without changing sorts of a gas inlet casing, only by replacing a rotor casing.
Thus, a gas inlet casing can be applied to these kinds of compressors in common. Namely, even if the rotor-casing is replaced by another one for constituting a different kind (capacity) of the compressor, it is not necessary to exchange the gas inlet casing.
Further, conventional compressors are apt to be of a large size, as a positioning means to position the internal volume ratio ( v i) adjusting valve is prolonged toward a gas discharge side, penetrating a gas outlet casing so as to be used for positioning the valve. Contrary to the above, the second embodiment comprises a positioning means for positioning the internal volume ratio-adjusting valve including:
a hollow shaft 38 , and a rotation rod 40 that is placed so as to intersect with the hollow shaft with right angles in order that the rod can transmit a rotational driving movement of the rod to the hollow shaft, via a bevel gear pair 39a/39b.
According to the above configuration, there is no need to prolong a positioning means as in conventional approaches; a driving mechanism to position the internal volume ratio-adjusting valve 37 can be formed as a compact one.
[0047] Incidentally, for the connection part between the rotation rod 40 and the hollow shaft 38, a crossed helical gear pair may be applied instead of a bevel gear pair; a bevel gear pair tends to have a play to some extent between meeting gears, while a crossed helical gear has little play.

Industrial Applicability [0048] According to the present invention regarding a liquid injection type screw compressor, a liquid leakage, such as an oil leakage or a water leakage, that flows back to a gas inlet side from a compression room which is formed screw rotors is further effectively prevented in comparison with conventional compressors of the same kind. In addition, suction resistance of the gas inhaled from the gas inlet can be lowered, and volumetric efficiency as to the inhaled gas can be improved;
further, cast modeling of a rotor casing can be simplified and a manufacturing cost can be reduced.
Thus, the invention greatly contributes to a practical compressor industry.

Claims

1. A liquid injection type screw compressor, comprising:
a pair of rotors including a male rotor and a female rotor, a rotor casing defining a pair of bores including a male rotor bore and a female rotor bore that accommodate the male rotor and the female rotor, a gas inlet and a gas outlet that are connected to the pair of the bores, the gas inlet being provided at a first end part of the rotor casing, while the gas outlet being provided at a second end part of the rotor casing, and a lip part that is provided on a surface of the bores and protrudes inside so as to prevent the liquid on the surfaces of the bores from back-flowing toward the gas inlet, wherein the lip part is placed in a range between a suction seal line and a line that is apart from the suction seal line within one screw pitch distance of the screw rotors, and wherein the lip part is arranged to face the suction seal line.

2. The liquid injection type screw compressor according to claim 1, whereby surfaces of the bores comprise a main bore face that comes in contact with rotor tooth tips with a slight fit tolerance, and an expanded bore face that faces rotor tooth tips with a clearance larger than the fit tolerance, during at least a gas suction process out of whole working processes;
wherein the suction seal line exists between the main bore face and the expanded bore face.

3. The liquid injection type screw compressor according to claim 1, the compressor further comprising:
a straight development-line portion of the suction seal line in a development view, lying at right angles to a bore intersection line that is defined as a common generating line of the male rotor bore and the female rotor bore, a lip-entering-edge of the lip part that is placed apart from the suction seal line toward the gas inlet side in a rotor axis direction, whereby the lip-entering-edge in response to the above-mentioned straight-line portion is bent so as to protrudes toward the suction seal line, and a lip ending edge of the lip part whereby the lip ending edge in response to the straight-line portion is placed parallel thereto so as to form a straight line portion of the lip ending edge in a development view, and a thickened lip part which is wide in the rotor axis direction in response to the straight-line portion.

4. The liquid injection type screw compressor according to claim 3, wherein the straight line portion of the suction seal line in a development view lies at right angles to the bore intersection line, and starts from a cross-point of the bore intersection and the suction seal line on the male bore surface as far as a point on the suction seal line on the female bore surface, the lip-entering-edge of the lip part is placed apart from the suction seal line toward the gas inlet side in the rotor axis direction, and the lip-entering-edge in response to the above-mentioned straight line portion is bent so as to protrudes toward the suction seal line; wherein, a part of the lip-entering-edge in response to the straight line portion starts from a cross-point of the bore intersection line and the lip-entering-edge on the male bore surface, as far as a point on the lip-entering-edge on the female bore surface, the lip ending edge of the lip part in response to the straight-line portion is placed parallel thereto so as to form the straight line portion of the lip ending edge in a development view; wherein a straight line portion of the ending edge starts from a cross-point of the bore intersection line and the lip-ending-edge on the male bore surface, as far as a point on the lip-ending-edge on the female bore surface, and the thickened lip part in response to the straight-line portion is provided.

5. The liquid injection type screw compressor according to claim 1, wherein the rotor casing with the gas inlet casing is formed in one piece or a plurality of divided-pieces from the lip ending edge toward a gas downstream side.

6. The liquid injection type screw compressor according to claim 1, wherein a labyrinth structure is embodied on the inner surface which faces rotor tooth tips in the lip part.

7. The liquid injection type screw compressor according to claim 1, wherein different outer diameters are applied to a pair of the male rotor and the female rotor so that the outer diameter of the male rotor is greater than that of the female rotor, and the number of the male rotor teeth is fewer than that of the male rotor teeth in a case when the same outer diameter is applied to a pair of the male rotor and the female rotor.

8. The liquid injection type screw compressor according to claim 1, wherein the lip part is provided at the lower side of the casing bore surfaces.

9. The liquid injection type screw compressor according to claim 1, the compressor further comprising:
a slide valve acting as a capacity control valve, and an internal volume ratio adjusting valve;
whereby, the slide valve has a cut-out part, at a discharge end thereof, which regulates a gas discharge throat between a discharge end part of the slide valve and an end face of the rotor casing, and a valve driving rod that is prolonged toward the gas inlet casing on a rotor end face so that the valve driving rod protrudes across the gas inlet casing, through a storage space of the internal volume ratio adjusting valve that is provided on a side of the gas inlet casing which comes in contact with the rotor end face, the valve driving rod being connected to a drive source in order that the slide valve can move forward and backward along an axis of the driving rod by means of the drive source and the driving rod, whereas the internal volume ratio adjusting valve in the storage space is placed adjacent to a gas inlet side end face of the capacity control valve, with a positioning means that adjusts variably the position of the internal volume ratio adjusting valve, along a direction to/from a gas discharge side, wherein an internal volume ratio is adjusted such that the positioning means shifts the internal volume ratio adjusting valve to a predetermined position, while an internal gas capacity is adjusted by by-passing an inhaled gas back to the gas inlet side through a gap between the internal volume ratio adjusting valve, and the capacity control valve that slides to and fro along the driving rod direction by means of the drive source, through the driving rod.

10. The liquid injection type screw compressor according to claim 9, wherein the positioning means comprises:
a hollow shaft which is placed concentric to the driving rod, having a screw part on an outer surface of the hollow shaft so that the screw part is engaged into a corresponding screw part inside the internal volume ratio adjusting valve, and a rotation rod that is placed so as to intersect with the hollow shaft in order that the rod can transmit a rotational driving movement of the rod to the hollow shaft, via a connection part; whereby, the rotational movement transmitted to the hollow shaft is transformed into a to-and-fro movement of the internal volume ratio adjusting valve, through the mentioned screw engagement, so that the internal volume ratio adjusting valve is positioned to a predetermined position.

11. The liquid injection type screw compressor according to claim 10, whereby the connection part between the hollow shaft and the rotation rod is formed with a bevel gear pair or a crossed helical gear pair.

12. The liquid injection type screw compressor according to any one of claims 1, 2 and 5 to 11, wherein the lip part has a curved line that extends along an inner wall of the rotor casing from a point of a bore intersection line that is defined as a common generating line of the male rotor bore and the female rotor bore.

13. The liquid injection type screw compressor according to claim 12, wherein the lip part has a thickened section formed by protruding convexly towards the gas outlet from the point of the bore intersection line.

14. The liquid injection type screw compressor according to any one of claims to 13, wherein the lip part is arranged along with the suction seal line.

15. The liquid injection type screw compressor according to claim 1, wherein the lip part is arranged in a curved line and extends from an inner wall of the casing which surrounds a periphery of the male and female rotors so as to extend from a cross-point with a bore intersection line of the mail and female rotor bores to a suction side of the male and female rotors.

16. The liquid injection type screw compressor according to claim 11, wherein the lip part is protruded convexly in a curved line at a discharge side from a cross-point with a bore intersection line of the mail and female rotor bores.