Patent Publication Number: US-10309402-B2

Title: Oil-free screw compressor having atmosphere open hole formed in casing and communication hole formed in shaft seal device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national phase application in the United States of International Patent Application No. PCT/JP2015/076916 with an international filing date of Sep. 24, 2015, which claims priority of Japanese Patent Application No. 2014-198966 filed on Sep. 29, 2014 the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an oil-free screw compressor. 
     BACKGROUND ART 
     In the oil-free screw compressor, air is compressed by a pair of male and female screw rotors which is rotatable in an oil no-supply state and in a non-contact state. In the oil-free screw compressor, there may be a case where compressed air produced in a rotor chamber is leaked along a rotary shaft or a lubricant supplied to a gear which drives the rotary shaft or a bearing which supports the rotary shaft flows into the rotor chamber. To prevent such drawbacks, a shaft seal device is disposed between the rotor chamber and the bearing. The shaft seal device includes: an air seal portion which seals compressed air from the rotor chamber; and an oil seal portion which seals a lubricant from the bearing. 
     When the rotor chamber is brought into a negative pressure state during an unloading operation, there may be a case where a lubricant which is supplied to the bearing or the like flows into the inside of the rotor chamber after passing through the oil seal portion although an amount of the lubricant is insignificant. To prevent such flowing in of the lubricant into the inside of the rotor chamber, an atmosphere open passage is provided for making an air ventilation gap formed on a rotor-chamber-side end portion of the oil seal portion and an atmosphere side of a casing communicate with each other. When the rotor chamber is brought into a negative pressure state, atmospheric air is introduced into the ventilation gap through the atmosphere open passage thus preventing the lubricant from flowing into the rotor chamber. 
     The oil-free screw compressor provided with the above-mentioned shaft seal device is disclosed in JP 2011-256828A and JP 59-51190A, for example. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In an oil-free screw compressor disclosed in JP 2011-256828A, a seal box portion formed between an air seal and a visco-seal or a communication hole of a seal box communicates with an atmosphere open hole formed in a casing. With such a configuration, the flowing in of a lubricant into a rotor chamber is prevented. In the oil-free screw compressor disclosed in JP 59-51190A, a plurality of small holes disposed between a fixed-type screw seal and a gas-use shaft seal device communicate with an atmosphere open hole formed in a casing. All of the atmosphere open holes, the communication holes and the small holes described in the above-mentioned two prior arts are formed for preventing a lubricant from flowing into a rotor chamber when the rotor chamber is brought into a negative pressure during an unloading operation. 
     However, in JP 2011-256828A, an annular space having a groove shape is formed on an outer peripheral surface of the shaft seal device and hence, when an open cross-sectional area of the annular space is small, a pressure loss is generated. Further, in JP 2011-256828A, an O-ring is disposed on both an air seal and a visco-seal of the shaft seal device, and the communication holes and the annular space having a groove shape are formed between such two O-rings. With such a configuration, a space in the shaft seal device in the axial direction is restricted by two O-rings and hence, it is difficult to properly ensure a width of the annular space in the axial direction. Accordingly, it is difficult to property cope with the lowering of a pressure loss with the use of the annular space formed in the shaft seal device disclosed in JP 2011-256828A. 
     In the same manner as JP 2011-256828A, also in JP 59-51190A, an annular space which is formed on an outer peripheral surface of a shaft portion is disclosed. In a cross-sectional view which shows the fixed-type screw seal in JP 59-51190A, a plurality of communication holes are formed in the fixed-type screw seal. In the fixed-type screw seal shown in the drawing, an open cross-sectional area of one communication hole is set substantially as large as an open cross-sectional area of the annular space, and a plurality of communication holes are formed. With such a configuration, a total open cross-sectional area of the communication holes becomes larger than the open cross-sectional area of the annular space by an amount corresponding to the number of communication holes approximately. Accordingly, also in JP 59-51190A, a large pressure loss is generated in the annular space rather than in the plurality of communication holes. 
     In this manner, in both of the oil-free screw compressors disclosed in JP 2011-256828A and JP 59-51190A, a large pressure loss is generated at the position of the annular space which makes the atmosphere open hole and the communication hole communicate with each other and hence, there is a possibility that an action of preventing flowing-in of a lubricant to the rotor chamber does not sufficiently function during an unloading operation. Nevertheless, neither one of JP 2011-256828A nor JP 59-51190A has taken this point into particular consideration. 
     Accordingly, it is an object of the present invention to provide an oil-free screw compressor which can prevent flowing-in of a lubricant during an unloading operation by reducing a pressure loss in an annular space which makes an atmosphere open hole and a communication hole communicate with each other as much as possible. 
     Means for Solving the Problems 
     To solve the above-mentioned technical problems, according to the present invention, an oil-free screw compressor having the following configurations is provided. 
     That is, an oil-free screw compressor includes: a pair of female and male screw rotors which meshes with each other in a non-contact manner; a casing having a rotor chamber in which the screw rotors are housed; a bearing which supports rotary shafts of the screw rotors; a shaft seal device which includes an oil seal portion disposed on a bearing side and an air seal portion disposed on a rotor chamber side and shaft-seals the rotary shafts; an atmosphere open hole which is formed in the casing; at least one communication hole which is formed in the shaft seal device in a state where the communication hole is positioned between the oil seal portion and the air seal portion; and an annular space which makes the atmosphere open hole and the at least one communication hole communicate with each other, wherein the annular space includes an inner peripheral annular space annularly formed on an inner peripheral side of the casing, and wherein assuming an open cross-sectional area of the inner peripheral annular space in a partial cross section taken along an axial direction of the rotary shaft as S 1 , assuming a total open cross-sectional areas of the communication holes as S 2 , and assuming an open cross-sectional area of the i-th communication hole out of the communication holes as S 2 i, and assuming the number of communication holes as n (n being a natural number of 1 or more), a following relationship is satisfied. 
     
       
         
           
             
               
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     With the above-mentioned configuration, the inner peripheral annular space which forms the annular space is formed on a casing side and hence, it is possible to alleviate the restriction imposed to ensure a width of the opening portion of the inner peripheral annular space in the axial direction of the rotary shaft. Accordingly, the open cross-sectional area S 1  of the inner peripheral annular space can be increased compared to the case where the annular space is formed on a shaft seal device side. As a result, the following relationship is easily satisfied. 
     
       
         
           
             
               
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     Accordingly, a pressure loss in the annular space can be lowered and hence, it is possible to prevent flowing-in of a lubricant during an unloading operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view showing the schematic configuration of an oil-free screw compressor according to a first embodiment of the present invention; 
         FIG. 2  is a partial cross-sectional view showing a shaft seal device and an area around the shaft seal device in the oil-free screw compressor shown in  FIG. 1 ; 
         FIG. 3  is a view for describing an inner peripheral annular space which makes an atmosphere open hole and communication holes communicate with each other; 
         FIG. 4  is a longitudinal cross-sectional view showing the schematic configuration of an oil-free screw compressor according to a second embodiment of the present invention; 
         FIG. 5  is a partial cross-sectional view showing a shaft seal device and an area around the shaft seal device in the oil-free screw compressor shown in  FIG. 4 ; 
         FIG. 6  is a view for describing an inner peripheral annular space and an outer peripheral annular space which make an atmosphere open hole and communication holes communicate with each other. 
     
    
    
     MODE FOR CARRYING OUT THE INVETION 
     An oil-free screw compressor  1  according to a first embodiment of the present invention is described with reference to  FIGS. 1 to 3 . First, the schematic configuration of the oil-free screw compressor  1  according to the first embodiment is described in detail with reference to  FIG. 1 . 
     In the oil-free screw compressor  1 , a pair of male and female screw rotors  16  which meshes with each other is housed in a rotor chamber  15  formed in a casing  12 . The casing  12  is, for example, formed of a casing body, a discharge-side casing portion, and a suction-side casing portion. 
     The casing  12  has: a suction port  17  through which air which is an object to be compressed is supplied to the rotor chamber  15 ; and a discharge port  18  through which compressed air compressed by the screw rotors  16  in the rotor chamber  15  is discharged. A rotary shaft  21  is formed on respective end portions of the screw rotor  16  on a discharge side and a suction side. A drive gear  28  and a timing gear  27 , which separate from each other, are mounted on the respective end portions of the rotary shafts  21  on the discharge side and the suction side. A rotational drive force of a motor not shown in the drawing is transmitted to one screw rotor  16  by way of the drive gear  28 . The rotational drive force transmitted to one screw rotor  16  is transmitted to the other screw rotor  16  by way of the timing gear  27 . Due to the rotation of the pair of screw rotors  16  in a non-contact state and also in a state where the screw rotors  16  mesh with each other, air is sucked in through the suction port  17 . Air sucked in through the suction port  17  is compressed to a predetermined pressure, and compressed air is discharged from the discharge port  18 . 
     On a discharge side of the casing  12 , a shaft seal device loading space  10  on a discharge side is formed. In the shaft seal device loading space  10  on a discharge side, ball bearings (angular ball bearings in two rows)  19  and a bearing (roller bearing)  22  which rotatably support the rotary shaft  21  on a discharge side and a shaft seal device  20  on a discharge side are loaded. Also on a suction side of the casing  12 , a shaft seal device loading space  10  on a suction side is formed. In the shaft seal device loading space  10  on a suction side, a bearing (roller bearing)  22  which rotatably supports the rotary shaft  21  on a suction side and a shaft seal device  20  on a suction side are loaded. 
     An atmosphere open hole  24   a which connects the outside (atmosphere side) and an inner peripheral side of the casing  12  and communicates with an atmosphere is formed in the casing  12 . An oil supply hole  26  for supplying a lubricant to the bearings  19 ,  22  and the timing gear  27  is formed in the casing  12 . 
     The shaft seal devices  20  loaded in the shaft seal device loading spaces  10  on a discharge side and a suction side are formed substantially in symmetry with respect to the rotor chamber  15 . Hereinafter, the shaft seal device  20  on a discharge side and an area around the shaft seal device  20  are described in detail with reference to  FIGS. 2 and 3 . 
       FIG. 2  is a partial cross-sectional view of the shaft seal device  20  on a discharge side and the area around the shaft seal device  20  in the oil-free screw compressor  1  shown in  FIG. 1 . 
     The bearing  22 , a first shaft seal portion  30  which seals a lubricant, and a second shaft seal portion  40  which seals compressed air are loaded in the shaft seal device loading space  10  in order from a bearing  22  side to a rotor chamber  15  side. An end portion of the bearing  22  loaded in the shaft seal device loading space  10  on a side opposite to the rotor chamber  15  is restricted by a stopper  29 . The first shaft seal portion  30  and the second shaft seal portion  40  are integrally connected to each other due to the fitting structure described later so that the shaft seal device  20  is formed. 
     To facilitate detachable assembling of the shaft seal device  20  in the shaft seal device loading space  10 , a clearance slightly larger than loose fit (JIS B 0401) is formed between the shaft seal device loading space  10  and the shaft seal device  20 . When a clearance slightly larger than a loose fit is formed, shaft seal ability is sacrificed. Accordingly, an O-ring  35  is disposed between an oil seal  31  and the casing  12  and an O-ring  46  is disposed between a packing case  41  and the casing  12 . As a matter of course, a size of the clearance is set within a range where the O-rings  35 ,  46  can exhibit shaft seal ability. It is preferable that the O-rings  35 ,  46  be disposed separately such that the O-ring  35  is disposed in a recessed portion (annular groove)  34  of the oil seal  31  and the O-ring  46  is disposed in a recessed portion (annular groove)  45  of the packing case  41 . The recessed portion (annular groove)  34  of the oil seal  31  and the recessed portion (annular groove)  45  of the packing case  41  are formed on outer peripheral surfaces of the oil seal  31  and the packing case  41  along a circumferential direction respectively. Due to the provision of the O-ring  35  of the oil seal  31  and the O-ring  46  of the packing case  41 , leakage of compressed air between the casing  12  and the first shaft seal portion  30  and leakage of compressed air between the casing  12  and the second shaft seal portion  40  can be prevented respectively. 
     In a portion of the casing  12  which is disposed between a position corresponding to the O-ring  35  and a position corresponding to the O-ring  46  and opposedly faces the oil seal  31 , the atmosphere open hole  24   a  is formed. The atmosphere open hole  24   a  penetrates the casing  12 , and makes the shaft seal device loading space  10  and the outside (atmosphere side) of the casing  12  communicate with each other. 
     The first shaft seal portion  30  is formed of the non-contact oil seal  31  having an oil seal portion  32 . The oil seal portion  32  is, for example, the visco-seal  32  where a spiral groove is formed on an inner peripheral surface of the oil seal  31 . When the rotary shaft  21  is rotated, the visco-seal  32  generates a pumping action due to viscosity of air existing between an inner peripheral surface of the visco-seal  32  and an outer peripheral surface of the rotary shaft  21 . Since a lubricant is pushed toward the bearing  22  due to a pumping action, the flowing out of the lubricant in a direction toward the rotor chamber  15  can be prevented. The spiral groove of the visco-seal  32  is not illustrated in the drawing. Since the spiral groove of the visco-seal  32  is formed on the inner peripheral surface of the oil seal  31 , the oil seal  31  is made of a metal material which enables easy cutting of the oil seal  31 . 
     On an end portion  36  of the oil seal  31  on a rotor chamber  15  side, a fitting projecting end portion  33  which projects toward a rotor chamber  15  side and has a cylindrical outer peripheral surface is formed. The fitting projecting end portion  33  is formed such that the fitting projecting end portion  33  is fitted in a fitting recessed end portion  44  of the packing case  41  described later by tight fit (JIS B 0401) or transition fit (JIS B 0401). The oil seal  31  and the packing case  41  are integrally connected to each other by the fitting structure. A gap between the fitting recessed end portion  44  and the fitting projecting end portion  33  is extremely small so that it is regarded that the gap does not exist in effect between the fitting recessed end portion  44  and the fitting projecting end portion  33 . Accordingly, leakage of compressed air from the gap can be prevented. 
     The second shaft seal portion  40  includes: a first air seal  40 A disposed on a bearing  22  side; and a second air seal  40 B disposed on a rotor chamber  15  side. 
     The first air seal  40 A is formed of a packing case  41 , a non-contact seal ring  42 , and a resilient body  43 . A projecting portion  49  which projects toward the inside in a radial direction is formed on an end portion of the packing case  41  on a rotor chamber  15  side. A cylindrical seal ring accommodating space  48  is formed between the end portion  36  of the oil seal  31  and the projecting portion  49  of the packing case  41 . In the seal ring accommodating space  48 , the resilient body  43 , and the seal ring  42  which is supported by the resilient body  44  in such a manner that the seal ring  42  is biased in an axial direction of the rotary shaft  21  (a direction of the bearing  22  in this embodiment) are accommodated. A size of the seal ring  42  is set such that an inner diameter of the seal ring  42  is slightly larger than an outer diameter of the rotary shaft  21 . The seal ring  42  is formed, for example, using a material equal to a material for forming the rotary shaft  21  (for example, stainless steel) as a base material, and a film having a small friction coefficient is applied to a surface of the base material by coating. The resilient body  43  is a metal resilient member (for example, wave spring, wave washer, a compression coil spring or the like). 
     The seal ring  42  resiliently supported by the resilient body  43  can move in a radial direction even when the rotary shaft  21  is deflected. A first air seal portion  61  of the second shaft seal portion  40  is formed between an inner peripheral surface of the seal ring  42  and an outer peripheral surface of the rotary shaft  21 . A large pressure loss is generated when compressed air passes through the fine gap of the first air seal portion  61  and hence, leakage of compressed air can be suppressed. 
     The second air seal  40 B is disposed on a rotor chamber  15  side of the first air seal  40 A. The second air seal  40 B is formed of a non-contact seal ring  52  and the resilient body  53 . A gas seal accommodating space  58  is formed at an end portion of the shaft seal device loading space  10  in the casing  12  on a rotor chamber  15  side. The resilient body  43  and the seal ring  52  supported by the resilient body  53  in a state where the seal ring  52  is biased by the resilient body  53  in an axial direction of the rotary shaft  21  (a direction of the bearing  22  in this embodiment) are accommodated in the gas seal accommodating space  58 . The gas seal accommodating space  58  has a cylindrical shape having an inner diameter size smaller than that of the first air seal  40 A. 
     The seal ring  52  also can move in a radial direction, and a second air seal portion  62  is formed between an inner peripheral surface of the seal ring  52  and an outer peripheral surface of the rotary shaft  21 . A large pressure loss is generated when compressed air passes through the fine gap of the second air seal portion  62  and hence, leakage of compressed air can be suppressed. 
     The second shaft seal portion  40  includes the second air seal  40 B in addition to the first air seal  40 A. With such a configuration, shaft seal ability of the second shaft seal portion  40  is enhanced. In the first air seal  40 A and the second air seal  40 B, by using the same seal ring for forming the seal rings  42 ,  52  and by using the same resilient body for forming the resilient bodies  43 ,  53 , the reduction of cost can be realized. 
     Next, an inner peripheral annular space  24   g  which makes the atmosphere open hole  24   a  and the communication holes  31   a  communicate with each other is described with reference to  FIG. 3 . 
     On an inner peripheral side of the casing  12 , an inner peripheral annular groove  24   b  is formed such that the inner peripheral annular groove  24   b  overlaps with an inner end portion of the atmosphere open hole  24   a . The inner peripheral annular groove  24   b  forms a part of the inner peripheral annular space  24   g . The inner peripheral annular groove  24   b  is an annular groove formed on an inner peripheral surface of the casing  12  along a circumferential direction. The inner peripheral annular groove  24   b  has, for example, an approximately semicircular shape in a partial cross section taken along the axial direction of the rotary shaft  21 . 
     A tapered expanding portion  24   c  on a rotor chamber  15  side is formed on a rotor chamber  15  side of the inner peripheral annular groove  24   b  in the axial direction of the rotary shaft  21 . A tapered expanding portion  24   c  on a bearing  22  side is formed on a bearing  22  side of the inner peripheral annular groove  24   b  (hereinafter, the tapered expanding portions  24   c  on the rotor chamber  15  side and the bearing  22  side being simply referred to as tapered expanding portions  24   c  disposed on both sides). The tapered expanding portions  24   c  disposed on both sides are formed by chamfering both end portions of the inner peripheral annular groove  24   b  in the axial direction of the rotary shaft  21  in a C-surface shape or an R-surface shape. In the tapered expanding portions  24   c  disposed on both sides, the respective end portions project in a tapered shape toward a rotor chamber  15  side and a bearing  22  side. The tapered expanding portions  24   c  which are disposed on both sides of the inner peripheral annular groove  24   b  and are chamfered in a C-surface shape are formed in an approximately right triangular shape in a partial cross section taken along the axial direction of the rotary shaft  21 . In  FIG. 3 , the inner peripheral annular space  24   g  which has the inner peripheral annular groove  24   b  and the tapered expanding portions  24   c  disposed on both sides is formed in the casing  12 . An annular space  25  according to the first embodiment is formed of the inner peripheral annular space  24   g . The inner peripheral annular space  24   g  is a space which surrounds the shaft seal device  20  in the circumferential direction (corresponding to “annular space” described in claims)  25 . The inner peripheral annular space  24   g  communicates with the atmosphere open hole  24   a  and hence, the inner peripheral annular space  24   g  is opened in atmosphere. 
     Sizes of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  are set by taking into account projecting heights of the O-rings  35 ,  46  disposed in the recessed portion  34  of the oil seal  31  and the recessed portion  45  of the packing case  41 , respectively. That is, the sizes of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  are set such that the O-rings  35 ,  46  disposed in the recessed portions  34 ,  45  of the shaft seal device  20  are not damaged at the time of loading the shaft seal device  20  in the shaft seal device loading space  10  or at the time of unloading the shaft seal device  20  from the shaft seal device loading space  10 . An inclination angle θ of an inclined surface of each of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  which are chamfered in a C-surface shape with respect to an axial direction of the rotary shaft  21  is set to 30 degrees to 45 degrees, for example. A height H of each of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  which are chamfered in a C-surface shape with respect to an outer peripheral surface of the shaft seal device  20  is set to 1 mm or more, for example. 
     On the other hand, in the oil seal  31  of the shaft seal device  20 , at least one (usually, a plurality of) communication hole (communication holes)  31   a  is/are formed. The communication holes  31   a  penetrate the oil seal  31  in a radial direction. Although the shape of the communication hole  31   a  is not limited, for example, the communication hole  31   a  is a round hole having a circular opening cross section in a direction perpendicular to a length of the communication hole  31   a . With respect to the communication holes  31   a  which do not limit the present invention, for example, four communication holes  31   a  are disposed at equal intervals in the circumferential direction at an angle of 90 degrees. 
     The respective communication holes  31   a  communicate with the atmosphere open hole  24   a  through the inner peripheral annular space  24   g  formed in the casing  12 . Accordingly, the communication holes  31   a  on a shaft seal device  20  side, and the inner peripheral annular space  24   g  and the atmosphere open hole  24   a  on a casing  12  side communicate with an atmosphere thus forming the atmosphere open passage  24 . 
     As shown in  FIGS. 2 and 3 , a ventilation gap  50  is disposed in a gap in the axial direction of the rotary shaft  21  between the visco-seal  32  of the first shaft seal portion  30  and the seal ring  42  of the second shaft seal portion  40 . The ventilation gap  50  has a flow passage cross-sectional area larger than a shaft seal cross-sectional area of the air seal portion  60  in the direction orthogonal to the rotary shaft. The ventilation gap  50  which communicates with the respective communication holes  31   a  communicates with the atmosphere open passage  24  opened to an atmosphere. Accordingly, the ventilation gap  50  is opened to an atmosphere through the atmosphere open passage  24 . 
     In manufacturing the casing  12  by casting, a tolerance attributed to casting is taken into consideration. In this case, as shown in  FIG. 3 , in the axial direction of the rotary shaft  21 , a width L 1  obtained by adding a width of the inner peripheral annular groove  24   b  and widths of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  (that is, a width of an opening portion of the inner peripheral annular space  24   g ) is set to a predetermined size slightly larger than an opening diameter D 2  of the communication hole  31   a . By taking into account a tolerance attributed to casting, the width L 1  of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  is set slightly larger than the opening diameter D 2  of the communication hole  31   a  by 3 mm or more, for example. Even when a tolerance which falls within a designed range is generated in the manufacture of the casing  12  by casting, the respective communication holes  31   a  never fail to overlap with the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  and hence, the deviation of the rotary shaft  21  in the axial direction can be absorbed. Further, the inner peripheral annular space  24   g  on a casing  12  side and the respective communication holes  31   a  on a shaft seal device  20  side can be made to communicate with each other with certainty. In the manufacture of the casing  12  by casting, a cast-in hole can be used as the atmosphere open hole  24   a . However, the atmosphere open hole  24   a  may be formed by machining. The inner peripheral annular groove  24   b  having an approximately semicircular shape can be simultaneously formed at the time of manufacturing the casing  12  by casting and hence, the number of man-hours for forming the inner peripheral annular groove  24   b  can be reduced. 
     During an unloading operation, the inside of the rotor chamber  15  assumes a negative pressure. The negative pressure performs an action of sucking a lubricant in the bearing  22  into the inside of the rotor chamber  15  through a gap formed between an outer peripheral surface of the rotary shaft  21  and an inner peripheral surface of the shaft seal device  20 . In view of the above, the atmosphere open passage  24  which is opened to an atmosphere and the ventilation gap  50  are disposed so as to prevent a lubricant in the bearing  22  from flowing into the rotor chamber  15 . However, due to a pressure loss generated in the atmosphere open passage  24  during an unloading operation, in an actual operation, a pressure in the ventilation gap  50  does not become an atmospheric pressure. 
     There is a possibility that a pressure loss is generated at various portions in the atmosphere open passage  24  ranging from the outside of the casing  12  (atmospheric side) to the ventilation gap  50 . The present invention is intended to reduce a pressure loss at such portions by focusing on an open cross-sectional area S 1  of the inner peripheral annular space  24   g  and a total open cross-sectional area S 2  of the communication holes  31   a . The atmosphere open hole  24   a  is formed such that a pressure loss in the atmosphere open hole  24   a  is smaller than a pressure loss in the inner peripheral annular space  24   g  and a pressure loss in the communication holes  31   a.    
     Assuming an open cross-sectional area of the inner peripheral annular space  24   g  in partial cross section taken along the axial direction of the rotary shaft  21  as S 1 , a total open cross-sectional area of the communication holes  31   a  as S 2 , an open cross-sectional area of the i-th communication hole out of the communication holes  31   a  as S 2 i, and the number of communication holes  31   a  as n (n being a natural number of 1 or more), the following relationship is established. 
     
       
         
           
             
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     Further, assume an open cross-sectional area of the inner peripheral annular groove  24   b  in partial cross section taken along the axial direction of the rotary shaft  21  as Sr and an open cross-sectional area of the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b  in partial cross section taken along the axial direction of the rotary shaft  21  as Sc. Since the inner peripheral annular space  24   g  is formed of the inner peripheral annular groove  24   b  and the tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b , the open cross-sectional area S 1  of the inner peripheral annular space  24   g  is expressed as Sr+Sc. 
     Two O-rings  35 ,  46  are disposed in the shaft seal device  20  in a spaced-apart manner from each other in the axial direction of the rotary shaft  21 . The restriction is imposed on the arrangement of the annular space  25  in the axial direction of the rotary shaft  21  due to such a spaced-apart distance between these two O-rings  35 ,  46 . Further, the communication holes  31   a  are formed in the circumferential direction of the oil seal  31  and hence, the number n of the communication holes  31   a  can be easily increased. Accordingly, there is a tendency that a total open cross-sectional area S 2  of the communication holes  31   a  is increased. Accordingly, a large pressure loss is generated in the annular space  25  rather than in n pieces of communication holes  31   a.    
     In view of the above, the present invention has focused on a casing  12  side where the small restriction is imposed on the arrangement of the annular space  25  in the axial direction of the rotary shaft  21 , and is characterized by arranging the inner peripheral annular space  24   g  which functions as the annular space  25  surrounding the shaft seal device  20  in the circumferential direction on a casing  12  side. With such arrangement, it is possible to properly ensure a width L 1  of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  compared to the case where the annular space  25  is arranged on the shaft seal device  20  side. 
     As expressed in the following formula (1), the open cross-sectional area S 1  of the inner peripheral annular space  24   g  (that is, the open cross-sectional area S of the annular space  25 ) is set larger than the total open cross-sectional area S 2  of the communication holes  31   a . With such setting of the open cross-sectional areas, a pressure loss in the inner peripheral annular space  24   g  (annular space  25 ) can be set smaller than a pressure loss in the communication holes  31   a.    
     
       
         
           
             
               
                 
                   
                     
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     Accordingly, a pressure loss in the inner peripheral annular space  24   g  which forms the annular space  25  can be reduced and hence, flowing-in of a lubricant during an unloading operation can be prevented. 
     Next, a second embodiment of the present invention is described in detail with reference to  FIGS. 4 to 6 . In the second embodiment, constitutional elements having identical functions as the constitutional elements in the first embodiment described above are given the same symbols, and the repeated description of these constitutional elements is omitted. 
     An oil-free screw compressor  1  according to the second embodiment includes: a shaft seal device  20 ; and an inner peripheral annular space  24   g  and an outer peripheral annular space  31   b  which form a space  25  surrounding the shaft seal device  20  in the circumferential direction in a peripheral portion of the shaft seal device  20  (corresponding to “annular space” described in claims). 
     As shown in  FIG. 5 , the outer peripheral annular space  31   b  which forms a portion of the annular space  25  is formed on an outer peripheral side of an oil seal  31  of the shaft seal device  20 . The outer peripheral annular space  31   b  is an annular groove formed on an outer peripheral surface of the shaft seal device  20  along the circumferential direction such that the outer peripheral annular space  31   b  faces the inner peripheral annular space  24   g . The outer peripheral annular space  3 l b  is formed by machining, for example. Although the shape of the outer peripheral annular space  3 l b  is not limited, for example, the outer peripheral annular space  3 l b  has a rectangular shape in partial cross section taken along the axial direction of the rotary shaft  21 . A width L 3  of an opening portion of the outer peripheral annular space  3 l b  in the axial direction of the rotary shaft  21  is set equal to or larger than an opening diameter D 2  of the communication hole  31   a  and smaller than a width L 1  of an opening portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21 . 
     The respective communication holes  31   a  communicate with the outer peripheral annular space  31   b  and a ventilation gap  50 . The ventilation gap  50  communicates with an atmosphere open hole  24   a  through the respective communication holes  31   a  and the outer peripheral annular space  31   b  on a shaft seal device  20  side and the inner peripheral annular space  24   g  on the casing  12  side. Accordingly, the communication holes  31   a  and the outer peripheral annular space  31   b  on the shaft seal device  20  side, and the inner peripheral annular space and the atmosphere open hole  24   a  on the casing  12  side communicate with an atmosphere thus forming an atmosphere open passage  24 . 
     In the same manner as the above-mentioned first embodiment, in manufacturing a casing  12  by casting, a tolerance attributed to casting is taken into consideration. In this case, as shown in  FIG. 6 , in the axial direction of the rotary shaft  21 , a width obtained by adding a width of the inner peripheral annular groove  24   b  and widths of tapered expanding portions  24   c  disposed on both sides of the inner peripheral annular groove  24   b , that is, a width L 1  of an opening portion of the inner peripheral annular space  24   g , is set to a predetermined size slightly larger than a width L 3  of an opening portion of the outer peripheral annular space  31   b . By taking into account a tolerance attributed to casting, the width L 1  of the open portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  is set slightly larger than the width L 3  of the opening portion of the outer peripheral annular space  31   b  by 3 mm or more, for example. Even when a tolerance which falls within a designed range is generated in the manufacture of the casing  12  by casting, the opening portion of the outer peripheral annular space  31   b  never fails to overlap with the opening portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  and hence, the deviation of the rotary shaft  21  in the axial direction can be absorbed. Further, the inner peripheral annular space  24   g  on a casing  12  side and the outer peripheral annular space  31   b  on the shaft seal device  20  side can be made to communicate with each other with certainty. 
     In the second embodiment, the annular space  25  is formed of the inner peripheral annular space  24   g  on the casing  12  side and the outer peripheral annular space  31   b  on the shaft seal device  20  side. With such a configuration, the annular space  25  is formed on both sides, that is, on the casing  12  side and on the shaft seal device  20  side. Accordingly, compared to the case where the inner peripheral annular space  24   g  is formed only on the casing  12  side, an open cross-sectional area S 1  of the inner peripheral annular space  24   g  can be made small so that it is possible to alleviate the restriction imposed on the arrangement of the inner peripheral annular space  24   g  on the casing  12  side. Accordingly, a degree of freedom in arranging the inner peripheral annular space  24   g  in the casing  12  can be increased. 
     In the same manner as the first embodiment described above, assume a total open cross-sectional area of the communication holes  31   a  as S 2 , an open cross-sectional area of the i-th communication hole out of the communication holes  31   a  as S 2 i, and the number of communication holes  31   a  as n (n being a natural number of 1 or more). As expressed by the following formula (2), an opening cross-sectional area S of the annular space  25  which is obtained by adding an open cross-sectional area S 1  of the inner peripheral annular space  24   g  and an open cross-sectional area S 3  of the outer peripheral annular space  31   b  is set larger than a total open cross-sectional area S 2  of the communication holes  31   a . With such setting of the open cross-sectional areas, a pressure loss in the inner peripheral annular space  24   g  and the outer peripheral annular space  31   b  (annular space  25 ) can be set smaller than a pressure loss in the communication holes  31   a.    
     
       
         
           
             
               
                 
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     Accordingly, a pressure loss in the inner peripheral annular space  24   g  and the outer peripheral annular space  31   b  which form the annular space  25  can be lowered and hence, flowing-in of a lubricant during an unloading operation can be prevented. 
     In the above-mentioned embodiment, the shaft seal device  20  on a discharge side has been described. However, the present invention is also applicable to the shaft seal device  20  on a suction side. The structure of the second shaft seal portion  40  in the shaft seal device  20  is not limited to the above-mentioned embodiment. The number of air seal portions and the directions of seal rings can be changed as desired. As the second shaft seal portion  40 , a known seal member such as a labyrinth seal can also be used in place of the seal ring  42 ,  52 . As the oil seal portion  32  of the first shaft seal portion  30 , the so-called visco-seal  32  is exemplified. However, a known seal structure such as a labyrinth seal can also be used. 
     In the above-mentioned embodiment, the oil seal  31  and the packing case  41  are respectively formed of a unitary member. However, provided that the oil seal  31  and the packing case  41  have the integral configuration at the time of assembling the oil seal  31  and the packing case  41 , the oil seal  31  and the packing case  41  may be respectively formed of two or more members split in the axial direction of the rotary shaft  21 . The oil seal  31  may be formed of an oil seal portion  32  and a body portion which supports the oil seal portion  32  thereon. A surface of the rotary shaft  21  may be formed of a base material of the rotary shaft  21  per se or may be formed of any one of various films applied to a surface of the base material. The mode of the rotary shaft  21  according to the present invention includes a mode where the rotary shaft  21  is used in a single form or a mode where a sleeve not shown in the drawing is fixed to an outer peripheral surface side of the rotary shaft  21 . 
     As described above, to exemplify the technical feature of this disclosure as an example, the above-mentioned embodiment has been described, and the accompanying drawings and the detailed description are provided for describing the embodiment. 
     Therefore, the constitutional elements described in the accompanying drawings and the detailed description include not only constitutional elements which are indispensable for solving the problems of the present invention but also constitutional elements which are not indispensable for solving the problems of the present invention and are provided for exemplifying the above-mentioned technique. Accordingly, it should not be construed that the fact that constitutional elements which are not indispensable for solving the problems of the present invention are described in accompanying drawings and the detailed description readily verifies that the constitutional elements which are not indispensable for solving the problems of the present invention are also included in the constitutional elements which are indispensable for solving the problems of the present invention. 
     Although this disclosure has been sufficiently described in conjunction with the preferred embodiment with reference to accompanying drawings, it is apparent for those who are skilled in the art that various modifications and variations can be made based on this disclosure. It should be construed that such modifications and variations are also embraced by the present invention so long as such modifications and variations fall within the scope of the invention called for in claims. 
     As can be clearly understood from the description made heretofore, in the oil-free screw compressor  1  according to the present invention, the annular space  25  which is a space surrounding the shaft seal device  20  in the circumferential direction includes the inner peripheral annular space  24   g  annularly formed on an inner peripheral side of the casing  12 . Assuming an open cross-sectional area of the inner peripheral annular space  24   g  in a partial cross section taken along an axial direction of the rotary shaft  21  as S 1 , assuming an open cross-sectional area of one communication hole  31   a  as S 2 , and assuming the number of communication holes  31   a  as n, S 1 ≥n×S 2  is satisfied. With such a configuration, the inner peripheral annular space  24   g  which forms the annular space  25  is formed on a casing  12  side and hence, it is possible to alleviate the restriction imposed to ensure a width L 1  of the opening portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21 . Accordingly, the open cross-sectional area S 1  of the inner peripheral annular space  24   g  can be increased compared to the case where the annular space  25  is formed on a shaft seal device  20  side. As a result, S 1 ≥n×S 2  is easily satisfied and hence, a pressure loss in the annular space  25  can be reduced whereby flowing-in of a lubricant during an unloading operation can be prevented. 
     The present invention has the following technical feature in addition to the above-mentioned technical feature. 
     That is, the annular space  25  which is a space surrounding the shaft seal device  20  in the circumferential direction further includes the outer peripheral annular space  3 l b  which is disposed so as to opposedly face the inner peripheral annular space  24   g  on an outer peripheral side of the shaft seal device  20 , and has a larger width in the axial direction of the rotary shaft  21  than the communication holes  31   a , and assuming an open cross-sectional area of the outer peripheral annular space  31   b  in a partial cross section taken along the axial direction of the rotary shaft  21  as S 3 , S 1 +S 3 ≥n×S 2  is satisfied. With such a configuration, the annular space  25  is formed on both on a casing  12  side and on a shaft seal device  20  side and hence, compared to the case where the inner peripheral annular space  24   g  is formed only on the casing  12  side, the open cross-sectional area S 1  of the inner peripheral annular space  24   g  can be made small so that it is possible to alleviate the restriction imposed on the arrangement of the inner peripheral annular space  24   g  on the casing  12  side. Accordingly, a degree of freedom in arranging the inner peripheral annular space  24   g  in the casing  12  can be increased. 
     The casing  12  is formed of a cast product, and a width L 1  of the opening portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  is larger than an opening diameter D 2  of the communication hole  31   a  by an amount set by taking into account a manufacturing tolerance of the cast product. With such a configuration, even when a tolerance which falls within a designed range is generated in the manufacture of the casing  12  by casting, the respective communication holes  31   a  never fail to overlap with the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  and hence, the deviation of the rotary shaft  21  in the axial direction can be absorbed. 
     The casing  12  is formed of a cast product, and a width L 1  of the opening portion of the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  is larger than a width L 3  of the opening portion of the outer peripheral annular space  3 l b  in the axial direction of the rotary shaft  21  by an amount set by taking into account a manufacturing tolerance of the cast product. With such a configuration, even when a tolerance which falls within a designed range is generated in the manufacture of the casing  12  by casting, the outer peripheral annular space  31   b  never fails to overlap with the inner peripheral annular space  24   g  in the axial direction of the rotary shaft  21  and hence, the deviation of the rotary shaft  21  in the axial direction can be absorbed. 
     The casing  12  is formed of a cast product. The inner peripheral annular space  24   g  has: the inner peripheral annular groove  24   b ; and the tapered expanding portions  24   c  having a tapered shape which are formed on both end sides of the inner peripheral annular groove  24   b  in the axial direction of the rotary shaft  21 . The inner peripheral annular groove  24   b  has an approximately semicircular shape in partial cross section taken along the axial direction of the rotary shaft  21 . With such a configuration, the inner peripheral annular groove  24   b  can be simultaneously formed at the time of manufacturing the casing  12  by casting and hence, the number of man-hours for forming the inner peripheral annular groove  24   b  can be reduced. 
     The tapered expanding portions  24   c  respectively have an inclination angle θ and a height H set by taking into account projecting heights of the O-rings  35 ,  46  mounted on an outer peripheral side of the shaft seal device  20 . With such a configuration, the O-rings  35 ,  46  disposed in the recessed portions  34 ,  45  of the shaft seal device  20  are not damaged at the time of loading the shaft seal device  20  in the shaft seal device loading space  10  or at the time of unloading the shaft seal device  20  from the shaft seal device loading space  10 .