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

1. An oil-free screw compressor comprising:

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,

wherein 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 , a total open cross-sectional area of the communication holes as S 2 , an open cross-sectional area of the i-th communication hole out of the communication holes as S 2 i, and a number of communication holes as n (n being a natural number of 1 or more), a following relationship is satisfied:

S

⁢

⁢

1

≧

S

⁢

⁢

2

=

∑

i

=

1

n

⁢

⁢

S

⁢

⁢

2

⁢

i

wherein the casing is formed of a cast product, and

a width of an opening portion of the inner peripheral annular space in the axial direction of the rotary shaft is larger than an opening diameter of the communication hole by an amount set by taking into account a manufacturing tolerance of the cast product.

2. The oil-free screw compressor according to claim 1 , wherein the annular space further includes an outer peripheral annular space which is disposed so as to opposedly face the inner peripheral annular space on an outer peripheral side of the shaft seal device, and has a larger width in the axial direction of the rotary shaft than the communication holes, and

wherein an open cross-sectional area of the outer peripheral annular space in a partial cross section taken along the axial direction of the rotary shaft as S 3 , a following relationship is satisfied

S

=

S

⁢

⁢

1

+

S

⁢

⁢

3

≧

S

⁢

⁢

2

=

∑

i

=

1

n

⁢

⁢

S

⁢

⁢

2

⁢

i

.

3. The oil-free screw compressor according to claim 2 , wherein

the casing is formed of a cast product, and

a width of an opening portion of the inner peripheral annular space in the axial direction of the rotary shaft is larger than a width of an opening portion of the outer peripheral annular space in the axial direction of the rotary shaft by an amount set by taking into account a manufacturing tolerance of the cast product.

4. The oil-free screw compressor according to claim 3 , wherein

the casing is formed of a cast product,

the inner peripheral annular space has: an inner peripheral annular groove; and tapered expanding portions having a tapered shape which are formed on both end sides of the inner peripheral annular groove in the axial direction of the rotary shaft, and

the inner peripheral annular groove has an approximately semicircular shape in partial cross section taken along the axial direction of the rotary shaft.

5. The oil-free screw compressor according to claim 4 , wherein the tapered expanding portions respectively have an inclination angle and a height set by taking into account a projecting height of an O-ring mounted on an outer peripheral side of the shaft seal device.

6. The oil-free screw compressor according to claim 2 , wherein

the casing is formed of a cast product,

the inner peripheral annular space has: an inner peripheral annular groove; and tapered expanding portions having a tapered shape which are formed on both end sides of the inner peripheral annular groove in the axial direction of the rotary shaft, and

the inner peripheral annular groove has an approximately semicircular shape in partial cross section taken along the axial direction of the rotary shaft.

7. The oil-free screw compressor according to claim 6 , wherein the tapered expanding portions respectively have an inclination angle and a height set by taking into account a projecting height of an O-ring mounted on an outer peripheral side of the shaft seal device.

8. The oil-free screw compressor according to claim 1 , wherein

the casing is formed of a cast product,

the inner peripheral annular space has: an inner peripheral annular groove; and tapered expanding portions having a tapered shape which are formed on both end sides of the inner peripheral annular groove in the axial direction of the rotary shaft, and

the inner peripheral annular groove has an approximately semicircular shape in partial cross section taken along the axial direction of the rotary shaft.

9. The oil-free screw compressor according to claim 8 , wherein the tapered expanding portions respectively have an inclination angle and a height set by taking into account a projecting height of an O-ring mounted on an outer peripheral side of the shaft seal device.

10. The oil-free screw compressor according to claim 1 , wherein

the casing is formed of a cast product,

the inner peripheral annular space has: an inner peripheral annular groove; and tapered expanding portions having a tapered shape which are formed on both end sides of the inner peripheral annular groove in the axial direction of the rotary shaft, and

the inner peripheral annular groove has an approximately semicircular shape in partial cross section taken along the axial direction of the rotary shaft.

11. The oil-free screw compressor according to claim 10 , wherein the tapered expanding portions respectively have an inclination angle and a height set by taking into account a projecting height of an O-ring mounted on an outer peripheral side of the shaft seal device.