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

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 S1, a total open cross-sectional area of the communication holes as S2, an open cross-sectional area of the i-th communication hole out of the communication holes as S2i, 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.