Oil-free screw compressor and design method therefor

A method of designing 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 shaft;
• a ventilation gap which is positioned between the oil seal portion and the air seal portion and is formed between an outer peripheral surface of the rotary shaft and an inner peripheral surface of the shaft seal device;
• and
• an atmosphere open passage which makes an atmosphere side of the casing and the ventilation gap communicate with each other,
• wherein a most narrowed portion of the atmosphere open passage, the air seal portion and the oil seal portion are set such that a following relationship is established
• (La/Sa2.5)/(Lh/Sh2.5)>|P2|/ΔPb
• wherein an effective open cross-sectional area as Sh and an effective narrowed length as Lh in the most narrowed portion where the atmosphere open passage is most narrowed;
• a shaft seal cross-sectional area in a direction orthogonal to a rotary shaft as Sa and an effective shaft seal length as La in a fine gap in the air seal portioned;
• an absolute value of a negative pressure in the rotor chamber during an unloading operation as |P2|;
• a minimum differential pressure in the oil seal portion during the unloading operation as ΔPb;
• wherein
• the atmosphere open passage has an atmosphere open hole formed in the casing, and at least one communication hole formed in the shaft seal device, and
• an annular space which surrounds the shaft seal device in a circumferential direction is formed by both of or either one of an inner peripheral side of the casing and an outer peripheral side of the shaft seal device, and the atmosphere open hole and at least one of the communication holes communicate with each other through the annular space, and
• the most narrowed portion of the atmosphere open passage is smaller one between an open cross-sectional area of the atmosphere open hole and a total open cross-sectional area of the at least one communication hole.