OIL-FREE SCREW COMPRESSOR AND DESIGN METHOD THEREFOR

摘要

Provided is an oil-free screw compressor that can prevent lubricating oil from flowing in and maintain compression performance. The oil-free screw compressor is provided with: a casing 12 that has a rotor chamber 150; a bearing 22 that supports the rotating shaft 21 of a screw rotor; a shaft sealing device 20 comprising an oil seal section 31 and an air seal section 60; a ventilation gap 50 that is positioned between the oil seal section and the air seal section; and an open air passage 24 that connects the air side of the casing and the ventilation gap. The oil-free screw compressor is set so that the expression (La/Sa2.5)/(Lh/Sh2.5)>|P2|/ΔPb is satisfied when the effective opening cross-sectional area at the most constricted section 24d of the open air passage is denoted by Sh, the effective constriction length at said most constricted section 24d is denoted by Lh, the shaft seal cross-sectional area at a minute gap in the air seal section is denoted by Sa, the effective shaft seal length at the minute gap is denoted by La, the absolute value of the negative pressure in the rotor chamber during unloading operation is denoted by |P2|, and the minimum differential pressure of the oil seal section during unloading operation is denoted by ΔPb.