Patent ID: 7722346

Claim:
An oil supply method of supplying operating gas dissolvable lubricating oil to a two-stage screw compressor integrating a low-pressure stage compressor and a high-pressure stage compressor in a compact one unit for compressing operating gas, wherein lubricating oil supplied to bearings and a shaft seal element of the two-stage compressor is supplied to a series of compression cavities produced by a pair of meshing rotors of the high-pressure stage compressor as the rotors rotate after the lubricating oil has lubricated the bearings and the shaft seal element, and the lubricating oil is supplied to the compression cavities at a position where the internal volume ratio Vi of the compression cavities is larger than 1 (not including 1), where Vi=maximum volume of the compression cavity/volume of said compression cavity when the oil is supplied to said compression cavity, wherein oil pressure of supplying said lubricating oil to the bearings and the shaft seal element is determined to suffice the following formula: P high >P oi1 ≧P max.int ×Vi κ 0.3˜0.5 MPa where P high : discharge pressure of operating gas from the high-pressure stage compressor, P oil : oil supply pressure to the bearings and the shaft seal element, P max.int : maximum intermediate pressure, i.e. maximum pressure of operating gas at the suction side of the high-pressure stage compressor in assumable operation condition, Vi: internal volume ratio, i.e. maximum volume of the compression cavity/volume of said compression cavity when the oil is supplied to said compression cavity, κ: specific heat ratio of operating gas, P loss : pressure loss in the bearings and the shaft seal element.