Patent ID: 8277207

Claim:
A method of supplying operating gas dissolvable in lubricating oil to a two-stage screw compressor having a low-pressure stage compressor and a high-pressure stage compressor integrated together, a common rotor shaft connecting rotors of the low-pressure stage compressor and the high-pressure stage compressor in series, with a first bearing residing at an axial end part of the common rotor shaft at a suction side of the low-pressure stage compressor, and an intermediate bearing residing at the common rotor shaft between the low-pressure stage compressor and the high-pressure stage compressor, the high-pressure stage compression having a pair of meshing rotors, the method comprising the steps of: supplying the lubricating oil to the first bearing and the intermediate bearing through a first oil passage; and supplying the lubricating oil to compression cavities, which are confined between the pair of meshing rotors of the high-pressure stage compressor, though a second oil passage that receives the lubricating oil that has lubricated the first bearing, wherein the lubricating oil is supplied to the compression cavities at a position where Vi of the compression cavities is larger than 1, Vi being an internal volume ratio at a maximum volume of the compression cavity/volume of the compression cavity when the oil is supplied to the compression cavity, and wherein an oil supply pressure in the step of supplying the lubricating oil to the first bearing and the intermediate bearing through the first oil passage is controlled to satisfy the following: P oil ≧P max.int ×Vi K +P loss +ΔP, where: P oil is an oil supply pressure to the bearing parts, P max.int is a maximum intermediate pressure of operating gas at the suction side of the high-pressure stage compressor, K is a specific heat ratio of operating gas, P loss is a pressure loss in bearing parts, and ΔP is a pressure difference required to inject oil through the second oil passage into the compression cavities.