1991 年 26 巻 6 号 p. 488-494
Two-stage screw compressors incorporating oil injection systems are extensively applied to helium refrigeration systems used in cooling superconducting equipment. The investigation of the optimum condition and the reliability of the two-stage oil injection type helium screw compressor have been carried out, in order to determine ways of enhancing compressor performance with the high reliability. In result, it was found that the compressor volumetric efficiency is increased by reducing the mass flow ratio of the helium gas/lubricating oil while accelerating the compressor revolution speed. And it was proved that the compressor volumetric efficiency is significantly influenced by the sound speed in the mixture of gas and lubricating oil. Furthermore, authors were able to derive a formula for predicting the change of the compressor volumetric efficiency, when the mass flow ratio of gas/lubricating oil and the revolution speed are changed. It was also found that the shaft power is decreased by setting the ratio of the high- and low-stage theoretical volumetric flow rates to 1:4 and utilizing a low viscosity lubricating oil. In result, the advanced experimental machine achieved an isothermal efficiency level of 60% without any trouble.