The Thermodynamic Performance of Oil-Flooded Helium Screw Compressors

Abstract

In recent years the cryogenic community has become interested in the application of hermetic screw compressors for helium refrigeration/liquefaction systems. In the past the screw compressor had not been an attractive compressor for cryogenic systems because of the problems associated with both dry and oil-flooded screw compressors. However, oil separation technology has evolved to a point where oil can successfully be separated from helium gas. The screw compressor has exhibited excellent reliability and performance characteristics for various refrigeration gases, and preliminary test results reveal that the same is true for helium gas. The present paper presents the thermodynamic performace characteristics of 30-kW (40-HP) and 113-kW (150-HP) oil-flooded hermetic screw compressors with various built-in volume ratios operating with helium gas at pressure ratios between 2.0 and 18.0, and for suction pressures of 0.5 atm, 1.0 atm, and 2.0 atm. The thermodynamic parameters of interest are the volumetric efficiency and the isentropic efficiency. It is found that the volumetric efficiency decreases with pressure ratio; however, depending on the built-in volume ratio of the compressor, the maximum value of the isentropic efficiency occurs in a narrow range of values for the pressure ratio at which the optimum isentropic efficiency occurs. The results of the tests indicate that oil-flooded screw compressors may successfully be used for helium service.