A ONE-DIMENSIONAL MODEL OF AN OIL-INJECTED HELIUM SCREW COMPRESSOR FOR CRYOGENIC REFRIGERATION SYSTEMS

Oil flooded rotary screw compressors are widely used in the field of helium cryogenics to provide the thermodynamic availability necessary to operate a cryogenic plant. Due to helium's low normal boiling point (NBP, approximately 4.2 K for helium-4) and its inability to freeze (except at high pressures), helium is the only working fluid that can be used in refrigeration systems where temperatures approaching absolute zero are required. The process of cooling helium from ambient temperature (around 300 K) to its NBP is highly energy intensive. For large and efficient helium refrigerators, approximately 250 W of actual input power is required for 1 W of cooling at 4.2 K. Currently, in large systems, compressor skid losses account for approximately one-half of the input power or two-thirds of the total system losses. The focus of this study is to numerically investigate the performance of a helium twin rotary screw compressor in order to better understand the influences of different operating parameters on the overall compressor efficiency. A 1-D model of the gas-oil compression process through a rotary screw compressor is developed and validated against published data. The 1-D model is derived from an energy and mass balance of the gas and oil mixture. Primary parameters investigated in the present study include oil-gas mass ratio, pressure ratio, discharge temperature, and isothermal efficiency. Based on the simulation results, it is observed that the discharge temperature and the isothermal efficiency can exhibit significant variations (up to 25%) depending on the oil-gas mass ratio. The results from the present study will be used to gain insight into how to improve compressor efficiency and specific design changes necessary to enable these improvements.