Scale limitations of gas bearing supported turbocompressors for vapor compression cycles

Abstract

An analysis of single-stage centrifugal compressors supported on dynamic, gas lubricated bearings for driving vapor compression cycles suggests that the maximum feasible compressor power is limited, primarily by the bearing load capacity, in the range from 80 kW to 250 kW. The analysis further suggests that the maximum feasible compressor power depends significantly on working fluid selection. Low-pressure fluids result in large impellers with good efficiency but heavy machines, the power of which is limited by the bearing load capacity. High-pressure fluids require small machines rotating at high speeds, the power of which is limited by the rotordynamic eigenfrequencies. Consequently, there is a family of fluids that allows maximization of the power range of gas bearing supported turbocompressors. Finally, the analysis suggests that the maximum compressor power can be expressed as a function of the reduced evaporation pressure, regardless of the working fluid choice. Maximum COP is reached at a reduced evaporation pressure decreasing nearly linearly from 0.22 to 0.11 as the temperature lift between the evaporator and the condenser increases from 20 K to 50 K. The feasible maximum compressor power increases with increasing specific compressor speed, bearing tip speed (NDm) and rotordynamic stability safety margin.