Small, High-Speed, Oil-Free Radial Turbo-compressors for Cooling Applications: Refrigerant Selection

Emerging energy-efficient, oil-free radial centrifugal compressors seem promising for small to mid-capacity air conditioning and refrigeration applications. Compressor design must balance trade-offs among impeller diameter, bearing type, and refrigerant selection. This paper guides refrigerant selection for single-stage systems delivering 10-250 kWtherm of cooling at a representative evaporating temperature of 0°C. The viable oil-free bearing types, the impeller diameter for maximum compressor efficiency and the resulting system efficiency were determined for a set of representative refrigerants with normal boiling temperatures ranging from -51.7°C to +49°C. For residential scale cooling systems, low pressure refrigerants enable high energy efficiency with small size, high-speed centrifugal compressors and vapor-lubricated bearings. For commercial scale cooling systems, medium pressure refrigerants enable high energy efficiency with reasonable size, high-speed centrifugal compressors and vapor-lubricated bearings. For commercial scale cooling systems, low pressure refrigerants enable higher energy efficiency with larger size, high-speed centrifugal compressors and magnetic bearings compared to medium pressure refrigerants with vapor-lubricated bearings. The preferred refrigerant enables combinations of system first and operating costs that are attractive for the intended markets, while meeting equipment reliability, operability and other constraints.