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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.
Dominique Bonvin, Timm Faulwasser, Alejandro Gabriel Marchetti, Predrag Milosavljevic