Magnetic-bearing centrifugal compressors

This new compressor technology - presented at the 2003 AHR Expo in Chicago - may have a significant effect on the future of mid-range chillers and rooftop applications in water-cooled, evaporatively cooled and air-cooled chilled-water and direct-expansion systems. A digitally controlled magnetic-bearing system, consisting of both permanent magnets and electromagnets, replaces lubricated bearings traditionally used in centrifugal compressors. The frictionless compressor shaft is the compressor's only moving component, which rotates on a levitated magnetic cushion. Only a very small amount of oil is required to lubricate system components other than bearings, such as seals and valves. According to HPAC Engineering, for many installations, compressor maintenance costs have been cut by more than 50%. Sources of vibration and noise are also noticeably decreased. The use of permanent magnets instead of rotor windings makes the motor smaller and lighter than induction motors and significantly improves the motor's efficiency (96-97% instead of 92%). The integrated variable-frequency drive (VFD) allows improvement of the part-load performances. According to ARI testing, a 150-ton frictionless chiller has a full-load COP of 5.6 and a 60%-load COP of 9.4. Primarily introduced by Turbocor (a Canadian manufacturer), this technology has been adopted by several other compressor manufacturers in the US and Europe.