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Floating rotors could boost compressor efficiencies

30 June, 2008

SKF has developed an advanced drive system for compressors that uses magnetic bearings and high-speed permanent magnet motors to boost compressor efficiencies from the 50–60% typical for screw compressors, to more than 75% for centrifugal compressors driven by the new technology. The company estimates that if 10% of the five million screw compressors in use around the world were replaced by more efficient machines using the new technology, the annual energy savings could amount to around 60GWh.

Traditional centrifugal compressors are based on low-speed drives, mechanical gears and oil-film bearings, resulting in high running costs because of their high losses, wear, and need for maintenance.

SKF compressor drive

SKF’s new compressor drive (above) uses a permanent magnet motor, operating at an efficiency of around 97%, to drive a rotor "floating" on magnetic bearings, which spins the compressor impeller at speeds of around 60,000 rpm. These drives experience almost no friction or wear, and need little maintenance. They also minimise the risk of oil contamination, and result in compressors that are about half the size of traditional designs.

According to Daniel Westberg, manager of SKF’s magnetic bearings business, compressors driven by the new technology will be up to five times more reliable than conventional centrifugal compressors of a similar rating. And whereas a conventional compressor will produce around 85% of useful output power compared to its input power, a high-speed compressor will produce some 94% of useful power.

Westberg cites the example of a typical 175kW air compressor operating around the clock. The higher efficiency of the new technology would result in annual savings of more than 238MWh – worth €11,900 a year (for electricity costing €0.05/kWh). Further savings would accrue from the reduced maintenance costs. For larger installations, such as gas pipeline compressors, the savings would be proportionately higher.

A key factor in making the new drives feasible has been the advances in magnetic bearing technology in recent years. These bearings have expanded from specialised applications to more affordable, general-purpose duties and now use simpler, more effective controls.

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