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Using magnetic bearing orbit information to maximize centrifugal compressor efficiency at off design conditions.

Number: pap. 1498

Author(s) : THORNTON W. T., BRASZ J. J.

Summary

Active magnetic bearings used on oil-free centrifugal refrigeration compressors have lower stiffness than conventional oil-lubricated journal or rolling element bearings. The lower stiffness of these bearings makes them sensitive to internal flow instabilities that are precursors of rotating stall or compressor surge. At operating conditions far away from surge the internal flow is very stable and the magnetic bearings keep the shaft centered, resulting in a minimal bearing orbit. The internal flow instabilities that arise when the compressor approaches the surge limit result in some radially fluctuating forces on the shaft. The active magnetic bearings correct for these fluctuating radial forces on the shaft. The bearing orbit increases with the size of these radial forces. At high flow conditions capacity is controlled by compressor speed for the imposed head. At low flow conditions a combination of variable speed and a means of range extension (e.g. inlet guide vanes, variable-geometry diffuser, or flow recirculation) are required to control capacity at the imposed pressure ratio and guarantee stable compressor operation. When operating with flow recirculation optimum compressor efficiency occurs close to surge at incipient stall conditions when the smallest amount of flow is recirculated with the compressor remaining safe from surge. The positional feedback system of the active magnetic bearing control loop system indicates the bearing orbit which relates to compressor efficiency. The bearing orbit signal can be used to determine which speed/geometry combination gives the highest compressor efficiency. If the bearing orbit is below a minimum value the compressor runs too close to choke and a reduction in speed combined with an opening of the diffuser throat area would increase compressor efficiency. If, on the other hand, the bearing orbit is above a maximum value the compressor runs too close to surge and an increase in speed combined with a closing of the diffuser throat area is required to increase compressor efficiency and guarantee surge-free compressor. Experience with this control scheme will be illustrated for a newly developed 350 ton two-stage centrifugal compressor where the variable geometry hardware is replaced with a controlled internal flow recirculation.

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Pages: 8 p.

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Details

  • Original title: Using magnetic bearing orbit information to maximize centrifugal compressor efficiency at off design conditions.
  • Record ID : 30013865
  • Languages: English
  • Source: 2014 Purdue Conferences. 22nd International Compressor Engineering Conference at Purdue.
  • Publication date: 2014/07/14

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