Oil-Free Centrifugal Capacity Control
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. Optimum compressor efficiency occurs close to surge at incipient stall conditions when maximum internal flow pressure recovery occurs. The positional feedback system of the active magnetic bearing control loop system indicates the bearing orbit which relates to compressor efficiency. At high flow conditions capacity is controlled by compressor speed for the imposed pressure ratio. 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. 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.