centrifugal compressor, range extension, variable speed, chillers
Centrifugal compressors used on water-cooled chillers require stable operation over a wide range of flows at greatly varying pressure ratios. These operational requirements are dictated by variations in cooling demand and ambient conditions. Variable-speed centrifugal compressors are known to maintain their peak efficiency at the varying operating conditions much better than fixed-speed compressors. Replacing a fixed-speed centrifugal compressor with a variable speed one can reduce the annual energy consumption of a chiller by 40-45%. The majority of centrifugal chillers sold today are therefore inverter driven. Lower speed operation maintains and sometimes even increases compressor efficiency along a wide band of capacity and head combinations which fits quite naturally with most of the chiller operating requirements. However, the variable speed compressor will eventually surge when forced to operate at lower capacity while maintaining head. Some variable-geometry compressor features are necessary to enable stable compressor operation at these conditions. Variable-geometry inlet-guide-vanes and/or variable-geometry diffusers have to be added to variable speed centrifugal compressors to allow stable operation at all possible centrifugal chiller operating conditions. The inherent mechanical complexity of variable-geometry hardware has a negative effect on compressor cost and reliability. What is less appreciated is that compressor efficiency also suffers from variable geometry hardware. The inlet guide vanes introduce additional flow blockage and frictional losses at compressor inlet while the clearances needed for the movement of the variable geometry diffuser hardware introduce flow leakage passages resulting in parasitic flow leakage losses. Moreover, these losses affect compressor performance under all operating conditions, even those where variable speed control without variable geometry flow passage reduction results in stable compressor operation. This paper describes the application of the newly developed IntraFlowTM technology on a recently introduced two-stage variable-speed centrifugal refrigeration compressor. The concept will be explained in detail and test results will be shown. The compressor is stabilized and surge is postposed by injecting a small amount of flow upstream of the throat area of the vaned diffuser of the first stage compressor. The increase in stable operating range using this technique is substantially larger than what can be obtained with variable geometry inlet guide vanes. Using this technology the compressor also achieves higher efficiency due to the elimination of the blockage, friction and leakage losses that accompany the variable mechanical geometry surge/capacity control concepts. The amount of flow to be injected is controlled by an externally mounted flow control valve which increases reliability and serviceability.