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Part load operation of screw compressors

 

A screw compressor is a fixed geometric compression device. It will capture a fixed volume of gas on the suction side and decrease that volume of trapped gas to expel at a higher pressure on the discharge end of the compressor. The ratio of the volume in the cavity when suction port closes to the volume in the cavity when discharge port uncovers is known as volume ratio or volume index, Vi. Operation of the screw compressor is the most efficient if discharge port is uncovered in the instant when pressure in the cavity has been raised to the pressure of the discharge line. Energy efficiency of this compressor will suffer if trapped gas will be under compressed or over compressed compared to the discharge pressure.

Operating pressures (suction, discharge) of screw compressor can change. To keep operation of this compressor at peak efficiency, compressor manufacturers have developed approaches to vary the volume ratio of their machines. The general principle behind variable volume ratio (or variable Vi) of screw compressor is that location of the discharge port is varied so the developed pressure of the gas during the compression process more closely matches the condensing pressure of the refrigeration system.

Refrigeration loads of every plant vary. To match these loads, capacity of screw compressors should be changed as well. The most common device for achieving a variation in refrigeration capacity with screw compressors is capacity control slide valve. In contrast to the volume ratio slide valve, that effectively repositions the location of the discharge port, capacity control slide valve functions to reposition the location of the suction port (technically, the slide valve repositions the starting point for compression process).

Even though the slide valve can provide smooth changes of capacity, the method results in reduced efficiency at part load. Energy use per unit of refrigeration gradually increase when screw compressor with variable Vi unload from 100% capacity to 50%. When capacity of this compressor is below 50%, energy efficiency will reduce rapidly (exponentially). Why does it happen?

Typical reasons for the drop in efficiency associated with unloading by slide valve are; (1) the friction of the gas venting back to suction and (2) the changing of the Vi of the compressor which is assumed to be perfectly matched to the external conditions at full load. Several compressor manufacturers claim that their compressors with variable Vi can keep required Vi when compressors unload from 100% to 50%. It means that in this range of capacity only gas friction has an affect on compressor efficiency. Below 50% capacity gas friction and changes of the Vi will reduce efficiency more rapidly.

Today, many refrigeration plants have screw compressors with VFDs to minimize losses related to slide valve unloading. Usually one compressor with VFD is used for each suction pressure. Installation of several compressors with VFDs for one suction pressure will not improve energy efficiency of the refrigeration plant because VFD itself uses 3 - 3.5% of additional energy.

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