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View Subscription Details: Letter ( 05/08/2006 ) - HTML Format Compressors

The types of compressors used in industrial refrigeration are screw and reciprocated. Customers often prefer the screw compressor because of lower maintenance costs. The screw compressor might run for 30,000 to 60,000 hours before overhaul, while plant operators or contractors have traditionally serviced reciprocated compressors annually.

The efficiency of the reciprocated and screw compressors is of the same order of magnitude at full load. At part load, the reciprocated compressor with cylinder unloaders almost always enjoys a higher efficiency than screw compressor. In general, part load performance of screw compressor degrades with lower suction and higher discharge pressure.

If your refrigeration plant has reciprocated and screw compressors, let the screw type handle the base load at full capacity and reciprocating compressors will provide the trimming at part load.

The screw compressor can work against higher compression ratios than can the reciprocated compressor. Thus, a screw compressor is capable of operating single stage from a low temperature evaporator in two-stage system during periods of low capacity demands.

The most common device for achieving a variation in refrigeration capacity with screw compressor is the slide valve.
Even though the slide valve can provide smooth changes of capacity, the method results in reduced efficiency at part load. Major reason for the drop in efficiency is the changing of the built-in volume ratio Vi of the compressor which is assumed to be properly matched to the external conditions (suction pressure and discharge pressure) at full load. The recommendation to plant operators is to operate screw compressors as close to the full load as the mix compressors allow.

Modern screw compressor has additional control system, by which the built-in volume ratio Vi of the compressor can be controlled. This is variable volume ratio Vi. Compressor with variable Vi always works at its highest efficiency, even at variable operating pressures in refrigeration plant. The Vi control is particularly effective with the compressor working at 100 % capacity and it acts as a mechanical stop to the slide valve. In this way the capacity slide valve adopts the size of the discharge gas outlet opening, maintaining the same compression ratio in the compressor as the pressure ratio from discharge to suction side in the refrigeration plant. At reduced compressor capacity, the Vi control system has little effect.

In general, a two-stage system is more efficient than economized single-stage system. However, economized single-stage system is more efficient than regular single-stage system. Typically, economized compressor loses economized operation at 70-80 % of slide valve position. Below this position the compressor operates as regular single stage one and efficiency will drop by 10 % to 15 %. This is one more reason to operate screw compressor as close to full load as possible.
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