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TRG-TRC004-EN 39
period four
Review
notes
Period Two reviewed various methods of varying compressor capacity.
Reciprocating compressors typically use cylinder unloaders that match the
compressor capacity to the evaporator load by deactivating piston–cylinder
pairs.
Refrigeration systems using scroll compressors generally unload by using
multiple compressors, cycling them on and off as needed to satisfy the
evaporator load.
A common method of unloading a helical-rotary compressor is to use a slide
valve that is an integral part of the compressor housing. By changing the
position of the slide valve along the compressor rotors, the volume of
refrigerant vapor being delivered by the compressor can be controlled to match
the evaporator load.
Finally, centrifugal compressors generally use inlet vanes to “preswirl” the
refrigerant before it enters the impeller, lessening the ability of the impeller to
take in the refrigerant. As a result, the compressor’s refrigerant pumping
capacity decreases to balance with the evaporator load.
Alternatively, the capacity of any of these types of compressors can be
controlled by varying the rotational speed of the compressor motor. It is most
often applied, however, to centrifugal compressors.
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Figure 53