Compressors
The purpose of the compressor in the vapour compression cycle is to compress the low-pressure dry gas from the evaporator and raise its pressure to that of the condenser.
Compressors may be divided into two types, positive displacement and dynamic, as shown in Figure 4.1. Positive displacement types compress discrete volumes of low-pressure gas by physically reducing the volumes causing a pressure increase, whereas dynamic types raise the velocity of the low-pressure gas and subsequently reduce it in a way which causes a pressure increase.
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ROLLING PISTON
Figure 4.1 Chart of compressor types
105
36000.00
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104 |
6000.00
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10°
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0.25 |
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10“ |
0.25
Figure 4.2 Approximate range of capacity covered by various compressor types
Figure 4.2 shows the approximate range of refrigeration capacities covered by various types. The most easily recognizable positive displacement type is the reciprocating or piston compressor, and being easily visualized, it will be used as a reference for descriptions of the compression process, and compressor features before moving on to other types.
The first refrigeration piston compressors were built in the middle of the 19 th century, and evolved from the steam engines which provided the prime mover (see Figure 4.3). Construction at first was double acting, but there was difficulty in maintaining gas-tightness at the piston rod, so the design evolved
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Figure 4.3 Double acting ammonia compressor and steam engine (Vilter) |
Further into a single-acting machine with the crankcase at suction inlet pressure, leaving only the rotating shaft as a possible source of leakage, and this was sealed with a packed gland. Today, the majority of compressors are completely sealed, with the motor enclosed.
Posted in Refrigeration and Air Conditioning