Introduction - A machine providing gas at high pressure is called a compressor and work be done upon the gas by external agency . A refrigerant compressor takes gas in refrigerant gas at low pressure or evaporator pressure . compresses it and delivers the high pressure gas to condenser . Since the process of compressing the gas requires that work should be done upon it . it will be clear that compressor must be driven by some form prime mover ..

Types of compressor 

For the vapour  leaving the evaporator the compressor serves as the re-claiming agent . The vapours  are sucked from evaporator compressed to the pressure corresponding to a saturation temperature which will be higher than the coolant available to condense it . The coolant may be ambient air or ambient water naturally available .The other function of the compressor in the refrigerant cycle is to continuously circulate the refrigerant through the system .In fact the capacity of the compressor determines the capacity of the refrigerating system . compressor is the costliest item of a refrigeration system . It thus need of the day to develop a cheap and reliable compressor .

There are mainly three types of compressor used for refrigeration systems ;
1. reciprocating compressors     2. centrifugal compressors 
3. rotary compressors 

The reciprocating compressor find application and are very suitable for refrigerant which require relatively small-displacement volumes and high condensing pressure . 

The centrifugal compressor find application and are very suitable for refrigeration which require relatively large displacement volumes and low condensing pressure .

The rotary compressor find application and are very suitable for refrigerants having moderate or low condensing pressure .

Reciprocating compressor 
Reciprocating compressor are available in sizes as small as 1/2 KW  which are used in small domestic refrigeration up to about 150 KW for large capacity installations . The reciprocating compressor are classified as ;
1. Open type 
2. Sealed or hermetic type and 
3. Semi-sealed or semi-hermetic types .

Open types compressor is flexible in the sense that by varying the speed of the compressor different capacities can be obtained .It can be operated by any prime-mover such as electric motor petrol/diesel engine , steam engine etc. In the field the motor can be easily changed in case of motor burn out .The refrigeration system is not affected by motor burnt out  .

Sealed or hermetic type , however has no need for the shaft seal .These are usually make in small capacities .In India up to 1 KW sealed compressor for R-12 and upto 2.5 KW for R-22 are available . Though 5 KW sealed compressor are quite in use abroad .

Semi-bolted or semi-hermetic compressor differ from those of sealed type since motor is accessible repair just by removing the bolted plate . So motor can be repaired without any difficulty .In the sealed or hermetic compressor the seal has to be cut for repairs of motor or compressor .Their-after sealed is again welded to bring back to original condition .This is quite cumber -some and costly affair as compared to the repair done in semi-bolted .

This article was taken by Refrigeration & air conditioning ( P.L. Ballaney )


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