All refrigeration and air-conditioning systems have four basic parts: the compressor, condenser, throttling or expansion valve and the evaporator. In refrigeration and air-conditioning units the heat is taken from the low temperature reservoir and thrown to the high temperature reservoir; hence this process requires external power which is given to the compressor.
The compressor sucks low pressure and low temperature refrigerant from the evaporator and compresses it to high pressure and high temperature gaseous state. The larger the size of the refrigeration plant, the larger the compressor will be and the more power will be required.
There are two type of compressors used commonly in the refrigerating and air-conditioning units. These are:
1) Reciprocating compressor: The working of reciprocating compressor is very similar to the reciprocating engine used in automotives. The difference is that while the engine generates power, the compressor consumes power and compresses the refrigerant. The reciprocating compressor is comprised of the piston and the cylinder arrangement connected by the connecting rod to the motor shaft. When the shaft of the motor rotates, the piston performs the reciprocating motion inside the cylinder, absorbing and compressing the refrigerant.
Reciprocating compressors can be used for small as well as large refrigerating and air-conditioning units. Their power consumption is more compared to rotary compressors, and they also make more noise.
Reciprocating compressors are of two types: i) Open type, and ii) Hermetics – totally sealed (welded) and semi-hermetic. The speed of the open compressor can be adjusted as per the capacity requirements. If it is a multi-cylinder compressor, a certain number of cylinders can be bypassed to adjust the capacity and reduce power consumption.
2) Rotary compressors: In rotary compressors the compression of the refrigerant is achieved by the rotary motion of the rotors instead of the reciprocating motion of the piston. There are two commonly used types of rotary compressors: the rolling piston type and rolling vane type.
- In the rolling piston type a rotor is fixed on the eccentric shaft, which rotates in the cylinder. A vane placed in the slot inside the cylinder acts as the dividing line between suction and discharge of the refrigerant.
- In the vane type of rotary compressor the rotor is concentric with the shaft and rotates inside the cylinder. The cylinder is offline with respect to the motor shaft. Depending upon the capacity of the compressor there may be multiple vanes on the shafts. As the refrigerant enters the compressor in gaseous state it gets trapped between successive vanes and gets compressed. The back flow of gas is prevented by the oil film between the surface of the cylinder and vane tip.