If compression is achieved by trapping a refrigerant vapor in an enclosed space and then reducing its volume, then it is called a Positive-displacement type compressor.

Types of Compressor

1. Positive displacement type
2. Roto-dynamic type

Positive displacement type.

• In positive displacement type compressors, Compression is achieved by trapping a refrigerant vapor into an enclosed space and then reducing its volume.
• Since a fixed amount of refrigerant is trapped each time, its pressure rises as its volume is reduced.
• When the pressure rises to a level that is slightly higher than the condensing pressure, then it is expelled from the enclosed space, and a fresh charge of low-pressure refrigerant is drawn in and the cycle continues.
• Since the flow of refrigerant to the compressor is not steady, the positive displacement type compressor is a pulsating flow device.
• However, since the operating speeds are normally very high the flow appears to be almost steady on a macroscopic time scale since the flow is pulsating on a microscopic time scale, positive displacement type compressors are prone to high wear, vibration, and noise level.

Depending upon the construction, positive displacement type compressors used in refrigeration and air conditioning can be classified into:

1. Reciprocating
2. Rotary type with sliding vanes (rolling piston type or multiple vane type)
3. Rotary screw type (single screw or twin-screw type)
4. Orbital compressors
5. Acoustic compressors