TYPES OF COMPRESSORS

TYPES OF COMPRESSORS

There are two generic principles for the compression of air (or gas)

1. positive displacement compression
2. dynamic compression

• Positive displacement compressors include, for example, reciprocating (piston) compressors, orbital (scroll) compressors and different types of rotary compressors (screw, tooth, vane).

• In dynamic compression, air is drawn between the blades on a rapidly rotating compression impeller and accelerates to a high velocity. The gas is then discharged through a diffuser, where the kinetic energy is transformed into static pressure. Most dynamic compressors are turbo compressors with an axial or radial flow pattern. All are designed for Large volume flow rates.

Positive displacement compressors

• A bicycle pump is the simplest form of a positive displacement compressor, where air is drawn into a cylinder and is compressed by a moving piston. 

• The piston compressor has the same operating principle and uses a piston whose forward and backward movement is accomplished by a connecting rod and a rotating crankshaft. If only one side of the piston is used for compression this is called a single-acting compressor. If both the pis-ton’s top and undersides are used, the compressor is double acting.

Dynamic compressors

• In a dynamic compressor, the pressure increase takes place while the gas flows. The flowing gas accelerates to a high velocity by means of the rotating blades on an impeller. 

• The velocity of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser. Depending on the main direction of the gas flow used, these compressors are called radial or axial compressors.

Comparison: turbocompressor and positive displacement

• At constant rotational speed, the pressure/flow curve for a turbocompressor differs significantly from an equivalent curve for a positive displacement compressor. 

• The turbocompressor is a machine with a variable flow rate and variable pressure characteristic. On the other hand, a dis-placement compressor is a machine with a constant flow rate and a variable pressure.

• A displacement compressor provides a higher pressure ratio even at a low speed. Turbocompressors are designed for large air flow rates.