In a sense they function like pumps, they perfom mechanical work on a process fluid. This results in the reduction of fluid volume, but an increase in the fliud pressure. The compressors transfer this energy either continuously or intermittently.
Continuous energy transfer compressors, are either ejector or dynamic. The dynamic compressors are either axial or centrifugal, with centrifugal being radial or mixed flow.
With intermittant energy transfer compressors, we have either positive displacement, which it usualy reciprocating or rotary, which can be lobe or vane or liquid piston.
The ejector type compressors are entirely static pieces of equipment. They convert the pressure of the working fluid into velocity. This is achieved thermodynamically, using adiabatic expansion of a motive fluid through a nozzle. The motive fluid velocity is very high and as it expands it creates a suction thus pulling in process fluid. The mixture of working and motive fluids pass through the convergent throat and then divergent diffuser. The high velocity is converted to pressure across the throat.
This lack of moving parts make it very simple in construction, hence very reliable and low maintenance costs. However it is very inefficient, as the very transition of the fluid across the constriction causes a shock wave that causes pressure loss.
Dynamic compressors consist of single or multiple stages of compressors. Each stage comprising of a rotor or impeller which is in motion rotating against a static row of blades, known as a stator or diffuser. The are run at high speeds, converting kinetic energy to pressure. Axial flow compressors usually have the intake and the dis-charge along the same plane, the axis of the compressor rotors. With radial and mixed flow compressors, also known as centrifugal compressors, the energy transfer takes place mainly because of centrifugal forces acting on the working fluid. The radial flow compressor has a complex fluid flow path, but can achieve a very high pressure ratio.
The mixed flow compressor resembles a radial flow compressor with an inducer, with the flow path being at an angle to the flow path. Their pressure ratios lie between that of radial and axial.
