Most common Types of compressors, displacement compressors and dynamic compressors

There are two generic working principles for the compression of air (or gas), so compressors can be subdivided into two groups: The first group operates in the principle of dynamic compression; and the second group of compressors operate in positive displacement principle:

In dynamic compression group, compressors are oil free compressing systems. The effective quantity delivered by a dynamic compressor depends on the operating pressure.

For examples: compressors using the dynamic compression principle are turbocompressors of axial or radial design. In a dynamic compressor, the pressure increasing takes place while the gas flows. Single or multi bladed wheels are used to impart a high velocity of flow to the air drawn in. The flow rate of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser. Thus converting the kinetic energy of the flow velocity into pressure. All dynamic compressors are designed for large volume flow rates.

The second group consists of compressors which operate in accordance with the positive displacement principle. The air is drawn into one or more compression chambers, which are then closed from the inlet. Gradually the volume of each chamber decreases and the air is compressed internally. So static pressure is built up through this positive displacement process, later the compressed air can be discharged to compressed air distribution system.

Positive displacement compressors include piston compressors and rotary compressors (vane compressors and screw compressors).

These displacement compressor systems are supplied in three versions: oil free (“non lubricated”), oil lubricated and oil flooded versions.

Displacement compressors by far are the most widely used types, and the common types are listed as follows:

• Piston compressors: oil free and oil lubricated versions. 
• Vane compressors: oil lubricated and oil flooded versions. 
• Screw compressors: oil free and oil flooded versions. 

Oil free compressors are very commonly used in clean industries, such as food, pharmaceutical, electronics and semiconductor industries, etc.

But in other industries it is also a better choice to use oil free compressors, in where it is possible, to keep oil pollution away from the compressed air.

It is also not correct to say that the oil free compressors make the compressed air oil free, so the compressed air purification could be completely removed. In most cases, the atmosphere air is not clean enough, it contains solid particles, air borne molecules, oil vapor and oil aerosol. But the treatment of compressed air is much simpler than for compressed air from the other type of compressors.