Compressors can be subdivided into two groups: The first group operates in accordance with the principle of dynamic compression, and these compressors are oil free compressing systems.
The effective quantity delivered by a dynamic compressor depends on the operating pressure. Examples of compressors using the dynamic compression principle are turbocompressors of axial or radial design. With dynamic compression, single or multi bladed wheels are used to impart a high speed of flow to the air drawn in, thus converting the kinetic energy of the flow velocity into pressure.
The second group consists of compressors which operate in accordance with the displacement principle. Here, static pressure is built up through a moving part which draws in, compresses and expels a volume of air into, inside and from an enclosed space. The group of compressors operating in accordance with the displacement principle includes, e.g., piston compressors and rotary compressors. The quantity delivered by these compressors is not determined by the pressure, if internal leakage losses and delivery losses through limited volumetric efficiency are ignored. These compressor systems are supplied in oil free ("non lubricated"), oil lubricated and oil flooded versions.
Displacement compressors represent by far the most widely used types: Piston compressors, oil free and oil lubricated Vane compressors, oil lubricated and oil flooded Screw compressors, oil free and oil flooded The following sections provide only a schematic sketch of these systems as there is sufficient literature available on the subject. Compressors and compressed air treatment form one unit. Impurities such as water and dust are contained in the ambient air already. In addition, compression introduces oil into the compressed air unless one operates with oil free compression systems. Using oil free compressors is, however, intimately linked with the ultimate purpose of the compressed air and is not required in each case. It is not the purpose of this section to discuss the advantages or disadvantages of oil free compressors. It is sufficient to point out that the use of oil free compressors, where this is possible, keeps oil pollution away from the compressed air. It would be quite incorrect to state that the use of oil free compressors makes it unnecessary to treat the compressed air or that compressed air purification could be completely ignored.
The assumption is correct however that the compressed air generated from oil lubricated compressors can be treated like any other compressed air right up to the standard of sterile air. The statement that "compressor and compressed air treatment form one unit" is true, irrespective of the design of compressor. It is a statement of principle.
