Rotary Screw Compressors
Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package. Rotary compressor uses a special type of air compressor filter to work effectively.
Reciprocating Air Compressors
Reciprocating air compressors are examples of positive displacement machines. This means that they increase the pressure of the air by reducing its volume. This also means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element.
The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. Centrifugal compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous
Axial compressors are rotating, air-foiled base compressors where the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axial-centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit.