Compressors supply the air flow for all equipment in a system. Air compressors work in two phases: the compression operation and the release operation. There are two types of compressors, positive displacement and dynamic (also called centrifugal).
The rate at which a compressor can deliver a volume of air is noted in cubic feet per minute (cfm). Because atmospheric pressure plays a role in how fast air moves into the cylinder, cfm will vary with atmospheric pressure. It also varies with the temperature and humidity of the air. To set an even playing field, makers calculate standard cubic feet per minute (scfm) as cfm at sea level with 68° F air at 36% relative humidity. Scfm ratings are given at a specific pressure—3.0 scfm at 90 psi, for example. If pressure is reduced, scfm goes up, and vice versa.
Positive displacement air compressors
Positive displacement compressors take in air and mechanically reduce the space occupied by the air to increase pressure. Dynamic compressors use the mechanical action of rotating impellers to transfer pressure to the air.
Positive-displacement compressors can further be divided into rotary and reciprocating types. In rotary screw compressors, filtered air enters the inlet of the air end where male and female rotors unmesh. The air is trapped between the rotors and the air end housing. This space is reduced as the rotors remesh on the opposite side of the air end. Thus, the air is compressed and moved to the discharge port. Cooling fluid injected into the housing mixes with the air to seal, lubricate, and remove the heat generated by compression. This fluid forms a thin film between the rotors that virtually eliminates metal-to-metal contact and wear. The fluid is separated from the compressed air, cooled, filtered, and returned to the injection point. The compressed air passes through an aftercooler to reduce its temperature and is ready for the air treatment equipment.
Because 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 screw air compressors are easy to maintain and operate. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life.
Oil-free rotary screw air compressors use specially designed air ends to compress air without oil in the compression chamber yielding true oil-free air. Oil-free rotary screw air compressors are available as air cooled and water cooled and provide the same flexibility as oil flooded rotaries when oil-free air is required.
Reciprocating air compressors use a piston within a cylinder as the compressing and displacing element. Single-stage and two-stage reciprocating compressors are commercially available. Single-stage compressors are generally used for pressures in the range of 70-100 psig and two-stage compressors are generally used for higher pressures in the range of 100-250 psig.
The reciprocating air compressor is single-acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double-acting.
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections.
Dynamic air compressors
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.
Adjusting the inlet guide vanes is the most common method to control capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced. The centrifugal air compressor is an oil free compressor by design. The oil-lubricated running gear is separated from the air by shaft seals and atmospheric vents.
They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants.