Types of Air Compressors

Reciprocating, rotary screw and rotary centrifugal air compressors

Sponsored LinksThe three basic types of air compressors are:

Reciprocating

Rotary Screw

Rotary Centrifugal

These types are further specified by:

the number of compression stages

cooling method (air, water, oil)

drive method (motor, engine, steam, other)

lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the compressed air)

packaged or custom-built

Reciprocating Air Compressors

Reciprocating air compressors are

positive displacement

machines, meaning that they increase the pressure of the air by      reducing its volume. This 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.

Single-stage and two-stage reciprocating compressors are commercially available.

Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig.

Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig.

Note that

1 HP ~ 4 CFM at 100 psi

and that 1 to 50 HP are typically for reciprocating units. Compressors 100 hp and above are typically Rotary Screw or Centrifugal      Compressors.

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

.

Load reduction is achieved by unloading individual cylinders. Typically, this is accomplished by throttling the suction pressure to the      cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying speed in engine-driven units through      fuel flow control.

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.

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 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.

The oil free rotary screw air compressor utilizes 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 air cooled and water cooled and provide the same flexibility as oil      flooded rotaries when oil free air is required.

Centrifugal Compressors

The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotatingimpeller 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.