Air compressor

Classification

Hydraulic source

• Internal combustion engine: Petrol, petrol without oil, diesel
• Electric: AC, DC

Drive type

• Direct drive
• Belt drive

Compressors may be classified according to the pressure delivered:

• Low-pressure air compressors, which have a discharge pressure of 150 pounds per square inch (10 bar) or less
• Medium-pressure compressors which have a discharge pressure of 151 to 1,000 pounds per square inch (10.4 to 68.9 bar)
• High-pressure air compressors, which have a discharge pressure above 1,000 pounds per square inch (69 bar)

There are numerous methods of air compression, divided into either positive-displacement or roto-dynamic types.

• Single-stage reciprocating compressor
• Multi-stage reciprocating compressor
• Single stage rotary-screw compressor
• Two-stage rotary screw compressor
• Rotary vane pump
• Scroll compressor
• Centrifugal (roto-dynamic or turbo) compressor
• Axial compressor, often used in jet engines.
• Another way of classification is by lubrication type: oil lubricated and oil-free. The oil-less (or oil-free) system has more technical development such as they do not require oil for lubrication. oil-less air compressors are also lighter and more portable than oil-lubricated models but require more maintenance. On other side Oil-lubricated air compressors are the more traditional type of air compressor. They require oil to lubricate the motor which helps prolong the compressor's life. One of the benefits of oil-lubricated compressors is that they tend to be more durable and require less maintenance than oil-free compressors.

Positive displacement compressors

Positive-displacement compressors work by forcing air through a chamber whose volume is decreased to compress the air. Once the pressure is greater than the pressure outside the discharge valve, a port or valve opens and air is discharged into the outlet system from the compression chamber. Common types of positive displacement compressors are

• Piston-type air compressors, which compress air by pumping it through cylinders by reciprocating pistons. They use one-way valves to admit air into the cylinder on the induction stroke and prevent it from leaving by the same route, and out of the cylinder through the exhaust valve to the high pressure side on the compression stroke, again using a non-return valve to prevent it leaking back on the next induction stroke. Piston compressors can be single or multi-stage, and may also have one or more sets of cylinders in parallel (at the same pressure). Multi-stage compressors provide greater efficiency than their single-stage counterparts for high compression ratios, and generally use interstage cooling to improve efficiency.

The capacities for both single-stage and two-stage compressors are generally specified in Standard Cubic feet per Minute (SCFM) or litres per minute and pounds per square Inch (PSI) or bar. To a lesser extent, some compressors are rated in actual cubic feet per minute (ACFM). Still others are rated in cubic feet per minute (CFM). Using CFM alone to rate a compressor is ambiguous because it represents a flow rate without a pressure reference. i.e. 20 CFM at 60 PSI.

Single stage compressors usually fall into the fractional through 5 horsepower range. Two-stage compressors normally fall into the 5 through 30 horsepower range.

• Rotary screw compressors provide positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is decreased as the screws turn. Rotary screw compressors can be single-stage or two-stage.
• Vane compressors: use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume. This type of compressor delivers a fixed volume of air at high pressures.

Roto-dynamic or turbo compressors

Roto-dynamic air compressors include centrifugal compressors where Rotating vanes impart kinetic energy to a gas and stationary passages convert velocity into a rise in pressure, and axial compressors, where rotor blades impart the kinetic energy and stator blades convert it to a rise in pressure.