What is an air compressor?
There are many types of air compressors that effectively increase the pressure of the air by reducing its volume. These mechanical devices work like pumps, but they alter the density of the air, an action which is easier to achieve with gases, which are compressible.
With many air compressors, the compression takes place in stages, which acts to increase the eventual discharge pressure. It will also increase the temperature of the air, so cooling between stages is often used.
How does an air compressor work?
Most air compressors work in two specific ways to achieve compression.
Positive Displacement Compressors
The positive displacement compressor uses air displacement to compress air and generate power. A defined amount of air is trapped in the compression chamber. The chamber is then slowly mechanically reduced in volume, resulting in a rise in air pressure and potential energy.
Positive displacement compression models are available in a variety of different designs and capacities, but the operating principle remains the same.
Dynamic air compressors use fast rotating blades to draw in the air. This air intake is then compressed to build the pressure, storing it as kinetic energy ready for use. The dynamic compressor has a narrow operating range, making it more suited to constant operation.
Rotary screw compressors
A rotary screw compressor uses positive displacement to hold air between rotors and then compress the air as it moves down the rotors. They usually operate with helical or spiral lobes to achieve variable speed and variable compressor displacement.
These rotary compressors are usually oil-cooled. They are unlikely to experience extreme operating temperatures and are easy to maintain and operate. Oil-free options use a different air end to compress air without the need for oil, providing the same compression efficiency as oil-operated compressors.
Rotary screw air compressors combine good air output and volume with longevity. They are typically used in larger industrial applications and automated processes.
Rotary Vane Compressors
Rotary vane air compressors are positive displacement units using radial vanes mounted on a cylindrical rotor which is positioned eccentrically to rotate inside a cylindrical housing, creating a reduction in volume from the inlet port to the discharge port.
The rotary vane compressor provides improved efficiency and constant pressure at a reduced cost. Its compact size means it is used in applications including air-powered tools, cooling operations and a range of commercial and industrial applications.
Centrifugal or radial air compressors are dynamic compressors which operate using a diffuser to slow and cool incoming air to increase pressure, transferring energy from rotating impeller blades. They are usually multi-phase in design to produce high levels of energy from a compact unit.
The centrifugal compressor operates at high speed and is typically used in higher-capacity applications. Some models can produce oil-free air with the running gear separated from the compressed air by seals and vents.
The centrifugal compressor is capable of providing up to 1,000 horsepower and beyond. It is used for demanding applications like construction sites and heavy industrial manufacturing.
Reciprocating air compressors
Reciprocating air compressors are displacement compressors using a piston and cylinder arrangement to build pressure by compressing and displacing the air. Multi-stage designs are used to achieve higher levels of pressure.
These positive displacement piston compressors are extremely efficient but create more noise. They have more moving parts than other compressors and may require more maintenance.
The reciprocating compressor is not suitable for continuous use and is typically used in smaller applications such as vehicle repair shops and even for home use.
Axial flow air compressors work by accelerating the air and then using a diffuser to increase the air pressure. A row of rotors accelerates the air which is then diffused using static blades, yielding the pressure increase.
Axial compressors are driven by an electric motor or by steam or gas turbines. They provide advanced levels of energy efficiency with a continuous flow but are typically more expensive than other compressor models. Applications include higher-value environments, including engines for ships and planes.
Rotary Scroll Compressors
Rotary scroll air compressors use two interlocking spirals, one stationary and one rotating, to form air pockets with differing pressures. The air is compressed along the surface of the scroll.
Commonly used for refrigerant applications, the scroll compressor offers a continuous flow, reduced noise levels and fewer moving parts, which in turn reduces maintenance requirements.
Diaphragm air compressors or membrane air compressors bring air into the compression chamber using a rotating membrane. This pressurises the air, which is then stored in a tube for later use.
Total separation of the compression chamber and the oil chamber means these units also qualify as oil-free compressors for applications where clean air is required.
Mixed Flow Compressors
Mixed flow or diagonal air compressors are dynamic compressors operating both axial and radial flow paths to create a diagonal compressor stage. The exit radius is greater than the inlet radius, similar to a centrifugal design.
The mixed-flow compressor is currently used in light jet aircraft.
There are many types of air compressors used across the industry today. Choosing the right air compressor package is dependent upon your application, required CFM output (flow rate) and operational environment.
Compressed air is often known as ‘the fourth utility, but there is often little understanding about how this valuable power source works, providing energy savings. The multiplicity of designs and operational methods demonstrate just how complex the medium is.
Most air compressor types are ultimately flexible; they can be powered by gas, electricity and some even by steam turbines. They have fewer maintenance requirements than other sources and usually operate with reduced noise levels. They can work at high temperatures and are also safer in more hazardous environments. With an estimated 70% of all manufacturers using air compression systems, they deserve more recognition!