How does an air compressor work and which one should you choose

Understanding how an air compressor works is fundamental to finding the right one for your business. Once you identify a compressor suitable to your needs, my suggestion is to rent it: you may save up to 50% on energy consumption!

How does an air compressor work

The air compressor is a device that raises the pressure of a gas / steam using mechanical energy, which it then converts into potential energy or pressure energy.

There are two main types of air compressors:

• Roto-dynamic type, where the compression is caused by the speed given to the body (the most common ones are centrifugal compressors)
• Positive-displacement type, where compression is caused by mechanical movements (the most common one is the piston-type)

Roto-dynamic compressors

Within a roto-dynamic compressor, the air flow is compressed using the kinetic energy imparted to the gas by ad hoc mechanisms. The most common roto-dynamic compressor is the centrifugal compressor. Centrifugal compressors’ main characteristic is to increase the discharge pressure related to the increase of their spinning speed (which is usually quite significant >10000 rpm) while the air flow rate remains constant and is independent form speed. They are called multi-stage compressors, because there are usually different size impellers, which increase the air pressure gradually until it reaches the set value.

These compressors are usually utilized in oil-free processes and with medium-high air flow rates. They are reliable devices but their maintenance is expensive. They are used for instance by food production companies, pharmaceutical and chemical companies.

Positive-displacement compressors

Positive-displacement compressors are much more widely used than roto-dynamic compressors, and they withdraw a constant volume of gas regardless of upstream and downstream pressure. Its air flow rate is directly proportional to spinning speed, while pressure remains constant as it depends on the compression ratio between inlet volume and discharge volume.

The most common positive-displacement compressor is the piston type. It is a device which is made of pistons that move constantly within a cylinder chamber, which, through the help of one-way valves, regulates and compresses the air flow.
These compressors are suitable for small factories and artisanal use, because their big dimension and high noise would not allow them to be used on large-scale processes.

They are very reliable and have limited maintenance costs. They are available on the multi-stage compressors market and work efficiently with working pressure of > 20 bars.

Finally, piston-type compressors can be classified as oil-free because of their use in food supply chains, chemical and health companies.

Rotary vane compressors belong to the family of rotary positive-displacement compressors, those devices which squeeze air from a bigger chamber to a smaller chamber by using rotating elements, increasing thus the air’s pressure and its discharge. The compression group is made of a cylinder, the stator, where there is a slotted rotor with longitudinal slots in which the vanes slide; it is covered by two lids.

The vanes are pushed against the stator by centrifugal force; they never touch the stator but rather float on an oil film that prevents their wear. The rotor mast is supported by a bush as there are is no axial thrust to compensate. The compressor’s spinning speed may change from 1000 rpm to maximum 3000 rpm and it depends on the specific features of the electric engine connected to the compressor through an elastic joint.

This very reliable and efficient compressor delivers a fixed volume of air at middle to low pressure, but it is not oil-free.

Rotary screw compressors are volumetric rotary compressors made of two parallel rotors with external helical profiles (screws) which enables the two rotors to engage, one into the other. The two rotors are fitted in a stator made from two cylinders which intersect longitudinally and in which the rotors turn with a critical minimum clearance.

The screws are supported by ball or roller bearings, and usually one rotor engages the other through meshing helical profiles. They are sometimes controlled by external gears to decrease friction. During rotation, the screws’ profiles uncover the inlet port at the suction side of the stator, through which air is let in to fill up the volume between the profiles until its maximum capacity.

The mashing rotors force the gas through the compressor to compress it, and on the opposite side, the gas exits at the end of the screws through the outlet port.

Rotary screw compressors are widely used thanks to the technology utilized for manufacturing. There are numerous manufacturers and assemblers, who supply a variety of these compressors: single-stage, two-stage for high pressure, with oil or oil free. The spinning speed is usually higher than that of the engine, thanks to gear boxes of the pulley ratio.

To guarantee a saving of energy, frequency converters have been used with compressors, because as they change the screws’ speeding speed, they also change the air-flow rate, allowing for better efficiency at part load.

Get in touch with author: vincenzo.grecomanuli@progettoenergiaefficiente.it