How Do Air Compressors Work? @DirectAir

Air compressors are an extremely valuable tool for both DIY and industry, depending on the job at hand, there are several types to choose from.

Direct Air have created this handy graphic on how air compressors work, this particular image features how a Hydrovane series rotary sliding vane air compressor functions.

Air compressors have a large variety of use, from gas-filled cylinders for industrial and scuba diving purposes to generating power for pneumatic tools and spray guns.

Below is an in-depth guide to how air compressors work to inform and help make the proper decision for your business.

Dual Phase vs. Single Phase Compressors

These are the most common type of air compressor; both essentially function in the same manor. Dual phase simply has one more step in the process.

To directly explain, single-phase uses one canister for the compression process and the air is compressed once, dual phase uses two and compresses the air twice.

Not to be confused with the number of cylinders a compressor has, many single phase machines have two cylinders.

How Do These Operate?

Single phase compressors draw air into the cylinder, and this is then compressed by a single piston using a vacuum system. Typically, the air is compressed at around 8.2 Bar. It is then sent to a storage tank awaiting use.

Dual phase uses the same concept but instead of one phase to compression, there are two. After the initial stage, the air is sent to the second cylinder to be compressed again at around 12.1 Bar.


As these fundamentally operate in similar ways, they can be used for the same tasks. Most commonly for powering pneumatics drills or similar tools.

Single phase compressors are more commonly used with DIY while dual-phase typically are used within industries.

Oil Lubricated vs Oil-Free

When looking to purchase an air compressor, it is important to check if they use oil or not as these can be suited to different jobs.

For air to be properly drawn into the cylinder, the piston needs to be well lubricated with oil.

There are two different means to ensure adequate lubrication, oil-free and oil-based.

The Difference between Oil-Free and Oil-Based

Oil-free compressors use non-stick methods to lubricate the cylinder (such as Teflon). Therefore there should be no maintenance in relation to lubrication.

Oil-based requires oil to be regularly added and changed to keep your machine running properly.

Oil-free are typically lighter in weight and require less mechanics so they are more compact, less complex and more affordable.

However, oil-based are more durable machines. They often have a longer lifespan than their counterparts as their lubrication can not wear down. They do however tend to emit more heat and more noise.


Oil-free compressors are commonly for home use, being more lightweight and affordable.

Oil-based are suited for commercial and industrial use.

Oil-based have several uses in a commercial setting. These include, but are not limited to vehicle repairs and painting, woodwork, dentistry tools and other medical use and pneumatic tools.

Oil-free compressors within domestic use are commonly used for small-scale jobs such

as blowing up balloons and home workshop jobs.

They are also used commercially within catering as the risk of food being contaminated with oil is too high.

Variable vs. Fixed Air Compressors

When choosing the right air compressor, you must consider if you require a fixed or variable speed.

The main difference is how the motor gains power. The compression element is similar between all compressors but the way in which the motor operates affects the usability, lifespan and efficiency.

How Do Variable Speed Compressors Work?

Also referred to as VSD (Variable Speed Drive) or VFD (Variable Frequency Drive), these automatically operate by adjusting the speed of the motor in relation to the demand for air. The system converts voltage from the main power supply into a variable frequency.

The convertor draws through power and this is then converted twice. Initially to AC power, then DC. The AC is cleaned through a capacitor then a transistor converts it to DC. This is then sent to the motor which controls the speed.

This allows a VSD to be closely controlled.

Rotary Screw, Scroll and Piston Compressors

We have previously mentioned piston compressors, there are generic machines for one type of compressor called the scroll compressor, also known as a reciprocating air compressor.

Another type is a rotary screw compressor which does not use pistons but instead operates in a different manor.

Scroll Compressors

These are the most common air compressor; a variation of piston compressor and they are widely available and the most affordable.

A piston travels downwards and decreases pressure within the internal cylinder with the use of a vacuum. With this sudden change in pressure, the door to the cylinder is forced open and air drawn through.

When the piston travels back up, air is forced out of the cylinder at high pressure. This continues in a reciprocating scroll pattern, which is where it gets its name.

Rotary Screw Compressors

These reply on rollers for compression, unlike a piston. Rollers are located in the central shaft and one side of the roller is always in contact with the wall. These then rotate at extremely high speeds producing the same effect as scroll compressors.

Low Noise Air Compressors

It is not unheard of that air compressors generate too much noise which in turn can lead to complaints from staff and others.

Low noise air compressors are available with a decibel as low as 40dB. These are available in both oil-based and oil-free.

These have an acoustic chamber purely for containing noise and therefore reducing the decibels emitted from the machine.


There are a variety of factors to consider before choosing your air compressor. Consider the application that your air compressor will be used for. Will you require continuous use, or will this be used sporadically?

You budget will also dictate what machine you will purchase, keep in mind that it will not just be the initial cost, but also the maintenance of the compressor.