Top Tip: So many types of air compressor. What's the difference?

what type air compressor to choose

What do we mean by positive displacement compression or dynamic compression? When is oil-free technology preferable to oil-injected? Why does a VSD compressor save more energy than a fixed speed one? We have put together a quick guide for those who need to know what’s what, or are just interested in air compressors!

Essentially, there are two generic principles for the compression of air or gas: positive displacement compression and dynamic compression. Positive displacement compressors include, for example, piston compressors, scroll compressors and rotary screw compressors.
Dynamic compressors are frequently called turbo compressors. Those with radial design are called centrifugal compressors. A dynamic compressor works at a constant pressure, unlike, for example, a displacement compressor, which works with a constant flow. Let us explain the principles behind each of these technologies.

Displacement Compressors

This is arguably the most common type of compressor grouping that you will see in an industrial setting, and includes all of our favourites! : Piston compressors (Reciprocating), Screw compressors (Double Rotor) and Scroll compressors (Single Rotor).

Positive displacement compressors have a cavity that allows a volume of gas into the machine at atmospheric pressure. This chamber then becomes smaller, decreasing the volume of the air and, at the same time, increasing the pressure of the air.

If you look at reciprocating compressors, the simplest form is a bicycle pump. A piston moves up the cylinder, shrinking the space above (or below in the case of double acting piston units), meaning that the fixed volume of air has to increase its pressure to be able to fit into the cavity.

A single action (compressing only on half the stroke) single cylinder compressor would produce pulses of compressed air as the piston moves up and down the cylinder, so typically with reciprocating compressors you will have multiple cylinders working out of phase with each other to mitigate this.

In a double rotor compressor (Screw, Lobe or Tooth), the air is trapped and sealed (typically with oil, but sometimes with water, special Teflon coatings or extremely small tolerances) between two shafts that look very similar to gears. As this air moves across the rotors it has less and less space available, and, once again, the pressure will increase to allow the given volume of air to fit into the cavities in the compression chamber. These types of compressors tend to produce a flow of compressed air that does not pulse, as air is constantly drawn into the rotors as it is constantly being ejected at a higher pressure.

There are a few single rotor compressor types - Vane, Liquid ring and Scroll. Atlas Copco has the SF scroll compressors as their main offering in this category. However some of our vacuum pumps also use this technology…but in reverse. A single spiral shaped rotor rotates against a similar fixed spiral in the compressor and, as these spirals move against each other, the cavity trapping air between them becomes progressively smaller. This decrease in volume forces the fixed volume of intake air to increase in pressure.

Displacement compressors can sometimes be referred to as Constant Flow Compressors, as the compressor will produce the same flow at a given motor RPM, no matter the outlet pressure.

Dynamic Compressors

These are a bit more exciting – think in the realm of Turbos and Jets! In the compressed air industry, you are unlikely to find too many ejector or axial compressors, because typically these are used for powered flight. However, radial dynamic compressors are definitely of interest to us and are more commonly called Centrifugal or Turbo compressors. In a dynamic compressor, the pressure increase is achieved by accelerating gases to high velocities using an impeller, and then smashing this fast flowing air into a solid surface (called a volute or diffusor). You will typically find a centrifugal or turbo compressor being used in chemical and petrochemical applications, power generation, industrial gases, and even fertilizer plants.

Air quality: Oil-Free vs Oil-Injected air compressors

Atlas Copco has an extensive range of products and can feasibly meet any compressed air requirement out there. So why don’t we start with air quality, the main categories of which are Oil-Flooded / Oil-Injected technology and Oil-Free compressor technology.

Most air compressors on the market rely on oil in the compression chamber for lubrication, sealing and cooling, but, of course, this means that some of the oil will get mixed into the air and that microscopic droplets of oil will carry through the machine and end up in the air network - and eventually in the process.

Some available solutions for oil-free air actually use an oil-injected unit with banks of filters but this cannot be 100% guaranteed to be free of oil. If your process cannot have any oil in contact with the product, then an oil-free compressor is the only way to ensure that there is no chance of product contamination by oil. In simple terms, an oil-free compressor guarantees that no oil or grease can enter the compression area of the compressor. The method of compression will be similar in nature, but the oil-free compressor is designed to use alternative methods of sealing to ensure that oil cannot enter the compression space.

In terms of cost, oil-free compressors are more expensive in the first place, but their running costs are usually lower in terms of consumables and, of course, there is no risk of product contamination. Atlas Copco was the pioneer in Oil-Free compressor technology and was also the first of the major compressor manufacturers to meet the Class 0 ISO 8573.1 standard, which guarantees Oil-Free air.

Fixed Speed Compressors vs. Variable Speed Driven Compressors

We’ve heard about it, but why does Variable Speed Drive (VSD) technology save energy when compared to fixed speed? To put it simply, the clue is in the name! Fixed speed units run at one speed are very efficient when operating fully on load 100% of the time, because you are running the motor and producing air. The cost comes when the unit unloads (stops making air). Although eventually it will shut down, it spends some of its time turning the motor and using energy whilst not actually producing anything, and therefore it is wasting energy. The VSD or, to use the proper name variable speed drive unit, turns its motor relative to the amount of air required: if the demand increases then the motor speeds up, and if the demand decreases the motor slows down, so you are only using the energy required to produce the air required – therefore no energy is wasted. In fact, a VSD can reduce energy consumption by 35% or even 50%, so it is definitely worthwhile getting an air usage audit to see if your application would gain from using one.

Check out the more detailed comparisson of fixed speed vs. variable speed compressed air technology.

At the end of the day, there is a need in the market place for a mix of both technologies depending on the air requirement for the given application.

We hope that this brief overview offers some insights into air compression, a process which is often overlooked, but much depended on in manufacturing.

If you’re interested, we could go on about air compressors for hours! Please get in touch on 01 4505978 or contact

Piston compressor VSD Variable Speed Drive High pressure compressors Screw compressors Air compressors Oil-lubricated compressors Multistage centrifugal compressors