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

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 energy over 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, a displacement compressor, which works with a constant flow. Let us explain the principles behind each of these technologies.

Displacement Compressors

This is 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), Tooth 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 and, at the same time, increasing the pressure of the gas.



If we look at piston compressors, the simplest form is a bicycle pump. A piston moves up the cylinder, shrinking the space above meaning that the air has to increase in pressure to be able to fit into the smaller cavity.

A single acting piston compressor compresses on one end of the piston only whereas a double acting compressor compresses on both ends of the piston. Piston compressors can incorporate multiple stages of compression to achieve the desired pressure which makes them particularly suitable for high pressure applications. Both lubricated and oil-free designs are available with special designs and are able to compress gases other than air.


In a double rotor compressor (Screw or Tooth), the air is trapped and sealed (typically with oil, but sometimes with water, special Teflon coatings or extremely small tolerances) between the male and female rotor profiles. As the rotors rotate and mesh the air is pushed along the rotors into a smaller and smaller space, and, once again, the pressure will increase to allow the given volume of air to fit into the cavities in the compression chamber.


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. In a scroll compressor a single spiral shaped rotor oscillates against a similar fixed spiral 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 the gas using an impeller then slowing the fast moving air in a diffuser and volute to transfer the kinetic energy into pressure. You will typically find a centrifugal or turbo compressor being used in chemical and petrochemical applications, power generation, industrial gases, steel or glass manufacturing plants and even fertilizer plants, in fact anywhere that needs high volumes of air.


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 microscopic droplets of oil will carry through the machine and end up in the air network – and eventually in the process or product.

Some available solutions for oil-free air use an oil-injected compressor with banks of filters but this cannot be 100% guaranteed to be free of oil. If your process cannot tolerate any oil in contact with the product, then an oil-free compressor is the only way to ensure that there is no chance of 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.


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 compressors run at one fixed speed and are very efficient when operating fully on load 100% of the time, when the motor is running and compressed air is being produced. The inefficiency comes when the unit unloads and (stops making air). Although eventually the compressor will stop, it spends some of the time turning the motor and using energy whilst not actually producing anything, and therefore wasting energy. A VSD or, to be precise, the variable speed drive unit, turns the motor relative to the amount of air required: if the demand increases then the motor speeds up, if the demand decreases the motor slows down, and uses only the energy required to produce the air required – therefore no energy is wasted. In fact, a VSD can reduce energy consumption by up to 35% or even 50%, depending on the VSD technology. It is therefore definitely worthwhile getting anair usage audit to see if your energy consumption could be reduced by using a VSD compressor.

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.