The classic saying ‘what goes in, must come out’ couldn’t be truer for compressed air. You can invest in a compressor with the strictest specification but that won’t guarantee you quality, clean compressed air.

Remember that manufacturers test their products in controlled environments. So unless you also control external factors you may not be able to achieve the quality of air promised.

The air around us is not only a mixture of gases but contains water vapour, dust, pollen and bacteria. In an industrial environment the ambient air is likely to have higher levels of oil particles, depending on the proximity of the compressed air system to other processes.

For example, Direct Air have previously visited a site near to a major motorway. They had also placed their compressor house next to the factory’s vehicle loading bay, so exhaust fumes were entering the system. This was causing contamination to the final product and damage to the compressed air system.


Any environment which is not 100% sterile can impact the quality of your compressed air. The air will remain the same quality following compression, unless the air is treated!

So, how do we remove unwanted substances from our compressed air?


Filters work in compressed air systems to separate the air particles from contaminants. Different filter types should be used throughout the system, as particles come in different sizes and no single filter is efficient for the entire particle size range. The most difficult particles to separate are between 0.1μm and 0.2μm.

Coalescing filters are probably the most important purification equipment in the system. They can remove oil and water aerosols to very low levels, as small as 0.01 micron. And with an activated carbon filter, oil can get reduced to 0.003 mg/m3.


Following compression, compressed air gets very hot to anywhere between 70 and 200°C! An after-cooler tends to get placed directly after the compressor. It not only lowers the temperature, but also reduces the water content. This removes most of the condensation away from the system as soon as possible. This now tends to get included as standard in compressor installations and can be either water or air cooled.


Dryers reduce the moisture levels below the point where condensation can occur later in the system. Typically, a refrigerated dryer would be used as this removes water by cooling the air to near freezing temperature. For even dryer air, a desiccant dryer reduces water content by absorbing the water with adsorbent desiccant material.

It needs to be noted that refrigeration and absorption dryer remove water vapour, not water in liquid form. So, coalescing filters get installed in front of them in the compressed air system.

Water separator

If the water separator is correctly sized, it can achieve up to 90% efficiency in separating condensation from the compressed air.

It’s important to first understand the level of air quality required for your system. You can then determine what combination of air treatment solutions suit your applications.