Unfiltered compressed air frequently contains dust, oil, rust, moisture and other harmful substances, and therefore requires filtration. In the first stage of filtration, the compressed air passes through a tube-shaped mesh filter, which creates a coalescenceeffect. Here bigger particles are adsorbed on the filter and the water will condense into larger droplets, which can then pass into the separation chamber. The compressed air is slowed down, which makes the particles condense on a honeycomb-like pad, allowing the water droplets to travel to the bottom of the drainage system and through an automatic or electric drain valve to the discharge. In the first filtration stage more than 95% of the water droplets, oil and large particles are removed. This practice is most common for removing water, but is also used for removing oil.

In the second filtration stage the air is passed through fiber made of cotton, generating thousands of small vortices and accelerating the air.