When the air is compressed, the partial pressure of water vapour is proportionately increased. The air is also heated by compression, and when cooled between stages in the inter-cooler coils, the relative humidity increases, and when it exceeds 100% will tend to condense out onto the surface of the tubes and as droplets carried by the air-stream. The air from the inter-cooler coils is led into the large diameter vertical axis tube of a separator, where it changes direction by about 90 degrees and is slowed down considerably. When the airflow changes direction towards the outlet at the top of the separator casing, the denser droplets have a tendency to hit the walls and coalesce into a film, which will flow downwards to the bottom of the separator and collect there where it can be periodically discharged through a drain valve. This reduces the water content of the outlet air, which is then compressed again in the next stage cylinder, cooled again, and the water that condenses out is again removed by the next separator.