This is an excerpt from The Compressed Air Manual, our comprehensive reference book for all things compressed air. Written by our engineers and compressed air experts, it contains all things you need to know about the compressed air industry.
There are several ways to remove water and moisture from compressed air.
The first method is overcompression, where the pressure is increased to a level above the required compressed air pressure; at this higher pressure, the free water droplets are removed. In the next step, the pressure is decreased to the required compressed air pressure. From this stage, only water vapor is present in the compressed air, as the relative humidity drops below 100%.
A second method is cooling, as the compressed air is cooled down to a lower temperature. At this lower temperature, the relative humidity exceeds 100% and free water droplets are formed. Those free water droplets are collected and removed. In the next step the temperature of the compressed air is increased again. From this stage, only water vapor is present in the compressed air, once the relative humidity drops below 100%.
A third method, chemical drying, is often used. In this method, the moisture is removed by absorption or adsorption in a certain substance. With absorption, the moisture is captured in a hygroscopic liquid or powder. The moisture is attached to this material and needs to be removed and replaced by new material for further operation. With adsorption, the moisture is captured in hygroscopic beads. The moisture molecules are transported into the pores through diffusion and are accumulated through physical binding and capillary condensation. In this case, when the beads are saturated, they have to be regenerated before they can start adsorbing again. The regeneration can be done by heating the beads or by sending extreme dry air over the beads. In both cases the water retaining forces are broken and the water molecules are removed.