An air dryer’s job is to remove moisture before it reaches the supply tank. Easy enough, right? So as warm, moist air comes off the compressor, it mixes with the cool walls of the air dryer. Some of this moist air will condense on the walls and drain down into the purge-valve. At that point, the air is forced through a filter that removes the moisture, collects solid particles and aerosol contaminates. This treats the air. Then it expels the treated air through the drying via a one-way valve.