The air dryer (Fig. 25) is a desiccant-type in-line filtration system that removes most liquid and water vapour from compressor discharge air before it reaches the air brake reservoirs. This results in only clean, dry air being supplied to the air brake system, aiding in the prevention of air-line freeze-ups.

Air dryers utilize a replaceable desiccant material that has the ability to strip water vapour from moisture laden air. The desiccant material is regenerative, in that its absorptive properties are renewed each time the compressor is reloaded.

The air dryer end cover is equipped with an automatic drain valve, controlled by the air-system governor, and is also equipped with an integral heating element.

Air dryers do not remove all the moisture. The reservoirs still need to be drained daily when in use.