The operation of an air compressor produces a large amount of moisture, and although the excess moisture can be drained, it cannot be entirely eliminated. Water often remains as a vapor or an aerosol, which can be difficult to remove.The use of compressed air dryers can assist in removing excess moisture from compressed air.

By cooling compressed air, refrigerated dryers remove moisture mechanically from the stream. Air that is cool cannot hold as much water vapor as warm air, so as the air cools, excess water vapor condenses out and becomes liquid water. The condensate must then be removed from the system.

Generally, compressed air dryers come in three different types: refrigerated compressed air dryers, membrane compressed air dryers, and desiccant compressed air dryers.

A refrigerant air dryer is a specific type of compressed air dryer that is used to remove moisture from compressed air, which always contains bulk liquid in the compressed air.