Desiccant air dryers are used when the compressed air application requires a pressure dew point below 0°. The regenerative desiccant air dryers consist of two pressure vessels. Both vessels are filled with desiccant. One vessel is removing moisture from the compressed air.

Wet air passes directly through the desiccant bed which adsorbs the moisture. When this vessel is saturated with moisture the valves will switch and lead the air to the other standby vessel. During adsorption in the other vessel, the first vessel will be regenerated. It's a cyclic process.

The desiccant medium has a finite capacity for adsorbing moisture before it must be dried out, or regenerated. To do this, the tower containing saturated desiccant medium is depressurized and the accumulated water is driven off.
How this happens depends on the type of desiccant air dryer.

• Heatless dryers use only compressed air as a purge
• Blower purge dryers use a combination of air from an external blower, heat, and minimal compressed air
• Blower zero purge dryers use a combination of air from an external blower, heat, and zero compressed air
• Heat of compression dryers use the heat of the compression
• Heated purge dryers use heat and small amount of compressed air