Desiccant dryers can be heatless, heated internal or heated external. In a desiccant dryer the air flows through one of two towers filled with a drying agent that absorbs any moisture. The opposing tower is offline and being regenerated with purge air.
Desiccant dryers have better dew point capacity than refrigerated dryers and they can be used in remote and hazardous environments. They tend to have high set-up costs, the desiccant bed needs to be replaced every 3-5 years, they require more filtration for airborne oil vapor and particulate carry over, and often need purge air. Most often they consume purge air, making them less efficient than refrigerated.