Heatless dryers do not use any source of heat for regeneration other than heat given off during the drying, or adsorption, cycle. This is referred to as “the heat of adsorption.” However, heatless dryers will consume up to 15% of the process air they are drying during the regeneration cycle. Heated dryers, on the other hand, utilize an external heat source for regeneration and require little or no process air. Donaldson’s three heated desiccant dryer types use different methods for heating, and subsequent cooling and repressurizing, the air used for regeneration.