Heatless desiccant dryers are used to treat unheated compressed air to regenerate the desiccant bed. This dryer is the most expensive to operate because of the large amount of compressed air that's consumed for purging during the regeneration cycle. Heated desiccant dryers have the lowest energy operating cost of the three types. A heating element supplements the drying action of the purge air. The heater can be mounted internally to heat the desiccant bed directly or externally to heat the purge air that's blown through the bed. Internal heating elements shorten the life cycle of desiccant beds and may cause dew point spikes. However, the amount of air required is reduced because the heat supplements the air's drying action.