The molecules of water vapour jump out of the wet desiccant into the dry air, thereby reactivating the bed. One advantage of heatless dryers is that they can be pneumatically controlled, this allows them to be installed into hazardous areas. Heat Reactivated Dryers. These dry compressed air in much the same way as heatless dryers, the only difference is the way the desiccant bed is reactivated. Heat reactivated dryers rely on a separate heat source to warm the bed up to the appropriate temperature.
Moisture is given off in the same way as boiling a kettle.
Then a much smaller quantity of air is passed through the wet desiccant bed to carry the water molecules away. There are different ways of applying heat to the desiccant bed and different ways of producing the purge. Each manufacturer claims theirs is better and more efficient than anyone else's. These use heater elements which are buried inside the desiccant bed, usually inside a protective tube which prevents the desiccant abrading the electric heater elements. If you've ever seen a 30kW heater element short to earth you'll know why the protective tube is needed.