A desiccant is a drying agent. More practically, a desiccant is a substance with a high capacity to remove (adsorb) water or moisture. It also has a high capacity to give off that moisture so that the desiccant can be reused. DESICCANT-TYPE DEHYDRATORS are basically composed of cylindrical flasks filled with desiccant. Compressed air system dehydrators use a pair of desiccant towers. One tower is in service dehydrating the compressed air while the other is being reactivated. A desiccant tower is normally reactivated when dry, heated air is routed through the tower in the direction opposite to that of the normal dehydration airflow. The hot air evaporates the collected moisture and carries it out of the tower to the atmosphere. The air for the purge cycle is heated by electrical heaters. When the tower that is reactivating has completed the reactivation cycle, it is placed in service to dehydrate air, and the other tower is reactivated. Another type of desiccant dehydrator in use is the Heat-Less dryer. These units require no electrical heaters or external sources of purge air. Figure 14-24 shows the compressed air entering at the bottom of the left tower (view A). The compressed air then passes upward through the desiccant, where it is dried to a very low moisture content. The dry air passes through the check valve to the dry air outlet. Simultaneously, a small percentage of the dry air passes through the orifice between the towers and flows down through the right tower. This dry air reactivates the desiccant

Figure 14-24.-Heat-Less desiccant dehydrator.

and passes out through the purge exhaust. At the end of the cycle, the towers are automatically reversed, as shown in view B.

Refrigeration and Desiccant Type of Air Dehydrator (Type III)

Some installations may use a combination of refrigeration and absorption for moisture removal. Hot wet air from the compressor first enters a refrigeration-type dehydrator where low-temperature R-12 removes heat from the airstream and condenses water vapor from the air. The cold, partially-dried air then flows into a desiccant-type dehydrator, where the desiccant absorbs additional moisture from the air.