A desiccant air dryer for compressed air uses special drying agent beads like silica gel and activated alumina to absorb water vapor from an air stream. These air dryer units are often regenerative so that the drying agent doesn’t need replacing in order to provide vapor-free compressed air consistently. Wet air passes directly through the desiccant medium which adsorbs the moisture. The desiccant medium has a finite capacity for adsorbing moisture before it must be dried out, or regenerated. To do this, the tower containing saturated desiccant medium is depressurized and the accumulated water is driven off. How this happens depends on the type of desiccant air dryer. They consist of an air-to-air heat exchanger and an air-to-Freon heat exchanger. The compressed air from the compressor first passes the air-to-air heat exchanger. In this phase the incoming air is pre-cooled by the outgoing air and the outgoing air is heated. Regenerative-desiccant-type dryers are the primary choice for compressed air users within the pharmaceutical, chemical, and food and beverage manufacturing industries.
They feature a porous desiccant that adsorbs the moisture by collecting it in its myriad pores, allowing large quantities of water to be retained by a relatively small quantity of desiccant. The desiccant medium has a finite capacity for adsorbing moisture before it must be dried out or regenerated. Normally, the desiccant is contained in two separate towers. Compressed air to be dried flows through one tower while the desiccant in the other is being regenerated. This is accomplished by reducing the pressure in the tower and passing previously dried purge air through the desiccant bed.
The regeneration air may also be heated, either within the dryer or externally, to reduce the amount of purge air required. Regeneration air may also be supplied by a blower and dryers of this type normally have a regeneration cycle, which can be based upon time, dew point or a combination of the two. The regeneration method depends on the type of desiccant dryer: heatless desiccant dryers or heated blower purge dryers.
