Desiccant Dryer
The term "desiccant dryer" refers to a broad class of dryers. Other terms commonly used are regenerative dryer and twin tower dryer, and to a lesser extent adsorption dryer.
The compressed air is passed through a pressure vessel with two "towers" filled with a media such as activated alumina, silica gel, molecular sieve or other desiccant material. This desiccant material attracts the water from the compressed air via adsorbtion. As the water clings to the desiccant, the desiccant "bed" becomes saturated. The dryer is timed to switch towers based on a standard NEMA cycle, once this cycle completes some compressed air from the system is used to "purge" the saturated desiccant bed by simply blowing the water that has adhered to the desiccant off.
The duty of the desiccant is to bring the pressure dew point of the compressed air to a level in which the water will no longer condense, or to remove as much water from the compressed air as possible. A standard dew point that is expected by a regenerative dryer is −40 °C (−40 °F), this means that when the air leaves the dryer there is as much water in the air as if the air had been "cooled" to −40 °C (−40 °F). Required dew point is dependant on application and −70 °C is required in some applications. Many newer dryers come equipped with a dew dependant switching (DDS) which allows for the dryer to detect dew point and shorten or lengthen the drying cycle to fulfill the required dew point. Often times this will save significant amounts of energy which is one of the largest factors when determining the proper compressed air system.
The regeneration of the desiccant vessel can be during three different methods:
- Heatless "pressure-swing" drying, which uses part of the dry compressed air coming from the other vessel to dry the desiccant in the vessel being regenerated at lower pressure. 17-20% purge rate
- Heated dryer, which uses a hot air blower, so there is no loss of compressed air. >7% Purge Rate.
- Heat of compression, which can only be used with an oilfree compressor.
Read more about this topic: Air Dryer