Desiccant dryers utilize chemicals beads, called desiccant, to adsorb water vapor from compressed air. Silica gel, activated alumina and molecular sieve are the most common desiccants used. (Silica gel or activated alumina is the preferred desiccants for compressed air dryers.) The desiccant provides an average -40°C pressure dew point performance. Molecular sieve is usually only used in combination with silica gel or activated alumina on -75°C pressure dew point applications.

Desiccant dryers are configured with two pressure vessels, filled with desiccant, switching valves to direct the compressed air flow and controls for proper switching of the dryer vessels.

Basic operation of a desiccant dryer consists of one drying cycle and one regeneration cycle commonly referred to as the NEMA cycle, which is continuously repeated. For example, a 10 minute NEMA cycle consists of a five minute drying cycle and a five minute regeneration cycle.

During the drying cycle, compressed air, at full pressure, flows through one desiccant vessel. As the air flows through the desiccant bed, microscopic pores on the surface of the desiccant beads "strips" the water vapor and lubricant molecules from the air, thereby reducing the relative humidity of the air. The relative humidity of the dried air is equivalent to a pressure dew point of -40°C or lower.

Desiccant dryers are available in two basic designs: heatless and heated. Since the drying cycle on all desiccant dryers is similar, the difference between heatless and heated designs is found in the regeneration methods.

Heatless dryers utilize a combination of dry purge air (approximately 14 percent of the compressed air leaving the dryer at 100 psig), depressurization and the "heat of adsorption" for desiccant regeneration. Heatless dryer cycles are usually 10 minutes (five-min. drying, five-min. regenerating). Heatless dryers are the most popular desiccant dryers used in industry and offer several advantages, including:
Consistent pressure dew point performance;
Three to five year desiccant life,provided prefilters are properly maintained;
Simple, long life switching valves requiring minimal maintenance;
Simple and reliable operation; and
Lowest purchase prices of all desiccant dryers.

The single disadvantage of the heatless design is the relatively high purge air consumption, which results in the highest operating costs and reduces the amount of compressed air available for use in the plant. Microprocessor controls are available to match purge consumption to actual compressed air demand, which can actually reduce operating costs.

When compressed air is not available for purge consumption or when utility costs are very high, heated dryers become the preferred alternative to heatless designs.

Heated desiccant dryers are available in three configurations: internally heated, externally heated and heat of compression. All three configurations regenerate the desiccant bed with a combination of heat to absorb the water vapor molecules from the desiccant beads and purge air, which delivers the heat to the desiccant bed and carries the moisture out of the bed. Benefits will vary for each of the three configurations depending on applications, so consult the supplier to determine the best format for specific applications.

Maintenance of desiccant dryers varies depending on the dryer style. Heatless dryers will require desiccant replacement every three to five years while desiccant is replaced every one to two years on heated dryers.

In addition, switching valves require inspection and possible rebuild annually. Blower and venturi intake filters must be cleaned or replaced and the blower motor bearings lubricated per the manufacturers instructions.

In short, compressed air systems can produce dry air, provided a comprehensive plan is developed to establish the air quality requirements. To devise an appropriate plan, identify the source of the moisture and contaminants and analyze the dryer construction features and system layout before selecting a specific system.

Development of this plan can be simplified with the selection of a qualified compressed air system supplier. Qualified suppliers should be capable of understanding individual compressed air requirements, be an expert on the application and function compressed air system components and provide sound direction on the total system installation.

Since all systems require maintenance and occasional repairs, the system supplier also should have a qualified service organization available to service systems regularly.

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