How Does a Desiccant Dryer work?

The operation of a desiccant dryer vs other types of compressed air & gas dryers is quite unique.  Van Air uses different types & sizes of desiccant depending on the application and the customers’ dew point requirement.  Most of the applications we review are from customers requesting a -40F pressure dew point. We use either activated alumina or silica gel in our dryers to achieve a -40F pressure dew point. Activated alumina is highly porous material and has the ability to trap the moisture vapor from the compressed air or gas stream in its pores. Silica gel falls into the molecular sieve class of desiccants. A molecular sieve is a material with very small holes of a specific size that are able to trap water molecules.  These desiccants perform the task of removing or adsorbing the moisture that enters the desiccant air dryer.

There are a different types of desiccant dryers that can be used depending on the application.  For instance, if a customer has a small intermittent flow application we recommend they use a cartridge type dryer. Cartridge type dryers are available for flows in the range of 1 to 35 scfm. These dryers use either color change silica gel or molecular sieve desiccant in a removable cartridge located inside the dryer. Basically the wet compressed air enters the dryer and flows thru the cartridge of silica gel or molecular sieve to dry the compressed air or gas stream to a -40F pressure dew point.  The dryers have either a sight window or moisture indicator so the user can determine the condition of the silica gel.  If the color of the silica gel in the sight window or moisture indicator is blue you’re good to go.  When the color changes to pink it is time to change out the cartridge.  The cartridge dryer is very simple to operate and maintain and has no moving parts.

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When looking at larger flow applications that require continuous -40F to -100F pressure dew points the heatless dryer is the perfect fit.  Unlike the cartridge type dryer where you replace the desiccant, a heatless dryer has (2) pressure vessels or towers filled with activated alumina desiccant. The life cycle of the desiccant is upwards of 3 years before it must be replaced.  The towers alternate between a drying mode and a regeneration mode.  Only one of the towers of activated alumina is used at a time for adsorbing moisture.  The activated alumina in the tower that is not drying the air or gas is being regenerated or renewed.  The regeneration process is accomplished by taking a portion of the dry air or gas from the outlet of the tower that is in drying mode; expanding it to atmospheric pressure; and then diverting it to the tower that is in regeneration mode.  The extremely dry air acts like a magnet and literally sucks the moisture out of the pores of the desiccant.  The towers then switch and the drying tower goes into regeneration mode and the regenerated tower is now ready to dry the wet compressed air or gas stream.

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