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Both types of compressed air dryers contain substances that are used to absorb or adsorb moisture and extract water vapour from an enclosed environment; these substances are known as desiccants.
For example, it's common to find small packets of desiccant chemical inside the packaging of electronic or photographic equipment, placed there to maintain the atmosphere inside the package in a very dry state. This protects the equipment from moisture damage.
Common desiccants - drying agents - are silica gel and aluminum oxide.
The compressed air stream will flow into, through the desiccant pellets or bead bed, and out into the application.
There is the potential for dust from the desiccant to travel downstream with the compressed air, so if the application is sensitive, that dust must be removed with a general purpose filter. Check with the supplier of the desiccant as to the dust issue.
In order to ensure that the desiccant in the dryer lasts as long as possible, you'll want a general purpose filter upstream from the dryer.
Deliquescent dryers, usually of an in-line design, generate a slurry of water and desiccant chemical in the bottom of the dryer housing while drying the compressed air, and that slurry must be drained periodically.
A desiccant dryer is usually a larger, whole-plant drying system, and rather than the desiccant deliquescing into a water chemical slurry in the bottom of the tank, a different desiccant would be used, and this chemical desiccant will be "regenerated" either by directing dry air through the desiccant bed from another desiccant "tower", or by depressurization of that tower and sometimes the application of heat.
The application of the dry air or heat strips moisture from the desiccant so that, at a later time, when moist air is again directed through that particular "tower" on its way to the plant, the desiccant is dry and ready to strip moisture and water vapour from the compressed air again.
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The use of a desiccant dryer often means that the dew point of the compressed air, after it leaves the dryer, is measured at the minus degree level.
When functioning properly after compressed air has been through an appropriately sized desiccant dryer, there should not be any condensate formed in the air lines downstream of the dryer, as the ambient temperature in the rest of the plant should be higher than the dewpoint of the desiccant-dried air. Any minute quantities of water vapour left in the compressed air should never condense into free water in the air lines under these circumstances.
The effectiveness of the desiccant chemical can be severely diminished or eliminated if air borne oil enters the desiccant bed.
If your compressor has oil carryover into the compressed air stream, and most reciprocating compressors do, you will want to put a coalescent filter upstream from the desiccant dryer, and after the general purpose filter.
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