Desiccant Air Dryer

Desiccant dryers are sometimes confused with deliquescent dryers.

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 - compressed air 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 leaving water and water vapor behind.

Desiccant Dust

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 compressed air desiccant 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.

Desiccant Dryers Often Bigger

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. The regeneration would take place by either by directing dry air through the desiccant bed from another desiccant tower (hence the term Twin Tower Desiccant Dryer) or by depressurization of that tower and sometimes the application of heat to dry the desiccant.

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.

The use of a desiccant dryer often means that the dew point of the compressed air , after it leaves the dryer, is at the minus degree level.

When the desiccant dryer is functioning properly, after compressed air has been through an appropriately sized desiccant dryer, there should not be any condensate formed in the downstream air lines. This is because the ambient temperature in the rest of the plant should be at higher temperature than the dew point 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.

Oil Carry Over

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 air filter upstream from the desiccant dryer, and after the general purpose filter.