A desiccant dryer can sometimes be confused with a deliquescent dryer. 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.
Many of have found small packets of chemicals inside the packaging of electronic or photographic equipment, placed there to absorb water vapor and to maintain the atmosphere inside the package in a dry state. This protects sensitive equipment from moisture damage. These are a form of desiccant dryer.
Common desiccants – compressed air drying agents – are silica gel and aluminum oxide.
Desiccant Dryer Function
The compressed air stream will typically flow from the compressor tank or the pump itse, into, and then through the desiccant chemical, be they pellets or a bead bed. The compressed air flows out of the desiccant dryer into the air mains, leaving water and water vapor behind.
Desiccant Dryer Dust
There is the potential for dust from the desiccant to travel downstream with the compressed air.
If the air-using application is sensitive to that, desiccant dust must be removed with a general purpose filter before the air gets to the equipment. Check with the supplier of the desiccant as to the dust issue for their media, and their filter recommendation to deal with it.
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 to strip out any free water, and as any airborne debris before it gets to the desiccant chemical.
In operation, deliquescent dryers, typically smaller and often in an in-line installation, will generate a slurry of water and desiccant chemical that drips down to the bottom of the desiccant dryer housing. That slurry must be drained periodically. To ensure this happens, consider an auto-drain to clear the buildup regularly, without having to rely on a maintenance person to remember to do it.
Desiccant Dryers Are 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 material would be used, and this chemical desiccant will absorb water, and will need to be regenerated.
The desiccant chemical regeneration would take place by either by directing dry air through the wetter desiccant bed from another desiccant tower. This happens if the system is of the Twin Tower Desiccant Dryer type, or by depressurization of that single tower and sometimes the application of heat to dry the desiccant.
The application of the dry air or heat removes collected moisture from the desiccant material. In time, and after going through the drying cycle, moist compressed air is once again directed through that particular tower with the desiccant bed dry and ready to strip moisture and water vapor from the compressed air stream 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 From The Compressor
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 from the compressor pump into the compressed air stream a coalescent air filter can be installed upstream from the desiccant dryer, and after the general purpose filter to remove oil from the air stream.
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