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Some dryer vendors use the terms deliquescent and desiccant as if they were interchangeable in describing their equipment.
A deliquescent dryer contains a desiccant.
The desiccant used in deliquescent dryers will, upon being exposed to water or water vapour in a compressed air stream, deliquesce; that is, the surface of the desiccant chemical, often beads or pellets, will liquefy, and the resulting liquid will flow to the bottom of the vessel where it must be expelled from time to time.
The liquid expelled from the drain on the dryer contains both water and the desiccant chemical in solution. Depending on what chemical is used the draining of this liquid into the city sewers may be illegal and the waste liquid may require storage and proper disposal.
A desiccant dryer will contain a chemical to strip the water vapor from the air as well, but the desiccant in this type of dryer will not deliquesce (dissolve) as it takes the water from the air. Rather, the desiccant will become saturated to the point where it can no longer trap water, and until it is dried down, it will no longer work.
The desiccant in the deliquescent dryer pressure vessel will adsorb the water in the air. In chemistry, adsorption of a substance is defined as the concentration of an adsorbate (in this case water) on a particular surface of adsorbent (the desiccant). This results in the formation of a liquid or sometimes a gas film on the surface of the solid pellet or bead.
Deliquescent dryers require recharging periodically. The desiccant contained within will, depending on flow of air and vapor content, ultimately be consumed completely.
Some factors that will affect the consumption of the desiccant are the type of adsorbent, type of adsorbate, the size of the adsorbent bead or pellet, the concentration of the adsorbate in the compressed air stream, and the temperature of that air stream.
A strict chemical definition of a deliquescent is that it's a water soluble chemical that will continue absorbing water from the surrounding air until it completely dissolves into a liquid.
This is why you will usually want to have a "water trap", also known as a general purpose air filter plumbed in line and just upstream from the deliquescent dryer. Otherwise, any liquid water transported into the dryer by the compressed air stream will make short work of the desiccant chemical requiring a recharge more often than desired.
As we've mentioned frequently elsewhere in this site, the compressing of air generates heat, and hot, moist compressed air will consume the desiccant chemical in the deliquescent dryer much more quickly. Try to ensure that air flow to the dryer is as cool as possible.
When you open a bag of desiccant chemical to recharge your deliquescent dryer, if you don't use it all at one time, be very sure that the remaining chemical is sealed tight against air ingress. Just having the bag open will, over time, render the desiccant inside into a puddle in the bottom of the container as water vapor is sucked from the surrounding air into the remaining desiccant. A deliquescent dryer can be expected to reduce the compressed air dew point by 20 - 30 deg. F, or so. It depends on the type of chemical used. Check with the vendor to determine which desiccant chemical suits your application.
Unlike other forms of compressed air dryers, a deliquescent unit doesn't "guarantee" a dewpoint. The amount of water vapour in the air that exits the dryer is completely predicated on how much water vapour is in the air going into the dryer. Where you need to have a specific dew point for the air exiting a dryer, you must use a different drying system.
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