Typical absorbents are sodium chloride, calcium chloride, and urea tablets or beads. The deliquescent dryer that uses one or a combination of these expendable chemicals does not provide a fixed, outlet pressure dew point. Instead, the dew point varies directly with the dryer inlet temperature - the higher the inlet temperature, the higher the outlet pressure dew point. Thus, deliquescent dryers can be installed outdoors in combination with aftercoolers to prevent freeze-up in outdoor pipelines and equipment.
The outlet relative humidity depends on the inlet dew point. Typically, these dryers will lower the inlet dew point by 20 degrees. The calcium chloride tablets must be replaced on a regular basis, as they dissolve in contact with the water in the compressed air to form a brine solution that must be drained and disposed of.
This type of dryer was popular at one time because of the low capital cost compared to other dryers at the time. Although this type of dryer will reduce the dew point, it cannot produce a minus pressure dew point; and it has a tendency to contaminate downstream systems.
These dryers are still in operation, although we would not recommend this type of system for instrument or process air applications because of potential downstream contamination and poor dew point performance.
Absorbent chemicals used in air dryers are known as "deliquescent desiccants." They attract moisture by the chemical reaction known as absorption.