Deliquescent Compressed Air Dryers
A deliquescent dryer is basically a tank full of salt tablets.As the compressed air passes through the salt, the salt attracts water and dissolves into a brine that can be drained off. These are the least expensive dryers to purchase and maintain because they have no moving parts and require no power to run. The operating cost consists of the cost of more salt tablets plus the freight to bring them in.
The pressure dew point of the compressed air leaving a deliquescent dryer varies with the temperature of the compressed air entering the dryer. In summer, if you have compressed air entering the dryer at 90°F, the pressure dew point of the exiting compressed air is 70°F. This is better than not having a dryer but not by much. In winter these dryers work much better and can be used to prevent freezing of condensate in piping that is exposed to outside temperatures. The fact that they need no electricity to run and the water that they they remove from compressed air is part of a salt brine with a lower freezing point also makes them desirable for winter operation outdoors. By controlling the temperature of the compressed air entering the dryer, they can keep freeze-ups from happening down to temperatures of 12 to 15°F.
The salt used to make the tablets for deliquescent is not common table salt, and is formulated to be less corrosive to metal. However, the issue of corrosion is still there. Eventually the dryer will corrode and need to be replaced.An afterfilter to capture any salt particles that the compressed air may pick up as it passes through the dryer will prevent piping downstream of the dryer from being corroded.
Resources: The Basics of Compressed Air Dryers