Deliquescent Dryers:
These use the natural absorption properties of common salt (Sodium Chloride) although Calcium Chloride is sometimes used. For use in this type of dryer the salt is compressed to form a hard tablet - about the size of a pin pong ball.
The principle is quite easy to demonstrate, simply pour some common salt on to a surface and leave it a few days. When you come back the salt will have started to turn into a liquid. It's because common salt has the irritating ability to absorb moisture that salt cellars get bunged up so easily.
In fact more than one company sells 'domestic dryers' for use in airing cupboards etc. Simply place the magic pellets into a container and pour the resulting liquid down the drain a few days later. The magic pellets are usually Calcium Chloride.
If you apply this technology to compressed air you can create a compressed air dryer.
A deliquescent dryer is an air receiver with an additional access hole at the top and an additional drain at the bottom. The compressed air flows through the air receiver, in at the bottom and out of the top. If the receiver is then filled with salt tablets, the wet compressed air must pass over the surface of many hundreds if not thousands of these salt tablets. The moisture is slowly removed from the compressed air. The result is that a white liquid forms at the bottom of the receiver, (salt water) this is simply drained away. Every few months or so, the receiver must be isolated and then refilled with new deliquescent.
Unlike any other type of dryer, a deliquescent dryer cannot achieve a 'target' dewpoint. It will only create a dewpoint suppression, usually no more than 11 deg C.
This means that if the dewpoint of the compressed air going into the dryer is 30 deg C, then it will leave the dryer with a dewpoint of 19 deg C. If it approaches the dryer with a dewpoint of 20 deg C, it will leave the dryer with a dewpoint of 9 deg C.
This type of dryer is very useful in warm climates (such as some parts of the USA) where it sells well. It is not very forgiving when temperatures fluctuate between +30 deg C and -10 deg C and there is the additional on-cost for the deliquescent pellets. In the UK this type of dryer is by far the most expensive to run.
Unfortunately common salt turns to a mush fairly quickly, for this reason the tablets used in deliquescent dryers is compacted to a high degree. The result is that the tablets still degrade when in contact with water, but in a controlled manner.