This is an in-line filter which is installed into a compressed air pipe. The manufacturers claim that the compressed air that comes out of the filter-system is drier than the compressed air that went in. This type of 'dryer' uses the natural properties of air which have been wrapped into marketing hype, proving that a good salesman can sell anything. To understand these dryers it is important to understand what happens to compressed air when it is expanded.
Lets look at some actual figures.
One cubic metre of saturated free air at 25 deg C will contain 22.83 grammes of water vapour. If we compress the cubic metre of air to 7 barg it gets hot. C. We now have a puddle on the floor comprising of 17.64 grammes of water. After we have mopped the puddle up, we can reverse the whole thing and expand the compressed air from 7 barg back to atmospheric pressure. The expanded air now contains only 5.19 grammes of moisture instead of its original 22.83 grammes.