Filter-System Dryers
We have made up the name 'Filter-System-Dryer' so that various manufacturers do not get too upset with us. 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. In fact its a dryer with no moving parts.......
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 (1.013 bara) at 25 deg C will contain 22.83 grammes of water vapour. If we compress the cubic metre of air to 7 barg (8.013 bara) it gets hot. Once it has cooled back down to 25 deg C, (the same temperature that it was before it was compressed), the pressurised air will now only occupy 0.126 cu metre, instead of the original 1 cu metre.
The compressed air must shed some water by way of natural condensation as this reduced volume is only physically capable of holding 2.88 grammes of water vapour. (At 25 deg C with 100% saturation). Therefore we end up with a puddle on the floor comprising of 19.95 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 - and wait for it to warm up to 25 deg C.
The expanded air now contains only 2.88 grammes of moisture instead of its original 22.83 grammes. Instead of its original relative humidity (RH) of 100% it now has an RH of 12.6%. equivalent to a dewpoint of -6 deg C. This means that by compressing air and expanding it again we have effectively dried the air without using a dryer.
A good marketing man can make use of this.
If we make a compressed air filter with a 7 barg pressure drop, then the air coming out of the other side will have a -6 deg C pressure dewpoint. We magically have ultra-dry air from a 'dryer' with absolutely no moving parts - the problem is that we have lost all of our air pressure.
A Filter-System-Dryer manufacturer may not be so obvious about their claim. They may also use terminology which can be mis-understood, such as referring to a dewpoint depression of between 2-5 deg C. I wonder how many people have thought this meant they would get a pressure dewpoint of between 2 and 5 deg C.
A real example may only have a pressure drop across the housing of about 10 to 12 psi. This pressure drop results in a slight expansion of the compressed air which in turn provides a marginal reduction in pressure dewpoint. Advertising literature may say something to the affect that....
'Using our unique Filter-System-Dryer will reduce water condensation problems in your compressed air system by reducing the system pressure dewpoint. A dewpoint depression of between 2 and 5 deg C can easily be achieved without any running costs.'
Amazingly, people buy them.