Coalescent filters are sometimes used as a part compressed air treatment when the presence of compressor oil or lubricating oil in the air stream creates an issue.
Coalescent filters contain filter elements that function similar in concept to the general purpose (30 - 40 Micron sized) elements found in the general purpose air filter. They, too, filter out debris moving down the air stream from the compressor, through the filter element, and out to your air tool or air valve.
Photo courtesy of Norgren.com
The source of the oil in the compressed air stream is quite often the compressor itself. Oil lubricated compressors use a lubricating oil either to relieve friction between mechanical parts in the compressor pump, or in the actual process of compressing the air like rotary screw compressors do.
Rotary screw compressors have oil separation equipment at the compressor discharge, and the oil sumps of reciprocating compressors are supposed to keep the oil in the sump and out of the air. Yet, in both cases, some of that lubricating oil can still get into the compressed air stream. It is this oil, which is normally not compatible to seals in downstream air components, which has to be removed by the coalescent filter as the oil is negative to downstream air seals and components.
The coalescent element inside a coalescent compressed air filter is often made of glass fibers.
The coalescent element is designed to force the compressed air stream through tortuous air paths. In so doing, the coalescent element will trap free oil onto the filter material. And, by the process of adsorption, "coalesce" oil vapor in the compressed air stream into tiny droplets of oil which gather and drip down - usually on the inside of the coalescent element - into the "quiet zone" at the bottom of the filter bowl.
When the filter bowl is drained, either manually or with an auto drain, the contents of the filter bowl, the trapped water and oil mix removed from the compressed air stream, are eliminated.
Adsorption is operative in most natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins and water purification.
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are filled. But atoms on the (clean) surface experience a bond deficiency, because they are not wholly surrounded by other atoms. Thus it is energetically favorable for them to bond with whatever happens to be available. The exact nature of the bonding depends on the details of the species involved.... "
There. Much more information on adsorbing than you probably really need.
Essentially, adsorbing means that the compressed air contaminate being filtered out is collected on the outside of the fibers of the filter element, a factor of the high speed twists and turns through which the compressed air has to travel to pass through the element, rather than the oil being "absorbed" into the fibers of the element, much like water is absorbed into the fibers of a paper napkin.
Some coalescent filters have mesh sizes as small as 0.01 Micron stripping the compressed air stream of particles down to that 0.01 Micron in size. A micron is one millionth of a meter.
Though it is more expensive to plumb your air supply to your application in this way in the short term, the payback will be many fold in longer, downstream filter element life, and less maintenance time troubleshooting why your machine or system lacks sufficient air as a result of the 5 micron or coalescent filter elements becoming plugged solid.
Some manufacturers can replace a general purpose element in their standard filter bowl housing with a coalescent filter element. It become difficult to tell then if the filter contains a regular element or a coalescent element. Care should be taken to ensure the right filter is installed in the correct locale.
Others coalescent filter manufacturers design their filters to require a specific filter housing, to handle what is often a larger element than the norm.
Regardless of the filter shape and size, if you have oil in your compressed air stream, you will likely need to get it out. A colescent filter will do that for you.