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Coalescent filters
Coalescent filters contain filter elements that function similarly to the general purpose (30 - 40 Micron sized elements) found in the general purpose air filter, by filtering out debris moving down the air stream from the compressor, through the filter element, and out to your air tool or air valve.
The coalescent element, by having extremely fine mesh size 'holes', brings an additional function not available from the general purpose element, that of removing oil from the compressed air stream.
The source of the oil to be removed is usually the compressor itself. Oil lubricated compressors (the majority) use a lubricating oil in the process of compressing the air.
Despite having oil removal equipment at the compressor discharge, 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.
The coalescent element inside a coalescent compressed air filter is often made of glass fibers.
Because of the tortuous air path the compressed air has to travel through the element, it 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 are eliminated.
The waste material from the coalescent filter bowl is considered "hazardous waste" in terms of it's impact on the environment, so care must be taken in the disposal of it.
Wikipedia defines adsorb as follows: “ Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process.
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, I expect.
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 that 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.
Because coalescent filters are so very efficient in trapping air borne contamination, you definitely want to have a general purpose 5 Micron filter unit upstream from the coalescent unit, and then upstream from the 5 Micron unit, you'll want a general purpose filter with a 30-40 micron element to remove the larger debris.
Though it's 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 bowl housing with a coalescent unit. Others can't, and require a specific filter housing to handle what is often a larger element than the norm.
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