The oil that lubricates a pneumatic system's compressor can oftentimes find its way into the rest of the system. This uninvited guest is all the more troublesome because it has been converted into aerosol form - a mist of fine particles - by the shearing action of pistons and other moving parts in the compressor. Aerosol oil particles are also created when the compressor's heat turns the oil into a vapor that cools and collects into condensation aerosols of less than a micron in diameter. Though seemingly insignificant, this tiny amount of oil can bring a pneumatic system to its knees. Airflow of 100 standard feet per minute containing just one part-per-million of oil can accumulate to 150 ml in just a month. Over time this accumulation becomes detrimental to the system, coating its components and affecting its delicate tolerances. These tolerances are especially crucial in apertures where the slightest change in dimension can cause a system malfunction. Oil can also cause seals to swell which, in turn, slows down the system and can even result in an all-out seizure. The presence of aerosols has other ways of diminishing the quality of the final product, either by undermining blow-molding processes or by leaving imperfections in air-applied coatings and finishes.

There are filters available that are designed to remove oil from the pneumatic system. Any filters that are not designed specifically to trap oil particles will be insufficient for the cause. This is because they typically only clear the system of particles down to the 5 parts-per-million range, which still allows too many particles for the system's well being. Even the filters designed specifically for the task of removing oil vary in effectiveness. They include absorbent filters, mechanical separators and microfiber filters. Absorbent filters use absorptive materials such as wool, felt, and cotton. Though these materials are initially effective, they lose their absorbent quality after encountering oil and are in constant need of replacement. Mechanical separators have a longer life span than absorbent filters but are less generally effective at trapping the smaller oil particles that make up the aerosols. Microfiber filters, on the other hand, have a longer life span and are able to catch aerosols as they pass through the air stream. Their dense matting of microscopic fibers allows them to intercept aerosols despite their various forms and behaviors. In times when the system's velocity is low aerosol particles fall from gravitational effect. This descent actually aids in their interception. When velocity is greater, the fiber media seizes both the particles that move with the airflow as well as those that are heavy enough to resist. Even oil particles so small they no longer behave as a liquid and randomly disperse through the system can be captured. With microfiber filters in place, aerosol oils will no longer have a place to hide.

Source: Clearing Oil from Compressed Air
Paul Barnes
Industrial Technology, January 2001
http://www.industrialtechnology.co.uk/2001/jan/hankison.html