Coalescing Filter Technology

Coalescing filters provide clean air for many applications including paint spraying, clean rooms, air bearings and other precision pneumatic systems. Clean compressed air is essential in such industries as food processing, electronics, health care, photographic, dairy and instrumentation. Clean air in these and other critical applications means more than air free of solid particulate contamination. Air used in these industries must also be free of sub- micronic water and oil aerosols . . . contamination beyond the control of conventional filter systems.

Why Sub-micronic Contamination Is A Problem.

A micron, identified by the symbol “ mm’’ and also known as a micro-meter, is one millionth of a meter (or about 0.000039 inches) in size. A typical strand of human hair is approximately 80 microns in diameter, while a grain of table salt is approximately 100 microns in size. The lowest level of visibility to the human eye is 40 microns. Sub-micronic contaminants in compressed air systems plug orifices of sensitive pneumatic instrumentation; they wear out seals, erode system components, reduce the absorptive capacity of desiccant air/gas dehydrators, foul heat transfer surfaces, reduce air tool efficiency, and damage finished products. The results include: product rejects, lost production time and increased maintenance expense. For example, trace amounts of sub-micronic oil can cause serious fish eye blemishing in automotive finishing operations. OSHA regulations state that air may not contain more than five milligrams (mg.) of oil mist particles per cubic meter of plant air, or one ounce of oil in 200,000 cubic feet – yet another reason why clean compressed air is a concern in industrial applications. Air contamination is particularly threatening in precision applications where the cleanliness of the workplace and the purity of the product are critical. In close-tolerance systems where extremely fine orifices and parts clearances are the norm, it is vital for the system air to be free of all suspended liquid aerosols as well as solids.

Water, Oil And Solids Are Three Contaminant Threats

The contaminants of greatest concern in precision compressed air systems are water, oil and solids. Water vapor is present in all compressed air; it is a condensation by-product of the compression process. In fact, compressed air is saturated with water until it is dried. A 25 hp compressor delivering 100 standard cubic feet of air per minute (SCFM) at 100 psig can produce 18 gallons of water per day. Water aerosols in compressed air range from 0.05 to 10 mm. While air dryer systems can be used effectively to remove water from compressed air, they will not remove the second major liquid contaminant – oil. Oil is also present in compressed air systems. It is largely introduced into the air stream by the air compressor. The amount of oil introduced in this fashion varies by the type of compressor used. Estimates of the hydrocarbon content of discharge air from typical compressors are, in parts per million (ppm): Screw – 25 to 75 ppm at 200° F, Reciprocating – 5 to 50 ppm at 350° F, Centrifugal – 5 to 15 ppm at 300° F. At a concentration of 25 ppm, a typical compressor flowing 100 SCFM for 35 hours will introduce eight ounces of oil into the pneumatic system. Even if an oil-less compressor is used, oil contamination of the air stream remains a problem because ambient air contains 20-30 ppm of hydrocarbon aerosols from industrial and automotive sources. Oil-less compressors can condense approximately 10 ppm of hydrocarbons during the compression cycle. This is enough oil to gum-up air line components and to collect in air dryer systems. Oil aerosols emitted by air compressors are generally 2 mm and smaller. The third contaminant found in compressed air is solid matter including rust and scale. Solid particulates, combined with aerosol water and oil, can clog and shorten the life of air system components as well as filter systems. Rust and scale contaminants typically found in compressed air systems are 0.5 to 5 mm in size.

Coalescing Filters Meet The Need For Clean Compressed Air

Standard nominal five micron in-line filters cannot remove sub-micronic contaminants as required in critical applications. The lowest particle removal limit for these general purpose filters is 2 mm. Eighty per-cent of aerosol contaminants are under 2 mm in size. Coalescing filters, however, are specifically designed to remove sub-micronic oil, water and solid contamination from compressed air. Standard graded porosity coalescing filters remove over 99.9% of all aerosols in the 0.3 to 0.6 mm range. What’s more, these filters are over 99.98% effective at removing all aerosols and eliminating all solid particles larger than 0.3 mm. Thus, oil aerosols at a 20 ppm contamination level are reduced to a 0.004 ppm concentration – acceptable for virtually all pneumatic applications.

Advances In Fiber Technology Led To Today’s Highly Effective Coalescing Filter Systems

Early coalescing filters were expensive and caused high pressure drop in many applications. These filters were also not easily adapted to meet specific system needs, so the benefits of coalescing technology were difficult to integrate into many air applications. Advances in glass fiber technology and advanced studies of aerosol behavior led to the development of more effective and reliable coalescing filters. Specifically, the fiber breakthrough that advanced general filter technology was the replacement of cellulose elements with sub-micronic glass fibers. While the early filters could remove particles as small as 2 mm, glass fiber filters are effective against contaminants as small as 0.001 mm. These modern coalescing filters provide a low-cost, adaptable means of removing aerosols and solids from critical air streams.

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