Compressed Air Filters
In addition to moisture, compressed air may contain numerous other contaminants that are undesirable at the point of use. These may be ingested into the compressor from the ambient air in the form of airborne dust, vehicle exhaust, or chemicals for example. Additionally, they may be generated in the compressed air system such as oil from compressors, rust from tanks and piping, and desiccant dust from air dryers.
These contaminants are removed using filtration. Several types of filters are available, but the majority fall into one of three primary categories.
Coalescing (sometimes called oil removal) filters operate by converting liquid aerosols (fine mists) into larger droplets that can be easily drained away from the filter. The most common contaminants removed by coalescing filters include water and oil.
It is important to note that water removed by these filters is in the aerosol form, or a fine mist of liquid, in contrast with air dryers that remove water in the vapor (gas) state.
Coalescing filters, are usually rated by the remaining concentration of contaminants after the filter. When used for oil removal, some filters can reduce oil concentration to less than 0.01 parts per million by weight, to provide compressed air that contains less oil than the ambient air in most facilities.
Particulate filters remove dirt, dust, rust, and similar solids from the air system. They are generally rated by remaining particle count (number of dust particles per unit of volume) at various micron sizes. Uses for these filters can include prefilters for refrigerated dryers to prevent rust contamination, or post filters to desiccant dryers to prevent abrasive desiccant dust from passing into the system.
Vapor filters or vapor adsorbers are used to remove final traces of oil and any tastes or odors in the compressed air for particularly sensitive applications such as chemical, pharmaceutical, dental, and food processing and packaging applications.
These filters rely on chemical attraction to remove the contaminant with an appropriate media (typically some form of activated carbon). These filters can be cartridge style units similar to particulate and coalescing filters, or can be offered as pressure vessels containing bulk filter media for longer service intervals. In the case of a pressure vessel unit, a particulate filter must follow to prevent carbon dust from traveling down line.
Specialized filters are available to remove practically anything, including specific chemicals and bacteria, but these are rare in an industrial setting.