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Types of Compressed Air Filters

Most compressed air filters fall into one of the categories listed below.

Coalescers (Oil Removal)

BUY Compressed Air Coalescer Assemblies

Oil removal filters typically employ the inertial impaction and diffusion filtration mechanisms to remove oil in the liquid state. These filters are commonly known as coalescers, since they rely on the combination or coalescing of small droplets into larger droplets in the filtration media. These larger droplets then fall away from the element due to gravity. Coalescers are available in a variety of efficiency ratings in order to suit the many applications of compressed air.

Some designs have built-in pre-separation of gross contaminants, such as water and large oil droplets or solid particulates.

Other coalescers are designed with pleated elements, providing significantly greater surface area than a non-pleated design, and the inherent reduction in pressure drop and increase in element life.

Still other coalescers are recommended for use in combination with one or more filters, feeding the air-stream to a coarse coalescer prior to a high efficiency coalescer. This type of installation, while inherently higher in pressure drop than the single filter, provides another approach to reducing the inlet contamination to the high efficiency coalescer and results in cleaner air at the outlet.

All coalescing filters should be installed with automatic condensate drains to effectively remove the filtered condensate.

Particulate Filters (Particulate Removal)

BUY Compressed Air Particulate Filter Assemblies

Particulate filters typically employ the interception mechanism of filtration to remove solids from the air-stream. Like oil removal filters, they come in a variety of efficiency ratings. While typically rated as a percent efficiency at a stated particle size, "absolute" ratings are also available. An absolute rating at a given particle size simply means that no particles at the specified size or larger will pass downstream of the filter.

Seek clarification of any "nominal" filtration ratings. Most references to nominal do not include a definition of efficiency; and those that do can be misleading.

Dirt-holding capacity and the related pressure drop determine the life of a particulate removal filter. Therefore, differential pressure gauges are common, and pleated element designs are generally preferred.

Particulate filters are often installed with manual drains. Depending on location in the air system and the likelihood of liquid presence, an automatic drain may be advisable.

Adsorbers (Vapor Removal)

BUY Compressed Air Adsorber (Carbon) Assemblies

Adsorbers typically employ the adsorption mechanism of filtration to remove hydrocarbon tastes and odors from the air-stream. Most use a special activated carbon to accomplish this.

It is never advisable to install a carbon filter without suitable pre-filtration, as carbon filters quickly become ineffective when challenged with liquid oil or water. Although these filters are often used as "final polishing" filters in an air treatment system, they often tend to pass small carbon fines downstream. Therefore, make sure there is a particulate filter either built into the carbon filter or downstream.

Adsorbers (Vapor removal filters) are available utilizing several configurations of filtration media. While most hydrocarbon vapor removal filters use activated carbon, the amount of carbon used can vary dramatically. Everything from carbon impregnated paper to packed beds of carbon are available for these applications. Be careful in estimating element life for vapor removal filters. Since the life of the element is directly related to the volume of carbon in the element, the relative life of one design versus another can vary by orders of magnitude.

Most carbon filter elements are changed when tastes or odors are detected downstream. However, filters with color change indication of element saturation are available at a slight premium.

Combination Filters

Filters that combine particulate and oil removal mechanisms; or particulate, oil and vapor removal into a single unit are available. Be sure to apply such filters within their ratings. A comparison of initial cost, installation cost, element replacement costs and average system pressure drop costs can aid in the decision to purchase a combination filter rather than multiple filters.

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