Compressed Air Filtering Basics

Sources of contamination

Contamination of compressed air results from several sources. The air contamination in industrial areas is very significant, with the average metropolitan environment containing in the order of 4 million dirt particles per cubic foot, as well as water vapor, which condenses out after compression and cooling.

Next, the compressor contributes its share of the pollution consisting of wear particles and, if oil-lubricated, carbonized compressor lubricant.

Most contamination in a compressed air system can be removed simply by filtration. It is important however to select the correct type of filtration to obtain reliable results.

Mechanical separation of particles relies on three mechanisms of filtration:

  1. Direct interception
  2. Inertial impact
  3. Diffusion or Brownian movement

The first method, direct interception, affects the larger particles in an air stream, which are literally sieved out.  These present no real problem other than clogging filter material.

The second method, Inertial impact occurs when a particle traveling in an air stream is deflected around the first, second or even third fiber in the filter material, but is eventually unable to negotiate the torturous path between the fibers and cannot change direction as quickly as the air stream. It therefore collides with a fiber and remains attached to it.

The third method, diffusion or Brownian movement, affects the very fine particles which are subject to inter-molecular and electrostatic forces which cause them to spiral in the air stream, thus increasing their effective diameter.

Air Filtering though diffusion

There is a critical particle size as a result of particles falling between the inertial impaction and diffusion mechanisms. This critical size is 0.3 micron and it is very relevant when considering removing fine particles, either liquid or solid. This particle size is typically used in filter efficiency and integrity tests.

If a filter tested on this particle size proves to be 100 % efficient, then it is quite safe to state that this filter is capable of removing any particle above this size.

There are different ratings for these types of filters. They will have either a nominal or an absolute filtration rating.

Research has shown that every cubic meter of air in industrial areas contains about 140 million dirt particles. In heavy industrial areas the problem is even worse. Furthermore, up to 80 % of this contamination is smaller than 2 micron, allowing it to pass straight through the compressor intake filters, which range between 5 -10 microns.

Compress this air to 100 psig, and you have a staggering 980 million dirt particles per cubic meter, not to mention oil, operating dirt, scale, rust and condensed moisture.

Dirt and other contamination can have a serious effect on the wear and efficiency of pneumatic machinery. It will increase maintenance and downtime, and can be a determining factor in the early replacement of the pneumatic equipment.

The coalescing type filter, as depicted in these photos, is typical of a low capacity coalescing type filter.  The flow pattern is the same as that of larger filters, however the filter media sometimes has an anti-re-entrainment barrier on the outside of the filter cartridge to prevent liquid contaminants from getting picked up by the outgoing, clean airflow.

The air enters through the inlet port and travels down through the inner core of the filter, passes through the filter media and leaves through the discharge port.  Moisture and oil collect in the bottom of the housing and are expelled by a condensate drain valve, and other solid contaminants collect in the filter element.


This type of a filter frequently has a built-in, float type auto drain trap and the some manufacturers have a moisture level sight glass mounted on the side.  This is to provide visual assurance that the moisture drain is functioning.  This unit is not shown with a differential pressure gauge which is usually attached to the top of the filter. 

The purpose of this type of gauge is to show the condition of the filter media by registering the pressure difference between the inlet side of the filter and the outlet side.

The manufacturers’ recommendations should be followed in respect to their suggested maximum allowable pressure drop before change out of the filter cartridge.  A high pressure drop across filters must be compensated for with higher initial system pressure.  Running with a dirty filter could be costly, from a fuel efficiency standpoint.

It is important to note that, as the airflow of the coalescing type filter is from the inside out.  The particulate type filter, used for solid dirt and dust removal, is most often from the outside-in, as this configuration takes advantage of the larger surface area of pleated filter media, giving the filter element a longer life as it has more area to catch the dirt and dust particles before a high differential pressure is detected.  If the media is not a pleated type media, the particulate filter may have an inside out flow as well.

The compressor inlet filter is a dry, pleated particulate filter, with a 5-10 micron rating.

Typical grades and specifications of filtration would be:

specifications of filtration

If you have any questions about this article or anything mobile compressor related, please contact us.