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Basics of Coalescing Filtration

Basics of Coalescing Filtration

A coalescing filter is used to remove water and oil aerosols from compressed air.

These contaminates are usually compressor lube oils, residual cutting oils and pipe dope from the pipe joints, condensed moisture, carbonized oils and solutions of dissimilar oils.

Coalescing filters should always flow from the inside of the filter element to the outside of the filter element. This allows for the oil and water droplets to collect on the fibers of the filter element and meet at crossover points and become larger and larger droplets (the process of coalescing) as they travel downward and outward to the base of the filter element. The large droplets drop off the base of the filter element into the sump area of the filter housing and ultimately into an automatic float drain where the liquids are drained out of the system.

A coalescing filter may remove liquid contaminants by three methods:

Direct Impact – typically 1 micron and larger size particles

Interception – typically between 0.1 and 0.6 micron

Diffusion – typically 0.1 micron and smaller size particles

Coalescing filter efficiency rating

Coalescing filters perform best when located at the highest pressure in the compressed air system and the lowest temperature. It is best to locate coalescing filters as close to the point of use as possible in order to ensure no additional condensing of liquids takes place.

A coalescing filter by nature will also remove all solid contaminates from the compressed air system as well as liquids. It is the solid contaminates such as rust particles, dust, solder particles, welding flash etc. That will determine the useful life of the filter element as these particles are permanently trapped in the filter element blocking off the filter media and creating a restriction of flow and an increasing differential pressure drop in the compressed air system.

Balston filters are ideal for the continuous removal of liquid contaminate to the submicron level from compressed air systems and protecting sensitive equipment and instrumentation from damage and malfunction. 


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Parker Hannifin Corporation
Gas Separation and Filtration Division
4087 Walden Ave.
Lancaster, NY 14086
1-800-343-4048 (US and Canada)
1-978-858-0505
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