Filters are an important part of your compressed air system. Installing the right type of filters and maintaining them regularly will ensure a consistent supply of clean air to your operations and reduce pressure drop. Filters start at the intake of the compressor and finish at the point of use.
Dirty compressor intake filters will reduce compressor efficiency and allow dirt particles to enter the compressor, loading up the separator and oil filters, contaminating oil, and possibly reducing the life of the air compressor.
Filters downstream of lubricated air compressors will collect contamination in the piping such as rust and remove oil in the air that could collect in the piping affecting the operation of pneumatic components.
If the compressor is located in an environment with a lot of airborne particulates, that will impact the cleanliness of your compressed air. In industrial applications, dust and fumes from production processes can contaminate compressed air. Outside air can be problematic, too — especially in the spring, when pollen counts rise, and in the fall, when mold and leaf dust are in the air. If the compressor is located outside, or factory doors are kept open, these seasonal considerations will impact compressed air cleanliness and compressor performance.
Understanding Filtration Efficiency for Compressed Air
There are two elements to filtration efficiency for an oil-lubricated air compressor: particulate filtration efficiency (measured in microns) and oil carryover (measured in parts per million, or PPM).
Dry contaminants in compressed air are measured in micron size. A micron is one-millionth of a meter, or 0.001 mm. The human eye can see particles as small as 50-60 microns, or a bit less than the diameter of a human hair. Contaminants in compressed air systems can be much smaller than this. About 80% of industrial contaminants are in the fine or ultrafine range.
- Fine particles are defined as less than 2.5 microns.
- Ultrafine particles are defined as less than 0.1 microns.
Filtration efficiency for dry particulate is measured by the size of the particle the filter can capture. A basic intake filter may remove particulates in the 30-40 micron range and above—enough to take out most pollen and coarse particulates, but not finer particles. Advanced dry particulate filters (such as HEPA filters) and coalescing oil filters can remove particles down to 0.01 microns. Filter efficiency will be expressed in a percentage, the higher the percentage of the particle size rating the more particles they will remove. Oil removal of a given filter is rated in remaining oil going downstream measured in PPM or parts per million.
How Clean Does Compressed Air Need to Be?
Most compressed air applications benefit from clean, dry air. But how clean compressed air needs to depend on how it is being used. Air used to inflate tires can tolerate quite a bit of contamination, while air used in medical applications or food and pharmaceutical production will require ultraclean air.
- For pneumatic systems, basic filters with efficiencies between 0.3 and 1 microns are generally sufficient.
- Paint lines and similar applications may require filtration efficiency down to 0.1 microns with low oil carryover.
- Medical, food processing and pharmaceutical applications require ultraclean air, down to 0.01 micron, along with the removal of all vapors, aerosols, and gases.
What Kind of Air Compressor Filter Do You Need?
Your air compression system may require more than one type of filter. The combination of filters you need will depend on the type of air compressor you have, the purity of the air you require, and what you are trying to filter out.
- Dry particulate filters, as their name implies, remove only dry dust and particulate.
- Coalescing filters remove both particulate and aerosolized liquids.
- Adsorption filters remove gases, vapors, and smell.
Types of Air Compressor Filters
Filters commonly used for air compression include:
- Intake filters
- Inline filters
- Oil filters
Intake filters are positioned at the air intake to filter contaminates out of atmospheric air before it enters the compressor. These filters are usually rated to capture course particulates such as dust and pollen. Intake filters generally remove dry particulate only.
Inline filters are positioned after the compressor to provide additional filtration for already compressed air. A standard particulate inline filter will remove dry particulate down to 1 micron in size. However, they will not remove oil mists. Oil-lubricated air compressors (such as rotary screw air compressors) will need an inline filter rated for both particulate and oil, such as a coalescing filter. Inline filters may be placed directly after the air compressor, finer filters should be placed after the compressed air dryers or immediately before the end application. Read more on inline filters.
Oil filters are used to remove particulate from liquid oil in oil-lubricated air compressors (such as rotary screw air compressors) oil contamination as expressed previously can reduce the life of the air compressor.
Dry Particulate Filters
Dry particulate filters remove dry particulate from the airstream. They may be intake filters or inline filters.
Particles are trapped by the filter media through direct interception, inertial impact, or diffusion. Large particles are directly blocked by the fibers in the filter media. Smaller particles are intercepted as they move erratically through the media via Brownian motion (diffusion). These particles are held in the media through electrostatic attraction.
Coalescing Inline Filter
A coalescing filter is another type of inline air compressor filter. An oil coalescing filter removes both oil mists and dry particulate.
A coalescing filter works by trapping mists and aerosols in layers of fine mesh. Aerosolized oil and water droplets collect on the surface of filter media and coalesce into larger and larger droplets until they are heavy enough to fall. The liquid is collected at the bottom of the filter and drained away. Fine particulates fall out with the liquid, while coarser particulates remain trapped in the filter media.
Coalescing filters provide superior filtration for both particulates and aerosols. They can remove aerosolized droplets and particles down to 0.01 micron and remaining oil to .008 PPM or lower. They may be used alone or in combination with other filters.
Adsorption filters are used to trap vapors, gases, and odors. These filters are used for high-purity applications that require the removal of trace gases and vapors along with sub-micron particulates. Activated carbon is the most common material used for adsorption.
In adsorption technologies, molecules of a gas, liquid, or dissolved solid adhere to the surface of a material. Adsorptive materials like activated carbon have millions of tiny micropores, which increase the available surface area for adhesion. Molecules bond to these surfaces and are trapped within the micropores.
Activated carbon inline filters can remove unwanted vapors, gases, and noxious odors from compressed air. They should be used in combination with a compressed air dryer a coalescing filter to remove oil mists and dry particulate from the air before it hits the adsorption filter.
Air Compressor Oil Filters
Oil filters are used for oil-lubricated industrial air compressors, such as oil-injected rotary screw compressors. They remove dirt, rust, and other course contaminants from liquid oil as it circulates through the compressor. This protects the compressor from wear and tear on bearings and other lubricated parts. An air compressor oil filter is similar to the oil filter you might find on your car or lawnmower.
The air compressor oil filter has a few important characteristics.
- A bypass valve, which will supply oil to the compressor if the filter is clogged (because dirty oil is better than no oil!).
- A high-pressure housing and special seals to withstand fluctuating pressures in compressed air applications.
Maintaining Air Compressor Filters
How often should air compressor filters be changed? It is important to maintain air compressor filters for the optimal performance of your compressed air system. Clogged filters will quickly sap the energy, every 2 PSIG in pressure drop is 1% of the cost of running the air compressor, many compressors are running at a higher pressure to compensate for pressure drop. Filters also lose their filtration efficiency over time, especially if you have abrasive contaminants in the airstream. Microtears and holes in filter media will allow contaminants to make it past the filter, where they can end up in the air supply this is also true with the oil separator in the air compressor.
The maintenance schedule will depend on your operations (air compressor runtime) and the overall cleanliness of the environment in which the compressor operates. Filters will need to be changed much more often for an air compressor running 24/7 in a dusty room than for a compressor operating intermittently in a relatively clean space.
- Monitor intake air filters and change them when they appear dirty or worn. Intake filters are typically changed after 2,000 hours of use, but they will need to be changed more frequently if your environment is very dusty.
- Inline filters should be changed at least once a year or every 8,000 hours of operation. Monitor pressure drop (the differential pressure before and after the filter) to determine filter loading. If you see excessive pressure drop before 8,000 hours of operation, your filters need to be changed more frequently. (Check your service manual for your specific inline filter model.)
- Compressor oil filters should be changed every 2,000 hours at minimum, or more often if the oil appears to be dirty. At least once a year, oil should be completely drained and flushed. Always replace your oil filter after flushing.
Proper air compressor filter maintenance will go a long way towards preventing problems with your air compressor. Clean air and oil filters will save energy, reduce wear and tear on the air end and extend the life of your air compressor.
Need help in selecting the right filter for your air compressor? The air experts at Fluid-Aire Dynamics can help, give them a call today.
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