Not all compressed air is created equal. Neither are all compressed air systems.
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Your compressed air needs to provide the flow, pressure and level of air quality required by your processes. A well-designed compressed air system supplies that air as economically as possible. A poorly designed system misses one or more of those goals.
So what makes a well-designed air compression system?
Air quality is a measure of how few contaminants are in the air stream. As mentioned above, compressed air dryers can help remove one type of contaminant—moisture. But filtration is needed to remove other types of contaminants from your air lines.
Particulates are typically solid particles (e.g. dry and wet dust, pollen, and rust from pipe corrosion) and large liquid drops.
Aerosols include both oil aerosols and smaller droplets of water.
Vapor consists primarily of oil and other hydrocarbons in a gaseous form.
The air quality level for all compressed air applications can be defined by the air quality classes in the ISO -1: standard. (See the post The ABC’s and 1-2-3s of Air Quality Classes for details about air quality levels and classes.) This standard specifies the allowed levels of contamination for each contaminant type at each air quality class. To meet these levels, you need to use the right types of air filters with the right degrees of filtration.
Particulate filters remove particulates from the air stream by trapping contaminants on the media. The filter element design and material determine the size of the particles it can trap, so particulate filters have varying degrees of filtration.
Coalescing filters remove water and aerosols of liquids like oil by coalescence. Coalescence is the process of trapping smaller droplets along the media until they combine into larger drops that fall out of the air flow. The structure and type of media determine how much aerosol can be removed, so they also have varying degrees of filtration.
Some filter designs (e.g., Sullair SX series threaded and flange filters) act as both particulate and coalescing filters. You only need one filter with a single element to handle both types of filtration.
Activated carbon filters are used to remove oil and hydrocarbon vapors from compressed air and reduce oil carryover to .003 mg/m³. These filters are most typically used where removing taste or odors is critical, e.g. food processing and pharmaceuticals.
Coarse particulate filters can only remove large particles from the air stream.
In most cases, coarse filters don’t provide high quality air. However, they can help remove moisture and keep large particles from fouling compressed air tools and instruments. (Unless those tools or instruments require higher quality air, of course.)
Typically, fine filters are the minimum size needed where applications or equipment need high quality air.
Superfine filters are used when the highest quality air is needed; for example, for food production, pharmaceuticals, spray painting, and instrumentation.
All degrees of filtration can collect larger particles and aerosols than they are optimized for. That might tempt you to just use filters with the highest degree of filtration needed—but you shouldn’t.
When filtering larger contaminants with finer filters, the buildup and loss of efficiency can happen much more quickly. (An active carbon filter could become clogged in hours, for example.) This can lead to unnecessary costs for replacement elements.
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All filter elements have a maximum usable saturation point and eventually need replacing. Over time, contaminants build up on filter elements (including coalescing filters, as some solid particles, however small, are always present). Eventually, this buildup reduces the efficiency of the element. This efficiency loss causes larger pressure drops and allows more contaminants to pass through.
You shouldn’t try to save money by delaying or not replacing elements. If you don’t replace filter elements once they reached their maximum useable saturation point, your overall production costs could increase in three ways:
These typical filtration examples for compressed air systems assume combo particulate/coalescing filters unless otherwise noted.
In systems without dryers, install a coarse or fine filter immediately after the compressor.
If you need a stricter level of filtration, add a superfine filter after the fine filter.
In systems using a refrigerated dryer, install a fine filter between the compressor and the dryer.
If you need a stricter level of filtration, install a superfine filter after the dryer.
If an active carbon filter is needed, install it after the superfine filter.
In systems using a desiccant dryer, install a fine filter between the compressor and the dryer.
If you need a stricter level of filtration, install a superfine filter between the fine filter and the dryer.
Immediately after the dryer, you should install a fine particulate filter (e.g. the Sullair SX, FXFRHT and FWFRHT particulate series filters) to remove any dust coming off the desiccant bed.
If an active carbon filter is needed, install it after the fine particulate filter.
While typical filtration arrangements can serve as guidelines, your filtration should always be designed for your process needs. Too little filtration and the air stream could contain too many contaminants to provide the correct quality of air. Too much filtration and you could be paying more for your compressed air system and its upkeep than you need to.
Clean compressed air is key in the health of your pneumatic tools and they can be the final barrier before a paint gun to save that paint job. We ALWAYS suggest to use one inline when painting or spraying powder.
Not all inline air filters are created equal and some are much more efficient than others. Compressed air filters are designed to separate moisture and dirt, debris, and oil that has made its way out of your air tank. A large air filter dryer system attached at the compressor tank outlet will be the most efficient and make the biggest difference in the quality of the compressed air coming out at your hose end. These multi-stage systems also create the least amount of pressure drop or restriction in your air system. We suggest installing a multi-step system that includes a water trap and a desiccant-type filter. Both of these linked together will get the air clean and dry enough for the majority of home hobbyists. These types of systems do cost a little more, but they are reusable and the desiccant can be dried out and reused or replaced for a nominal amount. Remember to regularly check, drain, and clean your dryers and filters as they can cause a large drop in PSI at the outlet if they become clogged.
Last chance filters are a nice peace of mind for using when you need additional filtration. We most commonly use this style filter when painting or powder coating as moisture or oil and dirt in the air can mix with your coating and cause poor results. We offer small disposable inline last chance filters that thread directly onto your paint or powder gun and filter the air just before it enters the gun. These are consumable filters and will greatly reduce the pressure in the line if they are used past their life cycle. We suggest putting a new filter on each time you are doing painting or powder coating.
If you’re doing a lot of painting or powder coating and can’t afford a multi-stage filtration system, or want less time replacing the small disposable filters, you can look into installing a Desiccant Snake. These are one of the best last-chance style inline air filters for dirt and oil and also act as a moisture separator. The line is filled with desiccant and filtration material that gives some peace of mind when painting or powder coating. These last a lot longer and can be installed inline just before the paint gun to give you clean, dry air. We also suggest desiccant snakes for users that have a small lower end compressor that tends to put out excessive moisture or contaminants. Desiccant snakes are inexpensive and will last you through multiple paint or powder projects.
The moral of the story is that you can never have too much filtration in your air system and extra filters can be added when you need it. Remember to check filters for the change in output before and after installing to make sure they don’t cause too much restriction in your system to overwork your compressor or drop the compressors efficiency.
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