If you have acquired or purchased a smaller air compressor for your home or garage workshop, chances are it did not come equipped with a compressed air filter. Do you need a compressed air filter, then? If so, why?
In the market place drive to offer the consumer the cheapest possible off-shore manufactured air compressor, not including a compressed air filter saved the compressor company a few dollars per air compressor, and allows them to keep their wholesale prices, and ultimately the consumer sell price a little lower.
In so doing, for most users of air compressors, they have not done you any favors.
How compressed air filters work
The cap in the compressed air filter directs the incoming compressed air down into the filter bowl, and against the filter bowl, which forces the air into a tight, cyclonic spiral inside the bowl.
The compressed air, reputedly moving at nearly the speed of sound (I say reputedly as I’ve never actually measured it’s speed) literally throws free water and debris against the side to the bowl. This contaminates runs down the inside of the bowl to the bottom quiet zone, where it’s either manually or automatically drained off, depending on the type of drain in the filter bowl.
The air continues spiraling inside the bowl while it also drives through the filter element, back up to the cap, and out downstream to the application. Any debris that hasn’t been thrown out will be captured by the filter element, as long as the debris is larger than the holes in the element, the holes were measured in microns of size.
About Compressed Air Filters – Why Install One?
Why would you consider adding a compressed air filter to a compressor air line?
Compressed air is filthy. Your air compressor is sucking in vast quantities of ‘fresh’ air, and that air is full of grit and dust. The intake filter on your air compressor filters out much of that debris. Some always gets through.
As the air compressor pumps, that still-dirty air is pumped into the tank. From there, it heads out through the coupler on the compressor into your air line, and into your air tool.
Dirty compressed air acts as an abrasive, wearing inner components of your air tools, particularly of concern if you use your air tools a lot.
Depending on the element capability in the air compressor filter, as the compressed air passes through the filter element, any debris still in the air line is removed down to the filter element size, thereby reducing particulates getting to the air tool.
The compressed air filter comes before other components!
Modern compressed air treatment units (FRL’s) are now usually modular, or they can be nippled together.
Ensure that when you assemble an FRL unit to treat your compressed air, that you make sure that the air travels through the filter before it arrives at other downstream components.
The filter will remove free water, and any airborne debris before it can get to and damage sensitive air-driven equipment.
Water Is Generated Right Away
Compressors turn water vapor in the intake air into free water inside the compressor. Compressors also increase the amount of water vapor in the compressed air in the tank. Both free water and water vapor laden airWater Is Generated Right Away enter your air line and through it to your air tools.
As soon as any air compressor starts pumping air into the tank, it is also pumping free water and water vapor into the tank too.
How much water ends up in your air compressor tank?
That is determined by a number of factors; how long the compressor is running at one time, how hot the compressor gets, the volume of compressed air being used, the ambient temperature and humidity of the air flowing into the air compressors intake port… many factors, few of which you have any control over.
Essentially, there could be lots of water pumped into the compressor tank along with the compressed air.
Water getting to an air tool is a problem. If your air tool is a paint sprayer, water getting to the sprayer is catastrophic to the paint job.
Compressed air filters remove much of that free water, and some of the water vapor from the compressed air stream, as well.
If you use your air tools a lot, or if you are working with an air tool that requires freedom from water spatters and water vapor, then you want to install a compressed air filter.
How Do You Use The Compressed Air?
What have you got hanging on the end of your air hose from the compressor? An air tool? A spray paint gun? Just a blow gun?
Regardless, whatever you are blowing compressed air into from the compressor will likely be negatively affected by the water coming from the compressor tank along with the air.
Do your equipment a favor and add a compressed air filter.
What Size Of Compressed Air Filter?
A rule of thumb is that the intake port on the compressed air filter should be the same size as the air line that is feeding it. That will work for most small air compressors, as the capacity of even a mini-air filter easily outstrips the amount of air that a small air compressor can deliver.
If your air compressor is more than a couple of HP in motor size, rather than use a mini-filter similar to the one shown in the photo above, consider purchasing a regular sized air filter that has the flow capacity of your compressor.
What’s flow capacity? Well, typically an air compressor can pump out about 3-4 CFM of compressed air at 90 PSI for each HP of motor size. The compressed air regulator will have a flow capacity shown on the box or the documentation that comes with the filter.
Just make sure that the capacity of the filter exceeds the flow capacity of the compressor and you are fine.
And yes, periodically you want to replace the filter element in the compressor filter as it clogs up with crud brought down from the compressor by the flowing compressed air.
About Compressed Air Filters – Where Install One?
As a rule of thumb of mine if it is a small DIY type air compressor, install a compressed air filter right at the coupler of the compressor. Rather than disassembling the piping, if you want to be crafty, by using another connector and coupler installed in the filter head you can then simply plug the filter into the existing coupler on the compressor with the connector in the filter, and plug the air line into the coupler on the other side of the filter head.
If you have long air line runs, know that water will condense out in the lines, and end up spraying out your air tool, even if you have a filter at the compressor.
I suggest you run the air line to a filter installed on the workshop wall, and then use another, shorter air line, to get air to your air tool.
About Compressed Air Filters – All compressed air has water in it
If water coming down your compressed air stream is a problem for your work or your air tools, and it almost always is, do yourself a favor. Install a compressed air filter.
The modest up-front expense may save you significantly in the long term.
Got a question about compressed air filters? Ask it below.
When To Use What Type Of Compressed Air Filter?
There are general purpose air filters, coalescent air filters and activated charcoal air filters. When do you use which type? This contribution by Norgren, with my thanks.
Three main types of filters exist: the general purpose filter for water and particles, the coalescing oil removal filter for oil aerosols, and the activated carbon filter for the removal of oil vapors.
Use general purpose filters for main headers, branch lines, tools, cylinders, valves and valve circuits, air agitators, packaging machines, etc.
Use oil removal filters when sub-micron cleanliness or oil-free air is required, such as for the supply to fluidic devices, painting or coating processes, instrumentation, air gauging equipment, air bearings, and medical applications or areas where typical synthetic compressor oils may damage commonly used Buna rubber such as non-lubricated power circuits.
Finally, use activated carbon filters for systems where the oil vapors in the air are not acceptable, such as instrumentation, medical, or packaging systems where the compressed air may come into contact with the product, such as pharmaceuticals.
Some excellent advice from Norgren regarding compressed air filters….
“Filters have their flow measured in terms of the pressure drop across them. As the flow increases, the pressure drop also increases. These pressure drops are energy losses in the system.
A well-designed filter not only removes water and particles efficiently but also has a low-pressure drop at a given flow.
Be selective. Not all manufacturers quote their flows under the same conditions.
If a higher inlet pressure is used or a higher pressure drop is quoted then the apparent flow will be higher. This does not necessarily mean it’s a better unit. Often the only way to compare units is to test them under the same laboratory conditions.”
Replacement compressed air filter
I have a Speedaire stock # 47028a I know it’s out of production. What can I do to change the filter, if possible. If not, what would be a comparable replacement?
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First off, I’m guessing he’s asking about the filter element…as opposed to the whole air filter.
Unfortunately, as air components age, the manufacturer may drop or re-engineer a product, and all of a sudden, there is no support for it nor can you get parts; in this case, a filter element for a perfectly good filter housing.
Most general purpose compressed air filters have at 25-40 Micron air filter in them.
If you are replacing a general purpose filter, it will be OK to visit your local hardware store (better if it was a local fluid power distributor) and ask them for a general purpose filter that is the same pipe size as yours. And, to prevent this issue for the foreseeable future, ask them for a couple of replacement elements for that filter too.
Typically there is pressure drop across a compressed air filter. The element, even when clean, creates a flow impediment that lowers downstream pressure. If you are seeing an increasing pressure drop across that air filter, that’s the indication that the element is getting plugged up. When the pressure drop is negatively affecting the work, then the filter element needs to be cleaned or changed out.
Here is more information on NPT sizing, and here is much for information about compressed air filters.