Air? Sure, you’re thinking of force behind a nail gun or tire pressure, but what about the air itself? It is invisible and harmless, right?
Until it isn’t. Just as any other fluid flows through your lines, water vapor, oil mist and microscopic dust does too. Invisibility makes them hard to see.
That is, until they clogs a pneumatic tool, mar a paint job or pollute a food processing line. Filter selection is frequently considerd an afterthought. You buy the compressor, then it’s a rubber stamp decision.
And that’s expensive. A filter element has a micron rating; it’s the thing that stops stuff, and it’s the thing that lets stuff through. It’s the difference between clear and cloudy, between long lasting and seizing up in half the time.
Understanding Micron Ratings For Compressed Air Filters
The Hidden Dangers of Contaminated Air
The natural instinct for most folks is to take the smallest thing available because “smaller must be better”. Not so fast. Smaller means a more aggressive restriction on air flow.
This will also limit how much volume flows past to your tool. The trick with filters is knowing where to strike the balance of filtration accuracy versus restrictive flow. A micron rating essentially mean how small the particles are.
Understanding Micron Ratings and Filtration Accuracy
How small is a micron? It’s one millionth of a meter. For comparison, a human hair averages around 70 microns thick.
A grain of sand measures approximately 74 microns. That means when people discuss 0.01 micron and 5 micron filters they’re referring to something that can’t be seen with the naked eye. Why does it matter?
The scale depends on what contaminant(s) you’re attempting to filter out. The average size of large oil mist and water droplet is approximately 3 to 5 microns. At that size they are considered relatively large.
A typical coalescing filter, one with a 5 micron rating, should removes most of these particles. This is your initial protection against liquids. The trash can in a crowded office.
It picks up the easily recognizable stuff before it clogs the work place. By stopping at a 5 micron filter, you’re missing the fines; even if you get most of the water out, some ultrafine particulate matter and oil vapor still hang around in that air stream. The small particles poses a danger to precision equipment.
They’ll damage seals on an air motor. They can also clog the holes in a spray gun nozzle or other tiny openings. This is when you need to step up your filtration game.
For the finer contaminants, you’d have to move up to a 1 micron (or even a 0.01 micron) filter. The idea is to combine the smaller drops into larger ones which will then drain off. It also catches solid particles which would not pass through a coarser filter.
For an application such as pharmaceutical manufacturing or even electrostatic painting, this degree of cleanness are not optional. Here there’s no room for compromise. Yet this purity comes with a price.
Balancing Filtration Precision with Airflow Restriction
The finer the filter, the tighter its pore size and the greater the pressure it puts against airflow. This causes a pressure drop. Run a 0.01 micron filter on a high volume application and you may discover that your tools runs sluggish.
They aren’t receiving sufficient air volume to operate at their best. It is a tradeoff between capacity and cleanliness. Match the filter to the job.
Choosing the Right Filter for Your Application
For example: For something as simple as an impact wrench or inflating tires, a 5 micron filter will do just fine. It cleans the air sufficiently so it’s dry enough for most uses and removes the water that causes rust. In this case adding a.01 micron filter is overkill.
It won’t give any practical benefit other than wasting money and reducing amount of air available to your tools. However, if you’re spraying paint with an HVLP spray gun for automotive refinishing, the 5 micron filter wont cut it. Fisheyes will form in your clear coat due to the leftover oil vapor.
For something like this, you really want that final stage filtering of.01 microns so the air is completely clean and free of anything harmful. Here, the small amount of pressure drop are a fair trade-off for a perfect finish. To address this balancing act, many systems employ a staged approach.
The Benefits of a Staged Filtration Approach
They install a coarse stage (like a 5 micron coalescing filter) in line to capture most of the oil and water. Downstream, they add a fine filter (like a 0.01 micron activated carbon or final filter) to polish the air. This shields the finer filter from rapid overload by big dirt loads.
It allows for high flow rates while extending life of the costly element. Companies such as Donaldson and Parker Hannifin have solid cartridge lines that offer there own solution. They have the ability to cover a range of industries from industrial machinery to food grade air.
There’s no need for guessing which micron rating goes with what. This has been engineered for you. The other consideration with micron ratings is maintenance.
Filters that has smaller micron ratings capture more stuff. This means they gets plugged up more quickly. A 5-micron element needs to be changed less often than a 0.01-micron one.
Maintenance and Monitoring Filter Performance
Let’s say you don’t follow the maintenance schedule. Guess what? Your filter is now a restriction point in the system.
It raises the pressure drop, and your compressor is working harder to force air through it. Doing so not only reduces how long the compressor last but also increases its energy consumption. But that also defeats the purpose of filtration!
If the filter becomes too clogged, it could even crack and stop filtering altogether. The best way to know this has happened is by monitoring both the before and after filter pressure gauges. When one significantly drops, it’s time to swap cartridges.
Finally, micron rating is a functional specification, not a badge of honor. In some contexts, a higher number does not mean better; it means higher precision. The trick is choosing the right rating for how sensitive your final use is.
Waste money over-filtrating. Ruin work under-filtrating. Identify your contaminants…
Know what’s in your compressed air. Assess your downstream equipment, What level of cleanliness does it need? Select the filter that provides that level of cleanliness while not choking the airflow.
Clean air is basis for reliable pneumatic performance. Once you know the variables, clean air is a simple equation. Get the micron rating correct and the rest falls into place.