Compressed air quality: how clean do you want your compressed air?

Get high quality compressed air by installing the right equipment and remove water, oil and dust from your compressed air.

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The quality of your compressed air is very important for the good operation of your equipment like air tools and pneumatic machines. Although it’s important, compressed air quality is often neglected or not given enough attention.High compressed air quality will dramatically reduce maintenance and breakdowns on air tools like grinders and nailers, and machines with air cylinders and moving parts. It will also reduce rust and dirt in your compressed air piping system.Dirt, water and rust will get stuck inside your pneumatic equipment. Valves will get stuck or wear down, same for cylinders and air tools. It’s like the difference of breathing air on a dusty construction site, or in the middle of a nice forest.

What is quality air?

Alright, so we need quality compressed air. But what is exactly quality air? There are several things we look for when checking air quality:

  • Water  in the air
  • Dewpoint / pressure dewpoint
  • Oil in the air
  • Dust
  • Contaminations like smells and micro organisms

All those things together contribute to the overall ccompressed air quality. Of course, use your common sense and decide which factors are important to you.

Water in the air is a common problem, probably important for all compressed air systems. Oil-free air on the other hand is not a big issue for many people, but is is a big issue for people working in the chemical or food industry.

On the ‘clean compressed air‘ page there’s a list of stuff that will determine the quality of your air, plus info how to get rid of it.

Bottom line: think for yourself what is important for you. Don’t believe the salesman that tries to sell you expensive equipment to get super high quality air, when you don’t really need it.

Figure out what is important for YOU. Check the manuals of your equipment, it will often tell you what quality of compressed air is needed (like the dewpoint / amount of water and oil that is allowed, are there any special filter requirements?).

Equipment to create quality compressed air

Equipment to ‘upgrade’ your air quality is available from most compressor manufacturers. For example: compressed air dryers, oil filters, dust filters, water traps.

It doesn’t really matter what kind of compressor you have. There are only two types of compressor, as far as air quality is concerned: oil-free compressor and oil-lubricated compressors. The oil-free compressor is more expensive but doesn’t use any oil, so the compressed air produces is completely oil-free.

A more expensive compressor will not give you better air quality. All compressors suffer the same problems with regards to water and dust in the air. The only exception being oil. Oil-free compressors give oil-free air, and piston compressors tend to ‘spit-out’ more oil compared to screw-type compressors.

Costs and paybacks / pros and cons

In short, this is what I could come up with, in terms of pros and cons of quality compressed air:

Pros of high quality air

  • Less maintenance of pipe system , easier piping installation (clean, dry air)
  • Less maintenance of equipment
  • Less breakdowns, resulting in higher machine uptime

Cons of high-quality air

  • Cost of equipment like filters and air dryers
  • Pressure drop over dryer (will result in increased energy consumption)
  • Pressure drop over filters (will result in increased energy consumption)
  • Replacement filters costs (filters need to be replaced every once in a while)

As I said before: figure out what is important for you. What do you really need? Remember, the less stuff you have (dryers, filters, etc), the less can go wrong or break down. Keep it simple.

You could buy a standard refrigerated compressed air dryer, or a more expensive desiccant compressed air dryer.  If you don’t need the very low dew point, why spend the money on an expensive air dryer? Do you need an air dryer at all?

On the other hand, if the air is used as breathing air, of course you need all the dryers and filters (and re-humidifiers)!  It all depends on the application…

Quality classes

To make it easier to compare air quality between systems and to rate equipment like filters and dryers, an ISO standard is produced, to classify the compressed air quality. Class 0 is the highest quality, super-clean air.



Particle size in micron

WATER @ 7 bar / 100 psiPressure Dewpoint


(including vapor)




Max number of particles per m3

° C

° F

Mg /m3



As specified

As specified

As specified








































How to use the table

The ISO standard gives quality ratings for solid particles, for water and for oil. To completely ‘belong’ to a class, all requirements must be met (solids, water and oil).

For example, air with 2.000 particles of 2.0 um and a pressure dewpoint of -20 degrees Celsius will be class 5.

2.000 particles of 2.0 um will give us class 5 for solid particles/dirt. -20 degrees Celsius will give us class 3 for water. The overall class of the compressed air will be 5, although for water only it will be higher: class 3.

Again, use your commons sense. You might not be interested in oil in your air, but dust and water might be disastrous for your equipment. Here’s a small list of general recommendations for air quality classes for different kinds of equipment / use.

Air tools with air motors (like grinders):

Dirt: 2

Water: 5

Oil: 4

Other hand tools:

Dirt: 5

Water: 5

Oil: 4

Machines with pneumatic cylinders:

Dirt: 4

Water: 4

Oil: 4

General workshop air:

Dirt: 4

water: 5

Oil: 4

Compressed air quality testing

It is possible to test the quality of the air on-site. But this is usually quite an expensive option, as the equipment needed to measure all the variables is quite expensive.

However, if you still want to have some assurance about the quality of your compressed air: buy filters and equipment that are rated to a specific quality class.

If you buy a dirt filter of class 2, you can be sure that the air quality will be according to class 2, for dirt. If you need the same quality for water and oil, you need to also install the correct dryer and oil filters. Keep in mind that there are ‘best practices’ when designing your compressed air system. Do you install the oil filter before the air dryer or after the air dryer?

Also, keep in mind that filters will get dirty. Depending on how well your system is designed, the kind of compressor and the amount of dust, water and other contaminants in the surrounding air, filters will clog up sooner or later.

So there is no way of telling in what air quality class you are when your filters are all 1 year old. But with regular maintenance (and keeping to the manufacturers recommendations), you should be ok.