Importance of the Compressed Air Quality

How do you know what quality of compressed air you need? Which filters should you use? What do you do about oil and water vapour? Find the answers here.

Estimated time to read: 7 mins

March 8, 2023

Why air quality is important for compressed air

Moisture, dust and even little droplets of oil are always present in atmospheric air. Always. And in greater amounts at certain times of year, or even certain times of day. You’ll know if you’ve ever experienced a humid summer or a misty morning.

If there’s always moisture and oil in the aur, you may wonder why it’s important to have good quality air for your air compressor. The reason? The more of these airborne nasties in the air, the faster different parts of your system will deteriorate. As well as this, it’s possible for them to have an impact on the product you’re producing.  

Compressed air: How clean is clean?

You may be wondering just how clean your air needs to be. There are plenty of factors. The biggest three:

1.    What you’ll be using your compressed air for.

2.    Whether the product you produce will be coming into direct contact with compressed air.

3.    The environment where you’ll be using your compressed air.

In fact, there are all kinds of other highly scientific factors you’ve probably never heard of and might not even understand. The good news is that a bunch of clever people who spend their day  on calculators speaking scientific sounding words have worked out exactly how clean your compressed air needs to be depending on what you’re using it for. They created the International Standard ISO 85731: Compressed air for general use. It tells you how much clean your compressed air needs to be for different purposes. 

Quality class Solid particle content Water content Oil content

Max. size


Max. amount


Dew point




Max. amount


1 0.1 0.1 -70 0.003 0.01
2 1 1 -40 0.11 0.1
3 5 5 -20 0.88 1.0
4 40 10 +3 6.0 5
5 - - +7 7.8 25
6 - - +10 9.4 -

Compressed air: Filters

According to ISO 85731, you need to make sure your compressed air only has contaminants of a certain size in it. Fortunately, a lot of very clever people with white lab coats and clipboards have already developed different kinds of filters for removing different sized contaminants from compressed air. Time for a quick rundown of how filters work. 


First of all, no single filter is able to efficiently remove particles of every size and sort. It’s best to pass your air across a range of different filters, with each one removing contaminants of different sizes. Inlet filters help to root out different sized nasties before they travel into the inner workings of your compressor. Air dryers help to remove water vapor and condensation that form during the air compression process. A special filter called a coalescing filter is used to stop oil and water vapour from travelling further into your network. It basically allows the tiny little droplets to form into larger droplets that gravity moves to a separate part of your system. And while this sounds pretty straightforward, everything from the size of the particle to the air speed and the distribution of fibres with different diameters comes into play when you remove different contaminants. It’s understandable that you’ll need more than one filter.

Why not just use a single filter that will remove the smallest particles – and stop the larger ones too? Your filter will quickly become blocked. You’ll spend more time replacing it than actually using it.

It’s important to know that compressed air drops in pressure every time it passes through a filter. The finer the filter, the tighter the structure, the faster it will become clogged and need to be replaced, and of course, the greater the amount your pressure will drop. This is why you’ll find systems that compress air more than once. Contaminants are still removed at different steps, with the air compressed again and again so that it exits your system at the pressure you need.

Compressed air: Oil filters

If you have an oil-injected air compressor , you’ll almost certainly have an oil separator to help remove oil from your compressed air. But when it comes to very fine, ultra miniscule traces of oil? There’s yet another filter. This one contains an adsorption material, usually something called activated carbon, that helps to remove any last traces of oil.

Compressed air: Air dryers and adsorption dryers

As for removing water vapour? Our recommendation is to use an air dryer under almost all circumstances. This is because water vapour, condensation and moisture will inevitably lead to:

  • extra maintenance, which means extra downtime;
  • corrosion and rust in your boiler and pipes;
  • moisture contaminating and even washing out the oils and lubricants in your pneumatic tools and cylinders;
  • any blasting materials you use, like sand, becoming sort of clumped together and less effective, in a weird kitty-litter way;
  • a weird sort of fogginess when you spray surfaces with compressed air.

A refrigeration air dryer will do the trick here. They’ll deliver air quality of Class 4 or higher, with a dew point of 3°C or warmer. If you need compressed air that is Class 3 or lower, you’ll need an adsorption dryer. It’s a different approach, with the material being used to adsorb moisture, but will get the results you need.

We also recommend raising the reliability and lowering the environmental impact of this technology with our ALUP AVSD Dryer. Among other benefits, it has variable speed settings, lowering costs while raising sustainability.

The right air compressor system

Wondering whether you need to pay an arm and a leg for a system to remove trace amounts of oil? It usually depends on whether your product comes into direct contact with compressed air or whether oil vapour will contaminate your working environment. You may require a systematic method to be used, in order to determine your ideal equipment choice.

We wholeheartedly agree that it’s best to use Class 0 compressed air to eliminate air contamination in critical processes. Your filters will need to be dimensioned to handle the air flow properly, and have a larger capacity to stop the pressure dropping too much as blockages occur. But if you don’t need Class 0 compressed air? Make life easier on yourself. Use a less complex system. You may still need different types of filters or additions to your system to reach the air quality you need, but this should be possible with less stress and a lower price tag.

Ready to start your journey into the world of filters? Before you become lost on the internet and in the world of cutting-edge research, you may want to have a chat with us at ALUP. We are full of information on everything from filters to air compressor choices. We would be more than happy to give you targeted advice, insights and recommendations. Get in touch today.

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