How classy is your compressed air?
To make it easier to talk about compressed air quality, we use quality classes. Each class stands for a particular level of water, oil or dust.
Almost every piece of pneumatic equipment in your factory will have some guidelines about the quality of the compressed air.
Now instead of listing the maximum amount of dust, oil and water by numbers (which can become confusing quickly), the manufacturer can simply give you the required quality class for each contamination.
This makes life easier for everyone involved:
For machine manufacturers they can simply state the required quality class that is needed for their machine.
For authorities: in their regulations, they can state the needed quality class needed for specific applications, instead of specifying all different contamination levels individually.
For filter and dryer manufactures (and for you, when selecting a filter or dryer): instead of having to specify each and every characteristic of the filter or dryer, they can state the output quality class of the equipment.
The big advantage of this that’s it is much easier to put together a compressed air system that delivers the right quality to the end user.
Because all compressed air filters and dryers will state the quality class that it produces, it makes our life much easier.
The official compressed air quality classes can be found in ISO 8573. Here’s the full table:
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Explanation
The table has columns for dust (grey), water (blue) and oil (yellow).
For each quality class (horizontal rows, numbered 0 to 6), the maximum amount of dust, water and oil is specified.
For dust particles, there’s a sub-division into different particles sizes. For water and oil, we’re talking about the total water or oil content (liquid + vapor).
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What quality do you need?
The compressed air quality that YOU need, depends entirely on what you use your compressed air for.
Here are some examples of typical compressed air classes for different applications:
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System layout
When laying out a compressed air treatment system, we can’t just put together some filters and a dryer in any random order.
For example, do we first filter the air, or do we first dry the air? Do we remove oil first, or do we remove dust first?
There are some best practices, or rules to follow to make sure our dryers and filters work optimally and you get the highest quality air from your equipment.
Rule 1: Remove liquid water first
Always remove liquid water first. Compressed air filters (any kind) will perform better if there’s no free water (liquid water) in the compressed air.
Compressed air filters work best when there’s no liquid water in the compressed air. Less is better.
For refrigerated air dryers, removing liquid water first improves the capacity and lowers energy use.
Also, liquid water quickly saturates the desiccant material in desiccant compressed air dryers. The dryer is designed to remove water vapor from the air, not liquid water.
In short: remove liquid water first.
Rule 2: Dry the air second
After removing the liquid water, dry the air further using a compressed air dryer.
Particulate (dust) and oil removal filters work (much) better with dry air.
Use a refrigerated air dyer or a desiccant air dryer, depending on your needs.
Rule 3: Cascade filters
Have multiple filters in series, going from course to fine.
By cascading filters, we filter the air in stages. First we remove relatively large dust particles, then smaller ones, and lastly the finest dust particles.
Doing this, prevents quick ‘loading’ of the filter. A filter has a maximum dust-holding capacity. As time goes by, more and more dust gets trapped in the filters. The pressure drop over the filter increases and the filtering efficiency will drop.
See the manual of your filter for information of the level of pre-filtering that is needed / recommended.
Rule 4: Desiccant air dryers
Desiccant air dryers need some extra attention, for two reasons:
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The desiccant material in the dryer is sensitive. It can’t handle liquid water and it should never come in contact with oil.
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The desiccant material creates desiccant dust that needs to be removed downstream.
For this reason, we always need a particulate filter and a coalescing filter upstream of the air dryer – this will remove all liquid water and oil for us.
Secondly, we need a particulate filter downstream of the dryer to remove desiccant dust.
Rule 5: Activated carbon filters
Activated carbon filters are special. They are the only kind of filters that removes vapors, odors and even tastes from the compressed air.
But they are very sensitive to liquid water and oil. So they must always have a fine coalescing filter upstream to protect them.
Rule 6: Colder is better
Treating the compressed air when it’s cold is better than when it’s still hot.
It’s also better to keep the compressor outlet temperature low, if possible.
‘low’ is relative here. By ‘low’ I actually mean around 80 degrees Celsius as an optimal temperature (to prevent condensate forming in the air compressor).
At higher compressor temperatures, oil carry-over will increase very quickly.
Also, colder air can hold less water vapor compared to warm air, so it’s easier to treat cold compressed air as it puts less load on compressed air dryers and there’s less chance of condensate forming again downstream of our treatment system.
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Cost of Air Treatment
Always remember that cleaning compressed air costs money. Not only in the purchase price of the equipment, but also in increased operational costs.
Not only do we have electricity cost of compressed air dryers, and filter element replacements for compressed air filters… we also have the pressure drop problem.
As we saw before: 0.1 bar pressure drop, inceases our energy use by 0.725%.
For example, a 100 kW air compressors may use around $30.000 in electricity per year (continuous operation).
The extra costs for a 1 bar pressure drop is $2.100 per year! And 1 bar pressure drop is pretty easy to ‘achieve’ !
Besides this, many desiccant air dryers use purge air to regenerate the desiccant – sometimes up to 20% of compressed air supply.
We also have condensate water drains that purge compressed air as well.
And we have extra costs for cleaning and disposing of oil-contaminated condensate water.
Bottom line: air treatment costs real money. Know the level of quality that you need.