Talking About Compressed Air Quality

Atmospheric air that is sucked into a compressor always contains a certain amount of moisture, dust and oil aerosols. The amount and ratio of these contaminants depend on the season, weather and where the compressor is located. After compression, the air must be disposed of these ingredients as quickly as possible in order to avoid unnecessary detrition of not only piping but also other pneumatic devices.

Why is it important to consider the air quality?

Compressed air can contain unwanted substances, for example, water in drop or vapor form, oil in drop or aerosol form, as well as dust. Depending on the compressed air's application area, these substances can impair production results and even increase costs. The purpose of air treatment is to produce the compressed air quality specified by the consumer. When the role of compressed air in a process is clearly defined, finding the system that will be the most profitable and efficient in that specific situation is simple. It is a question, among others, of establishing whether the compressed air will come into direct contact with the product or whether, for example, oil mist can be accepted in the working environment. A systematic method is required to select the right equipment.

What are the different air purity classes?

International Standard ISO 85731 "Compressed air for general use", determines the extent to which compressed air needs to be cleaned (see table below). It defines which other devices can be used to achieve the required compressed air quality class.

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 -

How do I remove condensation from the compressor?

Moisture or condensation should be removed from your compressor as quickly and efficiently as possible to prevent damage to your compressed air system. A cyclone can be installed directly inside the compressor or just behind it. To drain the condensate, an automatic condensate drain can be connected to air filters and air dryers directly into the oil-water seperator (if required).

How do filters help?

A flter basically separates the air particles from contaminant particles. The particle-separating capacity of a filter is a result of the combined sub-capacities (for the different particle sizes) as set forth above. In reality, each filter is a compromise, as no filter is efficient across the entire particle size range. Even the effect of the stream velocity on the separating capacity for different particle sizes is not a decisive factor. Generally, particles between 0.1μm and 0.2μm are the most difficult to separate (Most Penetrating Particle Size).2.35.png As stated above, the total capturing efficiency of a coalescence filter can be attributed to a combination of all occurring mechanisms. Obviously, the importance of each mechanism, the particle sizes for which they occur and the value of the total efficiency heavily depend on the particle size distribution of the aerosol, the airspeed and the fiber diameter distribution of the filter media. Oil and water in aerosol form behave similar to other particles and can also be separated using a coalescing filter. In the filter, these liquid aerosols coalesce to larger droplets that sink to the bottom of the filter due to gravitational forces. The filter can separate oil in aerosol as well as in liquid form. However, oil in liquid form will, due to the inherent high concentration, result in high pressure drop and oil carry-over. If oil in vapor form is to be separated, the filter must contain a suitable adsorption material, usually activated carbon. All filtering inevitably results in a pressure drop, which is an energy loss in the compressed air system. Finer filters with a tighter structure cause a higher pressure drop and may get clogged more quickly, which demands more frequent filter replacement and consequently higher maintenance costs. The quality of the air in regards of the amount of particles and the presence of water and oil is defined in ISO 8573-1, the industry standard for air purity. To eliminate the risk of air contamination in a critical process, it is recommended that only compressed air classified as Class 0 be used. Additionally, filters must be dimensioned so that they not only handle the nominal flow properly, but also have a larger capacity threshold in order to manage some pressure drop due to a certain amount of blockage.


How do air dryers help?

Condensing or adsorption dryers are used to remove moisture from the compressed air. Condensing dryers are used when the maximum air quality required is class 4, meaning the dew point is less than 3°C. If compressed air with less moisture (a lower pressure dew point) is required, an adsorption dryer must be installed.