Most anyone designing, using and maintaining a pneumatic system recognizes that proper filtration is necessary to keep equipment running at peak performance. The question is, what degree of filtration of necessary for a given system? Applying the Goldilocks principle, specifying filters that are too coarse can permit harmful contamination to pass through a filter and damage downstream components. But too fine of filtration and the system creates unnecessary pressure drop at the filter that wastes energy. To get compressed-air cleanliness that’s “just right,” a good guideline is ISO Standard 8573 that defines the quality of compressed air.
Part 1 of the standard specifies purity classes for compressed air and how they relate to certain contaminants. Parts 2 through 9 specify test
methods 
for measuring levels of solids, oil aerosols, liquid water, oil vapor, solvents, biological contaminants and other substances in the compressed-air stream.
The standard specifically classifies three types of contaminants in compressed air: solid particles, water and oil. The tables shown below explain how they’re broken into specific classes for each type. Note that the size of solid particles is defined as the greatest length between two ends of a particle. The concentration of total oil includes liquids, vapor and aerosols. And for all
three types of contaminant,
Class 0 is defined as being more stringent than Class 1, and is specified by the equipment user or supplier
In practice, air cleanliness is expressed as a three-digit purity level for particles, water and oil.For example, air containing less than 10,000 particles sized between 1 and 5 µm, with a pressure dewpoint below 7° C, and containing less than 1 mg per cubic meter of oil would be considered
having an ISO 8573 Class 4:5:3 purity level.
Users should understand that the classification system is intended to provide a guide to different levels of air purity in compressed-air systems, not how air is filtered and treated. Nor does it define the purity levels needed for specific components or equipment.
That said, the standard does note that pneumatic-powered compon
ents, tools and machines have traditionally been supplied with air filtered by general-purpose filters with nominal particle-capture ratings of 40 or 5 microns. The purity levels they attain would be considered Class 7 and 6, respectively. These ratings are not absolute particle-removal ratings, according to the standard; air delivered by such filters remove at least 95% of the particles of a given rated size. Such filters give satisfactory service while minimizing pressure drop and power losses.
In most industrial settings, compressed air is used to power or pilot devices su
ch as valves, cylinders, grippers and motors. In these applications, the goal behind contamination control is to attain a compressed-air purity level that protects components from corrosion and excessive wear. Here, Class 7:4:4 air is likely suitable, and can be produced using a general-purpose, 40-micron particle filter and a refrigeration dryer with an oil trap.
On the other hand, compressed air that comes into direct contact with foods like beverages, meat and vegetables usually requires higher purity levels, perhaps Class 1:4:1 for solids, water and oil. The specific degree of compressed-air purity in such applications is left to equipment manufacturers, regulatory bodies and relevant industry associations—such as those governing food processing or pharmaceutical manufacturing. Seek assistance from qualified application engineers for help with specific filtration and treatment options.
