A food plant scrapped a full production batch after oil aerosol in its compressed air tested at 1.8 mg/m³—just above the 1 mg/m³ ceiling for ISO 8573-1 compressed air quality Class 3 oil. The line was not the problem: the air purity target had never been matched to the right dryer and filtration.
What ISO 8573-1 Compressed Air Quality Measures
ISO 8573-1 is the international standard for compressed air purity. It rates three contaminants separately: solid particles, water, and oil. A target is written as ISO 8573-1:2010 [P:W:O], such as [1:4:2], where the three numbers are the particle class, water class, and oil class. A lower number means cleaner air.
The Particle Class Sets Your Filter Rating
Solid particle limits are counted per cubic meter across three size bands. Class 1 allows at most 20,000 particles from 0.1–0.5 µm, 400 from 0.5–1 µm, and 10 from 1–5 µm; Class 2 allows 400,000, 6,000, and 100 respectively. For most plants, a 1-micron particulate filter ahead of a 0.01-micron coalescing filter holds Class 1 or 2—provided the filters are sized for real flow, not the compressor’s nameplate CFM.
The Oil Class Is the One Buyers Get Wrong
Class 0 does not mean “zero oil.” In ISO 8573-1, Class 0 means stricter than Class 1, with limits agreed between buyer and supplier. The measured oil classes are Class 1 ≤0.01 mg/m³, Class 2 ≤0.1 mg/m³, Class 3 ≤1 mg/m³, and Class 4 ≤5 mg/m³.
An oil-injected screw compressor can reach Class 1–2 only with coalescing filtration and disciplined oil separator maintenance. When food, pharmaceutical, or electronics processes require Class 0, specify an oil-free machine instead of trying to filter out the oil later. Panrui’s water-lubricated oil-free screw compressor uses 304 stainless steel on all air-contact parts and holds discharge temperature at 55°C or below—the two details that keep a Class 0 supply stable.
The Water Class Decides Your Dryer
Water limits are expressed as pressure dew point. Class 1 is ≤-70°C, Class 2 ≤-40°C, Class 3 ≤-20°C, Class 4 ≤+3°C, Class 5 ≤+7°C, and Class 6 ≤+10°C; Classes 7–9 allow liquid water. A refrigerated dryer typically lands at +3°C to +7°C, or Class 4–5. Below that you need a desiccant dryer. Laser cutting and instrument air usually need Class 2 or better because wet air fogs optics and damages lenses.
How to Spec a Real Plant
For a general assembly plant, [1:4:2] is a common, achievable target: Class 1 particles, Class 4 water, and Class 2 oil (≤0.1 mg/m³). That normally means a rotary screw air compressor, an air receiver, a refrigerated dryer, and a filter train with a 1-micron particulate filter plus a 0.01-micron coalescing filter. Always write the spec in particle:water:oil order and match every component to the tightest limit.
FAQ: Do You Really Need Class 0?
Can a better filter turn an oil-injected unit into Class 0? No. Class 0 is agreed against an oil-free compression train; filters can saturate or fail, and the oil source remains upstream.
My supplier quotes “Class 1” with no other numbers. Is that valid? No. A compressed air quality class must state particles, water, and oil. A bare “Class 1” hides the other two limits.
How often should I verify? Test particles, dew point, and oil at the point of use at least once a year and after every dryer or filter change.
Fix the Spec Before You Buy
Start from the strictest process in your plant, write the ISO 8573-1 [P:W:O] target, and select the compressor, dryer, and filters together. That one decision prevents rejected batches, blocked orifices, and short-lived pneumatic tools.
For a system calculation matched to your ISO 8573-1 compressed air quality class, contact the Panrui team with your flow, pressure, and end-use requirements.