Why one number is not enough
Compressed air carries three families of contaminant: solid particles, water and oil. ISO 8573-1 assigns each a class, and a specification is written as three classes in a fixed order. A statement like class 1:4:1 describes particles, water and oil respectively - a clean dry line for a demanding process, not a single grade of air.
What the class numbers mean
| Contaminant | What the class controls |
|---|---|
| Particles | The size and count of solid contamination remaining in the air after filtration |
| Water | The moisture content, usually expressed as pressure dew point - the temperature at which water would condense at line pressure |
| Oil | Total oil content in liquid, aerosol and vapour form, including oil vapour that passes a coalescing filter |
Lower numbers are cleaner. The exact limits behind each number are defined in the standard, and because they are pressure-dependent for water, a dew point figure is only meaningful when the pressure is stated alongside it.
Where the classes come from in a real system
- Particles - set by the filter chain. A coarse pre-filter protects the coalescing stages; a fine final filter sets the delivered class.
- Water - set by the dryer. A refrigerated dryer reaches a pressure dew point a little above freezing; an adsorption dryer goes considerably lower, at a cost in purge air and energy.
- Oil - set by the compressor type and the coalescing stages. Oil-free compression starts from a different baseline than oil-injected plus filtration.
The mistake that costs money
Buying a filter to meet an oil class while ignoring the dryer is common: the air is oil-free but wet, and the water causes the corrosion and tool failures the specification was meant to prevent. Quality classes are a package, and the weakest of the three decides what the process actually sees.
Monitoring, not just purchase
A class is a promise about performance, and performance depends on condition. Coalescing elements load up, drains fail, dew point sensors drift. A system specified to a tight class needs a maintenance schedule that includes differential pressure checks and drain testing, otherwise the class is only true on the day of commissioning.
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