At the outlet of a compressor, the air is not yet clean. Even after the air/oil separator, the air receiver and the dryer, it contains water, very fine oil particles and dust. The fine filters, fitted at the compressor outlet, are there to retain them: this is what is called filtration.
Three filters, three roles
A basic set of precision filters comprises three filters of different models, fitted one after the other. Each has a specific function:
- The first filter carries out an initial filtration of the moisture present in the compressed air.
- The second filter provides additional filtration after the air has passed through the first, and acts as complementary protection.
- The third filter removes the very fine oil particles and oil aerosols still present in the compressed air.
Each filter has a different grade: you always go from the coarsest to the finest, so that the last, finest filter does not clog too quickly.
At 16 bar, five high-pressure filters
This set of three filters is a basic filtration system, suited to installations working at 8 bar.
For a 16 bar high-pressure compressor, such as those in the L series for laser cutting, a system of five precision filters is generally used. And these filters must be high-end models designed for high pressure: a filter intended for an 8 bar network has no place on a 16 bar installation.
Which filtration for which application?
The level of filtration depends on what the air will come into contact with:
| Application | Usual filtration |
|---|---|
| Workshop pneumatic tools | filtration of particles and water |
| Blowing off parts, painting | an added filter that retains oil aerosols, or even an activated carbon filter for odours |
| Laser cutting, air in contact with the optics | full high-pressure filtration |
The ISO 8573-1 standard expresses these requirements as purity classes. Our guide to compressed air quality according to ISO 8573-1 explains it simply, and our article Which air dryer should you choose for your compressor? deals with the question of water.
Maintaining fine filters
- Drain the filters regularly: the water and oil they retain build up at the bottom of the bowl.
- Replace the cartridge when the pressure drop increases. A differential pressure gauge shows when the filter is starting to restrict the air.
- A clogged filter makes the pressure drop at the point of use and forces the compressor to work harder.
Choosing your filters
The filter must be sized for the compressor's air flow and rated for the network pressure. Find our fine filters in the consumables, and state the air flow, pressure and connections in your quote request: we will propose the right set.
To see where filtration fits into the installation, read How does a screw compressor work?.