Compressed-air quality: ISO 8573-1 classes and choosing a filter
The ISO 8573-1 standard defines compressed-air purity by three contaminants — solid particles, water and oil — each rated with a class (1 = purest). A specification is written [particles : water : oil], for example [1:4:1]. Knowing the target class for each contaminant tells you which filter grade and which dryer to choose.
The three contaminants — and what treats them
| Contaminant | ISO class based on | Equipment that sets the class |
|---|---|---|
| Solid particles | Particle count per m³, by size | Particulate or coalescing filter |
| Water | Pressure dew point (vapour) | Dryer (+ separator for liquid water) |
| Oil | Total oil (aerosols + vapour), mg/m³ | Coalescing filter (aerosols) + activated carbon (vapour) |
Key point — a coalescing filter handles particles and liquid/aerosol oil, but not water vapour or oil vapour. Water vapour is the dryer’s job; oil vapour is the activated carbon’s job.
Oil classes (the most relevant to filters)
“Total” oil covers aerosols, liquid and vapour.
| ISO 8573-1 class | Maximum total oil |
|---|---|
| Class 1 | ≤ 0.01 mg/m³ |
| Class 2 | ≤ 0.1 mg/m³ |
| Class 3 | ≤ 1 mg/m³ |
| Class 4 | ≤ 5 mg/m³ |
Water classes (pressure dew point)
The dryer sets the water class, not the filter.
| Class | Pressure dew point |
|---|---|
| Class 1 | ≤ −70 °C |
| Class 2 | ≤ −40 °C |
| Class 3 | ≤ −20 °C |
| Class 4 | ≤ +3 °C |
| Class 5 | ≤ +7 °C |
| Class 6 | ≤ +10 °C |
A refrigerated dryer typically reaches Class 4 (+3 °C); Classes 1 to 3 require a desiccant dryer. See Choosing a refrigerated dryer and Water in the compressed-air system.
Solid-particle classes
Particle classes are defined by the number of particles per cubic metre, counted across three size bands (0.1–0.5 µm, 0.5–1 µm and 1–5 µm). The lower the class, the cleaner the air. Walker Alpha filter grades are validated down to particle Class 1 (grade XA).
Choosing a Walker Alpha filter grade
Walker Filtration Alpha filters share the same body; only the element changes with the grade. So you pick the grade by target air class:
| Grade | Retention | Max. oil carryover | ISO class (part. / oil) | Typical role |
|---|---|---|---|---|
| X25 | 25 µm | 10 mg/m³ | — | Pre-filter, coarse particles, pipe scale |
| X5 | 5 µm | 5 mg/m³ | 3 / 4 | General network coalescing |
| X1 | 1 µm | 0.1 mg/m³ | 2 / 2 | Most common coalescing, up/downstream of a dryer |
| XA | 0.01 µm | 0.01 mg/m³ | 1 / 1 | High efficiency — instrument air, oil Class 1 |
| AC | activated carbon | 0.003 mg/m³ (vapour) | 1 | Oil vapour and odours — breathing, food air |
Typical install order — water separator → X1 (1 µm) → XA (0.01 µm) → AC (carbon). Each stage protects the next: never place an XA or AC without a coalescing filter upstream, or it saturates prematurely. Activated carbon must not run in oil-saturated air and will not remove CO or CO₂.
In practice — application examples
| Application | Target quality (indicative) | Recommended treatment |
|---|---|---|
| Workshop pneumatic tools | Oil class 3–4 | X5 or X1 |
| Instrument air, precision pneumatics | Oil class 1–2 | X1 + XA + dryer |
| Food, pharmaceutical, paint, breathing | Oil class 1 + odour-free | X1 + XA + AC + dryer |
For flow rates and prices by size, see the ranges: X1 coalescing, XA high-efficiency and AC activated carbon. For the water class, see the compressed-air dryers.
References
- ISO 8573-1:2010 — Compressed air, Part 1: contaminants and purity classes
- Walker Filtration — Alpha range (coalescing and particulate)
- CAGI — Compressed Air & Gas Institute