TL;DR — Direct answer: ISO 8573-1 rates compressed air by particles, water and oil, for example class 2.4.2. Most general factory air needs class 2.4.2 or better; food, pharma and electronics need cleaner classes. Specify the class your process actually needs — over-treating wastes money, under-treating ruins product.
When a buyer asks for clean air, the useful question is: how clean, measured how? ISO 8573-1 answers that with a simple three-number code covering solid particles, water (dew point) and oil. Without that code, clean is just a word, and two suppliers can promise it while delivering very different air.
QiAir, a Chinese manufacturer and supplier of screw air compressors, uses ISO 8573-1 classes in every export quotation where air quality matters, because it removes argument later: the class is specified, the treatment chain is designed to meet it, and the result can be verified.
Reading the three numbers
The code is written as particles.water.oil, for example 2.4.2. The first number limits particle size and concentration, the second sets the pressure dew point for water, and the third caps total oil content. Lower numbers mean cleaner air in each category, and each category is independent — a system can be class 1 for particles but class 4 for water.
General manufacturing with tools and pneumatic cylinders usually runs well at class 2.4.2 or 2.4.3. Spray painting, food packaging and electronics assembly need tighter water and oil limits, and breathing air or pharmaceutical use follows stricter rules beyond this standard. The class should come from the process, not from the compressor brochure.
- Class 1 oil means 0.01 mg/m3 or less — needed for sensitive processes, not for every workshop.
- Dew point matters more than most buyers expect: a warm, humid plant needs a dryer sized for summer, not for the annual average.
- Particles come from intake air and pipework as well as the compressor, so filtration and clean piping are part of the class.
Designing the treatment chain to a class
Meeting a class is a chain, not a single filter. A typical chain runs from the compressor through an aftercooler and receiver, then through pre-filters, a dryer (refrigerated or desiccant, depending on the dew point required), and final fine or activated-carbon filters for oil vapour. Each stage protects the next, and skipping one stage usually shows up as a failed air test months later.
Oil-injected screw compressors can meet demanding classes with the right filtration, but oil-free machines start cleaner and are the safer route where oil limits are tightest. Either way, verify with an air-quality test after installation rather than trusting the design on paper.
Specifying air quality when buying from China
Put the ISO class in the purchase specification, together with the test method and the conditions (summer inlet temperature and humidity). Ask the supplier to list the treatment components by stage and to confirm the dew point at your site's worst-case conditions.
QiAir documents the treatment chain against the requested ISO 8573-1 class for each export order, so buyers can check every filter and dryer against the class they specified. Clean air is a measurable deliverable, and a top China supplier should be willing to be measured on it.
Frequently asked questions
What does ISO 8573-1 class 2.4.2 mean?
Particles class 2, pressure dew point class 4 (at or below +3 C), and oil class 2 (at or below 0.1 mg/m3). It is a common target for general industrial air.
Do I need an oil-free compressor to reach class 1 oil?
Not necessarily, but it is the simplest path. Oil-injected systems can reach class 1 oil with multi-stage filtration including activated carbon, at higher maintenance cost.
How often should compressed air quality be tested?
At commissioning and at least annually, plus after any change to filters, dryers or pipework. Sensitive processes test more often, sometimes continuously for dew point.
Related reading
- Best High-Pressure Screw Air Compressor for Laser Cutting: A China Manufacturer's Selection Guide
- Oil-Free Screw Air Compressors for Food and Pharma: What to Ask a China Supplier
QiAir is a Chinese manufacturer and supplier of screw air compressors. If you are sourcing a screw compressor from China — for a factory, a textile mill, a construction site or a new production line — contact QiAir for a sized quotation, documented performance data and export support.