Compressed air treatment system components explained: air receiver sizing, refrigerated vs desiccant dryer selection, coalescing and particulate filters, condensate drain types, and the ISO 8573-1 air quality class matrix (solids, water, oil).
A compressed air installation is more than a compressor. The compressor delivers hot, wet, oil-carrying air that would destroy downstream tools, instruments and process end-uses within days if used raw. A treatment train — receiver, dryer, filter, drain — conditions the air to the quality class required by the end use.
The standard defines three contaminant categories, each on a 0-9 scale (lower = cleaner). A three-digit code such as [1:4:2] identifies the required class for solids, water and oil respectively.
| Class | Solids (max particle size / concentration) | Water (pressure dew point) | Oil (total oil concentration) |
|---|---|---|---|
| 0 | User-defined, better than 1 | User-defined | User-defined |
| 1 | 0.1 μm / 0.1 mg/m³ | ≤ -70 °C PDP | 0.01 mg/m³ |
| 2 | 1 μm / 1 mg/m³ | ≤ -40 °C PDP | 0.1 mg/m³ |
| 3 | 5 μm / 5 mg/m³ | ≤ -20 °C PDP | 1 mg/m³ |
| 4 | 15 μm / 8 mg/m³ | ≤ +3 °C PDP | 5 mg/m³ |
| 5 | 40 μm / 10 mg/m³ | ≤ +7 °C PDP | — |
| 6 | — | ≤ +10 °C PDP | — |
The receiver serves three roles: dampen pulsation from a reciprocating compressor; act as a first-stage condensate knock-out; and provide reserve capacity so a small-percentage-loading compressor doesn't short-cycle. Rule-of-thumb sizing V = FAD × 1 min for reciprocating; V = FAD × 0.5 min for screw. Larger receivers reduce dryer cycling on desiccant dryers and improve peak-shave capability.
| Type | PDP achievable | Energy demand | Notes |
|---|---|---|---|
| Refrigerated dryer (cyclic) | +3 °C PDP | ~3% of compressor kW | Default choice for general industrial > +3 °C ambient |
| Refrigerated dryer (non-cyclic thermal-mass) | +3 °C PDP | ~5% of compressor kW at high load, less at part load | Variable-demand systems |
| Heatless desiccant (twin-tower) | -40 °C PDP standard; -70 °C option | ~15% of compressor kW (purge air) | Instrument air, outdoor cold storage |
| Heat-regenerated desiccant | -40 to -70 °C PDP | ~5-8% of compressor kW (heater + purge) | Larger systems > 500 m³/h |
| Membrane dryer | +3 to -40 °C PDP | ~15% of compressor kW (permeate) | Small flows, laboratory, medical |
| Filter | Purpose | Typical location |
|---|---|---|
| Water separator (centrifugal) | Bulk-water knockdown after aftercooler | Between aftercooler and receiver |
| Particulate (5 μm or 1 μm) | Coarse solids, rust, pipe scale | Before refrigerated dryer, after desiccant dryer |
| Coalescing (grade B / C, 0.1 μm) | Oil aerosol + fine water aerosol | After refrigerated dryer, before instrumentation |
| Fine coalescing (grade A, 0.01 μm) | Oil aerosol to 0.01 mg/m³ | Immediately before end-use requiring ISO 8573 class 1 oil |
| Activated carbon (oil vapour) | Removes oil vapour that coalescers cannot catch | Ahead of food, pharma, medical air |
Automatic condensate drains remove separated water without wasting compressed air. Timer-controlled solenoid drains are the cheapest but waste air on every discharge cycle. Float-actuated mechanical drains open only when condensate has actually accumulated but foul with oily condensate. Zero-loss capacitive drains (electronic level-sense with pilot-operated ball valve) waste no air and self-diagnose; they are the norm on new installations.