CDA stands for clean dry air. It is not a specialty gas bought in cylinders; it is atmospheric air that has been compressed and then stripped of water, oil and particles. The real question is how clean "clean" has to be — two plants can both call their supply CDA and still be several quality classes apart.
What Is CDA? Same Composition as Air, Defined by What Has Been Removed
The composition of CDA is simply that of air: about 78% nitrogen, 21% oxygen and 0.93% argon, with the remainder made up of trace amounts of carbon dioxide and noble gases. What sets it apart from ordinary compressed air is not what has been added but what has been taken out — water vapour, oil and particles are reduced below a specified class.
So CDA is not a fixed specification but a relative term. The industry usually sorts compressed air into three tiers by application, and CDA generally falls into the last two:
| Category | Typical uses | Common ISO 8573-1 requirement |
|---|---|---|
| Plant air | Pneumatic tools, general blow-off | Moisture is the main concern, roughly water Class 4–5 |
| Instrument air | Pneumatic control valves, sensors, instruments | Commonly interpreted as [2:≤4:3] |
| Process air | Air comes into direct contact with the product or process | Pressure dew point often specified at −40°C to −70°C, i.e. water Class 1–2 |
When semiconductor, display-panel and electronics plants say CDA, they usually mean process-air grade: a desiccant dryer in the drying stage, oil down to Class 1, plus particulate filtration at the point of use. The actual figures differ from plant to plant, so purchasing and acceptance must always be based on a written specification — never on the three letters "CDA" alone.
Where CDA Is Used: Four Concrete Applications
- ▸Pneumatic valves and cylinders: on/off valves and wafer-handling mechanisms on process tools are mostly driven by CDA. Once moisture in the air condenses inside a valve body, solenoid valves stick or respond sluggishly, and the cause of the downtime is hard to spot at a glance.
- ▸Blow-off and drying: parts, fixtures and carriers are blown dry with CDA after cleaning. If the air carries oil, the longer you blow, the dirtier the surface actually gets.
- ▸Instrumentation and control: pneumatic control valves and pressure transmitters depend on a stable air supply, and water and oil make readings drift and diaphragms age.
- ▸Purging inside equipment: electrical cabinets and optical modules are held at a slight positive pressure with CDA to keep out external dust and humidity. This use often demands cleaner air than driving cylinders does.
Reading ISO 8573-1: What the Three Numbers Mean
Compressed air quality is normally described with ISO 8573-1. The standard divides contaminants into three groups, classifies each one separately and writes the result as three numbers: [particles:water:oil]. ISO 8573-1:2010 [1:2:1], for example, means particles Class 1, water Class 2 and oil Class 1. The lower the number, the cleaner the air, and the three classes are independent of each other.
| Class | Particles 0.1–0.5 μm (count per m³) | Particles 0.5–1 μm | Particles 1–5 μm | Water (pressure dew point) | Oil (total oil) |
|---|---|---|---|---|---|
| Class 0 | Specified by the user or supplier; must be stricter than Class 1 | As specified | As specified | As specified | As specified |
| Class 1 | ≤20,000 | ≤400 | ≤10 | ≤ −70°C | ≤0.01 mg/m³ |
| Class 2 | ≤400,000 | ≤6,000 | ≤100 | ≤ −40°C | ≤0.1 mg/m³ |
| Class 3 | Not specified | ≤90,000 | ≤1,000 | ≤ −20°C | ≤1 mg/m³ |
| Class 4 | Not specified | Not specified | ≤10,000 | ≤ +3°C | ≤5 mg/m³ |
There are three common misreadings of this table:
- 1Class 0 does not mean "zero". It means "specified separately by the parties involved and stricter than Class 1". A specification that states Class 0 without giving a number has effectively specified nothing.
- 2Oil means the total of liquid oil, oil aerosol and oil vapour. An oil-free compressor only means that no oil is added during compression; the intake air itself already contains hydrocarbons, so the discharge still needs filtration.
- 3Water classes are based on pressure dew point, not atmospheric dew point; the difference between the two is covered in the next section.
Dew Point: Pressure Dew Point and Atmospheric Dew Point Are Not the Same Thing
Dew point is the temperature at which air, as it cools, starts to form condensation; the lower the figure, the drier the air. But the same air gives a very different dew point when measured at line pressure than when measured after being expanded to atmosphere: the higher the pressure, the more readily water vapour condenses, so pressure dew point is always higher than atmospheric dew point.
| Pressure dew point (at 0.7 MPa(g)) | Dew point after expansion to atmospheric pressure | Water vapour concentration by volume |
|---|---|---|
| +3°C (typical of refrigerated dryers) | approx. −21°C | approx. 950 ppm |
| −40°C (typical of desiccant dryers) | approx. −57°C | approx. 16 ppm |
| −70°C (water Class 1) | approx. −83°C | approx. 0.33 ppm |
The most common mistake in practice is a specification that says "dew point −40°C" without stating which kind. The supplier quotes an atmospheric dew point, the user accepts against a pressure dew point, both sides insist they are right, and yet the numbers are more than ten degrees apart. The specification should state explicitly "pressure dew point, at X MPa(g)".
A dew point meter is well suited to monitoring the condition of a dryer, but it can only see water. Once a specification moves into ppb or ppt territory, or starts to control carbon dioxide, organics, acids and bases, it is beyond anything dew point can describe; for more on that, see the XCDA technical deep dive.
What Equipment Makes Up a CDA System
A CDA system: each device handles one contaminant
Top row left to right, bottom row right to left; tags show the ISO 8573-1 class each device typically reaches
The order matters. If the coalescing filter does not stop oil aerosol first, oil fouls the desiccant; without a particulate filter after a desiccant dryer, desiccant dust travels into the header.
Each piece of equipment in a CDA system handles just one type of contaminant. Get the order wrong and the downstream equipment is overwhelmed by whatever slips through from upstream.
| Equipment | Removes | Achievable class (typical) | Notes |
|---|---|---|---|
| Aftercooler + moisture separator | The bulk of the liquid water | Not yet classifiable | Removes only liquid water; the water vapour remains, so the dew point does not drop |
| Coalescing filter (pre-filter) | Oil aerosol, water aerosol and particles | Oil Class 2–4 | Mandatory ahead of a desiccant dryer; oil aerosol deactivates the desiccant |
| Refrigerated dryer | Water vapour | Water Class 4–6 | Condensation still forms if the piping passes anywhere colder than the dew point |
| Desiccant dryer | Water vapour | Water Class 1–3 | Consumes part of the compressed air during regeneration |
| Particulate filter (after-filter) | Particles, including desiccant dust from abrasion | Particles Class 1–2 | Mandatory after a desiccant dryer |
| Activated carbon adsorber or catalytic oil remover | Oil vapour | Oil Class 1 | Liquid oil, water and particles must be removed upstream first |
The item most often overlooked is the last one. A coalescing filter stops oil aerosol but not oil that has already vaporised; to reach oil Class 1 (≤0.01 mg/m³), an activated carbon or catalytic oil-removal stage usually has to follow it.
Choosing a Dryer: Refrigerated or Desiccant
| Type | Typical pressure dew point | Compressed air used for regeneration | Best suited to |
|---|---|---|---|
| Refrigerated | +3°C to +10°C | None; consumption is mainly electricity | General pneumatic tools, indoor mains at a stable temperature |
| Heatless desiccant | −40°C, capable of −70°C | Approx. 15%–20% of rated flow | Small to medium flows that need a low dew point |
| Heated desiccant | −40°C to −70°C | About half that of a heatless dryer, plus heater power | Medium to large flows |
| Blower purge desiccant | −40°C to −70°C | Very little; mainly blower and heater power | Large flows where compressed air losses matter |
Heatless dryers have a detail few people notice: on a fixed cycle, the purge flow is set by the dryer's rated flow and does not fall with actual air demand. When the dryer runs at half load, purge air can climb to 30%–40% of the air actually consumed. Adding dew point demand control, so the towers do not switch until the desiccant is actually loaded, is the most direct way to recover that lost air.
What to Watch in CDA Piping
- ▸Material: carbon steel and galvanised steel pipe corrode over time, and the rust flakes become a particle source downstream. Plants with high cleanliness requirements mostly use stainless steel or aluminium alloy pipe. Do not use PVC for the mains — if it fails under pressure, it shatters and throws fragments.
- ▸Drainage: in systems with refrigerated dryers, the mains need a slope with condensate drains at the low points; otherwise, when the weather turns cold, condensate runs along the piping into the tools.
- ▸Branch connections: taking branch lines off the top of the main keeps water that collects at the bottom of the pipe from pouring straight into the branches.
- ▸Point-of-use filtration: keep a particulate filter at the tool inlet to catch particles generated by the piping itself.
If some tools are to be upgraded to XCDA, the piping downstream of the purifier belongs to an entirely different class, and both the materials and the installation methods have to be redone. For details, see upgrading from CDA to XCDA in practice.
When CDA Is Not Enough
When the air blows directly onto wafers, reticles or optical components, and tool specifications start listing ppt-level limits for H₂O, organics, acids and bases, the requirement has moved beyond what ISO 8573-1 can describe. What is needed then is XCDA: a gas purifier added in series after the CDA supply to push molecular contaminants down to the ppt level.
Conversely, for most pneumatic, blow-off and instrument applications, CDA is entirely sufficient. The pragmatic approach is tiered supply: CDA serves most uses, and XCDA goes only to the tools that genuinely need it.
FAQ
Q: What is CDA gas made of? Is it the same as ordinary air?
Yes, it is the same. CDA is air — about 78% nitrogen, 21% oxygen and 0.93% argon. The difference is that water vapour, oil and particles have been reduced below a specified class, so it is not a cylinder gas but a utility that the plant produces itself with air compressors, dryers and filtration equipment.
Q: Can CDA and nitrogen be used interchangeably?
It depends on the application. When oxygen has to be excluded — to prevent oxidation or combustion, or to store oxygen-sensitive materials — nitrogen is required. When all you need is a clean, dry gas, CDA costs far less and poses no asphyxiation risk to personnel. Nitrogen itself can also pick up impurities again along its supply path; for more on this, see what is a gas purifier.
Q: Does ISO 8573-1 Class 0 mean completely oil-free?
No. Class 0 is defined as "specified separately by the user or supplier and stricter than Class 1", and has no fixed value of its own. Whenever you see Class 0, ask for the actual value and the measurement method; a specification that states Class 0 without a number cannot be judged at acceptance.
Q: Does air from a refrigerated dryer count as CDA?
That depends on your definition. A refrigerated dryer delivers a pressure dew point of roughly +3°C to +10°C, corresponding to water Class 4–6, which is enough for general pneumatic tools. But the CDA that semiconductor and electronics plants talk about mostly calls for a desiccant-dryer-grade dew point; and whenever the piping passes somewhere colder than the dew point, such as an outdoor pipe rack in winter, air from a refrigerated dryer will still condense inside the pipe.
Q: Is a −40°C dew point good enough?
For general pneumatics, instrumentation and most blow-off uses, yes. Where piping runs through cold areas, or the process is sensitive to moisture, a pressure dew point of −70°C is often required. Keep in mind that however low it goes, dew point describes only one species, water; once a specification starts to set limits on carbon dioxide, organics, or acids and bases, switch to concentration-based limits and the matching measurement methods rather than pushing the dew point ever lower.
Q: How can we confirm that our existing CDA really meets specification?
Not from the dryer nameplate alone. At a minimum, measure pressure dew point, oil and particles at the point of use, and record the line pressure and flow at the time of measurement; for dew point, it is best to look at the trend over a full season, because full load in summer is usually the worst case. If what you need to verify is ppt-level XCDA, the measurement methods are entirely different — see XCDA measurement, sampling and acceptance.