Quick answer: A purge system feeds a small, continuous flow of clean, dry air into an enclosure so it stays at slight positive pressure: outside air cannot leak in, and the dry air lowers the inside dew point. It only works if the purge air is genuinely dry (filtered compressed air from a working dryer, whose dew point after expansion is well below the enclosure's coldest surface), flows continuously in every operating state including shutdown, and the enclosure is sealed enough to hold pressure. In hazardous areas, pressurisation is also a safety protection method with its own standards; keep the two purposes separate.
Quick answer
- Purge = dry air + slight positive pressure → no humid air ingress.
- Specify the purge air dew point at the enclosure, not just "compressed air".
- Make sure purge continues when the plant or cooling is off.
How it works
- Dry, filtered air enters through a regulator and flow device.
- The enclosure is held slightly above outside pressure.
- Leaks flow outward, so humid outside air does not enter.
- The inside dew point approaches that of the purge air.
What the purge air must be
| Property | Why |
|---|---|
| Dew point well below the coldest enclosure surface | Otherwise the purge air itself can condense inside |
| Clean (oil- and particle-filtered) | Oil and dust contaminate contacts and electronics |
| Continuous supply | Purge loss lets humid air in immediately |
| Controlled pressure and flow | Too little does not hold pressure; too much wastes compressed air |
Compressed air from a refrigerant dryer is typically around +3 °C pressure dew point at line pressure; expanded to atmospheric pressure its dew point is much lower (about −20 °C at 7 bar(g)), which is usually dry enough. Problems arise when dryers fail or are bypassed, so monitor the purge air; see compressed air dew point meters.
Ask about every operating state
| State | Question |
|---|---|
| Normal running | Is pressure held with doors closed? |
| Plant shutdown | Does the compressor and purge keep running? |
| Maintenance | What happens when doors are opened, and how long to re-dry after closing? |
| Cooling unit running | Does a cold surface (cooler, heat exchanger) fall below the purge air dew point? |
| Power or air failure | Is there an alarm? |
Safety purge vs moisture purge
In hazardous (explosive) atmospheres, pressurised enclosures are a recognised protection method under IEC 60079-2 ("Ex p"), with requirements for purge volumes, monitoring and interlocks. That design belongs to the hazardous-area specialist. A moisture purge in a non-hazardous area is simpler, but it must not be confused with or weaken a safety purge.
Alternatives and combinations
- Cabinet dehumidifier: removes moisture without a compressed-air supply; see our electrical cabinet dehumidifiers.
- Heater: keeps surfaces warm; see enclosure heater vs dehumidifier.
- Sealing and vents: see why IP-rated enclosures condense.
Purge suits sites with reliable dry compressed air, many enclosures or dusty atmospheres; dehumidifiers suit sites without compressed air.
FAQ
Frequently asked questions
What is purge air for an electrical enclosure?
A small, continuous flow of clean, dry air that keeps the enclosure at positive pressure so humid outside air cannot leak in.
Is plant compressed air dry enough for purging?
Often, if it comes from a working dryer and is filtered, because expansion lowers its dew point. Monitor it, as dryer faults make it wet.
Does purge air replace a dehumidifier?
It can, where reliable dry air is available. Otherwise a cabinet dehumidifier is simpler.
Sources
- IEC 60079-2, Explosive atmospheres: equipment protection by pressurized enclosure "p".
- ISO 8573-1, Compressed air purity classes.
