Dew point is the temperature at which water vapour condenses, but in compressed air the measurement depends on pressure. Understanding the difference between pressure dew point and atmospheric dew point prevents costly dryer mistakes.
The two measurements
Pressure dew point (PDP) is the temperature at which condensation occurs at the system working pressure, and it is the standard way to specify dryer performance. Atmospheric dew point is measured at ambient pressure and is always lower than the PDP for the same moisture content, because compressing air concentrates the water vapour.
Why it matters
A dryer rated at -40°C PDP is not the same as -40°C atmospheric dew point. Confusing the two leads to an under-specified dryer that lets moisture into the system. Always specify the dryer in pressure dew point at the working pressure.
Choosing a dryer
- General plant air — 3°C to 7°C PDP refrigerated dryer
- Outdoor or cold lines — -20°C to -40°C PDP adsorption dryer
- Critical process air — -40°C to -70°C PDP adsorption dryer