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Understanding Pressure Dew Point

Understanding Pressure Dew Point

What Happens During Compression

Air always contains water vapor. When compressed, its temperature rises and its ability to hold water increases. As the air cools after compression, excess vapor condenses. PDP predicts when this condensation will occur under pressure, making it essential for system design and quality control.

Why PDP Is Different from Atmospheric Dew Point

Atmospheric dew point is measured at ambient pressure, but compressed air operates under higher pressure. PDP accounts for this, giving a realistic measure of moisture risk inside your system.

ISO 8573 Water Classes

ISO 8573-1 defines air purity classes for particles, oil, and water. For water, classes are based on PDP:

  • Class 4: PDP ≤ +3 °C (≈ 37 °F) – typical for general industry
  • Class 3: PDP ≤ −20 °C
  • Class 2: PDP ≤ −40 °C

Most industrial applications aim for Class 4, which prevents liquid water under normal indoor conditions.


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