Pressure dew point

Pressure dew point (PDP) is the temperature at which water vapor in compressed air begins to condense, evaluated at the air’s actual line pressure. It is the standard rating used by every reputable dryer manufacturer because it reflects what the air will do in your plant, not in a hypothetical atmospheric expansion.

What it is

Compression concentrates moisture: 100 SCFM of saturated 80 deg F ambient air contains the same total water as 100 SCFM of compressed air, but the compressed air occupies one seventh the volume at 100 psig. That means seven times the moisture per unit volume, and a much higher dew point at compressor discharge. A dryer’s job is to lower that pressure dew point to the spec required by downstream use.

Why it matters

Specifying “dew point” without “pressure” is ambiguous and a frequent source of dryer mis-sizing. A +38 deg F pressure dew point at 100 psig converts to roughly -16 deg F atmospheric dew point — the same air, but a number 54 deg F different. Always confirm the rating context before comparing dryers.

How it is used in our catalog

All air dryers in our catalog list pressure dew point at rated inlet flow and rated inlet pressure. Refrigerated dryers cluster around +38 deg F PDP; desiccant dryers at -40 deg F or -100 deg F PDP; membrane dryers across a wide configurable range.

Common questions

Why is PDP higher than atmospheric dew point for the same air?

Higher pressure forces water molecules closer; at any given temperature, more of them condense. Expanding the same air to atmospheric pressure separates the molecules and the dew point drops.

Does PDP change down the distribution line?

Pressure dew point follows pressure. As distribution pressure drops, PDP drops slightly. The risk is the line temperature dropping below PDP, which is when liquid water reappears.