Dew point

Dew point is the temperature, at a given pressure, below which water vapor in air condenses. Lower dew point means drier air. Compressed-air systems live or die by dew point because liquid water in distribution lines corrodes piping, damages tools, and contaminates product.

What it is

Air can hold a temperature-dependent amount of water vapor. As air cools toward its dew point, the relative humidity climbs toward 100 percent; below the dew point, vapor condenses. In a compressor discharge line, hot saturated air leaves the aftercooler at near 100 percent relative humidity and continues to drop water as it travels through cooler piping. Dew point quantifies how dry the air actually is.

Why it matters

A plant air system at 100 psig and a +38 deg F pressure dew point will never see liquid water as long as ambient piping stays above 38 deg F. Drop the pressure dew point to -40 deg F and the air is dry enough for instrument applications and freeze-resistant outdoor runs. Specifying the wrong dew point either wastes energy (drying too low) or produces a system that floods distribution lines every winter.

How it is used in our catalog

Every air dryer in our catalog is indexed by guaranteed pressure dew point at rated flow. Refrigerated dryers typically deliver +38 deg F PDP; desiccant dryers deliver -40 deg F or -100 deg F PDP depending on configuration.

Common questions

Pressure dew point or atmospheric dew point?

Compressed-air systems use pressure dew point because that is the actual operating condition. Atmospheric dew point describes the same air if it were expanded to 1 atm, and is always lower than the pressure dew point.

What dew point do I need?

General plant air, +38 deg F PDP. Instrument air, -40 deg F PDP. Critical applications (pharma, semi), -70 deg F or below.