Relative Humidity in HVAC&R: What It Can and Cannot Predict About Condensation, Corrosion, and Drying

Relative humidity influences condensation, corrosion, and drying across nearly every branch of HVAC&R engineering, yet no single RH reading is sufficient to predict when any of these processes will occur.

Condensation and the dew point

A steel-roofed carport offers a familiar illustration. Vehicles parked underneath may be splashed by water dripping from the roof, particularly during cool mornings or after a change in weather. The mechanism is the dew point: if the underside of the steel roof falls below the dew-point temperature of the surrounding air, water vapour condenses on the metal, and droplets can grow large enough to fall.

Psychrometry lets engineers predict these conditions. By establishing the air temperature and relative humidity, the dew-point temperature can be calculated; comparing it with the roof's surface temperature indicates whether condensation is possible. The prediction must also account for changing weather, roof temperature, ventilation, and the thermal properties of the structure. A single measurement of ambient humidity cannot, on its own, determine exactly when dripping will begin.

Corrosion: more than a 50 % threshold

The suggestion that iron rusts only when relative humidity exceeds 50 % is an oversimplification. Corrosion depends on the metal, surface condition, contaminants, temperature, and the availability of moisture. Salts and other pollutants can allow corrosion to occur under conditions where clean metal might remain relatively unaffected.

Humidity nonetheless remains an important factor in corrosion control. Keeping equipment dry, preventing condensation, and managing exposure to corrosive contaminants can help extend service life.

Drying: RH is one variable among many

Relative humidity also influences how quickly a wet surface dries. In general, lower ambient humidity increases the potential for evaporation, while higher humidity reduces the difference between the moisture conditions at the surface and in the surrounding air.

There is no single mathematical expression based on RH alone that reliably predicts drying time in every situation. Air temperature, air velocity, surface area, material porosity, the initial quantity of water, and the supply of heat all influence the result. Psychrometric calculations can help estimate evaporation potential, but accurate drying predictions often require additional material data and empirical relationships.

Where the principles apply

The same interactions between moisture, temperature, and airflow are relevant to cold rooms, building envelopes, electrical enclosures, and industrial drying processes. Understanding them can help prevent damage and improve operating efficiency.

Photo credit: Wirestock / Magnific.com.

Authors: Eugene le Roux, FSAIRAC, and Eamonn Ryan. Source: RACA Journal.