Psychrometric Glossary
Key terms in moist air and psychrometrics for HVAC engineers.
Adiabatic (Evaporative) Cooling Cooling air by evaporating water with no heat exchange with the surroundings. Enthalpy stays constant, temperature falls and the humidity ratio rises. Desiccant Dehumidification Lowering air humidity with a hygroscopic sorbent that captures water vapor without condensation. The answer where cooling below the dew point would be too cold or too costly. Dew Point The temperature to which air must be cooled at constant humidity ratio to reach saturation. It decides where condensation forms on glazing, ducts and exchangers. Enthalpy of Moist Air The total heat content of one kilogram of dry air together with its water vapor, in kJ/kg. The enthalpy difference across a component gives its duty. Heat Recovery (HRV) An exchanger passing energy, and sometimes moisture, from extract air to supply air. The single biggest lever on the heating demand of ventilation. Humidity Ratio The mass of water vapor per kilogram of dry air, in g/kg. The horizontal axis of the Mollier chart and the basis of every moisture balance. Psychrometrics The thermodynamics of moist air, calculating and plotting the state of a mixture of dry air and water vapor. The foundation of every HVAC design. Relative Humidity The ratio of the actual vapor pressure to the saturation pressure at the same temperature. It shifts with temperature even when the moisture content does not. Saturation Curve The boundary of the chart at 100 % relative humidity, where air carries all the moisture it can at that temperature. No air state lies beyond it. Wet-Bulb Temperature The equilibrium temperature of a water-wetted surface evaporating into the surrounding air. The practical limit of adiabatic cooling and what a psychrometer reads.
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