Psychrometrics

Psychrometrics is the branch of thermodynamics that studies the properties of moist air and the processes that involve air–water vapor mixtures. It focuses on parameters such as dry-bulb temperature, wet-bulb temperature, relative humidity, humidity ratio, dew point, and enthalpy, and describes how these properties change during heating, cooling, humidifying, or dehumidifying.

Moist air is treated as a mixture of dry air and water vapor at a given pressure. Key quantities include the dry-bulb temperature (the temperature measured by a standard thermometer), the wet-bulb temperature (lower due to evaporative cooling), and the humidity ratio or specific humidity (the mass of water vapor per mass of dry air). Relative humidity expresses the actual vapor pressure as a percentage of the saturation vapor pressure at the same temperature. The dew point is the temperature at which water begins to condense at the given pressure. The enthalpy of moist air combines sensible and latent heat content.

A psychrometric chart graphically represents these relationships at a given pressure, with axes for dry-bulb temperature and humidity ratio or relative humidity. Lines of constant enthalpy, constant relative humidity, and constant moisture content help visualize processes such as sensible cooling, dehumidification, humidification, and mixing of air streams. Measurements often use dry-bulb and wet-bulb readings (psychrometers) or modern sensors, and results are read from the chart or calculated with formulas.

Applications of psychrometrics are widespread in HVAC design and operation, energy and comfort analysis, meteorology, textile drying, and food processing, where controlling moisture content and air temperature affects performance and safety.