What Is Psychrometrics?
Psychrometrics is the study of the thermodynamic properties of moist air -- mixtures of dry air and water vapor. The name comes from the psychrometer, a device consisting of a dry-bulb and wet-bulb thermometer used to determine humidity. The dry-bulb temperature is the ordinary air temperature, while the wet-bulb temperature is measured by wrapping a moist wick around the thermometer bulb and allowing it to evaporate. The evaporative cooling effect makes the wet-bulb reading lower than the dry-bulb, and the difference (wet-bulb depression) indicates the humidity level.
Psychrometric calculations are fundamental to heating, ventilation, and air conditioning (HVAC) engineering. Every aspect of air conditioning design -- from sizing equipment to designing duct systems to controlling indoor comfort -- depends on understanding the psychrometric properties of air. The psychrometric chart is the graphical representation of these properties, with dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis, showing curves for relative humidity, enthalpy, wet-bulb temperature, and specific volume.
Key Formulas
The Magnus formula gives the saturation vapor pressure as a function of temperature. The actual vapor pressure is found from the wet-bulb temperature using the psychrometric equation (Sprung's formula). Relative humidity is the ratio of actual to saturation vapor pressure at the dry-bulb temperature. The humidity ratio W is the mass of water vapor per unit mass of dry air. Enthalpy includes both sensible heat (temperature-dependent) and latent heat (moisture-dependent).
Human Comfort Zone Reference
| Parameter | Summer Comfort | Winter Comfort |
|---|---|---|
| Dry-bulb Temperature | 23 - 26 °C | 20 - 24 °C |
| Relative Humidity | 40 - 60% | 30 - 50% |
| Humidity Ratio | 8 - 12 g/kg | 4 - 8 g/kg |
| Dew Point | 12 - 17 °C | 2 - 10 °C |
| Enthalpy | 45 - 58 kJ/kg | 30 - 42 kJ/kg |
HVAC Applications
- Cooling coil sizing: The enthalpy difference between supply and return air determines the total cooling load, including both sensible and latent components.
- Dehumidification: Air must be cooled below its dew point to remove moisture. The psychrometric chart shows the relationship between cooling and moisture removal.
- Economizer control: Comparing outdoor and indoor enthalpy determines whether free cooling with outdoor air is beneficial.
- Evaporative cooling: The wet-bulb temperature determines the theoretical limit of evaporative cooling, making it effective in dry climates but ineffective in humid ones.
Frequently Asked Questions
What is the difference between dew point and wet-bulb temperature?
The dew point is the temperature at which air becomes saturated if cooled at constant pressure and moisture content -- condensation begins at this temperature. The wet-bulb temperature is the equilibrium temperature reached when water evaporates into the air at constant pressure. The dew point is always less than or equal to the wet-bulb temperature, which is always less than or equal to the dry-bulb temperature. At 100% relative humidity, all three temperatures are equal.
Why does high humidity make it feel hotter?
The human body cools itself primarily through evaporation of sweat. When relative humidity is high, the air already contains significant moisture, reducing the rate at which sweat can evaporate. This impairs the body's cooling mechanism, making the apparent temperature (heat index) feel higher than the actual air temperature. At 35 degrees Celsius with 70% humidity, the heat index can exceed 45 degrees Celsius.
How accurate is the Magnus formula?
The Magnus formula (also called the August-Roche-Magnus formula) provides saturation vapor pressure accurate to within 0.4% for temperatures between 0 and 60 degrees Celsius. For more precise calculations, especially at extreme temperatures, the Buck equation or the Hyland-Wexler correlations used by ASHRAE are preferred. For typical HVAC applications in the comfort range, the Magnus formula is more than adequate.