Psychrometric Properties Calculator
Enter the dry-bulb temperature, relative humidity, and atmospheric pressure to calculate various psychrometric properties of air.
Calculated Properties:
Wet-Bulb Temperature: °C
Dew Point Temperature: °C
Specific Humidity: kg water/kg dry air
Enthalpy: kJ/kg dry air
Specific Volume: m³/kg dry air
Vapor Pressure: kPa
Saturation Vapor Pressure: kPa
Understanding Psychrometric Properties
Psychrometrics is the study of the thermodynamic properties of moist air. It's crucial in fields like HVAC (Heating, Ventilation, and Air Conditioning), meteorology, and industrial drying processes. Understanding these properties allows engineers and scientists to design efficient systems for climate control, predict weather patterns, and optimize industrial operations.
Key Psychrometric Properties Explained:
- Dry-Bulb Temperature (Tdb): This is the temperature of air measured by a standard thermometer, unaffected by the moisture content of the air. It's the most commonly cited temperature.
- Relative Humidity (RH): Expressed as a percentage, relative humidity indicates the amount of moisture in the air relative to the maximum amount of moisture the air can hold at that specific dry-bulb temperature and pressure. 100% RH means the air is saturated.
- Atmospheric Pressure (P_atm): The pressure exerted by the weight of the atmosphere. It varies with altitude and weather conditions and affects all other psychrometric properties. Standard atmospheric pressure at sea level is 101.325 kPa.
- Wet-Bulb Temperature (Twb): This is the temperature indicated by a thermometer with a wet wick covering its bulb, exposed to a stream of air. Evaporation of water from the wick cools the bulb, so the wet-bulb temperature is always equal to or lower than the dry-bulb temperature. The difference between dry-bulb and wet-bulb temperatures indicates the air's capacity to evaporate water.
- Dew Point Temperature (Tdp): The temperature at which air becomes saturated with water vapor, and condensation begins to form (e.g., dew on grass, fog). If the air is cooled further below its dew point, water vapor will condense into liquid water.
- Specific Humidity (w) / Humidity Ratio: This is the mass of water vapor present per unit mass of dry air (e.g., kg water/kg dry air). It's an absolute measure of moisture content, unlike relative humidity.
- Enthalpy (h): Represents the total energy (sensible and latent) contained in a unit mass of moist air. Sensible heat is related to temperature, while latent heat is related to the phase change of water vapor. Enthalpy is critical for energy balance calculations in HVAC systems.
- Specific Volume (v): The volume occupied by a unit mass of dry air plus its associated water vapor (e.g., m³/kg dry air). It's the reciprocal of density and is important for calculating airflow rates.
- Vapor Pressure (Pv): The partial pressure exerted by the water vapor in the air mixture. It's a direct measure of the amount of water vapor present.
- Saturation Vapor Pressure (Psat): The maximum possible vapor pressure that can exist at a given dry-bulb temperature. When the vapor pressure equals the saturation vapor pressure, the air is saturated (100% RH).
How the Calculator Works
This calculator uses established psychrometric equations to determine various air properties based on your input. The core calculations involve:
- Calculating the Saturation Vapor Pressure (Psat) at the given dry-bulb temperature using the Arden Buck equation.
- Determining the actual Vapor Pressure (Pv) based on the relative humidity and saturation vapor pressure.
- Deriving the Dew Point Temperature (Tdp) by inverting the saturation vapor pressure equation using the actual vapor pressure.
- Calculating the Specific Humidity (w) from the vapor pressure and atmospheric pressure.
- Estimating the Wet-Bulb Temperature (Twb) using an empirical formula that relates dry-bulb temperature and relative humidity.
- Computing the Enthalpy (h) and Specific Volume (v) using standard thermodynamic formulas that account for both dry air and water vapor properties.
Example Calculation
Let's consider a typical summer day scenario:
- Dry-Bulb Temperature: 30 °C
- Relative Humidity: 65 %
- Atmospheric Pressure: 101.325 kPa (standard sea level pressure)
Using these inputs, the calculator would yield results approximately as follows:
- Wet-Bulb Temperature: ~24.5 °C
- Dew Point Temperature: ~22.5 °C
- Specific Humidity: ~0.0174 kg water/kg dry air
- Enthalpy: ~74.6 kJ/kg dry air
- Specific Volume: ~0.883 m³/kg dry air
- Vapor Pressure: ~2.76 kPa
- Saturation Vapor Pressure: ~4.24 kPa
These values indicate warm, humid air. The relatively high dew point suggests that condensation will occur if surfaces are cooled below 22.5 °C, which is important for HVAC design to prevent mold growth.