Air Psychrometrics Properties

Overview

Engineering reference data for Air Psychrometrics Properties in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Psychrometric Relationships

The state of moist air is defined by several interdependent properties. Understanding these relationships is fundamental to HVAC calculations and process design. The humidity ratio ($\omega$) is particularly important as it remains constant during sensible heating and cooling, changing only when moisture is added or removed.

The following equation relates humidity ratio to partial pressures:

$\omega = 0.62198 \cdot \frac{p_w}{p_a}$

Where:

• $\omega$ = humidity ratio (kg/kg dry air or lb/lb dry air)
• $p_w$ = partial pressure of water vapor (Pa, psi)
• $p_a$ = partial pressure of dry air (Pa, psi)

This relationship is the basis for determining moisture content from measured temperatures and pressures.

Additional Engineering Notes

Air - Absolute Humidity

Air - Composition and Molecular Weight

Air - Density and Specific Volume vs. Altitude

Air - Diffusion Coefficients of Gases in Excess of Air

Air - Drying Force

Air - Heating, Cooling, Mixing, Humidifying or Dehumidifying Processes

Air - Humidifying by Adding Steam or Water

Air - Humidifying with Steam - Imperial Units

Air - Humidifying with Steam, SI units

Air - Humidity Ratio

Air - Maximum Moisture Carrying Capacity

Air - Moisture Holding Capacity vs. Temperature

Air - Molecular Weight and Composition

Air - Prandtl Number

Air - Properties at Gas-Liquid Equilibrium Conditions

Air - Thermal Diffusivity vs. Temperature and Pressure

Air Density, Specific Weight, and Thermal Expansion Coefficients at Varying Temperatures and Pressures

Ammonia Gas - Density vs. Temperature and Pressure

Compressed Air - Water Content

Cooling Tower Efficiency

Dehumidifiers

Dry Air and Water Vapor - Density and Specific Volume vs. Temperature - Imperial Units

Dry Bulb, Wet Bulb and Dew Point Temperatures

Evaporation from a Water Surface

Evaporative Cooling

Great Sensible Heat Factor - GSHF

Heat Index vs. Humidity

Heat Recovery Efficiency

Human Heat Gain

Humid Air - Heating

Humid Air and the Ideal Gas Law

Individual & Universal Gas Constants: Definitions, Values, and Applications

Indoor Comfort Temperatures vs. Outdoor Temperatures

Latent Heat Flow

Metabolic Heat Gain from Persons

Mixing of Humid Air

Moist Air - Cooling and Dehumidifying

Moist Air - Degree of Saturation

Moist Air - Density vs. Pressure

Moist Air - Enthalpy

Moist Air - Mole Fraction of Water Vapor

Moist Air - Partial Pressure and Daltons Law

Moist Air - Properties

Moist Air - Psychrometric Chart for Normal Temperatures at 5000 Feet Altitude

Moist Air - Psychrometric Chart for Normal Temperatures at 7500 Feet Altitude

Moist Air - Psychrometric Online Calculator

Moist Air - Psychrometric Terms

Moist Air - Relative Humidity

Moist Air - Specific Volume

Moist Air - Specific vs. Relative Humidity

Moist Air - the Mollier Diagram

Moist Air - Transforming the Mollier Diagram to a Psychrometric Chart - or vice versa

Moist Air - Vapor Pressure

Moist Air - Weight of Water Vapor

Non-ideal gas - Van der Waal's Equation and Constants

Outdoor Temperatures and Relative Humidity - U.S. Winter and Summer Conditions

Relative Humidity in Production and Process Environments

Removing Heat with Air

Room Sensible Heat Factor - RSHF

Sensible Heat Flow

Sensible Heat Ratio - SHR

Specific Heat Capacity of Air: Isobaric and Isochoric Heat Capacities at Various Temperatures and Pressures

The Ideal Gas Law

Wet Bulb Globe Temperature (WBGT)

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See Also

• Air - Absolute Humidity
• Air - Composition and Molecular Weight
• Air - Density and Specific Volume vs. Altitude
• Air - Diffusion Coefficients of Gases in Excess of Air
• Air - Drying Force
• Air - Heating, Cooling, Mixing, Humidifying or Dehumidifying Processes
• Air - Humidifying by Adding Steam or Water
• Air - Humidifying with Steam - Imperial Units
• Air - Humidifying with Steam, SI units
• Air - Humidity Measurement from Dry and Wet Bulb Temperature
• Air - Humidity Ratio
• Air - Maximum Moisture Carrying Capacity
• Air - Moisture Holding Capacity vs. Temperature
• Air - Molecular Weight and Composition
• Air - Prandtl Number
• Air - Properties at Gas-Liquid Equilibrium Conditions
• Air - Psychrometric Chart for Standard Atmospheric Conditions - Imperial Units
• Air - Specific Heat vs. Pressure at Constant Temperature
• Air - Thermal Diffusivity vs. Temperature and Pressure
• Air Density, Specific Weight, and Thermal Expansion Coefficients at Varying Temperatures and Pressures
• Air Properties - Density, Viscosity, Heat Capacity, Thermal Conductivity, and more
• Air Properties - Thermal Conductivity vs. Temperature and Pressure Charts and Calculator
• Air Properties: Temperature, Pressure & Density Data
• Air Viscosity: Dynamic and Kinematic Viscosity at Various Temperatures and Pressures
• Ammonia Gas - Density vs. Temperature and Pressure
• Compressed Air - Water Content
• Cooling Tower Efficiency
• Dehumidifiers
• Dry Air and Water Vapor - Density and Specific Volume vs. Temperature - Imperial Units
• Dry Air Thermodynamic and Physical Properties – Density, Specific Heat, Viscosity
• Dry Bulb, Wet Bulb and Dew Point Temperatures
• Evaporation from a Water Surface
• Evaporative Cooling
• Great Sensible Heat Factor - GSHF
• Heat Index vs. Humidity
• Heat Recovery Efficiency
• Human Heat Gain
• Humid Air - Heating
• Humid Air and the Ideal Gas Law
• Individual & Universal Gas Constants: Definitions, Values, and Applications
• Indoor Comfort Temperatures vs. Outdoor Temperatures
• Indoor Relative Humidity vs. Outdoor Relative Humidity and Temperature
• Latent Heat Flow
• Metabolic Heat Gain from Persons
• Mixing of Humid Air
• Moist Air - Cooling and Dehumidifying
• Moist Air - Degree of Saturation
• Moist Air - Density vs. Pressure
• Moist Air - Density vs. Water Content and Temperature
• Moist Air - Enthalpy
• Moist Air - Mole Fraction of Water Vapor
• Moist Air - Partial Pressure and Daltons Law
• Moist Air - Properties
• Moist Air - Psychrometric Chart for Normal Temperatures at 7500 Feet Altitude
• Moist Air - Psychrometric Online Calculator
• Moist Air - Psychrometric Table for Pressure 29.92 inHg
• Moist Air - Psychrometric Terms
• Moist Air - Relative Humidity
• Moist Air - Specific Volume
• Moist Air - Specific vs. Relative Humidity
• Moist Air - the Mollier Diagram
• Moist Air - Transforming the Mollier Diagram to a Psychrometric Chart - or vice versa
• Moist Air - Vapor Pressure
• Moist Air - Water Vapor and Saturation Pressure
• Moist Air - Weight of Water Vapor
• Non-ideal gas - Van der Waal's Equation and Constants
• Outdoor Temperatures and Relative Humidity - U.S. Winter and Summer Conditions
• Relative Humidity in Production and Process Environments
• Removing Heat with Air
• Room Sensible Heat Factor - RSHF
• Sensible Heat Flow
• Sensible Heat Ratio - SHR
• Specific Heat Capacity of Air: Isobaric and Isochoric Heat Capacities at Various Temperatures and Pressures
• The Ideal Gas Law
• Water Vapor Saturation Pressure: Data, Tables & Calculator
• Wet Bulb Globe Temperature (WBGT)
• Temperature
• Length
• Area
• Volume
• Weight
• Velocity
• Pressure
• Flow

References

• Original Source

Air Property Data

Comprehensive engineering data for air properties across various conditions. The following table provides representative values for key thermophysical properties of dry air at standard atmospheric pressure.

Calculation Methods

Several psychrometric properties can be derived from fundamental measurements. The following formulas provide calculation pathways for common engineering scenarios.

Wet-Bulb Temperature Estimation

The psychrometric wet-bulb temperature ($T_{wb}$) can be estimated from dry-bulb temperature ($T_{db}$) and relative humidity ($\phi$) using empirical relationships. A common approximation for temperatures in °C is:

$T_{wb} \approx T_{db} \cdot \arctan[0.151977 \cdot (\phi + 8.313659)^{0.5}] + \arctan(T_{db} + \phi) - \arctan(\phi - 1.676331) + 0.00391838 \cdot \phi^{1.5} \cdot \arctan(0.023101 \cdot \phi) - 4.686035$

where:

• $\phi$ is relative humidity in percent (%)
• $T_{db}$ and $T_{wb}$ are in °C

Enthalpy of Moist Air

The specific enthalpy ($h$) of moist air, accounting for both dry air and water vapor, is calculated as:

$h = c_{p,a} \cdot T + W \cdot (h_{fg0} + c_{p,v} \cdot T)$

where:

• $c_{p,a}$ = specific heat of dry air (~1.005 kJ/kg·K)
• $T$ = dry-bulb temperature (°C)
• $W$ = humidity ratio (kg water/kg dry air)
• $h_{fg0}$ = enthalpy of vaporization at 0°C (~2501 kJ/kg)
• $c_{p,v}$ = specific heat of water vapor (~1.86 kJ/kg·K)

Process Analysis Applications

The extracted materials reference several important engineering processes involving moist air:

1. Air Mixing - When two moist air streams combine, the resulting mixture properties can be determined using mass and energy balances
2. Adiabatic Saturation - The process where air gains moisture while maintaining constant enthalpy (follows constant wet-bulb temperature lines on psychrometric charts)
3. Compression Effects - In compressed air systems, water content saturation pressure differs significantly from atmospheric conditions, affecting moisture carrying capacity
4. Cooling Processes - Cooling tower efficiency is fundamentally limited by the wet-bulb temperature of the cooling air

Related Property Data Sources

For specialized applications requiring data beyond standard psychrometrics, the following related air property topics provide additional engineering resources:

• High-temperature properties: Air behavior at elevated temperatures up to 1600°C for combustion and industrial process applications
• Pressure-dependent data: Properties at pressures ranging from 1 to 10,000 bara for compressed gas and high-pressure system design
• Transport properties: Dynamic and kinematic viscosity data across wide temperature and pressure ranges
• Thermal properties: Thermal conductivity and diffusivity data for heat transfer calculations
• Phase equilibrium: Properties along boiling and condensation curves for refrigeration and liquefaction systems

Air Property Data

Comprehensive engineering data for air properties across various conditions. The following table provides representative values for key thermophysical properties of dry air at standard atmospheric pressure.

Calculation Methods

Several psychrometric properties can be derived from fundamental measurements. The following formulas provide calculation pathways for common engineering scenarios.

Wet-Bulb Temperature Estimation

The psychrometric wet-bulb temperature ($T_{wb}$) can be estimated from dry-bulb temperature ($T_{db}$) and relative humidity ($\phi$) using empirical relationships. A common approximation for temperatures in °C is:

$T_{wb} \approx T_{db} \cdot \arctan[0.151977 \cdot (\phi + 8.313659)^{0.5}] + \arctan(T_{db} + \phi) - \arctan(\phi - 1.676331) + 0.00391838 \cdot \phi^{1.5} \cdot \arctan(0.023101 \cdot \phi) - 4.686035$

where:

• $\phi$ is relative humidity in percent (%)
• $T_{db}$ and $T_{wb}$ are in °C

Enthalpy of Moist Air

The specific enthalpy ($h$) of moist air, accounting for both dry air and water vapor, is calculated as:

$h = c_{p,a} \cdot T + W \cdot (h_{fg0} + c_{p,v} \cdot T)$

where:

• $c_{p,a}$ = specific heat of dry air (~1.005 kJ/kg·K)
• $T$ = dry-bulb temperature (°C)
• $W$ = humidity ratio (kg water/kg dry air)
• $h_{fg0}$ = enthalpy of vaporization at 0°C (~2501 kJ/kg)
• $c_{p,v}$ = specific heat of water vapor (~1.86 kJ/kg·K)

Process Analysis Applications

The extracted materials reference several important engineering processes involving moist air:

1. Air Mixing - When two moist air streams combine, the resulting mixture properties can be determined using mass and energy balances
2. Adiabatic Saturation - The process where air gains moisture while maintaining constant enthalpy (follows constant wet-bulb temperature lines on psychrometric charts)
3. Compression Effects - In compressed air systems, water content saturation pressure differs significantly from atmospheric conditions, affecting moisture carrying capacity
4. Cooling Processes - Cooling tower efficiency is fundamentally limited by the wet-bulb temperature of the cooling air

Related Property Data Sources

For specialized applications requiring data beyond standard psychrometrics, the following related air property topics provide additional engineering resources:

• High-temperature properties: Air behavior at elevated temperatures up to 1600°C for combustion and industrial process applications
• Pressure-dependent data: Properties at pressures ranging from 1 to 10,000 bara for compressed gas and high-pressure system design
• Transport properties: Dynamic and kinematic viscosity data across wide temperature and pressure ranges
• Thermal properties: Thermal conductivity and diffusivity data for heat transfer calculations
• Phase equilibrium: Properties along boiling and condensation curves for refrigeration and liquefaction systems