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# Moist Air - Partial Pressure and Daltons Law

## The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.

Daltons Law states that

• the total pressure of a mixture of gases is equal to the sum of the partial pressures of the constituent gases

The partial pressure is the pressure each gas would exert if it alone occupied the volume of the mixture.

Daltons Law of partial pressure for moist air can be expressed as

p = pa + pw                            (1)

where

p = total pressure of air (Pa, N/m2)

pa = partial pressure dry air (Pa, N/m2)

pw = partial pressure water vapor (Pa, N/m2)

With the partial pressure for dry air and water vapor (1) can be modified to:

p = ρa286.9T + ρw455T                                    (2)

where

ρa = density of air (kg/m3)

ρw = density of water vapor (kg/m3)

## Related Topics

• ### Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

## Related Documents

• ### Air - Composition and Molecular Weight

Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.
• ### Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
• ### Air - Maximum Moisture Carrying Capacity

Maximum water content in humid air vs. temperature.
• ### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.
• ### Air and Steam Mixtures

Air in the steam will lower the surface temperatures in heat exchangers - and less heat will be transferred.
• ### Humid Air and the Ideal Gas Law

Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).
• ### Moist Air - Degree of Saturation

Humidity ratio of moist air to humidity ratio of saturated moist air.
• ### Moist Air - Density vs. Pressure

Density of moist air vs. pressure ranging 75 - 1000 mmHg.
• ### Moist Air - Density vs. Water Content and Temperature

Density of the mix of dry air and water vapor - moist humid air.
• ### Moist Air - Enthalpy

Sensible and latent heat of moist air.
• ### Moist Air - Properties

Psychrometric table with humid air properties like saturation pressure, specific volume, enthalpy and entropy.
• ### Moist Air - Relative Humidity

Relative humidity in moist air is the ratio of partial vapor pressure to air pressure.
• ### Moist Air - Specific Volume

Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air
• ### Moist Air - Water Vapor and Saturation Pressure

Saturation pressure of water vapor in moist air vs. temperature.
• ### Moist Air - Weight of Water Vapor

Weight of water vapor in air

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## Citation

• The Engineering ToolBox (2004). Moist Air - Partial Pressure and Daltons Law. [online] Available at: https://www.engineeringtoolbox.com/moist-air-daltons-law-d_678.html [Accessed Day Month Year].

Modify the access date according your visit.

8.21.8

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