= 0.0022 p w / T (2)
p w = partial pressure of water vapor (Pa, N/m 2 )
ρ w = density of water vapor (kg/m 3 )
R w = 461.5 - individual gas constant water vapor (J/kg K)
T = absolute dry bulb temperature (K)
The amount of water vapor in air influences the density. Water vapor is relatively light compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air .
When vapor content increases in moist air the amount of Oxygen and Nitrogen are decreased per unit volume and the density of the mix decreases since the mass is decreasing.
Based on specific volume of moist air the moist air density can be calculated
ρ = 1 / v
= (p / R a T) (1 + x) / (1 + x R w / R a ) (3)
v = specific volume of moist air per mass unit of dry air and water vapor (m 3 /kg)
x = m w / m a = specific humidity or humidity ratio (kg h2o /kg dry_air )
m w = mass of water vapor (kg)
m a = mass of dry air (kg)
p = pressure in the moist air (Pa)
The density of dry air with the same total pressure p as in the mixture (dry air + vapor partial pressure) - can be calculated as a reference as
ρ ref = p / R a T (4)
ρ ref = reference density of dry air with the same pressure as in the air vapor mixture (kg/m 3 )
The density of moist air related to dry air can be expressed by combining (4) and (3) as
ρ = ρ ref (1 + x) / (1 + x R w / R a ) (5)
The gas constant ratio between water vapor and air is
R w / R a = (461.5 J/kg K) / (286.9 J/kg K)
and inserting the ratio in (5) the density of moist air related to dry air can be expressed as
ρ = ρ ref (1 + x) / (1 + 1.609 x ) (6)
Note! As we can see from (6) - increased moisture content reduces the density of moist air - dry air is more dense than moist air.
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Densities of solids, liquids and gases. Definitions and convertion calculators.
Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.
Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.
Air can be humidified by adding water or steam.
Estimate the amount of steam required (lb/h in 100 cfm) in humid air.
Using steam to humidify air.
Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
The mass of water vapor present in moist air - to the mass of dry air.
Maximum water content in humid air vs. temperature.
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Dry air is a mixture of gases where the average molecular weight (or molar mass) can be calculated by adding the weight of each component.
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Thermodynamic properties of dry air - specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity at temperatures ranging 175 - 1900 K.
Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC).
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Saturation pressure of water vapor in moist air vs. temperature.
Weight of water vapor in air
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