Moist Air - Specific Volume
Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air
When discussing specific volume of moist air its necessary to differentiate between
- specific volume of moist air per mass unit of dry air , or
- specific volume of moist air per mass unit of dry air and water vapor
Specific Volume of Moist Air per Mass Unit of Dry Air
Specific volume is defined as the total volume of dry air and water vapor mixture per kg of dry air (SI-units). The specific volume can be expressed as
vda = V / ma (1)
where
vda = specific volume of moist air per mass unit of dry air (m3 /kg)
V = total volume of moist air (m3)
ma = mass of dry air (kg)
When dry air and water vapor with the same temperature occupies the same volume the equation for an ideal gas can be applied.
pa V = ma Ra T (2)
where
pa = partial pressure of dry air (Pa)
Ra = 286.9 - the individual gas constant dry air (J/kg.K)
T = temperature of the moist air (K)
Combining (1) and (2):
vda = Ra T / pa (3)
The partial pressure of the air can be expressed as
pa = p - pw (3b)
where
p = pressure in the moist air (Pa)
pw = partial pressure water vapor (Pa)
Combining (3) and (2):
vda = Ra T / (p - pw ) (3c)
The ideal gas law can also be applied to the water vapor as
pw V = mw Rw T (4)
where
pw = partial pressure of water vapor (Pa)
Rw = individual gas constant water vapor ( 461.4 J/kg K)
T = temperature of the moist air (K)
The mass of the water vapor can be calculated by using the humidity ratio and the mass of air as
mw = x ma (5)
where
x = mw / ma = specific humidity or humidity ratio (kgh2o /kgdry_air)
mw = mass of water vapor (kg)
ma = mass of dry air (kg)
Combining (4) and (5):
pw V = x ma Rw T (6)
Combining (6) with (1):
vda = x Rw T / pw (7)
Transforming (7):
pw = x Rw T / vda (8)
Combining (8) with (3c):
vda = Ra T / (p - (x Rw T / vda )) (9)
(9) can be transformed to express the specific volume of moist air per unit mass of dry air as
vda = (1 + x Rw / Ra ) Ra T / p (10)
Specific Volume of Moist Air per unit Mass of Dry Air and Water Vapor
Specific volume is defined as the total volume of dry air and water vapor mixture per kg of dry air and water vapor (SI-units). The specific volume can be expressed as:
v = V / ma + mw (11)
where
v = specific volume of moist air per mass unit of dry air and water vapor (m3 /kg)
Combining with (5):
v = V / ma (1 + x) (12)
Combining with (1):
v = vda / (1 + x) (13)
and the specific volume of moist air per unit mass of dry air and water vapor (moist air) can be expressed as
v = (Ra T / p) [(1 + x Rw / Ra )/ (1 + x)] (14)
(14) can used to express the inverse of the specific volume - the density - of moist air as
ρ = 1 / v
= (p / Ra T) (1 + x) / (1 + x Rw / Ra ) (15)
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 - Density vs. Pressure and Temperature
Air density at pressure ranging 1 to 10 000 bara (14.5 - 145000 psi) and constant selected temperatures. -
Air - Density, Specific Weight and Thermal Expansion Coefficient vs. Temperature and Pressure
Online calculator, figures and tables showing density, specific weight and thermal expansion coefficients of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units. -
Air - Diffusion Coefficients of Gases in Excess of Air
Diffusion coefficients (D12) for gases in large excess of air at temperatures ranging 0 - 400 °C. -
Air - Dynamic and Kinematic Viscosity
Online calculator, figures and tables with dynamic (absolute) and kinematic viscosity for air at temperatures ranging -100 to 1600°C (-150 to 2900°F) and at pressures ranging 1 to 10 000 bara (14.5 - 145000 psia) - SI and Imperial Units. -
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 - Humidity Ratio
The mass of water vapor present in moist air - to the mass of dry air. -
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 - Prandtl Number
Prandtl number for air vs. temperature and pressure. -
Air - Properties at Gas-Liquid Equilibrium Conditions
Properties of air change along the boiling and condensation curves (temperature and pressure between triple point and critical point conditions). An air phase diagram included. -
Air - Thermal Conductivity vs. Temperature and Pressure
Online calculator with figures and tables showing air thermal conductivity vs. temperature and pressure. SI and imperial units. -
Air - Thermal Diffusivity vs. Temperature and Pressure
Figures and tables withdry air thermal diffusivity vs. temperarure and pressure. SI and Imperial units. -
Dry Air and Water Vapor - Density and Specific Volume vs. Temperature - Imperial Units
Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC). -
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 - 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 - 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. -
Moist Air - Properties
Psychrometric table with humid air properties like saturation pressure, specific volume, enthalpy and entropy. -
Moist Air - Psychrometric Table for Pressure 29.92 inHg
Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more. -
Moist Air - Psychrometric Terms
Dry and wet bulb temperature, specific volume, relative humidity, enthalpy and more. -
Moist Air - Specific vs. Relative Humidity
Specific humidity of moist air vs. relative humidity, water vapor and air density. -
Moist Air - Weight of Water Vapor
Weight of water vapor in air
