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Moist Air - Specific Volume

Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air

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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)

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