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PSYCHROMETRICS 451

dharm

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Specific humidity W :

Specific humidity =Mass of water vapour

Mass of dry air

or W

m

m

v

a

=

Also, ma =

pV

RT

a

a

...(10.2)

and mv =

pV

RT

v

v

×

× ...(10.3)

where pa = Partial pressure of dry air,

pv = Partial pressure of water vapour,

V = Volume of mixture,

Ra = Characteristic gas constant for dry air, and

Rv = Characteristic gas constant for water vapour.

From equations (10.2) and (10.3)

W pV

RT

RT

pV

R

R

p

p

v

v

a

a

a

v

v

a

= ×

×

×=×

But Ra = R

Ma

0 F==

HG

I

KJ

8.3143

28. 97
    0.287 kJ/kg K in SI units

Rv =

R

Mv

0 F==

HG

I

KJ

8.3143

18

0.462 kJ/kg K in SI units

where R 0 = Universal gas constant,

Ma = Molecular weight of air, and

Mv = Molecular weight of water vapour.

∴ W = 0.

0.

287

462

.p

p

v

a

= 0.622 p

pp

v

tv−

i.e., W = 0.622

p

pp

v

tv−

...(10.4)

The masses of air and water vapour in terms of specific volumes are given by expression as

ma = V

va

and mv = V

vv

where va = Specific volume of dry air, and

vv = Specific volume of water vapour.

W = v

v

a

v

...(10.5)

Degree of saturation (μ) :

Degree of saturation =

Mass of water vapour associated

with unit mass of dry air

Mass of water vapour associated with

saturated unit mass of dry saturated air