TITLE.PM5

PSYCHROMETRICS 453

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\M-therm\Th10-1.pm5

Enthalpy of moist air
It is the sum of enthalpy of dry air and enthalpy of water vapour associated with dry air.
It is expressed in kJ/kg of dry air
h = hair + W. hvapour
= cptdb + W. hvapour
where h = Enthalpy of mixture/kg of dry air,
hair = Enthalpy of 1 kg of dry air,
hvapour = Enthalpy of 1 kg of vapour obtained from steam tables,
W = Specific humidity in kg/kg of dry air, and
cp = Specific heat of dry air normally assumed as 1.005 kJ/kg K.
Also hvapour = hg + cps (tdb – tdp)
where hg = Enthalpy of saturated steam at dew point temperature,
and cps = 1.88 kJ/kg K.
∴ h = cptdb + W[hg + cps(tdb – tdp)] ...(10.11)
= (cp + cps W) tdb + W(hg – cpstdp)
= cpmtdb + W(hg – cpstdp) ...[10.11(a)]
where cpm = (cp + cps W) is the specific heat of humid air or humid specific heat.
The value of cpm is taken as 1.021 kJ/kg dry air per K. It is the heat capacity of (1 + W) kg
of moisture per kg of dry air.

hvapour (^) ~− hg at dry bulb temperature. So,
h = cptdb + W hg. ...(10.12)
However, a better approximation is given by the following relationship :
hvapour = 2500 + 1.88tdb kJ/kg of water vapour ...[10.12 (a)]
where tdb is dry bulb temperature in ºC, and the datum state is liquid water at 0ºC.
∴ h = 1.005 tdb + W(2500 + 1.88 tdb) kJ/kg dry air. ...[10.12 (b)]
Adiabatic saturation process
In an insulated chamber when unsaturated air flows over a long sheet of water (Fig. 10.1),
the water evaporates, and the specific humidity of the air increases. As the evaporation takes place
Water h (at t )fdb 2
Make-up water
at tdb 2
1 2
1 kg of dry air
at t + W kg of
water vapour at h
db 2S
2S
2
Unsaturated air Saturated air
1 kg of dry air
at t and t + W kg of
water vapour at t and h
db db 1
db f
12
2
Insulated chamber
Fig. 10.1. Adiabatic saturation process.
both the air and water are cooled. The process continues until the energy transferred from the air
to the water is equal to the energy required to vapourise the water. When this point is reached,