Page 116 - Industrial Ventilation Design Guidebook

4.2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 8 i

air D (rnr/s), and the heat conductivity of humid air A (W/m °C). In Table 4.7
the thermodynamic properties of saturated air, including the diffusion factor
and the heat conductivity are presented.
Substituting Eqs. (4.112) and (4.113) into Eq, (4.111), we obtain

from which we observe that the heat transfer factor a has been reduced. The
only factor in Eq. (4.116) depending on the airflow conditions of the measure-
ment is the power n in the Lewis number. Because the value of the Lewis num-
ber is very close to 1, the effect of n is very small.
The wet bulb temperature9 M can be solved for from Eq. (4.116) when the
state of the air, the temperature t, and the partial pressure of water vapor p h
are known. Inversely, if the temperature t and the wet bulb temperature 8 M are
known, the partial pressure and consequently the humidity of air can be found
from Eq. (4.116).

Example 6
Given the temperature of air 9 = °C and the wet bulb temperature
0 M =10 °C, calculate the humidity x, when the pressure of air is (a) /; = 1
bar and (b) p = 0.90 bar.
By solving for the steam pressure p h from Eq. (4.116), we obtain

The diffusion factor D, which is contained in the Lewis number, is in-
versely related to the total pressure:

From Table 4.7 we obtain the diffusion factor at the temperature of 10 °C and
6
2
pressure p = 1.0 bar: D(10 °C, p = 1.0 bar) = 23.3 • 10~ m /s. The diffu-
sion factor at the same temperature but a pressure p = 0.9 bar is, according
2
6
to Eq. (4.118), D(10 °C, p(0.9 bar)) = 25.9 • 10" m /s.
For heat conductance there is a dependency on pressure almost like that of
Eq, (4.118), so for good accuracy it is a valid approximation that D/A = g(T).
From Table 4.7 we have A(10 °C, p(1.0 bar)) = 0.02466 W/m K, and so
D/A = 9.45 • 10 m K/J. Vaporization heat and the pressure of saturated
steam are, from Table 4.7;

Temperature T is taken as a mean boundary layer temperature:

Heat capacity,