Page 108 - Industrial Ventilation Design Guidebook

4,2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 73

(a) Saturated air, <p = 100%

p' h (20 °C) = 0.0234 bar
2 3 = 01816 k H
x = 0.6220 - Q 82^'°- 0 0234 °- § 2Q/kg d.a.
p h = 0.01731 kg/m 3
5
o = (0.825-0.0234). IP -0.028964 = Q 953 k . 3 ( &}
Pt U 53 kgtol
8.314 -293.15 '* (~ x J
p - 0.970 kg/m 3
/.^ = 1.006 • 20 + 0.01816 • (2501 + 1.85 • 20) = 66.2 kj/kg da.

(b) Humid air, <p = 50%
p' h(2Q °C)= 0.0234 bar
p h = 0.0117 bar
01
=
x = 0.6220 • 0825' ooii7 0-00895 kg H 2O/kg d.a.
Ph = 0.0865 kg/m 3
5
n - (0.825 - 0.0117) • 1Q • 0.028964
Pi
8.314 - 293.15
3
= 0.967 kg/m f= fife = ~|1
I x 0.00736 I
^ - 1.006 • 20 + 0.00895 • (2501 + 1.85 • 20) = 42.8 kj/kg d.a.

Comparing Examples 2a and 2b we notice that the total air pressure has
effects on the humidity x, partial density of dry air p it total pressure or pres-
sure of humid air, and enthalpy b k. Knowing the total pressure is therefore es-
sential in calculations of the thermodynamic properties of humid air.
Pressure and humidity have also an effect on the mass flows. We continue
Examples 2a and 2b by calculating the dry air mass flow in a fan when the hu-
3
mid air volume flow in the fan is 0.8 m /s. According to Eq. (4.91) and the cal-
culations above, we obtain

Thus the total mass flows m— (m,• + m^) differ in different cases. Water va-
por flow fhf, is obtained by multiplying the dry air mass flow by the corre-
sponding humidity x (Eq. 4.93). As a basic quantity in humid air mass and
energy balance calculations, we use dry air mass flow m it and the effect of hu-
midity on the energy balance is noted in the enthalpy h k (Eq. 4.87).