Page 105 - Industrial Ventilation Design Guidebook
P. 105
70 CHAPTER 4 PHYSICAL FUNDAMENTALS
On the other hand, the differential of the total pressure is
which, substituted in Eq, (4.101), gives
This differs from the water pressure equation of Clapeyron, which lacks the
last term. If p i = 0 or dp i - 0, then Eq. (4.102) is identical to the Clapeyron
equation, as it should be.
Considering that v^ » v v and v h = 1/pf, and using Eq. (4.78), an ap-
proximation to Eq. (4.102) is obtained:
On the other hand,
so according to Eq. (4.103) we can write
3 3
The specific volume of water is approximately v v = 10~ m /kg, and an esti-
mate of Eq. (4.105) at a temperature of 50 °C is
Integrating Eq. (4.105) with the help of this value we can examine the effect of
air on the vapor pressure. When the partial pressure of air p, = 0 and when
5
the partial pressure of air is 10 Pa, a ratio of the vapor pressures correspond-
ing to these situations at the temperature of 50 °C is obtained:
or
When p t• — 0, a situation is described where water and water vapor are in an
equilibrium without the presence of dry air. The corresponding vapor pressure
can be found in tables for vapor:
