Page 103 - Industrial Ventilation Design Guidebook

68 CHAPTER 4 PHYSICAL FUNDAMENTALS

When an energy balance is written, an enthalpy flow of humid air H is
needed. This can be written according to the Eqs. (4.85) and (4.93) as
H = rhjhi + m hb h = w i(b i + xh h)

or briefly, using definition (4.86),

In energy balance calculations, which we will handle later on, we use Eq. (4.94).

Example I
3
A fan takes +15 °C humid air at 0.5 m /s. What is the dry air mass flow
thf taken by the fan when the outdoor humidity is x = 0.009 and the outdoor
pressure is p = 1.0 bar ?
Partial pressure of water vapor is calculated from Eq. (4.84):

Dry air partial pressure is

Partial density of dry air is calculated from Eq. (4.76):

Dry air mass flow is calculated from Eq. (4.91):

Water vapor flow through the fan is

4.2.3 Water Vapor Pressure in the Presence of Air
The equilibrium between water and water vapor in the case of humid air is il-
lustrated in Fig. 4.7.

FIGURE 4.7 Equilibrium between water and vapor in the presence of air.