Page 123 - Industrial Ventilation Design Guidebook

4.2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 85
has to be iterated because steam pressure p h and therefore humidity x are at
this stage unknown. When humidity is low, as in this example, an approxima-
tion is pCp^piCpi. The calculation can be repeated when needed using the
steam pressure from Eq. (4.117). In this example, we don't do that. For calcu-
lating the density and heat capacity, we will use an approximation p cp - p,, so
p = p, and cp = Cpi. From this approximation it follows that

whose value is

From Eq. (4.76) we get the partial pressure of dry air:

The Lewis number in cases a and b is as follows:

It is assumed that n = 0.5, so Le = Le = 1.073

n
Thus Le — 1.017. Substituting all values into Eq. (4.117), we obtain