Page 178 - Industrial Ventilation Design Guidebook

I 40 CHAPTER 4 PHYSICAL FUNDAMENTALS

with C A = p A/(RT).
Instead of the mean velocity v x weighted with the molar fractions, a veloc-
ity weighted with the mass fractions—the mass center velocity u x—can be
used. Using Eq. (4.271), the above equation becomes

Equations (4.308) and (4.309) give a formula for the diffusion resistance
(
force f £>:

Substituting the formulas for forces f ^ and f mx (Eqs. (4.310) and (4.305a))
in Eq. (4.306) gives

In the steady-state case Eq. (4.311) is simplified to

The aim is to solve this equation for the term v Ax, or actually p Av Ax, which is
the diffusion flow density of component A.
An important case is

This case applies to drying processes (B = dry air), condensation, and ab-
sorption, such as the diffusion of sulfur dioxide gas through a calcium
oxide.
When Eq. (4.313) is valid, pu x = p Av Ax + p Bv Bx = PA^AX* and u x =
V +
(PA/P) AX (p ~ PA PB) • Therefore, in this case

Substituting Eq. (4.314) in Eq. (4.312) and solving for the mass flow density
gives

This can be written as