Page 169 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER I 3 I

FIGURE 4.36

The equations are valid for the case j A = -/g :

By using Eq. (4.253), the approximation (4.249) becomes

r DiVi
In practice the mass transfer factors are often presented without stating the
experimental assumptions by which j A — -j B or ; B = 0 has been obtained.
The designer has to decide on the suitability of the experiments from which the
quantity k' c or k c, is measured.
An idea of the approximate nature of Eq. (4.249) or the equivalent Eq.
(4.259) can be gained by comparing a pure-diffusion case (Fig. 4.36a] with the
case involving a diffusion boundary layer (Fig. 4.366).

4.3.7 Heat and Mass Transfer Differential Equations
in the Boundary Layer and the Corresponding Analogy
We will consider flow through a solid element. Introducing the notations for
molar flow density, partial density, and the reaction rate gives an equation for
the mass balance:

where
2
I Ax is the molar flow density of component A (mol/m s) in the x direction
M A is the molar mass of component A (kg/mol)
3
P A is the partial density of component A (kg/m )
3
r A represents the formation rate of component A by chemical reactions (mol/rn s)
The corresponding equation for component B is