Page 167 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER 1 29

FIGURE 4.35 Diffusion with equal measures, j A = j B : fully permeable surface.

Considering the case of Eq. (4.244), it is normal to describe a real mass
transfer case by taking into consideration the boundary layer flows and the
turbulence by using a mass transfer factor k' n which is defined by

For Eq. (4.243), which can be written similar to Eq. (4.244) as

it is seen that by defining a logarithmic concentration difference

Eq. (4.243) can be written in an identical form:

Based on this, it is normal to define a mass transfer factor consistent with this
case, analogous with Eq. (4.245):

Assuming that the relation of Eqs. (4.243) and (4.244) represents cor-
rectly the ratio of the real mass transfer flows, if it is valid that