Page 165 - Industrial Ventilation Design Guidebook

4,3 HEAT AND MASS TRANSFER 1 2 7

Thus

Radiation heat transfer decreases by

4.3.6 Mass Transfer Coefficient
Consider a binary mixture consisting of components A and B. If component A moves
with a velocity of V A and the component B with a velocity of V B there is a force
against the motion of component A that is proportional to the velocity difference
( V A — v s ). This is the physical content of Pick's law in the steady-state condition.

where j A is the molar flux density (mol/m" s), D AB is the diffusion factor
(m~/s), C A is the concentration of component A (mol/m ), and z is a coordi-
nate parallel to the flux (m). Note that / A — C AV A. On the basis of the force
and counterforce, D AB = D BA.
Note: Equation (4.241) characterizes diffusion when the mixture element is
in steady state with no turbulence. Diffusion in a pipe can be represented by Eq.
(4.241) in convective mass transfer; the flow and turbulence are important.
An important convective flow is created from vaporization alone, if no
other component is absorbed from the gas and is replacing the vaporizing
component. In drying technology, for example, the diffusion process is consid-
ered to be diffusion between water vapor (A) and dry air (B) (a mixture of ni-
trogen and oxygen), and only a small amount of dry air replaces the vaporized
water, if the volume of the water in the form of liquid is very small. With good
accuracy ; B = 0, and the diffusion caused by the concentration gradient
dc B/dz is fulfilled with convective flow (Stefan flow) according to

where C BV represents the convective flow that cancels the diffusion. Therefore
the Stefan flow is

The net flow of component A with Stefan flow taken into consideration is

With constant temperature (c = C A + C B = constant),