Page 183 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER I 45

To calculate the conductance of the boundary layer we first calculate the
mass transfer factor using Eq. (4.300):

Assume that n = 0.4. Table 4.7 gives (0 = 70 °C):

Boundary layer conductance is, from Eq. (4.322),

Total conductance is, from Eq. (4.325),

3
6
3
Vaporization at time t = 2 x 0.407 x 10~ (27.7 x 10 -7.44 x 10 ) =
2
3
16.49 x 10~ kgm~ s. The humidity change rate is calculated from Eq.(4.328)
when the square mass of the dry substance of the paper web is
3
2
m"-40.5x10- kg m- .
3 2 1
Heat capacity C" = 40.5 x 10~ ( 1400 + 0.2 x 4186) = 90.6 J rrr K" ,
The temperature rise, Eq. (4.326), is

The velocity of a coated paper web is 17ms and the width of the IR
drier is 0.4 m. Thus the delay time in one drier is 0.4/17 = 0.0235 s. This
yields an indication of the processes inside the drier, using the above calculated
values:

An accurate indication is achieved by carrying out the calculations in
small time steps, such as At = 0.004 s, and then by calculating the vaporiza-
tion, humidity change, and corresponding temperature rise at each time step.
This is the numerical solution of differential equations (4.326) and (4.328).
The results of a calculation of this type are shown in Table 4.12.