Page 242 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 231

Figure 11 Radiation absorptivity of sheet glass with surface reflection deducted. 1

90 at a common edge, the view factor is shown in Fig. 16. If surfaces do not extend to a
common intersection, the view factor for the missing areas can be calculated and deducted
from that with surfaces extended as in the figure, to find the net value for the remaining
areas.
For spaced cylinders parallel to a furnace wall, as shown in Fig. 17, the view factor is
shown in terms of diameter and spacing, including wall reradiation. For tubes exposed on
both sides to source or receiver radiation, as in some vertical strip furnaces, the following
factors apply if sidewall reradiation is neglected:

Ratio C/D 1.0 1.5 2.0 2.5 3.0
Factor 0.67 0.793 0.839 0.872 0.894
For ribbon-type electric heating elements, mounted on a back-up wall as shown in Fig.
18, exposure factors for projected wall area and for total element surface area are shown as
a function of the (element spacing)/(element width) ratio. Wall reradiation is included, but
heat loss through the backup wall is not considered. The emission rate from resistor surface
2
will be W/in. Q/491A, where
Q Btu/hr

A ft 2