Page 163 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER i 25

When P] = 1 - oti,

Equation (4.234) is valid for two coaxial cylinders and spheres. If A 2»A 1,
a-, ~ e 2, as almost all the radiation from surface 2 is reflected back to it.

For two planes whose dimensions are large compared with the distance
between them,

It is difficult to estimate the absorption ratio. Approximately o^ = €i(T 2 ) and
= € T
«2 2( i)-
When the absorption relations are not dependent on temperature, the fol-
lowing approximations can be used (a = e = constant = 1 -p). For two co-
axial cylinders and spheres,

For two parallel planes, A l ~ A 2,

WhenA 2 »A l5

In HVAC technology the following formula is used for small temperature dif-
ferences with sufficient accuracy (see Fig. 4.33):

Example 4
Radiation beat transfer. The radiation heat transfer between two parallel
planes is reduced by placing a parallel aluminum sheet in the middle of the
gap. The surface temperatures are 9 l = 40 °C and 0 2 = 5 °C, respectively; the
emissivities are e-j = e 2 = 0.85. The emissivity of both sides of the aluminum