Page 197 - Mechanical Engineers' Handbook (Volume 4)

186 Heat-Transfer Fundamentals

Table 18 Normal Total Emissivity of Nonmetals a
Surface
Temperature Normal Total
Materials (K) Emissivity
Asbestos, board 310 0.96
Brick
White refractory 1370 0.29
Rough Red 310 0.93
Carbon, lampsoot 310 0.95
Concrete, rough 310 0.94
Ice, smooth 273 0.966
Magnesium oxide, refractory 420–760 0.69–0.55
Paint
Oil, all colors 373 0.92–0.96
Lacquer, flat black 310–370 0.96–0.98
Paper, white 310 0.95
Plaster 310 0.91
Porcelain, glazed 295 0.92
Rubber, hard 293 0.92
Sandstone 310–530 0.83–0.90
Silicon carbide 420–920 0.83–0.96
Snow 270 0.82
Water, deep 273–373 0.96
Wood, sawdust 310 0.75

a Adapted from Ref. 19.

concentrated in the longer-wavelength range. A comparison of absorptivities for a number
of different materials is given in Table 19 for both solar and low-temperature radiation.

3.3 Configuration Factor
The magnitude of the radiant energy exchanged between any two given surfaces is a function
of the emisssivity, absorptivity, and transmissivity. In addition, the energy exchange is a
strong function of how one surface is viewed from the other. This aspect can be defined in
terms of the configuration factor (sometimes called the radiation shape factor,, view factor,
angle factor, or interception factor). As shown in Fig. 20, the configuration factor, F ,is
i j
defined as that fraction of the radiation leaving a black surface, i, that is intercepted by a
black or gray surface, j, and is based on the relative geometry, position, and shape of the
two surfaces. The configuration factor can also be expressed in terms of the differential
fraction of the energy or dF , which indicates the differential fraction of energy from a
i dj
finite area A that is intercepted by an infinitesimal area dA . Expressions for a number of
i j
different cases are given below for several common geometries.
to infinitesimal area dA
Infinitesimal area dA j
j
cos cos
dF di dj i j dA j
R 2
to finite area A
Infinitesimal area dA j j