Page 196 - Mechanical Engineers' Handbook (Volume 4)

3 Radiation Heat Transfer 185

Table 17 Normal Total Emissivity of Metals a
Surface Normal
Temperature Total
Materials (K) Emissivity
Aluminum
Highly polished plate 480–870 0.038–0.06
Polished plate 373 0.095
Heavily oxidized 370–810 0.20–0.33
Bismuth, bright 350 0.34
Chromium, polished 310–1370 0.08–0.40
Copper
Highly polished 310 0.02
Slightly polished 310 0.15
Black oxidized 310 0.78
Gold, highly polished 370–870 0.018–0.035
Iron
Highly polished, electrolytic 310–530 0.05–0.07
Polished 700–760 0.14–0.38
Wrought iron, polished 310–530 0.28
Cast iron, rough, strongly oxidized 310–530 0.95
Lead
Polished 310–530 0.06–0.08
Rough unoxidized 310 0.43
Mercury, unoxidized 280–370 0.09–0.12
Molybdenum, polished 310–3030 0.05–0.29
Nickel
Electrolytic 310–530 0.04–0.06
Electroplated on iron, not polished 293 0.11
Nickel oxide 920–1530 0.59–0.86
Platinum, electrolytic 530–810 0.06–0.10
Silver, polished 310–810 0.01–0.03
Steel
Polished sheet 90–420 0.07–0.14
Mild steel, polished 530–920 0.27–0.31
Sheet with rough oxide layer 295 0.81
Tin, polished sheet 310 0.05
Tungsten, clean 310–810 0.03–0.08
Zinc
Polished 310–810 0.02–0.05
Gray oxidized 295 0.23–0.28

a Adapted from Ref. 19.

the outer limit of the atmosphere of approximately 1353 W/m (429 Btu/ft hr). This solar
2
2
irradiation is called the solar constant and is greater than the solar irradiation received at
the surface of the earth, due to the radiation scattering by air molecules, water vapor, and
dust, and the absorption by O ,H O, and CO in the atmosphere. The absorptivity of a
2
3
2
substance depends not only on the surface properties but also on the sources of incident
radiation. Since solar radiation is concentrated at a shorter wavelength, due to the high source
temperature, the absorptivity for certain materials when exposed to solar radiation may be
quite different from that which occurs for low temperature radiation, where the radiation is