Page 158 - Industrial Ventilation Design Guidebook

I 20 CHAPTER 4 PHYSICAL FUNDAMENTALS

FIGURE 4.30 Radiative bodies.

when the radiant energy d$> passes through a surface element dA in the direc-
tion of its normal vector, in a space angle element dot.
For a black object,

A gray object absorbs only part of the incoming radiation: incoming
= aL v d v = outgoing = eL mv dv.
From Kirchhoff's law,

The radiation intensity from a surface to a semispace is temperature dependent on

The radiation intensity absorbed by the surface is

Emissivity is strongly dependent on the surface quality. The emissivity of a
rough surface is greater than that of a smooth surface, increasing the rate of ab-
sorption. Emissivity values are found in textbooks. Care must be taken when
using these values, as they usually denote total emissivities. The emissivity is
considered constant in the spectrum, and this may be a poor approximation.
For example, the emissivity of white paper is high ( « 0.93 ) at room tem-
perature and, according to Kirchhoff's law, the absorption factor at the same
temperature is also high. At relatively low temperature, as in this case, the ra-
diation is concentrated in the long wavelengths, according to Wien's law.
However, when the same paper receives radiation from the sun, at a radiation
temperature of 6000 K, when the absorption factor is small, this radiation has
a short wavelength. A white object is a good reflector; this is why white cloth-
ing is used in the tropics.