Page 159 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER 1 2 I
When a low-temperature heating radiator is painted, the color is selected
according to heat radiation. At this relatively low temperature, the radiation
lies almost outside the visible region. The color may be deceptive. Snow is a
good reflector of visible radiation; still, its total emissivity and therefore its ab-
sorption in normal conditions is as high as 0.98. Visible radiation passes
through glass; its emissivity at the temperature 20 °C is 0,98. Glass radiation
lies within the 300-2800 nm range. An important feature of glass is that it is
opaque to longwave radiation, which is produced by low-temperature emit-
ters. It is this phenomenon that is termed the greenhouse effect.
The radiation emitted by the sun, due to its high temperature, has a short
wavelength. Glass is transparent at this wavelength, allowing the radiation to
pass through into the interior of the building. This energy is absorbed by the
room surfaces, causing them to rise in temperature and to become low-
temperature emitters. The radiation from these low-temperature surfaces is
longwave, to which glass is opaque, and thus the radiation cannot escape
through the glass to the outdoors, resulting in a rise in the space temperature.
The transmission of radiation through the glass depends on the spectral char-
acteristics of the nature of the glass.
Example 3
Silica glass transmits 92% of the radiation in the wavelength region of
0.3-2.7 (xm, and it is impervious to other radiation. Determine the wave-
length of the sun's radiation that the glass transmits when the sun is
treated as a blackbody, T = 5600 K. What happens for a blackbody at a
temperature of 295 K?
According to Eq. (4.204), at the median point of the spectral energy, A 50,
T = 4107 |xrn K, and therefore the median point of the spectral energy of the
sun is

Calculations using Planck's radiation law show which part of the radiation en-
ergy remains in the wavelength range:

4.3.5.3 Lambert's Cosine Law
The radiation power in the direction of the normal vector to the surface
dA in a space angle do) (see Fig. 4.31) is

and in the direction ft,