Document dQ3ovD608NxLm3JkL6JJm4759

HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 difference of the fourth powers of the absolute surface temperatures' (7\4 -- 7Y). The proportionality factor (a FAFE) may be conveniently separated into three parts: o = the Stefan-Boltzmann radiation constant. = 1730 X 10-11 Btu per hour per square foot per degree Fahrenheit absolute temperature to the fourth power. F* = the angle factor is dimensionless and ^ 1. This factor accounts for the relative geometry of the two surfaces, and is called the shape factor. A value of Fa = 1 may be used in the cases of large parallel planes, long concentric cylinders or small bodies in large enclosures. (For other values see References.) Fk =[the emissivity factor is also dimensionless and ^ 1. This factor accounts for the absorption and emission characteristics of the surfaces for the radiation which exists. Individual emissivities (e) should be taken from Table 6 and applied, for either radiation or absorption, as follows: a. For a small body in a large enclosure, use the emissivity of the small, body only: Fb = i. b. For rectangles or disks, either parallel or perpendicular and with a common side, use the product of the emissivities: Fb = ei X e* c. For large parallel planes, long concentric cylinders or large enclosed bodies, use both emissivities in the equation: Fb = -- + ------------ The radiation under black-body conditions, or for an emissivity of 1.0; is given in Table 76 for cold surfaces as low as -- 39 F to warmer surfaces as high as 139 F. The emissivities of a number of surfaces ordinarily encountered in engineering practice are shown in Table 6; For radiation table at higher temperatures, and further discussion of radiation calcu lations, see Chapter 45. Combined Convection and Radiation It should be noted that the previous equations and tables give the heat transfer by convection and by radiation computed separately: In many Heat Insulation in Air Conditioning, by R. H. Heilman (Industrial end'Engineering Chemistry, VoL 28, July 1936. p. 782). ' CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER Table 6. Radiation Factors or Emissivities For the determination offactor Fs in Equation S Class ___ 1 Surfaces A small hole in a large box. . sphere, furnace, or enclosure Fraction of Black-Body Radiation At 50-100 F At 1000 F Solar Radiation 0.97 to 0.99 0.97 to 0.99 0.97 to 0.99 2 Black non-metallic surfaces such as asphalt, carbon, slate, paint, paper.................................. 0.90 to 0.98 0.90 to 0.98 0.85 to 0.98 3 Red brick and tile, concrete and stone, rusty steel and iron, dark paints (red, brown, green, etc.).................................... 0.85 to 0.95 0.75 to 0.90 0.65 to 0.80 4 ' Yellow and buff brick and stone, firebrick, fire clay-----................. 0.85 to 0.95 0.70 to 0.85 0.50 to 0.70 5 White or light-cream brick, tile, paint or paper, plaster, white wash..........-.................................... 0.85 to 0.95 0.60 to 0.75 0.3 to 0.5 6 Window glass.. ............................... 0.90 to 0.95 Transparent 7 Bright aluminum paint; gilt or bronze paint 0.4 to 0.6 0.3 to 0.5 8 Dull brass, copper, or alumi num; galvanized steel; pol-. ished iron................... ................. /. 0.2 to 0.3 0.3 to 0.5 0.4 to 0.65 - 9 Polished brass, copper, monel metaL ........................................ ^ 0.02 to 0.05 0.05 to 0.15 0.3 to 0.5 10 Highly polished aluminum, tin plate, nickel, chromium............. 0.02 to 0.04 0.05 to 0.10 0.10 to 0.40 practical cases it is desirable to treat convection and radiation as a single combined process, using a first-power equation: 2rc = A (h -- It) (4) where qTC is the total heat flow due to radiation and convection, in Btu per hour. Values of hrC, the surface or film conductance for combined radiation and convection are given in Chapter 4, Table 1 and Fig. 1. Complete tables for the combined heat transfer of steam and hot water radiators, pipes, coverings, etc., will be found in the appropriate chapters. When dealing with the effect of operating temperatures upon the com bined heat transfer of a given piece of equipment (as for instance a steam radiator), another-form of equation is frequently used: Src. = B A (1, -- /,)" (5) Values of in this equation usually range from 1.3 to 1.5 (see Chapter 13). The chief advantage of this equation is the convenience of representing 79