Document 7MKGLJ0zB0e1pvZ60Y8exDj6B

106 CHAPTERS 1949,Guide, which radiant exchange takes place between the surfaces of solids, as sche- ?r = oAlFt.Fz (TV - TV) (3) matically shown in Fig. 3. Gaseous and luminous radiation are not con ,sidered in this discussion. Equation 3 states that the net radiation per unit transfer area of surface 1 q,/A Btu per (hour) (square foot), which sees surface 2 through a non-absorbing medium, is proportional to the difference of the fourth powers of the absolute surface temperatures (TV -- Tt*). The proportionality factor (oFaFe) may be conveniently separated into three parts: a = the Stefan-Boltzmann radiation constant >= 1730 X 10-11 Btu per (hour) (square foot) (Fahrenheit degree absolute temperature to the fourth power). Fi <= the configuration factor which is dimensionless and S 1. This factor accounts for the shape and relative position of the two surfaces.' The value of Fa = 1 may be used in the cases of large parallel planes, long concentric cylinders or smaller bodies in large enclosures. Fb <= the emissivity factor which is also dimensionless and g 1. This factor ac- Fio. 3. Radiation Between Surfaces counts for the absorption and emission characteristics of the surfaces for the radiation which exists, individual emissivities (e) should be taken from Table 3 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 = . " b. For large parallel planes, long concentric cylinders or large enclosed bodies, use both emissivities in the equation: The radiation under black-body conditions, or for an emissivity of 1.0, is given in Table 4' 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 3. For radiation table at higher temperatures, and further discussion of radiation calcu lations, see Chapter 31. 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 practical cases it is desirable to treat convection and radiation as a single combined process, using a-first-power equation: - ffrfl = hro A (ll fa) (4) where qn is the total heat flow due to radiation and convection, in Btu Fundamentals of Heat Transfer ' ` ^ praedriahtoiounr.anVdacluoensveoctifotnh, aere gsiuvrefancemoCr hfialmptecro6n,du(Tctaabnlceedfoarndc'oFmi?binqeTd Complete tables for the combined heat transfer-of stZ Ld hof water' radiators, pipes, covenngs, etc., will be found in the Wh Almg .ith th, eff, ,,( ope,,^ Table 3. Radiation Factors or Emissivittes, e. For the determination of factor Fb in Equation S Class Surfaces FractioRnaodpiaBtiloanck-Body . , At.50-100 F At 1000 F Absorptivity RaSdfoioalrtaison 1 A small hole in a large box, sphere, furnace, or enclosure 0.97 to 0.99 0.97 to 0.99 0.97 to 0.99 2 Black non-metallicsurfaces 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.)---------------------r-- 0.85 to 0.95 0.75 to 0.90. 0.C5 to 0.80 4 . Yellow and buff brick and stone, firebrick, fire clay.------------- -- 0.85 to 0.95 0.70 to 0.85 0.60 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___ i---- ------ -- 0.90 to 0.95 -- Transparent4 7 Bright aluminum paint; gilt or bronze paint________________ 0.4 to 0-6 0.3 to 0.5 S 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 Reflects about'8 per cent bined heat transfer of a given piece of equipment (as for instance a steam radiator), another form of equation is frequently used: qn = B A (h - l,) (5) Values of n in this equation usually range from 1.3 to 1.5 (see Chapter 25). The chief advantage of this equation is the convenience of representing heat transfer performance on logarithmic coordinates, and the factor B should be regarded as a simple constant of proportionality. HEAT-FLOW RESISTANCE In most of the steady-state heat transfer problems encountered in air conditioning applications, more than one of the heat transfer mechanisms are effective, and the thermal current' flows through several resistances in.