Document KR06dX5e870LnLQr1JbMxQv6K

94 CHAPTER 5 ig53 Guide Table 2. Approximate Unit Conductances fob Thermal Convection fob VtCTmS < r P* C*---- /rfTf- - T ' Cash System Heat Transfer Equation* and iiw Limits of Application Forced Convection encb NOMENCLATURE AND DIMENSIONS FOR TABLE 2 cp = heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder diameter, feet. / = subscript denoting film. Heat Transfer 95 0 = body force per unit mass, feet per hour per hour. (For static system on earth, g = 32.2 X 36001 feet per hour per hour.) G = 3600 UmP = mass flow per unit cross-sectional area normal to flow, pounds per (hour) (square foot of flow cross-section). IfGr = Grashof modulus, dimensionless (IVg, = D'p*l}&tg/p*). hp = average unit thermal convective conductance from the leading edge of surface to the position x, Btu per (hour) (square foot) (Fahrenheit degree). ha = local unit thermal convective conductance, at the position x from the leading edge of surface, Btu per (hour) (square foot) (Fahrenheit degree). k = thermal conductivity, Btu per (hour) (square foot) (Fahrenheit degree per foot thickness). 1 = a dimension of the system, feet, m = a subscript denoting mean. P = pressure, atmospheres. Pp = pressure (atmospheric) atmospheres. t = temperature, Fahrenheit. T = temperature, Fahrenheit, absolute, u = fluid velocity, feet per second. V = volume, cubic feet. x = a dimension of the system, feet. 1 dV 0 = coefficient of cubical expansion (/3 = y(fjfp)p)', for perfect gases 0 = 1/T. At = difference between wall and fluid temperatures, Fahrenheit degrees. M = fluid viscosity, pounds per (hour) (foot). p = density, pounds per cubic foot. w = infinity, referring the quantity to a point not directly affected by the phenomenon in question. Table 3. Radiation Factors or Emissivities, e* For the determination of factor Fe in Equation 3 Class Surfaces Fraction op Black-Body . Radiation At 50-100 F At 1000 F Absorptivity fob . Solar Radiation 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-metallic surfaces such as asphalt, car bon, 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, 0.85 to 0.95 0.60 to 0.75 0.30 to 0.50 6 0.90 to 0.95 Transparent* - 7 Bright aluminum paint; gilt or bronze paint.... 0.40 to 0.60 0.30 to 0.50 8 Dull brass, copper, or aluminum; galvanized steel; polished iron................................................... 0.20 to 0.30 0.30 to 0.50 0.40 to 0.65 9 Polished brass, copper, monel metal....................... 0.02 to 0.05 0.05 to 0.15 0.30 to 0.50 10 Highly polished aluminum, tin plate, nickel, chromium................................................... ............. 0.02 to 0.04 0.05 to 0.10 0.10 to 0.40 * Emissivities of other materials may be found in Reference 4. n Reflects about 8 percent. fourth powers of the absolute surface temperatures (TV -- TV). The pro portionality factor (vFaFe) may be conveniently separated into three parts (excepting in some problems involving interreflections, where it is not possible to divide the product (FAFE) into separate terms): a -- the Stefan-Boltzmann radiation constant = 1730 X 10-1* Btu per (hour) (square foot) (Fahrenheit degree absolute temperature to the fourth power). Fa = the geometrical factor which is dimensionless and ^ 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. Fe = the emissivity factor which is also dimensionless and ^ 1. This factor ac counts for the absorption and emission characteristics of the surfaces for the