Document Qked1593YapRJrD8mMweKjXn4

100 CHAPTER 5 1951 Guide Table 2. Approximate Unit Conductances for Thermal Convection for Several Flow Systems (<Concluded) System .Heat Tbanbfeh Equation* and its Limits or Application |RepebENCB Forced Convection 9 Same as Case 7. General Equation (Laminar Flow) 3 10 Same as Case 7. For < 600,000 Ac (aver***) -- 2Ax Equation for Air (Laminar flow) hex - 0-0562(7',)"- For (^) < 500,000 3 Equation for Air Free convection past a heated horizontal cylinder. T1 10* < iVGr < 107 Equation for Air e 1 __ Free convection past a single vertical sur face. JK 101 < WGr < JO' ` (KT'(fya 10* < NGr < iQi. Equation for Air Free convection past a heated horizontal surface (face up). 10* < Nqt< 10T Free convection past a heated horizontal surface (face down). Equation for Air --(^r(7r 10* < nQt < 107 diviBdeFdlubidyp2r.operties should be evaluated at the arithmetic mean fluid temperature, tj = Usurface +1fluid) b These expressions are suitable approximations to longitudinal flow in other than right circular cylinders, provided t-he hydrauiio diameter is employed as the conduit dimension parameter. For non-circular crossseetecrti.ons, the hy- draulic diameter is equal to four times the cross-sectional area divided by the wetted perim 0 For low rates of heat transfer by free convection the, exponent decreases towards zero, and for higher rates, increases towards053. The above equations employing an exponent equal to 0.25 are applicable in the intermediate range indicated. NOMENCLATURE AND DIMENSIONS FOR TABLE 2 Cp => heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder diameter, feet. f = subscript denoting film. Heat TranInsaf.ev.r AVA G 3c6r0o0ssi-wsec=tionfl)u. id "ass velocity, pounds per (hour) (square foot of flow" Ngt Grashof modulus, dimensionless. A. (FahreSdegree)CtanCe ^ COnVection' Btu Per 0>our) (square foot) tChfctoCeltty' Bt" Per (8<IUare f00t) (Failrenhei` degree i m P Fo ( T a dimension of the system, feet, a subscript denoting mean. pressure, atmospheres, pressure (atmospheric) atmospheres, temperature, Fahrenheit, temperature, Fahrenheit, absolute, . x A J" = = = fluid velocity, feet per second, a dimension of the system, feet, an increment of the quantity referred to. fluid viscosity, pounds per (hour) (foot). '---- nmiriHfl 'net* cubic foot. Table 3. Radiation Factors or Emissivities, * For the determination of factor Fzin Equation 3 Class Surfaces Fraction of 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, carbon, 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, 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. 0.85 to 0.95 0.60 to 0.75 0.3 to 0.5 6 0.90 to 0.95 7 Bright aluminum paint; gilt 0.4 to 0.6 Transparent 0.3 to 0.5 8 Dull brass, copper, or alumi num: galvanized steel; pol0.2 to 0.3 0.3 to 0.5 0.4 to 0.65 9 Polished brass, copper, monel 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 Emissivitiea of other materials may be found in Reference 4. * Reflects about 8 percent. parts (excepting in some problems involving interreflections, where it is not possible to divide the product (FaFe) into separate terms): o = the Stefan-Boltzmann radiation constant = 1730 X 10-11 Btu per (hour) (square foot) (Fahrenheit degree absolute temperature to the fourth, power). Fa at 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. F* = the emissivity factor which is also dimensionless and ^ 1. This factor ac counts for the absorption and emission characteristics of the surfaces for the