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102 . CHAPTER 5 1948 Guide b. For large parallel planes, long concentric cylinders or large enclosed bodies, use both emissivities in the equation: ,Fb = ei e. The radiation under black-body conditions, or for an emissivity of 1.0, is given in Table 7* 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 31. Fig. 3. Radiation Between Surfaces Fig. 4. Heat Transfer Conditions in an . Insulated Cold Water Line NOMENCLATURE AND DIMENSIONS FOR TABLE 5 ep = fluid unit heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder diameter, feet. G = 3600 Fsp = fluid mass velocity, pounds per (hour) (square foot of flow cross-section). p = density, pounds per cubic foot. . Ac = unit conductance for thermal convection, Btu per (hour) (square foot) (Fahrenheit degree)'. k = unit thermal conductivity of the fluid, Btu per (hour) (square foot) (Fahren heit degree per one foot thickness). Ra = hydraulic radius of the flow cross-section = flow cross-section area per wetted perimeter, feet. . t = fin spacing, feet. t = average fluid film temperature, Fahrenheit degree. ti-- h = temperature difference surface to main fluid, Fahrenheit degree. V, = fluid velocity, feet per second. (i = fluid viscosity, pounds per (hour) (foot) = viscosity in centipoises X 2.42. 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 Fundamentals of Heat Transfer 103: Table 5i Approximate Unit Conductances for.Thermal Convection for Several Flow Systems . Expressed in Convenient Empirical Form . Case System Unit Conductance' Equation** Forced Convection Longitudinal flow in cylinders, turbulent 1. region. Fluid being heated. For (-^-) > 3000 he " 0.0036 * 2. For longitudinal air flow in cylinders case 1 reduces to*. For ("^-) > 3000 Ac = 0.00486 (1 + O.OII) 3. For longitudinal water flow in cylinders case 1 reduces toe. . For (-5^-) > 3000 4. Air' flow normal to a single right circular A. - 0.45 (-^-) + 0.178 G cylinder. 5. Air flow, over staggered pipe banks. Ac-- 0.061 )** 0>a 6. Air flow over single spheres. 7. Air flow over plane surfaces. G-a " 0.040 0 < / < 250 F he - 1 + 0.22 V% . For V < 16 fps or he - 0.53 Vf* 16 fps < Vb < 100 fps 8. Air flow normal to finned cylinders. 0 < < < 250 F Feee Convection** 0. Single horizontal right circular cylinder In air. Ac = 0.23 (--) 10. Vertical surfaces in air. he - 0.3 11. Top surface of horizontal plates to air. i 0 7 6 0 X 12. Bottom surface of horizontal plates to air. he = 0.2 Heat Transmission, by W. H. McAdams. *>Fluid properties should be evaluated at the arithmetic`mean fluid temperature, it m (^surface *fluid) divided by 2. These expressions are applicable to longitudinal flow.in other than right circular cylinders provided the hydraulic radius is employed as the conduit dimension parameter. For non-circular cross-sections'0 TM 4J?a* ;' <*For low rates of heat transfer by free convection the exponent decreases towards zero, and for higher rates increases towards 0.33.` The following equations employing an exponent equal to 0.25 are applicable in the intermediate range..