Document dYZ56B1MN55kJoj9k3znBpO1b

104 CHAPTER 5 1946 Guide b. For rectangles or disks,' either parallel or perpendicular and with a common side, use the product of the emissivities: Fb = e* X e. c. For large parallel planes, long concentric cylinders or large enclosed bodies, use. both emissivities in the equation: Fe ei 6j The radiation under black-body conditions, or for an emissivity of 1.0; is given in Table 74 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. Surfaces Insulated Cold Water Line NOMENCLATURE AND DIMENSIONS FOR TABLE $ cp = fluid unit heat capacity at constant pressure, Btu per (pound) (degree Fahrenheit). .. D = cylinder diameter, feet. G = 3600 Yep -- fluid mass velocity, pounds per (hour) (square foot of flow cross-section). p = density, pounds per cubic foot. \ he -- unit conductance for thermal convection,. Btu per (hour) (square foot) (degree Fahrenheit). ' k = `unit thermal conductivity of the fluid, Btu per (hour) (square foot) (degree Fahrenheit per one foot thickness). Rm = hydraulic radius of the. flow cross-section = flow cross-section area per wetted perimeter, feet. ` ' $ = fin spacing, feet. t = average fluid film temperature, degree Fahrenheit. it--ti ~ temperature difference surface to main fluid, degree Fahrenheit. V8 = fluid velocity, feet per second. (a = 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 105 Table 5. Approximate Unit Conductances for Thermal Convection for Several Flow Systems3 Expressed in Convenient Empirical Form Case System Unit Conductance Equation!* Forced Convection Longitudinal flow in cylinders, turbulent 1. region. Fluid being heatedo. For > 3000 he " ,0.0036 G*/&* 2. For longitudinal air flow in cylinders case 1 reduces toe. For (-^-) > 3000 *-ftc = 0.00486 (1 + 0.010 3. For longitudinal water flow in cylinders case 1 reduces toe. For (-pp~) > 3000 4. Air flow normal to a single right circular fc> = 0.45 (-5- ) + 0.178 (-5-)' " cylinder. 5. Air flow over staggered pipe banks. / fr \*41 A* = 0.061 6. Air flow over single spheres. 7. Air flow over plane surfaces. G,* Ac 0 040 "pTa 0 < t < 250 F he - 1 + 0.22 VB For V% < 16 fps or Ac - 0.53 V.* * 16 fps < Ve < 100 fps . . 8. Air flow normal to finned cylinders. 0 < t < 250 F ' Free Convection^ 0. Single horizontal right circular cylinder in air. " -23 Vo--) 10. Vertical surfaces in air. he = 0.3 11. Top surface of horizontal plates to air.' Ac = 0.4 (i--/*> * 12. Bottom surface of horizontal plates to air. he = 0.2 </i-*)- Heat Transmission, by W. H. McAdams.- bFluid properties should be evaluated at the arithmetic mean fluid temperature. It (^surface *1" *fluid) divided by 2. s - Thee 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 D * 4 Rq. dFor 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. `