Document EvGnKRE49NwXyYngQon73bNM4

1p` I if .1 .1 HEATING VENTILATING AIR CONDITIONING GUIDE 1943 CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER I: Table 5. Approximate Unit Conductances for Thermal Convection for i;: Several Flow Systems3 Expressed in Convenient Empirical Form Case System Unit Conductance Equation!* Forced Convection 1. Longitudinal flow in cylinders, turbulent region. Fluid bemg heated. For ( 4rG ) > 3000 h = 0 0036 C*4/(4rH)*- 2. For longitudinal air flow in cylinders case 1 reduces toe. For ( 4'h<') > 3000 The increased rate of heat transfer due to forced convection can -be calculated from Equation 2b: 3fc = 1 + 0.225 V (2b) where gtc = heat transfer by forced convection, Btu per square foot per hour per degree Fahrenheit temperature difference. V = velocity of air, feet per second. This equation is approximately correct for large surfaces exposed to air currents at temperatures of approximately 70 to 80 F. Problems in either forced convection or natural convection may be solved by the simple first-power equation if the convection coefficient is expressed as a unit conductance: h = 0.00486 (1 + 0.010 i 3. For longitudinal water flow in cylinders case 1 reduces toe. 4h For () > 3000 3 .M i 4 Air flow norma! to a single right circular *- '45(4-) + 0.178 C- (A)"" i cylinder. 4 5. Air flow over staggered pipe banks. / k \-*i h = 0.061 f) G qc = h A 0, - tt) (2c) where qc = heat transmission by convection, Btu per hour. A = surface area, square feet. h -- tt = temperature difference between the surface and the air, degrees Fahrenheit. h -- unit conductance given in Table 5. Thermal Radiation Equation 6. Air flow over single spheres. G*- * - -40 0 < * < 250 F The relation shown in Equation 3 is usually applicable to systems in < which radiant exchange takes place between the surfaces of solids, as sche- . ffr = oAxFaFe (TV - 2Y) (3) 7. Air flow over plane surfaces. 1 = 1 + 0.22 Fa ' For F# < 16 ft per second or h - 0.53 Fa0'* 16 ft per second < Fa < 100 ft per second; matically shown in Fig. 3. Gaseous and luminous radiation are not consid ered in this discussion. Equation-3 states that the net radiation current per unit transfer area of surface 1, qr/A Btu per hour per square foot, which sees surface 2 through a non-absorbing medium, is proportional to the 8. Air flow normal to finned cylinders. h = 6,2 (/"36G00"\) 04 !** . 0 < / < 250 F NOMENCLATURE AND DIMENSIONS FOR TABLE 5 Free Convection*! Cp = fluid unit heat capacity at constant pressure, Btu per pound per degree Fahrenheit. 9. ' ,8 - 0.23 (-)" Single horizontal right circular cylinder in air. D = cylinder diameter, feet. G -- V9y = fluid mass velocity, pounds per hour per square foot of flow cross section. 10. Vertical surfaces in air. h = 0.3 (AO*-** 11. Topi surface of horizontal plates to air. h - 0.4 (Al) *" 12. Bottom surface of horizontal plates to air. h = 0.2 (A/)0'*5 Y = density, pounds per cubic foot. < h = unit conductance for thermal convection, Btu per hour per square foot per degree Fahrenheit. k = unit thermal conductivity of the fluid, Btu per hour per square foot per degree Fahrenheit for one foot thickness. rj, = hydraulic radius of the flow cross section0. "Heat Transmission, by W. H. McAdams. bFluid properties should be evaluated at the arithmetic mean .fluid temperature, tt (Surface + *fluid) divided by 2. . oThese expressions are applicable to longitudinal flow in other than right circular cylinders provided the hydraulic radius is employed as the conduit dimension parameter. For right circular cylinders 4rH = D.' 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. = flow cross section area per wetted perimeter, feet. s = fin spacing, feet. ' / = average fluid film temperature, degree Fahrenheit. . At = temperature difference surface to main fluid, degree Fahrenheit. Vs = fluid velocity, foot per second. .(jl = fluid viscosity, pounds per hour per foot. = viscosity in centipoises X 2.42 = viscosity in pounds per hour per foot. 76 77 .