Document rBLMyk7yJy5ZK2enp8BBQMzzv

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 The increased rate of heat transfer due to forced convection can be calculated from Equation 2b: where 2fc = 1 + 0.225 V (2b) qic -- 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: where 2c = h A (<I - t,) (2c) 2c = heat transmission by convection, Btu per hour. A = surface area, square feet. -h -- h = temperature difference between the surface and the air, degrees Fahrenheit. h = unit conductance, from Table 5, Btu per square foot per hour per degree Fahrenheit temperature difference. .. . . --_ NOMENCLATURE AND DIMENSIONS FOR TABLE 5 . cp = fluid unit heat capacity at constant pressure, Btu per pound per degree Fahrenheit. D = cylinder diameter, feet. G = 3600 Kg-f = fluid mass velocity, pounds per .hour per square foot of flow cross-section. 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. . rH = hydraulic radius of the flow cross-sectionc. = flow cross-section area per wetted perimeter, feet. s = fin spacing, feet. t = average fluid film'temperature, degree Fahrenheit. At = temperature difference surface to main'fluid, degree Fahrenheit. = fluid velocity, feet per second. . li = fluid viscosity, pounds per hour per foot = viscosity in centipoises X 2.42. Thermal Radiation Equation . The relation shown in Equation 3 is usually applicable to systems in which radiant exchange takes place between the surfaces of solids, as sche- 2r = gAiFaFb (TV - 7Y) (3) 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 - 76 . CHAPTER 3. FUNDAMENTALS OF HEAT TRANSFER 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 h = 0.0036 * 2. For longitudinal air flow In cylinders case 1 reduces too. For ("--) > 3000 h = 0.00486 (1 + 0.011) 3. For longitudinal water flow in cylinders ease l reduces too. For > 3000 4- Air flow normal to a single right circular A = 0.45 (--) + 0.178 0- (--)" cylinder. 5. Air flow over staggered pipe banks. A = 0:061^J 6. Air flow over single spheres. 7. Air flow over plane surfaces. 8. Air flow normal to finned cylinders. G4J *" 0.040 -^- 0 < / < 250 F h = 1 + 0.22 For Vt < lfrfps or h = 0.53 PV-* 16 fps < Vs < 100 fps /G h " 62 V3600 / 0 < t < 250 F s\M Free Convection* `o*(f)"9. Single horizontal right circular cylinder in air. 10. Vertical surfaces in air. A = O.3(A0, 11. Top surface of horizontal plates to air. * A = 0.4 (AO 12. Bottom surface of horizontal plates to air. H = 0.2 (A0* " Heat Transmission, by W. H. McAdams. bFluid properties should be evaluated at the arithmetic mean fluid temperature, t{ = (^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 D = 4 re. . 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. 77