Document ZB8jQpdq4aJ46KXEwY39q40m0

,/3j 104 CHAPTERS 1949 Guide ; Table 2. ' '' Appbohmate Unit Conductances fob Thebbal Convection .fob . Sevebal Flow Systems* Expressed in Convenient Empirical Form CASS; . System ' Unit Conductance Equation* ... Forced Convection 1- Longitudinal flow In cylinders, turbulent region. Fluid being heated*. - 0.0225 For (-^-) > 3000 2. For longitudinal air flow in cylinders 1 he - 0.0036 <?*/** . reduces to*.' For > 3000 fto - 0.00486 (I + 0.010 AA 3. For longitudinal water flow in cylinders case 1 reduces to*. For (-if-) > 3000 a - 4. (A)Air flow normal to a single right circular cylinder. He - 0.45 + 0.178 (A)"-" 5. Air flow over staggered pipe banka. ho.- 0.081 (A )*'" C Fundamentals of Heat Transfer 105 The heat transmission by free or natural convection for objects sur rounded by air can be conveniently expressed as in Equation 2a: /i W i V" . __ , where qa = heat transmission by convection, Btu per (square foot) (hour). q __ a constant depending upon the shape of the surface. D = diameter of pipe or circular duct or height of vertical wall, inches. (Effect of diameter or height becomes constant at 24 in.) f> -- average of wall surface and surrounding air temperature, Fahrenheit degrees absolute. .. t _ it -- temperature excess between wall surface and surrounding air, Fahren heit degrees, - For horizontal cylinders, the value of C = 1.016 has been well estab lished by various investigations. For vertical plates, the value of C = 1.394 has been fairly well established. Suggested values2 of C for hori zontal plates warmer than the surrounding air are 1.79 when facing upward and 0.89 when facing downward: 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: 6. Air flow over single spheres. 7. Air flow over plane surfaces. 8., Air flow normal, to finned cylinders. * - 0-040 AA 0 <t < 250 F h - 1 + 0.22 K. For V < 16 fps or Ac 0.53 K,-* 16 fps < F. < 100 fps (-rr^- 0 <1 < 250 F ' Free Convections V'-. ' Single horizontal right circular cylinder in air. *-<>* (^-r ;, :; 10. Vertical surfaces in air. *0 - 0.3 11. Top surface of horizontal plates to air. ho - 0.4 12. Bottom surface of horizontal plates to air. ho = 0.2 * Heat Transmission, by W. H. McAdams. k Fluid properties should be evaluated at the arithmetic mean fluid temperature, tf *=. (I*urfae +1 fluid) divided by 2. ' * ; > '' : ' 9 These expressions are applicable to longitudinal flow in other than right circular cylinders provided"the hyrfrpnHn ro/finft fa o^iploytsf] ftp tha 'iifc dm*>n*^rin For non-circular croea-eectiona D 4 fig - d For low rates of-heat transfer by free convection the exponent decreases towards zero, and for. higher . .. jrnatetbssininctreearmseesdtioawtearradnsg0e.33 The following equations employing an exponent equal to 0.25 are applicable where q0 a heat transmission by convection, Btu per hour. A = surface area, square feet. lt -- = temperature difference between the surface and the fluid Fahrenheit degrees. * h0 = unit conductance, from Table 2, Btu per (square foot) (hour) (Fahren heit degree temperature difference.) Thermal Radiation Equation The relation shown in Equation 3 is usually applicable to systems in NOMENCLATURE AND DIMENSIONS FOR TABLE 2 Cp = fluid unit heat capacity at constant pressure, Btu per (pound) (Fahrenheit degree). D = cylinder, diameter, feet. G = 3600 Vtp = fluid mass velocity, pounds per (hour) (square foot of flow :cross-section). p = density, pounds per cubic foot. ha = unit conductance for thermal convection, Btu per (hour) (square foot) (Farhrenheit degree). . k = unit thermal conductivity of the fluid, Btu per (hour) (square foot) (Fahrenheit degree per one foot thickness). Rn = hydraulic radius of the flow cross-section = flow cross-section area.per wetted perimeter/feet. s = fin spacing, feet. t = average fluid film temperature, Fahrenheit degree, h--fi = temperature difference surface to main fluid, Fahrenheit degree. Vt = fluid velocity, feet per second. P = fluid viscosity, pounds per (hour) (foot) = viscosity in centipoises X 2.42.