Document qdgDYBaeBE6VoNVY5RNXNK9gR

92 CHAPTER 5 1953'Guide perature .'difference : t,) which is the temperature of i the surface less that of-the fluid. The particular fluid temperature to use for. a given system will be noted under the discussion, of that system. The propor tionality factor is termed the unit thermal convective conductance (sometimes called the film coefficient for convection), A0, Btu per (hour) (square foot) (Fahrenheit -degree). Fig.. 1 shows the conditions, associated .with con vection. ... The heat transmission by Tree or natural convection for objects sur rounded byair can be Conveniently-expressed as in Equation 2a:`- jr A . - td}" -. (2a) where heat transmission by convection, Btu per (square foot) (hour). a constant depending upon the shape of the surface, diameter of pipe or circular duct or height of vertical wall, inches. (Effect of diameter or height becomes constant at 24 in.), average of wall surface and surrounding air temperature,.Fahrenheit degrees absolute. temperature excess between wall surface and surrounding air, Fahren heit degrees. . For horizontal cylinders, the. value of C = 1.02 has been well estab lished by various investigations. For vertical plates, the value of C = 1.39 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: q = KA (!, - t,) ... .... ... (2b) where q -- heat transmission by convection, Btu per hour. A = surface area, square feet. fi -- j = temperature difference between the surface and the fluid, Fahrenheit degrees. .' ' ' . Ac = unit convective conductance, from Table 2, Btu per (square foot) (hour) (Fahrenheit degree temperature difference). Thermal Radiation Equation . The relation given by Equation 3 is applicable to systems in which radiant exchange takes place between the surfaces of solids, as schemati se = .tA.FaFe (7V - TV) (3) cally shown in Fig. 3. Gaseous and luminous radiation are not considered in this discussion. Equation-3 states that the net radiation per unit trans4 fer area of surface 1, qj A Btu per (hour) (square foot), which sees surface 2 through a non-absorbing medium, is proportional to the difference of the