Document 7RmpLVQbwLk4ojbDZQzNJzEke

98 CHAPTER 6 1951 Guide i perature difference, ((,--lt) which is the temperature of 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), hc, Btu per (hour) (square foot) (Fahrenheit degree). Fig. 1 shows the conditions associated with con vection. The heat transmission by free or natural convection for objects sur rounded by air can be conveniently expressed as in Equation 2a: i-'Gri&r-*- (2a) where -j = heat transmission by convection, Btu per (square foot) (hour). C = 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.). V',,. = average of wall surface and surrounding air temperature, Fahrenheit degrees absolute. t, -- tt -- 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 values* of G 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 the1 simple first-power equation if the convection coefficient is expressed as a unit conductance: where q = h, A (ti -- ti) (2b) 9 = heat transmission by convection, Btu. per hour, A = surface area, square feet. ti-- tt -- temperature difference between the surface and the fluid, Fahrenheit degrees. . - A. = 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- qr = itA,FaFe (TV -- 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 trans fer area of surface 1, q,/A Btu per (hour) (square foot), which sees surface 2 through a non-absorbing medium, is proportional to the difference of the fourth powers of the absolute surface temperatures (2V -- TV). The pro portionality . factor (<jFaFe) may be conveniently separated into three Heat'Transfer 99 Table 2. Appboximate IjNIT^CoNDUOTANCES FOB THERMAL Several Flow Systems CONVECTION FOB Expressed in'Convenient Empirical Form J.----------------:----------------------- ---------- Cask Stbtkm Heat Transfer Equation* and its Limits of Application Refer* ENCE General Equation 3 1--x--*LoneUudinal flow in a circular cylinder*. 2" Same as Case X.D 3 Same as Case l.D 4 Flow normal to a single cylinder. .6 Same as Case 4. 6 Same as Case 4. 7 Uo,rEH 1 8- Flow along a flat plate.' Same as Case 7. where 4.4D < x and (22) > 2200 Equation for Air <ja. he = 5.4 X lfr-HTf)0 * where 4.4D < x and (22) > 2200 Equation for Liquid Water (60-400 F) 4. = 13.9(H)* " ^ where 4.4D < x and (22) > 2200 General Equation -(f)" ofr where 1000 < ---- < 50,000 n Equation for Air he (average) = 0-211(T where 1000 < ^ < 50,000 M Equation for Liquid Water (60-400 F) ho (avera*) ** 35.4(if)*-,w ^ where 1000 < -- < 50,000 n General Equation (Turbulent Flow) A? -0.0295 (?)"()- where (j) > 500,000 Equation for Air (Turbulent Flow) *= - 0SHT,)" where (^) > 500,000 (lT(n(t) " 1J5 ltd 3 3 4 '4 4 3 3