Document DMm8pXvKOV5VE9zKjwOQEb2ZN

98 3 I -i !!! i.; 'if' ji 1! : !I CHAPTER S 1952 Guide Heat Transfer 99 - The conductance hT thus defined is a function of the shape-eniissivity factor, as .well as the temperatures of the radiator and receiver. - Fig: 7 shows a plot of the equivalent conductance for two black bodies (t.e., with emissivities equal to unity) which exchange energy only with one another. Combined Convection and Radiation !^ It should be noted that the previous equations and tables give the heat transfer by convection and by radiation computed separately. In many practical cases it is durable to treat convection and radiation as a single combined process, using a first-power equation: . <7,, = hro A (<1 -- /,) (5) where is the total heat flow due to radiation and convection, in Btu per hour: Values of Arc, the surface or film conductance for combined Fio. 5. Geometrical Factor F for Direct Radiation Between Adjacent Rectangles in Perpendicular Planes* Pro. 7: Equivalent Conductance for Radiation Between Two Black Bodies ' Exchanging Energy Only with One Another i radiation and convection, are given in Chapter 9, (Table 1 andFig. 4). 1 Complete tables for the combined heat transfer of steam and. hot; water I! radiators, pipes, coverings, etc., will be found in the appropriate chapters. i . HEAT-FLOW RESISTANCE In most of the steady-state heat transfer problems encountered in`air conditioning applications, more than one of the heat transfer mechanisms fira effective) and the thermal current'flows through several resistances in senes or in parallel. In using the resistance concept, the calculations inf volved are analogous to the application of Ohm's Law in electricity, viz:, "fie heat flow or thermal , current is directly proportional to the thermal Potential or temperature difference, and inversely' proportional' to the Fig. 6. Geometrical Factor P for Direct Radiation Between Opposed V thermal resistance: . . '' Parallel Rectangle and Discs of Equal Size* From Radiant Heat Transmission, by H. C. Hottel (Mechanical Engineering, July 1930, pp. 700 to 702). tfre -- R