Document YrVyLNVVkaqG4zBLKN1oQ1ZXV

y8 CHAPTER 5 1953 Guide Fig. 4. Geometrical Factor F For Direct Radiation Between an Element dA and a Parallel Rectangle* RADIATION between adjacent RECTANGLES IN PER PENDICULAR PLANES 'RATIO (|.ENGTH OF UNIQUE SIDE OF THAT RECTANGLE ON WHOSE AREA THE HEAT TRANSFER EQUATION IS SASed) Length of common SlOOY/XIN SKETCH z-ratio (Length of UNIQUE SIDE OF OTHER RECTANClOI (LENGTH OF COMMOnI SIOQ-Z/X IN SKETCH ' Heat Transfer -99 : The conductance hr thus defined is a function of the shape-emissivity factor, as well as the temperatures of the radiator and receiver.' ; Fig. 7 shows a plot of the equivalent conductance for two black bodies (i.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 desirable'to treat convection and radiation as a single combined process, using a first-power equation: <7ro -- Arc A (tl /,) (6) . where grc is the total heat flow due to radiation and convection, in Btu per hour. Values of Arc,', the surface or film conductance for combined Fig. 5. DIMENSION RATIO Z Geometrical Factor F for Direct Radiation Between Adjacent Rectangles in Perpendicular Planes* l radiation between parallel I PLANES,DIRECTLY OPPOSED I-2-3-* DIRECT RADIATION i 6CTWCEN THE PLANES,F | PLANES CONNECTED BY MON-CONOUCTING BUT RCRAOIATING WALLS F 1-5 DISCS y-t 2 = 1 RECTANGLE 12-6 SOUARES 4-0 LONG,NARROW RECTANGLES T IT pT ITjTTTTT Fig. 6. RATIO SIDE OR OIAMETER OISTANCE BETWEEN PLANES Geometrical Factor F for Direct Radiation Between Opposed Parallel Rectangle and Discs of Equal Size* ' From Radiant Heat Transmission, by H. C. Hottel (.Mechanical Engineering, July 1930, pp. 700 to 702) T, Fig. 7. Equivalent Conductance for Radiation Between Two Black Bodies Exchanging Energy Only with One Another radiation and convection, are given in Chapter 9, (Table 1 and Fig. 4). Complete tables for the combined heat transfer of steam and hot water radiators, pipes, coverings, etc., will be found in the appropriate chapters. 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 are effective, and the thermal current flows through several resistances in series or in parallel. In using the resistance concept, the calculations in volved are analogous to the application of Ohm's Law in electricity, viz., the heat flow or thermal current is directly proportional to the thermal potential or temperature difference, and inversely proportional to. the thermal resistance: tl -- It Qn -- R (6)