Document NEkY1MVKm4yEvQ6odLy46e46p

96 CHAPTER 5 .1952 Guide Fig. 3. Radiation Between Surfaces radiation which exists. Emissivities or absorptivities (e) for many common surfaces, are given in Table 3. The value of Fb for large parallel planes, long concentric cylinders, or large enclosed bodies is 1 (l/et + l/ -- 1).' - The radiation under black-body conditions, or for an-emissivity of 1.0, is given in Table 4a for cold surfaces as low. as --39 F to warmer surfaces as high as 139 F. Some net radiation exchange solutions for several common radiation systems are given in Table 5. There are several methods by which the geometrical factors FA can be determined. One. method involves the use of a mechanical geometrical integrator (Reference 7). . Photographic and other methods are given in References 8 and 9. Equivalent Conductance for Radiation Although Equation 3 is a suitable equation for describing radiant exchange, it is not convenient for computations where other modes of energy transfer , are operative. For such, cases, it is convenient to define an equivalent conductance for radiation by the equation: q, = k, A (ti - tj) . (4) TAble 4. Heat Transmission by Radiation for Black-Body Conditions* Expressed in Btu per (square foot) (hour) Temp F- Deo 6 -1 -2 -3 -4 -5 -6 -7 -8 -9 -30 -20 -10 0 59.3 65.2 71.4 78.0 0 0 10 20 30 .40 50 60 70 80 90 " 100 110 120 , 130 . 78.0 ' 85.0 .92.4 100 109 118 127 137 148 159 170': 183 196 211 .58.7 64.7 70.8 77.4 -H 78.7` 85.7 93.3 101 110 119 128 138 149 160 171 184 197 212 58.2 64.1 70.1 76.7 +2 79.4 86.5 94.0 102 111 120 129 139. . 150 161 173 185 199 214 57.7 63.5 69.5 76.0 +3 80.1 87.2 94.8 103 112 121 130 140 151 162 174 187 200 215 57.2 62.9 68.9 75.4 +4 80.8 88.0 95.6 104 112 122 131 142 152 163 175 188 201 217 56.7 62.3 68.3 74.7 +5 81.5 88.7 96.4 105 113 123 132 143 153 164 176 189 . 203 218 56.2 61.7 67.7 74.0 +6 82.2 89.4 97.2 105 114 123 133 144 154 166 178 191 204 220 55.7 61.1 67.1 73.4 +7 82.9 90.2 08.0 106 115 124 134 145 155 167 179 192206 221 55.2 60.5 66.4 72.7 +8 83.6 90.9 98.8 107 116 125 135 146 156 168 180 193 207 222 54:7 59.9 65.8 72.1 +9 84.3 : 91.7 99.6 108 117 126 136 147 157 169 182 195 209. 224 * Exam-pU: Radiation from walls of room at 32 F to surface at --25 F for effective emissivity of 0.95 =* (102 -- 62.3) 0.95 = 37.7 Btu per (square foot) (hour). Heat Transfer 97 - Table 5. - Net Radiation Solutions System Solution Remabxs Two infinite parallel planes. el e3 ^2 ei*e2' 7 \--------.(TV - TV) Considering interrefiections. (Reference 5). ci a \ G)L+\_1'(ri` m Considering interrefiections. A (References) Cl Ci One radiation shield between two infinite parallel planes. '1 , .A . n + 1 Considering interrefiections. . (Reference 5) ere23 *en where is the net radiation ex change without the shields'. n radiation shields between two infinite parallel planes. - T = 1----- -T77------- -.(TV - TV) Considering interrefiections and diffuse surfaces. (Ref . ei 3i\ / erence 5)- Two concentric spheres or two infinitely . long cylinders. - = .1 m.Fa(7V - TV) Surface diffuse, neglecting interreflection. ` (Refer ence 5) Two areas dAi and dAt . OI* *1 2-Jn = (f) Neglecting inteireflections. (Reference 5) where N is the length of cylinder from which qr is exchanged Tube of in inite length parallel to an infinite wa ?_ f Surfacesare perfect radiators. See-Fig. ,4 * n . (Reference 4) Surface ele nent tMand rectangle above and para] el to it, with one corner of reotangle co atained in normal to dA. -t S. Surfaces are perfect ra- `' . See Fig. 5 ' (Reference 4) fry diators. ^'planes* rectflnles >n perpendicular Surfaces are perfect ra diators. -See Fig. '6' (Reference 4) Opposed ps rallel rectangles and disks of equal size