Document ZnbvaZw4kGgnzRx6jg21Rd8bZ

96 CHAPTER 5 1954 Guide: Heat Transfer radiation which exists. Emissivities or absorptivities (e). for many common surfaces, are given in Table 3. . The value of Fe for large parallel planes, long concentric cylinders, or large enclosed bodies igl e (1/ei + 1/e, -- 1). The radiation under black-body conditions, or for an emissivity of 1.0, is given in Table 47 for cold surfaces as low as -- 39 F to wanner 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 8). Photographic and other methods are given in References 9 and 10. 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 radiaiion by the equation: q, = h, A (ti -- t) (4) Table 4. Heat Transmission by Radiation fob Black-Body Conditions* Expressed in Btu per (square fool) (hour) Temp F Deo 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 59.3 65.2 71.4 78.0 0 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 58.2 64.1 70.1 . 76.7 +i. 78.7 85.7 93.3 101 110 119 128 138 149 160 171 184 197 212 +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 95.6 104 112 122 131 143 152 163 175 188 201 217 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 98.0 108 115 124 134 145 155 167 179 192 206 221 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 . +? 84.3 91.7 99.6 108 117 126 136 147 157 169 182 195 209 224 * Example: Radiation from walla 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). 97