Document XRKR4Nay61qBMY16ea0vrpwGJ

. 282 CHAPTER 15 1949 Guide Costing Load 283 For horizontal surfaces the magnitude of K is determined by the time of year, the time of day (sun's position), .and the latitude of the location con cerned. For vertical surfaces, a fourth factor is needed: the azimuth of the surface. The azimuth (see Fig. 3) is the angle, measured clockwise, from the south to the exterior side of the wall in question. Complete tabulated calculations are available for magnitudes of the factor K, extending over all latitudes, all azimuths, all months of the year, and all hours of the day.3 Illustrative excerpts are given in Tables 7 and 8. Data of this type are valuable for all manner of . problems involving solar radiation, and not only for cooling-load calculations. The reader is warned that values of K from Tables 7 and 8 include only the direct radiation; sky radiation must be calculated separately and added to the direct radiation (see Table 6). The use of Tables 5, 6, 7, and 8 requires that the relation between solar Table 7. Values of K fob Horizontal Planes in North Latitudes* During the Period Mat 2-Auqust 10' Local Mean Sun Time** a.m. p-m. 7 6 4 3 9 \ 12 25 .145 .365 .570 .747 .882 .967 .696 . North Latitude--Decrees 30 .171 .382 .578 .746 .876 .957 .985 35 .196 .395 .581 .740 .863 .940 .966 40 .034 .220 .406 .580 .729 .843 .915 .940 45 .070 .242 .414 .574 .712 .817 8S4 .906 50 .106 .262 .418 .564 .689 .785 .845 .866 t Figured for solar declination of 20 deg. b The relation between local mean sun time and civil time may be obtained from Weather Bureau offices in various localities. called the absorptivity or the emissivity; see Table 6 and Equation 3, Chapter 5. Convection is calculated as explained in Chapters 5 and 6. The daytime radiation emitted to the sky by a surface is not usually calculated as a separate item in practical air-conditioning work. Its effect may be roughly accounted for by an adjustment of the magnitude of the absorptivity factor for solar radiation, and this approach is often fol lowed in making emissivity measurements for building materials exposed to the sun and sky. When a building surface is exposed to a clear night sky, there is an appre ciable net exchange of low temperature radiant energy from the surface to the sky, but this is not considered for the usual comfort cooling calcula tion. When peak load conditions occur at night, nocturnal radiation is of particular importance for glass sections. For further information, see Brunts equation as given by Haurwitz.4 Table 8. Values of K for Vertical Planes in North Latitudes During the Period Mat 2-August 10 The azimuth angle a is always measured clockwise from the south to the exterior side of the wall in question. KAzimuth Scales for Values op ot Ordinart Type altitude and mean sun time be available in convenient form. Table 9 provides this information. The preceding discussion has been concerned only with incident radiation. When radiation is incident upon a surface, part is reflected, part is ab sorbed, and, if the material transmits radiant energy, part is transmitted. Moreover, the building surfaces themselves send out radiant energy to the TUUU* Deg A.M. ----- 0 15 30 45 60 75 90 . 105 120 135 150 165 180 i P.M> 180 165 6 5 30 8 4 93 10 ' 11 12 .940 .831 .666 .455 .213 .043 496 .589 .788 474- set 484 440 .908 ..832 .699 .518 403 .066 .175- 404 MS' .766 473 .9S8 408 ..814 .770 .674 .532 454 .151 j06t 470 460 j819 .736 Mt .814 .664 .6S1 .593 .495 .363 .207 .036 .137 Ml 444 457 .est. ..684 -V470 .482 .462 .410 431 .229 .111 .016 .139 464 46t 495 470 .243 .276 .289 .283 .258 .217 .158 .089 .015 Ml .13S 494 443 .000 .045 .087 .123 .ISO .168 .174 .168 .150 .123 .087. .045 .000 sky and to the surroundings. The heat balance on an outer surface, for a unit time interval, may be expressed as follows: 5 .908 .Wo .579 .349 .095 .185 .836 4/f .934 493 484 408 6 7 6 5 .940 .908 .845 .859 .693 .751 .493 .592 .260 493 .010 444 .167- .070 477 40t .679 414 .835 Ml 455 .8*1 470 .895 440 408 45 8 4 .814 .809 .750 .639 .485 .298- .090 .its 419 Jit. .680 .783 414- Heat entering! f Incident direct f Incident sky _ fReflected direct 9 3- .664 .701 .689 .631 .S30 492 .228 .048 .135 M9 481 .683 .884 19 2' .470 .540 .574 .568 424 .444 434 .201 .054 .096 J40 488 470 outer surface / (solar radiation ( radiation \ radiation n 12 .243 433 .411 .455 .468 .449 .400 423 .225 .ill 411 .131 443 .000 .109 .211 .299 466 .408 .423 .408 .366 .299 .211 .109 .000 Reflected sky fRadiation emitted (Air-to-surface ! { radiation (to sky by surface (convection (2) The convective heat transfer may be either to or from the surface, de pending upon whether the air temperature is higher or lower than the sur t. A P. M.V 360 345 330 31S 300 ' 285 270 255 240 225 210 195 180 .A;m. --t-*. 180 195 210 225' 240 255 270 285 300 315 .330 345 360 Azimuth Scales for Values op K in Italicized Ttfe face temperature. No term has been included to represent radiation ex change with other objects in the surroundings, as this effect is not usually of significant magnitude relative to the others. In practical calculations, the difference between incident and reflected radiation is computed by multiplying the incident radiation by a factor * Values for 30 deg latitude may be employed,over the range 25 deg to 35 deg. Values for 45 deg latitude may be employed over the range 40 deg to 50 deg. Interpolation will yield values for the range 35 deg to 40 deg latitude. h The relation between local mean sun time and civil time may be obtained from Weather Bureau offices in various localities. - Values in ordinary type are for azimuths 0 to 180 deg. Values in italic type are for azimuths 180 to 360 deg.