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280._________________________________________________ CHAPTER IS___________________________ 1948 Guide . j Table 19. Values of the Azimuth-Difference Angle, y, Degrees, for Window-Reveal Shading Calculations (See Equation 12) Computed for solar declination of 18 deg--August 1 ' N. Latitude Degrees Mean Sun Time 30 ' 40 50 - - 6 a. m. 7 8 9 10 11 12 N 1 p. m. 2 3 4 5 6 6 a. m. 7 8 9 10 11 12 N 1 p.m. 2 3 .4 * 5 6 6 a. m. 7 8 9 10 11 12 N 1 p. m. 2 3 4 5 6 Window Orientation NE : 61 54 . 47 39 28 7 Shade 59 50 40 27 14 Shade 57 45 33 20 3 Shade E 74 81 88 v 84 73 52 0 Shade SE . 29 ` 36 43 51 62 83 45 Shade S Shade 6 17 38 90 38 17 6 Shade ' 76 85 85 74 59 35 0 Shade -- 78 90 78 ' 65 48 26 0 Sh__ade 31 40 50 61 76 80 45 10 Shade _ 33 45 57 70 87 71 45 19 Shade Shade 5 16 31 55 90 55 31 16 5 Shade Shade 0 12 25 42 65 ,90 65 42 25 12 0 Shade SW Shade 45 83 62 51 43 36 29 Shade 45 80 76 61 50 40 31 Shade 19 45 71 87 70 57 45 33 W' Shade NW Shade 0 52 73 84 88 81 74 . Shade 7 28 39 47 54 61 Shade 0 35 59 74 85 85 76 Shade 14 27 40 50 59 Shade 0 26 48 3 65 20 78 33 90 45 78 57 Solar altitudes are given in Table 15 for various latitudes and an August 1 design day. Values of the angle y are given in Table 19 for the August 1 design day. Special cases not covered, by the tabulated data may be solved analytically13; however, the design conditions chosen will yield a satisfactory approximation if used without correction for estimates at any time during the summer period. Example 6. Estimate the instantaneous rate of sky and solar radiation heat, gain from a west window 3 ft wide by 5 ft high with a setback of 6 in. for August 1 and a north latitude of 40 deg at 3 p. m. (sun time). From Table 15, the instantaneous rate of sky and solar radiation heat gain per square foot of sunlit glass is 172 Btu per hour. From the same table, the solar altitude is 45.5 deg. From Table 19, the angle y is 16 deg, from Equation 13, the fraction of the total window area that is receiving direct solar radiation is: Gf = 1 - 0.1 tan 45.5 - 0.167 tan 16 + 0.0167 tan 45.5 tan 16. = 1 - 0.102 - 0.048 + 0.005 = 0.855. Although the sky radiation is not reduced in proportion to the shaded portion, since the sky radiation is small this fact may be neglected, and hence instantaneous sky-and solar-radiation heat gain for this window is: 2 = 3 X 5 X 0.855(172) = 2206 Btu per hour. Cooling- 'Load 281 Table 20. Effectof Shading Upon Total (Radiation, Conduction and. . Convection) Rates of Instantaneous Heat. Gain Through Single Thickness of Common Window Glass* * ' Type of Shading Finish Outside Shading Screen: metal slats 0.050 in. wide, spaced 0.063 in. apart and set at 17 deg angle with horizontal. Canvas Awning Outside Venetian Blind: slats at 45 deg, ex- tended as an awning without sides to cover approximately, two-thirds of window. Inside Roller Shade: fully drawn.' Outside Venetian Blind: slats at 45 deg, fully covering window; Inside Venetian Blind: slats at 45 deg, fully covering window. Inside Roller Shade: half drawn. Inside Roller Shade: half drawn. Dark . Dark . Light Aluminum Aluminum Aluminum Buff Dark Fraction of Gain ` Through Unshaded.:; - Window 0.20 0.35 0.25-0.35 0.35-0.50 Approx. 0.45' ' Approx. 0.3 0.65-0.80 Approx. 0.7 0.90-0.95 These factors are not accurate for heat absorbing glass or glass blocks. A window such as the one in Example 6 above would customarily be provided with an additional shading means for use particularly when directly sunlit. Conventional shading, devices include awnings, shades, and screens of various.types. In Table 20 are given the ratios of the instantaneous rates of heat-gain from Windows shaded by different;indoor and outdoor, shading.fixtures to the rate of heat gain from unshaded glass according to tests 13 at the A.S.H.V.E. Research Laboratory. There are a number of variables affecting these ratios such as color, fit, solar altitude, and angle of incidence of the solar radiation. These values, therefore,. must be considered as approximate, only, and will have to be used with considerable judgment. An inside shade is effective to. the extent of its reflectivity, since the portion of the solar radiation directly, transmitted by the glass that is absorbed by the shade is transferred by convection to the room air and by radiation to the solid surfaces of the room. ' Instantaneous Heat Gains vs. Instantaneous Cooling Loads The difference between instantaneous heat gain and instantaneous cooling load has been mentioned previously; its practical,importance is. sufficient to warrant further consideration. Fig. 6 offers a simplified schematic illustration showing how the radia tive part of the instantaneous heat gain is first absorbed by solid objects and is not encountered by the conditioning equipment as a cooling load Fig. 6. Origin of the Difference Between the Magnitudes of the Instantaneous Heat Gain and Instantaneous Cooling Load The radiation absorbed by the interior furnishings and structure reaches the conditioning.equipment after a considerable delay in time. '