Document 6BzLXvpdxqOGQXKNYjagprLr9

282 CHAPTER 12 1950 Guide ' Tablb 20. Approximate Solab Declinations in Degrees Date April 1 April 15 May 1 May 15 Declination 4.5 10.0 15.0 19.0 .Date June 1 June 15 July 1 July 15 Declination 22.0 23.5 23.0 21.5 Date Aug. 1 Aug. 15 Sept. 1 Sept. 15 Declination 18.0 14.0 8.5 3.0 exposure walls. (Steady-state calculations, as suggested there, take no account of the time lag present with periodic cycles of air temperature.) The results of experimental observations1' on heat gains through glassblock sections are given in Table 21. Tabulated heat-gain rates are based upon a constant indoor-air temperature of 78 F and a maximum outdoor*air temperature of 95 F for the design day chosen. The values given are approximate, and intended for estimates only; they are averages for.four representative glass block designs, two with smooth exterior faces and two with ribbed exterior faces. Shading of Glass Areas--Design Tables' The effects and possibilities of shading should be carefully, investigated whenever the heat gain from glass is a large portion of the cooling load. Vertical glass, which is not mounted in the plane of the building surface, is partially shaded by the setback. If a vertical window of height l and width w be set back from the plane of the building a distance s, the fraction of the total area of the window which receives direct solar radiation is: Ot- 1 - r- tan-g - r, cot T + c.0t-7 (11) sin y. sin y . where ft =* s/l, ft = */to, 0 = solar altitude, and y is the difference between the azimuth angles of the wall in question and of the horizontal projection of the aim's rays, (y is always equal to or less than 90 deg.) Solar altitudes are given in Table 18 for various latitudes and an August Table 21, Total Instantaneous Rates of Heat Gain fob Glass Blocks1* on Design Day of August 1 {Value* tabulated include solar and sky radiation, convection, and conduction averaged for four types of blocks) Mean Sun Time Total Instantaneous Rate of Heat Gain. Btu pea Hour for Each Square Foot of Sunlit Glass Block ___________________ - Vertical Surface Faring North Latitude, Deg 7 a.m. 8 9 10 11 12 noon 1 p.m. 2 -3 4. 5 6 7 East West 30.to 45 61 78 ' 74 58 45 37 30 24 20 16 13 11 8 30 to 45 . 5.0 6.5 7.5 11 22 35 55 77 * 86 55 19 30 -4.5 0.0 5.0 11 17 22 25 26 24 20 15 . 9.5 3.5 ' South 35 - 2.0 ' 2.0 7.0 ' 15 22 28 32 32 30 26 20 14 7.0 40 --0.5 4,0 10 18 26 34 39 39 37 32 25 18 11 - 45 1.0 5.0 . 12 21 32 41 46. sS. 4457 41 34 . 26 18 Cooling toad 283 : Table 22; Values of the Azimuth-Difference Angle, y, Degrees, fob . Window-Reveal Shading Calculations (See Equation 11) Computed for solar declination of 18 deg--August 1 N. Latitude Degrees 30 40 GO Mean Sun Time 6 a.m. 7 8 9 10 11 12 N 1 p. ra. 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 NE 61 54 47 39 28 .7 Shade E 74 81 88 84 73 ' 52 0 Shade . . 59 50 40 27 14 Shade ... 76 85 85 74 59 35 0 Shade 7'-- 57 45 33 20 3 Shade __ 78 90 78 65 48 26 0 Shade .... "" Window Orientation SE - 29 36 43 51 62 83 45 Shade 31 .40 50 61 76 80 45 10 Shade ... 33 45 57 70 87 71 45 19 Shade .,, S Shade 6 17 38 90 38 17 6 Sh. ade -- 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 1 design day. Values of the angle 7 are given in Table 22 for the August 1 design day. Special cases not covered by the tabulated data may be solved analytically;14 however, the design conditions chosen will yield a satisfac tory approximation if used without correction for estimates at any time during the summer period. Example 9. 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 18, 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 alti tude is 45.5 deg. From Table 22, the angle 7 is 74 deg, from Equation 11, the fraction of the total window area that is receiving direct solar radiation is: 0.1 tan 45.5 _____ x 0.0167 tan 45.5 cot 74 sin 74 sm 74 .. ' = 1 - 0.108 - 6.048 + 0.005 = 0.851. 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: 5 = 3 X 5 X 0.851(172) = 2200 Btu per hour.