Document N2Mn5K116wMg7w76z9a99Bq0R

304 CHAPTER IS 1949 Guide Table 20. Approximate Solas Declinations in Degrees Date April 1 April 15 May 1 May 15 Declination 4.5 ro.o 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 observations11 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 outdoorair 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 -- rt tan ft -- rt tan y + r,rt tan $ tan y (11) where r, =. e/l, rt - a/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 sun's rays. (7 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 op Heat Gain fob Glass Blocks11 4 on Design Day of August 1 (Values tabulated include solar and sky radiation, connection, and conduction averaged for four types of blocks) -' Mean Sun Time . Total Instantaneous Rate-of Heat Gain, Btu PEa Houa foe Each Souakb Foot of Sunlit Glass Block ':: ! : _______________________________________________________ ' '-- Vertical Surface Facing ;v* East' North Latitude. Deg 8 9 10 fl 12 noon 1 p.m. 2 3 4 -~ 5. . 6 7" : 30.to 45 61 78 74. 58. 45 37 30 ' 24 20 16 13 11 8 West 30 to 45 5.0 . . 6.5 7.5 ' 11 22 35 ^ 55 77 86 55 19 30 -4.5 0.0 5.0 1I7t / 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 45 -0.5 4.0 . 10 . 18 26 34 39 39 37 32 . 25 18 11 1.0 .. . ` 6.0- 4 7i .12 21 . 32 41 ' 46 ' 47 45 . . .41 --. 34 . 26 18 Cooling .Load - 305 Table 22.,' Values of the Azimuth-Difference Angle, y. Degrees,' for Window-Reveal Shading Calculations (See'Equation .11) Computed for solar declination of. IS deg*--August l N. Latitude Degrees 30 40 50 Mean Sun Time 6a.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 v NE 61 54 47 39 28 7 Shade _ 59 50 40 27 14 Shade 57 45 33 20 \3 Shade E 74 81 88 84 73 52 0 Shade 76 85 85 74 59 35 0 Sh, ade , .... 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 S Shade 6 17 38 90 38 17 46 Shade -Shade 5 16 31 55 90 55 31 16 5 Shade 33 45 57 70 87 71 45 19 Shade -1". ' 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 67 45 33 W Shade 0 52 73 84 88 81. 74 Shade 0 35 59 74 85 85 76 0 26 48 65 78 90 78 NW : Shade 7 28 39 '47 .'54 61 , Shade 14 27 40 50 59: Shade 3 20 33 45 57. 1 design day. Values of the angle yare given in .Table 22 for the August 1 design day.- Special cases not covered by the tabulated data may be solved analytically14; 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 splar radiation heat gain from a west window 3 ft wide by '5 ft high with a setback of 6 in. for August l 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 y is 16 deg, from Equation 11, the fraction of the total window area that is receiving direct solar radiation is: Gt = 1 -- 0.1' tan 45.5 -- 0.167 tan 16 + 0.0167 tan 45.5 tan 16 : 1 ,. = 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.: g = 3 X 5 X 0]855(172) = 2206 Btu per hour. A window such as.the one in Example 9 above would customarily be pro vided with an additional shading means.for use particularly when directly