Document OBwx61amdX2Bme3g6Qr7g11M

178 Latitude 30 Deg north 40 Deg north 50 Deg north CHAPTER 13 1959 Guide Table 6 .. . Values of the Wall Solar Azimuth, y, for Variously Oriented Walls and Solar Altitude Computed for 18 Dog Declination, North (August I) Sao firne Solar Altitude P Degrees Azimuth Angie y, Pegreei AM -+ 1 6 &.m. 7 8 9 6 pjn. 5 4 3 10 2 11 1 12 5 a.m. 6 7 8 7 p.m. 6 5 4 93 10 2 11 1 12 5 a.m. 6 7 8 7 p.m. 6 5 4 93 10 2 11 1 12 9.0 21.5 34.5 47.5 60.0 72.0 78.0 0.5 11.5 23.0 34.5 45.5 56.0 64.5 68.0 4.5 13.5 23.5 33.0 42.0 50.0 56.0 58.0 N- KE 74 81 88 Shade 66 ' 76 85 Shade 29 36 43 51 62 83 Shade 21 31 40 . 50 61 76 Shade 67 78 90 Shade 22 33 45 57 70 87 Shade E 16 9 2 6 17 38 90 24 14 5 5 16 31 55 90 23 12 0 12 25 42 64 90 SE s 61 54 47 Shade 39 84 28 73 7 52 45 0 69 59 50 Shade 40 85 29 74 14 59 10 35 45 0 68 57 45 90 33 78 20 65 3 48 19 28 45 0 r PM -* N NW W sw s sw Shade 45 Shade 80 45 Shade 71 45 SE By linear interpolation, the diffuse irradiation is id - 25 + (33 - 25) = 26.6 Btu per (hr) (sq ft). The total solar irradiation is I, - 152.0 + 26.6 - 178.6 Btu per (hr) (sq ft). PERIODIC HEAT FLOW THROUGH WALLS AND ROOFS The calculation of heat flow, through a structural section of a building exposed to the weather, requires consideration of the diurnal cycles of solar irradiation and air temperature. These cycles and other factors lead to a periodic variation in the instantaneous rate of heat flow into the weather sur face, and a related periodic variation in the rate of heat flow into the air-conditioned space. Because of heat capacity and other factors, these heat-flow cycles are, in general, out of time phase and unequal in amplitude. In order to calculate the rate of heat entry into the weather surface of a building, it is necessary to know: 1. The intensity of direct solar radiation striking the surface. 2. The absorptivity (or reflectivity) of the surface for direct solar radiation. 3. The intensity of diffuse or sky solar radiation striking the surface. 4. The absorptivity (or reflectivity) of the surface for diffuse or sky solar radiation.* 5. The rate at which the surface emits radiation to the sky and other surroundings. 6. The rate at which the surface absorbs the low temperature radiation emitted by the sky and other surroundings by virtue of their temperatures and radiating characteristics. 7. The temperature of the surrounding air. 8. The temperature of the outer building surface. 9. The unit convective conductance for beat transfer be tween the air and the building surface. The Sol-Air Temperature The complex interrelationship of the above factors can be considerably simplified through the use of the sol-air tem perature concept. The sol-air temperature U is the tempera ture of the outdoor air, which, in the absence of all radiation exchanges, would give the same rate of heat entry into the surface as would exist with the actual combination of incident solar radiation, radiant energy exchange with the sky and Table 7 .... Approximate Solar Declinations in Degrees Dote Oedinotion Date Declination April 1 May 1 May 15 June 15 4.5 10.0 15.0 19.0 22.0 23.5 July l July 15 - Aug. 1 Aug. 15 Sept. 1 Sept. 15 23.0 21.6 18.0 14.0 8.5 3.0 -f- Cooling Load 179 Table 8....... Summer Design Sol-Air Temperatures Used for Tables 9 and 10 Sol-Air Temperature t. Fahrenheit Degrees Any Surface0 Horiz. North East Sooth West Ratio*; 0 0.225 0 0.225 0.125 0.225 0.125 0.225 0.125 1 AM 2 3 4 6 7 8 9 10 11 1 PM 2 3 4 5 6 7 8 9 10 11 24 Hr Avg. 77 76 76 75 74 74 74 75 77 80 83 87 90 93 94 95 94 93 91 87 85 83 81 79 83.1 77 76 76 75 74 74 76 91 106 119 129 137 142 144 140 132 120 107 96 90 85 83 81 79 100.5 77 76 76 75 74 74 74 75 77 80 83 87 <90 93 94 95 94 93 .91 87 85 83 81 79 83.1 77 76 76 75 74 75 110 123 126 125 117 108 92 93 95 95 94 93 91 87 85 83 81 79 93.0 77 76 76 75 74 80 93 100 103 104 100 96 92 93 - 94 95 94 93 91 87 85 83 81 79 88.4 77 76 76 75 74 74 74 75 82 93 102 110 114 115 111 104 99 95 91 88 85 83 81 79 89.0 77 76 76 75 74 74 74 75 78 86 93 99 104 105 104 100 96 94 91 87 85 83 81 79 86.2 77 76 76 75 74 74 74 75 77 80 83 89 96 110 124 135 141 139 125 103 85 83 81 . 79 93.0 77 76 76 75 74 74 74 75 77 80 83 87 92 102 111 119 120 118 111 94 85 83 81 79 88.4 a -- aarf&ce fcwptiviiy, rod -- 0.0; dark wells -- 0.0, and li*ht walla -- 0.6. fm -- unit convective conductance m 4.0 Btu per (hr) (F deg). b Value* in this oolumo are magnitude* oi , the outdoor air temperature. other outdoor surroundings, and convective heat exchange with the outdoor air. . The sol-air temperature data** *19 as developed by Mackey and Wright for an industrial atmosphere were used as a basis for preparing Table 8 showing summer design sol-air tem peratures. Sol-air temperatures may also be estimated from experimental observation of surface temperatures of walls and roofs which appear in the literature.11' u Both analytical and experimental studies have been made on the problem10 of heat flow through walls and roofs. Those concerned with a further study of the details of cooling load estimates in particular relation to periodic heat' flow will find much of value and interest in the reports of experimental studies of these problems.11 " " "10 The reader may also refer to the Cooling Load chapter of The Guide 1952 for the theory of heat flow through walls and roofs. PRACTICAL TABLES FOR CALCULATING SOLAR HEAT GAIN THROUGH WALLS AND ROOFS The analytical10 method reported by Mackey and Wright was used by Stewart1* to obtain temperature differentials based on Table 8 and shown in Tables 9 and 10. These ana lytical procedures, as well as those using Tables 9 and 10, presented here, yield generally higher rates of heat gain than reported for-Pittsburgh in early ASHAE experimental studies. Current authoritative opinion indicates a preference for analytical calculations. Thermal and physical properties of materials used in these tables are given in a paper.10 The rate of heat flow is obtained by multiplying the overall heat transmission coefficient of the structure by the equivalent temperature differential obtained from the tables. Tables 9 and 10 were developed by using an outside surface conductance of 4.0 and an intide film conductance of 1.65 Btu (hr) (sq ft) (F deg). A- reduction was made in the tem perature differentials for roofs amounting to some 20 percent of solar radiation as explained by Stewart.10 This was to compensate for several factors, one of which is the radiant heat lost to the sky which is not included m the Mackey and Wright method. Experimental work by Parmelee17 and pre vious work by Brunt10 give data showing the magnitude of thU radiant heat loss from a roof or wall to the sky. Temper ature differentials for roofs probably would be reduced below those shown in Table 9 whenever the radiant heat lost to the sky is included in calculation of sol-air temperature. The temperature differentials for roofs were based on an in side surface conductance of 1.65 because the charts prepared by Mackey *nrl Wright10 used this value, and it was not practicable to .repeat their work using a different film co efficient. An examination of the values given in their paper indicates that the temperature differential would be changed very little even if a value 1.20 were used instead of 1.65. To obtain the heat-flow rates through roofs, more accurate