Document MJrMM50DpKdYVMZqGQQmJbZRk

188 latitude 20 Deg north 40 Deg north 60 Deg north CHAPTER 13 1960 Guide Table 6 .... Values of the Wall Solar Azimuth, y, for Variously Oriented Walls and Solar Altitude Coaputod for It Oog OoeBoaHoa, North lAogotf 1) Son Thao Sotor Attitude fi Dogroos Azimvtit Anglo y, Oeprees AM --* i 6 &.m. 7 8 8 6 p.m. 5 4 3 10 2 11 1 12 5 a.m. 6 7 8 7 pjn. 6 5 4 93 10 2 11 1 12 5 am. 6 7' 8 7 p.m. 6 5 4 93 10 2 11 1 12 T' PM -- N NE 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 . 74 81 88 Shade 66 76 85 Shade 67 78 90 Shade 29 36 43 51 62 83 Shade 21 31 40 50 61 76 Shade 22 33 45 57 70 87 Shade N NW E 16 9 2 6 17 38 90 24 14 5 5 16 31 55 90 23 12 0 12 25 42 64 90 w SE S 61 54 47 Shade 39 84 28 73 7 52 45 0 69 59 50 40 85 29 74 14 59 10 35 45 0 68 57 45 90 33 78 20 65 3 48 19 26 45 0 $W S Shade 45 Shade 80 45 Shade 71 45 SE By linear interpolation, the diffuse irradiation is U 25 + (33 - 25) - 26.6 Btu per (hr) (sq ft). The total solar irradiation is It = 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 heat transfer be tween tbe air and the building surface. The Soi-Air Temperature The complex interrelationship of the above factors can be considerably simplified through the use of the sol-air tem perature concept. The sot-air temperature t, is the tempera ture of the outdoor air, which, in the absence of ail 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 Dale O*dinol*on Dot* Dedtnotion April 1 April 15 May 1 May 15 June 1 June 15 4.5 10.0 15.0 19.0 22.0 23.5 July 1 July 15 Aug. 15 Sept. 1 Sept. 15 23.0 21.5 18.0 14.0 8.5 3.0 Cooling Load Table 8 .... Summer Design Sol-Air Temperatures Used for Tables 9 and 10 Sol-Air fMptnfm t, faftmlml Decreet Aay Sorfeca* Horf*. North East Sooth 189 West Ratio*! y- 0 0225 0 0.225 0.125 0.225 0.125 CU25 0.125 12 Midnight 77 77 77 77 77 77 77 77 77 1 AM 76 76 76 76 76 76 76 76 76 2 76 76 76 76 76 76 76 76 76 3 75 75 75 75 75 75 75 75 75 4 74 74 74 74 74 74 74 74 74 5 74 74 74 75 80 74 74 74 74 6 74 76 74 110 93 74 74 74 74 7 75 91 75 123 100 75 75 75 75 8 77 106 77 126 103 82 78 77 77 9 80 119 80 125 104 93 86 80 80 10 83 129 83 117 100 102 93 83 83 11 87 137 87 108 96 110 99 89 87 12 Noon 1PM 2 3 90 142 90 92 92 114 104 96 92 93 144 93 93 93 US 105 no 102 94 140 84 95 94 111 104 124 111 95 132 95 95 95 104 100 135 119 4 94 120 94 94 94 99 96 141 120 5 93 107 93 93 93 95 94 139 118 6 91 96 91 91 91 91 91 125 111 7 87 90 87 87 87 88 87 103 94 8 85 85 85 85 85 85 85 85 85 9 83 83 83 83 83 83 83 83 83 10 81 81 - 81 81 81 81 81 81 81 11 79 79 79 79 79 79 79 79 79 24 Hr Avg. t_ 83.1 100.5 83.1 93.0 88.4 89.0 86.2 93.0 88.4 * a BUrfaoo absorptivity, A'lmwiswuhw. roof " 0.9,-dirk wiQj m 0.9, Mud light mBs = * Values in this columnere magnitude* of 4. theoutdoor sir texnperatwe. < unit eoovBetivs conduct*nee -- 4.0 Btu pw (tr) (F deg). other outdoor surroundings, and convective heat exchange with the outdoor air. The sol-air temperature data*- * 10 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* a Both analytical and experimental studies have been rn*A* 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.10* u* " u* **15 The reader may also refer to the Cooling Load chapter of The Gums 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 ASHRAE 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 Tbe 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 inride 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 in the Mackey and Wright method. Experimental work by Parmelee57 and pre vious work by Brunt10 give data showing the magnitude of this 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 differential* for rook were based on an in side surface conductance of 1.65 because the charts prepared by Mackey and 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