Document Lp9pyYQROpbRBoL6DEO9z1bbw

. 268 CHAPTER 15 " '1948 Guide Table 10. Summer Design Sol-Air Temperatures for New York, N. Y. (North Latitude 4046'; Elevation 180 Ft) Mean Sun Time Sol-Air Temperature, te Fahrenheit Decrees. Any Surface** - Horizontal North East South b'.. Ratio: -------. /o 0 0.25 0.25 0.25 0.25 12 midnight 1 a.m. j 2 3 4. 5 6 7 8 9 10 11 12 noon - . 1 p.m. 3 5 6 8 9 10 11 24-hr avg, tm 79 78 ` 77 ' 77 ' 76 76 76 80 ; . -82 86 88 90 92 93 94 94 94 93 90 88 85 83 82 81 84.8 ' 79 78 77 77 76 76 81 96 110 127 137 148 155 154 152 . 144 136 121 106 93 85 83 82 81 106.4 79 78 77 77 76 76 80 85 85 90 92 94 97 98 99 99 99 103 106 102 85- 83 82 81 88.5 79 78 - 77 77 76 76 - 89 106; 114 120 114 106 97 98 .99 99 99 97 93 90 85 83 . 82 81 92.3 79 78 77 77 76 76 77 82 86 97 104 111 115 117 - 112 102 : 99 . .97 93 90 85 83 82 81 , - 90.7 West 0.25 79'. 78 77 77 76. 76 77 82 85 : 90 92 94 98 108 . 126 136 145 ' 140 134 115 85 83 82 81 . 96.5 6 = surface absorptivity, dimensionless. So = unit convective conductance. Btu per (hr) (sq ft) (F deg). - . ^Values in this column are magmtudesof to, the outdoor air temperature. . Table 11. Summer Design Sol-Air Temperatures for Lincoln, Nebr. (North Latitude 4050'; Elevation 1225 Ft) .Mean Sun Time b Ratio*:----- U 12 midnight 1 a.m. 2. 3 4 5 6 7 8 9 10 11 12 noon 1 p.m. 2 3 4 5 6 7 '8 9 10 11 ` 24-hr avg. tm Sol-Air Temperature, te. Fahrenheit Degrees Any Surface** Horizontal North East South 0 0.25 0.25 0.25 ` . 0.25 89 88 86 84 84 82 81 82 88 - 93 96 100 102 104 106 107 107 106 105 102 98 94 92 90 . . - . 94.4 89 88 86 84 84 82 87 103 124 143 160 172 178 180 178 170 158 142 126 109 99 94 92 90 121.6 89 88 86 ' '84 84 82 88 93 94 98 102 106 108 110 112 113 112 113 117 113 98 94 92 ` 90 98.6 89 88 86 . 84 ' 84 82 100 125 137 142 138 129 115 110 112 113 112 110 108 104 98 94 92 90 105.9 89 - 88 86 84 * 84 .82 82 85 " : 92 104 115 125 130 132 131 126 117 110 108 104 98 94 92 90 102.1 ,*t>surface absorptivity, dimensionless. ` :/o = unit convective conductance. Btu per (hr) (sq ft) (Fdeg).' - bValues in this column are magnitudes of to, the outdoor air temperature. West' 0.25 89 88 86 84 84 82 82 85' - 92 98 102 106 108 119 137 150 158 157 149 , 128 98 94 92 90 106.6 ^Cooling Load :269 all things:considered, a .simple interpolation.between the four points of the compass should serve for all but the most accurate calculations! An example of ,.the. use of the tables in determining sol-air temperature is given. ' : ' " Example S. Find the.,summer design sol-air temperature in New York, N. Y., for a west wall which'has a solarabsorptivity of 0.7, at a sun time of 3:00 p. m. ^ ...... Q7 .............. . ... ....__ ... Use/0 = 4; then, yr-for this wall is or 0.175. ; . From Table 10: for = 0, /e = 94 F; for -- 0.25, - : Jo Jo: ' interpolation for. this wall: . 1, = 136 F; and by linear <e = 94 + (136 - 94) = 94 + 29 = 123 F. . 0.25 ' l Sol-air temperatures are especially, helpful in., the calculation of periodic heat transfer, as will be illustrated in the material which follows. PRINCIPLES OF PERIODIC HEAT FLOW Calculation principles for periodic heat flow are dealt with briefly in this section; in the section which follows, practical tables will be given to facilitate rapid design estimates; . .' .r .Time Lag . . _ _............ ....... __......... ....................................... ...___ The fundamental" analysis '6f, periodic heat flow is complicated when compared with steady-state calculations on account of the time-variable storage of heat from point , to point through, a wall or roof.. The cyclic variation of outdoor"conditions,produces a related cyclic .variation.,of temperature and heat flow throughout each structural section exposed to the weather. The cyclic .variations undergo a progressive shift in phase and decrease in amplitude in going through a wall with constant cohditions maintained In themdoob space. "- ' : ' By a shift in phase is meant that as the cyclic.temperature wave passes through the wall, the time of occurrence of the maximum temperature at any point shifts farther and farther behind the time of the outer-surface maximum :for successive'positions through the wall. "The resultant time lag between the outer-surface and inner-surface maximum,temperatures is important,-for it may be-the determining factor in fixing the time of the maximum cooling load. By a decrease in amplitude is meant that as the cyclic temperature wave passes through the wall, "the. jdifferen.ee between the maximum tempera ture of a cycle and the mean temperature of the cycle, which is the ampli tude of the wave by definition, decreases progressively as the wave passes through the wall. The magnitude of the temperature a,mplitude at the . inner wall surface is necessary for the determination of the instantaneous rate "of heat transfer to the indoor space. Practical design data for periodic heat flow comprise a means of determining the time lag and amplitude decrement for'different wall constructions and any given outdoor cycle of sol-air temperature. Both analytical and experimental studies have been iiiade on this problem7. While the' Unalyticalsolution has been written, it is far .too detailed for direct use in rapid practical work; and,theiextensive numerical work required to establish a basis for simplified calculations, has been only