Document OzgXp8p1gyaxe6wY64NO6Kn4j

:* i li . , t '}* ' 268 CHAPTER 12 : 1950Guide Table 13. Summer Design Sol-Air Temperatures Used for Tables 14 AND 15 Soi^Axa Thmprbatttb* t9 Fahrenheit Degrees HranSuh tdu Any Sur Horit. North face*' East South West 0 0.225 0 0.225 0.125 0.225 0.125 0.225 0.125 12 Midnight 1 AM 2 3 77 77 77 77 77 77 77 77 77 ' 76 76 76 76 76 76 ' 76 76 76 76 76 76 76 76 76 76 76 76 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 1 PM 2 3 90 142 90' 92 92 114 104 96 92 93 144 93 93 93 115 105 110 102 94 140 94 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 (m . 83.1 100.5 83.1 93.0 88.4 89.0 86.2 93.0 88.4 ft,* 6 a surface absorptivity, dimensionless: roof ** 0.9; dark walls -- 0.9, and light walls TM 0.5. -- nnit " convective conductance = 4.0 Btu per (hr) (F deg). b raluee in this oolumn are magnitudes of to, the outdoor air temperature. The design sol-air temperature at a time earlier than 9:30 p.m. by the time lag (at 4:00 p.m.) for a west wall in New York, N. Y., is, from Table 10, t\ -- 94 + ~~ (145 -- 94) = 129.7 F 0.25 From Equation 6, the instantaneous design rate of heat gain is <. 4 - 0.41 [(93 - 80) + 0.44 (129.7 - 93)1 .A = 11.9 Btu per hour for each square foot of sunlit surface. From Table 12, thetime lag is 5.5 hr. From Table 10, the time of maximum rate of heat entry for a west wall is 4:00 p.m. The time of maximum instantaneous rate of heat gain by the enclosure, as far as the west wall is concerned; is then 4:00 p.m. plus 5.6hror9:30pjn. (This is sun time.) The computed rate is thereforethe maximum. Cooling Load FACTOR X 269 Fig. 4. Approximate Value of the Amplitude Decrement Factor a for Use in Equation 6 Composite Walls or Roofs, Constant Indoor Temperature A composite wall or roof is made up of two or more layers of different materials. Since the analytical solution for this type of construction bm not been reduced to simple and practical terms, it is necessary at present to utilize approximate procedures. In accord with the results of com parative calculations, the following procedures are suggested.7 To find the time lag for a composite construction: a. Find the time lag for each layer from Table 12. b. Add'the individual time lags, recognizing that this sum will always be less than the true time lag for the actual composite wall. c. To the sum from (b), add an arbitrary additional lag of 1 to 1 hr to obtain the estimated lag for the actual construction. For two-layer and light construction walls, the J-hr value will be suitable while, for walls of three or more layers, or very heavy construction, the 1-hr value is preferred. For intermediate conditions, indi vidual judgment is the only guide. (Computed time lags should not be considered to be accurate closer than to about the nearest hour by this method.) To find the amplitude decrement factor, X, for composite construction: Having determined the time lag and the orientation, use Fig. 4. (Note also that the factor X for a composite construction should never exceed the product of the fac tors for the individual layers.) One valuable result of the analytical studies made to date on composite walls has been the demonstration of the effect of the order of the materials: ^ac^ors rem&iiung the same, the use of the material of lower density on the weather side will increase the time lag and decrease the instan taneous maximum rate of heat gain. Two examples are given to show how to estimate, roughly, the instan taneous rate of heat gain through sunlit composite walls or roofs. Example 4- Estimate the maximum instantaneous design rate of heat gain from a horizontal roof in New York, N. Y., that is made up of black built-up roofing on the 'y'