Document aD4BYkpLwGp5jmgw0rMxjxz0Y

276 CHAPTER 12 1952 Guide time lag to the time of maximum sol-air temperature (from Table 9) for the particular wall or roof. . The magnitude of the. second term in Equation 5 relative to the first term indicates the relative.portion of the structural heat in-flow assignable to periodic heat flow. The periodic term is continually passing through a cyclic variation from zero to a. positive maximum, to zero, to a negative maximum, to zero again and so on over each 24-hour cycle. Surface^ with Table 10. Periodic Heat Flow Data fob Homogeneous Walls ob Roofs Material Over-all CoKFFI* Thick- nebs, In. - ClENT, Btu per (hr) (sq n)F) Thermal Resist^ ANCE OP (hb) (sq rr) (F)/Btv L k Tins Lao. Hr. Factor, X, in Equation 5 Horisontal and North East South West Stone Solid Concrete ' Common Brick Face Brick Insulating Board 8 0.67 12 0.55 16 . 0.47 24 0.36 2 4 6 8 12 16 . 4 8 12 16 0.98 0.84 0.74 ` 0.66 0.54 0.46 r 0.60 0.41 0.31 , 0.25 4 . 0.77 H 1 2 . 0.68 0.48 0.30 M 6.42 1. 0.26 2 0.14 4 0.08 6 0.05 0.64 0.96 1.281 1.92 0.17 0.33 0.50 0.67 1.00 ' 1.83 0.80 1.60 2.40 3.20. 0.44 0.62 1.25 5 2.50 1.51 3.03 6.05 12.1 18.2 - 5.58.0 10.5 15.5 1.1 .2.5 3.8 5.1 . 7.8 10.2 2.3 5.5 8.5 12.0 . 2.4 0.17 0.45 1:3 0.08 0.23 . 0.77 2.7 . 5.0 0.51 0.28 0.17 0.06. 0.86 0.19 0.10 . 0.03 0.93 0.79. 0.61 ' 0.49 0.29 0.17 0.87 0.68 0.46 0.33 0.17 0.09 0.83 0.51 0.26 0.13 0.75 . 0.39 0.17 0.08 0.81 0.70 1.0 1.0 0.08 1.0 0.99 0.91 1.0 1.0 1.0 0.83 0.64 1.0 1.0 1.0 0.74 0.49 ` 0.48 0.26 0.15 0.05 0.92 0.78 0.58 0.46 0.26 0.15 0.81 0.49 0.25 0.12 . 0.78 1.0 0.99 0.96 1.0 1.0 . 1.0 0.81 0.61 0.42 0.22 0.13 - 0.04 . 0.89 0.72 0.51 0.39 0.22 0;12 0.78 0.44 0.21 0.10 0.74 1.0 0.99 0.94 1.0 1.0 1.0 0.76 0.55 ' * Based upon an outdoor.surface conductance .of 4.0 and an indoor surface conductance of 1.85;Btu per (hour) (square foot) (Fahrenheit degree). different exposures pass through these cycles with maximum points at dif ferent times of day. .... An example in the use of Tables 9 and 10 follows: Example 7.- . Find the instantaneous rate of heat gain through an 8 in. west wall of common brick (at = 0.7, /ero -- 4.0) located at 40 deg north latitude at 9:30 p.m. sun time. The indoor air temperature is constant at 80 F.. Use sol-air data for an industrial atmosphere. . Solution: From Table 10,. U -- 0.41, the time lag is 5.5 hr, and X = 0.44. By linear interpolation on the basis of ajfaa in Table 9, U = 83.1 + (92-9 " 83.1) = 90jO F. The design sol-air temperature at a time earlier than 9:30 p.m. by the .time lag (at 4:00 p.m.) is, by interpolation, from Table 9, W " 94 + oiF (13.7 --94) = m-i F- Cooling. Load From Equation 5, the instantaneous design rate of heat gain is , 277 J = 0.41[(90.0 - 80) + 0.44(124.1 90.0)] = 10.3 Btu per (hr) (sq ft). From Table 10, the time lag is 5.5 hr. From Table 9,' the time of mainmiim rate of.heat entry for a west wall is 4:00 pm. plus 5.5 hr or 9:30 p.m. (this is sun time) The coniputed rate is therefore the maximum rate. ...: -. ; .. 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 has not been reduced to simple and practical terms, it is necessary at present Fig. 2. Approximate Value or the Amplitude Decrement Factor X fob Use in Equation 5 to utilize approximate procedures. In accord with the results of com parative calculations, the following procedures are suggested.11 To find a time lag for a composite construction: a. Find the time lag for each layer from Table 10. I 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 ) to 1 hr to obtain the estimated lag for the actual construction. For two-layer and light construction .walls, the 1-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, individual 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. 2. (Note also that the factor X for a composite construction should never exceed the product of the factors for the individual layers.) One valuable result of the analytical studies made to date on composite waffs has been the demonstration of the effect of the order of the materials. Other factors remaining the same, the use of the material of lower density