Document gaJZ2YM3LKz1jMkY4DmpKGobN

276 CHAPTER 12 1951 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. Surfaces with Table 10. Periodic Heat Flow Data fob Homogeneous Walls ob Roofs Material ThickNESS, OVEB-ALL COEFFIClENT. Btu FEB (HB) (9Q ' ANCB OF (HB)| (SQ FT) (F)/Btu L k Time Lao. Factob, X, in Equation 5 Horizon- tal and North East South West Stone Solid Concrete Common Brick Face Brick Insulating Board 8 0.67 0.64 5.5 0.51 0.36 0.48 0.42 12 0.55 0.96 8.0 0.28 0.19 0.26 0.22 16 0.47 1.28 10.5 0.17 0.10 0.15 0.13 24 0.36 1.92 15.5 . 0.06 0.03 0.05 0.04 2 0.98 0.17 1.1 0.93 0.87 0.92 0.89 4 0.84 0.33 2.5 0.79 0.68 0.76 0.72 6 0.74 0.60 3.8 0.61 0.46 0.68 0.51 8 0.66 0.67 5.1 0.49 0.33 0.46 0.39 12 0.54 1.00 7.8 0.29 0.17 0.26 0.22 16 0.46 1.33 10.2 0.17 0.09 0.15 0.12 4 0.60 0.80 2.3 0.83 0.75 0.81 0.78 8 0.41 1.60 5.5 0.51 0.39 0.49 0.44 12 0.31 2.40 8.5 0.26 0.17 0.25 0.21 16 0.25 3.20 12.0 0.13 0.08 0:12 o.io 4 0.77 0.44 2.4 0.81 0.70 0.78 0.74 M 0.68 0.62 0.17 1.0 1.0 1.0 1.0 1 0.48 1.25 0.45 1.0 0.99 0.99 0.99 2 0.30 2.50 1.8 0.98 0.91 0.96 0.94 X 0.42 1.51 0.08 1.0 1.0 1.0 1.0 1 0.26 3.03 0.23 1.0 1.0 1.0 1.0 2 0.14 6.05 0.77 1.0 1.0 1.0 1.0 4 0.08 12.1 2.7 0.83 0.74 0.81 0.76 6 0.05 18.2 5.0 0.64 0.49 . 0.61 0.55 * Based upon an outdoor surfaoe conductance of 4.0 and an indoor surface conductance of 1.65 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, /wo = 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 at/fao in Table 9, m'= 83.1 + (92.9 - 83.1) = 90.0 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, C .= 94 + U37 - 94) = m.! F. Cooling Load , prom Equation 5, the instantaneous design rate of heat gain is 277 ? = 0.41[(90.0 -- 80) + 0.44(124.1 -- 90.0)] =-105 Btu per (hr) (sq ft). A From Table 10, the time lag is 5.5 hr. From Table 9, the time of maximum-rate f heat entry for a west wall is 4:00 p.m. plus 5.5 hr or 9:30 p.m. (this is sun time). The computed rate is therefore the maximum rate. Composite Walls or Roofs, Constantllndoor 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 FACTOR X Fig. 2. Approximate Value op the Amplitude Decrement Factor X for 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 the time lag for a composite construction: a. Find the time lag for each layer from Table 10. 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 $-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 factort 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 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. Other factors remaining the same, the use of the material of lower density