Document 93DOpp8r552rjDJ5b1DkmjK03

302 CHAPTER 13 1957 Guide Table 8. Summeb Design Sol-Aib Temperatures Used for Tables 9 and 10 Sol-Aib Temeebatuhe < Fahrenheit Deobeeb ___________ .---------------------- . 24 Hr Ayg *a -- surface absorptivity, dimensionless: roof 13 0.9; dark walla * Q.9, and light walls = 0.5. /<*> *" o** convective conductance = 4.0 Btu per (hr) (F deg). b values in this column are magnitudes oi to, the outdoor air temperature. of walls and roofs which appear in the literature.11-12 Both analytical and experimental studies have been made 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 mpruocbnlemU1s.v1o0x- uno-c1h2aav1a.!---1--4-----1--5------T- he reader may also refer to the Cooling Load pc'hroabple-tem-r so-1f0-Tuh- ew-G"u1id4-e15 19T5h2ef'roeradth'eer m-t'h--aeyoarylsoofre`hfeerattof"lotnwe t"uh-ur--ou--uu--gu-lh6 --wwaa-lls and roUoOfIsS. PRACTICAL TABLES FOR CALCULATING SOLAR HEAT i GAIN THROUGH WALLS AND ROOFS The analytical10 method reported by Mackey and Wright was used by Stewart16 to obtain temperature differentials based on Table 8 and shown in Tables 9 and 10. These analytical procedures, as well as those using Tables 9 and 10, presented here, yield generally higher rates of heat ga Cooling Load . 303 than reported for Pittsburgh in early A.S.H.A.E. experimental studies. Current authoritative opinion indicates a preference for analytical calcu lations. Thermal and physical properties of materials used in these tables are given in a paper.16 The 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 inside film conductance of 1.65 Btu (hr) (sq ft) (F deg). A reduction was made in the temperature differentials for roofs amounting to some 20 percent of solar radiation as explained by Stewart.16 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. Recent experimental work by Parmelee17 and previous work by Brunt18 gives data showing the magnitude of this radiant heat loss from a roof or wall to the sky. Temperature differentials for roofs probably would be reduced be low those shown in Table 9 whenever the radiant heat lost to the sky is in cluded in calculation of sol-air temperature. The temperature differentials for roofs were based on an inside surface conductance of 1.65 because the charts prepared by Mackey and Wright10 used this value, and it was not considered practicable to repeat their work using a different film coefficient. An examination of the values given in their paper indicates that the tem perature differential would be changed very little even if a value 1.20 were used instead of 1.65. But to obtain the heat flow rates through roofs, more accurate values will be obtained if the overall heat transmission coefficient is calculated using 1.2 as the inside film conductance for summer. The roof coefficients of transmission for summer shown in Table 11 are based on surface conductances/Cr<, of 4.0 for an outside roof surface and 1.20 for an inside ceiling surface. The outside conductance 4.0 is used for summer because it corresponds to a wind velocity of approximately 7.5 mph averaged for rough and smooth surfaces, and is mor? representative of summer wind velocities. Also, the lower wind velocity should be used in order to be on the safe side in determining the sol-air temperature. The inside conductance 1.20 is used because the convective portion of the film conductance for downward heat flow from a horizontal surface is appre ciably less than the winter conductance for heat flowing upward. Since there is little difference in wall transmission coefficients for summer, based on the conductances of 4.0 and 1.65, and the winter coefficients, based on 6.0 and 1.65, it is recommended that the overall coefficient U, for walls, be taken directly from the tables in Chapter 9 in which they are based on an outside film conductance of 6.0, corresponding to a 15 mph wind velocity. Advantages of Equivalent Temperature Differential Method The advantages of the equivalent temperature differential method of determining the total heat transmission are given in following paragraphs, and are apparent from Examples 6 to 7. mis *tal. sensible heat flow is obtained by multiplying the overall heat transTables^g coej lenl> and l-be equivalent temperature differential indicated- in tion temperature differentials listed for a few representative types of construcmj.may be used on all classes of walls and roofs, even though the overall heat trans- TM1011. coefficient is different, provided the structure has thermal and physical ^Perties similar to one of those listed in Tables 9 and 10. loom Adiustlllents can be made, according to instructions given in the footnotes, for "I and outdoor conditions different from those on which the tables are based.