Document M84EExNqGOzDwy0VeEKZNxZV

498 CHAPTER 27 1965 Guide And Data Boole la - K X /o. = 0.657 X 273 - 152.0 Btu per (hr) (sq ft). By linear interpolation, the diffuse irradiation is: U = 25 + 18/90 (33 - 25) - 26.6'Btu per (hr) (sq ft). The total solar irradiation is: It - 152.0 -f 26.6 - 178.6 Btu per (hr) (sq ft). PERIODIC HEAT FLOW THROUGH WALLS AND ROOFS The calculation of heat flow through.a structural section, of a building exposed to the weather requires consideration of the diurnal cycles of solar irradiation and air temperature. Various analytical techniques have been employed for com bining the effects of the heat capacity of the structural section of the building and the variation in outdoor weather. One of the most promising analytical techniques is that employed by Buchberg.**-0 This method employs the transfer function concept for the building wall or roof. If the frequency response for various wall and roof configurations is determined; the data can be used with the Fourier series of outdoor weather to predict the periodic variation of heat flow to the interior of the space. Usually, the fundamental and three or lem har monics are sufficient to represent the'eombination of outdoor temperature and solar irradiationorsol-air temperature. The Sol-Air Temperature . . .. The complex interrelationship of-the above factors can be considerably simplified through the.use of the sol-air tem perature concept. The Bol-^air temperature't, is the tempera ture of the outdoor air, which, in the absence of all radiation exchanges, would give the same, rate of heat entry; into the surface as would exist with the actual combination of.incident solar radiation, radiant energy exchange with, the sky and other outdoor surroundings, and convective heat exchange with the outdoor air, or t, = t* + (al/fj). The sol-air temperature data**-0'" as developed by Mackey and Wright for an industrial atmosphere were used as a basis for preparing Table 7 which shows summer design sol-air tem peratures. Sol-air temperatures may also be estimated from experimental observation of surface temperatures of walls and roofs which appear in the literature.**-0 Both analytical and experimental studies have been made on the problem.0 of heat flow through walla 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 tire reports of experimental studies of these problems.1"**'**-*4 ... PRACTICAL TABLES FOR CALCULATING SOLAR HEAT GAIN THROUGH WALLS AND ROOFS The analytical0 method reported by Mackey and Wright was used by Stewart0 to obtain temperature differentials based on Table 7 and shown in Tables 8 and 9. These ana lytical procedures, as well as those using Tables 8 and 9, presented here, yield generally higher rates of heat gain than those reported for Pittsburgh in early.ASHRAE experi mental studies. Current authoritative opinion indicates a preference for analytical calculations. Thermal and physical properties of materials used in these tables are given in a paper.0 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 8 and 9 were developed by using an.outside surface conductance of 4.0 and an inside film conductance of 1-65 Air-Conditioning Cooling Load leBtvd* 80 Deg north 40 Deg north 50 Deg north Table 5 .... Values of the Wall Solar Azimuth, y, for Variously Oriented Walls and Solar Altitude Computed tor 0*9 Peefaofroo, Norrt [Avgvti I) Son Tim Solar Atiitvdm P Degree* Azustrffa Angie y, Degree* AM --* i 6 aun. 6 pjn. 93 12 5 a.m. 7 pxn. 84 12 5 a.m. 7 p.m. 84 1 12 9.0 21.6 34.5 47.5 78.0 0.5 23.0 34.5 64.5 68.0 4.5 13.5 23.5 33.0 56.0 58.0 H HE E 74 81 88 Shade 66 76 85 'Shade 29 36 43 51 62 83 'Shade 21 31 40 50 61 76 Shade 67 78 90 Shade 22 33 45 57 70 87 Shade 16 9 2 6 17 38 90 . 24 14 5 5 16 31 55 90 23 12 0 12 25 42 64 90 SE 5 61 54 47 Shade 39 84 28 73 7 52 45 0 69 59 50 Shade 40 29 74 14 59 10 35 45 0 68 67 45 90 33 78 20 65 3 48 19 26 45 0 T PM --* N NW . W sw S 499 SW Shade 45 Shade 45 Shade 45 SE Btu (hr) (sq ft) (F deg). A reduction was made in the tem perature differentials for roofs amounting to some 20 percent of solar radiation as explained.by Stewart.0 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. Experimental work by Parmelee** and per vious work by Brunt*7 give data showing the magnitude of this radiant heat loss from a roof or wall.to the sky. Temper ature differentials for roofs probably would be reduced below those shown in Table 8 whenever the radiant heat lost to the sky is included in calculation of sol-air temperature. The temperature differential* for roofs-were based on an in side surface conductance of 1.65 because the charts prepared by Mackey and Wright** used this value, and it was not practicable to repeat their work using a different film co efficient An examination of the values given in their paper indicates that the temperature differential would be changed very little even if a value 1.20 were used instead of 1.65. To obtain the heat-flow rates through roofs, more accurate Table 6 .... Approximate Solar Declinations in Degrees Dote April 1 April 16 May 1 May 15 June 1 Jane 15 Declination - 4.5 10.0 15.0 19.0 22.0 23.5 Date July i . July 15 Aug. 1 Aug. 15 Sept. 1 Sept. 15 DcdtnoRon 23.0 . 21.5 18.0 14.0 8.5 3.0 values will be obtained if the overall heat transmission co efficient is calculated by using 1.2 as the inside film conductance for summer. '..................' - ` The roof coefficients of transmission for summer shown in Table 10 are based on surface conductances /. of 4.0 for an outride roof surface and 1.08 for an inside filing surface. Since there is little difference in wall transmission coefficients for summer (based on the conductances of 4.0. and 1.63) arid the winter coefficients (based on 6.0 and 1.63) it is recom mended-that the overall coefficient U,-for walls, be taken directly from the tables in Chapter 24 in which they are based on an outride film conductance of 6.0, corresponding to a 15 mph wind velocity. Advantages of. Equivalent Temperature Differ ential Method The advantages of the-equioalent temperature differential method of determining the total heat transmission arc given in following paragraphs, and are apparent from Examples 6 to 7. 1. The total sensible heat flow a obtained by multiplying the overall heat transmission coefficient U, and the equivalent temperature differential indicated in Tables 8 and 9. r 2. The temperature differentials listed for a few representa tive types of construction may be used on all classes of walls, and roofs, even though the overall heat transmission coefficient is different, providedthe structure has-thermal and physical properties milar to one of those listed in Tables 8 and 9-. . . 3. Adjustments can be made, according to instructions given in the footnotes,' for room and outdoor conditions differ ent from those on which the tables are based..j j .ii .*