Document Vj3BDjDr2mp0dp3MxD91pwvrj

200 CHAPTER 13 1960 Guide Table 18----- Description of Gloss Block Patterns Type I A, D B C ffoOow Qau Dock Und for Vertical WoIj --Smooth Face 8mooth Wide vertical ribs or flutes Wide horizontal ribs or flutes None U --Semi-Light Diffusing ; D Narrow vertical ribs or flutes B, C Etched or stippled None 'ype HI--Light Diffusing A, D: Narrow vertical ribs or flutes B,C Etched or stippled E Glass fiber screen Type IV --Light Diffusing A, D: Close pitch deep horizontal corrugations B, C Vertical light diffusing prisms E None Type IVA--light Diffusing Same as IV except corrugations vertical Tyi* V --Light Directing . ,D: dose pitch deep vertical corrugations B;-C:. Horizontal light directing prisms ' E: None- Hollow dots Block Us*d for Horizontal Skyffgfcf* Type VI --light Directing (sfcyiight) A: Smooth surface B: Light-directing prisms C, Dr light-diffusing surface E: Clear glass fiber screen Type VII--Light-diffusing (skylight) A: Smooth surface B, C: Stippled surfaee D: Narrow-ribbed surface E: Green-tinted glass fiber screen The total instantaneous heat gain is, from Equation 2a, ? - 86 X 0.65 + 26 X 1.4 925 Btu per (hr) (sq ft). Effect of Deviations from Design Conditions Ii the indoor temperature differs from 80 F, or the design outdoor dry-bulb temperature differs from 95 F, corrections can be made to the convection and radiation gain values for flat glass, rolled figured glass, and glass block according to the schedule in Table 24. The effect of the humid industrial-type atmosphere is to cause a considerable reduction in heat gain, if all factors except solar intensity remain the same. Reference 22 gives heat gain values for four orientations at 40-deg north latitude on August 1 for several types of flat glass and glass block, and solar intensities typical of humid industrial atmospheres. These data show that the following approximate reductions, based on total gain for the day, can be expected: 20 percent walls; 10 percent for south facing flat glass; 5 percent for south facing glass block walls. Shading of Glass Areas--Shade Factors The shade factor is defined as the total gain from a shadeglass combination minus the convection and radiation gain from single unshaded common window glass divided by the total incident solar radiation transmitted by single unshaded common window glass. In equation form, factor (Total Gain from\ /Convection & Radiation \ Shade Glass l -- l Gain from Single j Combination / \Unshaded Common Glass/ Total Solar Energy Transmitted by Single Unshaded Common Glass Design shade factors for a number of combinations are listed in Table 25. The total gain for a shaded window is found by multiplying tire values of Table 12 by the appro priate shade factor and adding to these values the correspond ing values from Table 13. The values for sunlit windows are average values based upon calculations for several orientations. Actually the shade factor varies somewhat with orientation and time of day, because the proportion of direct to diffuse incident solar radiation as well as the profile angle varies. Generally these variations may be neglected. There are a number of variables affecting these ratios such as color, fit, solar altitude, and angte of incidence of the solar radiation. These values, therefore, must be considered as approximate, only, and will have to be used with consider able judgment. An inside shade is effective to the extent of its reflectivity, since the? portion of the solar radiation directly transmitted by the; glass that is absorbed by the shade is transferred by convection to the room air, and by radiation to the solid room surfaces. A more complete analysis' of shade factors may be found in Reference 25. Roof Overhang, Canopies, and Other Horizontal Projections ` In some cases, a considerable reduction in heat transfer through windows can be attained by the use of a suitable horizontal projection. This is particularly true for S, SE, and SW orientations. In Table 26 are tabulated the required projections to produce a shadow height of 10 ft on a window or wall. The projection necessary for other shadow heights can be found by direct ratio. For example, a shadow height of 5 ft would require one-half the projection needed for a shadow height of 10 ft. Note that the table is for the period from April 11 through September 1. The choice of projection should not necessarily be based on providing complete shading during all daylight hours. Reference should be made to Tables 12 through 15 to deter mine during which hours of the day the solar gain is suffi ciently large to indicate the desirability of shading. This and the architectural practicability of the required projec tion should be deciding factors. Vertical glass, which is not mounted in the plane of the building surface, is partially shaded by the setback. If a ver tical window of height l and width w be set back from the plane of the building a distance a, the fraction Gf of the total area of the window which receives direct solar radiation is: <7,-1 r, tan fi rtft tan g tan y (5) Cooling Load 201 Table 19 .... Instantaneous Rate of Heat Gain Due to Transmitted Direct and Diffuse Solar Radiation by Unshaded Walls of 8-in. Hollow Glass Block of Type 1 Pattern hr Char Ahnoxpbons and 18 O*g Oedinoihr^ North (Aupuxt Tl Note For totaf tmfoatanoou* hoot Qabi add the-- value* to Fafate 21 value* latitude San Time AM -* i liutonteneoer Heat Coca co Btv per (hr) (*q fl) N NS i SE S sw W 30 Deg north 40 Deg north 50 Deg north 6 p.m. 84 93 10 2 l 12 5 a.co. 7 p.m. 5 84 93 2 12 5 a.ro. 6 7 8 7 p.m. 6 5 4 3 10 2 l- 12 4 45 55 12 5 59 94 29 5 42 94 38 5 25 59 34 2 4 S 6 22 33 44 5 6 12 27 24 9 6 8 12 13 10 6 67 89 66 76 87 3 3 0 0 00 5 50 67 17 4 54 98 36 4 44 5 34 . 90 47 5 4 4 5 18 59 47 10 5S 6 8 29 35 15 5 6 e 6 13 22 18 7 6 6 6 6 13 17 13 6 4 28 27 4 1 11 5 53 77 22 3 22 4 44 101 44 4 44 4 28 86 57 7 4 4 5 12 57 60 16 55 5 6 29 54 25 6 5 6 6 14 34 32 10 6 6 6 6 20 34 20 6 NW 2 3 4 6 6 6 0 4 4 5 6 6 6 1 2 4 4 5 5 6 6 T PM --* N NW W SW S SE E N where ft -- t/l, ft \a/w, 0 -- solar altitude, and y is the wall solar azimuth (See Fig. 1) INSTANTANEOUS HEAT GAINS VS. INSTANTANEOUS COOLING LOADS The difference between instantaneous heat gain and in stantaneous cooling load has been mentioned previously; its practical importance is sufficient to Warrant further consideration. Fig. 4 offers a simplified schematic illustration showing how toe radiative part of the instantaneous heat gain is first absorbed by solid objects, and is not encountered by the conditioning equipment as a cooling load until some later time, when it finally appears in the air stream entering the equipment. While it is true that some lag also is inherent in convective heat transfer and toe time required to change the air in the conditioned space, tiffs is usually of toe order of a few minutes to perhaps half an hour. Heat storage in the interior furnishings and structure increases according to the proportion of toe instantaneous heat gain which is in the form of radiation, and also increases as the thermal capac itance of tiie objects and materials involved is increased. Constituents of the total instantaneous heat gain which have appreciable radiation components include those due to glass areas, exposed walls and roofs, lighting, appliances and people. A large difference in the time-incidence of the peaks be tween various spaces or parts of the same space indicates the necessity for zoning. In a building having an east and west exposure, where solar heat gains form a fair share of the cooling load, toe times of individual zone peaks are apt to be some hours apart, and the peak load of one plus the off-peak load of the other will be substantially less than their combined peak loads. Proper zoning will permit operation to take full advantage of this condition or of similar con ditions of nonsimultaneous peaks, and will result in a lower total load and in savings in equipment. A factor similar in effect and closely related to toe non- 1Te radiation absorbed by the interior fomb&ngt end ttnefun reoche* ft* condnkxung equipment after a eonadtrobU deby in time. Fig. A..., Origin of the Difference Between the Magnitudes of the Instantaneous Heat Gain and Instantaneous Cooling Load