Document xa9YxdMQgbKkEyVXDGV54750

190 CHAPTER 13 1959 Guide Table 18 .... Description of Glass Block Patterns Type I --Smooth Face A, D: Smooth B: Wide vertical ribs or flutes C: Wide horizontal ribs or flutes E: None Type II --Semi-Light Diffusing A, D: Narrow vertical ribs or flutes B, C: Etched or stippled E: None Type III--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 Type V --Light Directing A, D: Close pitch deep vertical corrugations B, C: Horizontal light directing prisms E: None Hollow dot* Block Utodfor Horizontal SkyUghh Type VI --Light Directing (skylight) A: Smooth surface B: Light-directing prisms C, D: Light-diffusing surface E: Clear glass fiber screen Type VII--light-diffusing (skylight) A: Smooth surface B, C: Stippled surface D: Narrow-ribbed surface E: Green-tinted glass fiber screen window glass. Table 24 gives corrections to be applied for other design temperatures. Example 9: Find the total instantaneous heat gain through an east wall of 8-in. hollow glass block of Type V pattern at 8 a.ra. and 50-deg north latitude. The design temperatures are 80 F indoors and 95 F maximum outdoor dry-bulb, with a clear atmosphere. Solution: The. gain due to transmitted solar radiation is found from Table 19 for Type I pattern. The factor for Type V is found from Table 20. The convection and radiation gain is found from Table 21 (note the footnote). The total instantaneous heat gain is, from Equation 2a, q = 88 X 0.65 + 26 X 1.4 - 92.3 Btu per (hr) (sq ft). Effect of Deviations from Design Conditions If 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 fiat 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 for all types of glass and glass block in east or west facing 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 gls In equation form, (Total Gain from\ /Convection & Radiation \ Shade Glass J -- I Gain from Single ] factor 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 the 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. yhere are a number of variables affecting these ratios such as color, fit, solar altitude, and angle 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 II 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- V? Cooling Load 191 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 I Pattern For Chor Atmosphere* and 18 Deg Declination, North (August I) Note; For total instantaneous Seat gain add these vatue* to Tablo 21 vetoes tolftidi Sun Time AM --* i Instontoaeoot Hoot Gobi ia Bto per (hr) (*q ft) N NE E SE S SW W 30 Deg north 40 Deg north 50 Deg north 6 a.m. 7 8 9 6 p.m. 5 4 3 10 2 1 12 5 a.m. 7 8 7 p.m. 6 5 4 93 2 1 12 5 a.m. 6 7 8 7 p.m. 6 5 4 93 2 1 12 4 45 55 12 2 2 5 59 94 29 4 3 5 42 94 38 5 4 5 25 59 34 6 5 6 12 27 24 9 6 6 8 12 13 10 . 7 6 67 898 1 3 3000 5 50 67 17 2 4 54 98 36 4 4 5 34 90 47 54 5 18 59 47 10 5 6 8 29 35 15 5 6 - 6 13 22 18 7 6 6 6 13 17 13 4 28 27 4 5 53 77 22 4 44 101 44 4 26 86 57 11 3 44 7- 4 5 12 57 60 16 5 5 6 29 54 25 6 6 6 14 34 32 10 6 6 6 20 34 20 2 3 4 5 6 6 7 0 4 4 5 6 6 6 1 4 4 5 5 6 6 NW 2 4 5 6 6 6 0 4 4 5 6 6 6 1 4 5 5 6 6 T PM -- N NW W SW S SE NE 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. INSTANTANEOUS HEAT GAINS VS. INSTANTANEOUS COOUNG 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 the radiative part of the instantaneous heat The rwRafroo absorbed by the inferior furnishing* and stmetwo roaches the condifioasng otpmpmont after a consdaraMa delay in tine. Fig. A.... Origin of die Difference Between the Magnitudes of the Instantaneous Heat Gain and Instantaneous Cooling Load 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 the time required to change the air in the conditioned space, this is usually of the order of a few minutp-Q to perhaps half an hour. Heat storage in the interior furnishings and structure increases according to the proportion of the instantaneous heat gain which is in the form of radiation, and also increases as the thermal capac itance of the objects and materials involved is increased. Constituents of the total instantaneous heat gain which have appreciable radiation components include those due to 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, the timaa 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 t*kpi full advantage of this condition or of similar con ditions of nonsimult&neous peaks, and will result in a lower total load and in savings in equipment. A factor gimil.r in effect and closely related to the non-