Document 37zOKnQBv2dDJmObEV89QwGbJ

298 CHAPTER 13 1954 Guide 'siderable 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--Design Tables The effects and possibilities of shading should be carefully investigated whenever the heat gain from glass is a large portion of the cooling load. Vertical glass, which is not mounted in the plane of the building surface, is partially shaded by the setback. If a vertical window of height l and width to be set back from the plane of the building a distance s, the fraction of the total area of the window'which receives direct solar radiation is: : ,, r, tan 0 , nr, tan 0 tan y Gi=l-- -------------- ri tan y -I-------------------------------- cos y cos y (5) where r, = s/l, r, = s/ui, 0 = solar altitude, and y is the wall solar azimuth (see Fig. 1). Values of ft and y for various latitudes and August 1 are given in Table' 6. Special cases not covered by the tabulated data may be solved analy tically;21 however, the design conditions chosen will yield a satisfactory-; approximation if used without correction for any time during the summer, period. Example 11:. Estimate the total instantaneous rate of heat gain for a west win dow 3 ft wide by 5 ft high, with a setback of 6 in., for August X and 40 deg north; latitude at 3:00 pm (sun time). -T- Solution: From Table 13, the instaneous rate of heat gain, due to transmitted? direct and diffuse solar radiation, is 180 Btu per hr. From Table 6,0 is 45.5 deg and.;' -r.is 16 deg. From Equation 5, the fraction of the total window area that is receiving- direct solar radiation is: / Gt = 1 0.1 tan 45.5 ,, , 0.0167 tan 45.5 tan 16 ------ ;-- -- 0.167 tan 16 + ---------------------- rj---------- - cos 16 cos 16 = 1 - 0.106 - 0.048 + 0.005 = 0.851 In this instance the convection and radiation heat gain is due principally? to temperature difference, so that shading has but a small effect on that?: portion of the absorbed radiation. Hence, the factor 0.851 is applieflf only to the Table 13 value. Note also a small error results from the facfi that the diffuse radiation is not shaded to the same extent as the direct- radiation. The total instantaneous heat gain therefore is: 9 = 3 X 5 (0.851 X 180 + 26) = 2690 Btu per (hr)(sq ft) _f> A window such as the one used in Example 11 would customarily be pW"-, vided with an additional shading means for use particularly when direct!?; sunlit. Conventional shading devices include awnings, shades, and of various types. ' -J": Recent experimental and analytical work conducted at the A.S.H.Yl8| Research Laboratory26 as well as earlier experimental work,24-26 and oth^? research22 to determine the effectiveness of various types of window shade!*; Cooling Load 299 h_e_a__t _t_ra__nsfe.r. from thuei sahoasdoerptotivtihtye oofutthdeoosrhsaadnedtoinsdoo_loa__r--rs,r-awwdatiuasutiuiousneOdantod determine these ratios. There are a number of variables affecting these ratios such as color, fit, solar altitude, and angle of incidence of the solar radiation. These values, Table 24. Effect of Shading Upon Instantaneous Solar Heat Gain Through Single Thickness of Common Window Glass Type of Shading Canvas awning sides open Canvas awning top and aides tight against building Inside roller shade, fully drawn* Inside roller shade, fully drawn4 Inside roller shade, fully drawn4 Inside roller shade, half drawn* Inside roller shade, half drawn4 Inside roller shade, half drawn4 Inside Venetian blind, slats.set.at.45 degb Inside Venetian blind, slats set aC45 degb l iside Venetian blind, slats set at 45 deg^ -ctside Venetian blind, slats set at 45 degb Outside Venetian blind, slats set at 45 degb Outside Venetian blind, slats set at 45 degb' c extended as awning fully covering window Outside Venetian blind, slats set at 45 deg, extended as. awning covering H of window4 Finish on Side Exposed to Sun Dark or medium Dark or medium | White, cream I Medium Dark White, cream Medium Dark White, cream Diffuse reflecting aluminum metalj Medium Dark White, cream White, cream I bite, cream Fraction of Gain Through Unshaded Window 0 25 0.35 0.41*, 0.62 0.81 0.71 0.81 0.91 0.56* 0.45* 0.65* 0.75* 0.15* 0.15 .0.43 Outside shading screen solar altitude 10 deg Dark*- I Green Outeide shading screen solar altitude 20 deg tint. 1 Outside shading screen solar altitude 30 deg 0.52 0.46 Outside shading screen solar altitude, above 40 deg 0.40 0.35 jfttoaioioocnpmto`.r`reeHVs. vveLF*oaeenuowlannlecerittuitriwiwsdadaseinnihahrrdeeniabtofcceedoliittienntrtsrrrrddaaaarassoynvvnrsl*es*saalellrrffuuaeerrrccosrsff,seemhuuunnallmtlltdpyysseeahhdddsfaarrusdtaaiodnlewwl^gys_bnnL_effbbd_uuaaee_ro'llannttllpwwuyywddtaesendqdeccee.uoorrnvnaaevv.aswwse.e_oll_rranan_atwwtS_ssunoii..s_nndme_dd_teIIoo0_off_.wwwr5_ss5..hlhl_aaaaittIIesnlfttdadsiissrhfreoaaiasrwdfsusenhsjslualmuymlmsfewecdadliryo0a.st7htwer7daan. nt(_u*.tss_shulm_eae'_ti0sot_.c7cs_7coe_.unt pIsaia*dt.ne9*0>0t0rv0..a.w112udb555ieelfei.gga.s.))|oduuljaussreeso00tr..ss!a22s3aad24lammiatees- Uy drawn. e Commercial shade with wide slats. The sun may shine on window through si ^he exposed portion of glass as unsshlatdee.d.The sun may shine on window through sides, of shade. Estimate Commercial shade, bronze. Msheatdael ds.lats 0.05 inches wide 17 per inch and set at .. -v* *ugie wjin Don- Aasomtns*otoaMuCllan.CeorttomawabAmlemlwttcsimtoeolsuamermodctrlseeiecaagsisr0laa.ppalb0alrrle5utoolilumtggoiunmrrwicdeenhiess4nuesss0umsiivvmdbeewseellhgsyyilotah'sggwdaorredeem4.1eaa00e.tteedSddrreSeliaraaggletattessctell0ootoo.mm0s0ww.o50eel7s5sadd7ooirniillirraarnceearrhcyccheaasttesllttsssaiiwttooruuweillddaaiaederrles,srlroa,.1wy7.se5dapreteor allowed to pass between slats, and this inch, set at 17 deg angle with horizontal. pass between slats and this amount be- romcomfiJre"sT^trppoprrmaooaglgptirrfeteeiurrsssdisstneiivvpseeRabllpyyeeefggleorrrrweeeinaan4ttceRe0err*dfaa9pe.rttgAllnoosowwwmsseoooldlaaiir--al'titud* e. . used with mcounsstidbeeracbolensjuiddegremdeanst. apApnroxinimsidaetes, hoandley,isaenfdfewctiilvlehtaovethteo ebxe- ent of its reflectivity, since the portion of the solar radiation directly mnsmitted 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. INSTANTANEOUS HEAT GAINS VS. INSTANTANEOUS COOLING LOADS difference between instantaneous heat gain and instantaneous cools load has been mentioned previously; its practical importance is suffi-