Document wgqXeq7M2gGq9V03wM870r5No

where '; (?M) instantaneous rate of heat Sow, Btu per (hour) (square foot), transmittance of glass for direct and diffuse solar radiation, respectively, TD, /D, * incident direct and diffuse solar radiation, respectively, Btu per (hour) (square foot). absorptance of glass for direct and diffuse solar radiation, respectively, ap, ad s tgo = R.- emissivity of glass at temperature tm. low temperature radiant energy falling on glass from outdoor surround ings, Btu per (hour) (square foot). low temperature radiant energy emitted by a surface with emissivity Kgo : equal to 1.0 at temperature t&,. /co ; outdoor convective conductance, Btu per (hour) (square foot) (Fahren heit degree). f*o : temperature of outdoor surface of glass, Fahrenheit degrees. <. = temperature of outdoor air, Fahrenheit degrees. rate at which glass stores energy, Btu per (hour)(square foot). 3`; Table 12. Tbansmittanceb and Absohptances of Common Window Glass fob Dibect and Diffuse Solab Radiation Angle op Incidence, 0,-DEO Single Sheets D TWO AIR-SPACED SHEETS aD tD Outdoor Sheet Indoor Sheet -------------- Fob Direct Radiation 0 20 : 40 50 60 70 80 00 0.87 0.87 0.86 0.84 0.79 0.67 0.42 0.0 0.05 0.05 0.06 0.06 0.06 0.06 0.06 0.0 0.76 0.76 0.74 0.72 0.66 0.52 0.25 0.0 0.06 0.06 0.06 0.07 0.07 0.07 0.07 0.0 Fob Diffuse or Set Solar Radiation 0.04 0.04 0.04 0.05 0.05 0.05 0.05 0.05 f ,l! is ... Si *1; 0.79 | 0.06 I o.ss 0.07 0.05 % rt'4l <s Transmissivity and absorptivity vary with both wave length of the; : incident radiation and incident angle. Values of t and a for a single sheet ' . - of the average ordinary drawn window glass are given in Table 12 for.a- ; standard distribution of solar energy.4 Values for two air-spaced sheefcyg are also given. Normal incidence transmittance values for some coig&J monly-used types and combinations are given in Table 15. Some vanter-V tion in these values can be expected in practice due to variations' in;:; manufacture and in solar energy distribution. However, a change1 inj? transmissivity causes an approximately equal and opposite change lipj'-/ absorptivity. Hence, the total heal flow is not greatly altered. Trans-JS mittance data for other types of glass and various patterns of 8-in. glass|; block are given in A.S.H.V.E. research papers.19'10 ** **23 As stated earlier in this chapter, present data as to the value of!?*S| are inadequate, so for the present it is suggested that /,, be increased include radiation, and the term qA -- be disregarded. It is'n^i practicable to give values of <S in this chapter. However, for ordinap? glass, the value of <S is small. Fig. 3 is a graphical solution, for single glass, of Equation 2b. absorbed solar radiation is considered, although low temperature radiatioJi Cooling Load 289 exchange and heat storage can be added algebraically to aj, if such data are available. The small thermal resistance of the glass has been neglected; The heat flow rates are for a 75 F indoor temperature, an indoor surface conductance for convection/oi as given by Equation 3, anu an equivalent surface conductance for radiationas given by Equation 4. Indoor surfaces seen by. the glass are assumed to radiate'as a black body at room air temperature. Ux = 0,27.(iti:- h) (3) where tsi = temperature of indoor surface of glass, Fahrenheit. ti = temperature of indoor air, Fahrenheit. A more complete treatment of the problem is given in an A.S.H.V.E. research paper.22 Example 8: Find-the .total heat gain at 10 a.m. sun time for a single unshaded Bheet of common window glass in a wall facing 18 deg east of south on August 1 at 50 deg north latitude. The indoor temperature is 75 F, the outdoor temperature is 83 F. Use clear atmosphere radiation values and = 4.0. Solution: From Example 2, K is 0.557; hence, the angle of incidence, 0, is 56 deg 9min. From Example 4,Id -- 152.0,Id = 26.6. By interpolation in Table 12, rD is found to be 0.81, aD is 0.06; and ad are 0.79 and 0.06, respectively. The heat gain due to transmitted solar radiation is (q/A), = 152.0 X 0.81 + 26.6 X 0.79 = 144.1 Btu per (hr)(Bq ft). FigT. h3e: heat gain by convection and radiation from the indoor surface is found from from which to + ttJt foo 83 + 0.06 (152.0 + 26.6) 4 85.7 F (q/A),, = 115 Btu per (hr)(sq ft). From Equation 2a the total heat flow is (q/A) = 144.1 + 11.5 = 155.6 Btu per (hr)(sq ft). Design Tables for Flat Glass Tables 13 and 14 give design values of instantaneous rates of heat gain jor single unshaded cofnmon window glass for a solar declination of 18 deg. .this corresponds to a nominal August 1 day. ' The tables are based upon Th]Slar intensity values for-a clear atmosphere as given in Table 4. lable 13 represents the first bracketed term of Equation 2a; therefore, the yahies are dependent only upon values of I and t. Table 14 is the second j1 Equation 2a, and is based, upon a 80 F indoor temperature and a i b temperature cycle, with a 95 F maximum as tabulated. The w heat gain is the sum of the Table 13 and Table 14 values. In preparing able 14, convection and radiation heat exchange were' combined, and a ombmed surface conductance of 4.0 used. Corrections to be applied for