Document JNOjnnB0ZjyzK37VpyjRdOoY2

310 CHAPTER 13 1957 Guide sources, so long as the temperature of these sources is not over approxi mately 450 F. The complete heat-balance for a glass section can be expressed for a unit time interval as follows: fTotal heat flow, 1 _ Transmitted, "Heat flow by convective and radiative exchanges at (2a) ]1 through glass section I solar radiation _the indoor surface The second term of the right side of Equation 2a can also be expressed by a heat balance equation as follows: Heat flow by convective-] rAbsorbed-] ["Radiative exchanges be- "| [and radiative exchanges =I solar | ] tween outer surface of glass I at the indoor surface J Lradiation J Land outdoor surroundings J Convective exchanges "| ["Heat storage-] [between outer surface of | | within the | (2b) glass and outdoor air J Lgiass sectionj Equations 2a and 2b can be. combined and expressed in symbolic terms by Equation 2c. Tabular values of the two bracketed'terms of Equation 2a are presented later in this section for various types of glass for specific design conditions. (q/A) = [tdId + tjId] -)- [auld + add + e,JE, -- tgoRzo -- /co (t,0 -- to) -- S3, Btu per (hr) (sq ft) (2c) where (q/A) = instantaneous rate of heat flow, Btu per (hour)(square foot). td, Td = transmittance of glass for direct and diffuse solar radiation, respectively. Id, Id = incident direct and diffuse solar radiation, respectively, Btu per (hour) (square foot). ,od ad = absorptance of glass for direct and diffuse solar radiation, respectively- <co => emissivity of glass at temperature t#,. R, = low temperature radiant energy falling on glass from outdoor surround ings, Btu per (hour) (square foot). Rgo = low temperature radiant energy emitted by a surface with emissivity equal to 1.0 at temperature go. fto = outdoor convective conductance, Btu per (hour) (square foot) (Fahren heit degree): (to = temperature of outdoor surface of glass, Fahrenheit degrees. to = temperature of outdoor air, Fahrenheit degrees. S = rate at which glass stores energy, Btu per (hour)(square foot). Transmissivity and absorptivity vary with both wave length of the incident radiation and incident angle. Values of r and a for a single sheet of the average ordinary drawn window glass are given in Table 12 for standard distribution of solar energy.4 Values for two air-spaced sheets are also given. Normal incidence transmittance values for some com monly-used types and combinations are given in Table 15. Some varia tion in these values' can be expected in practice due to variations f> manufacture and in solar energy distribution. However, a change ']11 transmissivity causes an approximately equal and opposite change absorptivity. Hence, the total heat flow is not greatly altered. Trans mittance data for other types of glass and various patterns of 8-in. jfl&f block are given in A.S.H.A.E. research papers.19-2021 2223 ] ;r|f; 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 raitiation, and the term tJR. -- be disregarded. It is not Cooling Load 311 practicable to give values of S in this chapter. However, for ordinary glass, the value of S is small. Fig. 3 is a graphical solution, for single glass, of Equation 2b. Only . absorbed solar radiation is considered, although low temperature radiation 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 fei as given , by Equation 3, and an equivalent surface conductance for radiation /,, as given by Equation 4. Indoor surfaces seen by the glass are assumed to radiate as a black body at room air temperature. /.i = 0.27. (t,s - t,) (3) where /<*- tgi = temperature of indoor surface of glass, Fahrenheit. t; = temperature of indoor air, Fahrenheit. A more complete treatment of the problem is given'in an A.S.H.A.E. research paper.22 Example 8: Find the total heat gain at 10 a.m. sun time for a single unshaded sheet of common window glass in a wall facing 18 deg east of south on August i 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, 8, is 56 deg 9 min. 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; re and oa 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)(sq ft). ^gThe heat gain by convection and radiation from the indoor surface is found from from which = gg + 0.06 (152.0 + 26.6) fto .4 85.7 F (q/A),, = 11.5 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 rSF.^ngle unshaded common window glass for a solar declination of 18 deg. Ihis corresponds to a nominal August 1 day. The tables are based upon Jjhe solar intensity values for a clear atmosphere as given in Table 4. fable 13 represents the first bracketed term of Equation 2a; therefore, the yalues are dependent only upon values of I and r. Table 14 is the second term of Equation 2a, and is based upon a 80 F indoor temperature and a