Document BQ6JwD64xKB3Jpo6bB4NxxyL

286 CHAPTER 13 1953 Guide y ' where (q/A) = instantaneous rate of heat flow.Btu per (hour)(square foot). - td, rd = transmittance of glass for direct and diffuse solar radiation, respectively. Id, Id = incident direct and diffuse solar radiation, respectively, Btu per (hour) (square foot). , ap, ai = absorptance of glass for direct and diffuse solar radiation, respectively. e,,, = 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 t#,. fco = outdoor convective conductance, Btu per (hour) (square foot) (Fahren heit degree). (, = 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). | -i 1 I I?r - Table 12. Tbansmittances and Absobftances or Common Window Glass for Direct and Diffuse Solar Radiation Angle op Incidence, 0, DEG 0 20 50 60 90 - .Single Sheets TWO AIR-BPACED SHEETS td 0.87 0.87 0.86 0.84 0.79 0.67 0.42 0.0 0.79 d . td Outdoor Sheet Indoor Sheet Fob Direct Radiation 0.05 0.05 0.06 0.76 0.76 0.74 . 0.06 0.06 0.06 0.06 0.72 0.07 0.05 0.06 0.66 0.07 0.06 0.52 0.07 0.06 0.0 0.25 0.0 0.07 0.0 0.05 . 0.05 Fob Difpuse ob Sky Solar Radiation | 0.06 0.68 0.05 pr 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 fora 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 in manufacture and in solar energy distribution. However, a change in transmissivity causes an approximately equal and opposite change in absorptivity. Hence, the total heat flow is not greatly altered. Trans 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.19-20-21 2223 As stated earlier in this chapter, present data as to the value of R. are inadequate, so for the present it is suggested that be increased to include radiation, and the term eJEt, -- be disregarded. It is hot 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 1 I m. Cooling Load 287 exchange and heat stofage'can be added algebraically to ajt 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/Ci as given by Equation 3, and an equivalent surface conductance for radiation /ti as given by Equation 4. Indoor surfaces seen by the glass are assumed to radiate as a black body at room air temperature. .. . , U, = 0.27.,,, . <,)" (3) . (4) 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 sheet 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, 9, 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, <*,, is 0.06; r<j and aa 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). The heat gain by convection and radiation from the indoor surface is found from Fig. 3: ,,,, , 0.06 (152.0 + 26.6) Sd -I----------------- 85.7 F from which (q/A)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 Tab les for Flat Glass Tables 13 and 14 give design values of instantaneous rates of heat gain for single unshaded common window glass for a solar declination of 18 deg. This corresponds to a nominal August 1 day. The tables are based upon the solar intensity values for a clear atmosphere as given in Table 4. Table 13 represents the first bracketed term of Equation 2a; therefore, the values are dependent only upon values of 7 and r. Table 14 is the second term of Equation 2a, and is based upon a 80 F indoor temperature and a dry-bulb temperature cycle, with a 95 F maximum as tabulated. The total heat gain is the sum of the Table 13 and Table 14 values. In preparing Table 14, convection and radiation heat exchange were combined, and a combined surface conductance of 4.0 used. Corrections to be applied for