Document K6o5JOd4axJaVaOvM4q820Eoo

288 CHAPTER 12 1952 Guide are also given. Normal incidence transmittance valuesfor some com monly-used types and combinations are given in Table. 18. ..Some yaria-: tion in these, values can be expected in. practice due to variations in manufacture and in solar .'energy distribution. ^However, a .change jp" transmissivity causes an-approximately equal and `opposite, change.-in absorptivity. Hence, tbei'total heat flow is not' greatly- altered. ` Trims-, mittance data for other types of glass, arid various patterns of 8-in- glass blrick are given in A.SJI.V.E. research papers.16'17 20 ... A . As stated earlier in this chapter,- present data as to the value , of Rl are inadequate, so for the present it is suggested that f,, be. increased to. include'radiation, and the term e^R. f&Rt. be disregarded. - It is not practicable to give values of S in this chapter. However, for ordinary glass, the value of & is small. ,, Fig. 4 is a graphical solution, for single glass, of`Equation' 7b. Only Table 15. Tbanbmittances and Absobptances or Common Window Glass fob Dibect and Diffuse Solas Radiation Angle of Incidence, $, DEG Single Sheets .TWO AIB-SPACED SHEETS ftD D Outdoor Sheet Indoor Sheet Fob Direct Radiation 0 0.87 0.05 0.76 0.06 0.04 20 0.87 .. 0.05 0.76 0.06 0.04 40 0.88 0.06 0.74 0.08 0.04 50 0.84 0.08 0.72 0.07 0.05 60 0.70 .. 0.06 . 0.66 . 0.07 . 0.05 70 0.67 0.06 0.52 6;07 -0.05 80 0.42 0.06 0.25 , 0.07 0.05 90 . 0.0 * 0.0 0.0 0.0 0.05 Fob Diffuse ob Set Solab Radiation f . 0.79 0.06 0.68 0.07 0.05 absorbed solar radiation is considered, although low temperature radiation exchange and heat storage can be added algebraically to J 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-8, and an equivalent surface conductance for radiation fri as given by Equation 9. Indoor surfaces seen, by the glass, are.assumed to radiate as a.black body at room air temperature. ..... /ri = 0.162 [(^9)`:- (Mo?)4] / (9) where : t,i = temperature of indoor surface of glass, Fahrenheit degrees, fi = temperature of indoor air, Fahrenheit degrees. A more complete treatment of the problem is given in an A.S.H.V.E. research paper.18 . Example IS: Find the total heat gain at 10 a.m.'sun time for a single unshaderi sheet of common window glass in a wall facing 18 deg east of south on August 1 at; Cooling Load 289 50 deg' n'O'rth 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 9 min. From Example 4, Id = 152.0, Id = 26.6. By interpolation in Table 15, td is found to be 0.81, aD is 0.06; t& and d are 0.79 and 0.06, respectively. The heat gain due to transmitted solar radiation is (g/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. 4: from which t,, + f = gs + 0^6. (152.0 + 26,6) _ ^ ? y feo 4 (.g/A),, = 1U Btu per (hr)(sq ft). ' Fiq. 4. Convection and Radiation Heat Flow fob Vebtical Sinole Glass From Equation 7a the total heat flow is (g/A) = 144.1 4- 11.5 = 155.6 Btu per (hr)(sq ft). Design Tables for Flat Glass Tables 16 and.17 give design values of instantaneous rates of heat gainior 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 Hie solar, intensity values for a clear atmosphere as given in Table 5. Table 16 represents the first bracketed term of Equation 7a;.therefore, the values are dependent only upon values of 7 and t. Tabie.17 is the second term of Equation 7a, and is based upon a 75 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 16 and Table 17 values. In preparing Table 17, convection and radiation heat exchange were coinbined, and a' combined surface conductance of 4.0 used. Corrections to be applied for