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288 CHAPTER 12 1951 Guide are also given. Normal incidence transmittance values for some commonly-used types and combinations are given in Table 18. 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 heal 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.18'17 "19 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 zgjh -- e-o/2^ be disregarded. It is not practicable to give values of S in this chapter. However, for ordinary glass, the value of S is small. Fig. 4 is a graphical solution, for single glass, of Equation 7b. Only Table 15. Tradmittances and Absorptances of Common Window Glass bob Direct and Diffuse Solar Radiation Angle os* Incidence, 9, DEO .0 20 40 0 60 70 80 90 Single Sheets TWO AIR-SPACED SHEETS D D D Outdoor Sheet Indoor Sheet Fob Direct Radiation 0.87 0.05 0.76 0.06 0.04 0.87 0.88 0.05 0.06 0.76 0.74 0.06 0.06 0.04 0.04 0.84 0.06 ' 0.72 0.07 0.05 0.76 0.06 0.66 0.07 0.05 0.67 0.06 0.52 0.07 0.05 0.42 0.06 0.25 0.07 0.05 0.0 0.0 0.0 0.0 0.05 Fob Diffuse ob Set Solab Radiation 0.79 0.08 0.68 .0.07 0.05 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 fl, as given by Equation 8, and an equivalent surface conductance for radiation fTi as given by Equation 9. Indoor surfaces seen by the glass are assumed to radiate as a black body at room air temperature. U = 0.27 (,, - ,)" (8) <> where tti = temperature of indoor surface of glass, Fahrenheit degrees. ti -- temperature of indoor air, Fahrenheit.degrees. A more complete treatment- of the problem is given in an A.S.H.V.E. . research paper.19 Example IS. hind 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 Cooling,TLoada 289 CO a north latitude. The indoor temperature is 75 F, the outdoor temperature is 5" V Use clear atmosphere radiation values and = 4.0. Zhaion From Example 2, A is 0.557; hence, the angle of incidence, 0 is 56 deg o Jn F?om Emmple4,/D = 152.0,7, = 26.6. By interpolation in Table 15, rD is found to be 0.81, aD is 0.06; r,, and a, are 0.79 and 0.06, respectively. The heat gain due to transmitted solar radiation is: (g/A)r = 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: +to ajt = gg + 0.06 (1527) + 26.6) = ^ p from which Fio. 4. Convection and Radiation Heat Flow fob Vertical Single Glass From equation (7a) the total heat flow is (q/A) = 144.1 + 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 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 5. Table 16 represents the first bracketed term of Equation 7a; therefore, the values are dependent only upon values of I and r. Table 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 heal gain is the sum of.the Table 16 and Table 17 values. In-preparing Table 17, convection and radiation heat exchange were combined; and a combined surface conductance of 4.0 used. Corrections to be applied for