Document 0q9zd48GwQr3NO3joyEmmX5gk

276 CHAPTER IS i 1948, Guide Similar equations with other numerical constants apply to other types of glass or glass areas in other arrangements. (Heat:absorbing glass or glass in multiple layers, for example). Equation 12 may be clarified further by explanations of the numerical constants involved:' -1.04 = Over-all unit conductance for heat transfer under summer conditions, Btu ' per (Hour) (square foot) (Fahrenheit degree)." (Note that this is the trans mittance U taken at 1.13 for winter conditions in Chapter 6.) 0.022 = Fraction of Id which is absorbed and then transferred to the indoor space from the indoor glass surface, dimensionless. (Note that, of the total absorption, part goes.inside and part goes outside.) 0.0165 = Fraction of 7S which is absorbed and then transferred to the indoor space from the indoor glass surface, dimensionless. 0.778 = transmissivity of the glass for sky radiation, dimensionless. Magnitudes of the quantities I&, Ia and to entering into Equation 12 would depend upon the time of the day, time of the year, atmospheric conditions, latitude of the receiving surface, and the orientation of the receiving surface. Principles and data needed for the calculation of Id and Is have been given previously in this chapter. Weather data, such as the sol-air temperatures in Tables 10 and 11, give data on ta throughout a design day. Table 15 has been prepared to expedite practical calculations of the heat gain through glass areas; tabulated values are magnitudes of the sum (0.022 Id + 0.0165 Is + raid + 0.778 Is) from Equation 12 for a solar declination of 18 deg. This declination corresponds to a nominal August 1 design day, although the values tabulated may be used with safety to represent average conditions from July 15th to August 15th. It is possible to use the heat-gain data of Table 15 for other single thickness glass materials than common window glass through the intro duction of approximate correcting factors. Table 16 gives such factors. The range of transmissivities in Table 16 extends from high-transmission crystal-like glass to low-transmission heat-absorbing glass. The trans missivity of 0.87 is that of common window glass. By using two or more layers of glass separated by air spaces, the absorptivity is increased and the transmissivity decreased. Values for some combinations appear in the literature u. A rough approximation which holds fairly well until the angle of incident radiation becomes large is that the transmissivity, of the combination is the product of the trans missivities of the component layers, while the absorptivity of the com bination is the absorptivity of the outer glass plus the- product of the absorptivity of the inner glass and the transmissivity of the outer. Heat-gain quantities from Table 15 may also be used for other times of the year than August 1 if a correction is made for solar declination. Solar heating calculation for glass areas can also be based upon these data. Table 17 gives solar declinations for various times of the year. The rule for procedure is the following: To find the radiation heat gain for some declination of the sun other than 18 deg use the latitude in Table 15 equal to that of the locality in question plus (18 deg--solar declination involved). For example, consider that the instantaneous heat gain is required for Philadelphia in the middle of June. The latitude of Philadelphia is 40 deg North. The solar decimation in mid-June is about 23.5 deg. The section of Table 15 to be used is then that for a latitude of 40 + (18 -- 23.5) = 34.5 deg; say 35 deg. Caution: This method gives only approximate values and its use should be limited. It may be used for Eastern and Western exposures for all hours 8:00 a.m. to 4:00 p.m. Cooling Load 277 .raiAni/uituus rvAriis of meat (jain ube io Solar and Sky for Single Sheets of Unshaded Common Window Glass a- h Computed for Solar Declination of 18 Deg--August 1 Note: To determine the total instantaneous rate of heat gain add the term 1.04 (to-ti) to the values shown in the table (see Equation 12). ------------------------------------------------------------------------------- . V Sun Time ` Solar Altitude e Dec Instantaneous Rate of Heat Gain, Btu per Hour - for Each Square Foot of Unshaded Glass N NE . E SE S SW W NW Horizontal 25 Deg North Latitude SO Deg North Latitude 35 Deg North Latitude 5 a.m. 9 12 1 p.m. 1.5 11.5 23.0 34.5 43.5 56.0 64.5 68.0 64.5 56.0 45.5 34.5 23.0 11.5 1.5 40 Deg North Latitude 7 IS 17 6 23 106 120 62 15 141 181 118 14 122 194 147 15 76 172 156 16 30 125 144 16 16 53 110 16 16 16 62 16 16 16 22 16 16 16 16 15 15 15 15 14 14 14 14 15 11 11 11 23 5 5 5 71 1 1 111 11 5 11 5 11 5 11 19 42 14 15 14 15 14 15 66 16 16 16 85 22 16 16 94 62 16 16 85 110 52 16 66 144 125 30 42 156 172 76 19 147 194 122 118 181 141 62 120 106 1 . 6 17 18 2 24 82 145 196 235 261 269 261 235 196 145 82 24 2