Document 065rx5KMrLDExBx6jrBZ1VnEm

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1943 The percentage values in this table were obtained by dividing the total amount of heat actually entering through the shaded window by the total amount of heat calculated to enter through a bare window (solar radiation plus glass transmission, based on observed outside glass tem perature). For bare windows on which the sun shines, the transmission of heat from outside air to glass may be small or negative as the glass temperature is raised by the solar radiation absorbed. In calculating the total heat gain through windows on the sunny side of buildings, it is sufficiently accurate to proceed as.outlined herewith:' Consider the total heat gain as that resulting from solar radiation and' neglect the heat transmission through the glass caused by the difference between the temperatures of the inside and outside air. This method should be used except at times when the calculated heat gain per square foot due to normal transmission exceeds the solar intensity. At such times, solar radiation may be neglected.and the total heat gain considered as resulting from normal transmission. The solar heat transmission through windows or skylights may be expressed by the formula: where Ha = Aajl (2) Ha = solar radiation transmitted through a window, Btu per hour. Ac = net area of glass exposed to sun's rays, square feet. / = percentage of solar radiation (expressed as a decimal) transmitted to the inside (Table 7). For bare windows, / = 1. I = intensity of solar radiation striking surface, Btu per hour per square foot (Tables 2, 3, 4 and 5). In Equation 2, / = 1 for bare windows because the tests from which Table 7 was obtained showed that approximately all of the solar radiation impinging on a bare window became a part of the heat load in the room. This was because almost all of the heat absorbed by the glass flowed into the room by conduction. Other tests9 have indicated that in the case of a building having floors of high heat capacity such as concrete floors on which the solar radiation falls, some of the heat entering a bare window is , absorbed by the floor and does not immediately become a part of the cooling load,.but is delivered back to the air in the building at a slow rate. The maximum solar intensity on any surface is of limited duration as shown in Fig. 1. In the case of windows the total energy impinging on the glass before and after the time of maximum intensity is further reduced by increased shading of the glass from the frame, or wall. The cooling load due to solar radiation therefore does not have to be calculated as a steady load. Another point which should be noted is that the maximum solar radiation load bn the east wall occurs early in the morning when the outside temperature is low. Tests have been made which indicate that solar radiation through window glass is the most important factor to contend with in the cooling of an office building. At times it was shown to account for as much as 75 per cent of the total cooling necessary. Because of the importance of ,, Research Report No. 1002--Cooling Requirements of Single Rooms in a Modern Office Building, by F. C. Houghten. Carl Gutberlet. and Albert J. Wahl (A.S.H.V.E. Transactions, Vol. 41; 1935, p. 53). 156 CHAPTER 7. COOLING LOAD the sun load, cooling systems should be zoned so that the side of the building on which the sun is shining can be controlled separately from the other sides of the building. If buildings are provided with awnings so that the window glass is shielded from sunshine, the amount of cooling required will be reduced and there will also be less difference in the cooling requirements of different sides of the building. The total cooling load for a building exposed to the sun on more than one side is of course less than the sum of the maximum cooling loads in the individual rooms since the maximum solar radiation load on the different sides occurs at different times. In determining the total cooling, load for a building if the time when the maximum load occurs is not obvious, the load should be calcu- Table 8. Heat Gain Through Glass Blocks3 Solar Radiation Heat Gain (Direct plus Sky) Btu per Sq Ft per Hour Total Heat GAiNb (Solar Radiation plus Normal Transmission) Btu per Sq Ft per Hour Side EastC Westc N. Latitude Degrees 40 40 30 South 35 40 Eastc Westc 45 40 40 30 South 35 40 45 Sun Outside Time TempF 7:00 74 65.0 8:00 76 63.0 9:00 79 40.0 10:00 83 24.0 11:00 87 12 KX) 90 ilHX) 93 15.5 10.0 7.0 2:00 3:00 4:00 94 95 95 6.0 5.0 4.5 5:00 6:00 7:00 93 91 89 4.0 2.5 1.5 - 0.0 5.0 6.0 7.0 10.0 15.5 24.0 40.0 65.0 63.0 23.5 0.0 1.0 3.0 5.5 8.5 12.0 14.0 12.0 8.5 5.5 3.0 1.0 0.0 2.8 4.4 7.1 11.3 15.2 17.4 15.2 11.3 7.1 4.4 2.8 0.7 3.0 6.5 10.2 14.7 18.7 21.0 18.7 14.7 10.2 6.5 3.0 0.7 0.0 5.0 11.0 13.4 17.1 21.8 24.8 21.8 17.1 13.4 11.0 5.0 3.0 0.7 61.0 77.5 73.5 57.5 45.0 36.5 30.0 24.0 19.5 15.5 12.5 10.5 8.0 -4.5 -2.0 -0.5 0.0 . 2.0 4.0 5.0 . 5.0 7.0 10.0 6.5 11.0 15.0 18.0 7.5 10.5 22.0 16.5 21.5 25.0 22.0 25.5 28.0 33.8 31.8 . 38.5 35.0 55.0 77.0 26.0 24.0 20.0 32.0 29.8 25.5 39.0 36.5 31.5 85.5 15.0 20.0" 25.2 55.0 9.5 13.5 18.0 18.5 3.5 7.0 11.0 1.0 5.0 12.0 20.8 32.0 40.8 46.0 47.0 45.0 40.5 33.5 25.5 18.0 aFor August 1. blnside temperature,'78 F. ' CFor east and west walls these values can be applied to all latitudes between 30 and 45 deg N without excessive errors. lated for various times of day to determine the times at which the sum of the loads on the different sides of the building is a maximum. The direct solar and scattered sky radiation penetration through glass block panels is given in Table 8 for various times of the day for south, east and west exposures for different latitudes on August 1. This table also gives the total heat gain into an air conditioned space when 78 F is maintained indoors, resulting from the effect of both radiation and air to air transmission. These values result from A.S.H.V.E. Laboratory data10 and apply for expected design radiation intensity, and for a design day having a maximum temperature of 95 F. The resulting heat gains are averages for four typical glass block designs, two having smooth exterior faces, and the other two having exterior ribbed faces. Loc. Cit. Note 1. 157