Document QgL0O1vQYE2reGdaoVMJqQx8

298 CHAPTER 12 1952 Guide y is 16 deg. From Equation 10, the fraction of the total window area that is receiving . direct solar radiation is: O^f = 1 -- 0-.-1-c-ot-a-s-n- 1--465.5-- 0.1,,67 tan 16 + --0.0167---t-ac--on-s--415--6.5--t-a---n---1-6 = 1 - 0.106 - 0.048 + 0.005 = 0.851 In this instance the convection and radiation heat gain is due principally "'to temperature difference, so that shading has but a small effect on that portion of the absorbed radiation. Hence, the factor 0.851. is applied only to the Table 16 value. Note also a small error results from the fact Table 27. Effect of Shading upon Instantaneous Solas Heat Gain Through Single Thickness of Common Window GlaSs Type op Shading Finish on Side Exposed to Sun Fraction op Gain Through Un shaded Window Canvas Awning Inside Roller Shade, Fully Drawn* Inside Roller Shade, Fully Drawn* Inside Roller Shade, Fully Drawn* Inside Roller Shade. Half Drawn* Inside Roller Shade, Half Drawn* Inside Roller Shade, Half Drawn* Inside Venetian Blind, Slats set at 45 degb Inside Venetian Blind, Slats set at 45 deg.b Inside Venetian Blind, Slats set at 45 degb Inside Venetian Blind, Slats set at 45 degD. Outside Venetian Blinds Slats set at 45 degb Outside Venetian Blind, Slats set at 45 deg, extended as an awning0 Outside Shading Screen,1} solar altitude 0*20 deg Outside Shading Screen, solar altitude 20-40 deg Outside Shading Screen,*1 solar altitude, above 40 deg Dark White Medium color Dark color White - Medium color Dark color White Medium Aluminum Dark color Cream Any color Dark color Dark color Dark color 0.25-0.35 0.45 0.63'' 0.80 0.72 0.81 0.90 0.62 0.74 0.70 . 0.86 0.30 0.40 0.75-0.43 0.43-0.22 0.22 * Roller shades are assumed to be opaque. Some white shades may transmit -considerable solar radia tion. For white translucent shades fully drawn use 0.55, and for half drawn use 0.77. b Venetian blinds are fully drawn and cover window. It is assumed that the occupant will adjust slats to prevent direct rays from passing between slats. c Commercial shade with wide slats. d Metal slats 0.05 in. wide, spaced 0.063 in. apart, and set at 17 deg angle with horizontal. At solar alti tudes below 38 deg some direct solar rays are allowed to pass between slats, and this amount becomes progres sively greater at low solar altitudes. that the diffuse radiation is not shaded to the same extent as the direct radiation. The total instantaneous heat gain therefore,is: g = 3 X 5 (0.851 X 180 + 26) = 2690 Btu per (hr)(sq ft) A window such as the one used in Example 16 would customarily be pro vided with an additional shading means for use particularly when directly sunlit. Conventional shading devices include awnings, shades, and screens of various types. Experimental work conducted at the A.S.H.V.E. Research Labora tory,21-22 and other research22 to determine the effectiveness of various types of window shades, have been used as the basis for the recommended ratios in column 3 of Table 27. A study of absorptivity of the shade to solar radiation and heat transfer from the shade to the outdoors and indoors, was used to determine these ratios. There are a number of variables affecting these ratios such as color,-fit, solar altitude, and angle of incidence of the solar radiation. These values, therefore, must be considered as approximate, only, and will have to be used with considerable judgment. An inside shade is effective to the ex- Cooling Load 299 tent of its reflectivity, since the portion of the solar radiation directly transmitted by the glass that is absorbed by the shade is transferred by convection to the room air, and by radiation to the solid room surfaces. INSTANTANEOUS HEAT GAINS VS. INSTANTANEOUS COOLING LOADS The difference between instantaneous heat gain and instantaneous cool ing load has been mentioned previously; its practical importance is suffi cient to warrant further consideration. Fig. 5 offers a simplified schematic illustration showing how the radia tive part of the instantaneous heat gain is first absorbed by solid objects, and is not encountered by the conditioning equipment as a cooling load until some later time, when it finally appears in the air stream entering the equipment. While it is true that some lag also is inherent in convective heat transfer and the time required to change the air in the conditioned space, this is usually of the order of a few minutes to perhaps half an hour. Heat storage in the interior furnishings and structure increases according to the proportion of the instantaneous heat gain which is in the form of radiation, and also as the thermal capacitance of the objects and materials involved is increased. Constituents of the total instantaneous heat gain which have appreciable radiation components include those due to glass areas, exposed walls and roofs, lighting, appliances, and people.. No comprehensive data are presently available for use in design load estimates to evaluate the interior load-lag effect, but several investiga tors13- M-2425 have made a study of the problem and have presented many useful data. Tables 12,13,16,17,21,23, and 24 are all based on instantane ous rates of heat transfer. Hence, practical judgment and experience offer the only basis of procedure. Until the needed data become available, it is recommended that the non-continuous load be averaged over two or three hours during the time of maximum load, when determining the total instantaneous cooling load where a large portion of the heat gain is radiant. This suggestion applies only to conditions near the time of maximum heat gain, as the heat stored within the structure would necessarily appear in the cooling load eventually; but if it appears at a time when the gain from outside is relatively low, the equipment will be able to maintain satisfactory conditions within the range of maximum capacity. LOAD FROM INTERIOR PARTITIONS, CEILINGS, AND FLOORS Whenever a conditioned space is adjacent to another space in which a different temperature prevails, the transfer of heat through the separating structural section must be considered. Calculations are made according to the relation: q = UiA ,(tb -- t,) Btu per hour. (11) where Ui = coefficient of overall heat transfer between the adjacent and the condi tioned space, Btu per (hour) (square foot) (Fahrenheit degree). A\ = area of separating section concerned, square feet. lb = air temperature in adjacent space, Fahrenheit degrees, fi = air temperature in conditioned space, Fahrenheit degrees. Magnitudes of XJ-, may be obtained from Chapter 9. . The temperature