Document gaYwQbYd1mv2vok9gR5O1Kj1Q

m T: 284 CHAPTER 12 1950 Guide. A window such as the one used in Example 9 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,14 and other research1' 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 23. A study of absorptivity of the shade to solar radiation and heat transfer from the shade, to the outdoors and indoors, wa3 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- Tablb 23. Effect of Shading upon Instantaneous Solar Heat Gain Thbouoh Single Thickness or Couiion Window Glass Ttpb or Shading 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* Inride Roller Shade, Half Drawn* Inside Venetian Blind, Slats set at 45 deg Inside Venetian Blind, Slats set at 46 deg Inside Venetian Blind, Slats set at 45 deg Inside Venetian Blind, 8lats set at 45 deg. Outride Venetian Blind, Slats set at 45 dig* Outride Venetian Blind, Slats sst at 45 deg, extended as an awning" Outside Shading Screen*, solar altitude 0-20 deg Outride Shading Screen, solar altitude 20-40 deg Outside Shading Screen, solar altitude, above 40 deg Finish on Sidb Exposbd to Suit Fraction qw Cash Through Un shaded Window Dark White ` Medium color Dark color White Medium color Dark color White Medium Aluminum Dark color Cream 0.25-0.85 0.45 0.63 0.80 0.72 0.81 0.80 0.62 0.74 0.70 0.86 0.30 Any color Dark color Dark color Dark oolor 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 oover window. It is assumed that the occupant will adjust elate to prevent direct rays from passing between elate. Commercial shade with wide slats. - d Metal slate 0.05 inches wide spaoed 0.053 inches apart and set at 17 degree angle with horizontal. At solar altitudes below 39 deg some direct solar ray* are allowed to pass between slats, and this amount beh comes progressively greater at low solar altitudes. 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. 6 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 Cooling Load 285 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 tors'. 16 have made a study of the problem and have presented much useful- data. Tables 14, 15 and 18 are all based on instantaneous 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 sug gestion applies only to conditions near the time of maximum heat gain, as the heat stored within the structure would necessarily appear in the cool ing load eventually; but if it appears at a time when the gain from outside Fig. 6. Origin of the Difference Between the Magnitudes of the Instantaneous Heat Gain and Instantaneous Cooling Load The radiation absorbed by the interior furnishings and structure reaches the conditioning equipment after a considerable delay in time. is relatively low, the equipment will be able to maintain satisfactory condi* tions withm 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: g = UiAi(b -- ti) Btu per hour. (12) where XJ-, = coefficient of overall heat transfer between the adjacent and the condi tioned space, Btu per (hour) (square foot) (Fahrenheit degree), At - area of separating Bection concerned, square feet, <b = air temperature in adjacent space, Fahrenheit degrees, = air temperature in conditioned space, Fahrenheit degrees. Magnitudes of XJi may be obtained from Chapter 9. The temperature tx may have any value over a considerable range, according to conditions in the adjacent space. The temperature in a kitchen or boiler room may be as much as 15 to 50 deg above the outdoor air temperature. It is recom mended that actual temperatures be measured in adjoining spaces wherever practicable^ Where nothing is known, except that the adjacent space is of conventional construction and contains no heat sources, it is recommended that the difference ((b -- 4) be taken as the difference between the out-