Document 8Vp2OR9LDBawbMOG89X1nDj5y

310 CHAPTER 13 1956 Guide Research Laboratory2* as well as earlier experimental work,24'28 and other research27 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 24. 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 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. Fig. 4. Obigin op the Diffebence Between the Magnitudes of the Instan taneous 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. 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. 4 offers a simplified schematic illustration showing how the radiative 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 investigators"' i4, 28. 29. so ]jave made a study of the problem and have presented many useful data. Tables 9,10,13,14,18,20, and 21 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 Cooling Load 311 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 = U,A,(.tb -- ti) Btu per hour. (6) where U% = coefficient of overall heat transfer between the adjacent and the condi tioned space, Btu per (hour) (square foot) (Fahrenheit degree). A i = area of separating section concerned, square feet, fb = air temperature in adjacent space, Fahrenheit degrees. ii = air temperature in conditioned space, Fahrenheit degrees. Magnitudes of (/, may be obtained from Chapter 9. The temperature (b 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 (fi, -- b) be taken as the difference between the out door-air and conditioned-space design dry-bulb temperatures minus 5 deg. In some cases it may be that the air temperature in the adjacent space will correspond closely to the outdoor air temperature at all times. Undei these latter conditions, the heat gain through the partition will be periodic in nature, and the value of a shaded wall should be used from Table 10. For floors directly in contact with the ground, or over an underground basement that is neither ventilated nor warmed, the heat transfer may be neglected for cooling-load estimates. LOAD FROM OUTDOOR AIR, VENTILATION AND INFILTRATION Ventilation. Data for determining the necessary ventilation rate have been presented previously in this chapter. Ventilation required is pri marily dependent upon the number of occupants and upon the materials and apparatus within the space which may give off odors. For spaces having ceiling heights 10 ft or less, the total requirement should be checked against the volume, and in no case should the ventilation air rate be less than one air change per hour. In spaces having ceilings higher than 10 ft where the occupant load is low, a check calculation can be made against the volume of the space below an assumed 10-ft ceiling. Infiltration must never be counted upon to provide ventilation, because on still days there will be little or no infiltration. Infiltration. The principles of infiltration calculations have been dis cussed in Chapters 11 and 12, with emphasis on the heating season. For the cooling season, infiltration calculations are usually limited to doors and windows. To compute cooling-load infiltration for windows by the crack method,