Document EvRwwX1bna67GYOnjZOg3XVKb

304 CHAPTER 13 1955 Guide' v-" eient 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 hot encountered by the conditioning equipment as a cooling load until some later time, when.it finally appeafi in the air stream entering the equipment. While it is true that some1 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 minute 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. 30 have 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 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 = t/iAi(<b -- ti) Btu per hour. ,where LI = coefficient of overall heat transfer between the adjacent and the condi tioned space, Btu per (hour) (square foot) (Fahrenheit degree). Ai = area of separating section concerned, square feet. fb = air temperature in adjacent space, Fahrenheit degrees. t, = air temperature in conditioned space, Fahrenheit degrees. Magnitudes of U; may be obtained from Chapter 9. The temperature <tb 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 oj conventional construction and contains no heat sources, it is recommended that the difference (fc -- U) be taken as the difference between the out: door air and conditioned-space design dry-bulb temperatures minus 5 deg- Cooling Load 305 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. . Under , 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 OUTSIDE 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 Fio. 4. Origin of the Difference 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. 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, use the data of Table 2, Chapter 11, for a wind velocity of 10 mph. Note that for double-hung windows the length of crack is three times the width plus twice the height; while for metal-sash windows the crack length is the total perimeter of the movable or ventilating sections. In calculating uindow infiltration for an entire structure, it is not necessary to consider the total crack length on all sides of the building, for the wind would not iL sTMultaneusly on all sides at once. In no case, however, should less ten half of the total crack length be figured. A knowledge of the pre' ailing wind direction will aid judgment in this consideration. Cooling-load infiltration for doors31 may be obtained from Table 3, to nfter 1 * ,Fr conditions other than those covered, the notes appended k tae table will provide a basis for estimates. The tabulated data may also ti US'i as ^e basis of estimates for interior doors between an air-condi- ?e^ aud a non-air-eonditioned space, umtration load must be included whenever the new air introduced