Document k9gGL7y6Z4ELzJRKyzynXg8JO

324 CHAPTER 13 1958 Guide effect. In the absence of reliable data this allowance is often a matter of experience rather than calculation. Where air conditioning supply and return ducts pass through uncon ditioned spaces, there will be a transfer of heat from these spaces to the air in the ducts, even though these duct are well insulated. An allow ance should be made for this heat gain and included in the heat estimate so that air can be supplied at a temperature low enough to offset the rise caused by this heat gain (see Chapter 31). There will also be some heat gain to the air in ducts passing through conditioned spaces, but since a cooling effect is produced in the space through which the duct passes, this is not a loss and usually can be compensated for by adjustment of air quantities between the various spaces. No comprehensive data are presently available for use in design load estimates to evaluate the interior load-lag effect, but several investigators'3' h, 26 27 23 have made a study of the problem and have presented many useful data. Tables 9, 10, 12, 13, 17, 19, 20, 21, 22, and 23 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 determin ing 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 neces sarily 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 = Ui.4 i((b -- ii) Btu per hour. (6) where Ui = 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, fs = air temperature in adjacent space, Fahrenheit degrees, fi = air temperature in conditioned space, Fahrenheit degrees. Magnitudes of U-, may be obtained from Chapter 9. The temperature fb 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 (fb -- U) 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. 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 Cooling Load 325 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 littleor 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 window 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 act simultaneously on all sides at once. In no case, however, should less than half of the total crack length be figured. A knowledge of the pre vailing wind direction will aid judgment in this consideration. Cooling-load infiltration for doors29 may be obtained from Table 3, Chapter 11. For conditions other than those covered, the notes appended to the table will provide a basis for estimates. The tabulated data may also be used as the basis of estimates for interior doors between an air-condi tioned and a non-air-conditioned space. Infiltration load must be included whenever the new air introduced through the system is not sufficient to maintain excess pressure within the enclosure to prevent the infiltration. Whenever economically feasible, it is desirable to introduce sufficient outdoor air through the air-conditioning equipment to maintain a constant outward escape of air, and thus eliminate the infiltration portion of the load. The pressure maintained must, of course, be sufficient to overcome wind pressure through cracks and door openings. When this condition prevails it is not necessary to include any infiltration load. When the quantity of new air introduced through the cooling equipment is not sufficient to build up the required pressure to offset infiltration, the entire infiltration load should be included in the cooling load calculations. Total Outdoor Air Load. Many cooling coil manufacturers publish tables giving psychrometric data based on the average conditions of the leaving TM various coil temperatures, air velocities, and entering dry-buib and ''et-bulb conditions. When these tables are used, it is necessary to cal culate the mixed air condition entering the coil, and determine from the ables what coil and air velocity will produce the desired leaving air con- itions as required for the space to be conditioned. When cooling coils re lsted as 80 to 95 percent efficient, the manufacturer indicates that 20