Document 1Q0L674YyOqr1bE7a40xYMmmj

620 CHAPTER 29 1951 Guide flooded; which may give sufficient moisture. In other cases, such as those in which cooling coils are sprayed, the spray water supply may be throttled. If the saturation efficiency of the sprays is too low, the spray water may be heated. The amount of heat put into the spray water by open or closed water heaters will be equal to that required to bring the dew-point temperature of the air entering the sprays up to that required before entering the preheater. It is possible, where clean steam is avail able, to introduce steam directly into the air stream to produce the desired dew-point temperature of supply air. However, the steam must be ex ceptionally clean, or objectionable odors will result. It should be noted that the quantity of outdoor air to be introduced is affected by infiltration and leakage. Infiltration will reduce the quantity to be introduced by the system, while leakage may have to be offset by an increase in the quantity of outdoor air. COOLING LOAD The method of determining the cooling load for a conditioned space or spaces is outlined in Chapter 12. As pointed out therein, many of the ' items of heat gain are variable and do not reach their maximum values ' simultaneously. Proper consideration of these peaks and the avoidance of pyramiding these peaks in the cooling load calculations are stressed. : Maximum solar heat gain on an east exposure is seldom coincident with the maximum outdoor wet-bulb. A large difference in the time-incidence of the peaks between various spaces or parts of the same space indicates the necessity for zoning. In a building having an east and west exposure, where solar heat gain forms a fair share of the cooling load, the times of individual zone peaks are apt to be some hours apart, and the peak load of one plus the off-peak load of the other will be substantially less than their combined peak loads. Proper zoning will permit operation to take full advantage of this con dition or of similar conditions of non-simultaneous peaks, and will result in a lower total load and in savings in equipment. A factor, similar in effect and closely related to the non-simultaneous occurrence of peak loads, is diversity. Typical of this is the case of a large department store where the air handling equipment serving a certain space must be sufficient to handle the load created by the throngs of people attending sales in that space. Under such a condition the number of people in other spaces is usually normal or below. While this means that the air handling equipment for certain departments must be large /enough to cope with the situation, the refrigeration equipment need be only large enough to handle the average maximum. If a system employ ing zone recirculating fans and a single central fan and dehumidifier were used, the saving would be reflected in the capacity of the central fan and dehumidifier. Another example of this diversity is found in an office building having restaurants and stores of certain types in the first story and basement. At noon, when the restaurants and stores are crowded, the offices are below normal occupancy. Heat lag should be-carefully considered in the cooling load calculations. In certain types of buildings the effect of solar radiation is still apparent several hours after the sun has shifted from that exposure. In other types having a much lighter construction, the heat gain due to solar ' radiation decreases markedly with the passing of the sun. Some walls, having been warmed by the sun, may radiate heat long after the passing Central Systems for Air Conditioning . . 621 of the sun, thus requiring lower inside temperatures to offset the radiant energy. Buildings have considerable heat storage capacity which can often be utilized to great advantage, and which has more than once provided an unexpected safety factor. If a space is kept below the design inside tem perature for some time, the interior walls, floors, furniture and fixtures begin to assume the temperature of the space. Where the time is suf ficient the entire mass, rather than merely its surface, may reach the room temperature. Thus, when a space has been precooled below the design maximum temperature for a period of time prior to the advent of the peak load, and the heat gain begins to increase the peak conditions, some of the increase is used in raising the temperature of the furniture, fixtures, etc., to the design conditions and the cooling load can be reduced accord ingly. However, unless very accurate data with regard to the mass, sur face, specific heat, etc., of the items within the space are available, due caution must be used in discounting the cooling load for this storage 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 ducts 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. ; HEATING LOAD Methods of calculating the heating load are shown in Chapter IT. Many of the factors outlined previously under Cooling Load, such as zoning, non-simultaneous peaks, and diversity, apply in the reverse man, ner due to the heating requirements instead of the cooling requirements. However, these factors affect the heating load from the standpoint of control of inside conditions, overall performance, and economy of opera tion more than from a capacity of equipment standpoint. It is not only necessary to heat a building or space to its design conditions when there is but the merest fraction of normal occupancy, and when there are prac tically no lights, internal heat, or solar radiation, but it is also necessary to provide capacity to heat the building quickly when sudden cold follows relatively warm weather, as may occur after a week-end or holiday shut down. However, in normal operation during week-ends and holidays, buildings are usually kept at a holding temperature to prevent the freezing of services. In many cases, less fuel is required to continue operation of the heating plant at a near-normal rate and maintain the building or space at a temperature of 50 to 65 F at such times, than to shut the sys tem down and then bring the temperature back to normal through forced operation of the heat generating equipment with a consequent loss in efficiency. AIR QUANTITY AND EFFECTUAL TEMPERATURE DIFFERENCE The difference between the room air temperature and the supply air temperature at the outlet to the room is known as the effectual tempera-