Document 71kLJ2wNNEV60abMgGgDbxY1V

690 CHAPTER 30 1956 Guide 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 warmed by the sun, may radiate heat long after the passing 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 to 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 32). 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 12. 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- Central Systems for Air Conditioning 691 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 fights, 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 operate the heat ing 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 system 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 TEMPERATURE DIFFERENTIAL The difference between the room-air temperature and the supply-air temperature at the outlet to the room is known as the temperature dif ferential. In the theoretical case of a dehumidifier having 100 percent saturating efficiency, and where this air is delivered directly to the room without temperature increases due to heat gain, then the temperature differential is the difference between room temperature and apparatus dew point temperature. If duct heat gains are considered a part of the room load, this still holds true. The apparatus dew point, as outlined pre viously, is fixed by the latent and sensible loads of the space, but in many cases, it is desirable to deliver more air to the spaces than is indicated by the difference between the room temperature and the apparatus dew point. It has been indicated that where a percentage of air is passed through the dehumidifier without being treated, the relationship is modified in direct proportion, and that if room air is passed through untreated, no effect on the heat balance results. Similarly, if room air is passed around the dehumidifier and mixed with the treated air, the heat balance is not adversely affected. Therefore, if the quantity of air passed through the dehumidifier is determined by the usual methods, room air can be passed around the dehumidifier and mixed with the dehumidified air, increasing the supply air quantity and temperature and decreasing the temperature differential. Thus if the difference between the room temperature and the apparatus dew point indicates that 10,000 cfm at 30 deg below room temperature will be required to hold conditions, that quantity can be passed through the dehumidifier and cooled to 30 deg below the room tem perature, then mixed with 10,000 cfm of room air, resulting in a supply-air quantity of 20,000 cfm and a temperature differential of 15 deg instead of 30 deg. Air supply outlets and grilles having a high induction ratio are available, and through their induction effect cause a large amount of room air to be mixed with the supply air within a short distance of the grille. A proper selection of outlets may make it possible to introduce air at low temperatures and high velocities without causing objectionable drafts or cold spots, but care must be used to see that too little air motion is not a result. Low temperature differentials may be required for this reason. While the use of a high temperature differential results in a saving in initial cost of fans and ducts, and in the Operating cost of fans, this differential should be carefully considered. If the sensible heat load of a space is sub jected to substantial variations, low temperature differentials should be considered, since systems employing low temperature differentials require less precision in controls. Reduction of air quantity by slowing down the fans for the winter season,