Document MGJXZjZpzwZ5Q0zZVp8Q3M1Zk

656 CHAPTER 30 .1954 Guide In cold weather no refrigerant is required in the cooling coil, and in hot weather no heating medium is circulated in the heating coil. ,With this scheme, the control of relative humidity in warm weather is hot always sufficiently precise to meet requirements, since the untreated air delivered through the upper coil may be so high in relative humidity that it cannot sufficiently compensate for the nearly saturated air leaving the lower coil. A reheater could be placed if desired, to the right of the lower coil to bring the air in the lower chamber to the desired relative humidity. The simple arrangement of Fig. 2 is admirable in winter and, except where close control of relative humidity is important, may be acceptable in summer. Another method of attaining temperature control in individual rooms with a year-round central air supply system, is to install a booster fan between the main air supply duct and the air delivery opening to each Fig 3 Arrangement fob Individual Room Temperature Control with Central Air Supply System zone or room, as shown in Fig. 3. Air can then be delivered, from; the central supply fan through the main duct at some desired condition, for instance, 60 F, 45 percent relative humidity. A double mixing damper near the intake opening of the booster fan, controlled by a thermostat in the room or zone that is served by the fan, is interlocked with an outlet exhaust damper in the spent air opening, so that as.more,of the room'.air is recirculated, and as less new air from the main air supply, duct is'.qC: livered into the room, the spent air outlet is throttled in proportion. ,In many large installations this principle is applied successfully for zoning different stories in multi-story office buildings, the main supply fan being on the roof, and each booster fan used for supplying the rooms of one orientation of each story. In other cases the booster fans serve only single offices, and therefore are small enough to be concealed above ceilings along: side the main supply duct. j;'. There may be installations in which the use of recirculated air for mixing with, new refrigerated and nearly saturated air to control temperature and relative humidity is objectionable. In such cases the general recircU; lation arrangements of Fig. 1 may be omitted, and heat transfer cons located in the ducts may be used. In some cases where general recircu lation is not acceptable, as for all the rooms in an entire building, use of the local circulation of Fig. 3 may solve the problem. Central Systems for Air Conditioning 657 HUMIDITY CONTROL In winter, room relative humidities in excess of 30 percent are seldom required in a system designed for comfort conditioning only, and a low saturating efficiency may be desirable, or even necessary, especially if the same volume of air is handled as in summer. With a spray type dehu midifier the main sprays may be shut off. and. only the eliminators need be 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. For summer operation and significance of apparatus dew-point see Chap ter 13, Cooling Load. COOLING LOAD The method of determining the cooling load for a conditioned space or spaces is outlined in Chapter 13. 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 m a lower total load and in savings in equipment. A factor similar in effect and closely related to the non-simultaheous 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 8Pace must be sufficient to handle the load created by the throngs of People attending sales in that space. Under such a condition the number ef 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 ?n|y large enough to handle the average maximum. If a system employ,ng zone recirculating fans and a single central fan and dehumidifier were . jSed> the saving would be reflected in the capacity of the central fan and oehurtudifier. Another example of this diversity is found in an office