Document RpDejQ8wDdM2bLzLm9XDJpYNV

680 CHAPTER 30 1955 Guide'/ * .Central Systems for Air Conditioning 681 111 cold weather no refrigerant is required in the cooling coil, and in hot weatherno heating medium is circulated in the heating coil. With this -' scheme, the control of relative humidity in warm weather is not 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 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 Fig. 3. Arrangement for Individual Room Temperature Control with Central Air Supply System 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. if zone or room, as shown in Fig. 3. Air can then be delivered from the 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 central supply fan through the main duct at some desired condition, for a building having an east and west exposure, where solar heat gain forms instance, 60 F, 45 percent relative humidity. A double mixing damper a fair share of the cooling load, the times of individual zone peaks are near the intake opening of the booster fan, controlled by a thermostat in apt to be some hours apart, and the peak load of one plus the off-peak load the room or zone that is served by the fan, is interlocked with an outlet of the other will be substantially less than their combined peak loads. exhaust damper in the spent air opening, so that as more of the room air Proper zoning will permit operation to take full advantage of this con is recirculated, and as less new air from the main air supply duct is de dition or of similar conditions of non-simultaneous peaks, and will result livered into the room, the spent air outlet is throttled in proportion. In in a lower total load and in savings in equipment. many large installations this principle is applied successfully for zoning different stories in multi-story office buildings, the main supply fan being A factor similar in effect and closely related to the non-simultaneous on the roof, and each booster fan used for supplying the rooms of one occurrence of peak loads, is diversity. Typical of this is the case of a orientation of each story. In other cases the booster fans serve only single large department store where the air handling equipment serving a certain !f: offices, and therefore are small enough to be concealed above ceilings along side the main supply duct. 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 There may be installations in which the use of recirculated air for mixing with new refrigerated and nearly saturated air to control temperature of people in other spaces is usually normal or below. While this means that the air handling equipment for certain departments must be large and relative humidity is objectionable. In such cases,the general recircu lation arrangements of Fig. 1 may be omitted, and heat transfer coils enough to cope with the situation, the refrigeration equipment need be ?n y large enough to handle the average maximum. If a system employ 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 ot the local circulation of Fig. 3 may solve the problem.. 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