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632 CHAPTER 29 1952 Guide upper one closes, the lower one opens; selecting between them, air in the required quantity from either the warmer chamber A or the cooler one B. 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 not always sufficiently precise to meet requirements, since the untreated ah' 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 Slimmer. . . Another method of attaining temperature control in individual rooms with a year-round central air supply system, is to install a booster fan Fig. 3. Abbangement fob Individual Room Tempebatube Contbol with Centbal Aib Supply System between the main air supply duct and the air delivery opening to each 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 de 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. 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 coils located in the ducts may be used. In some cases where general recircu- Central Systems for Air Conditioning 633 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. ~ APPARATUS DEW-POINT In ordinary practice, with commercial apparatus, complete saturation of the air is seldom obtained. Four-row finned cooling coils contact ap proximately 80 percent of the air, whereas six-row finned. coils contact approximately 95 percent of the air. In spray type dehumidifiers of good design the air leaves the dehumidifier at 1 to 2 deg higher wet-bulb tem perature than the spray water leaving the dehumidifier, and the difference between the dry-bulb and wet-bulb temperatures leaving the dehumidifier may be as low as 1 deg. A spray type dehumidifier having sufficient length of spray chamber and density of spray, together with proper arrangement of nozzles, may approach saturation very closely. As explained in Chapter 3, the slope of the line on the psychrometric chart connecting the room condition with the apparatus dew-point on the saturation line, determines the ratio of sensible heat absorbing capacity to the moisture absorbing capacity of the supply air. Therefore the room condition can be maintained as long as the supply air temperature lies on this line, but a greater volume of supply air must be used to satisfy the room load if the cooling coil does not contact 100 percent of the air. For a given room load, the same apparatus dew-point will be required whether the cooling appliance contacts all the air or only part of the air. From the. point of view of satisfying the given cooling load require ments, the air passing through the apparatus without being cooled below the dew-point temperature produces two effects: 1. The air quantity which must be passed through the dehumidifier must be in creased. ; Thus, if 20 percent of the air passing is contacted, then (20 4- 80) X 100 = 25 percent more air must be used than would be necessary if all of it were contacted. 2. Passing untreated air may change the room cooling load, which in turn may change the sensible heat factor. If return air only is passed through the dehumidi fier or if room air only is by-passed, the room load will not change, but if some outside air is passed through, the room sensible heat gain and room latent heat gain will be changed due to the addition of untreated outside air, which changes the sensi ble heat factor. When a load calculation is made, it is necessary to know the per centage of air affected in the dehumidifier, and calculation must be made accordingly. If the ventilation air is drawn through the dehumidifier before it goes into the room, only, that portion of the air not saturated must be included in the room load for the purpose of determining the apparatus dew-point and supply air quantity. It should be noted when evaluating the load added by untreated outside air that the temperature difference between room air and outside air, and the moisture content difference between room air and outside air, should be used, rather than the difference between Outside air and apparatus dew-point, since the rise from the apparatus dew-point to room condition is charged against the dehumidifier as the cooling and dehumidifying load. 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