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754 CHAPTER 29 1958 Guide rooms may be somewhat limited by static-pressure regulators and volume dampers at selected points along the mains. There are some installations where volume control at each room supply is desirable to maintain close control regardless of static-pressure changes in the ducts, and incidentally to facilitate balancing of the distribution system. . Air-capacity requirements are generally governed by summer cooling loads, and general practice is to supply the cold air to the cold-air duct at 25 to 30 deg below the established room temperature, this being satis factory with suitably designed room outlets. Fig. 8. Roou Air-Mixing Unit fob Dual Duct System Fan and Coil Units Another type of room convector used in connection with central cooling and heating plants is known as the Fan and Coil Unit (see Fig. 5, Chapter 25). These units, like induction units, are located around the periphery of a building, usually under windows, and are equipped with fans and a watertype heating-cooling coil. They normally take air for ventilation directly from outdoors, and have a manual damper for adjusting the quantity of outdoor air within certain limits. Positive closing of this damper when the fan stops, can be obtained by adding a damper operator actuated by the fan-motor switch. Hot water supplied during the heating season fre quently is varied in accordance with outdoor temperature. Cold water for use during the cooling season should be supplied at a fixed temperature low enough to provide the proper amount of dehumidification. These units sometimes are used with a primary-air system which supplies the outdoor ventilation air and handles the latent load. In such cases the primary air unit is controlled as explained under the section Unitary-Cen tral Systems, and may be of either the low- or high-pressure type. The combined heating-cooling coil of each unit may be controlled by a valve in the water supply line to each unit, actuated by a thermostat in the room or recirculated air of the unit. The thermostat is of the heating cooling type. EVAPORATIVE COOLING In climates where, on the hottest days, the outdoor wet-bulb depression is relatively great, it may be possible to replace mechanical refrigeration, or other cooling sources, and use the evaporative cooling effect. A well- Central Systems for Air Conditioning 755 designed air washer using recirculating sprays will reduce the entering drybulb temperature to within a degree or two of the entering wet-bulb condi tion. Thus, it may be possible that, with air entering at 100 F dry-bulb, 60 F wet-bulb, a leaving condition of 62 F dry-bulb, nearly saturated, can be obtained. Under some conditions of latent and sensible heat ioad, this evaporative cooling may be adequate. At times when the outdoor wet-bulb temperature is not low enough to permit the use of straight evaporative cooling, it is still possible to use pre-cooling convectors with refrigeration, well water, or a cooling tower, as the basic source of sensible heat removal to reduce the wet-bulb tempera ture of the air before it enters the spray chamber. Where internal heat loads are high, this scheme may be more economical than one using return air. Where the required supply air dew point is too low to permit straight evaporative cooling, and where the sensible heat load is not too great, intentional partial saturations, may be employed. That is, the low dew point of the outdoor air is utilized by permitting, some of this air to pass through the humidifying sprays untreated, or to bypass the humidifier. All of these remarks with regard to evaporative cooling are based on the assumption that all of the supply air will be taken from outside. Provision should be made in most cases for the return of some air from the conditioned spaces for control purposes, as well as for economy of fuel in winter. PRECOOLING Where sufficiently cold water from wells or streams is available, a saving in refrigeration may be obtained by the use, in location ahead of the de humidifier, of precooling coils through which the cold water is circulated. The resultant cooling of the air decreases the load to be carried by the dehumidifier arid refrigeration plant. In normal practice the water, after passing through the precooling coils, may be further utilized.in the refrig eration plant condenser. The economic advantages of this scheme are apparent, and it is frequently used. SENSIBLE COOLING WITH UNWETTED COILS Under favorable atmospheric conditions where a large wet-bulb depres sion exists and the dew point of the outdoor air is sufficiently low at all times, acceptable cooling may be obtained by removing only the sensible heat from the outdoor air delivered to the rooms. Under this condition of a great wet-bulb depression, a temperature-reducing coil may be located m the air stream and supplied with water from a cooling tower. When humidity control is desired, sprays to saturate or partially saturate the air may be used down stream from the unwetted coil. Saturation or partial saturation after the coil will reduce further the dry-bulb temperature and the air quantity required. This system has very definite application in hot dry climates. SELECTION OF TYPE OF SYSTEM Low buildings with large floor areas may be divided into sections or zones with separate central air-supply systems to facilitate temperature control. In the case of large department stores it may be possible to pro vide a single conditioner, with a fan delivering the conditioned air to local mixing fans which supply the various departments or spaces. This appli cation is limited by the practicability of running the large conditioned mr ducts to the various recirculating fans. Each vertical section of the