Document xje9kVYnQyDOQ0M40xOk7L8x0

866 CHAPTER 43 1949 Guide Selection of induction convectors should be made with due regard to noise level. The inductive capacity of the device increases with the jet velocity, but high jet velocities may result in objectionable noise. INDUCTION CONVECTORS--HIGH PRESSURE TYPE Another type of induction convector Fig. 5 employs nozzles which produce a high velocity air jet without objectionable noise. The term, high pressure* is to some extent inaccurate, since the air pressure at the nozzles, while several times that used with a low pressure induction con vector, is still less than the total resistance pressure of a conventional central system. The high velocity jet of primary air induces a flow of air from- the room through coils located in the secondary air stream and supplied with chilled water in summer and with hot water in winter. The chilled water removes a large portion of the sensible heat in summer and the hot water supplies the. sensible heat loss in winter. The primary air is delivered at a sufficiently low dew-point to; compensate for the latent heat gain in summer. In winter the primary- air is supplied at a sufficiently high dew-point to take care of latent heat losses. Control of temperature is obtained by throttling the water quantity supplied to the secondary .coil. The required flow of primary air is greatly reduced due to the fact that a portion of the sensible heat load is earned by the second ary air stream. Since the primary quantity is small, very high velocities can be maintained in the supply ducts without requiring fan power in excess of that for a conventional system. Therefore, the supply, ducts or pipes can be very small and can be run in chases, or furred in at columns along with the water pipes. The primary air is treated in the usual man ner to reach the required dew-point and a surface or spray dehumidifier or a dehydrator may be. used. The primary air quantity is sufficient foe ventilation purposes and frequently consists entirely of outdoor.air. The water piping for the coils can be so valved that hot water,will be sup plied to one zone that may require heating while cold water may be sup plied at the same time to a zone that requires cooling. V Central Systems for Air Conditioning 867 This system usually is limited in application to hotels, apartments, office buildings and other multi-room installations having a large perime ter with relation to the floor area. The units usually are installed beneath the windows, replacing direct radiation or thermally-circulating enclosed convectors. Where the spaces to be conditioned extend a considerable distance from the outer wall into the interior of the building, a separate system or zone for the conditioning of the interior portions may be required. EVAPORATIVE COOLING In climates where, on the hottest days, the outdoor wet-bulb depression is relatively great, it may be possible to dispense with refrigeration or other cooling sources by use of the evaporative cooling effect. A well 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 load, evaporative cooling may be adequate. At times when the outdoor wet-bulb temperature is not quite low enough to permit the use of straight evaporative cooling it is 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 temperature 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 saturation 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 by-pass 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 condi tioned 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 and refrigeration plant. In normal practice the water, after passing through the precooling coils may be further utilized in the refrigr 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 in the air stream and supplied with:water from a cooling tower.- When humidity control, is desired, sprays to saturate or partially saturate the. air