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HEATING VENTILATINC AIR CONDITIONING GUIDE 1944 selves and is drawn constantly to the atomizer by aspiration when com pressed air is supplied. This aspiration ceases and the flow of water stops when the air supply is cut off. The water should not be supplied under pressure to atomizers because of the possibility of leakage, drip, or coarse spray. These cannot occur when water is supplied by aspiration. High-Duty Humidifiers Water is supplied to high-duty humidifiers under high pressure (usually about 150 lb per square inch) through pipe lines from* a centrally-located pumping unit. The spray-generating nozzle which is of the impact type is located in a cylindrical casing. A drainage pan provides for the collec tion and return of unevaporated water which flows through a return pipe to a filter tank, from which it is recirculated. A powerful air current is forced through the humidifier by means of a fan mounted above the unit. The air enters from above, is drawn through the head, charged yvith moisture, and cooled. It then escapes from the opening below' at a high velocity in a complete and nearly horizontal circle. The spray is evapor ated and the resulting vapor diffused. This distribution of fine spray over the maximum possible area promotes complete and rapid vaporization even at high humidities. Spray Humidifiers This type of humidifier consists of an impact spray nozzle in a cylin drical casing with a drainage pan below it; The aspirating effect of the spray nozzle induces a moderate air current through the casing which distributes the entrained spray. The general method of circulating and returning the water is similar to that employed for high-duty humidifiers. A suitable pump and centrally-located filter tank are required. Self-Contained Humidifiers The self-contained or centrifugal humidifier has the ability to generate . and distribute spray without the use of air compressors, pumps, or other auxiliaries. These may be used either singly or in groups. In large installations, where suitable connections are provided to permit the cleaning and servicing of individual units without affecting the room as a whole, group control of the water and power may be employed. AIR DEHUMIDIFICATION WITH WASHERS Moisture removal from an air-vapor mixture can be accomplished by use of an air washer so long as the temperature, of the spray medium is lower than the dew-point of the air passing through the unit. The final dry-bulb temperature and the relative humidity of the air leaving a dehumidifier washer are dependent upon: the air velocity, the length of air travel through the sprays, the dry- and wet-bulb temperatures of the entering air, the spray temperature, the number of spray banks and nozzles, the. quantity of spray medium handled, and the effectiveness of the nozzles in breaking the spray into a fine mist. Both sensible and latent heat are removed in the process of dehumidi fication by cooling. Abstraction of sensible heat occurs during the entire time that the air is in contact with the spray medium. Latent heat removal takes place as condensation occurs. Therefore, the lower the 526 CHAPTER 27. SPRAY EQUIPMENT 527 Fig. 3. Approximate Well -Water T emperatures at Depths of 30 to 60 1