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730 CHAPTER 34 1951 Guide introduces moistened air; and (2) direct, which sprays moisture into the room or introduces moisture by means of steam jets. As in the cases of humidification by use of an air washer; the heat necessary for the vaporization of the moisture added ,to the air by direct humidification is secured either from heat stored in the spray water or by F ig . 3. A p p r o x im a t e W e l l W a t e r T e m p e r a t u r e s a t D e p th s op 30 t o 60 F t 1 Spray Apparatus 731 a transformation of sensible to latent heat in the air humidified. In the latter case, the enthalpy of the air remains constant, but the dry-bulb temperature of the air is reduced. Direct humidification is usually preferable where high relative humidi ties must be maintained, but where there is little cooling or ventilation required. In comfort air conditioning, where both humidification and ventilation are required, the indirect humidifier is preferable. In indus trial applications, where the cooling or ventilation load, is large and where very high relative humidities must be maintained, a combined system employing both direct and indirect humidifiers is sometimes used. Spray Generation Spray generation is obtained by. (1) atomization, (2) impact, (3) hy draulic separation, and (4) mechanical separation. Atomization involves the use, of a compressed air jet to reduce the water particles to a fine spray. With the impact method, a jet of water under pressure impinges directly on the end of a small round wire. Where hydraulic separation is employed, a jet of water enters a cylindrical chamber and escapes through an axial port with a rapid rotation which causes it immediately to separate in a fine cone-shaped spray. In the mechanical separation process, water is thrown by centrifugal force from the surface of a rapidly revolving disc and separates into particles sufficiently small to be utilized in certain types of mechanical humidifiers. Spray Distribution Spray distribution is obtained by (1) air jet, (2) induction, and (3) fan propulsion. The air jet which generates the spray in atomizers also carries the spray through a space sufficient for its distribution and evaporation, and this method of distribution is termed air jet. Where distribution is obtained by induction, the aspirating effect of an impact or centrifugal.spray jet is utilized to induce a current of air to flow through a duct or casing, and this air current distributes the spray. Fan propulsion obviously consists of the utilization of fans to entrain and distribute the spray. Industrial type direct humidifiers are commonly classified as (1) atomiz ing, (2) high-duty, (3) spray and (4) self-contained or centrifugal... Atomizing Humidifiers. Several types of atomizing humidifiers employ nozzles placed within the room and rely upon compressed air to effect com plete atomization of the water which is converted to vapor by the heat of the room air. Some of these nozzles depend upon an aspirating effect to draw the water into the nozzle and atomize it; others operate on a com bined air and water pressure. It is usual for nozzles of the water pressure type to be controlled by a diaphragm valve actuated by the pressure of the atomizing air. High-Duty Humidifiers. Water is supplied under high pressure'(usually about 150 psi) 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 collection 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 with moisture, and cooled. It then escapes from the opening below at a high