Document 937n37JMBa4xp1dD7QMBGz1oq

Heating Ventilating Air Conditioning Guide 1938 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 is secured either from heat stored in the spray water or by a transformation of sensible to latent heat in the air humidified. In the latter case the total heat of the air remains constant but the dry-bulb temperature of the air humidified is reduced. Spray Generation Spray generation is obtained by (1) atomization, (2) impact,' (3) hydraulic 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 suf ficiently 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) atomizing, (2) high-duty, (3) spray and (4) self-contained or centrifugal. Atomizing Humidifiers There are several types of atomizing humidifiers, all of which rely upon compressed air as the atomizing and distributing agency, similar to the familiar method used in ordinary nasal atomizers. Compressed air (ordinarily about 30 lb per square inch) is- supplied from a centrallylocated air compressor through pipe lines to the atomizing units. The air lines are usually horizontal and parallel to water lines which supply . water by gravity from a float tank. The water in the tank is maintained at a constant level slightly lower than the outlets of the atomizers therm selves and is drawn constantly to the atomizer by aspiration when comr 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 which cannot be permitted 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 504 Chapter 25. Spray Equipment for Humidification & Dehumidification 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 with moisture, and cooled to the wet-bulb temperature. It then escapes from the opening below at a high velocity in a complete and nearly horizontal circle. The spray is quickly evaporated and the resulting vapor is rapidly and thoroughly diffused. This effective distribution of fine spray over the maximum possible area insures complete and extremely rapid vapori zation even at the highest 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. The spray and high-duty types of humidifiers have many features in common but the latter, because of its finer spray and greater capacity, is often considered better adapted for producing high humidities. Seli-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. Where large quantities of power are generated in a limited space and where a comparatively high relative humidity is required, it is often feasible and economical to use a combination of direct and indirect humidification. The indirect humidification provides the desired quantity of ventilation and cooling, and the additional direct humidification pro vides for increase in humidity without interfering with the ventilation or the cooling effected by the indirect system. In general, it may be stated that direct humidification is most satis factory where high humidities are desired but where little cooling, ven tilation or air motion is required. Therefore, the indirect system is most applicable where either low or high relative humidities are desired with maximum cooling and ventilation effect. For conditions that require an unusually large amount of heat to be absorbed by ventilation, together with the maintenance of high humidities, it is often preferable to make use of the combination system of indirect and direct humidification. If the indirect system alone were used it would mean an unusually large volume of air to be handled, which might interfere, due to air motion, with production, even though it would result in greater cooling effect. If direct humidification alone were used, no ventilation would be obtained, with consequently higher room temperatures. 505