Document 6wa5LGq53464KzY213oNVeBBg

644 CHAPTER 37_______________________ 1946 Guide velocity of air flow through the water sprays, and the time of the contact of the air with-the spray water! Other conditions being the same, low velocity of air flow is more conducive to higher humidifying effectiveness. The air leaving the washer may require reheating to produce the required dry-bulb temperature and relative humidity. Method S'. The preheating of the air increases both the dry- and wetbulb temperatures, lowers the relative humidity, but does not alter the humidity ratio (pound water vapor per pound dry air). At a higher wetbulb temperature but the same humidity ratio, more water can be absorb ed per pound of dry air in passing through the washer* assuming that the humidifying effectiveness of the washer is not adversely affected by opera tion at the higher wet-bulb temperature. The analysis of the process occurring in the washer itself is the same as that explained under Method 1. The final desired conditions are secured by adjusting the-amount of pre heating. to give the required wet-bulb temperature at entrance* to the washer and then reheating the outlet air when necessary. Method 3. Evert if heat is added to the spray water, the mixing occur ring in the washer itself may still be regarded as adiabatic. The state point of the mixture should move in a direction determined by the specific enthalpy of the heated spray as explained in Chapter 3. By elevating the water temperature it.should be possible to saturate the air and even raise its temperature above the dry-bulb temperature of the entering air. APPARATUS FOR DIRECT HUMIDIFICATION Humidifiers may be divided, into the following general types, according to the method of operation: (I) indirect, such as the air washer, which 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 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 (!) . 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 Apparatus 645 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 them 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 with 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. 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