Document v61NoxDkZV45k27w1NEBNmQY

HEATINC .VENTILATING AIR CONDITIONING CUIDE 1941 explained in Chapter 1. The extent to which the final temperature approaches the thermodynamic wet-bulb temperature of the entering air, or the extent to which complete saturation is approached is conveniently expressed by a ratio known as humidifying efficiency or saturating ef ficiency and defined as follows: where eh = humidifying efficiency, per cent. h = dry-bulb temperature of the entering air, degrees Fahrenheit. It = dry-bulb temperature of the leaving air, degrees Fahrenheit. {' = thermodynamic wet-bulb temperature of the entering air, degrees Fahrenheit. The humidifying or saturating efficiency of a washer is dependent upon the number of spray banks and nozzles, the effectiveness of the nozzles in breaking an adequate quantity of water into a fine spray, the 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 velocities of air flow are more conducive to higher humidifying efficiencies. The following may be taken as representative humidifying or saturating efficiencies of air washers for the conditions stated: 1 bank--downstream.............. ....... .......... ................................... 60-70 per cent 1 bank--upstream................... ....................................................... 65-75 per cent 2 banks--downstream..................... ..... ......................................... 85-90 per cent 2 banks--1 upstream and 1 downstream........... .... ..................... 90-95 per cent 2 banks--upstream .................................................................... 90-95 per cent The air leaving the washer may require reheating to produce the required dry-bulb temperature and relative humidity. Method 2. 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 absorbed per pound of dry air in passing through the washer, assuming that the humidifying efficiency of the washer is not adversely affected by operation 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 preheating to give the required wet-bulb temperature at entrance to the washer and then reheating when necessary after passage through the washer. Method S. Even 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 1. By sufficiently elevating the spray water temperature it should be possible to completely saturate the air and even raise its temperature above the dry-bulb tem perature of the entering air. APPARATUS FOR DIRECT HUMIDIFICATION Humidifiers may be divided into the following general types, according to the method of operation: (1) indirect, such as the air washer, which 504 CHAPTER 26. SPRAY EQUIPMENT 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 (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 505