Document 375qLGdjob6RGX5rkp74YgXvD

494 Chapter 26 1945 Guide anticipated. For greater decrease in dry-bulb temperature, multi-stage washers should be utilized. A rise of 6 F should be the calculated maxi mum,for the spray water. . The width and height of a washer may be dictated by space limitations outside the washer, such as headroom, or by the inside'space requirements, such as face area needed by a bank of cooling coils. The length of a washer is. determined by the number of spray banks', or scrubber plates, 'and if cooling toils are installed in the unit, by the number of banks of .coifs. ; Roughly, a spray space of about 2 ft 6 in. in length is required for each 'bank of sprays; leaving eliminators.require about 1 ft 6 in., and entering eliminators about 1 ft. . The resistance to air flow through an air washer varies with the type of eliminators, number of banks of sprays, direction of spray, air velocity, type of scrubber plates, and size and type of cooling coils if located in the washer. Manufacturers should be consulted to obtain the resistance for a particular installation. HUMIDIFICATION WITH AIR WASHER Air humidification can be accomplished in three ways with an air washer. These are:. (1) use of recirculated spray water without prior treatment of. the air, (2) preheating the air and washing it with recircu lated spray water, and (3) using heated spray water. In any air washing installation the air should not enter the washer with a dry-bulb tempera ture less than 35 F in order to eliminate danger of freezing the spray water. Method 1. Except for the small amount of energy added from outside by the recirculating pump in the form, of shaft work, and for the small amount of heat leak from outside into the apparatus, including the pump and its connecting piping, the process would be strictly adiabatic. Evaporation from the liquid spray would therefore be expected to bring the air immediately in contact with it to saturation adiabatically; and, since the liquid is recirculated, its temperature would be expected to adjust to the thermodynamic wet-bulb temperature of the ehtering air. It does not follow from the foregoing reasoning that the whole air stream is brought to complete saturation, but merely that its state point should move along a line of constant thermodynamic wet-bulb temperature as explained in Chapter 1. The extent to which the final temperatureapproaches 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 ^ humidifying effectiveness or saturating. . effectiveness and is defined: where h = humidifying effectiveness, per cent. h = dry-bulb temperature of the entering air, degrees Fahrenheit. tt -- dry-bulb temperature of the leaving air, degrees Fahrenheit. t' = thermodynamic wet-bulb temperature of the entering-air, degrees Fahrenheit. The humidifying or saturating effectiveness 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 velocity of air flow is more conducive to higher humidifying effectiveness^ 495 The following may be taken as representative humidifying or saturating effectiveness of an air washer 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 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 when necessary after passage. through the washer. Method 3. 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 f 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 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 heatnecessary 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