Document OJzddqz1p21kYayxDj15O8zpe

652 ' CHAPTER '37-' 1948 Guide Essential requirements iri the airwasher operation are: uniform distri bution of the air across the chamber section, moderate air velocity of from 250, to 600 fpm in .the washer chamber, an- adequate, amount of spray water broken up irito fine droplets throughout the air stream, at pressures of. from 15'.to 30 psi, sufficient dength-. of. travel Through the water spray and wetted surfaces, and the elimination -of-'entrained mois ture from the outlet air: : : Expected performances, physical size. iength, number of sprays, etc., vary greatly, defending upon the functions of the. installation. In general, the width and height of an air washer are dictated by the space available. Washers of nearly equal height and width are desirable from an.air flow and economic standpoint although not necessary,,, The length ELEVATION ORAM 0 OVERFLOW ELEVATION Fig. 1. Typical Single-Bank Ant Washes Fig. 2. Typical Two-Bank Air Washer of washers varies considerably. A space qf approximately ft between spray banks is used and the first and last banks of sprays are located about 1 ft to ft from the entering or leaving end of the. washer. In addition air washers.are very often furnished with cooling coils or heating coils within the washer chamber and the use of these coils affects the overall length of the washer. Where increase of overall heat transfer between the air and water is required, multistage washers are used. These washers are equivalent to a number of washers in series and the water is often pumped from one stage to the other where conditions permit. The resistance to air flow through an air washer varies with the type of eliminator and wetted surfaces, number of banks of spray and their. direction, air velocity, size and type of other resistances such as cooling and heating coils, and other factors such as air density. Resistances vary from as low as 3T in. to higher than 1 in. and it is therefore necessary that the manufacturer be consulted in regard to the resistance of any par ticular washer design involved. Apparatus HipiDinCATION WITH AIR WASHER Air humidification can .be-accomplished in three ways with an-air washer. These are: (I) 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-bulbtemperature less than 35 F in order to eliminate danger of freezing the spray water. Method 1. Except for the small amdunt of energy added froth outside by the recirculating pump .in the-form of shaft work, arid, 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 frpm 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 entering 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 3. 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 effectiveness or saturating effectiveness and is defined: where. eh = Humidifying effectiveness, percent. <i = dry-bulb temperature of the entering air, Fahrenheit degrees. tt = dry-bulb temperature of the leaving air, Fahrenheit degrees. t' = thermodynamic wet-bulb temperature of the entering air, Fahrenheit degrees^ 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 Tbanfc--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 humidifying or saturating effectiveness of a washer, is dependent upon the essential items of design mentioned under Air Washers. Other conditions being the same, low velocity of air flow is more conducive to higher humidification effectiveness. 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 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. 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