Document zd5LxDxKbbKMga13M59k0Jgvm

642 CHAPTER 37 1946 Guide eliminator plates, which will cause from 4 to 6 reversals of the direction of air flow, should be installed for the purpose of removing drops of unvaporized water from the leaving air. If the air contains certain substances which may cause the spray water to become acidulated, corrosion resistant materials must be used in the washer construction. Essential items in air washer operation are: uniform distribution of the air across the chamber section above the level of the water in the sump; moderate velocities of air flow, 300 to 600 fpm in the spray cham ber; an adequate amount of spray water broken up into a fine mist throughout the air stream; sufficient length of air travel through the water spray and over thoroughly w.etted surfaces; and the elimination of free moisture from the air as it leaves t,he unit. -Fie. 1. Typical Single-Bank Air Washer Fig! 2. Typical Two-Bank Air Washer Washers are sometimes arranged in two or more stages to cool through long ranges or to increase the over-all efficiency of heat transfer between the air and the heating or cooling medium. A'multi-stage washer is equivalent to a number of washers in a series arrangement. Usually the catalog capacity of a washer is expressed in cubic feet of air per minute and is based upon an air velocity of 500 fpm through the gross inlet area of the unit. At this rating spray type washers handle about 2]/2 gpm of water per bank per square foot of area, that is, about 5 gpm per bank per 1000 cfm. These proportions of air, water, area, and velocity may be departed from to meet the needs of some particular job, but certain limiting relationships should be observed. For a single-stage air washer, a 15 deg drop in dry-bulb temperature of the air passing through the washer is about the maximum that should be anticipated. For greater decrease in dry-bulb temperature, multi stage washers should be utilized.' A rise of. 6 deg should be the calcu lated maximum for the spray water. ; .Spray Apparatus 643 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 coils are installed in the unit, by the number of banks of coils. Roughly; a spray space of about 2 ft 6 in. in length is required for each bank of sprays; i.e., 1 ft for entering and 1 ft 6 in. for leaving. 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 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, per cent. , , <i = dry-bulb temperature of the entering air, Fahrenheit degrees. It = 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 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 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 X'