Document Z4017JJdbreV7qvmMJqxJz5bL

596 CHAPTER >33LV; '' 1965GuideAndData Book1 spray water. In any air washing installation the air should not enter the washer with a wet-bulb temperature than 35 F. This is a precaution to eliminate the 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* leakage from out-: side into the apparatus, including the. pump and'its con necting piping, the process would be strictly adiabatic. Evap oration from the liquid spray would therefore be'expected to bring the air immediately in contact with it to saturation1 adiabatically; and, since the'liquid is recirculated, its tern** perature would be expected to adjust to the thermodynamic wet-hulb temperature of the entering air. . ', * '' It does not follow from' the foregoing reasoning 'thatthe whole air stream is . brought to complete saturation,' but merely that its state point should move along a line of con stant thermodynamic' wet-bulb temperature. The extent to which the living air temperature approaches the thermody namic wet-bulb temperature of the entering air, or the extent io which complete saturation is approached, is conveniently by a ratio known as humidifying effectiveness or saturating effectiveness, and is defined in Equation 1 as ;. where --xioo *1 -- * ci) - tk " humidifying or saturating effectiveness, percent, : . ' : tt = dry-bulb temperature of the entering air, Fahrenheit. dry-bulb temperature of the leaving air, Fahrenheit. - j' a `it^rmody^wmin wet-bulb temperature of the entering - air; Fahrenheit. ! The following may be taken as representative' of the hu-; midifying. or saturating effectiveness of a, spray-type air washer, for the arrangements stated: . Assangement 1 bank downstream 1 bank downstream 1 bank upstream 2 banks downstream 2 banks opposing each other 2 banks upstream Lbnotb Ft 4;, 6. 6 8 to 10 8 to 10 8 to 10 Effbchvb^ -nbss-^I- 60^60 j 60-75 ' 65-80 80-90 " 85-95 ` The'degree of saturation depends upon the extent of contact between the air and water. Other conditions bong the Eaine, a low velocity of air flow is conducive to higher humidifying per&ture.to ruse the'air*temperature,:both dry-bulb and wetbulb, above the dry-bulb temperature of the entering air.: *T-: ~ in' methods 1, 2,' or3, the air leaving the air' washer, may require reheating 'to' produce ,in 'toe'conditiohed.'space'thS' required dry-bulb temperature'and relative humidity. -Dehumidification and Cooling with Air Washers - 1 Wishers are alaqapplicable'to air cooling and dehumidified.' - tibni Heat and moisture removed from the^air, produce a risel in' the water temperature. ITtire' entering water temperature! b below tiie entering wet-bulb temperature, then a lowering' of both the dry-bulb and wet-bulb temperatures b-obtainecQ will result if toe leaving water temperature is'below the;ehtering'dew'point'temperature.' Moreover,'the. final water temperature b. determinal by the sensible and, latent heatpick-iip andtoe quantity of water circulated: How-.. evur'-tiib final temperature must nbt'erceed th&final required5dew'point,' oneor two degrees below the dew point beiiig' com-! mon practice! " * r \ `:The air leaving a.spray-type deKumidifier ^substantially saturated. .'Usually . the spread between' dry-' arid wet-bulb! temperatures is 'less than one degrfee/ The 'spread betweeh leaping aiV"'and leaving water will dependon the'difference between entering dry- and wet-bulb temperatora^and,on. certain features of dedgn; such as' length and height 6f spray, chamber, air velocity,Quantity of water, and character of toe spray pattern!'The rise in' water. temperature isusually'be-r tween`6 and l2 F deg, although higtter rises have beent'uaed successfully! The lower,rises,are ordinarily-selected when thh. wateris chilled by nnyhahieal refrigeration because of possible^ higher refrigerarittonip6ratures. It is often desirable to make an oMneirnif". analysis'of the effect of higher refrigerant'temrj perature compared with'the benefits of a greater'rise in prater temperature. With systems ^re^yi^ vTOt^ from a well or otibr source at an acceptable temperature, ii may.be,desirable to <bsign'Oil the.basis.of. a high rise and `minimum flow of water..''!! . '"w'p ',-!j . . : > u;Thembst common air washer arrangement for cooling and dehumidifying air has two spray banks and is eight to nine feet long. If the airwasher Bhs ability to cool and dehumidify the entering'air to a wet-bulb temperature equal to the leav ing water temperature, it is convenient to assign to such a wisher a perfdrmanto factor of 1:0.' The actual performance factor of any'washer is toe actual enthalpy change divided by enthalpy change in a washer of 1.0 performance factor. p)cid*tirm of toe] required performance factor, Fp, for Method S. The preheating of the air increases both ,the dry- and wet-bulb temperatures, and lowers'the relative hu midity,'.but does not alter,the humidity ratio (pounds of wa ter vapor per pound dry air). At a higher wet-bulb tempera ture, but tiie same humidity ratio, more water can be absorbed per pound of dry air in passing through toe washer, assum ing that the humidifying effectiveness of the.washer is not adversely affected by operation at the higher wet-bulb tem perature.*The analysis of the process occurring in the wisher itself isthe same as that explainedunder Method 1. The-final desired conditions are secured by adjusting the'amount'bf preheating to give the required wet-bulb temperature at the entrance to the washer. .' ; < Method S. Even if the heat is added to the spray water, the miring occurring in' the washer itself "may still tie re garded as adiabatic. The state point- of the mixture should move in a direction determined by the specific enthalpy of the heated spray. It is possible, by elevating the water tom* fig. 6 .... Graphical Solution of Conditions Produced -by a1 Dehumidifying Air-Washer' Washers, Evaporative Air-Cooling Equipment and Humidifiers 597 any washer application-involving cooling and dehumidifying can be made from Equation 2: --- > F, > (A, - A*)/(i - A,) ' . . (2) where A, ** enthalpy at entering air wet-bulb temperature,' Btu' per pound. ^ _ enthalpy at leaving air wet-bulb temperature at' actual condition, Btu per pound - Ai * enthalpy at wet-bulb temperature leaving a 'washer' with Fp "110, Btu'per pound ' ,`"- Knowing the performance factor of a particular air washer, the actual conditions of operation .can be graphically de-' termined as shown mFig. 6. Points 1 and 2 are plotted on the saturation curve1 representing total heat at'the'entering' and leaving air wet-bulb'temperatures, W and V. Point 5 represents.the condition at which leaving air wet-bulb and leaving water tempe&tures'Would be the same.'This point is' determined by solving Equation 1 for A*. ^ (A, - AJ &, = A, . ** A diagonal line is drawn through Point 5 with a negative dope equal to the water-to-air weight ratio. Points 3 and 4, at which this diagonal line intersects horizontalfines through' points 1 and 2,'show the required'entering and'leaving water' temperatures Un and W A check of toe solution can be made fromthefundamental heat'bialanceequation,- Beat absorbed by-the water--heat removed from the air.'; .The graphical method as' described can be ared to'arrive quickly at'solutions of air-washer cooling and dehumidifying' problems where"there are a number of unknown factors! in-' chiding quantity of:water to be used and the entering and,' leaving water temperatures. The actual performance factor of a particular washer must, however, be obtained-from1toe manufacturerYdata. " .. * -i Another method sometimes used to express'toe performance* of a dehiimidifier is given in terms of the relationship1of the' leaving to' entering spread between air ahdwater, tempera- . , ,W 7- U) * - C.) (3) Rq--7 ...Comparison of.PerfonnanceFoctor'for,': Given Set of Conditions where ti " thermodynamic wet-bulb temperature of entering air, 5 Fahrenheit. W * thermodynamic wet-bulb temperature of leaving air, Fahrenheit. tmt -- entering water temperature, Fahrenheit. = leaving water temperature, Fahrenheit. Performance factors expressed in these terms normally vary from about 70 percent for a single bank washer 6 ft long to practically 100 percent for a unit 8 ft long with two spray hanks, one opposed. - It must be remembered that the performance factors ob tained by there two methods are not equal as is seen in Fig. 7. The representative curves in Fig. 7 are based on, entering wet-bulb temperatures of 65 F and 70 F with a leaving dif ference between air and water of 1.5 F deg and a water tem perature rise of 10 F deg. The calculation of Pp is independent of rise in water temperature. However, the equation for Fpt does involve the difference' between entering and leaving water temperature. For example, the performance factor for 65 F entering wet-bulb temperature and 1.5 F degrees leaving difference falls from 0.925 for 10 F deg rise in water to 0.912 for 7 F deg rise, assuming the leaving difference is still 1.5 F deg. This emphasizes the desirability of stating which of the two performance concepts are being applied when considering factors. As an added precaution, it would be well to give the washer requirement in terms of leaving temperature dif ference between air and water for a given set of conditions? Air Oeaning . '! The dust removal efficiency of air washers depends largely on the size of toe particle, its density and tnrUnMHfy and itg solubility in water. Efficiency is highest for the larger more wettable particles, separation being accomplished almnat. entirely by impingement of particles on the wetted surface of the eliminator plates. Since force of impact increases with sire' of solid, this together with the adhesive quality of the wetted surface determines the washer's usefulness as a dust remover.. Experience has shown that the spray itself is relatively ineffeo-; tive in removing most atmospheric dusts. Air washers are of little use in removing soot particles because of the afr^nc? of, an adhesive effect from the greasy surface. They are afcn not' effective in removing smoke because of inadequate inertia of. the small particles (less than-one micron) to impinge-and be held on the wet plates. Instead, the particles follow the path of the air between the plates as they are unable!to pierce the .water film covering the plates. Cell-type washers, however, are efficient air cleaners. In practice, they remove, from 70 to 90 percent by weight of the airborne solid matter,, which includes most particles exceeding 5 microns in sir* and* many down to 1 micron. However, they should not be in! cotton mills and other installations where large volumes of. fibrous or linty material are present in the air, tmlpan it is, filtered.out completely ahrad of the washer. Maintenance ' r;'! . Uninterrupted performance depends largely on a regularcleaning and inspection schedule. Frequency varies with operating conditions; however a weekly inspection achwdnK is common, practice. The spray system requires the most: attention. Partially clogged nozzles are indicated by a rise in` spray pressure, while the symptom of eroded orifices is a fall in pressure. Strainers can minimize this problem.-For con4, tinuous operation a bypass around pipe line strainers or.du-. plex strainers is required. Air washer tanks should be drained and dirt deposits removed at regular intervals. Eliminatorsand baffles should be inspected periodically and repainted to