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air PommoM centlneed 1 Ga$>flow rate*, plus drift velocity 1 of particles, influence final results Collecttog area * flow rotk>m 3.4 so ft per eft Percent combustible*e% C*K> tfmXJO 30 40 Peek UtovoJis 50 The higher the applied voltogo, the more efficient will dust collection be Peo* aofffCgtv ContctipQ area * Horn ratio*3 4 Sc ft per eft Combustion or rva, 0f tt -tI olid, liquid ot gueou^j off waste products thst eooU wE lion problems. Some, likecsrbefo ide and water *apor, rented te tvjS air and adequately diffused, proZ$3 less. But to ibe untraloed eye th$ plume they form appears thrma This public reaction to stack diSjl makes the job of remorfag tk^H J Settllnp velocity of porftdes ho* fc long been used to rote preelpltotors tlonablc elements that much toada) One ol the biggest targets for reaction to sir pollution Is tkefli central-station power plants. YetJj class, they usually ymt wiUini isji more In solving powcr-geamtbijlj allied problems than other % 80h "" b 70 . Pack 0Ht* 4g hr I Arrcfttf com0v*fthtM/?1g,e/ eo xjjo I t--i---- --------- 2 4 6 e <0 Coiitc'ingarto+tloa ratloiAia ft per eh Ratio of effective collecting area to got (low affects unit efficiency And this instance proves ae aJSH What, then, Is the esse* lor lUiS lion, today? On the whole It's iff! encouraging- JH Air-Borne Solids. The msjor as the utility Industry sees It, 1* t&R been control of slr-bome soBdJl member, they burn tremendous ties of pulverised coal--over 105,928 ions in 1951 alone. Assuming stffS age ash content of 10% this ceuldjjfl out due preeautlons, show up uffl 10,000,000 tons annually of ffysshfl But to get the problem down mJH practical proportions--4he siqglsSB --consider a large modem boiler OOO.OOB-lb-steam-per-hr capscity. Tffl hausts (lue go* at about 500,0 efflj a temperature of 300 F or so. typical, overage figure io flyswfl Pollution Praeamtiom im Blatter CwhmSS H A Osman, ConsotWslsd Kdittfj At EE rtpr K. IS-J*. and EMyMfl2 nfrlitori ft* EUttrif CffMltlf tWw| ' j White, btR|) Corn. \9 Combustible content of flynsh obove Mechonicoi dust collectors show good 10% destroys preclpltotor results w results for cortaln particle ranges vtutopc con/not 7 "vp,n<` <" <rt:tn>,.o.le prsclpitator bv moon.llc mean, imn dim d.po.1, wllhom O'-vlouily ujual oblMItanoblo puH. Precptorw ^xtogr-1' ** 8Predlcllng corono voltop* *c vorlss with eoch dlffsrent ENOINEERINO AND MANAGEMENT SECTION lad Field Research Have Upped Performance of Dust Collectors ,iioa ol 2 V P*r cu II* dwl l**ing e6 amounts to about 140 lb or 100 tons per day. A els- l(r0| problem lor any plant. Yet equipment today that does a clean-up job of this stack Vide $!* Solid pertieles, making tek dust can and do range in sise e tiniest specks, too small to he iu t powerful optical microscope, irtjda large enough to pick up the fingers, Table II, p 76. The y^sptill panicles represent no probrceept under most unusual eondi0 ol concentration. The large ones be reduced in quantity both at fur* 'tnd at stack level. Upper limit of njrtduction has been set in many m by ordinances regulating smoke ^un.. Urge percentage of these ordinances 1 on ASME Test Code PTC 21* [l, which states a maximum figure of > per 1000 lb gas (equivalent to at flj gr per cu ft at 300 F) adjusted excess sir. If left to a clean-up level alone, this would require -jOeeter of 85% efficiency for the ev. [fee flue-dust load--well within cur. ji design limits (see below). [furnace Performance. But the utility dry doe* net leave the cleanup job f<the nsek alone. They've thoroughly ^ettigated furnace conditions and per. mince* with an eye to reducing the 'le before it gets a start. [Complete combustion is a basic re* , 'mem with the large, better-run i.trsl stations. They lake every precaun for obvious reasons of economy: to . . SMftt PW WN>* .*ecrch shows sparking Is good In Omclpttotori but only up to o point assure (l) proper fuel preparation (2) intimate mixture of air with fuel (3) correct Introduction of fuel to furnace (4j furnace of adequate design for fuel in question (S) ample fen capacity (or required excess air f6) constant maintenance of proper fuel-air ratios (7) mechanical or electronic aids to correct and signal faulty furnace conditions to operation. Tbete seven steps represent the best engineering practices in aasur* ing complete combustion. Over and above Instituting these cor rect operating steps the role of indi vidual fuels and furnace designs have received similar Intensive study. The more progressive companies take every advantage of facts like these--elog-tap furnaces con drain off a portion of tho fuel osh in a fused form; other furnace designs may separate out varying pro portions of ash from the Burgas stream; fuels of low solid residue result in low est gas dust-loading. Jest about every known way of holdfng down dust load ing at the furnace level can be found In use or under study. A reduction in the carbon percent in fiyash (preferably below 10%) is very desirable for clean ing boihot furnace and staek level. Stack level Clecm-up. The seme at tention to performance details carries over into the reothods (or collecting flue dust after it leaves the furnace. Hence, dust-collector manufacturers have been steadily raising their equipment's per formance levels in response to this de mand, Powen, special reprint, Cleon Air. Both mechonieel and electrical de signs report efficiencies up to 90% and, in some instances, better. A recent electrostatic precipitator design installed at Cpudey Station oi New York State Caa and Electric Co is said to give 98% efficiency handling 253,000 efm of 264-F gas from 0 560,000-lb-per-hr generator. Dust loadings run from 3.43 to 233 gr per standard cubic ft. Moreover, this unit features a new method for con tinuously cleaning off the collecting electrodes. But let's review precipitator theory first, to better understand these improvements. Theory. In practice, gas containing pertieles of dust in suspension passes between highly charged electrodes of the typical electrostatic precipitator. While under the Influence of these elec trodes electric forces set on the particles to forco them onto collecting plates or surfsoes. Under usual conditions the charge on h Hknioron-dki particle (1/2500 ol an in.) is about 25,000 elec trons, while the precipitating or collect ing field measures about 4500 v per fcm. The coulomb force amounts to about 300 times the gravitational force, as suming a spherical particle of unit den sity. So it is by far the dominant ele ment. The efficiency ol particle removal, though, depends mostly on the effective surface area of the collecting electrodes, the gas-flow rate through the precipita tor and drift velocity, to, of tho charged particles in the electrical field, fn turn, w is directly proportional to the particle radius so largo particles collect at a faster rate than do 6ne ones. Fig. 1 graphically illustrates the string of precipitator efficiency with gss flow for several values of to. Efficiency runs high at low gas rates for all values of to and falls off with Increasing gas (low, slowly for high values ol 10, more rapidly for low. The effective theoretical value of to for flyaah precipitators Is about 0.7 to 131 ft per see as against the practical and field efficiency measurements of 0.4 to 0.6 ft per see. This 2 to 1 discrepancy can be charged to the loss oJ precipita tor performance caused mostly by un even gas-flow end loss of particles from collecting surfaces by resuspension. From a practical application point the preeipltstor manufacturer needs data on the properties of furnace get and dust. For example, the corona and the spark ing characteristics that determine the strength of the electric field, Its coulomb force, depend on dieleclrlo properties ol the gas and resistivity properties of the dust. A typical pulverized-coal-fumsee gas after the preheater consists of 77% nitrogen, 4 to 5% oxycen, 12 to 15%' carbon dioxide and 4 to 10% woter with traces of sulfur dioxide and trloxlde. Table 1 shows the makeup of a typical dust. From such data, gas properties end resistivity can be determined. Furnoce-aoi moisture content and dewpoint ore primarily influenced by the amount of volatile matter In coal. They, in turn, itrondy Influence corona and electrical operating characteristics. Trace quantities of sulfur trloxlde are Complete (lue-dust dote, next page, should accompany specs AMtit ISM ENGINEERING AND MANAGEMENT SECTION