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however, dependent on maintenance of a thin, level fuel bed. Chain end Traveling Oralai. The grate in these stokers resembles.nothing so much as a wide belt conveyor, moving slowly from the feed end of the furnace to the ash-discharge end. Coal feeds from a hopper under control of a gate, which establishes fuel-bed thickness. Furnace heat Ignites coal end distills* tion begins. As the fuel bed moves along slowly, the coke formed is burned ond the bed gets progressively thinner. By the time the far end is reached, nothing remains but ash, which falls off the grate as it goes around the end sprocket to begin its return journey underneath. Orat* Contirvrtlen. Chain grates were originally developed for bituminous coal, and traveling grates for small sixes of onthrecite. Structural details differ, but these units perform the same functions, often side by side. As the name implies, the chain grote is in reality a wide chain, with grate bars forming the links. Links ore staggered. Fig. 13, and connected by rods extend. Ing across the stoker width. In the traveling grate there is a drive chain at eoch side with crossbars at intervals. The fingers, keys or dips that form the grate surface are attached to these crossbars. They overlap, Fig. 12, to prevent ash sifting through. It can be seen that stokers of this type ere true overfeeds, but the fuel bed differs considerably in appearance from the idealised one shown on p 76. At the feed end, we have virtually all raw coal, with Ignition beginning In the top layer. Further along the stoker, diitillotian has progressed and we have active burning. Then, as distlUa. tion is completed, we come to en area where carbon Is being burned out ond, at the end, we have nothing but ash. Thus, although the steps in the burn ing process are the same, the location and shape of the various tones differs markedly from the Idealised version with clear-cut tones one above the other. Ignition. With this method of firing on a moving fuel bed, rote of ignition tends to control grate speed and fuelbed thickness. Otherwise, available time within the furnace may be too short for complete combustion, or too long to permit full use of the grate. It is radiation, in the main, that ignites incoming fuel. Once the sur face layer Ignites It warms coal beneath by contact. If the rate of air flow through this part of the fuel bed is about right, surrounding coal ignites in short order. If air flow, including leakage, Is far more than needed, over all result is cooling of the fuel bed ond o slower ignition rate. It can be seen that for best results air-flow rate will differ along the grate. as the burning progresses, oad that the . gas coming off the bed will differ markedly in composition from feed cod to ash discharge. From this point of view, there are three broadly differ ent sections: (1) The part where most' volatile matter is distilled off, usually early In the grate's travel. Furnee** atmosphere in this region Is a reducing*k one -- the gas is rich in combustible volotiles but poor in air. (2) The active burning area, where mixture of sir, and combustibles comes closest to the idea). (3) The rear tons where the last chance for burning carbon occurs. Osi from this is usually lean--heavy orith excess air. Air Distribution. Amount ond control' of air is highly Important. So, too, b control of leakage around the grata* Should this occur at the combustion* chamber level It breaks down the pas ture difference across the fuel bed. Tbb drop in "pull" through the bed lower*, the ignition rote. In units with laieeo or induced draft, such leakage put* *. heavier load on the fans. For this rest son, considerable attention is given sealing arrangements. To take care of differing air rtqo * ments and fuel-bed resistances wool the grate length, the stoker ll toned scctionollxed, with o damper in ***" section to regulate air pressure-,A_ combustion enn enter from the bottew through both grates. Pig* H* or 102 (7M) POWER Dec*** pr-vi--'~Trr'77'-' y* ? ;v;'-''' . SEAL|Nb: 0fCRAtE -fVpi^Ai::!FvU ' ' t}4g between top and bottom fi S-? Fte* l4' 1 l*K*r, three lLc*1 damper and seal arrangements JVlde-admijslon units are illustrated. fbe difference in composition of the yj coming off the fuel bed colls for I'^jtoite measures to ovoid itratifica- 'boo nd insure thorough mixing, ardist. Earliest ond still commonest for mixing Is the turaoce arch. "Furnace layouts have traced a long pattern of development Early designs featured a long front arch to reflect ^st to the Incoming fuel for quick ignl- . 'Vempere In e/do'mhdbot centre/ e/r fto*s tocompertmehti \ > . -.Chain rideeon curved goo end to direct rich gas from the boU end toward the rear. The rear arch bss gradually become more prominent, utd the front arch shorter. A typical anfa design ol today. Fig. IS, has a modest front arch, a long rear one, ond / relatively narrow "throat" between gym. The rear arch directs the lean ^gsi forward end the narrow throat ` promotes mixing. The excess air from Cthe rear end thus helps to supply the War] 'Sand between bottom chain flight end /tear forini mot -' A* Joote sir deficiency of the rich gas from the front end and thereby reduces over-all excess air. Ovtrflra Jets. Some designers prefer 'Stoatptetee term coon portmentt er tu/eree. . ` <Mo*ob/optotee'/oem dampen V deub/epipe . controt oh togreta . % ' ;rotfor\ Ill 'll Ll 1 II. .11 |--vijVvW-- relatively open furnace with only a short front arch, and depend on high* velocity overfire jets to produce the 'turbulence and mixing required. They i reason that arches as mixing agents ; give best results at design rating, whereas the jets give positive mixing over the entire load range. A number 1 of such "open" furnaces have been In' stalled and results seem favorable. tokor Drivei. Drive can be opplled at either front or rear end. A worm and wheel or gear and pinion provide the ' necessary speed reduction. Power unit cpmportumte terme ptenua chamber/ etr onion from eideaindbee K . Air rtqv1r*iMMi'iAHsr a* fln rmvii from Ifront .ti. riar.- Wtvoro1 otr ll ' odnilrti'd a| ildu, fsrlwi milhodi or* iwd le boas elr.flow ofimitnDh'. / , At top, cron mombori and iloal bottom plait font, tight' lompartrnonti. Mlddlo .tftilgn hoi moI plates hlogod from trots rnombors and rolling-on lower'grata, i ' in bottom unit, I'poca batwooa grate* formi ptowwm chombor wlttrdsmporii abova e*a be o constant-speed motor with variable-speed transmission, a turbine nr s steam engine. Chain- ond traveling-grate stokers, n* a class, have long been considered f **11 suited for high-ash free-burning - *nle. The reasoning behind this is that the fuel moves in one plane, relatively , toe from agitation. Coking coals, it is , tot, tend to mot over, cut off air sup- Ply, and increase fuet-bed resistance. draper selection of coal sise and site t\AAAAA>j 15 .Oas at itektr front It rich, ol roor liloon. This mekasgood ml*lng vU 'tol. Arrival mey bo oidd to dlrdct flow or fvmoco mey bo opon, with loti : Wnsist goes a long way toward ossur- Mtisfactory operation. With natufurnace draft of about 0.1 to 0.1S coal having a top site of 1V4 In. and le fines below Vh in. gives good re* *hs. Heavier draft permits coals of IH-in top site, having the range of naturally occurring in crushing. I "ced-droft chain grates have about 10% air space in the grate as con- -tod to about 20% for natural-draft ** So cools with on increased per- J7****e of flnea and o smaller top site to * in.) can be burned satiifacy with forced draft. Compsaioilng ' food. One overfeed stoker employing a moving grate oper ates on what b known as the compensating-feed principle. The grate Is made up of alternate moving and sta tionary sections thot run the full length. Orate is inclined. Moving sec tions occupy the greeter area. Over the grate surface at the rear ere cover plata designed to retard ash flow ond give o tight rear-end draft seal. As coal moves progressively down the grate, Impelled by the moving sections, the stationary sections retard movement ond build up a compression force. When cracks and fissures develop this force relieves itself by shifting coal Into the open oreas of the fuel bed. -P?*U Docimbir 1946 PV) 102