typs of feeders

8/6/2019 Typs of Feeders

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STOCK EQUIPMENT COMPANY TECHBRIEF

31ULlH STOCK ModularFeeders Replace Older\ Volumetric Feeders'4-"Figure B - Doctor Type Table F eede r

The STOCK modular feeder has been developed tonllow application of gravimetric feed technology to%xisting lants currently feeding coal with mechani-XI olumetric-type feeders. Generally, five or moregeneric volumetric feeder types were used through:he years for delivery of coal to pulverizer or cy-:lone. The following describes the performance andnaintenance aspects of the various feeder types.Drag FeedersThe drag feeder (FigureA) was commonly used byWer Lehigh and Babcock& Wilcox Companyjuring the 1920s and 1930s. Generally, the chainNas about 6 overall width and from3-0" inimumo whatever length was required to connect the bun-cer with the discharge point. Due to the abrasiveNear between chain links and feeder casing, theseeeders required frequent maintenance and replace-ment of plate steel, suffered from occasional chainxeakage, and would occasionally cause pluggagesat the feeder inlet due to the rather constrictivep n i n g size. In an effort to eliminate pluggage con-i'tions, larger (16' width) feeder sizes were intro-iuced around 1955; however, they still suffered fromnechanical wear and occasional pluggage withramp material. Due to the clumping of coal be-ween links, feed tended to be erratic and fall to the

pulverizer/cyclone n clumps, resulting in occasionalfurnace pulsation, particularly when used on cy-clones.Doctor Blade Table FeederUse of a table feeder with scraper blade, known as adoctor blade table feeder (Figure B), was commonto the ore and rock industry prior to introduction ofthe belt feeder to coal handling. Foster Wheelertypically used the table feeder on double-ended tubemills with two scraper blades splitting discharge toeither end of the mill. Because the angle of reposeof the coal tended to valy with moisture and fuelsizing, the delivery rate tended to vary widely for afixed table (rotor) speed. The problem of unformdelivery was aggravated as the table polished withtime, and it was not uncommon for sticky coal to beundercut by the rotating table without actuallyfeeding fuel to the firing system.Controlled Discharge Table FeederB&W developed a controlled discharge table feeder(Figure C) about 50 years ago. At that time, com-bustion control was in its infancy and control of feedto pulverizers was often adjusted and set by hand.The B&W table feeder was an improvement overolder table feeder designs which scraped the outer


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layer of material from the table, and the angle ofrepose greatly affected the amount fed for a giventable speed. The B&W feeder used a fixed areaof discharge through which the coal was ex-truded. However, slippage of the table (especiallyafter wearing smooth) reduced he capacity to theextent that material eed rate had no clear correla-tion with feeder table speed. The coal rate could notbecontrolledby combustion control but, rather.relied on ratio metering with the control signalcoming fmm measuring he coal level in the pulver-izer bv cotmarina the Dressure dron across thepulvehzer 4 th & pr&ure drop across the primaryair orifce. This cancels out the effect of quantity ofprimary air and gives level only. This level is con-trolled by running the feeder at a speed generallytwice maximum pulverizer rate and. when the levelreaches normaloperation, the motor is stopped untilthe level of coal in the pulverizer decreases. A laterrefinement included a two-speed motor with'Stopped.' 'One-Half Speed' (100% capacity), and'Full Speed' (200% capacity) to better tune devia-tions caused by the StarUStop cycle. The cyclingtypically resulted in spotty feed which could beseenas peaks in superheat temperature records. Gener-ally, problems associated with the feeder had to dowith slippage of the table under the coal withouttransfer of motion to the coal.StationaryDrumPocketFeederThe pocket feeder has been usedby CombustionEngineering for over 50 years. It was made in anumber of sizes and isshown n Figure D. Gener-ally, these feeders have di iiulties in rapid responseto load changes as seen by the boiler. Problemsspecifically related to pocket feeders included:0 The drive unit required considerable mainte-nanceandwould occasionally fail, stopping coalflow to the pulverizers.0 The pockets (intended to deliver a specificvolume of coal per revolution) were oftenpartially packed with coalwhich made it diicultto determine how much fuel had been trans-f e d hrouah the feeder. This was ~rimarilvwhy many &ket feeders had batch& ccklscales installed mmediately upstream inthepulverizer system to totalize weight of fuel going

0 On occasion,coalor tramped material in thecoalwould bindbetweenthe rotor and housing,jamming the feeder and stopping coal Row.

With extremely dry, dusty coal, there had beenoccasions where the leveling bars employed onthe pocket feeders had ammed in the UPposition, allowing flooding of the mill with free-flowing fuels.Drum-type pocket feeders and apron feederswhose operation and operational problems closelyrelate to the pocket and drag feeders previously dis-cussed are shown in Figures D, E, F,and G.lt is important to note that the mechanical vol-umetric feeder types outlined had no way of re-sponding to fuel density variations except as trim-med by the combustion control master following timelag. The benefits of matching fuel to air flow asapplied to the combustion process are evidenced bythe rapid acceptance of belt-type gravimetric feedersduring the early 1960s. It issafe to say that most ofthe new units built since the early '60s have incorpo-

rated gravimetric machines to more closely relatefuel input to the boiler-to-boiler demand for Btu input.Due to the limited offset from centerline of bun-ker discharge to centerline of pulverizer inlet on jobsemploying the various mechanical volumetric feed-ers outlined, it has been difficult to retrofit grav-imetric technology to existing boilers without majorchanges to the plant's equipment location, andaeneralhr there was insufficient floor soace availableibr succ&ful installation and mainter;ance.Wth the developmentof the STOCK modularfeeder (shortened o accommodate these limitedoffsets), Stock is now in a position to supply gravi-metric feeders and their associated benefits to manyexisting installations currently running volumetricstyle.This modular feeder is available to accommo-date any offset from 4'-6' to Om,the smaller offsetsbeing satisfied by placing a return belt in the bottomof the gravimetric feeder to return coal to the offsetrequired.


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Figure C - Controlled DischargeArea Type Table FeederFigure D - Stationary DrumPocket Feeder

Figure E - Self Cleaning Pocket Feeder Figure F - Pocket Feeder With MagneticBelt Pulley. Remote Mounted

Figure G -


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StockQuality,StockReliability

MPC BATCH SCALES

KNIFE GATE VALVES COMPACTORS

NFPAFEEDERS

Y?-z3GmCONICAL DISTRIBUTORS . . . . 2 HANDLING EQUIPMENT

A WIDE VARIETY OF COALAND BULK MATERIAL VALVES

MPC CONTROLS LOW PRESSURE GRAVIMlRICFEEDERS

For additional information about Stock Equipment Companyor its products, please contad:Stock Equipment Company16490 Chillicothe RoadChagrin Falls, Ohio 44023-4398Telefax 216-543-5944

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