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FIG.51-VIBRATING GRATE.

FIG. 52-RECIPROCATING GRATE.

FIG. 53-OSCillATING GRATE.

amplitude of motion that causes the grate sections 10 oscil·

late. The entire assembly is inclined toward the discharge

end of the bJiler to ensure movement of the coal and

dumping of the ash.

The TRAVELING GRATE (Fig. 54) is the most popular

for spreader stokers feeding larger bJilers (over 75,000

pounds at steam per hour). The grate is made of overlap￾ping clips or bars in the form of a wide, Elfldless, conveyor

bell. It moves al speeds between 4 and 20 feet [1.2 and 6.1

metres] per hOur (depending on steam loward

the front end ot the bJller, discharging ash continuously

Into a hOpper. II Is designed 10 handle a wide r.lnge of

coals, as well as process wastes and municipal refuse.

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I PLENUM CHAMBER

CUTAWAY VIEW

ASH -

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SIMPLIFIED DRAWING

FIG. 54-TRAVELING GRATE.

CROSSFEED STOKERS (FIGS. 55 AND 56)

Crossfeed slokers (otten called rnass.Qurning slckers)

are well suited for a wide variety of solid fuels, including

peat, lignite, subbltuminous, free-burning bituminous, an￾thracite, or coke breeze. Their big advantage 15 thai lhey

offer smoketess combustion al most loads.

Coal is ted onto the grate from a hopper under control

of a gate. The gate establishes the thickness of lhe fUel

bee!. Furnace heat ignites the coal and dislillaUon begins.

/As the fUel bed moves slowly. the coke formed is bumed.

and the bed gets progressively thinner. By the time the far

end is reached, nothing remains bul ash, which fallsortthe

grate into a hOpper.

Because of the variation In fuel{)ed thickness, airflow

must vary along the bed. The grale surface is divided into

a series of lOnes. each wilh Individual dampers tor close

control 01 airflow. The furnace itself Is usually dBsigled

with long, rear refraclory arches to direct the lean fUrnace

gas fOlward, and to reflect heat 10 ignite the fUel qJickly.

High-velocity averfire air jets, located in fronl or lhe fUr￾nace above the grate. prlXllce the turbulence and mixing

required for goocl combusllon.

GRATE TYPES FOR CROSSFEED STOKERS

(FIGS. 55 AND 56)

Crossfeed stokers usually use traveling chain grates al￾though an Inclined vibrallng grate is sometimes used

The TRAVELING CHAIN GRATE (Fig. 55) is similar to

the IraveJing grate described fo~ spreader stokers. except

thai il is really a wide chain with grate bars forming lhe

links. The linkS are slaggered and connected boI rods ex￾tending across Ihe stoker width. A traveling gtate made

with overlapping clips (lhe same as for a spraader stoker)

Instead or a chain is somelimes used. This type has a

chain drive althe side of lhe grate.

31 71-97556-1

UNDERFEED STOKERS (FIG. 57)

In under1eed stokers, lhe coal is fed from the bottom or

6ide of the bed and Il'1e combustion zone propagales

downward. The comt:l!Jslible gases from the Volatile mat·

ter are passed through incandescent coke rather than

through green coal (as in O'Ierleed stokers), resulting in

more COf11,)lete combustion. The coal is pushed along in a

feed trough, or retort. Urder pressure of fresh coal from

behind, il rises, in the retort and spills over onto the bed at

either side of lhe trough. No air is sUJ:Plled In the felort; II

comes through ~nings, called ruyeres, in the grate sec￾tions adjoining the trough. At the top of the retort, the in·

coming air and furnace heat dry Il'1e raw coal. los the coal

moves from the retort to the bed, ignilion occurs and distil·

lation begins. The pressure 01' Ihe Incoming coal or the

grate molion moves the blJrnlng coal to the oomping

grates, where lhe astl is diSCharged. Underleed stol<ers in·

elude single-retort and muJDple·retort stokers.

FIG.57-UNDERFEED STOKER.

SINGLE·RETORT STOKERS {FIGS. 58 AND 59)

Single·retort stokers burn most bituminous coals as

well as anthracile. Practical considerations limit their size,

so they are used crlly in small plants. These slokers diNer

in the method of feeding coal. One type has a reciprocat·

ing ram and pusher block arrangement. The (am transfers

coal from the h~r 10 the retort, where pusher blocks

help dislriblJle il 10 the fuel bed. Another type has a relort

with a sliding bottom on which are mounted auxiliary

pusher blocks for advancing coal in the relort.

Single-relort stokers may have stationary grares (Fig.

58), but many have moving grates to provide fuel-bed agio

talion and 10 move the coal. An undulating grale (Fig. 59)

produces a wave-like motion which breaks up the coke for·

mations and keeps the fuel bed polOUS and free-burning.

In addition, all single-relort stokers have dumping grates

at the sides where the ash is discarded..

MULnPLE-RETORT STOKERS (FIG. 60)

Multiple-retort slokers are large-capacity units. They

consist 01' a series of inclined, feeding relorts, extending

from the 1ront to lhe rear ot the boiler, with tuyere sections

between them. Rams push Ihe coal inlO the front end of

the retorts below the fX>inl at air supply. Air is admilled

lhrough the luyeres in the grate sec\ions between and al

the top of the retorts. Incoming coal gradually forces its

wey up under the fire. secondary distributing pushers

move the whole mass slowly to the rear.

The fuel bed is characterized tIy l'1ills directly over the

retorts where the coat is coming in and by valleys (rela·

, -,' •

FIG. 55-CROSSFEED STOKER INSTALLATION

WITH A TRAVELING CHAIN GRATE.

The INCUNED VIBRATING GRATE (Fig. 56) consists

of a grate surlace mounted on, and in intimate contact

with, a grid of water lubes. These lubes are connected to

the boiler circulatory system to ensure positive cooling.

Waler cooling prevents grate deterioration, minimizes

clinker formation. and protects the grate from radiant heat

when auxiliary oil or gas firing is used.

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FIG. 56-CROSSFEED STOKER INSTALLATION

WITH AN INCLINED VIBRATING GRATE.

The entire structure is supportedtly a number of flexing

plates, allowing the grate 10 move freely in a vibralory

mode. Intermittent grate vibration moves the coal from the

feeding h~ onto the grate and graooal1y down the in￾clined surlace. Ashes arB discharged aulomalicaHy into

an ash pit. A liming device regulates the frequency of the

vibratory periods.

FIG. Sg-SECTIONAL VIEW OF A SINGLE￾RETORT STOKER WITH UNDULATING

GRATES. (Courtesy of Detroit Stoker

Company, Monroe, Michigan.)

FIG.S9-SECTIONAL VIEW OF A SINGLE·RETORT

STOKER WITH STATIONARY GRATES.

(Courtesy of Detroit Stoker Company,

Monroe, Michigan.)

•. -.'

FIG. SO-MULTIPLE·RETORT STOKER (SECTIONAL VIEW OF 1 RETORl).

lively thin sections at fuel baa) over fhe tuyere zones keeps the fuel bed porous. Dumping grates atlhe rear get

where the air is entering. The valleys form aseries at paral· rid at the ash.

leI, active burning lanes down the tength of the stoker. The multiple-retorl slake' was a nalural extension of

The reciprocating grate is built in sections. Adjacent the single-relor! idea. HOWElWJr, its pq:l'Jlarily has waned

sections move in q::lposile directions to cause stroking ac­ 10 lhe poinl where only one or two are sold each year

lion-when one section is moving forward, the other is Ihrou;tloul (he enlire industry. The multlple-relort s1ok.er

moving backward. This reciprocating movement distrib­ was usacl eXlensively tot' burning caking coals, tor which it

utes coal over the grate surface and at the same time is WEIll adapled. Recant successes in this same area by

33 71-97558-1