flame safeguard control phần 2 pps

The quantity of pulverized coal in the mill never exceeds a minimal amount, and piping belween the mill and furnace is short.. Usually the coal is simply blown into the furnace through a

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

, -;.••

over1eed stokers, which are much less costly to malnlain,

have just aoout obsoleled multlple-reton stokers

SUSPENSION FIRING

In susp80Slan firing, pulverized !powdered) coar is transported to the furnace in an air slream and injected

into the combuslion cl'larrt)er, along with primary air,

(hrou~ 8 nozzle The nozzle IS usually horizontal, and is

surroune!ed by art.air register Itlrou~ which secondary air

Is admitted

Within a fractioo of a IIbcbndafter a fine panicle of pow­

dered coal enters the combustion chamber, the heat pre­

sent raISes 1m lerrperature and distills aI'I' lhe volatile

maUer The volatiles, moslly hydrocartlOns, Ignile more

easily than the carbon c~1 of lhe COllI While the

volatiles bum, they heat the remaining carbon particles to

IncandesC8flG8 SecCl'ldary air sweeps past and &Crltls

the hoi carb::ln partiCles, grackJally burning them

Pulverized coal installatiOnS have lhe high heal effi­

Ciency and quick regulation dJlainable wilh gas and oil,

which are olher examples of suspension firing They also

represent an efficient method of burning a Cheaper fuel

The major disadvantage is the expensive pulverizing and

handling equipment required, which results In a relatively

hi~ ~raling cost for mechanical power They also re­

quire dusl calchers or precipitalors near urban areas to

keep fly ash from settling oyer the area

Pulverized coal units are economically feasible only for plants consuming more lhan a ton 01 coal per hour These

inslallations handle any type of bituminous coal They can

handle coke or anthracite in special cases, but it takes

much more power to grind these hard coals The power re­

quirement also increases rapidly with moisture conlenl, so

the coal is dried as much as possible before pulverizing

THE DIRECT·FIRING SYSTEM

A variety ol equipment is used to grind and lransp:ln the coal and inject it into the furnace Originally, the prepa­

ration pnase was entirely'separate The central system (or

nLR1t:ler 01 furnaces, and had a bin or bunker for sloring

ltie coal 10 await demand II could operale al optimum ca­

paCity wilhout the need for a back~ pulverizer In case of

an emergency The coal was a constant.gradeofflneness,

and the burners could be cOntrolled separately with ease

HOW8ller, the central system had lwo big disadvantages:

1 The storage bin was a potential fire hazard Sponta­

neCJl.JS iglitlon almost always occurs jf a binful of pow­

dered coal is left undisturbecl for several days

2 After Sloring the coal for a few hours, caking occurs

due to surface o:.cidation and the coal no 10nQlir flows

fteely

In the direct·firing system now used, the pulverizer SLP­

piles only one furnace, and has no storage bin Since there

Is no slorage capaCity, pulverizer operation fluctuates with

load demand ThIs system is simpler, involving less equip­

menl, so It rl!ldJces capltal outlay II also avoids lhe poten­

lIal fire hazard and caking of lhe bin The quantity of

pulverized coal in the mill never exceeds a minimal

amount, and piping belween the mill and furnace is short

The flexibility needed 10 handle a wide range of coal and load conditions is built into leday's dlrect·firlng systems

tem The funclions of amlll are (1) feeding raw coal at the proper rate, (2) grinding the COlli to the desired fineness, and (3) classifying the finished product so oversized parti­cles are returned to the mill's grincing zone

In most mills, air performs three functions: (1) it dries

lhe coal (2) II helps Classify the pulverized coal leaving the grinding elements, and (3l1t Iransports the finished prc:xj uct to the burners The air is si.Wled by tarceck:lrafl or negative pressure

In (he fOlCed-dlalt arrangement, a fan outside the mill

~lIes air under &nOl (tl pressure to perform these 3 functions tr 8 separate air heater is provided, the fan can

be located on the Inlet side 01 the heater and will handle cold air When aU combuSlioo air cemes from a single air heater (general practice in aU but large, central stallons), the fan moves only heated air and its size and power re­QUirements are mUCh greater In both cases, the fans han­dle air only, whiCh Is an advanlage, but the pulverizer mUSl

be kepi airtj~1

In the negative pt&ssure arrangement, an exhauster

fan rl!ldJces lhe air prBSSLl'e on the mill so that internal at­mospheric pressure can be used The fan may be com­bined with the pulverizer proper, or il may be mounled ellternally However, il has 10 handle air laden with coal, so

it must be rl.lQQQCl to resist wear

PULVERIZING MILLS (FIGS 61 THROUGH 66)

In a typical pulverIzing mill (Fig 51), a feeder moves the

raw coal from a hopper into a pulverizer at a definite ad-­justable rate The feedet mechanism can be a variable­stroke plunger, a re1iOlving screw, or a rotaling table A

ture coals before they get 10 the pulverizer The pulverizer

ilself crushes Ihe coal into a powder, usually by grinding

STOR"GE"'N "O'PER~ COA~

PIPING

TO BuRNERS FEEOER

but also by impact and attrition (wearing away by friction) drum, typically a steel barrel with a cast alloy·steelliner Air carries the pulverized coal to a cla::isi(ief, which deter­ Steel or special·alloy balls, about 1 fa 2 inches [25 to 50 mines the fineness 01 the coal going to the burners and re­ mm] in diameter, occupy about one-third 01 the drum vol­turns the ov~rsize particles to the pulverizer Finally, an air ume As the drum rotates, the balls are carried part of the stream carries the classified coal to the burners through way around it and then slide or drop back toward the bot·

There are several types of pulverizers Table IV lists gles with the balls Impact from the 1alllng balls and typical characteristics 01lhe most common types, which attrition and crushing from the sliding mass pulverize Ihe will now be·described -; coal The pulverized coal exits 1rom txlth endS of the drum, BALL M1L,L (FIG 6f) as shown (In another design, the raw coal enters one enc:t

A baO mill (or ,- tube mill) consists ' at a horizontal, rotaling and the pulverized coal leaves at the other end.)

TABLE IV -lYPICAl PULVERIZER CHARACTERISTICS PULVERIZING

MILL TYPE

•

SPEED (RPM)

CAPACITY (TONS OF COAL PER HOUR)

PRINCIPAL APPLICATION

FIGURE

NO

20-25

Bowl (SuctionB-) Medium

l

BOWL MILL (FIG 63)

A bowl mill is usually a suction machine An exhauster

keeps the txlwl under slight negallve pressure to draw In the raw coal Md convey pulverized coal 10 the burners /4s the txlwl rolates at a constanl speed, coal Is drawn Into II and ground between the rollers and grlndlng 00wl

The exhauster is a ruggedly buill steel-plale tan de­

signed tor handling abrasive materials A semishrouded tan wheel wilh so-called ·Whizzerft

blades handles lhe coarser coal particles This herps !o increase the life ot the maIn exhauster blades, sInce Ihey only have to corrveythe finer coal particles

Bowl mills are also manufaclured for pressurized, rather than suclion, operation TIle pressurized mills are built in larger sIzes (LP to 100 tons ot coal per hour) tor the electric utility industry In lhese mills, the rollers are In­clined more 10 the horizontal than they are in lhe suction FIG 62-BALl MILL PULVERIZER

design

PULVEI'lIZEO·COAL DELIVEI'lY PIPE FIG G3-BOWL MILL PULVERIZER

BALl-AND-RACE MILL (FIG 64)

bottom ring and 1 set of balls that comprise the grinding

elements Each ring has a groove (race) 10 keep the balls

in place The oollom ring is driven by a yoke allached 10

the verlical main shatl of the uni\

FIG 64-BALL-AN[)"RACE PULVERIZER

This mill is designed for pressurized operation Raw

coal Is fed into the grinding zone to mix with partially

ground coal Primary air causes the coal 10 circulate

through the grinding elements, where some of it is pulver­

Ized in each pass between the rings and balls A$ the coal

becomes fine enough 10 be picked ~ by the air it is car.­

ried 10 the classifier Oversize coal is removed and r&

turned to the pulverizer

Maintenance on this type of mill depends on the

abrasiveness of the coal being burrl8d Grinding elements

might have to be replaced annually, or they might last len years; the average is two years

ROLLAND-RACE Mill (FIG 65;

equally spaced around Ihe mill The rolls are mounted on axles and IiI inlo a concave grinding ring The roll assem blies are allached, by a pivoted connection, 10 an over· head stalionary Irame which maintains their posilion, (in anolher design, the rolls are not1ixed in position, but rA­volve about the axis 01 the mill in planetary fashion.) Springs apply force 10 the roll axles, thus Supplying grind­ing pressure

This Iype of mill is bum in large sizes for use in utility industries The mill shown weighs 150 tons and stands over 22 teet [6.7 metres) high, Each roll assernbly weighs

10 tons The diameter of lhe grinding rillQ is 89 inches

12.26 metres]

ATIRITION MILL (FIG 66)

impact and allrition forces 10 pulverize coal (Attrition is the process of wearing away by friction.) Coal and primary air enter the crusher-dryer section where the coal is reduced

by swing hammers and an adjustable crusher-tJlock as­sembly Here also, flash drying and turbulenl action

36

FIG 66-ATTRITION PULVERIZER

remove surface moisture1rom the coal Dried, granulated coal then passes through a grid section which removes oversize particles for recnJshing

In Ihe pulverizing section, the impeller pegs, statiOllary pegs, ard moving pegs are all tungsten·carbide-1aCed for exlreme hardness The coal is pulverized by (1) impact of the coal on the impeller pegs (2) rubbing between the sta· tionary pegs and moving pegs and (3) rubbing of coal on coal A classifier assembiy returns the coarse particles for further pulverization The integra' exhauster draws in coal

of the desired fineness and delivers the coal-air mixture 10 the burners

The allrition mill has these advantages:

1 Low capital cost per unit 01 output

2 Minimum space requirements

3 Direct drive (no speed reduction) between the prime mover and the pulverizer

4 Quiet operation

5 Lightweight parts to facilitate maintenance

6 Small coal inventory within the mill

The main disadvantage of an attrition mill is the high main­tenance cost incurred wilh abrasive coals

PULVERIZED-COAL BURNERS (FIG 67)

Burners for pulverized coal are comparatively sirrple Usually the coal is simply blown into the furnace through a horizontal nozzle by the same air wnich has passed through the pulverizer and has transported the coal to the burners in pipes: this is the primary air SE!con::lary air is usually preheated and SUl=Plied by forced·draft through a

"windbox" opening around the burner

•

.;­

WITH STUODEO TueES

FIG 67-TYPICAl PULVERIZEO-COAl BURNERS

Internal ribs or vanes in the nozzle, in lhe format rifling, heip 10 control air turbulence and the resulling lIame impart a rotary motion to the mixt\Jre of coal and primary shape

air This provides fuel·air premixing and considerable tur· Most industrial pulverized-coal burners fire info lhe bulence, which are required 10r efficienl combustion M combuslion chamber horizontally from one 01 the walls justable vanes in the secondary air regislef, or wil"'d:x:lx, These burners generally are arranged In a maTVler 10 pro­

mote turbulence Two methods used 10 increase turbu­ bustion chalTtler throw their flames against each other In lence are opposed firing and tangeflfjal firing tangential firing, the burners throw their tlames inlo the

COMBINATION BURNERS

same lime There are prd:lably as many Iypes of col'Tbina­

Comtination,burners, also:callad multlfuel burners, are

tion burners as there are combinations of the different gas, burners that.are capable 9t burning gas or oil, or even pul­

oil, and pulverized-coal bJrners Table V shows the mostverb:ed coal They can Oe 'divided Inlo two classes-con­

common combination gas-oil bumers, which are now the

most prevalent of the modern bJrners

burner, also called a dual-fuel burner, is a combination

bJrnel that is designed 10r rapidly and conveniently CONVERSiON BURNERS

changing from one fuel 10 another by automalicaly or Periodic changes in the fuel sLWly and price picture manual,ly qJeninl;j and closing valves A Simultaneous often make it adVisable 10 change fuels Therefore, by tar

HORIZONTAL ROTARV

In OIOor b.rn" ,

nO, ACI popJlilf lor modarn l:>u,nal"S (Fig ",

H"IPE

PRE~IXING

(ASPIRATINGI

Raql.l~85 aTh Inl,,!

10 bl.ltner al'lO SI·

IoIULTANEOUS (IOlgs, 13 and 14),

-GA,S FliNG /NOLlLE"

Widely usod,

p.C~AOIed :60 10 1CO BOHP!, also SI~ULTA,NEOUS (Fill

the rrosl prevalenttypEl of burner is the conversion ldual­

tueO burner thai turns aUher gas Of oil These burners are

typically used on steam generators rated between 20 and

700 boiler horSBlXlwer output (840 thousand to 29.3 mil­

lion 8tuh input at 80 percent efficiency) Most mOdern gas

burners are deSigned so thallhey can be adapted to con­

version burners by the addition of oil controls, and vice­

versa Packaged combination burners, which Include all

necessary conversion com~nents and automatic con­

trols, are also available Combination gas-oil burners can

be divided inlo 3-ptpe or 4-pipe burners, depending on the

number 01 inlets to the burner

J-PIPE CONVERSION BURNERS

As the name implies, these conversicn burners require

three pipes to deliver (1) primary comb Jstion air, (2) oil,

and (3) gas Low pressure air-aulomizing, meChanical at­omizing, and horizontal rotary oil burners are adaptable to 3-pipe systems

LOW PRESSURE AIR-ATOMIZING OIUPREMIXING GAS BU RNERS (FIGS 68 and 69)

The low pressure air·atomizing oil burner is readily adaptable as a convarsi::m burner because gas may be fed through the alomizirlg air passage This requires only the insertion 01 a 3-way valve Although there are really four pipes, the atomizing air and gas p:pes are connected 10 the 3-way valve, so there are only three pipes actually con­nected to the burner Therelore this system is commonly referred to as a 3-pipe conversion burner

To burn oil, the 3-way valve is positioned to shut off the gas and admit atomizing air When the valve is Iurned 10

38

FIG.S8-LOW PRESSURE AIR-ATOMIZING

OIL/PREMIXING GAS, 3-PIPE CONVERSION BURNER (PIPING

ARRANGEMENT) (From Combustion

Cleveland, Ohio.)

•

~'O"'l'~" ~~"t'

.~ ToU

FIG 59-LOW PRESSURE

VIEW) (From Combustion Handbook by

Ohio.)

the opposite position, II shuts off the atomizing air and ad­

mils gas The gas enters [he burner lhrOLQ'l the same an­

nular space used for atomizing air when burning oiL This system cannot be used for simultaneous burning of gas

and oil

A premixing gas burner of lho asplraUng type Is used, and the corri:lustion air draws In the gas by venturi action

The gas Is usually supplied at at~rjc pressure (zero

gas) Masl at lhese burners have a retractable oil nozzle

Retracting the nozzle enlarges the atomizing air passage

so ellOlJgh gas can pass throu~ ii, even at zero gas

pressure

MECHANICAL·ATOMIZING OIUGAS RING BURNERS (FIGS 70 AND 71)

The adaptallon of a mechanical atomlzing oil burMr 10

a conllersiCYl b.Jmer is easily accomplished a.; ack:Iing a circular manilold carrying the gas orifices The gas ling is hinged SO that it can be swung out 01 firing p:.lSiUon for in­spection and cleaning

The general shape and location oflhe gas ring cause the entering air to swirl thrCll.q1 lhe space between the out­side of the ring and the air regisler, and belween the inner surface of the ring and the flame cone This causes a lur· b.Jlence of air and gas lor intimale mixing, as welt aspanly

inducing a sucllon of gas into the combustlCYl chamber The gas ring can sometimes be moved forward and back· ward with respect to the wall of the combustlCYl chamber to facilitate optlmum positioning for the desired <::peration Generally, the same controls can be used to regulate the gas flow, oil flow, and airflow SO gas and oil can be b.Jrned simultaneously as well as separately

This type of conversion burner Is widely used and is available as a paCkaged automatic burner Fig 70 shows

an internal view of one modal as II looks from inside the comb.JsliCYl chamber Four oil nozzles are in a cluster in the cenler 01 the b.Jrner, and the gas pans are evenly dis­tributed arounc:lthe burner A blower Slq)lies corrbustiCYl air Fig 71 shows a similar modal as it looks from outside the corrtluslion chamber It has been swung <::pen CYl its hir'l',JEl to reveallhe oil nozzles Both of the packaged mcx:l­els shown have nozzle mixing gas b.Jrners and lorced draft from a blower They can b.Jrn gas or liQhI oil, or both simultaneously The Model 119 is rated for 20 to 70 boiler horsepower, ana the Model 120 is raledlor 50 to 125boiler horsepower

HORIZONTAL ROTARY OIUGAS RING BURNERS (FIG 72)

A gas ring can also be added to a horizontal rotary b.Jrn­

er, b.Jt the oil b.Jrner has to be swung oul from its pan when gas is used For Ihis reason it Is nol as popular as Ofhl:lrs for combination b.Jrners Obviously, gas and oil

cannot be b.Jmed simultaneously

Normally, gas pressure is about 1 psig, b.J1 some mcx:l­els <::perale with hiQher gas pressures Some of these b.Jrners have their own coobusliCYl chatrtler and are

Mpust't-~ unlls completely assembled at the factory; Ihey are pushed ~ to a Scotch marine or similar type 01

boller, essentially making a steam generatOl CapaCilies -range from alx:lul 1010 380 boiler horsepower OUlput {420 thouSand to 15.9 million Btuh inpul at 80 percent efficiency)

4-PIPE CONVERSION BURNERS These conversion b.Jmers require four pipes 10 deliver (11 primary combusllon air, (2) afomlzlng air or steam, (3) oir, and (4) gas Generally, anoll b.Jmer Is made Inloa c0n­

version burner a.; adding a founh connectiCYl for the gas

FORWARD GAS PORTS BURNtll TILE

ORI1'ICE PL"TE A55El,IBLY

D1LNDlZLE5

RADIAL GAS PORTS

FIG 70-MECHANICAL-ATOMIZING OIL/NOZZlE NIXING GAS RING COMBINATION BURNER

(INTERNAL VIEW) (Courtesy of Notl.fl American Mfg Co., Cleveland, onio.)

Low

systems !l is also sometimes possi:;'!e

10 a gas burner to ~ke

LOW PRESSURE AIR-ATOMIZING OIUPREMIXING

GAS BURNERS (FIGS 73 AND 74)

This burner is sir'QUar to Ihe 3-pipe burner (Figs 68 and

69) except that it has a separate gas inlet, so

valve is nol reqUired, TIle metal We support shown in

wall-not when

burner has separate alomizing air and gas inlets, il may

used 10 !x.lrn oil alone, gas alone, or 011

simUltaneously

HIGH PRESSURE AIR- OR STEAM-ATOMIZING OIU

GAS RING BURNERS (AG 75)

An oil burner using high pressure air or steam 10r atomi­

zation can be simply adapted to a conversion burner by

mounting a ported, ring-shaped gas manifold around the

all burner nonla{s) This is the sarne merhod used to

adapt a mechanical.alomizing oil burner 10 a conversion

burner (FiQS 70 and 71) Gas and oil can be burnedsimul­

taneously as well as separately

This type of conversion burnel is well-suiled for burn­

ing heavy oil and is generally buill in larger sizes II is

widely used, and available as a packaged aulOTlalic

burner The MOdel 121 (shown) has a nozzte mixing gas

burner TIle all burner is equipped with a tip emulSion

'0

atomizer in which either compressed air 0' steam serves

as the atomizing medium The blower provides 10rced

nation gas and light oil, or combination gas and heavy oil

11 is raled lor 60 10 700 boiler oorsepower

GAS BURNER PLUS ATOMIZER

II is sometimes poS5ible to ad::J an atomizer to a gas burner to make ~ into a 4-pipe conversion burner An aspi· rating Iype of premixing gas burnet using a disptacement rod 10 adjust its capacity (Fig '5) can be easily adapted to

a conversion burner by' simply removing Ihe rod and re­placing II with an oil atomizer Many modern gas burners are simply combination bumers with the atomizer omitted; this facilitates Conversion to oillaler

OTHER TYPES OF CONVERSlON BURNERS (FIGS, 76 AND 77)

SEPARATE BURNERS IN ONE ASSEMBLY Two separate bumers are sometimes built into one as­sembly Fig 76 Shows a hOrizontal rolary oil burner com­bined with a Websler Kinetic forCed-draff gas burner Each burnar haS its own pilot and Is supervised sepa

rately.TIle oil burner can burn all grades 01 fuel oil; the gas burner uses low pressule gas Only one of the burners Is q:)Elrateel at a time capacities range from 75 to 300 boilgr horsepower Other combinations of mechanical-atomiz­Ing oil burners with separafe gas burners ate fairly common

Burrler$Arfd Boilers;

GAS SECTION BURNER ORIFICE PL TE

NOULES

'"

PILOT GAS Fl:EGULATOA

NORTH AMERICAN MODEL 120 PACKAGED AUTOMATIC BURNER

FIG 71-MECHANICAL-ATOMIZING OIL/NOZZLE MIXING GAS RING, COMBINATION

BURNER (EXTERNAL VIEW, SWUNG OPEN ON HINGE) (Courtesy of North American Mfg Co., C/8v8Iand, Ohio.)

PULVERIZED-COAL, OIL OR GAS COMBINATION

BURNERS (FIG 77)

Burners wilh circular air registers can be equipped to

fire any combination of the three principal fuets-coal, oil,

or gas However, combination pulverized-coaVoilliring (in

the same burner) is not recommended because coke may

form, reducing burner perlormance When pulverized coal

is burned, it is simply jelled into the combustion chamber

by Ihe same primary air that passed through the pulverizer

and transported the coal to the burner secondary air is

usually preheated and introduced through the circular reg­

ister around the burner by forced draft

In cerlain industrial processes, It may be desirable 10

bum gas and 011 at the same lime to achieve the proper flame characteristics and tElf1'1:l&rature The mechanical atomizlng oil/nozzle mixing gas ring comtinatfon burner

(Figs 70 and 71) and any 01 the 4-fipe conversion burners

(Figs 73 through 75) can burn gas and oil simultaneously Also, 3·fipe conversion bumers using air alomi.alia'1 can be adapted to simultaneous burning by (1) substltut· ing gas tor air as the atomizing medium, {2} milling lhe gas

and atomizing air as they enter the burner, or (3) inserling

an aspirator mixer in the combustIon air line 01 the burner The available gas pressure and the desired control char­acteristics determine which method Is applicable

FIG 74-LOW PRESSURE AIR-ATOMIZING

OIL/PREMIXING GAS, 4-PIPE CONVERSION BURNER (SECTIONAL VIEW (From cpmbustion HaadbQoJs by

FIG 73-l0W PRESSURE AIR-ATOMIZING OIL!

PREMIXING GAS, 4-PIPE

CONVERSION BURNER (PIPING

ARRANGEMENT) (From Combustion

Handbook by North American Mfg Co"

Cleveland, Ohio.)

FIG 75-HIGH PRESSURE AIR- OR STEAM-ATOMIZING OIL/NOZZLE MIXING GAS RING,

COMBINATION BURNER (EXTERNAL VIEW, SWUNG OPEN ON HINGE) (Courtesy of North

.,.,

'-'_.~~' -

:T""!'.'

FIG 76-SEPARATE BURNERS IN ONE

ASSEMBLY (HORIZONTAL ROTARY

OIL BURNER AND WE'BSTER '

KINETIC GAS BURNER)

44

A boiler is a pressure vessel into which water is fed and

10 which heat is applied II generates hoi water by absorb­

ing heal, or generales, stearn by absorbing enough heat

sd Ihal the water evapcirales The source at heat may be

Ihe burning fuel il'llhe boiler's furnace, hot gases from an

exlernaLprocess, or electric healing elements

01 a boiler and a burner, including auxiliaries such as

safety conlrols, combustion cootrols, fan or blowsr oil

pump, and oil heater The term implies that all of this

equipment is furnished by one SUl=Plier who assumes re­

sponsibility ·for the operating capabilities a1tha complete

assembly II is also frequently called a packaged OOiler or

packaged generator,

The types 01

steam generators

limited 10 a relatively low pressure of 160

and water temperatures not over 250 F [121 Cl

perature hot water (250 F to 430 F (121 C to 221 Cl

pressure of 55 psig to 350 psig) may be

BOILER CAPACITY RATINGS

The oldest measure 01 boiler capacity is the boiler

QlJired to generate one horsepower in a typical steam en­

gine at the time the unit was ad:lpted Boller capacity is

now defined as the equivalent at the heat reQUired to

evaporate 34.5 poundS [15.648 kg] of water per hour into

dry saturated steam al 212 F [100 C1 It is therefore

equivalent to 34.5 poundS of sleam per hour, or 33,479 Btu

per hour (Btuh) Boiler horsepower is still used as the com­

mon measure of capacity for small boilers Domestic (resi·

dential) boilers have capaCities up 10 about 9.5 BoHP

output (400,000 Btuh input at 80 percent efficiency)

Larger boiler capacity is almost invariably given in

pounds of steam evaporated per hour, with the sleam con­

ditions specified One pound 01 steam per hour at 212 F

[100 C] is equivalent 10 970.4 Btuh Maximum conlinuous

rating is the hourly evaporation that can be maintained for

24 hours A recent trend is toward rating iarge boilers in

kilowalls (kW) or megawatts (MW) of the turbine genera·

tor thus including the work done by the reheater

Boiler output is usually expresSed in BoH P or in Ib of

steam per hr at 80 percent efficiency (which is common)

with input in Bluh

GENERAL CLASSIFICATIONS OF BOILERS

By far the greatest nUl'l't:ler of boilers in use today are eilher flre·tube or water-lube boilers

A FIRE-TUBE BOILER (Fig 78) is generally made Lfl or

a large diameter sleel shell with It.bas Inslde the shell, all arranged so that waler surroundS the lI,bes and lhe hot

gases from the furnace flow throl.l'Jh lhe hbes The pres­sure on the rubes is always on the oulslde, as shown in Fig 78 which lends to collapse the tubes Therefore the

PRESSURE ON TUBES

FIG 78-TYPICAL FIREMTUBE BOILER

tLbes must be made thick and heavy which puts a practi­cal limit on the size of the boiler (aooul 35,000 Ib of steam per hr and 300 psi) However, in their size range, fire-tube boilers generally cost less [han water-lube boilers be­cause they are easier to make and easier 10 install This makes them espeCially adaptable to special processes and heating applications TIley also are better for use in areas where the water is bad and contains considerable sediment because scale is much easier to (emove from the outside of the fire-tubes than from the inside of water­tubes

A WATER-TUBE BOILER (Fig 79) is generally made

up of one or more drums (or headers) with connecting h.bes, arranged so thai water is contained in the drums and tubes and the hot gases from the furnace flow around the outside of the tubes The pressure on Ihe tLbes is from the Inside as shown in Fig 79, so the Ilbes can be made considerably thinner and lighter weight than fire-IL.bes To illustrate this point, il is a simple matler to collapse a

TABLE VI-TYPICAL BOILER CHARACTERISTICS

CAPACITY (OUTPUTa , MAX'MUM TYPE OF BOILl':R

OPERATING STEM"

PRESSUREd (PSIG)

FIG

'0

Cast·I,on Sectional '-" 10 " million L4l 10270 ~p to 9.300

" 81.62

T~~lu" (SISflllb 290 l/'lOuSan<:l I"

840 l/'IOuSanCI 7 loa:> 24010 Il90

Horl>;onl.~Afll~~TubUl4l

{HAT) Bf1C1<·Set

, 25 million 10

~-FUOO~!) "00 25 Il"oO\Jsar>c:llo million '010600 34' '02O.0CI0 ~ ST,&I FI's-'- o

"""" 420 Il'IOuSancllO 31 million '0 10 7.50 34510 :ie.CO? ,~ ~.~

W.l ••_T o eflnl· , Stq>A8IIsrrt:NCl '2.5 million (0

250 million 2ElO 10 !IllOO 10.lXXl 10 aX!.lXIIJ '''0 ",

nol appllCaQls

up 10 17."00

UP to 200 (2 l.4W) 9104.000 (40 l.4WI

1'2,1\3 11'.118 111·'20

a For a boaS' sllll:l.nC)/ 01 tlO

b Avan.Dls as

C No lrlr'Q8 buU! s.CflPlloI' _d.1

d \lIrlsn U8S<110' gs"s,.,Ing"'" _als, pr ss~.s Is U8~.1ly ~mlisd 10 1eo psill and wale' lalrpt.atUffl not ove' ~ F ('2' ct In 6Psclllly

d8'SIg08d High Ternp8ral~'s HOi Wals, (HTHWl bolls, , prsSt;.u'se can ~ ~p 10 3M PSIll atld ISIrpt"I~.n up ID 4X1 F (221 C),

S l!lOO psIg lor """sdlln uP 10 20 8c>HP: 4.50 I'JSIll tr"'" 20 to eo 80HP 2IXl PSI aDo-oS eo 90HP

',nlOfml.lK>r1 ~n.".N.b1s

II S " c:rllieal P'fl8II""S syslsms, al:o.< 30!0U.2 ~",

FIG 79-TYPICAL WATER-TUBE BOILER

dandelion slam by suction, bul il is nearly impossible to burst I! by blowing inlo it lherefore the waler-lube 001l9r can bebullt in larger sizes and for higher pressures than the fire-tube boiler, and it dominates the markel10r large

Table Vllisls typical CapaCity ranges and maximum cp­

stating steam pressures 10r the general classes of boilers,

as well as 10r the various types of fire-lube and waler-Iube boilers available These will be dlscussed in more detail in the following sections Fig SO shows typical capacity ranges, steam pressures, and sleam lemperatures in a handy chart Keep In mind lhat unlls are buill and used for capacities pressures, arc! temperatures well outside lhe areas indicated The chart is simply a guide for represen­talive installations and Should not be conslrued as estab­liShing limits in size or applicalion

'6

.00

'SO ""JO

10.0 SO '00 '10 '00 '00 '0 ' O l O l S O

,,'

'"

t - - - " , , , , , u ·~nE"T

, CAPAC'fY FO'l U~vER'IlOCOAL

Ilr"IJl.n,,"vE IlEA" H,,'U"TUIIU'" SlJccUS'vE c"'Ac'T¥ ~U

FIG SO-REPRESENTATIVE CAPACITIES, PRESSURES, AND TEMPERATURES FOR STEAM

GENERATORS

CAST-IRON SECTIONAL BOILERS

A cast·iron seclional b:::liler lFigs 81 and 82) is corn­

posed 01 casH ron secllons lhal fil together 10 form the

pressure vessel conlaIning lhe waler, When shipped In

sections it can be taken into a bUilding Ihrough normal·

sized door openings and assembled inside I"e building

This type of boiler usually uses an Inshol or upshOI gas

burner, or a mechanical-atomizing oil burner The large

furnace rElSulls in a large area of the pressure vessel being

exposed 10 the burner flame Either forced draft or nalural

draft can be used, bullhe draft is somewhal critical elle to

Ihe large furnace_ Removal of caibon and ash deplsits from Ihe boiler healing surfaces requires wire brushing

A cast·iron sectlonal b:::liler can be used 10 generale eilher hoi waler or steam The mallimum hot waler lem· perature is 250 F (121 C1, and the maximum ~ralilllJ

pressures are 160 psig for hoI waler and 15 psig tor sleam Sizes range l4> to 210 BoHP (8.8 million Bluh IrpA) for shop-assen'tlled boilers, and l4> IQ 270 BoHP (11.3 million Btuh inpul) for field-assembled boilers

FIG 81-TYPICAL CAST-IRON SECTIONAL

BOILER (EXTERNAL VIEW) (Courtesy

of the American Boilel ManufactutetS

Associalion, Alffngton, Virginis.)

FIG B2-TYPICAL CAST-IRON SECTIONAL

BOILER (CUTAWAY VIEW)

TUBELESS BOILERS

A ll lbeless boiler (Fig 83) is made up of a verlical steel

shell within another verlical sleel shell The waler is be­

tween the shells, and the interior of the inner shell forms

the furnace The heating surface is the exterior of the Inner

shell and a section of· the ouler shen over which the hot

gases pass Steel f1nl;\ may be altached 10 eilher or bolh 01

the shells to Increase the heating surface

This type of boiler is usually designed 10r use with an oit

burner and Is available as a packaged boIler{)urner Its

main aw1icalion is the general ion of steam for smalilaun­

dries and dry'cleaning planls, The maximum operaling

pressure is 125psig Sizes range 1rom aboul710 20 BaHP

(290,OOO 10 840.000 Bluh input)

FIG 83-TYPICAL TUBElESS BOILER (Courtesy

of the American Boiler Manufacturers Association, Alfington, Virginia.)

48

struction result in se"'e:r~1 types

HORlioNTAL.RETURN·TUBULAR (HRT) BOILERS (FIGS B4.THROUGH 86) Although, as we shall see, there are several other fire­

lube boilers with hOrizontal tubes, the name horizontaJ·re·

1he bliCK-set boiler wilh an external furnace It consists of a cylindrical shell, usualfy fusion-welded, suspended from a steel structure so It does not COO"iEl into contact with the brickwork around it The shell is filled with waler to a spe­

cific level; spaCe above the waler level serves for steam separation and storage Tubes of identical diameter run the length of Ihe shell through the water space The brick·

work forms the furnace and the rear reversing chamber

The hot furnace gases 'flow from the burner althe front 01 the boiler, between Ihe brickwork and the sheU,to the rear reversing chamber There the gases reverse direction and

flow forward lhrough Ihe lUbes in !he shell 10 the smokebol at lhe front, where they are exhausted through lhe slack The steel fronl on lhe boiler has smokebol doors that provide access for Cleaning

An HRT boiler may also be designed for lhree passes (Fig BS) An inner Shelt containing lhe tubes for the third pass fits into a larger outer shell lhe hot gases flow to the

IST~"SS

REVERSING CHAMBER BURNER IIlIICIO;WORIO;

FIG 84-HORIZONTAL-RETURN-TUBUlAR

(HRT) BOILER, BRICK·SET, 2 PASSES

rear at the burner between the brickwork and the outer· shell Therelhey reverse direction for lhe second pass and flow forward through the shorter tubes in lhe rower part of lheinner shell They change direction again in Ihe revers· ing chamber for the third pass, and flow through lhe full· length lubes in the outer shell to the smokebox althe rear

at the boiler

The brick setting may be designed to accommodale many kinds of burners, so an HRT boiler can handle many types offuels This is an asset when waste by-produclsare available for use as fuel

This boiler Is normally used to generate high pressure steam, up to 150 psig Sizes range from about 30 10 300 BaHP (1.25 million to 12.5 mlllion Bluh input)

FIREBOX BOILERS (FIGS 87 AND 88)

A firebox boiler consists of a horizonlal steel shell thai contains both the tire-lubes and a small, inlernal furnace

of cubical design (also called a fireboX) The short, first· pass bank of lubes is connecled between lhe rear of the furnace and the rear at the boiler lhe second-pass bank

of lubes extends the full length of lhe boiler, above thefur­nace and lhe tirst-pass bank Three-pass designs have another fult-rengih bank of tubes above the SElcond·pa.ss bank

The furnace is usually refractory lined, and it is otten also water-cooled A water-cooled firebox boiler in which the waler vessel extends to the base on both sides o1lhe furnace is known as a water-leg boiler The locomotive boi1er is a portable firebox boiler used on trains

This type of boiler can use a gas, oil, or cai'Ttlinalion burner, usually with forced dra'fl Larger sizes, above 120 BaH P (5 mIllion Btuh input) may require field installation

of refractory and insulatioo for the furnace floor II is avail· able as a pa.ckaged boiler-burner in sizes up to 600 BaHP (25 million Btuh il'l'Ul)

The tirebol boiler can be used to generate either hoi

water or steam The maximum hot water le~rature is

250 F [121 C) , and the maximum operating pressures are

160 psig for hal water and 200 psig for sleam

drical steel shell The shell conlains a cylindrical, internal furnace located (generally) In the lower portion: A bank or banks of fire-lubes run the length of the Shell al the sides

of, and above, the internal furnace The furnace and !L.bes are completely surrounded by water A Scotch boiler (s de­scribed by the number of passes, and whether il isdryback

or welback

AdIYbaCk boiler has a ceramic baffle in the rear revers­

ing chamber to direct the hot gases from the furnace to the second pass The baffle is separate from the pressure ves­sel ard is constructed of heat-resistant malerial (generally refractory brick and insulation) The dryback boiler is used

for stationary service

chamber 10 direct the hot gases from the furnace 10 the

FIG 8S-TYPICAL HORIZONTAL~RETURN·TUBULAR(HRT) BOILER BRICK-SET

(HRT) BOILER, BRICK-SET, 3 PASSES

FIG 87-TYPICAL FIREBOX BOILER (Courtesy

of the American Boiler Manufacturers Associ8lion, Arlington, Virginia.)

50