'''~
Il""T
<lAT1~
~,
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.
-'''EL'''~D
c.~",~
TUVll
_TOC.l'M(~
~
I
PLENUM
CHAMBER
CUTAWAY
VIEW
ASH
-
'I,

_~
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
MUL
nPLE-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.


'''D9D~
'''Ln
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-MUL
TIPLE·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
, -;.
••
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 ofpow-
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
tin
system) consisted
of

a large pulverizer supplying a
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
storagebin. 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.
The
pufverizing mlJlls the hean of the direct-firing sys.

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 parti-
cles 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
re-
QUirements are
mUCh
greater.
In
both cases, the fans han-
dle
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 at-
mospheric pressure can
be
used. The fan may
be
com-
bined
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 variable-
stroke plunger, a
re1iOlving
screw,
or
a rotaling table. A
crusher-dryer removes surtace moislure
from
high mois-

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
CRuS"ER·
DRYER
C~"SS'F'ERS
Pu~YER'lER
FIG.
61-TYPICAL
PULVERIZING
MILL.
34
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 vol-
turns 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·
pipes.
tom. Coal is fed in at both endS
at
the drum and intermin·
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.
Ball
Slow
20-25
4.so
Abrasive coals.
62
Bowl (SuctionB-)
Medium
75-225
4·20 Industrial steam-
generaling systems.
63

Ball-and-Race
Medium
75-225
4·20 IndUstrial steam·
generating systems.
64
Roll-and-Race
Slow
2().75
55·70
Gentral-station
txlilers
lor
utilities.
65
Altrition
High
above 225
6-32
Nonabrasive coals;
txllh
industrial plants
and utility staliOtlS.
66
,
",-
•
a Pressurized
txlwl
mitis are manufactured

in
larger sizes,
up
10
100 tons
01
coal per hour, for the electric utility
industry.
Ball mitis
use more
power
and
are noisier than other
types
at
pulverizers. However,
tJ:ley
can handle abrasive
coals at less than
halt
the
malnt~ance
costs.
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
·Whizzer
ft
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
In-
clined more
10
the horizontal than they are in lhe suction
FIG.
62-BALl
MILL
PULVERIZER.
design.
35
71-97558 1
PULVEI'lIZEO·COAL
DELIVEI'lY
PIPE
FIG.
G3-BOWL
MILL
PULVERIZER.
BALl-AND-RACE MILL

(FIG.
64)
A ball-and-race mill has 1 stationary top ring. 1 rotating
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\.
MAIN
SHAFT
CLASSIFIER
'~J:ciIONARY
DRIVING
YOKE
'SURNER,PIPE
'
SHUTOFF
VALVE

~,
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;
A roJ/-and-race mill has 3 large diameter, loroidal rolls
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
rA-
volve about the axis
01
the mill in planetary fashion.)
Springs apply force
10
the roll axles, thus Supplying grind-
ing pressure.
I
LLr"
PIPES
I.,'
CO"L
1
-'"
'U"'CY
'I'
PIPE
PULVERIZED_
:
-COAL
II' ,
,r~OELIVERY

FIG,
65-ROLl-AND-AACE
PULVERIZER.
The grinding ring
rOlales
at
a slow speed. This motion
is
transmilled
10
lhe foils, which rolate about their own axes.
Grinding occurs under the rolls in lhe replaceable grinding
ring. There
is
no metal-lo-metal conlact between grinding
elements
as
each roll rides on a thick layer
ot
coal. This
1ealure minimizes maintenance. Circulation of the coal
IS
the same
as
described for
the
bali-and-race mill.
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)
An attrition mill
is
a high-speed machine that combines
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
as-
sembly.
Here.
also, flash drying and turbulenl action
36
I
£)(HAUSTER
SECTION
Jj
-_.,~
.j;.~
~41,
CRUSHER·
I
ORYER
SECTION
:
REJECTOR
ASSEMeLY
I
~
GRIO
I~MOVING
.•
~Iil~~\""
.:;.i
.
".

.

I
~
II"'
I
~,
''"1
~
" ""
,

., ~
STATIONARY
IMPELLER
PE:G
""
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 main-
tenance 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
HOfUZONfAlllURNER
CIRCULAR
REGISTER
BURNER
INTERVANE
BURNER
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-
37
71-97558·1
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
In

opposed /iring,
bumefS
In
~lfe
walls
of
the com· corners
of
the
C~I,
COMBINATION BURNERS
DEFINITIONS
burner is one that is capable
01
burning two fuels at
Ihe
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
most
verb:ed coal. They
can
Oe
'divided
Inlo
two
classes-con-
common combination gas-oil bumers, which are now the
version
burn'f!tS and simultaneous burners, A conversion
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
"'-
TABLE
V-COMMON
TYPES
OF COMBINATION GAS.OIL BURNERS
TYPE
Of
CONVERSION
BURNER
TYPEOf
0"
BURNER
TYPE
OF
Oil
BURNEll
""IR_ATOMIZINO
STEAM_
lOW
PRESSURE
HIGH
PRESSURE
ATOMIZlNO
IIIEC10CAIUCAl
ATOMIZINCl

HORIZONTAL
ROTARV
3-f'IPE
PRE~IXING
(ASPIRATING)
Easlly
llOapleC
or
merl/ng
3-W/ll'fva

{Figs
Be
and
BB),
-
- -
-
[}A,S
"lING
(NOZZLE
MIXING)
-
-
-
W~Jy
,,"C'.
paCllaOle<:l
(20
10

'~BoHP).
also
SllIULTA,NEOUS
IFI"",
70
acd
7\)
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"
~IXIN)'I
-
WIcIoIy
"'"C',
pac~aOllC'
(60 10
YCO
BOHP),
also
SI~ULTANEOUS
(FIG,151
Widely
usod,
p.C~AOIed
:60 10
1CO
BOHP!,
also
SI~ULTA,NEOUS
(Fill
'"

-
-
2
SEPARATE
BURfolERS IN
t
ASSEMBLY
SEVERAL
TypeS
-
-
-
Fa~1y
a:.mmon_
In
<><100,
Inat&II&I'on~.
nol
80
papular
10'
rr>r>De1ll
~rs
(F"'_
18).
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 at-
omizing, 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 con-
nected 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
Handbook
by North Amen'can Mfg.
Co"
Cleveland, Ohio.)

•
~'O"'l'~"
~~"t'
.~


ToU
FIG.
59-LOW
PRESSURE AIR-ATOMIZING
OIL/PREMIXING
GAS,
3-PIPE
CONVERSION
BURNER
(SECTIONAL
VIEW). (From Combustion Handbook by
North American Mfg.
Co., CJevelsnd,
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 in-
spection
and
cleaning.
The
general shape
and
location
oflhe
gas
ring cause
the entering
air
to swirl thrCll.q1
lhe
space between the out-

side
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 dis-
tributed 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:l-
els 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:l-
els
<::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.
39
71-9755&-1
FLAM!!:
ELECTRODE
FORWARD
GAS PORTS
BURNtll.
TILE
ORI1'ICE
PL"TE
A55El,IBLY
D1LNDlZLE5
RADIAL
GAS

PORTS
NORTH
AMERICAN
MOD£l119
PACKAGEO
AUTOMATIC
IlIUIllN£Il

,
FIG.
70-MECHANICAL-ATOMIZING
OIL/NOZZlE
NIXING
GAS RING. COMBINATION BURNER
(INTERNAL VIEW). (Courtesy
of
Notl.fl American Mfg. Co., Cleveland, onio.)
Low
pressure air·atomizing,
high
pressure air-atomizino,
and
steam-alomizing oil burners are adaplable
to
4-pipe
systems
!l is also sometimes possi:;'!e
to
add
an

atomizer
10 a gas burner to
~ke
It a conversion burner.
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 the 3-way
valve
is nol reqUired,
TIle
metal
We
support
shown
in
Fig.
. 74 is used only when the burner ls installed in a thin metal

wall-not
when
itls
installed In a relractof)' wall.
Sincelhis
burner has separate
alomizing
air and
gas
inlets,
il
may
be
used
10
!x.lrn oil alone,
gas
alone,
or
011
and
gas
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
draft. This model
can
burn
gas, light oil, heavy oil, combi·
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 re-
placing
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 as-
sembly.
Fig.
76
Shows a hOrizontal
rolary
oil
burner com-
bined
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-atomiz-
Ing oil burners with separafe
gas
burners
ate
fairly
common.
Burrler$Arfd Boilers;
GAS SECTION
BURNER

ORIFICE
PL
TE
NOULES
'"
PILOT GAS
Fl:EGULATOA
'.
PILOT
~
SOLENOID
,
1 G
SVALVE
-".i

t "', J
BLO

ER

orOA
CABLE
_OILPV"'P"OTOR

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.
SIMULTANEOUS
GAS~OIL
BURNERS
In
cerlain industrial processes, It may be desirable
10
41
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 char-
acteristics determine which
method

Is applicable.
71-97558-1

,,~
[
:.h
••
,Bllme:.f.s::And:sdilir.si:jj{
:
._.'._"

',':
"~:>;Z;W""""'·'
.
"'.
:

,

-"-,,,,-,":',,"
:"-'c'::,,'-
'UOL
,

, ~ t'"
-
"tN~
i; +AO'

H

~
••
OUT
n
FIG.
72-HORIZONTAl
ROTARY OIL/GAS RING, CONVERSION BURNER (SECTIONAL VIEW).
TIL~""I'ORT\

_.
.
"._.
r!
_.
BUIINER
nc[\
o~
Ll"'l
I:.l
l
.TO~'NG
~
,,'
~
M:"''''
-
FIG.
74-LOW
PRESSURE
AIR-ATOMIZING

OIL/PREMIXING
GAS,
4-PIPE
CONVERSION BURNER
(SECTIONAL
VIEW.
(From
cpmbustion
HaadbQoJs
by
North American Mfg.
Co"
Cleveland, Ohio.)
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.)
"
-\,I~
TRAVIOLET

rLA

£
oETECTOR
__________
OIL
NOZZLES
~
•
OIL
NOZZLE
,OAflTEA
"'"",C'''"''''
S':
"'oF""
"OW"""""G~~~'
'~'f.~)
~
\ f -
r
",'
I;'
\.
./
J:iIoo=-o

,
-
':::AS
SOLENOIQ


LVE
OIL
SOLENOID
V"'LVE
_ ATOloO'ZING
""11
",05£
NORTH
AMERICAN
MOOEL 121
PACKAGEO
AUTOMATIC
BURNER

FIG.
75-HIGH
PRESSURE AIR-
OR
STEAM-ATOMIZING OIL/NOZZLE MIXING GAS RING,
COMBINATION
BURNER (EXTERNAL
VIEW,
SWUNG OPEN
ON
HINGE).
(Courtesy
of
North
American Mfg. Co., Cleveland, Ohio.)

71-97558-1

.,.,
'-'_.~~'

:T""!'.'

FIG.
77-PULVERIZEo-COAl,
OIL,
OR
GAS
COMBINATION BURNER.
FIG.
76-SEPARATE
BURNERS IN ONE
ASSEMBLY
(HORIZONTAL ROTARY
OIL BURNER
AND
WE'BSTER '
KINETIC
GAS
BURNER).
44
PART
II-BOILERS
INTRODUCTION
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.
A boiler·burner
(or
boI7er-burner unit)

is
a corrt:linalioo
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
boilers
described

in
this
reference are all
steam generators (designed basically tor generating
steam). Some
of
the smaller boilers can be used
for
gener-
ating
hot
water,
but
in
most
cases
this
type
01
operation is
limited
10
a relatively
low
pressure
of
160
psig
maximum
and

water
temperatures
not
over
250 F
[121
Cl. For large,
new
space
heating
systems involving
long
runs
of
distribu-
tion
piping
(such as airports and universities),
higher
tetTl'
perature
hot
water
(250
F
to
430 F
(121
C to
221

Cl
at
pressure
of
55
psig
to 350 psig)
may
be
used as the heat-
ing
medium.
The
High
Temperature
Hot Water (HTHW)
boilers
used
are special
applications
requiring consulla-
lion
with
the
boiler
manufacturer.
BOILER
CAPACITY
RATINGS
The

oldest measure
01
boiler
capacity
is the boiler
horsepower
(BoHP). whiCh
wa~
the
amount
of
steam
re-
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.
45
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
pres-
sure on the rubes is always
on
the
oulslde,
as shown in
Fig. 78.
which
lends
to
collapse the tubes. Therefore the
PRESSURE
ON

TUBES

·0·

FIG.
78-TYPICAL
FIREMTUBE BOILER.
tLbes must be made thick and heavy.
which
puts
a practi-
cal 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
be-
cause 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
water-
tubes.
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
71-97558-1
TABLE
VI-TYPICAL
BOILER CHARACTERISTICS
CAPACITY
(OUTPUT
a

,
MAX'MUM
TYPE
OF
BOILl':R
''''PUT,
",U
(9TU'S
PER
HRl
601ll':R
HORSEPOWER
(80HPI
L80FSTEAM
PER
jotOUR
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
'2.'
million
:Ill
10
300

',ooo'o'O.1XCI
,~
~-
FUOO~!)
"00
Il"oO\Jsar>c:llo
25
million
'010600
34'
'02O.0CI0
~
ST,&I
FI's-' o.
""""
420
Il'IOuSancllO
31
million
'0
10
7.50
34510
:ie.CO?
,~
~.~
PaCl<aged
(l,lbmlfld
SCOld'1
BoIa/"

421l'lhous.nClIO
42
million
'0101000
34510
~.OCIO
~
91·94
Venleal
b
e.<
It.,.JUrICl
10
'2.'
mlllK>r1
210300
!!Ill
'0
'O.IXCI
",,'
"
$1f
•
",·Tl.I:>8
c
84
million
10
37'
million

aX!
10
i5ICXXl
7.000
10
300.000
,
~.~
P.draQ8d
1,25
mlNlon
10
,00
million
2lJ
10
2300
1.000
10
eo,lXXl
""
~,~
W.l
••
_T o.
eflnl·
,
Stq>A8IIsrrt:NCl
'2.5
million

(0
250
million
2ElO
10
!IllOO
10.lXXl
10
aX!.lXIIJ
'''0
",
FI.Id-EroK:Jed
,8.6
mlI'l<;W1
'0
8100
million
4~'0
'90,000
",000
10
6,~.lXXl
""
I""""
,~
OUICk
Slumfl."
up
10
18.8

million
l4l10
435
UP
10
15,000
~
'"
Rs~
slum
Gflnfl.alors
AKIslanc

Typs
EISel.1e
EISCllOds
up
10
750
million
nol
"!'PlICabls
nol
appllCaQls
up
10
17."00
UP
to
200

(2
l.4W)
9104.000
(40
l.4WI
~p
10
600.000
~p
'08.900
31010
'38,000
,,~
,
,
1'2,1\3
11'.118
111·'20
a
For
a
boaS'
sllll:l.nC)/
01
tlO
per""nl,
_lch
Is
c:oTImoo,
b

Avan.Dls
as a
packaQ8d
b:IIIS'-(>,Irns.
C
No
lrlr'Q8.
buU!
s.CflPlloI'
_d.1
~allon".
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
~",
:::::P:":"I.~U~"~'l.'
'U

@
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
sleam generators.
Other general classifications include csst-iron sec-
tional,
tu~fess
(steel), and electric boilers. Recovery
steam
generstors
burn the by-products from a manufac-
turing process or energy system rather than the principal
fuels
(gas, oil, or coal). For this reason, they are classed
separately, ralher
than by the
type
of
boiler which they

usa
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 represen-
talive installations and Should not
be
conslrued as estab-
liShing limits
in
size or applicalion.
'6

00
'SO ""JO
I • ,
'!
!
10.0
SO
'00 '10 '00
'00
'0
'OlOlS.O
,,'
'"
t ",,,,,.u

·~nE"T
,

CAPAC'fY
FO'l
.U~vER'IlOCOAL
"00
"10
"00
A""0.',"AlE
U,",T
~Oll
NATUIIAlC.IICUlATIO,.

JIOOr ~
'000
CA'ACor¥.
'OooLS
OF
§lEA"
PEII
~II
,',·:,f ':~~,-W':':1_'U'~~~'~~~'U~~
,.




,.~~
~,
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
casHron 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.
47
71·97558·1
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 generalion
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
I
FIRE·TUBE BOILERS
•
~."
A fire-lube boiler is constructed
at
a large steel shell
with
thtck-walled lUbes inside. The
water
10
be
healed
sur-
rounds
the

lubes,
and
the hOi furnace gases pass
Ihroogh
the tubes. The location
and
size of the fumace. position
and
number
of
banks
of
lubes,
and
other variations in con-
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·
turn-tubular (or
HAT
tor short) is
commonly
used
10
refer to
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
~~=='=} d

:-
.
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 thefur-
nace
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.
SCOTCH BOILERS (FIGS.
89
AND 90)
A Scotch
boil"r
is essentially self-contained in a cylin-
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
de-
scribed 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 ves-
sel
ard
is
constructed
of
heat-resistant malerial (generally
refractory
brick
and
insulation).
The
dryback

boiler is
used
for stationary service.
A
w"tback.
boiler
has
a steel
baffle
in the rear reversing
chamber
10
direct
the
hot
gases
from
the furnace
10
the
49
71-97558-1
FIG.
8S-TYPICAL
HORIZONTAL~RETURN·TUBULAR
(HRT) BOILER. BRICK-SET.
I,
R£VEIlSlNG
C
"RR

UPSHOT
lIURN£RS
TuBES
SOdOKEBO"
'"

'

FIG.
86-HORIZONTAL-RETURN-TUBULAR
(HRT)
BOILER,
BRICK-SET, 3 PASSES.
FIG.
87-TYPICAL
FIREBOX BOILER. (Courtesy
of
the American Boiler Manufacturers
Associ8lion, Arlington, Virginia.)
50
FIG.
88-2·PASS
FIREBOX BOILER.
'"0

1
$"O"UO\~OT
o~.u
TU.U
,

,
.
FIG.
89-3·PASS,
DRYBACK, SCOTCH BOILER.
second pass. The rear reversing charmer
is
completely
submerged
in
water. The boiler gels
ifS
name from the wall
of
water belween the baffle and the rear
enc:I
of
the boiler.
The
Scotch boiter originated as a wetback boiler devel-
oped
lor
marine service, so this distinctive
desiQn
is atso
called a
Scotch marine boiler. I! proved to be extremely
satlsfactory
for
marine service because

of
lis compact-
ness.
and
it
can also
be
used tor.stallonary service.
The Internal furnace In a
Scotch boiler musl
be
de-
signed to resist the compressive forces
10
which It
Is
slb-
jected. A smaller furnace may
be
self-sL;flOrtlng
if
ilS walls
are thick enough. A larger furnace can
be
strengthened by
(1)
corrugating its walls,
(2)
dividing
I!

into sections with a
stiffening flange (Adamson ring) between sections.
(3)
us-
ing welded stiffening rings, or
(4)
Installing staybolls be-
tween the furnace and the
ouler
shell. For solid fuel, a
btidgewall may
be
built into the furnace al the end
ot
the
grate section.
Most Scotch boilers found
tOday
are modified designs
in the form of packaged fire-tube boilers, which will bedls-
cussed next.
PACKAGED FIRE-TUBE BOILERS
(FIGS.
91
THROUGH
94)
Most paCkaged fire-tube boil9fs are lineal descendants
of the basic Scotch design, and
represenllhe
bulk of fire-

tube boilers being buill
tOday.
They
are
complete assem-
blies thaI can
be
installed
"'ilh
a minimum
of
complications. One manufacturer engineers, builds.
1iretests before shipment,
and
guarantees the material,
workmanship, and performance
of
the packaged boiler.
The manufacturer also furnishes
and
assumes rasponsi-
bility for all
cOrTlJXlnents
in the assembled unit, including
the burner, boiler, controls,
and
auxiliary equipment.
Today's packaged lire-tube boilers are available in a
wide variety
of

designs. The basic flow pattern
lor
the hot
furnace gases
is
2-pass. 3-pass,
or
4-pass
(Fig.
91).
!n
all
cases. the 1irsl pass
is
through the internal furnace, and
{he succeeding
passes are through full-length banks
01
tubes. All patterns are available
in
either dryback or wet-
back designs. The wetback designs are also known as
modified Scotch marine boilers.
FIG.
91-TYPICAL
GAS nOW
PATTERNS

l-PA5S
OIlV


CK
2.P

:;:;
ORYBACK
).~A:;S
W[TIIACK
"He
.~£~C\
,.(n
O~

'"0
.~

lU."
''''0'''00'
~
-
'I
t
Iii
i)
<:,_,.~rE"
l
~.
-
~
p

~.::;:~~ "
~~"f;Y
~:
f1

-
_~





~
r
1/
,,":\
n
,,.
.~$$
O~'''CE
"URNA'.
RUR
"'''ER5'"G
, u
-
FIG.
90-2.PASS,
WETBACK,. SCOTCH BOILER.
FOR PACKAGED FIRE·TUBE
BOIL.ERS.

The
2-pass
boiler tends to
be
the
si~est
and may
have the lowest initial cost. However, the gases have a
relatively short travel time between the burner and
the
stack, resulting
in
less heat transfer and lower effiCiency
than for boilers with mare passes.
The
3-pass
boiler seems to
be
the mosllXlPUlar. The
additional
pass
Increases efficiency at the expense of
added complexity.
Designs vary greatly, from relatively
small-diameter furnace
lubes
surroundEld by a mullipliCJ/y
of
fire-tubes, to large furnace
tlbes

"'ith
fewer fire-tubeS.
51
71-97558-1
The 4·pass boiler resulted from
an
effort
10
funher
In-
crease the Iravel lime
of
the
gaSes
and
hence the
eft!-
clancy.
To
maintain gas velocities through all tha tubes,
builders decrease
Ihe
total cross-secl1onal area
of
each
succeeding pass,
usually
by
rEdJcing the nunt:ler
01

tLbes
In each
of
lhe
suCCessive passes, TI\e ad:1itlonal complex-
Ity results in the
highest
Initial
cosio
In recent years, Improvements in construclion have
procluced
mor.e
compact
aoo
less cosily packages. To-
day's
units develop a
boiler
horsepower from only 3 to 5
square
1ael
01
healing surlace.
Some
fire-tLbes have inter·
nal fins, which increase the ,surface area elQXlsed
10
the
hot furnace gases
by

350
percent, resulting
in
2-pass boil-
ers
of
great compactness. Larger openings
and
hinges
have
made
the
inlerior
surlaces more accessible for i.,
spe"ctlon
and
cleaning. Heat radiation
losses
have been
reduced
ttl· a minimum
by
effeclive shell design
and
Insulation.
Corrbuslion-control
EqJipmenl
has
been continuously
I~oved

A majority
of
packaged
fire-lube boilers
use
foread draft (usually blowers)
and
an
Increasing nurTtler
use modulatIng control. Some controls (Honeywell's
R4140 Flame Safeguard Programming Control,
for
ex-
an'JIle) have plug-in components for easy mainlenance
and
replacement
of
parts.
For convenlional, unpaCkaged, fire-lube boilers,
it was
il'Jl'OSSibla
10
know what firing, selling, and draft condi·
lions would be encountered
in
actual application. There-
fure, il became customary to assume that. under average
conditions, 10 square feel
of
healing surface would pro-

duce one BoHP.
This
mett\OO
of
rating was ultracortServa-
tive, so boilers were freqJenUy operaled
at
150
10
200
percent
of
• rating
.•
In conlrasl, makers
of
packaged fire-lube
roilers
con-
trol all the conditions affecting capacity and, in addilion,
shq:rtest Complete units.
Thus
lhey are able to assign a
true rating
of
continuous maximum ooiler output. When re-
placing an old boiler 'Nilh a packaged boiler, the raling
of
the old boiler cannot' be
used

The aCllJal performance
re-
quiremenls must be estimated
10
select the capacity
of
the
,new
unit.
A typical 3-pass, welback, packaged
fire-Ilbe
boiler is
shown In
Ags.
92
and
93. This packaged bOiler
is
a com-
pletely engil')eered, Integraled, packaged unit consisting
01
a boiler, forced-draft burner, blower, automalic cootrols,
and
accessories. The
burner
is capable
01
burning gas,
light
011,

or
heavy oil. TI\e
roilar
Is available
In
capacities
ranging from 40
10
700 BoHP (1.68 million to 29.3 million
Bluh
irp.,rt), It can
be
used to generate
sleam
from 15 to
150 psig,
or
hoi
waler
up
to 30
psig,
and
can be adapted
10
higher pressures if required.
A typical 4-pass, dryback, packaged 1ire-tlbe boiler is
shown In Fig. 94, The packaged
roiler
consists

of
a pres-
NOATH
AMIlAICAN
ATLAS
GENEAATOA
6'179
FIG.
92-TYPICAL
3-PASS, WETBACK,
PACKAGED FIRE-TUBE BOILER
(SHOWING HINGED BOILER DOORS
AND BURNER).
(Courtesy
of
North
American Mfg.
Co"
Cleveland, Ohio.)
sure vessel, burner,
burner
controls, forced-draft fan,
damper, air pump,
and
associaled components. The con-
version burner can
burn
either gas
or
heavy oiL

The
boiler
can
be
used to generale
sleam
from
15
10
150 pslg, or
hoi
water from 30
10
1S0'pSig,
or
highsr
if
specified. Available sizes range 1rom 50 through 125
BoHP
(2.1
million
10
5.25 million Btuh inpul).
VERTICAL
FIRE~TUBE
BOILERS
(FIG.
95)
The vertical fire-tube boiler design Is
one

of
the oldest
and
is still pq::lular
for
maT1'f
applications because
of
the
minimum
floor
area It requires. This
type
of
boiler
i:s
self-
contained
in
a v9l1ical, cylindrical shell. The hoI furnace
gases pass 1rom the furnace
allhe
roltom
of
/he boller
~
ward
through the vertical fire-lubes. Helical
fillings
inside

the lubes spiral
the
hot
gases against the
lube
surfaces to
increase heat transfer. Usually, the pressure vessel con-
taining the water elllends
down
10
the
base
around
Ihe fur-
nace, 10rmlng a water·leg,
By
usIng
a water·leg
conslruction at the
lop
of
the boiler, it is possible to
exlend
lhe
water level higher than the endS
of
the tubes; It Is then
called a
submerged vertical boller,
A vertical boiler is usually

used
to generate
high
pres-
sure steam, The
maximum
q:l9raUng pressure varies from
200
psig
for
the largest
rollers
up to 600
psig
for
the small-
est ones. Sizes range from 2 to 300 BoHP (84 thousand
10
12.5 million
Bluh
hlJut).
WATER·TUBE BOILERS
A water-tube
boiler
is
cons!ructed
of
one
or
mora

drums
ner,
so
water-tube boilers
canbebullt
for
higher pressures
or
headers, with connectIng tt.bes. The waler
10
be heated
and
much
greater outputs than firetube
rollers.
Because
passes through the tubeS,
and
the hot furnace gases
SUf-
of
their grealer healing surface area, they can proclUce
round the tubes. The walls
of
lhe
tubes can be much thin-
steam
much
faster.
They

elso have greater reserve
52
~L"NG~"
STE

OUTLET
INSU

TEO
FLUE
'''LucroR
I-+INGrO
OOORl
OIURV"T10N
PORT
'luE
VENT
S"OKEBO~
."UR·TYPE

ell
~:-_ ~

-:T;_.
.

'
TEfL
IlIlfFLE
.:

'
:

:.:.
'sT
P SS
1-+0,
REAR
G~~
P.EHRSlHG

r'HlF
CN

BER
I".q
L,::-::: :;;:::::::::-:~::::=-::::=::-:c·cO~'_TcH~A:M.",'c"c':A:'_A=T=~::::·:·:_'c·C'C·A:TCOC·-==,
J
FIG.
93-1l'P1CAL
3·PASS,
WETBACK,
PACKAGEO FIRE-TUBE
BOILER.
(Courtesy
of
North American
Mfg. Co., GJeveland, Ohio.)
capacities and can
be

fired above rated capacity for longer
periods. Bent-tube designs have largely replaeed
lhe
old
straight-tube
types. Classifications resull from
the
arrangement
of
the drums and lubes, methOd
of
assem-
bly, and
type
01
burner.
STRAIGHT-nJBE BOILERS (FIGS.
96
AND 97)
During the firsl quarter
of
the century, straight·rube boil·
ers (also called horizontal boilers) were quile popular. To-
day they are no longer
built
except
lor
special
applications. However, many are still in service.
In these boilers, the main

bank
of
waler-tubes is
straight
and on a slope
of
510
15 degrees Irom the horizon-
tal. They
are
further classified
by
the position
of
the drum.
In a
fongirudinaJ-drum boiler
(Fig_
96), the axis
of
the hori·
zontal drum is parallel
10
the cenler lines
of
the lUbes in the
main bank
(in
a horizontal plane).
In

a cross-drum boiler
{Fig. 97j, lhe
axisot
the horizontal drum is at right angles to
the center lines
of
the
tubes in the main bank. Both types
have large furnaces. The cross-drum type
has
an advan-
lage
When
it
is preferable
10
increase the boiler width
rather than its length
10
obtain necessary capacity.
A straight-tube boiler
01
either
type,
with the bank
of
lubes connected
by
IWD
inclined, rectangular headers

(a
fronl and a rear), is also caned a box·header boiler. If the
tube bank is divided into parallel sections. each
secUon
having a front and rear header,
and
tile
headers
are
all
connected
by
vertical rows
of
lubes
to
a. CCXTlmon steam
drum, the boiler is known
as
a S!Jctional·/Jeacler boiler.
53
BENT-TUBE BOILERS
Bent-tube boilers were found to
be
more flexible lhan
straight-lUbe boilers. Where headroom is limited,
they
can
bemade wide and low. Where floor space
isal

a premium,
they can be made narrow and high. They also allow more
surface
10
be exposed
to
the radiant
heat
of
the furnace,
and
can
be
built at a lower cost.
Mc:x:lern
water-lube boilers consist
of
thousands
of
feel
of
steel tubes
ot
various diameters and wall thicknesses.
In general, there
Is
maximum use
ot
vertical,
or

near verti·
cal
tubes.
Steel drums serve as convenient ecllecllng and
separating points
In
the steam-water circuit. In a sirrllie
water-tube circuil (Fig.
98),
sleam bubbles form in
the
heated risers. TIle resulting steamwater mixture weighs
less than the cooler water in the
unheated
downcomers.
so il is displaced. In the drum, steam but:tllas rise to
the
water's surface and steam is released. Moving the relative
positions
of
the drums provides flexibility in the shape
and
size
of
the boiler
desigl.
Boilers with lwo, three, and sometimes tour
dtlXTlS
were lXPular through the 1920's.
later

advances
maCie
it
practical to eliminate one and sometimes even two drums.
Boilers with two drums, and even with only one drum at the
top and one
or
two large headers
at
the bottom, became
usual designs. However, lhoUsands
of
the old multicltum
variety are still operating satisfactorily today.
To reduce radiant heat loss. waterwalls were
daVel-
cpecl; water-lubes are
irrt:leCJ:jed
righlln
the
walls to
cool.
the furnace. Baffles in the form
of
tangent tubes gJidelhe
flow
of
hot furnace
gases
in the dBsired direction.

7t-97558·'

FRONT
1l

LE
BUIlNEIl
"'SSEMBLY
1l0T",lly
,1l
O",MPl:R
&TH
PASS
3RD
PASS

,
CLEAVER
BROOKS
MODEL
CB
P"'CKACED
BOllER
FIG.
94-TYPICAL
4-PASS, DRYBACK, PACKAGED FIRE-TUBE
BOILER.
(Courtesy
of
Cleaver Brooks, Milwaukee, Wisconsin)

-;~
VERTICAL
FI
R£-TUeES
WATER_LEC
FURNACE
FIG.
9S-LONGITUDINAL-DRUM,
STRAIGHT-TUBE
BOILER.
FIG.
95-TYPICAL
VERTICAL
FIRE-TUBE BOILER.
54
I
•,
••
FRONT
NUOIR
FIG.97-CROSS-ORUM,
STRAIGHT-TUBE
BOILER.
"'~","AT'~

,XTUO,
FIG.
98-SIMPLE
WATER-TUBE CiRCUIT.
Other deSign trends Include

(1)
lighter cons/ruction to
prevent air
or
gas leakage,
(2)
elimination
ot
the Induced-
draft fan
by
keeping the furnace and txliler under pres-
sure, and
(3)
more use
of
heal
recO\lElry
equipment-air
heaters and
economizers-in
the flue
gas
exhaust.
The required
CapaCity
of
a water·tube txliJer deter-
mines how
it is assembled. It can usually be Classified as a

paCkaged, shop.assembled,
or
field-eret:led boiler.
PACKAGED
WATER-TUBE
BOiLERS
(FIGS.
99
THROUGH 102)
By
definition
of
the American Boiler Manufacturers
As-
sociation, a paCkaged boiler is
~a
boiler equipped and
shipped complete with fuel-burning equipment, mechani-
cal-draft equipment, automatic controls. and accesso-
ries. - Packaged boilers
are
buill in the small and
medium-size ranges.
MlX!ern paCkaged boilers have experienCed an ex·
traordlnary growth in popularity and size sInce their incep-
lion
in
tMe
late 1940's. Using
gas,

oil, or combination
gas-oil burners, Ihey have established an atlractive price
advanfage
In
the medium-size range (10,000 to 80,000
55
FIG.
99-A·TYPE
PACKAGED
WATER-TUBE
BOILER.
POl.ll'k:fs
of
steam
per
Mour).
For a boiler in Ihis size range,
it
is estimaled lhat a packaged boiler
COSls
only aboul
$1
per
pound
of hourly capaCity, while a field-erected boiler
would cost about $2
per
pound, a stoker-fired field-erected
unil would coot about $2.50
per

pound. and a p.Jlverized-
coal field-erected unit would cost
about
$3
per pound.
Controls hal/e played an important part in the success
of
paCkaged wateHube boilers. Users have come to
ex-
pect
higM
reliability in supervisory controls such as
flame
monitoring, low-water alarms, and ol/er-pressure cutoffs.
There has also been an increase in the use
of
annuncialor
panels to monifor
lhe
complete burner-bailer systems and
indicate the location
of
at'foI
troubles which might
c:levelcp.
These panels are a great eid
{o
~eralors
auerrpling
10

firM:l'
tMe
causes
of
shutdowns.
Today, most packaged water-tube boilers are
b.Jin
wilh
one
of
lhree basic
h.bearrangemenls-A.
0,
or
D.
When a
larger capacity is deSired
beyond
the maximum size limits
for these units. transportation limitations must be
consid-
71-97558-1