1. Steam boilers can be classified based on their orientation, tube configuration, firing method, circulation type, pressure, portability, and number of tubes. Common types include fire tube boilers like Cochran and water tube boilers like Babcock & Wilcox. 2. Boilers have various components like shells, tubes, furnaces, and mountings. Accessories like economizers and air preheaters are used to increase efficiency. 3. Modern high pressure boilers like once-through designs operate at supercritical pressures and temperatures but require high purity feedwater and advanced materials.

1. STEAM BOILERS
By
Prof. Ashish Bandewar

2. Definition
A closed vessel in which steam is produced from
water by combustion of fuel

3. Classification of boilers
• Horizontal, vertical or inclined
• Fire tube and water tube
• Externally fired and internally fired
• Forced circulation and natural circulation
• High pressure and low pressure
• Stationary and portable
• Single tube and multi tube

4. Horizontal, vertical or inclined
• If the axis of the boiler is horizontal, vertical or
inclined then it is called horizontal, vertical or
inclined boiler respectively

5. Fire tube and water tube
• If hot gases are inside the tube and water is
outside the tube, it is called fire-tube boiler.
• Examples: Cochran, Lancashire and
locomotive boilers
• If water is inside the tube and hot gases are
outside the tube, it is called fire-tube boiler.
• Examples: Babcock and Wilcox, Stirling,
Yarrow boiler etc

6. Externally fired and internally fired
• The boiler is known as externally fired if the
fire is outside the shell.
• Examples: Babcock and Wilcox, Stirling
• The boiler is known as internally fired if the
furnace is located inside the boiler shell.
• Examples: Cochran, Lancashire

7. Forced circulation and natural
circulation
• In forced circulation type of boilers, the
circulation of water is done by a forced pump
• Examples: Velox, Lamont, Benson boiler
• In natural circulation type of boilers, circulation
of water in the boiler takes place due to natural
convection currents produced by the
application of heat
• Examples: Lancashire, Babcock and Wilcox

8. High pressure and low pressure
• The boilers which produce steam at pressures
of 80 bar and above are called high pressure
boilers
• Examples: Babcock and Wilcox, Velox,
Lamont, Benson boilers
• The boilers which produce steam at pressure
below 80 bar are called low pressure boilers
• Examples: Cochran, Cornish, Lancashire
and locomotive boilers

9. Stationary and portable
• Stationary boilers are used for power plant-
steam, for central station utility power plants,
for plant process steam etc
• Mobile or portable boilers include locomotive
type, and other small unit for temporary use
at sites

10. Single tube and multi tube
• The fire tube boilers are classified as single
tube or multi-tube boilers, depending upon
whether the fire tube is one or more than one
• Examples of single tube boilers are Cornish
and simple vertical boiler

11. Purpose of boilers
• For generating power in steam engines or
steam turbines
• In textile industries for sizing and bleaching
• For heating the buildings in cold weather and
for producing hot water for hot water supply

12. Primary requirements of a boiler
• The water must be contained safely
• The steam must be safely delivered in desired
condition (as regard its pressure,
temperature, quality and required rate)

13. Boiler terms
• Shell: Consists of one or more steel plates bent
into a cylindrical form and riveted or welded
together. The shell ends are closed with end
plates
• Setting: The primary function of setting is to
confine heat to the boiler and form a passage for
gases. It is made of brick work and may form the
wall of the furnace and combustion chamber

14. • Grate: it is a platform in the furnace upon
which fuel is burnt
• Furnace: it is the chamber formed by the
space above the grate and below the boiler
shell, in which combustion takes place.
• Water space and steam space: the volume of
the shell that is occupied by the water is
termed as water space while the entire shell
volume less the water and tubes is called
steam space

15. • Mountings: The items which are used for
safety of boiler are called mountings
• Accessories: The items which are used for
increasing the boiler efficiency are called
accessories
• Water level: The level at which water stands
in the boiler is called water level

16. • Refractory: insulation material used for lining
combustion chamber
• Foaming: Formation of steam bubbles on the
surface of boiler water due to high surface
tension of water

17. • Scale: A deposit of medium due to extreme
hardness occurring on the water heating
surfaces of boiler because of an undesirable
condition in the boiler water
• Blowing off: The removal of mud and other
impurities of water from the lowest part of
the boiler. Accomplished with the help of
blow off cock or valve
• Lagging: Insulation wrapped on the outside of
the boiler shell or steam piping

18. Fire Tube Boiler
COCHRAN
BOILER

19. Contents:-
•Multi-tubular
•Internally furnace fired tube boiler (160)
•Portable, Very popular
•Size 2.7m dia. & 5.7m height of shell
•Steaming capacity- 3500kg/hr
•Working pressure- 20 bar

20. Features:-
•Compact, require min. floor area.
•Any type of fuel can be used
•Well suited for small capacity requirement
•ηth -70% (with coal firing)
•ηth -75% (with oil firing)
•Ratio of grate area to heating surface area varies
from 10:1 to 25:1

21. Mountings:
1.Pressure gauge
2.Water level indicator
3.Safety valve - to prevent pressure not increase
above design pressure(spring controlled)
4.Fusible plug - when shell temp. increases above
particular level it melts by creating opening through
which pressurized water falls on grate extinguishing
fire.
5.Blow off cock – when opened steam pushes water
at bottom to remove slurries, salted contaminants,
dust etc

22. 6. Stem stop valve – regulate steam supply outside
boiler
7. Feed check valve – high pressure feed water is
supplied to boiler through this valve, prevent
backflow of steam through the valve.

23. LOCOMOTIVE BOILER

24. •Internal furnace
•Multi-tubular fire tube boiler
•Used in locomotives
•Ideal portable unit for small power plant
•Pressure range – up-to 17.5 bar
•Steaming capacity - 4000-7000 kg/hr
•Water jacketed fire box
•Large heating surface required to produce steam at
high rate
•Cheapest of higher capacity portable boiler.
•Fire box is water jacketed with cyl. Shell
•Can use poor quality of fuel

25. LANCASHIRE BOILER

26. • Land type, stationary
• Fire tube, horizontal straight tube
• Internally fired, natural circulation
• Pressure range – 15 bar
• Evaporative capacity – 8000 kg/hr
• 7-9 m in length & 2-3 m in dia.
• Safety valve – dead weight type
Flue gas movement:-
Fire tube Bifurcate side flue
bottom flue
finally discharged through chimney.

27. CORNISH BOILER

28. • Similar to Lancashire boiler
• Except this has one fire tube inside
• Steam generation is less than Lancashire
All above boilers were fire tube boilers
1. Cochran boiler
2. Locomotive boiler
3. Lancashire boiler
4. Cornish boiler

29. WATER TUBE BOILER
BABCOCK-WICOX BOILER

30. • Water tube, horizontal, straight tube
• Draught loss is very less.
• Easy tube replacement
• All components are accessible for inspection even
during the boiler operation
• Tubes are inclined at 5 – 150
to promote water
circulation.
• Baffles are provided to pass flue gases
• Suited for all kind of fuels
• Pressure range – 11.5 to 17.5 bar(max 42 bar)
• Steaming rate – 20000 kg/hr( max 40000 kg/hr)

31. STERLING BOILER

32. • Water tube boiler patented by Alan sterling
• More than 2 steam drum connected in series
• More than 1 mud/water drum at bottom
• Joined by bent tubes to undue thermal stresses at
joints
• Can use solid, liquid or gaseous fuels.
• Lighter in construction and flexible in operation.
• Pressure range 50 bar
• Steaming capacity 20,000 kg/hr

33. Comparison of fire tube and water tube
boilers
ParticularsParticulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers
Position of water and hotPosition of water and hot
gasesgases
Hot gases inside the tubesHot gases inside the tubes
and water outside theand water outside the
tubestubes
Water inside the tubesWater inside the tubes
and hot gases outside theand hot gases outside the
tubestubes
Mode of firingMode of firing Generally internally firedGenerally internally fired Externally firedExternally fired
Operation pressureOperation pressure Limited to 16 barLimited to 16 bar Can go up to 100 barCan go up to 100 bar
Rate of steam productionRate of steam production LowerLower HigherHigher
SuitabilitySuitability Not suitable for largeNot suitable for large
power plantspower plants
Suitable for large powerSuitable for large power
plantsplants
Risk on burstingRisk on bursting Involves lesser risk ofInvolves lesser risk of
explosion due to lowerexplosion due to lower
pressurepressure
More risk on bursting dueMore risk on bursting due
to high pressureto high pressure
Floor areaFloor area For a given power itFor a given power it
occupies more floor areaoccupies more floor area
For a given power itFor a given power it
occupies less floor areaoccupies less floor area
ConstructionConstruction DifficultDifficult SimpleSimple

34. Cont…
ParticularsParticulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers
TransportationTransportation DifficultDifficult SimpleSimple
Shell diameterShell diameter Large for same powerLarge for same power Small for same powerSmall for same power
Chances of explosionChances of explosion LessLess MoreMore
Treatment of waterTreatment of water Not so necessaryNot so necessary More necessaryMore necessary
Accessibility of variousAccessibility of various
partsparts
Various parts not so easilyVarious parts not so easily
accessible for cleaning,accessible for cleaning,
repair and inspectionrepair and inspection
More accessibleMore accessible
Requirement of skillRequirement of skill Require less skill forRequire less skill for
efficient and economicefficient and economic
workingworking
Require more skill andRequire more skill and
careful attentioncareful attention

35. HIGH PRESSURE MODERN BOILER
•Pressure above 100 bar
•Temp. upto 600o
C
Advantages:-
•Scale formation is reduced due to high velocity of
water
•Small sized, light weight tube
•Time lag between start of firing & generation of
speed is reduced
•Can meet load changes quickly
•Efficiency of plant increase

36. LA MONT BOILER
High pressure water tube, forced circulation

37. • Circulating pump has higher pressure(2-3 bar)
than steam pressure
• Water from distributing header pass through
nozzle to RET
• Pressure of 150 bar and steam generation between
30,000 to 45,000 kg/hr.
• A centrifugal pump circulates the water equal to 8
to 10 times the weight of steam evaporated.
• Because of the large circulation of water
overheating is prevented.
• Main disadvantage of this boiler is formation and
attachment of bubbles on the inner surfaces of the
heating tubes

38. LOEFFLER BOILER

39. • Consist of two superheater (Radiant and
convective)
• ET only carries superheated steam.
• Suitable for marine application
• Pressure 150 bar and steaming capacity 1 lakh
kg/hr
• Problem of salt deposition in Benson boiler is
eliminated in Loeffler boiler as most of the steam is
generated outside from the feed water using part
of the superheated steam coming out from the
boiler.

40. • About 65% of steam from super-heater is used to
evaporate the feed water and 35% of steam is
supplied to H.P. turbine.
• Special design nozzle is used to distribute the
superheated steam through the water into
evaporator drum to avoid noise.
• This boiler can carry higher salt concentration than
any other type and is more compact.

41. BENSON BOILER

42. • The main difficulty experienced in the La Mont
boiler is the formation and attachment of bubbles
on the inner surfaces of the heating tubes.
• Benson in 1922 argued that if the boiler pressure
was raised to critical pressure (225 atm.), the
steam and water would have the same density and
therefore the danger of bubble formation can be
completely resolved.
• Benson boiler having temperature as high as 6500
C,
pressure up to 500 bar and steam generation
capacity of 1.5 lakh kg/hr are in use.

43. Advantages and Disadvantages
•There is no drum so weight is reduced by 20% than other
boilers.
•Pipe joints are welded, so erection of the boiler is easier and
quicker as all parts are welded at sites.
•Transportation is easy as there is no drum and majority of
parts are carried to sites without pre-assembly.
•Can be started very quickly.
•Insensitive to load fluctuation.
•Blow-down losses of Benson boiler are hardly 4% of natural
circulation boilers of same capacity.
Major problem is salt deposition when all remaining water is
converted into steam in transformation zone.
To avoid this the boiler is flashed out after every 4000 working
hours to remove salt.

44. VELOX BOILER

45. • When the gas velocity exceeds the sound-velocity, the
heat is transferred from the gas at a much higher rate
than rates achieved with sub-sonic flow.
• Boiler make use of pressurized combustion
• Atm. Air is compressed at 2-3 bar pressure.
• Compressed air and fuel are injected into CC above
sonic velocity.
• High heat release rates (40 MW/m3).
• Hot gases circulation is 10-20 times than feed water.
• Water gets separated from steam separator &
pumped back to feed water line
• Low excess air is required as the pressurised air is used
and the problem of draught is simplified.

46. • It is very compact generating unit and has greater
flexibility.
• It can be quickly started even though the separator
has a storage capacity of about 10% of the
maximum hourly output.
• It has thermal efficiency of 90 – 95 %
• Can operate only on liquid or gaseous fuels

47. SCHIMIDTH-HARTMANN BOILER

48. • Two pressure are used to effect an interchange of
energy.
• Primary circuit use distilled water at 100 bar pressure.
• Natural circulation is used in primary circuit.
• Secondary circuit is used to produce steam at 60 bar.
• Because of use of distilled water in primary circuit, salt
deposition problem is eliminated. And salt deposited
in secondary circuit can be easily brushed off.
• Wide fluctuation of load are easily taken by this boiler.

49. SUPERCHARGED BOILER

50. • Combustion is carried out under pressure in the c.c. by
supplying the compressed air.
• The exhaust gases from the c.c. are used to run gas
turbine as they are at high pressure.
• The heat transfer surface required is hardly 25 to 30%
of that required for conventional boilers.
• Rapid start is possible.
• Better response to control.
• G.T. output can be used to run other auxiliaries.
• Number of operators required is less than
conventional boiler plant

51. • ONCE THROUGH BOILER
• Steam drum is eliminated
• Entire operation (feed water heating, steam formation
& superheating) carried out in single tube.
• Capable of generating steam in the range of 225 bar
and 515o
C to 300 bar & 615o
C
• Steaming rate – 1.3lakh to 6.7 lakh kg/hr
 Difficulties:
• The purity of water and make-up water becomes more
and more important.
• Availability of suitable material to withstand such a
high pressure and temperature

52. RAMZIN SUB-CRITICAL ONCE THROUGH BOILER

53. COMBINED SUPER-CRITICAL PRESSURE CYCLE

54. Boiler accessories
• Feed pumps: Used to deliver feed water to
the boiler. It is desirable that the quantity of
water supplied should be at least equal to that
evaporated and supplied to the engine
• Two types of which are commonly used as
feed pumps are (1) reciprocating pump (2)
rotary pump

55. Injector
• Function of injector is to feed water into the
boiler
• It is commonly employed for vertical and
locomotive boilers and does not find its
application in large capacity high pressure boilers
• Also used where the space is not available for the
installation of feed pump

56. Economizer
• Is a device in which the waste heat of the flue
gases is utilized for heating the feed water
• Economizers are of two types
Independent type
Integral type

57. Air Pre-heater
• The function of the air pre-heater is to increase the
temperature of air before it enters the furnace.
• It is placed after the economizer.
• Flue gases pass through the economizer and then to
the air preheater
• Degree of preheating depends on
Type of fuel
Type of fuel burning equipment, and
Rating at which the boiler and furnace are
operated

58. Types of air preheaters
I. Tubular type
II. Plate type
III. Storage type

59. Super heater
• The function of a super heater is to increase
the temperature of the steam above its
saturation point
• The super heater is very important accessory
of a boiler and can be used both on fire tube
and water – tube boilers.

60. • Advantages of super heated steam
Steam consumption of the engine or turbine is
reduced
Erosion of turbine blade is eliminated
Efficiency of the steam plant is increased
Losses due to condensation in the cylinders and
the steam pipes are reduced.

61. Steam separator
• The function of a steam separator is to
remove the entrained water particles from
the steam conveyed to the steam engine or
turbine.
• It is installed as close to the steam engine as
possible on the main steam pipe from the
boiler

62. • According to principle of operation the steam
separators are classified as follows
Impact or baffle type
Reverse current type
Centrifugal type

63. Boiler mountings
• Pressure gauge
• Fusible plug
• Steam stop valve
• Feed check valve
• Blow off cock
• Mud and man holes

64. Pressure gauge
• To record the steam pressure at which steam is
generated in the boiler
• A bourdon pressure gauge in its simplest form consists
of a simple elastic tube
• One end of the tube is fixed and connected to the
steam space in the boiler
• Other end is connected to a sector through a link

65. Pressure gauge

66. Fusible plug
• To extinguish fire in the event of water level in
the boiler shell falling below a certain
specified limit
• It is installed below boiler’s water level

67. Working of Fusible plug
• When the water level in the shell falls below the top
of the plug the steam cannot keep it cool and the
fusible metal melts due to over heating.
• thus the copper plug drops down and is held with in
the gun metal body by the ribs.
• Thus the steam space gets communicated to fire box
and extinguishes the fire.

68. • Thus damage to the fire box which could burn up is
avoided
• By removing the gun metal plug and copper plug the
Fusible plug can be put in position again by inserting
the fusible metal usually lead or metal alloy

70. Steam stop valve
• A valve is a device that regulates the flow of a fluid
(gases , fluidized solids slurries or liquids) by opening
or closing or partially obstructing various
passageways
• Function : to shut off or regulate the flow of steam
from the boiler to the steam pipe or steam from the
steam pipe to the engine

71. Steam stop valve

72. Feed check valve
• To allow the feed water
to pass in to the boiler
• To prevent the back
flow of water from the
boiler in the event of
the failure of the feed
pump

73. Blow off cock
• To drain out water from
the boiler for internal
cleaning inspection or
other purposes

74. Mud and man holes
• To allow men to enter in to the boiler for
inspection and repair

75. Lancashire boiler
• Reliable, has simplicity of design, ease of
operation and less operating and maintenance
costs
• Commonly used in sugar-mills and textile
industries where along with the power steam
and steam for the process work is also needed

78. Cont…
• Consists of cylindrical shell inside which two
large tubes are spaced
• Shell is constructed with several rings of
cylindrical from it is placed horizontally over a
brick work which forms several channels for
the flow of hot gases
• The furnace is placed at the front end of each
tube

79. Locomotive boiler

80. • Consists of cylindrical barrel with rectangular
fire box at one end and smoke box at another
end
• Hot gases generated due to burning of coal are
deflected by an arch of fire bricks, so that walls
of the fire box may be heated properly
• The heat of the hot gases is transmitted into
water through the heating surfaces of fire
tubes

81. Babcock and Wilcox boiler

82. Cont…
• It consists of a drum connected to a series of
front end and rear end header by short riser
tubes
• To these headers are connected a series of
inclined (150
or more) water tubes
• A hand hole is provided in the header in front
of each tube for cleaning and inspection of
tubes

83. Cont…
• Feed valve is provided to fill the drum and
inclined tubes with water
• Through the fire door fuel is supplied to grate
where it is burnt
• The hot gases are forced to move upwards
between the tubes by baffle plates
• The water from the drums flows through the
inclined tubes via down take header and goes
back into the shell in the form of water and
steam via uptake header

84. Nestler boiler

85. Nestler boiler
• Fire tube type of fired horizontal axis boiler
• The boiler shell consists of two mild steel thick
plates with large number of fire tubes fitted
between two plates
• A bigger diameter furnace tube extending
from burner end to other end is used for
carrying hot flue gases from one smoke box to
other smoke box

86. • At the rare end smoke box chimney is
provided for the rejection of exhaust gases
• Hot gases passes through the furnace tube
and enter into the rear end smoke box and
pass through fire tubes to the front end
smoke box for final discharge through
chimney
• Water surrounding tubes get transformed into
steam and gets collected in steam space.
• Oil is first heated up to 80o
c by electric heater
before being supplied to burner for injection
into furnace tube.

87. • Blower is employed for atomization of furnace
oil into furnace
• Such a boilers are capable of generating steam
up to 10-11 bar.

88. Bent tube boilers
Straight tube boilers has many disadvantages like
1. They had less accessibility and poorer inspection
capability, considerable time, labour and expense
were required to open up or close the bolts in the
headers, and to remove and replace the gaskets
2. Inadequate design and imperfect fabrication of hand
hole caps (cleaning purpose) resulted in much
leakage
3. Circulation was sluggish sluggish due to low head,
and limited steam disengaging surface made
inadequate separation of steam and water reducing

89. • Bent tube boilers offers many advantages over
straight-tube boilers
• The notable among them being greater accessibility
for inspection, cleaning, and maintenance, and
ability to operate at higher steaming rates and to
deliver drier steam

90. Four drum stirling boiler

91. Five-drum form

92. • Water flows downwards from the mud drum
to headers feeding the tubes lining the walls
of the radiant surface
• The low density steam-water mixture rises up
to the steam drum at the upper side
• The steam is separated and flows to the
central drum, where it is removed
• Feed water enters the drum at the left and
mixes with the saturated liquid in the drum
• The cooled liquid flows down to mud drum

94. Cochran boiler
• One of the best types of vertical multi-tube boiler
• Consists of a cylindrical shell with a dome shaped top
where the space is provided for steam
• The furnace is one piece construction and is
seamless

95. Cont..
• Its crown has a hemispherical shape and thus
provides maximum volume of space
• The fuel is burnt on the grate and ash is collected
and disposed from the ash pit
• The gases of combustion produced by burning the
fuel enter the combustion chamber through the flue
tube

96. • They strike against fire brick lining which directs
them to pass through number of horizontal tubes,
being surrounded by water
• After which the gases escape to the atmosphere
through the smoke box and chimney
• A number of hand holes are provided around the
outer shell for cleaning purposes

100. • It is a component of steam generator
Basic requirements :
• Through mixing of fuel and air
• Optimum fuel-air ratios leading to most
complete combustion possible maintained
over full load range
• Ready and accurate response of rate of fuel
feed to load demand

101. Contd..
• Continuous and reliable ignition of fuel
• Practical distillation of volatile components of
coal followed by adequate action
• Adequate control over point of formation and
accumulation of ash, when coal is the fuel

102. Solid fuels fired
Hand fired Stoker fired Pulverized fuel fired
Underfeed stockers Overfeed stockers
Unit system Central system Both

103. Liquid fuel fired
• Injection system
• Evaporator system
• Combination of both

104. Gaseous fuel fired
• Atmospheric pressure system
• High pressure system

105. • Initial cost of equipment
• Sufficient combustion space and its liability to
withstand high flame temp
• Area of grate
• Operating cost
• Minimum smoke nuisance
• Flexibility of operation
• Arrangements for through mixing of air with fuel for
efficient combustion

106. • A stoker is a power operated fuel feeding
mechanism and grate
• A cheaper grade of fuel can be used
• A higher efficiency can be attained
• A greater flexibility of operations assured
• Less smoke produced
• Generally less building space is necessary
• Can be used for small or large boiler units
• Very reliable , maintenance charges are reasonably
low

107. • Practically immune for explosion
• Reduction in auxiliary plant
• Capital investment as compared to pulverized
fuel system is less
• Some reserve is gained by the large amount of
coal stored on the grate in the event of coal
handling plant failure

108. • Construction is complicated
• In case of very large units the initial cost may be
rather higher than with pulverized fuel
• There is always a certain amount of loss of coal
in the form of riddling through the gates
• Sudden vibrations in the steam demand cannot
be met to the same degree

109. • Troubles due to slagging and clinkering of
combustion chamber walls are experienced
• Banking and stand by losses are always present
• Structural arrangements are not so simple and
surrounding floors have to be designed for
heavy loadings
• There is excessive wear of moving parts due to
abrasive action of coal

110. • In overfeed stokers the coal is fed into the grate
above the point of air admission
• The fuel bed section receives fresh coal from
top surfaces
• The ignition plane lies between green coal and
incandescent coke
• The air enters the bottom of the grate under
pressure
• In flowing through the grate opening the air is
heated while it cools the grate

111. • The warm air then passes through a layer of hot
ashes and picks up the heat energy
• The region immediately above the ashes
contains a mixture of incandescent coke and
ash, coke content increasing upward direction
• As the air comes in contact with incandescent
coke, the O2 reacts with carbon to form CO2
• Water vapor entering with the air reacts with
coke to form CO2, CO and free H2

112. • Upon further travel through the incandescent
region some of the CO2 reacts with coke to
form CO
• Hence no free O2will be present in the gases
leaving the incandescent region
• Fresh fuel undergoing distillation of its volatile
matter forms the top-most layer of the fuel
bed

113. • Heat for distillation and eventually ignition
comes from
1.By conduction from the incandescent coke
below
2.From high temperature gases diffusing
through the surface of the bed
3.By radiation from flames and hot gases in the
furnace
4.From the hot furnace walls

114. • The ignition zone lies directly below the raw
fuel undergoing distillation
• To burn gases additional secondary air must
be fed into the furnace to supply the needed
oxygen
• The secondary air must be injected at
considerable speed to create turbulence and
to penetrate to all parts of the area above the
fuel bed
• The combustible gases then completely burn
in the furnace

115. • Fuel, coke and ash in the fuel bed move in the
direction opposite to that of air and gases
• Raw fuel continually drops on the surface of
the bed
• The rising air feed cools the ash until it finally
rests in a plane immediately adjacent to the
grate

116. Types of overfeed stokers
1 Travelling grate stoker
• Chain grate type
• Bar grate type
2 Spreader stoker

118. • A chain grate stoker consists of flexible endless
chain which forms a support for the fuel bed
• The chain travels over sprocket wheels one at
the front and one at the rear of furnace
• The front sprocket is connected to a variable
speed drive mechanism
• The grate should be saved from being
overheated, for this, coal should have
sufficient ash content which will form a layer
on grate

119. • Simple in construction
• Initial cost low
• Maintenance charges low
• Self-cleaning stoker
• Giving high release rates per unit volume of
the furnace
• Heat release rates can be controlled just by
controlling the speed of the chain

120. • Preheated air temperatures are limited to
1800
C maximum
• The clinker troubles are very common
• There is always some loss of coal in the form
of fine particles through riddlings
• Ignition arches are required
• This cannot be used for high capacity boilers

122. • In this type of stoker the coal is not fed into
furnace by means of grate
• The function of the grate is only to support a
bed of ash and move it out of the furnace
• From the coal hopper, coal is fed into the path
of a rotor by means of a conveyor
• And it is thrown into the furnace by the rotor
and burnt in suspension
• The air for combustion is supplied through the
holes in the grate

123. • The secondary air to create turbulence and
supply oxygen for thorough combustion of
coal is supplied through nozzles located
directly above the ignition arch
• Unburnt coal and ash are deposited on the
grate which can be moved periodically to
remove ash out of the furnace
• Spreader stokers can burn any type of coal

124. • A wide variety of coal can be burnt
• This stoker is simple to operate, easy to light
up and bring into commission
• The use of high temperature preheated air is
possible
• Operation cost is considerably low
• The clinking difficulties are reduced even with
coals which have high clinkering tendencies

125. • Volatile matter is easily burnt
• Fire arches etc. Are generally not required
with this type of stokers

126. • It is difficult to operate spreader with varying
sizes of coal with varying moisture content
• Fly-ash is much more
• No remedy for clinker troubles
• There is a possibility of some fuel loss in the
cinders up the stack because of the thin fuel
bed and suspension burning

127. Hand fired system
• Manual feeding system
• Very old system
• Used in small scale applications.

128. Hand fired system

131. • Air entering through the holes in the grate
comes in contact with the raw coal
• Then it passes through the incandescent coke
where reactions similar to overfeed system
takes place
• The gases produced then pass through a layer
of ash
• The secondary air is supplied to burn
combustible gases
• The underfeed principle is suitable for burning
the semi-bituminous and bituminous coal

133. • High thermal efficiency as compared to chain
grate stokers
• Combustion rate is considerably higher
• The grate is self cleaning
• Part load efficiency is high particularly with
multi retort type
• Different variety of coal can be used
• Much higher steaming rates are possible with
this type of stoker

134. • Grate bars, tuyeres and retorts are not
subjected to high temp as they remain contact
with fresh coal
• Overload capacity of the boiler is high as large
amount of coal is carried on the grate
• Smokeless operation is possible even at very
light load
• With use of clinker grinder, more heat can be
liberated out of the fuel

135. • Substantial amount of coal always remains on
the grate so that boiler may remain in service
in the event of temporary breakdown of the
coal supply system
• It can be used with all refractory furnaces
because of non-exposure of stoker mechanism
to the furnace

136. • High initial cost
• Require large building space
• The clinker troubles are usually present
• Low grade fuels with high ash content cannot
be burnt economically

138. Contd..

139. • Coal is reduced to a fine powder with the help
of grinding mill and then projected into the
combustion chamber with the help of hot air
current
• The amount of air (secondary air) required to
complete the combustion is supplied separately
to the combustion chamber
• The resulting turbulence in the combustion
chamber helps for uniform mixing of fuel and
air

140. • The amount of air which is used to carry the
coal and dry it before entering into the
combustion chamber is known as primary air
• The efficiency of the pulverized fuel firing
system mostly depends upon the size of the
powder

141. • Any grade of coal can be used since coal is
powdered before use
• The rate of feed of the fuel can be regulated
properly resulting in the economy
• Since there is almost complete combustion of
the fuel there is increased rate of evaporation
and higher boiler efficiency

142. • Greater capacity to meet peak loads
• The system is practically free from sagging and
clinkering troubles
• No stand by losses due to banked fires
• Practically no ash handling problems
• No moving parts in the furnace is subjected to
high temperatures
• This system works successfully with or in
combination with gas or oil
• Much smaller quantity of air is required as
compared to that of stoker firing

143. • Practically free from clinker troubles
• The external heating surfaces are free from
corrosion
• It is possible to use highly preheated
secondary air which helps for rapid flame
propagation
• The furnace volume required is considerably
less

144. • High capital cost
• Lot of flyash in the exhaust, which makes the
removing of fine dust uneconomical
• The possibility of explosion is more as coal
burns like a gas
• The maintenance of furnace brick work is costly
• Special equipment is needed to start this
system

145. • Skilled operators are required
• A separate coal preparation plant is necessary
• High furnace temps cause rapid deterioration
of the refractory surfaces of the furnace
• Nuisance is created by the emission of very
fine particles of grit and dust
• Fine regular grinding of fuel and proper
distribution of burners is usually difficult to
achieve

146. Self study topics
1. Schematic diagrams of modern steam
generators
2. Cyclone furnace
3. Fluidized bed combustion

147. THANK YOU