Definition of a boiler
• A boiler is a device used to generate
steam at a desired pressure and
temperature by transferring heat energy
produced by burning fuel to water to
change it to steam.
• It is a combination of apparatus used for
producing, furnishing or recovering heat
together with the apparatus for
transferring the heat so made available to
the fluid being heated and vaporized.
Function of a boiler
• The fluid is contained in the boiler
drum called shell and the thermal
energy released during combustion
of fuel is transferred to water and
this converts water into steam at the
desired temperature and pressure.
Applications of boilers
Power generation: Mechanical or electrical power
may be generated by expanding steam in the steam
engine or steam turbine.
Heating: The steam can be used for heating
residential and industrial buildings in cold weather
and for producing hot waters for hot water supply.
Industrial processes: Steam can also be used for
industrial processes such as for sizing and
bleaching etc. in textile industries and other
applications like sugar mills, cement, agricultural
and chemical industries.
Factors to be considered
for selection of good boiler
1) The working pressure and quality of steam required
2) Steam generation rate
3) Floor area available
4) Accessibility for repair and inspection
5) Comparative initial cost
6) Erection facilities
7) The portable load factor
8) The fuel and water available
9) Operating and maintenance costs
Requirements of an efficient boiler
1. The boiler should generate maximum amount of steam at a required pressure and
temperature and quality with minimum fuel consumption and expenses
2. Steam production rate should be as per requirements
3. It should be absolutely reliable
4. It should be light in weight
5. It should not occupy large space.
6. It should be capable of quick starting
7. It should conform to safety regulations.
8. The boiler components should be transportable without difficulty
9. The installation of the boiler should be simple
10. It should have low initial cost, installation cost and maintenance cost.
11. It should be able to cope with fluctuating demands of steam supply.
12. All parts and components should be easily accessible for inspection, repair and
13. The tubes of the boiler should not accumulate soot or water deposits and should be
sufficiently strong to allow for wear and corrosion
14. The water and gas circuits should be such as to allow minimum fluid velocity (for
low frictional losses)
Classification of boilers
Relative position of hot gases and water
•Fire tube boilers (Cochran, Lancashire, Cornish, Locomotive)
•Water tube boilers (Babcock and Wilcox boiler, Stirling boiler)
Method of firing
•Internally fired boilers ( Lancashire, Locomotive)
•Externally fired boilers (Babcock and Wilcox boiler)
Pressure of steam
•High pressure boilers(>80 bars-Cochran,Lancashire,Cornish, Locomotive)
•Low pressure boilers (<=80 bars-Babcock and Wilcox boiler, Lamont
Method of circulation of water
•Natural circulation boilers (Lancashire, Locomotive, Babcock & Wilcox
•Forced circulation boilers (Two large fire tubes Lancashire boiler, Single
large fire tube Cornish boiler, Cochran boiler, Many small tubes Locomotive
boiler, Babcock Wilcox water tube boiler)
Nature of service to be performed
•Mobile boilers (or) Portable boilers
Once through boilers
Position and number of drums
• Single drum boilers
• Multi-drum boilers(Longitudinal or crosswise)
Design of gas passages
• Single pass boilers
• Return pass boilers
• Multi-pass boilers
Nature of draught
• Natural draught boilers
• Artificial draught boilers
• Combustion of solid, liquid or gaseous fuels
• Electrical and nuclear energy
• Hot waste gases of other chemical reactions
• Steam boilers
• Mercury boilers
• Special boilers for heating special chemicals
Material of construction of boiler shell
• Cast iron boilers
• Steel boilers
Differences between Water-tube and Fire-tube boilers
Particulars Fire tube boiler Water tube boiler
water and hot
Hot gases inside the tubes and
water outside the tube
Water inside the tube and
hot gases outside the tubes
Mode of firing Generally internally fired Externally fired
Operating pressure limited to
Can work under as high
pressures as 100 bar
Rate of steam
consumption Lower Higher
Not suitable Suitable
Involves lesser risk on
explosion due to lower
Involves more risk on
bursting due to high
Floor area For a given power,
For a given power,
Differences between Water-tube and Fire-tube boilers
Particulars Fire tube boiler Water tube boiler
Construction Difficult Simple
Transportation Difficult Simple
Shell diameter Large for same power Small for same power
explosion Less More
Treatment of water Not so necessary More necessary
Various parts are not so
easily accessible for
cleaning, repair and
Various parts are more
Require less skill for
efficient and economic
Require more skill and
Features of Cochran boiler:
5)Fire tube boiler
6)Up to maximum steam pressure of 15 bar
7)Maximum evaporative capacity of 4000 kg
of steam per hour.
Construction of COCHRAN BOILER
Cochran boiler consists of a vertical cylindrical shell, fitted with a
hemispherical crown at its top which form the steam space, and a hemispherical
dome which forms the furnace of fire box.
A platform over which the fuel burns called fire gate is provided in the
furnace. Beneath the grate there is a space, called ash pit to facilitate the
collection of ashes. The fuel is charged through the fire door provided at the
front end of the furnace.
The combustion chamber at the rear end in the middle portion of the boiler
is lined with the fire bricks which prevents the overheating of the combustion
The furnace and the combustion chamber are interconnected by the elliptical
flue tube. The unburnt volatile matter leaving the furnace along with the hot
gases are burnt in the combustion chamber.
Number of flue tubes connects the combustion chamber and the smoke box
fitted at the front end. The chimney provided above the smoke box serves for
the escape of gases.
The man hole provided at the crown of the boiler facilitates the inspection
and repair of the interior of the boiler.
Working of COCHRAN BOILER
The Cochran boiler is filled with water to the specified level and maintained
at that level by charging with makeup water using a feed water pump and when
the water level drops below its specified level. The entire surface of the furnace
except the openings for the fire door and the combustion chamber will be
surrounded by water. The flue tubes will also be completely submerged in the
The hot gases from the furnace along with the unburnt volatile matter pass to
the combustion chamber through the elliptical flue tube where the unburnt
volatile matter burns completely. From the combustion chamber they pass
through the horizontal flue tubes to the smoke box. The gases from the smoke
box escape to the atmosphere through the chimney.
The hot gases while passing through the flue tubes transfer their heat to the
water which is also heated by the furnace directly, gets converted into steam
and accumulates in the steam space. The steam stop valve allows the steam
from the boiler to the steam supply pipe.
The Cochran boiler is mounted with the essential mountings and accessories
like steam stop valve, safety valve, pressure gauge, water level indicator,
fusible plug, blow off valve, feed check valve. The working pressure and steam
capacity of Cochran boiler are 6.5 bar and 3500 kg /hr respectively.
Advantages & Disadvantages of COCHRAN BOILER
1)Cochran Boiler occupies less floor space.
2)Construction cost of Cochran Boiler is Low.
3)Cochran boiler is semi-portable and hence easy to install
4)Because of self contained furnace no brick work setting is
1)The capacity of the Cochran boiler is less because of the
2)Cochran Boiler requires high head room space.
3)Because of the vertical design, it often presents difficulty in
cleaning and inspection.
BABCOCK & WILCOX BOILER
Features of Babcock & Wilcox boiler:
1)Horizontal, Straight & Stationary
4)Water tube boiler
5)Minimum steam pressure of 10 bar
6)Minimum evaporative capacity of 7000 kg
of steam per hour.
Construction of BABCOCK & WILCOX BOILER
Babcock and Wilcox boiler is a horizontal, externally fired, natural circulation, stationary, and water tube
boiler. The Babcock and Wilcox water tube boiler consists mainly four parts such as water and steam drum,
water tubes, chain grate stoker, superheater tubes.
The water and steam drum is suspended from iron girders resting on the iron columns, and is independent
of the brick work setting. This arrangement prevents unequal expansion troubles and facilitates repair of the
brick work. A number of inclined water tubes at a very low inclination are connected at right angles to the end
boxes called headers. The water tubes will be arranged in a number of vertical rows, each row consisting of
40 to 5 tubes. In each vertical row the tubes will be arranged one below the other in a serpentine form. There
will be a number of such vertical rows one behind the other. Each one such vertical row of inclined water
tubes are connected to one set of two headers. The header at the right end of the water tubes is called down
take header and the other at the left end of the water tubes is called uptake header. Each of the vertical rows of
water tubes which are arranged one behind the other are connected to one set of headers which are also
arranged one behind the other. Each set of the headers are inturn connected to the boiler drum by one set of
two tubes, on eat the uptake end and the other at the downtake end. A mud box is provided just below the
downtake header. Any sediment in the water, due to its heavier specific gravity will settle down in the mud
box and is blown off from time to time through the blow off pipe.
The grate is provided at the front end below the uptake header. The boilers of higher capacity are usually
provided with a chain grate stoker, which consists of a slowly moving endless chain of grate bars. The coal
fed on at the front end of the grate is burnt on the moving grate in the furnace and the residual ash falls at the
outer end of the grate into the ash pit. The boiler is fitted with a superheater. The superheater consists of
number of U-tubes secured at each end to the horizontal connecting boxes and placed in the combustion
chamber below the boiler drum. The upper box of the superheater tube is connected to a T-tube, the upper
branches of the T-tube being situated in the steam space in the drum. The lower box of the superheater tubes
is connected to the steam stop valve mounted over the drum through a vertical tube passing outside the drum.
Working of BABCOCK & WILCOX BOILER
The water is introduced into the boiler drum through a feed valve. A constant water level is
maintained in the boiler drum. The water descends at the rear end into the downtake headers and
passes up in the inclined water tubes, uptake headers and in the tubes connecting the uptake
header and the drum. Thus a circuit is established between the drum and the water tubes for the
flow of water.
The hot gases from the furnace grate are compelled by the baffle plate to pass upwards around
the water tubes lying in between the combustion chamber under the water drum, then downwards
around the water tubes in between the baffle plates, then once again upwards between the baffle
plate and the downtake header, and finally passes out of the boiler through the exit door and the
During this path of the hot gases, the hottest gases emerging directly from the grate come in
contact with the hottest portions of the water tubes. The water in these portions of the water tubes
gets evaporated. The water and the steam mixture from this portion of the water tubes ascend
through the uptake headers and reach the boiler drum.
The steam from the steam space in the boiler drum is led into the branches of T-tube, and then
it passes into the upper connecting box of the superheater, then through its U-tubes. Since the
superheater tubes are fitted in the combustion chamber and exposed to the hot gases, the steam
passing in it will be superheated. The superheated steam from the superheater tubes are passed to
the steam stop valve through the lower connecting box and the vertical tube fitted outside the
drum. From the steam stop valve the superheated steam is passed to the prime-mover. When the
superheated steam is not required the steam from the steam space directly passes out to the
prime-mover through the steam stop valve.
1) Pressure gauge
2) Fusible plug
3) Steam stop valve
4) Feed check valve
5) Blow off cock
6) Man and mud(sight)holes
7) Two safety valves
8) Two water level Indicators
Man and mud(sight)holes
These are used to allow men to enter
inside the boiler for inspection and
Two safety valves
The commonly used safety valves are:
1)Dead weight safety valve
2)Lever safety valve
3)Spring loaded safety valve
4)High steam and low water safety valve
Loading arrangement for Lever Safety Valve
Taking moments about the fulcrum F, we get
W AF W GF W VF p a VF l v ´ + ´ + ´ = ´ ´
a =p ´d
Advantages of economizer:
1. The temperature range between various parts
of the boiler is reduced which results in
reduction of stresses due to unequal
2. If the boiler is fed with cold water it may
result in chilling the boiler metal. Hot fed
water checks it.
3. Evaporative capacity of the boiler is
4. Overall efficiency of the plant is increased.