Study of Boilers for Mechanical Engineering

ME 4101: Introduction to Mechanical
Engineering
Steam Generators

Steam generator-Introduction
The primary function of a steam generator is to generate steam under
pressure, but modern steam generators are also required to do the following:
i.Ensure generation of exceptionally high-purity steam by eliminating all
impurities from saturated steam.
ii.Raise the degree of superheat of supplied steam as specified and maintain
the same temperature over a defined range of load.
iii.In large power stations after partial expansion in the turbine steam is
returned to the steam generator for further superheating and then
transmitted to the turbine for complete expansion. This new degree of
superheat is called “reheat” and should also be maintained constant over a
defined range of load.
iv.While executing the above duties, a steam generator must utilize the heat
of combustion of fuel as efficiently as possible.

Circulation
In drum-type subcritical boilers, water flows from
the drum through downcomers to the bottom of
the furnace, then moves up through risers or
evaporator tubes and returns to the drum.
The figure shows that the water in the
downcomers does not receive any heat while
risers, which form furnace walls, absorb the heat
of combustion of the fuel. As a result, the water
inside the riser tubes is heated and gradually
steam bubbles form in the water.
This mixture of water and steam in the riser tubes
has a much lower density than the density of
water alone in the downcomers.

Circulation
.
Working principal: https://www.youtube.com/watch?v=nL-J5tT1E1k
A density difference is thus generated which
results in a static head difference causing the
thermo-siphon effect and provides the driving
force for a downward flow in downcomers and
an upward flow in risers. Since the flow of fluid
is generated by density difference alone, this is
known as natural circulation in boilers. For
maintaining the continuous circulation of fluid,
the fluid flow must overcome friction losses in
downcomer and riser tubes, headers, bends,
etc. One way of overcoming resistance due to
friction is by increasing heat input to the
furnace

Forced/assisted circulation
Boilers
In the earlier sections, the density
difference between the saturated
liquid and saturated vapor starts
diminishing at 18 MPa or higher fluid
pressure, thus it is difficult to maintain
the natural circulation of fluid flow in
boiler tubes.
In such cases, fluid flow is ensured
with the help of forced/assisted
circulation using pumps.
The forced/assisted circulation applies
equally in both supercritical and
subcritical ranges.

Design
As discussed earlier, the chemical energy in fuel is converted to heat energy by
combustion, and low-temperature water in the steam generator receives this heat
of combustion and becomes high-energy steam for use in the steam turbine. The
salient features of this heat-transfer process encompasses the following circuits,
systems, and/ or areas:
a. Water and Steam Circuit
b. Furnace
c. Fuel-burning System
d. Draft System
e. Heat Recovery System

Furnace
In the furnace of a steam generator combustion of fuel takes place with
atmospheric air to release heat. Hence, it is essential to ensure the complete
combustion of fuel under all operating conditions to harness the maximum heat
potential of the fuel under combustion.
The furnace in a modern boiler is a large, water-cooled chamber in which fuel and
air are mixed and burned in suspension. Its purpose is to achieve the most
efficient transfer of heat energy from the fuel to the working fluid; water/steam.
The furnace houses the firing equipment and confines the combustion process. A
typical furnace has two entry and two exit ports. These are:
(1) fuel inlet,
(2) combustion air inlet,
(3) products of combustion (flue gas) outlet and
(4) a disposal port, usually in the form of an ash hopper.

Furnace
The combustion chamber in modern boilers
is constructed entirely of water-cooled tubes
which are formed into four walls and
surround the furnace.
Factors that must be taken into account
when designing a combustion chamber of a
furnace are :
 Capacity of the boiler
 type of circulatory system
 type of firing
 range of fuels to be burnt

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
⚫ Grate: it is aplatform 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
⚫ 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

⚫ Refractory: insulation material used for lining combustion chamber
⚫ Lagging: Insulation wrapped on the outside of the boiler shell or steam piping
⚫ Foaming: Formation of steam bubbles on the surface of boiler water due to high surface
tension of water
⚫ Scale: A deposit of medium dueto extreme hardnessoccurring 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

STEAM BOILERS, MOUNTINGS & ACCESSORIES
Classification of Boilers:
Boilers may be classified according to the following-
1. Relative position of Hot gases and Water
a) Fire tube boiler:
The hot gases passes through the tubes that are
surrounded by water. Fire tube boilers are also known by
certain common names-
i) Horizontal return tubular
ii) Locomotive fire box
iii) Scotch marine and,
iv) Vertical tubular
https://www.youtube.com/watch?v=I2ZifNtUJkU

b) Water tube Boiler:
The water passes through the tubes and the hot gases
produced by combustion of fuel, flow outside. This
type of Boilers designated by the following
common names:
i) Babcock and Wilcox Boiler (straight but inclined
tubes which connect the headers).
ii) Stirling Boiler (multitubular boiler having bent
tubes that connect boiler drums to headers).

2. Method of firing
a) Internally fired Boilers:
The furnace is provided inside the boiler shell and is
completely surrounded by water cooled surfaces.
This method of firing is used in:
• Lancashire Boilers
• Locomotive Boilers and
• Scotch Boilers

i) b) Externally fired Boilers:
The furnace is provided outside/ under the boiler. It has an
advantage that its furnace is simple to construct and can
easily be enlarged, as and when required. This method of
firing is used in Babcock and Wilcox Boiler.
3) Pressure of Steam:
i) High pressure Boilers:
Boilers producing steam 80 bar and above are called High
pressure boilers. E.g.
* Babcock and Wilcox Boiler
* Lamont Boilers
* Velox Boilers and
* Benson Boilers etc.

ii) Low pressure Boilers:
Boilers producing steam Lower than 80 bar are called
Low pressure boilers. E.g.
• Cochran Boilers
• Cornish Boiler
• Lancashire Boiler
• Locomotive Boiler

4. Method of circulation of water:
a) Natural circulation method
Circulation set up by convection current or by
gravity.
b) Forced circulation method
Circulation set up for high pressure steam through
pumps.

5. Nature of service to be performed:
a) Land Boilers:
Boilers which are used with stationary plants
b) Portable Boilers:
Boilers which can be readily dismentaled and easily carried
out from one site to another.
c) Mobile Boilers:
Boilers which are fitted on mobile carriages are called
Mobile Boilers. E.g. marine and locomotive boilers.

6. Once through Boilers:
The boilers in which no circulation of water takes place
i.e. the feed water leaves the tube as steam e.g.
Benson Boilers.
7. Position and No. of Drums:
Single or multidrums may be positioned longitudinally
or crosswise.
8. Design of gas passages:
a) Single pass
b) Return pass
c) Multipass

9. Nature of Draught:
a) Natural Draught:
when the fuel burns in the furnace of the boiler, with the
circulation of air, the draught is named as Natural
Draught.
b) Artificial Draught:
When the air is forced by means of forced fan, the draught
is named as Artificial Draught.

10.Heat Source:
a) Combustion of solid, liquid or gaseous fuel.
b) Electrical and nuclear energy.
c) Hot waste gases of other chemical reactions.
11. Fluid Used:
a) Steam Boilers- use water as a fluid.
b) Mercury Boilers- use mercury as a fluid.
c) Other Boilers- use special chemicals as a fluid.

12. Material of construction of Boiler Shell:
a) Cast Iron Boilers: Low pressure heating Boilers.
b) Steel Boilers: Low pressure heating Boilers.
c) Copper and Stainless steel Boilers: Miniature
Boilers.

Difference between the boiler
mountings and accessories:

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

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

Economizer
The exhaust from the boilers is generally in the temperature range of 200°C – 250°C, so there are
ahuge amount of losses from the boiler if any heat recovery devices are not installed after it.
If the exhaust gaseswhich are leaving the Boiler at such high temperature is made to pass through
the Economizerin order to provide the required sensibleheatto the water byincreasing its
temperature, it will reduce the heat load on the boiler to the greater extent.

ECONOMISER
Generally there are two types of economisers: condensing and non-condensing.
Condensing economisers are designedto accommodatethe corrosive liquidswhich
contain moisture in them. There are two types : heat exchanger type and spray type.
Non-condensing economisers are the most common economisers. They are generally of
fin type which located in the flue gasducting at the exit so that they can gain more heat
which is being wasted anyways.

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

Super heater
⚫ The superheater raises the temperature of saturated steam without
increasing its pressure. It consists of a small bundle of tubes. It is set in the
path of the hot flue gas of the furnace. Saturated steam passes inside the
heater tubes, and hot flue gases pass outside the tubes. Thus the transfer
of heat takes place in saturated steam from hot flue gases and raises its
temperature without increasing the pressure of steam.

⚫ 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.

Steam separator
⚫ The function of asteam 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

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

BOILER MOUNTINGS
1. WATER LEVEL INDICATOR
It is an main fitting in the boiler, Water level indicator indicates the
water levelinside the boiler. It isasafetydeviceupon which safe
working of the boiler depends.
Figure 3.9 shows the water level indicator. It
consists of a thick hard glass tube. The ends are
connected with two hollow blocks that are
connected with boiler end plate such that one
end of the blocks is connected with steam
space and other end with water storage. Parallel
to the glass tube, there is another metallic tube.
In case, the gauge glass breaks, the rush of
steam and water will push the steel balls
thereby blocking the passage to the position as
shown by black color stop and prevents water
and steam to come out of the boiler shell or
drum. The broken glass tube is replaced
immediately to function it asusual.

Pressure gauge
⚫ The function of the pressure gauge is to measure the pressure of
steam inside the boiler.
⚫ The gauge is usually mounted on the front top of the shell or drum such that
it should be clearly visible to the operator. (C).
The pressure gauge shown in fig is
bourdon pressure gauge. It consists
of a circular spring tube A. One
end of the bourdon tube is closed
and connected to a link L and the
other end is sequared to a Hollow
block B. The link L connects the
closed end of the tube to the
toothed sector C which is hinged
at O. The toothed sector gears
with pinion D which carries a
pointer P. The pointer moves on a
dial graduated in pressure units.

Fusible plug
⚫ The main purpose of a fusible plug is to extinguish the fire in the
furnace of the boiler when the water level in the boiler falls below an
unsafe level. Thus, explosion is avoided which may take place due to
overheating of the tubes and shell. It is generally fitted over the crown
of the furnace or over the combustion chamber. This is shown in Fig.
3.12. Plugs A and B are separated by fusible metal.
In normal case, when the boiler contains sufficient
water, the plug also remains covered by water. But
when the water level falls, the plug is exposed to
steam. Because the steam temperature is larger than
the water temperature, the fusible metal melts. Plug
B drops down and Thus the steam space gets
communicated to fire box and extinguishes the fire.

⚫ 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

Steam stop valve
⚫ A valve is adevice that regulates the flow of afluid (gases , fluidized
solids slurries or liquids) byopeningor 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

Feed check valve
⚫ When the level of water in the boiler falls, it is brought back to the specified
level by supplying the additional water called feed water. The pressure inside
the boiler will behightherefore the pressure of the feed water hasto be
raisedby apumpbefore it isfed into the boiler. The feed water under high
pressure is fed into the boiler through the feed check valve.
The function of a feed check valve is to
control the flow of water from the feed
pump to the boiler and to prevent the
backflow of water from the boiler to the
pump when the pump pressure is less
than the pressure or when the feed
pump ceases to work. Evidently feed
check valve is placed at the boiler end of
the delivery pipe of the feed pump.

Blow off cock
⚫ The function of a blow-off valve is to
periodically remove the sediments
deposited at the bottom of the boiler
while the boiler is in operation and to
empty the boiler while it is being
cleaned or inspected.
⚫ When the blow-off valve is opened the
water which is under the pressure of
steam, rushes out with tremendous
velocity thus carrying out the
sediments along with it.

MANHOLE
This is provided at suitable position on the boiler shell so that the man can
enter into boiler shell for inspection, maintenance and repairs. This hole is
usually made in elliptical shape of the size convenient for a man to enter
through this hole. The opening is closed by steam tight cover.

ANTIPRIMING PIPE
Function: The steam in contact with the water surface always contains water particles. A steam
scrubber also known as steam drier is used in a boiler to remove the water particles contained in
steam.
Construction and operation:
Refer Fig. 29.12. Anti-priming is a cast iron box which is fitted under the mounting block on
which the steam stop valve is to be bolted. When the steam with water particles passed between
closely fitted corrugated plates or the perforation made in the upper half of the anti-priming
pipe, the heavier water particles separate out and are collected at the bottom of the anti-priming
pipe. The water thus collected is later on drained to the boiler through the small holes ‘O’
provided at the bottom of the anti-priming pipe.

Cochran Boiler
Working Principal:
https://www.youtube.com/watch?v=hVcqqaEulVA

•
•
• Simple vertical,
portable, natural
circulation, low pressure
boiler
Suitable for small plants
require small quantity of
steam.
Size = 1 m Dia. x 2 m
high (evaporation
20kg/hr.)
•
•
•
• Size = 3 m Dia. x 6 m
high (evaporation
3000kg/hr.)
Heating surface= 10 to 25
times of grate area
Steam pressure= upto 20
bar
Efficiency = 70 to 75%

FIRE TUBE BOILER
LANCASHIRE BOILER
Working Principal: https://www.youtube.com/watch?v=xMAymzTl2gQ

LANCASHIRE BOILER
• Internally fired, horizontal,
natural draft and natural
circulation type boiler.
• Diameter of tubes is 0.4 times
the diameter of shell.
• Each flue tube has its own
furnace with grate of about 2
met. Length.
• Dampers
path of flue gases
flow and
are placed in the
the to
gas air
regulate
inflow.
• Openings are made in the
boiler for inspection.

Working of Lancashire Boiler
Lancashireboiler isashell andtube type, heatexchanger.The fuel isburned at the grate.The water is
pumped into the shell through the economizer which increases the temperature of the water. Now the
shell isfilled with water. The fire tube is fully immersed in the water. The fuel ischargedat the grate
produces exhaust gases.
These flue gasesfirst pass through the fire tubes from one end to another. These fire tubes transfer 80% to
90% of the heat to the water. The backward flue gasespass from the bottom passagewhere it transfers
8-10%heat to water.
The remaining flue gasespassfrom the sidepassagewhere it transfers 6-8%of the heatto the water. The
brick is the lower conductor of heat, so works asaheat insulator. The steam produces in the drum shell is
taken out from the upper side where it flows through the superheater if required. So the steam generated
is taken out for process work.

(Capacity and Utility)
•Made to withstand working pressure up to 20bar.
•Evaporative capacity 8000 kg/hr.
•Heating surface: grate area 24:30
•Shell size ranges from (2m diameter x 6m length) to (3m diameter x 10m
length)
•Widely used in sugar mills, chemical plants, power systems, process works.
•Lancashire boilers are employed to operate steam turbines, locomotives,
marines etc.
•Lancashire boiler has a high thermal efficiency of about 80-90%.

LANCASHIRE BOILER
Disadvantages
S.No Advantages
1 Simplicity of design
2 Good steaming quality
3 Can burn coal of inferior quality
4 Easy to clean and inspect
5 Less maintenance and operating
cost
Maximum working pressure is limited to 20
bar.
More floor area is required due to brick work
setting.
Cracks in setting due to large temperature
difference inside and out side.
Restricted grate area due to furnace being
inside
More time taken for developing steam
pressure due to large water capacity.
6 Stand against sudden heavy
demand for considerable time.

LOCOMOTIVE BOILER
Working Principal: https://www.youtube.com/watch?v=9mhYnQGZJuM

• Locomotive boiler is a horizontal drum axis, multi-tubular, natural circulation, artificial draft,
forced circulation, mobile, medium pressure, solid fuel fired fire tube boiler with an
internally fired furnace. It is used in railway locomotive engines and in marine. It is a mobile
boiler and has ahigh steam generation rate.
• There are three main parts in this boiler including a horizontal cylindrical boiler baller, a
smokebox, and adouble-walled firebox.

LOCOMOTIVE BOILER
(Utility/ Application)
•
•
• The locomotive boiler is so designed that it is capable of meeting sudden and
fluctuating demand of steam, which may be imposed due to variation in
power and speed.
Uses in railways, road rollers etc.
Also used in agricultural fields, saw mill plants and stationary power services
where semi- portability is desired.

LOCOMOTIVE BOILER
Advantages Disadvantages
Large rate
to70kg/sec)
of evaporation
per sq. met.
heating surface.
(55 Incapable of meeting very high
Of overloads because of danger of
being damaged due to overheat.
Freedom from brickwork, Max. steam pressure is limited to
chimney and special foundations, 20 bar.
which reduces the cost of
installation.
Reasonable low cost and Leakage occurs frequently at the
compactness makes it ideal for place where the tube joins the
portable unit. plate.
Large flat surface needs bracing
Accumulation of mud particles in
water lags cause scale formation.

BABCOCK & WILCOX BOILERS
Working Principal: https://www.youtube.com/watch?v=ae_QmSRhD5w

(Constructional features)
❖ It consist of welded steel high
pressure drum mounted at the top.
❖ Drum is connected with uptake
header and down take header.
❖ Water tubes connected to the
headers are inclined at 150 to the
horizontal.
❖ Water tubes are straight and 10cm
diameter expended into the bored
holes of header.
❖ Serpentine from of header provides
complete heating surface to the flue
gases.
❖ Furnace is arranged below the
uptake header.

(Constructional features)
❖ Unit is provided with chain grate
stroker.
❖ Speed of chain is adjusted with respect
to the complete combustion of coal.
❖ Deflectors to flue gases are provided in
the form of baffles.
❖ Mud Box: To collect the sedimentation
in water.
❖ Super heater tubes: To enhance the
super heated steam.
❖ Vents provided: safety valve, pressure
gauge, water level indicator, fusible plug
and feed check valve.
❖ Water tube and drum assembly hung on
steel girder frame called slings.

(Capacity and utility)
❑ Evaporative capacity ranges from 20000 to 40000 kg/hr
❑Operative pressure ranges from 11.5 to 17.5 bar.
❑Steamformed from such boilers are primarily used to run steam turbines and
generate electric power.
(Salient Aspects)
✔
✔
✔
✔
Capability to cope with high peak loads which are generally needed in thermal
power stations.
Inspection of the boiler can be carried even when the boiler is in operation.
Draught loss is minimum.
Replacement of defective tubes can be made easily.

⚫ Disadvantages
• High maintenance cost.
• It is not much suitable for impure and sedimentary water. In the case of
impure and sedimentary water, the scale may deposit in the tubes and this
leads to overheating and bursting of tubes. That’s why water treatment is
must before feeding into the boiler.
• A continuous supply of feed water is required for working. In the case, if feed
water is not continuously supplied even for a short period of time, the boiler
gets overheated. The water level must be carefully watched during the
operation of the Babcock and Wilcox boiler.

STIRLING BENT- TUBE BOILER
Working Principal: https://www.youtube.com/watch?v=Rv5X8KEvRWQ

STIRLING BENT- TUBE BOILER
1.
a)
Drums are interlinked to each other with
bent water tubes for the following
reasons:
To allow free expansion and contraction
of the tubes.
Tube replacement become easier.
b)
c) Flexibility in design with regards to
location of drums.
d) Tubes can enter the drums in
approximately radial direction.
2. Mud drum is usually 10 to 25cm larger in
diameter than asteam drum.
3. Entire unit is independent of brick work.
4. High steam pressures (60bar and 4500c).
5. Evaporation capacity up to 50000kg/hr.

COMPARISION
Between
WATER
TUBE
&
FIRE TUBE
BOILERS
WATER TUBE FIRE TUBE
Water passes through water Hot gases passes through
tubes. flues.
Water content: steam capacity
low
Water content: Steam capacity
high
Complexity in design requires Simple & rigid construction
quick examination by skilled hence greater reliability & low
hands. operating cost.
Operating pressure up to 200
bar.
Pressure ranges from 17.5 bar
to 24.5 bar
Evaporation rate ranges from
20,000 to 50,000kg/hr.
Evaporation rate 900kg/hr.
Increased heating surface area. Low heating surface area.
Low water to steam ratio Large water to steam ratio
Bigger in size, suitable for large
power plants
Smaller in size, used only for
small power plants
Transportation and installation Transportation and installation
is easy due to handling of is difficult due to large size of
dismentaled parts shell.
Externally fired boilers, furnace
size can be varied.
Internally fired boilers, furnace
size can not be varied.
Requires more floor area Requires less floor area

ESSENTIALS OF A GOOD BOILER
1. Heat generation capability should be at:
a) Required pressure
b) Required quality
c) Fast speed
d) Minimum fuel consumption
2. Economic :
a) Low initial cost
b) Low installation cost
c) Low operating cost
d) Low maintenance cost

Study of Boilers for Mechanical Engineering

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