ELEMENTS OF MECHANICAL

ENGINEERING
UNIT -1
STEAM FORMATION
and
BOILERS

Mr.MADHUKUMAR K C
Assistant Professor
Department of Mechanical Engineering
SJCE,JSSSTU,Mysuru
1
• Energy Resources:
• Energy
Energy is defined as the capacity to do work. It is primary
requirement for day to day activities of human beings.
Renewable Sources Of Energy:
Energy sources which are continuously produced in nature and
are essentially inexhaustible are called renewable energy
sources. Example:
1. Direct solar energy 2. Wind energy 3. Tidal energy 4. Hydel
energy 5. Ocean thermal energy 6. Bio energy 7. Geo
thermal energy 8. Peat 9. Fuel wood 10. Fuel cells 11. Solid
wastes 12. Hydrogen

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• NON-RENEWABLE ENERGY SOURCES:
Energy sources which have been accumulated over the ages
and not quickly replenish able when they are exhausted.
Examples: 1. Fossil fuels. 2. Nuclear fuels. 3. Heat traps.

ADVANTAGES OF RENEWABLE ENERGY

SOURCES:
1. Non exhaustible.
2. Can be matched in scale to the need and can deliver
quality energy.
3. Can be built near the load point.
4. Flexibility in the design of conversion systems.
5. Local self-sufficiency by harnessing locally available
renewable energy.
6. Except biomass, all other
Steamsources
Formation are pollution free. 3
DISADVANTAGES OF RENEWABLE ENERGY SOURCES:
1. Intermittent nature of availability of energy such as solar, wind,
tidal etc. It is a major setback in the continuous supply of energy.
2. Solar energy received at the earth is dependent on local
atmosphere conditions, time of the day, part of the year etc.
3. Sources such as wind, tidal etc. are concentrated only in certain
regions.
4. Technology is not fully developed to meet the present energy
requirements.
5. Systems such as solar cells require advanced technologies and
hence costlier.
6. Application to transport sector has been found to be not viable as
on today.
ADVANTAGES OF NON-RENEWABLE ENERGY SOURCES:
1. Initial cost are lower. Hence widely used.
2. Unit power costs are much lower and so are economical
3. Sources are highly reliable.
4. Power generation technologies are well established.
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DISADVANTAGES OF NON-RENEWABLE ENERGY
SOURCES:
1. The sources are getting depleted and soon will be
exhausted.
2. They pollute the atmosphere.
3. They are not freely available.

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 Steam Formation and Properties
• Definition of Steam:
Steam can be defined as it is a mixture of water and air or it
can also be defined as vapour of water.
Types of steam
• Wet Steam: It is defined as a two-phase mixture of entrained
water molecules and steam at saturation temperature.
• Dry Steam (dry saturated steam): As wet steam is heated
further, the water molecules in the steam get converted
into vapour. Dry steam is the steam at saturation
temperature having no water molecules in it.
• Superheated Steam: It is defined as the steam which is
heated beyond its dry state to temperatures higher than its
saturated temperature atSteamthe given pressure.
Formation 6
Formation of steam at constant pressure

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• Consider 1 kg of water at 0˚C taken in a cylinder, on which
a constant pressure P is exerted. Point A on the temperature-
enthalpy graph.
• When this water is heated its temperature rises till the
boiling point is reached. This temperature is called
saturation temperature (Ts). Point B on the graph.
• Further addition of heat, initiates the evaporation of water
while the temperature remains at saturation temperature
until all of water is converted into steam. Point C on the
graph.
• On heating the steam further, it increases the temperature of
steam above the saturated temperature to superheated steam.

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 STEAM PROPERTIES
Dryness fraction of steam:
A wet steam has different proportions of water molecules
and dry steam. Hence, the quality of wet steam is specified
by the dryness fraction which indicates the amount of dry
steam in the given quantity of wet steam and is denoted by
x.
It is defined as the ratio of mass of dry steam in a given
quantity of wet steam to the total mass of wet steam.
Let mg = mass of dry steam
mf = mass of water molecules Dryness fraction,
x= mg/ (mg+mf)
•The dryness fraction of wet steam is less than 1.
• The dryness fraction of dry steam is 1
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• ENTHALPY (h), kJ/kg: It is the amount of heat required
to raise the temperature of 1 kg of water from 0℃ to the
desired form of steam at constant pressure. It is the sum of
the internal energy and work done at constant pressure.
• Enthalpy of Dry Saturated Steam (hg): It is the amount
of heat required to raise the temperature of 1 kg of water
from 0℃ to 1 kg of dry saturated steam at constant
pressure.
ℎg = ℎf + ℎfg KJ/Kg
• Enthalpy of Wet Steam (h): It is the amount of heat
required to raise the temperature of 1 kg of water from 0℃
to 1 kg of wet steam to the specified dryness fraction, at
constant pressure.
ℎ = ℎf + xℎfg KJ/Kg
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• Enthalpy of Superheated Steam (hsup):
It is the amount of heat required to raise the temperature of 1 kg
of water from 0℃to 1 kg of superheated steam to the stated
saturated steam temperature, at constant pressure. It is the sum of
enthalpy of dry steam and the amount of superheat.
hsup=hg+Cps(Tsup-Ts)KJ/Kg
hsup=hf+hfg+Cps(Tsup-Ts) KJ/Kg
Where Cps is the specific heat of superheated steam.
Steam Properties:
• Ice melts.
• Water is heated beyond boiling point.
• Steam is defined as vapour of water.
• Vaporization. Gaseous phase.
•Steam is two phase mixture of water and steam.
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Specific volume (m3 /kg):
It is the volume occupied by the unit mass of a substance.
Specific Volume of Dry Saturated Steam (Vg):
It is the volume occupied by 1 kg of dry saturated steam at a
given pressure.
Specific Volume of Wet Steam (v):
It is the volume occupied by 1 kg of wet steam to the specified
dryness fraction at a given pressure.
Internal Energy of Steam:
The total heat energy of a dry saturated steam at a constant pressure
is the sum of the sensible heat and latent heat. But in latent heat
a portion is used for external work. Therefore, the actual energy
stored in the steam is the sensible heat and the internal latent
heat. This actual energy stored in the steam is called internal
energy of steam. It is defined as the difference between the
enthalpy of the steam and the external work of evaporation.
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• External work of evaporation
When water at the saturation temperature is heated at
constant pressure, it gets converted into steam by
absorbing the latent heat.
The work done, due to change in volume of steam from Vf to
Vg is called external work of evaporation.

External Evaporation =W= 100.P(Vg - Vf ) KJ

Where P = pressure in bar.
For wet steam of dryness fraction x, Ww = 100.P.x.Vg KJ
For dry steam Wg = 100.P.Vg KJ
For superheated steam Wsup = 100.P.Vsup KJ

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 BOILERS
• Definition of boilers:
Boiler is defined as a closed metallic vessel in which the
water is heated beyond the boiling state by the application
of heat liberated by the combustion of fuels to convert it
into steam.
Function of a boiler:
The function of the boiler is to supply the steam at the
required constant pressure with its quality either dry or as
nearly as dry, or superheated. The steam can be supplied
from the boiler at a constant pressure by maintaining the
steam generation rate and the steam flow rate.

Steam Formation
• CLASSIFICATION OF BOILERS:
According to the circulation of water and hot gases:-
a) Fire Tube boilers:-In fire tube boilers the hot flue gases
produced by the combustion of fuel passes through the tubes
which are surrounded by water.
Eg: - Cornish boilers, locomotive boilers, marine boilers
and Lancashire boiler.

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b) Water Tube boilers: -
In water tube boilers water circulated inside the tubes, while
the hot gases produced by the combustion of fuels pass
around the tubes.
Eg: - Babcock and Wilcox boiler, Stirling boilers…etc.

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 According to the location of furnace.
a. Internally fired boilers: - If the furnace is situated
inside the boiler shell, the boiler is called internally fired
boilers. Most of the fire tube boilers are internally fired.
b. Externally fired boilers:-If the furnace is situated
outside the boiler shell, the boiler is called externally fired
boilers. Water tube boilers are always externally fired.
According to the circulation of water.
I. Natural circulation: - In these boilers, water is
circulated by natural convection currents that are set up
due to the temperature difference.
II. Forced circulation: - water is circulated with the help
of pump driven by a motor. Forced circulation is used
only in high pressure and high capacity boilers like La mont
boilers and Benson boilers, etc....
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According to the axis of the shell
a) Vertical boilers: - Axis of the boilers shell is vertical.
Ex:-Cochran boilers.
b) Horizontal boilers:-Axis of the boilers shell is horizontal.
Ex:-Babcock &Wilcox boilers, Lancashire boilers.
According to their uses.
a) Stationary boilers
b) Locomotive boilers
c) Marine boilers, etc…..

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 BABCOCK & WILCOX BOILER
• It is a horizontal, externally fired, natural circulation, water tube
boiler.
It consists of mainly four parts:
(a) Steam and water drum – It is filled with three-fourths water. It
stores the feed water and steam.
(b) Water tubes – Number of water tubes are connected through
down take header and uptake header in which water circulates as
shown in fig.
(c) Baffle plates- It is placed across the water tubes. It deflect the
hot gases coming out from the furnace to allow the hot gas pass
around water tubes.
(d) Internal furnace– It burns the coal to produce hot flue gases.
(e) Super heater – It is set of U-tubes just below the boiler drum, it
converts the steam into superheated
Steam Formation steam. 19
Steam Formation 20
BABCOCK AND WILCOX BOILER

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BABCOCK AND WILCOX BOILER

Steam Formation 22
BABCOCK AND WILCOX BOILER

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Working: - Water is introduced into the boiler drum through
the feed valve. Water descends into the down take headers,
into the water tubes and then into the uptake headers. The
hot flue gases from the furnace pass over the water tubes.
The path of the hot gases is guided by the baffle plates as
shown in the fig., and passes out to the chimney. As the hot
gases pass over the water tubes, the water gets converted
into steam. This steam due to low density rises up the tube
through the uptake headers and reach the top of boiler drum.
This sets up a natural circulation of water. The steam
collected in the boiler is wet. This is made to pass through
the super heater U-tubes just below the boiler drum. The hot
gases on their way out pass over these tubes hence
converting the steam into superheated steam. The
superheated steam in then passed out to the turbine through
the steam stop valve. For safety the boiler consists of safety
valve, steam stop valve, blow-off pipe, and pressure gauge,
etc.
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Lancashire boiler

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Lancashire boiler

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Lancashire boiler

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Lancashire boiler

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It is a horizontal, internally fired, natural circulation, fire tube
boiler.
• This boiler consists of a large horizontal cylindrical shell
placed on the brick wall. Two large flue tubes are placed
inside the shell, which carry the hot flue gases.
• The boiler shell is filled with water to three-fourths of its
volume and the remaining space is the steam space.
• Hot flue gases from the combustion are made to pass through
the flue tubes.
• In the first run it passes from the front end to the rear end of
the boiler.
• At the rear end they are made to pass to the bottom central
channel.
• In the bottom central channel the hot gases travel from rear
end to the front end of the boiler. This is the second run.
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• At the front end the hot gases enter into side channels 1 and
2 and travel to the rear end of the boiler. This is the third
run.
• At the rear end hot gases coming out of channels 1 and 2 are
made to exit to the chimney through the rear exit passage.
• During the first, second and third pass the heat transfer takes
place between hot flue gases and the water in the shell,
converting water into steam.
• The steam gets accumulated in the steam space at the top.
• Super heater (set of U-tubes) is placed at the rear end of the
shell.
• For safety the boiler consists of safety valve, steam stop
valve, blow-off valve, pressure gauge, etc.
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 BOILER MOUNTINGS AND ACCESSORIES:
• MOUNTINGS:
1) Water level indicator: It indicates the level of water in the
boiler drum.
2) Pressure gauge: Indicates the pressure of the steam in the
boiler.
3) Safety Valves: When the pressure inside the boiler drum
exceeds the desired level, the safety valves blows off the excess
steam from the boiler.
4) Steam stop valve: It regulates the flow of steam from the boiler.
5) Feed check valve: It checks the level of water in the boiler
drum.
6) Blow off valve: Its function is to remove periodically the
sediments and impurities collected at the bottom of the boiler.
7) Fusible plug: It is a device used to extinguish the fire in the
furnace. Steam Formation 31
• ACCESSORIES:
1. Economizer: The function of an economizer is to heat the
feed water, before being supplied to the boiler, using the
products of combustion (flue gas) discharged from the
boiler.
2. Air preheater: The function of an air preheater is to
preheat the air being supplied to the furnace for
combustion.
3. A super heater: Is a device used to convert saturated steam
or wet steam into dry steam.
4. Feed pump: Pumps the water into the boiler at high
pressure.
5. Steam separator: It is used to separate the water particles
is present in the steam before enters the turbine or engines.
6. Steam trap: It is used to drain off the condensed water
accumulated in the steam pipes and steam separator without
allowing the escape of high pressure steam from it.
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