BASIC MECHANICAL
ENGINEERING
UNIT-2
A)
STEAM POWER PLANT, BOILER, STEAM TURBINES.
B)
STEAM TURBINES
C)
GAS TURBINE-POWER PLANTS
BOILER
An equipment used for producing steam is called steam generator
or boiler.
OR
 A closed vessel in which steam is produced from water by
combustion of fuel .

ACCORDING TO ASME (AMERICAN SOCIETY OF
MECHANICAL ENGINEERING)
Steam generating unit is “A combination of apparatus for
producing, furnishing or recovering heat together with the
apparatus for transferring the heat so made available to the fluid
being heated and vapourized.”
 The fluid (water) is contained in the boiler drum called shell and
the thermal energy released during combustion of fuel, which may
be solid, liquid or gaseous is transferred to water and this converts
water into steam at the desired temperature and pressure. The
steam thus generated is used for
1) Power generation
2) Heating
3)Utilization of steam for sizing and bleaching etc. and many
other industries like sugar mills, chemical industries.

TYPES OF BOILERS
High Pressure Boilers:
Benson boiler (High Pressure)
La-mount boiler (High Pressure)
CLASSIFICATION OF BOILERS
Based on the Position of hot gases and water
Fire tube, water tube
Based on the Axis of shell
Horizontal, vertical, inclined
Based on the Location of furnace
Externally fired, internally fired
Based on the Method of circulation
Natural circulation, Forced circulation
Based on the Mobility
Stationary, Portable
Based on the Pressure
High pressure, Low pressure
Based on the Tubes
Single tube, Multi tube
COMPARISION OF FIRE TUBE AND
WATER TUBE BOILERS
Criterion
Fire Tube
Water tube
Position of hot gases
and water
Hot Gases in tubes
surrounded by water
outside
Water in tubes and Hot
gases flow outside
Mode of firing
Internal fired
External fired
Floor area
For a given power it
occupies more floor
area.
For a given power it
occupies less floor area.
Capacity
10000kg.hr
50000kg/hr
Evaporation
Slow
Fast
Chances of explosion
Less
More
COMPARISION OF FIRE TUBE AND WATER
TUBE BOILERS
Criterion
Fire Tube
Water tube
Operating Pressure
range
15 to 20 bar
170 to 200 bar
Efficiency
80%
92%
Suitability
Not suitable
power plants.
for
large Suitable for large power
plants.
Risk on burning
Involves Lesser risk on
explosion due to lower
Pressure
Involves more risk on
bursting due to high
Pressure
Application
Not suitable for large
power plant
Suitable
Rate of steam
production
Lower
Higher
COMPARISION OF FIRE TUBE AND
WATER TUBE BOILERS
Criterion
Fire Tube
Water tube
Requirement of Skill
Requires Less skill for
efficient and economic
working
Requires more skill for
efficient and economic
working
Water Treatment
No so necessary
More necessary.
Construction
Difficult
Simple
Shell Diameter
More
Less
Transportation
Difficult
Simple
Accessibility of various Not easily accessible for Easily accessible
parts
cleaning, repairing and
inspection.
SELECTION OF A BOILER
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Operating Pressure
Steam Generating Rate
Availability of floor space
Accessibility for repair and inspection
Comparative Initial cost
Quality of steam required
Fuel to be used
Nature of the load on the boiler
Operating and maintenance cost
Availability of water
REQUIREMENT OF A GOOD BOILER
1.
Capable of producing maximum amount of steam with minimum fuel
consumption.
2.
It should occupy less space and be light in weight.
3.
Capable of quick starting and should rapidly meet the fluctuation in
load.
4.
Safe in working.
5.
Economical and should require very little attention during operation.
6.
All parts should be easily accessible for inspection and repairs.
7.
Components should be transported without difficulty.
REQUIREMENT OF A GOOD BOILER
8.
Installation should be simple.
9.
Tubes should not accumulate soot or water deposits and should be
sufficiently strong to allow for wear and corrosion.
10.
Water and gas circuits should be such as to allow minimum fluid
velocity for low frictional losses.
11.
Should have as less joints as possible to avoid leaks, which may occur
due to expansion and contraction.
12.
Velocities should be high for high heat transfer rates with minimum
pressure drop through the system.
13.
Should confirm to safety regulations laid down in Boiler Act.
ESSENTIALS OF GOOD BOILERS









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The boiler should produce the maximum weight of steam of
required quality at minimum expenses.
Steam production rate should be as per requirements
Should be reliable.
It should occupy minimum space.
It should be light in weight.
Should be capable of quick starting.
Should have an easy access to various parts of boilers for
inspection and repairs.
Boiler components should be transportable without difficulty.
Installation of boiler should be simple.
Tubes should not accumulate soot and water deposits.
FIRE TUBE BOILERS
COCHRAN BOILER:
It is one of the best type of vertical multi-tubuler boiler and has a no. of
horizontal fire tubes.

Shell diameter
2.75m

Height
5.79m

Working Pressure
6.5 bar (max pressure 15 bar)

Steam Capacity
3500 kg/hr (maximum capacity
4000 kg/hr)

Heating surface
120 m2

Efficiency
70 to 75%

Tubes diameter
6cm
WORKING
 Cylindrical
shell with a done shaped top where steam
is collected
 One piece construction and seamless
 The fuel is burnt on the grate and ash is collected and
disposed of from ash pit
 Gases of combustion produced enter the combustion
chamber through flue tube
 After striking brick lining flue gases passes through
no of horizontal tubes, being surrounded by water.
 Flue gases escape to atmosphere through smoke box
and chimney
Boiler mountings:
1.
2.
3.
4.
5.
6.
Water level gauge
Safety valve
Steam stop valve
Blow off cock
Manhole
Pressure gauge
Advantages:
1.
2.
3.
4.
Free from overheating
Light, Compact and occupy less space
Cost of construction is low
Easy to install and transport
BABCOCK AND WILCOX WATER-TUBE BOILER
It is an example of horizontal-straight boiler
 Designed for Stationary and marine purposes.

The Specifications of the boiler are:
 Diameter of the drum = 1.22 to 1.83 m
 Length = 6.096 to 9.144 m
 Size of the water tubes are 7.62 to 10.16 cm
 Size of the Super-heater tubes = 3.84 to 5.71 cm
 Working pressure = 140 bar (max)
 Steaming Capacity = 40000kg/h (max)
 Efficiency = 60% to 80%.
D = Drum
DTH = Down Take Header
WT = Water Tube
D = Doors
G = Grate
FD = Fire Door
MC = Mud Collector
WLI = Water level Indicator
BP = Baffle Plates
PG = Pressure Gauge
ST = Super-heater Tubes
SV = Safety valves
MSV = main stop valve
APP = Anti-priming pipe
L = Lower Junction pipe
U = Upper Junction pipe
FV = Feed Valve.
The angle of inclination of the water tubes to the horizontal is abt
15deg.
 A feed valve is provided to fill the drum and the inclined tubes
with the water level of which is indicated by the water level
indicator.
 Through the fire door the fuel is supplied to the fire grate and it is
burnt. The hot gases are forced to move upwards between the
tubes by the baffle plates.
 The water from the drum flows through the inclined tubes via
down-take header and goes back into the shell in the form of
steam and water mixture via UTH.
 The steam gets collected in the steam space of the drum
 The steam enters through the APP and flows in the ST, where it
converts from Wet steam to Dry saturated steam and it is taken
out from MSV and feed to the turbine.
 At the lowest point of the boiler the mud and dirt collects which is
pumped out through Blow-off Cock.

Advantages:
Generate Steam at high pressure and rate is high (140 bar and
40000kg/hr)
 For given output it occupies less space
 Parts are accessible for cleaning, inspection and repair
 Parts of boiler can be separated and transported easily
 Fuel is completely burnt due to large heating surface

Disadvantages:
Pure feed water required
 Cost is high

BOILER TERMS
Shell
 Furnace
 Grate
 Scale
 Blowing off
 Refractory
 Water Space
 Steam Space
 Water Level
 Mountings
 Accessories

BOILER MOUNTINGS & ACCESSORIES

MOUNTINGS

Water Level Indicator
Pressure Gauge
Safety Valves
Fusible Plug
Blow off cock
Feed check valve
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ACCESSORIES:
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Feed Pumps
Injector
Air Preheater
Economizer
Super heater
Steam Separator
Steam Trap
HIGH PRESSURE BOILERS
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o
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FEATURES OF HIGH PRESSURE BOILERS:
Method of water circulation
Type of tubing
Improved method of heating.
ADVANTAGES:
Increased evaporative capacity of the boiler.
Efficient usage of combustion heat.
Lesser floor space is required due to compactness.
Limited tendency of scale formation.
Early start is possible due to external heat.
Economical.
LAMOUNT BOILER
WORKING
 Water
is feed to the drum after passing through
economiser(Boiler accessory)
 Circulating pump pumps water to the tube evaporating
section which in-turn sends a mixture of steam and
water to the drum
 Water is drawn to super heater where dry steam is
converted to super critical steam
 Super heated steam obtained is supplied to the prime
mover
 45 to 50 tonnes of steam at 130 bar and 5000 C
BENSON BOILER
BENSON BOILER
The first high pressure Benson boiler was put into
operation in 1927 in West Germany.
 Maximum operating pressure is 500 atm
 Output = 150 tones/hr.
 Maximum temperature of steam = 700 degrees.
 No steam separating drum required

Working:
 Temperature of the feed water is raised in economiser
 Later water is partly evaporated in the radiant parallel
tube section
 In transit section mixture is converted into steam
 The steam is now passed through the convection super
heater and then supplied to the prime mover.
ADVANTAGES
1.
2.
3.
4.
5.
6.
Requires small floor area
Light in weight hence low cost
Can be started easily
Transfer of parts is easy, no drum is required
No pressure limit
Explosion hazards are less
STEAM AND GAS TURBINES
LAYOUT OF STEAM POWER PLANT
STEAM TURBINE
Steam
Turbine is a machine capable of transforming thermal
energy (from steam) to mechanical energy.
Steam Turbine is a prime mover in which, the potential energy
of the steam is transformed into kinetic energy.
PURPOSE OF STEAM TURBINES
Broadly speaking, Purpose of Steam turbines is
divided into two broad categories:
 Generating Electric Power
 General - Purpose units used for driving pumps,
compressors etc.
In both cases Steam Turbine will act as a prime mover.
PRINCIPLE OF WORKING

Steam is injected through nozzles over to the ring of moving
blades.

Thermal energy of steam is partly converted into kinetic energy
due to static pressure drop in nozzle.

High velocity steam leaving nozzle enters the moving blade and
the direction of steam flow gets changed from inlet to exit.

This change in direction of steam flow causes change of
momentum, which results in dynamic force acting as driving
thrust for rotation of shaft.
CLASSIFICATION OF STEAM TURBINE
•
According to the working principle or mode of steam action
•
•
•
According to number of stages
•
•
•
•
Condensing turbine
Back pressure (non-condensing) turbine
According to direction of steam flow
•
•
•
Single stage turbine
Multi stage turbine
According to the exhaust condition of steam
•
•
Impulse Turbine
Reaction Turbine
Axial flow turbine
Radial flow turbine
According to pressure of steam
High pressure turbine
Medium pressure
Low pressure
Movin
g
Blade
Steam
Nozzle
Stationary
Blade
DESCRIPTION OF COMMON TYPES OF STEAM
TURBINES
SIMPLE IMPULSE TURBINE
 REACTION TURBINE

SIMPLE IMPULSE TURBINE
 Longitudinal
section of
upper half of the turbine
 One set of nozzle followed
by a ring of moving blades
 Pressure
and
velocity
changes during flow of
steam through the turbine
IMPULSE TURBINE BLADE
Impulse Blade
Steam
In
Impuls
e Force
Steam Out
SIMPLE REACTION TURBINE
Differences between Impulse and Reaction turbines
METHODS OF REDUCING WHEEL OR ROTOR
SPEED
•
•
1.
2.
3.
Multiple system of rotors in series , keyed on a common shaft
and the steam pressure or jet velocity is absorbed in stages as the
steam flows over the blade
This is known as “Compounding”
Velocity Compounding
Pressure Compounding
Pressure Velocity Compounding
VELOCITY
COMPOUNDING
PRESSURE
COMPOUNDING
PRESSURE
VELOCITY
COMPOUNDING
GAS TURBINE
A gas turbine, also called a combustion turbine. It has an
upstream rotating compressor coupled to a downstream turbine,
and a combustion chamber in-between.
 The basic operation of the gas turbine is similar to the of the
steam power plant except that air is used instead of water
 Used to power aircrafts, trains, ships, electrical generators, or
even tanks
 It consists of three main components a compressor a combustion
chamber and a turbine.
 It is self contained, light weight not requiring cooling water and
generating fit into the overall shape of the structure .
 It is selected for power generation because of its simplicity, lack
of cooling water, needs quick installation and quick starting.

Compressor:
 A compressor is a device that is used to supply compressed air to
the combustion chamber.
 May be of single stage or multi-stage design.
Combustor:
 A combustor is a device inside which the combustion of fuel
takes place.
• For an efficient operation of gas turbine plant, it is necessary to
ensure good combustor performance.
• A good combustor should achieve completeness of fuel
combustion and the lowest possible pressure drop in the gas,
besides being compact, reliable and easy to control.
The ideal constant pressure type gas turbine works on Joule or
Brayton cycle.
TYPES
Open Cycle Gas Turbine Power Plant
In this type of plant the atmospheric air is charged into
the combustor through a compressor and the exhaust of
the turbine also discharge to the atmosphere.
1.
Closed Cycle Gas Turbine Power Plant
In this type of power plant, the mass of air is constant or
another suitable gas used as working medium, circulates
through the cycle over and over again.
2.
OPEN CYCLE GAS TURBINE
BRAYTON CYCLE:
• Popularly used for gas turbine power plant
• Consists of adiabatic compression process, constant pressure heat
addition, adiabatic expansion process and constant pressure heat
release process.
Thermodynamic cycle shows following processes:
1-2 : Adiabatic compression, involving (–ve) work, WC in
compressor.
2-3 : Constant pressure heat addition, involving heat Qadd
in combustion chamber or heat exchanger.
3-4 : Adiabatic expansion, involving (+ve) work, WT in
turbine.
4-1 : Constant pressure heat rejection, involving heat,
Qrejected in atmosphere or heat exchanger.
Working Principle
•
•
•
Fresh air enters the compressor at ambient
temperature where its pressure and temperature are
increased.
The high pressure air enters the combustion chamber
where the fuel is burned at constant pressure.
The high temperature (and pressure) gas enters the
turbine where it expands to ambient pressure and
produces work.
CLOSED CYCLE GAS TURBINE
 In
gas turbine plant of closed type the working fluid is
recycled and performs different processes without
getting contaminated.
 Working fluid is compressed in compressor and
subsequently heated up in heat exchanger through
indirect heating.
 High pressure and high temperature working fluid is
sent for getting positive work from turbine.
 The expanded working fluid leaving turbine is passed
through heat exchanger where heat is picked up from
working fluid.
 Even costly working fluids can also be used in closed
type
1-2 Isentropic compression (in a
compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (in a turbine)
4-1 Constant-pressure heat rejection
Advantages of closed cycle:
•
Wide variety of fuels and gasses can be used
•
Products of combustion do not come in direct contact with turbine blades
•
Heat transfer coefficients are higher at high pressures
Disadvantages:
•
External furnace for combustion process
•
More complicated design and costly
•
Coolant is used for pre cooling
METHODS TO IMPROVE THE PERFORMANCE
OF OPEN CYCLE GAS TURBINE PLANT

For the maximization of specific work output the gas turbine
exhaust temperature should be equal to compressor exhaust
temperature.
Methods to improve open cycle plant performance
1.
Regenerative gas turbine cycle
2.
Reheat gas turbine cycle
3.
Gas turbine cycle with intercooling
REGENERATIVE GAS TURBINE
CYCLE
A
counter flow heat exchanger called regenerator,
which shall preheat the air leaving compressor before
entering the combustion chamber, thereby reducing
the amount of fuel to be burnt inside combustion
chamber.
REHEAT GAS TURBINE CYCLE
 To
increase the net work output the positive work may
be increased by using multistage expansion with
reheating in between.
 In multistage expansion the expansion is divided into
parts and after part expansion working fluid may be
reheated for getting larger positive work in left out
expansion.
 For reheating another combustion chamber may be
used.
 Ambient air enters compressor at 1 and compressed
air at high pressure leaves at 2.
 Compressed
air is injected into combustion chamber
for increasing its temperature up to desired turbine
inlet temperature at state 3.
 High pressure and high temperature fluid enters high
pressure turbine (HPT) for first phase of expansion.
 Expanded gases leaving at 4 are sent to reheat
combustion chamber (reheater) for being further
heated.
 The fluid leaves at state 5 and enters low pressure
turbine (LPT) for remaining expansion up to desired
pressure value.
GAS TURBINE CYCLE WITH
INTERCOOLING
 Net
work output from gas turbine cycle can also be
increased by reducing negative work i.e. compressor
work
 Multi staging of compression process with intercooling
in between is one approach for reducing compression
work.
 First stage compression occurs in low pressure
compressor (LPC)
 Compressed air leaving LPC at ‘2’ is sent to intercooler
where temperature of compressed air is lowered down
to state 3 at constant pressure.
 Intercooler is a kind of heat exchanger where heat is
picked up from high temperature compressed air.