BME STEAM TURBINES AND STEAM BOILERS

UNIT 1
STEAM BOILERS
AND
STEAM TURBINES

FUNDAMENTAL OF THERMODYNAMICS
• Thermodynamics is the branch of science dealing with energy
transfer in the form of heat and work & effect of energy transfer on
the properties of the system.
• Thermodynamic System : A thermodynamic system is defined as
a region in space or a quantity of matter upon which attention is
focussed for the study of work & heat transfer.
Steam Boilers and Steam Turbines

• Density: It is defined as the mass of the unit volume.
ρ=m/v
• Specific Volume: The volume occupied per unit mass is known as
Specific volume.
Vs=V/m
• Specific Gravity: It is the ratio of density of the substance to the
density of reference substance.
• Pressure: Pressure is defined as force per unit area.
p=F/A
• Temperature: It is quantitative measurement of degree of
hotness or intensity of heat of body.

• Absolute Zero Temperature: It is defined as the point where no
more heat can be removed from the system, according to
thermodynamic temperature scale. This corresponds to 0 Kelvin
or -2730C.
• Absolute Temperature : The temperature below which the
temperature of any substance can not be changed is known as
Absolute Temperature.
• Thermometer : instruments used for measuring ordinery
temperature
• Pyrometer : instruments used for measuring high temperature

• Work :Work is done when the point of application of force moves
in the direction of the force.
W=F.x
• Energy: Energy is defined as the capacity to do work.
Types of Energy:
a) Potential Energy: It is the energy possessed by the body by
virtue of its position above ground level.
𝑃. 𝐸. = 𝑚𝑔𝑍
b)Kinetic Energy: It is the energy possessed by the body when it is
in motion.
𝐾. 𝐸. =
1
2
𝑚𝑣2

Internal Energy: It is the energy possessed by body due to its
molecular arrangement and motion of molecules.
• Heat: Heat is an energy which is transferred across the boundary
between systems, without transfer of mass, by reason of the
difference in temperature of the two systems and in the direction
of lower temperature.
• Specific Heat: The specific heat of a substance is broadly defined as
the amount of heat required to raise the temperature of a unit
mass of any substance through one degree.

• Enthalpy: The total heat content of the system is called as
enthalpy.
• Entropy: Entropy means transformation. Entropy is a property of
working substance which increases with the addition of heat and
decreases with removal of heat.
𝑑𝑆 = 𝑑𝑄/𝑇

LAWS OF THERMODYNAMICS
Zeroth law
• If the two bodies are separately in thermal equilibrium with third
body, then the two bodies are also in thermal equilibrium with
each other .
• Let us say TA, TB and TC are the temperatures of A, B and C
respectively.
• A and C are in thermal equilibrium. so… TA=TC
• B and C are in thermal equilibrium. So TB=TC
• A and B will also be in thermal equilibrium so…
• TA=TB

• Zeroth law: When a body ‘A’ is in thermal equilibrium with the
body ‘B’ and also separately with the body ‘C’ , then the two bodies
‘B’ and ‘C’ will be in thermal equilibrium with each other

FIRST LAW OF THERMODYNAMICS
• Law of Conservation of Energy
• The first law says that heat transfer is equal to work transfer when
the system executes a cycle transferring work & heat through its
boundaries.
• ∮ 𝛿𝑄 = ∮ 𝛿𝑊

FIRST LAW OF THERMODYNAMICS
• It is based on
• Energy Conversion
• Energy Transfer (work to heat OR heat to work)
• Statement:
• This may be stated as follows
• work and heat are mutually convertible into each other
• (i.e. work to heat or heat to work 100% convertible) without any
restriction on the direction of flow

Second law of Thermodynamics
• Is based on limitation on first law of
thermodynamics
Kelvin Plank statement
• It is impossible to construct an engine working
in cyclic process, whose sole purpose is to
convert all supplied heat into an equivalent
amount of work
• In other words heat engine working in a cyclic
process, can’t convert whole supplied heat into
work
• 100% heat is not converted into work

• It means that there is degradation of energy in the process of
producing work from the heat supplied.
• The Kelvin Planck statement of the second law of thermodynamic
is sometimes known as law of degradation of energy.
• Clausius statement
• It is impossible for a self-acting machine, working in a cyclic
process, to transfer heat from a body at a lower temperature to a
body at a higher temperature without the addition of an external
work

• Kelvin-Plank Statement: It is impossible to construct an engine
working in a cyclic process whose sole effect is to convert all the
heat supplied to it into an equivalent amount of work.
• In other words, heat of, itself, cannot flow from a colder to a hotter
body.
• Clausius Statement: heat can not flow itself from a cold body to a
hot body without the help of an external agency.
• Second law of thermodynamics is also called as law of degradation
of energy.

Second law of thermodynamics
A reservoir that:
• Supplies heat is a source
• Absorbs heat is a sink
Converting heat to work
requires the use of a device
called a heat engine
Heat engines come in many
forms, pure heat engines
(steam power plants) and
semi heat engines (gas turbines)
All have a working fluid

STEAM
• Steam is a gas formed when water passes from the liquid to the
gaseous state .At the molecular level, this is when H2O molecules
manage to break free from the bonds keeping them together.
• Dry Steam: Steam is said to be dry steam when molecules of steam
remains to be in gaseous state. It is transparent gas.
When the steam contains no moisture it is dry steam.
• Wet steam: When the steam contains moisture it is wet steam. Wet
steam is mixture of two states-liquid and vapour.

• SUPERHEATED STEAM:
When steam is heated after it has become dry and saturated, it is
called superheated steam and the process of heating is called
superheating. Superheating is always carried out at constant
pressure.
• Dryness fraction (x). The term dryness fraction is related with wet
steam. It is defined as the ratio of the mass of actual dry steam to
the mass of steam containing it. It is usually expressed by the
symbol ‘x’ or ‘q’.

If ms = Mass of dry steam contained in steam considered, and
mw = Weight of water particles in suspension in the steam
considered,
• 𝑥 =
𝑚𝑠
𝑚𝑠+𝑚𝑤
Degree of Superheat : It can be defined as the amount by the
temperature of superheated steam exceeds the temperature of
saturated steam at the same pressure.

Steam Boiler
A steam boiler is a closed vessel, generally made of steel, in which
water is heated by some source of heat produced by combustion of
fuel and ultimately to generate steam.
The steam produced may be supplied at low pressure for industrial
process work in cotton mills, sugar industries etc. and for producing
hot water which can be used for heating installations at much low
pressure.

• Function of main Parts:
1. Steam separator drum:
This drum is situated upside of the boiler. It is larger diameter drum
in which water and steam placed together. The one half of the drum
is filled with water and the other half is remaining for steam.
2. Water tubes:
Water tubes are situated bottom side of the drum. Water flows from
the drum to the tubes.

3. Uptake header:
Steam separator drum and water tubes are connected by the two
tubes. One is known as uptake header and the other one is known as
down take header. The steam from the water tubes to the drum flow
by the uptake header.
4. Down take header:
The water flows from the drum to the water tubes through down
take header. When the steam flows by uptake header to the drum, at
the same time water flows from drum to the water tubes by down
take header which maintains the flow of water.

5. Grate:
The place in the furnace, where the fuel is placed and burn known
as grate.
6. Furnace:
The furnace is the place where the fuel burns. This is situated at the
down side of the water tubes. When the fuel burns, the flue gases
generate. This gases flow upper side and passes through water tube,
which heat the water and convert it into steam.

7. Super heater:
Super heater is situated upper side of the water tube. One end of
super heater is connected to the drum and other end is for process
work. Steam flows from the drum to the super heater, where it
heated by the flue gases and send for the process work.
8. Baffles:
Baffles are provided between the water tubes. The main function of
baffles is to divert the flue gases, so it flows more than one time
through the tube and more heat is transfer.

Features of Babcock & Wilcox boiler:
1)Horizontal, Straight & Stationary
2)Externally fired
3)Natural circulation
4)Water tube boiler
5)Minimum steam pressure of 10 bar
6)Minimum evaporative capacity of 7000 kg of steam per hour.

Working
• Coal is fed to the grate through the fire door and is burnt.
• Flow of flue gases:
• The hot flue gases rise upward and pass across the left-side
portion of the water tubes.
• The baffles deflect the flue gases and hence the flue gases travel in
the zig-zag manner(i.e., the hot gases are deflected by the baffles to
move in the upward direction, then downward and again in the
upward direction) over the water tubes and along the superheater.
• The flue gases finally escape to atmosphere through chimney.

• Water circulation: That portion of water tubes which is just above
the furnace is heated comparatively at a higher temperature than
the rest of it. Water, its density being decreased, rises into the
drum through the uptake-header. Here the steam and water are
separated in the drum. Steam being lighter is collected in the
upper part of the drum.
• The water from the drum comes down through the down –comer
into the water tubes.
• A continuous circulation of water from the drum to the water
tubes and water tubes to the drum is thus maintained. The
circulation of water is maintained by convective currents and is
known as “natural circulation”. A damper is fitted as shown to
regulate the flue gas outlet and hence the draught.

COCHRAN
BOILER

Features of Cochran boiler:
1)Vertical, compact and requires minimum floor area
2)Multi-tubular, Internally fired
3) Any type of fuel can be used with this boiler
4)Natural circulation, Fire tube boiler
5) 70% thermal efficiency with coal firing and about 75% with oil firing.
6)Up to maximum steam pressure of 15 bar
7)Maximum evaporative capacity of 4000 kg
of steam per hour.
8)It is well suited for small capacity requirements

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 chamber plate.

• 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:
• Coal or oil can be used as fuel in this boiler. If oil is used as fuel, no
grate is provided but the bottom of the furnace is lined with
firebricks.
• 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.

Advantages
• Cochran Boiler occupies less floor space.
• Construction cost of Cochran Boiler is Low.
• Cochran boiler is semi-portable and hence easy to install and
transport.
• Because of self contained furnace no brick work setting is
necessary.

Disadvantages
• The capacity of the Cochran boiler is less because of the vertical
design.
• Cochran Boiler requires high head room space.
• Because of the vertical design, it often presents difficulty in
cleaning and inspection.

Boiler Mountings
Different fittings and devices necessary for the operation and safety of
a boiler are known as boiler mountings.
1)Water Level Indicator
2) Pressure Gauge
3)Safety Valve
4)Steam Stop Valve
5)Blow-off Cock
6) Fusible Plug

• Mounting of high pressure boiler: There are different fittings
and device which are necessary for the operation and safety of a
boiler. The various mountings used on the boiler
• 1. Water level indicators: The function of a water level indicator
is to indicate the level of water in the level constantly. It is also
called water gauge.

Water level indicator

-It is an important fitting, which indicates the water level inside the
boiler to an observer. It is a safety device, upon which the correct
working of the boiler depends.
-This fitting may be seen infront of the boiler. It is mostly employed in
the steam boiler.
-It has a strong glass tube fitted to two hollow gun metal castings with
the help of stuffing box. The lower end of this indicator communicates
with water and the upper end with steam in the boiler.

These are the devices attached to the steam chest for preventing
explosions due to excessive internal pressure of steam.
A steam boiler is usually provided with two safety valves.
These are directly placed on the boiler.
The function of a safety valve is to below off the steam when the
pressure of steam inside the boiler exceeds the working pressure.
There are four types of safety valves usually used in boilers:
Safety valves

Types of Safety Valves
Lever safety valve
Dead weight safety valve
Spring loaded safety valve
High steam and low water safety valve
The function of safety valve is to release the excess steam
when the pressure of steam inside the boiler exceeds the rated
pressure.
Matoshri Aasarabai Polytechnic, Nashik

It consists of a valve resting over a gun metal seat. The valve
seat is fixed on a mounting block, fitted over the boiler shell.
One end of the lever is hinged to a rod of the mounting block,
while the other end carries a weight. A short strut is placed
over the valve.

Steam stop valve

-It regulates the flow of steam from a boiler. This is generally mounted on
the highest part of boiler shell and performs function of regulating the
flow of steam from boiler.
- Steam stop valve generally has main body of cast steel. Valve, valve seat
and nut etc. are of brass.
- It can be easily operated by rotating the hand wheel which causes
lifting or lowering of spindle, thus causing opening or closing of valve

Steam stop valve
• Steam stop valve: A junction valve is a valve which is placed
directly over a boiler and connected to a steam pipe which carries
steam to the engine.
• If a valve is placed in the steam pipe leading steam to the engine
and placed near the engine. It usually termed as stop valve. The
larger sizes are called Junction valve and smaller sizes Stop valve
• 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.

-A pressure gauge indicates the pressure of steam in a boiler. It is
mounted at front top.
-Generally Bourdon type pressure gauge is being used for pressure
measurement.
-Pressure is continuously monitored so as to avoid occurrence of over
shooting of boiler pressure.
-Although safety devices to protect boiler against pressure rising
beyond a limit are provided but pressure gauges are also used for
monitoring pressure.
Pressure Gauge

Pressure Gauge

Blow-off Cock
It is used for periodical cleaning by discharging the water and
sediments from bottom of boiler. It is fitted to the bottom of boiler
shell.
Blow off cock has a plug of conical type put into the mating casing.
Plug position is altered for opening and closing the flow.
It also helps in regulating the salt concentration as frequent
draining helps in throwing out the salt deposited over period of
time.
It is also used for emptying the boiler when ever boiler is to be
cleaned

Fusible Plug
• It is a safety device used for preventing the level of water
from going down below a critical point and thus avoids
overheating. It is fitted to the crown plate of the fire box.
• It has gun metal body and a copper plug put with fusible
metal at interface of copper plug and gun metal body.
• A fusible plug must be kept in a good condition and
replaced annually.

Boiler Accessories
The accessories fitted with the boiler are the devices which are
responsible for increasing the efficiency of the boiler.

Super heater
It is an important device of steam generating unit. Generally boiler
generates wet steam.
By heating further it can be converted into dry-saturated steam.
The steam temperature can further be increased to any desired
degrees by passing it through super heater.
The super heater receives heat from furnace itself. Since the
temperature of superheated steam is more, it can do more
mechanical work.
Therefore, a super heater increases the efficiency of the boiler.

Super heater

Economizer
An economizer is used to heat the water which is being fed into the
boiler shell. The heat required for this purpose is extracted from the
waste flue gases going out of the boiler.
It is also a type of heat exchanger having exhaust gas and feed water.
It also help in removal of dissolved gases by preheating of water and
thus minimizing tendency of corrosion.
It is placed between the exits of the furnace and entry into the
chimney.
Thus economizer increases the efficiency of the boiler.

Economizer

Advantages of economizer:
1. The temperature range between various parts of the boiler is
reduced which results in reduction of stresses due to unequal
expansion
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 increased.
4. Overall efficiency of the plant is increased.

Air Pre heater
The function of an air pre heater is to heat the air before it is supplied to the furnace of
the boiler.
It is placed near chimney and above economizer.
There are three types of air pre heater:
1. Tubular type 2. Plate type
3. Regenerative type

Steam Nozzle
Steam nozzle is an insulated passage of varying cross-sectional area
through which heat energy (Enthalpy), pressure of steam is
converted into kinetic energy.

Steam Nozzle
Functions of Nozzle :-
1) The main function of the steam nozzle is to convert heat energy to
kinetic energy.
2) To direct the steam at high velocity into blades of turbine at
required angle.
Applications :-
1) Steam & gas turbines are used to produces a high velocity jet.
2) Jet engines and rockets to produce thrust (propulsive force)

Consider a non-viscous liquid in streamline flow through a tube AB, of
varying cross-section.
Let A1 and A2 be the area of cross-section at A and B respectively.
Continuity Equation

The volume of water entering A per second = A1V1
Volume = Area x distance
where V1 is the velocity of the flow of liquid at A

Assuming there is no loss of liquid in tube and for free steady flow,
Mass of liquid entering per second at A= Mass of liquid leaving per second at
B
or AV = constant.
This is the equation of continuity.

Types of Nozzles
1) Convergent nozzle
2) Divergent nozzle
3) Convergent - Divergent nozzle

Convergent Nozzle
• It is a nozzle with large entrance and tapers gradually to a smallest
section at exit.
• It has no diverging portion.

Divergent Nozzle :-
It is a nozzle with small entrance and tapers gradually to a large
section at exit.
It has no converging portion at entry.

• Convergent - Divergent Nozzle
• convergent - divergent nozzle is widely used in steam turbines.
• The nozzle converges first to the smallest section and then diverges up to
exit.
• The smallest section of the nozzle is called throat.
• The divergent portion of nozzle allows higher expansion ratio i.e.,
increases pressure drop.

Convergent - Divergent Nozzle :
• The taper of diverging sides of the nozzle ranges from 60 to 150 .
• if the taper is above 150 turbulent is increased.
• However if it is less than 60, the length of the nozzle will increases

Applications of Steam Nozzle
1) Nozzles are used in steam turbine, gas turbine,
water turbine etc
2) Nozzles are used for flow measurement. e.g. in venturimeter.
3)Nozzles are used to remove air from condenser.
4)Injectors for pumping feed water to boiler.

Mach number
• the ratio of speed of an object moving through a fluid and the local
speed of sound.
Where,
• M is the Mach number, v is the velocity of the source relative to the
medium, and vsound is the speed of sound in the medium.
• Mach number varies by the composition of the surrounding medium
and also by local conditions, especially temperature and pressure.

Mach number
Significance of Mach number :
• M< 1 , the flow is called subsonic.
• M=1, the flow is called sonic.
• M>1, the flow is called supersonic.
• M>5, the flow is called hypersonic.

Critical Pressure of Nozzle
1) The pressure for which the maximum discharge through nozzle
occur is called the critical pressure.
2) The pressure ratio of critical pressure to initial pressure is called
critical pressure ratio.
Critical pressure ratio is given by,
𝑃𝑐
𝑃1
=[
2
Ɣ−1
]
Ɣ
Ɣ
−1

Impulse Turbine
• impulse turbine is a type of steam turbine where the rotor derives
its rotational force from the impact force, or the direct push of steam
on the blades.
• The impulse turbine was first built in 1883 by the Swedish engineer
De Laval.
• The impulse turbine consists of a rotor mounted on a shaft that is free
to rotate.
• Attached to the rotor are a set of curved blades. Nozzles then direct
the high pressure and high temperature steam towards the blades of
the turbines.
• The blades catch the impact force of the rapidly moving steam and
rotate from this force.

(1) The steam first enters the impulse
turbine through a fixed Nozzle.
(2) The steam strikes the blades that are
free to rotate with a strong enough force
to move the blades.
(3) The steam exits the blade towards the
condensing system of the steam turbine
generator system.
4) The direction of the blades due to the
force of steam.

Reaction turbine
• A reaction turbine is a type of steam turbine that works on the principle that
the rotor spins, as the name suggests, from a reaction force rather than an
impact or impulse force.
• In a reaction turbine there are no nozzles to direct the steam like in the
impulse turbine.
• Instead, the blades that project radially from the outer edge of the rotor are
shaped and mounted so that the shape between the blades, created by the
cross-section, create the shape of a nozzle. These blades are mounted on the
revolving part of the rotor and are called the moving blades.

Reaction turbine
• The fixed blades, which are the same shape as the moving blades, are
mounted to the outer casing where the rotor revolves and are set to guide
the steam into the moving blades.
• Below is a simple diagram of reaction turbine blades:

• (1) The steam enters through a section of
curved blades in a fixed position.
• (2) The steam then enters the set of
moving blades and creates enough reactive
force to rotate them,
• (3) The steam exits the section of rotating
blades.
• (4) The direction of rotation.

Reaction turbine
• There are three main forces that act to move a reaction turbine.
• First, from the reactive force that is created on the moving blades as it
expands and increases in velocity as it moves through the nozzle
shaped spaces between the blades.
• Second, from the reactive force produced on the moving blades as the
steam passes through and changes directions.
• Third, and to a lesser extent, from the impact force of the steam on
the blades helps rotate the reaction turbine.

Difference between Impulse and Reaction
Turbine
1. In impulse turbine, there are nozzle and moving blades are in series
while there are fixed blades and moving blades are present in
Reaction turbine (No nozzle is present in reaction turbine).
2. In impulse turbine pressure falls in nozzle while in reaction turbine in
fixed blade boiler pressure falls.
3. In impulse turbine velocity (or kinetic energy) of steam increases in
nozzle while this work is to be done by fixed blades in the reaction
turbine.
4. Compounding is to be done for impulse turbines to increase their
efficiency while no compounding is necessary in reaction turbine.

Difference between Impulse and Reaction
Turbine
5)In impulse turbine pressure drop per stage is more than reaction
turbine.
6) Not much power can be developed in impulse turbine than reaction
turbine.
7)Efficiency of impulse turbine is lower than reaction turbine.
8)Impulse turbine requires less space than reaction turbine.
9)Blade manufacturing of impulse turbine is not difficult as in reaction
turbine it is difficult.

Compounding of steam turbines
• Compounding of steam turbines is the method in which energy
from the steam is extracted in a number of stages rather than a
single stage in a turbine.
• A compounded steam turbine has multiple stages i.e. it has more
than one set of nozzles and rotors, in series, keyed to the shaft or
fixed to the casing, so that either the steam pressure or the jet
velocity is absorbed by the turbine in number of stages.

• As we have seen , if the high velocity steam is allowed to flow through
one row of moving blades, it produces a rotor speed of about 30000
r.p.m. which is too high for practical use.
• Not only this, the leaving loss is also very high.
• It is therefore essential to incorporate some improvements in the
simple impulse turbine for practical use and also to achieve high
performance.
• This is possible by making use of more than one set of nozzles, blades,
rotors, in a series, keyed to a common shaft.

• So that either the steam pressure or the jet velocity is absorbed by
the turbine in stages.
• The leaving loss also will be less.
• This process is called compounding of steam turbine.

Types of compounding
• In an Impulse steam turbine compounding can be achieved in the
following three ways: -
• 1. Velocity compounding
• 2. Pressure compounding
• 3. Pressure-Velocity Compounding

Velocity Compounding
• The velocity compounded Impulse turbine was first proposed by C G Curtis to
solve the problem of single stage Impulse turbine for use of high pressure and
temperature steam.
• The rings of moving blades are separated by rings of fixed blades. The moving
blades are keyed to the turbine shaft and the fixed blades are fixed to the
casing.
• The high pressure steam coming from the boiler is expanded in the nozzle
first. The Nozzle converts the pressure energy of the steam into kinetic
energy

• It is interesting to note that the total enthalpy drop and
hence the pressure drop occurs in the nozzle. Hence, the
pressure thereafter remains constant.
• This high velocity steam is directed on to the first set (ring)
of moving blades. As the steam flows over the blades, due
the shape of the blades, it imparts some of its momentum to
the blades and losses some velocity.

velocity compounded
• Only a part of the high kinetic energy is absorbed by these blades. The
remainder is exhausted on to the next ring of fixed blade.
• The function of the fixed blades is to redirect the steam leaving from the first
ring moving blades to the second ring of moving blades. There is no change in
the velocity of the steam as it passes through the fixed blades.
• The steam then enters the next ring of moving blades; this process is repeated
until practically all the energy of the steam has been absorbed.
• A schematic diagram of the Curtis stage impulse turbine, with two rings of
moving blades one ring of fixed blades is shown in figure 1. The figure also
shows the changes in the pressure and the absolute steam velocity as it passes
through the stages.

velocity compounded
• where,
• Pi = pressure of steam at inlet
• Vi = velocity of steam at inlet
• Po = pressure of steam at outlet
• Vo = velocity of steam at outlet
• In the above figure there are two rings of moving blades separated by a single of
ring of fixed blades.
• As discussed earlier the entire pressure drop occurs in the nozzle, and there are
no subsequent pressure losses in any of the following stages. Velocity drop occurs
in the moving blades and not in fixed blades.

Advantages
• Velocity compounded impulse turbine requires a comparatively
small number of stages due to relatively large heat drop per stage.
• Due to small number of stages the initial cost is less.
• In two or three row wheel, the steam temperature is sufficiently lo,
hence a cast iron cylinder may be used , thus saving material cost.

Disadvantages
• The velocity compounded impulse turbine has low efficiency and
high steam consumption.

Pressure Compounded
• The pressure compounded Impulse turbine is also called as Rateau turbine,
after its inventor. This is used to solve the problem of high blade velocity in
the single-stage impulse turbine.
• It consists of alternate rings of nozzles and turbine blades. The nozzles are
fitted to the casing and the blades are keyed to the turbine shaft.
• In this type of compounding the steam is expanded in a number of stages,
instead of just one (nozzle) in the velocity compounding.

• It is done by the fixed blades which act as nozzles. The steam
expands equally in all rows of fixed blade. The steam coming from
the boiler is fed to the first set of fixed blades i.e. the nozzle ring.
The steam is partially expanded in the nozzle ring.
• Hence, there is a partial decrease in pressure of the incoming
steam. This leads to an increase in the velocity of the steam.
Therefore the pressure decreases and velocity increases partially
in the nozzle.

pressure compounded
• This is then passed over the set of moving blades. As the steam flows over the
moving blades nearly all its velocity is absorbed. However, the pressure
remains constant during this process.
• After this it is passed into the nozzle ring and is again partially expanded.
Then it is fed into the next set of moving blades, and this process is repeated
until the condenser pressure is reached.
• This process has been illustrated in figure.
• where, the symbols have the same meaning as given above.
• It is a three stage pressure compounded impulse turbine. Each stage consists
of one ring of fixed blades, which act as nozzles, and one ring of moving
blades. As shown in the figure pressure drop takes place in the nozzles and is
distributed in many stages.

Disadvantages of Pressure Compounding
• The disadvantage is that since there is pressure drop in the
nozzles, it has to be made air-tight.
• They are bigger and bulkier in size

Pressure-Velocity compounded Impulse Turbine
• It is a combination of the above two types of compounding. The total
pressure drop of the steam is divided into a number of stages.
• Each stage consists of rings of fixed and moving blades. Each set of
rings of moving blades is separated by a single ring of fixed blades.
• In each stage there is one ring of fixed blades and 3-4 rings of moving
blades. Each stage acts as a velocity compounded impulse turbine.
• The fixed blades act as nozzles. The steam coming from the boiler is
passed to the first ring of fixed blades, where it gets partially
expanded.

Pressure-Velocity compounded Impulse Turbine
• The pressure partially decreases and the velocity rises
correspondingly. The velocity is absorbed by the following rings of
moving blades until it reaches the next ring of fixed blades and the
whole process is repeated once again.
• This process is shown diagrammatically in figure .
• where, symbols have their usual meaning.

Control of pollution due to steam boilers
Due to in die boiler furnace and smoke and other particulate are
released into the atmosphere which causes pollution in air. So to
reduce it the following points must be considered:
• An economizer should be provided in the circuit for pre-heating
boiler feed water.
• The boiler should be provided with an Induced draft fan of
appropriate capacity.
• TheC02 monitoring instument should be provided to the flue
system.

• The damper should be located preferably nearer to the front side
of boiler and should be easily accessible such that the boiler
operator can access the damper easily.
• A cyclone separator of appropriate size should provided in the
circuit along with "bottom storage hopper and the duct collected
should be taken out from time to time.
• The Fuel should be fired uniformly and in less quantity at a time
such that the bed thickness does not exceed than design level of
boiler.

• Every time the fuel is fired, the damper should set to 'High' position
for a minute as this would suck more air required combustion or
burning volatile matter which reduce soot or black smoke formation
and then it should be set back to Low' till the next firing.
• Solid fuels like coal should be pulverized to an appropriately size.

BME STEAM TURBINES AND STEAM BOILERS

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