Basics of steam boilers section b

BASICS OF MECHANICAL ENGINEERING (ME-101 F)
Lectures on
STEAM GENRATION AND BOILERS
By
Priyanka Singh
M.Tech (Applied Mechanics)
Motilal Nehru National Institute of Technology, Allahabad

Definitions
Latent Heat:
Latent heat is the heat absorbed or releases during a change of state of
matter.
Latent Heat of Fusion:
The specific latent heat of fusion is the heat needed to change 1 kg of a
solid at its melting point into a liquid, or vice versa, without a change in
temperature.
Latent Heat of Vaporization:
The specific latent heat of vaporization is the heat needed to change 1
kg of a liquid at its boiling point into vapour or vice versa, without a
change in temperature.
Steam:
If water is heated beyond the boiling point, it vaporizes into steam, or
water in the gaseous state. The properties of steam vary greatly depending
on the pressure and temperature to which it is subject.

Heating Curve
Assume that unit mass of steam is generated from solid ice which is at-
20°C at atmospheric pressure (1.01325 bar).

Heating Curve - Latent Heat
• T1 is the melting point whereas
• T2 is the boiling point.

Regime P-Q: Initially ice is at point P, i.e. at -20°C. Temperature of ice
increases from -20°C to 0°C. Point Q represents 0°C temperature condition.
Regime Q-R: The ice starts melting in this regime. The heat supplied in this
regime is called latent heat of fusion as it is responsible to change in phase
not increasing the temperature. At point R total ice is convert into water.
Regime R-S: At point R temperature of water is 0°C. If any heat addition is
there, the temperature of water will increase and it will continue till point S is
reached where temperature become 100°C. This heat addition is called
sensible heat. Point S represents saturated liquid condition.
Regime S-T: At point S water starts boiling and additional heat is responsible
to convert the water into vapours (steam). This boiling is continue till whole
water is convert into vapours. The heat supplied is called latent heat of
vaporization. Point T shows the saturated steam condition.
Regime T-U: Point T represents saturated steam condition. If any heat
addition is there, volume of steam will increase very rapidly and it behave
like gas. The difference between superheated temperature and saturation
temperature is called degree of superheat.

• From Q to R and S to T, the temperature remains constant because
the heat supplied to the object is used to overcome the forces of
attraction that hold the particles together.
• Heat obsorbs during Q-R is called the latent heat of fusion.
• Heat obsorbs during S-T is called the latent heat of
vaporisation.

Wet Steam Quality and the Dryness Fraction
To produce 100% dry steam in an boiler, and keep the steam dry throughout
the piping system, is in general not possible. Droplets of water will escape
from the boiler surface. Because of turbulence and splashing when bubbles
of steam break through the water surface the steam space will contain a
mixture of water droplets and steam.
In addition heat loss in the pipes will condensate steam to droplets of water.
Steam - produced in a boiler where the heat is supplied to the water and
where the steam are in contact with the water surface of the boiler - will
contain approximately 5% water by mass.
Dryness fraction of Wet Steam
If the water content of the steam is 5% by mass, then the steam is said to be
95% dry and has a dryness fraction of 0.95.
Dryness fraction can be expressed as:
x = ms / (mw + ms) (1)
where
x= dryness fraction
mw = mass of water (kg)
ms = mass of steam (kg)

Types of Steam
In the sections that follow, we will discuss the types of steam
used in these applications.

Saturated Steam (Dry)
Saturated steam occurs at temperatures and pressures where steam
(gas) and water (liquid) can coexist. In other words, it occurs when
the rate of water vaporization is equal to the rate of condensation.
Unsaturated Steam (Wet)
This is the most common form of steam actually experienced by
most power plants. When steam is generated using a boiler, it
usually contains wetness from non-vaporized water molecules that
are carried over into the distributed steam. Even the best boilers
may discharge steam containing 3% to 5% wetness.
Superheated Steam
Superheated steam is created by further heating wet or saturated
steam beyond the saturated steam point. This yields steam that has
a higher temperature and lower density than saturated steam at the
same pressure. Superheated steam is mainly used in
propulsion/drive applications such as turbines, and is not typically
used for heat transfer applications.

Steam Generation Theory
 Within the boiler, fuel and air are
force into the furnace by the
burner.
 There, it burns to produce heat.
 From there, the heat (flue gases)
travel throughout the boiler.
 The water absorbs the heat, and
eventually absorb enough to
change into a gaseous state -
steam.
 To the left is the basic theoretical
design of a modern boiler.
 Boiler makers have developed
various designs to squeeze the
most energy out of fuel and to
maximized its transfer to the water.

 Water enters the boiler, preheated, at the top.
 The hot water naturally circulates through the tubes down to the
lower area where it is hot.
 The water heats up and flows back to the steam drum where the
steam collects.
 Not all the water gets turn to steam, so the process starts again.
 Water keeps on circulating until it becomes steam.
 Meanwhile, the control system is taking the temperature of the
steam drum, along with numerous other readings, to determine if
it should keep the burner burning, or shut it down.
 As well, sensors control the amount of water entering the boiler,
this water is know as feedwater.
 Feedwater is not your regular drinking water.
 It is treated with chemicals to neutralize various minerals in the
water, which untreated, would cling to the tubes clogging or worst,
rusting them.
 This would make the boiler expensive to operate because it would
not be very efficient.

 On the fire side of the boiler, carbon deposit resulting from improper
combustion or impurities in the fuel can accumulate on the outer
surface of the water tube.
 This creates an insulation which quickly decrease the energy transfer
from the heat to the water.
 To remedy this problem the engineer will carry out soot blowing. At a
specified time the engineer uses a long tool and insert it into the fire
side of the boiler.
 This device, which looks like a lance, has a tip at the end which "blows"
steam.
 This blowing action of the steam "scrubs" the outside of the water
tubes, cleaning the carbon build up.
 Water tube boilers can have pressures from 7 bar to as high as 250
bar.
 The steam temperature's can vary between saturated steam, 100
degrees Celsius steam with particle of water, or be as high as 600 -
650 degrees Celsius, know as superheated steam or dry steam
 The performance of boiler is generally referred to as tons of steam
produced in one hour.
 In water tube boilers that could be as low as 1.5 t/hr to as high as
2500 t/hr.

• Introduction
Boiler is a device which is used to produce steam at high pressure. Steam is being
used in thermal power plant, textile industries and for domestic uses during winter to
heat the room.
• Types of boilers
1. Horizontal, Vertical and Inclined boilers
2. Fire tube and water tube
3. Externally fired and internally fired
4. Forced circulation and natural circulation
5. High pressure and low pressure
6. Stationary and portable(moving)
7. Single tube and multi tube
• Horizontal, vertical or inclined
If the axis of the boiler is horizontal, vertical or inclined then it is called horizontal,
vertical or inclined boiler respectively.
BOILERS

• Fire tube and water tube
If hot gases are inside the tube and water is outside the tube, it is called fire-tube boiler.
Examples: Cochran, Lancashire and locomotive boilers.
If water is inside the tube and hot gases are outside the tube, it is called fire-tube boiler.
Examples: Babcock and Wilcox, Sterling, Yarrow boiler etc.
• Externally fired and internally fired
The boiler is known as externally fired if the fire is outside the shell.
Examples: Babcock and Wilcox, Sterling
The boiler is known as internally fired if the furnace is located inside the boiler shell.
Examples: Cochran, Lancashire
• Forced circulation and natural circulation
In forced circulation type of boilers, the circulation of water is done by a forced pump
Examples: Velox, Lamont, Benson boiler
In natural circulation type of boilers, circulation of water in the boiler takes place due to
natural convection currents produced by the application of heat.
Examples: Lancashire, Babcock and Wilcox.

• High pressure and low pressure
The boilers which produce steam at pressures of 80 bar and above are called high pressure
boilers.
Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers.
The boilers which produce steam at pressure below 80 bar are called low pressure boilers.
Examples: Cochran, Cornish, Lancashire and locomotive boilers.
• Stationary and portable
Stationary boilers are used for power plant-steam, for central station utility power plants,
for plant process steam etc.
Mobile or portable boilers include locomotive type, and other small unit for temporary use
at sites.
• Single tube and multi tube
The fire tube boilers are classified as single tube or multi-tube boilers, depending upon
whether the fire tube is one or more than one.
Examples of single tube boilers are Cornish and simple vertical boiler

Parts and terms in Boiler
• 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
• Grate
It is a platform 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.
• Refractory
Insulation material used for lining combustion chamber
• Combustion chamber
Part of furnace where combustion of fuel takes place.
• 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
• 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.
• Water level
The level at which water stands in the boiler is called water level.
• Refractory
Insulation material used for lining combustion chamber.

• Foaming
Formation of steam bubbles on the surface of boiler water due to high surface tension of
water.
• Scale
A deposit of medium due to extreme hardness occurring 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.
• Lagging
Insulation wrapped on the outside of the boiler shell or steam piping.
• Priming
It is the carryover of varying amounts of droplets of water in the steam (foam and mist),
which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on
the super heaters and in the turbines. Priming may be caused by improper construction of
boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes
aggravated by impurities in the boiler-water.
rate.

• Mountings
Equipment and devices, which are used for safety of boiler are called mountings, these are
required parts for the successful operation of a boiler.
Ex. Feed-check valve, safety Valve etc.
• Accessories
The items which are used for increasing the boiler efficiency are called accessories.
Ex. Super heaters, Steam separators etc.

BOILER MOUNTINGS
Important boiler mountings are as follows,
 Pressure gauge
 Safety valves
 Fusible plug
 Steam stop valve
 Feed check valve
 Blow off cock
 Mud and man holes
 Water level Indicator

PRESSURE GAUGE
 Fitted in front of the boiler to record the steam pressure at which steam is generated in the
boiler.
 Two types of pressure gauges are being used in boiler operations.
Diaphragm type
Bourdon Tube
A bourdon pressure gauge in its simplest form consists of a simple elastic tube, one end of
the tube is fixed and connected to the steam space in the boiler, other end is connected to a
sector through a link.

SAFETY VALVE
Safety valves are needed to blow off the steam when the pressure of the steam in the boiler
exceeds the working pressure. It is placed on the top of the boiler.
 Dead weight safety valve
 Lever safety valve
 Spring loaded safety valve
 High steam and low water safety valve
Dead weight safety valve
Lever safety valve

Spring loaded safety valve High steam and low water safety valve

FUSIBLE PLUG
To extinguish fire in the event of
water level in the boiler shell falling
below a certain specified limit. It is
installed below boiler’s water level
on the crown plate.
Fusible Plug

STEAM STOP VALVE
• A valve is a device that
regulates the flow of a
fluid (gases , fluidized
solids slurries or
liquids) by opening or
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
• To allow the feed
water to pass in to
the boiler
• To prevent the
back flow of water
from the boiler in
the event of the
failure of the feed
pump

BLOW OFF COCK
• To drain out water from the boiler for internal
cleaning inspection or other purposes

WATER LEVEL INDICATOR
A : End plate of boiler H & J
: Two balls
B & C : Hollow gun metal casting K
: Drain cock
D & E : Cocks L
: Guard glass
F : Gauge glass M,N,P&
R: Screwed caps
G : Hollow metal column X,Y
: Flanges
The function of water level indicator is to
show level of water present in the boiler.

BOILER ACCESSORIES
Accessories are the devices being used to increase the efficiency of the boiler. A large amount
of heat is being carried out by the flue gases, this is wastage of useful energy, which can be
recovered. Accessories are those equipment which recovers the wastage along with
smoothing the operation to increase the utilization of energy as well as reducing the cost of
operation. The waste recovery takes place by the help of flue gases, which has a large amount
of heat.
Accessories are not the mandatory parts or devices but being used for efficient operation.
Commonly used accessories are as follows,
• Steam Super heater
• Economizer
• Air preheater
• Steam separator
• Feed Pump
• Injector

STEAM SUPER HEATER
The function of a super heater is to increase the temperature of steam above its saturation
point. That means it gives assurance of the quality of steam. During superheating pressure of
steam remains same but the volume increases with its temperature, increasing the internal
energy which in turns prove to increase in kinetic energy, resulting in:
• Reduction of steam consumption of
turbine.
• Reduction in losses due to condensation
in steam pipes.
• Elimination of erosion of turbine blades
• Increase in efficiency.
There are two types of super heaters.
1. Convective Super heater
2. Radiant Super heater

ECONOMIZER
In best way it is known as feed water heater, that refers heating of feed water, which is
supplied to the boiler shell to get vaporized. It utilizes heat carried out but the waste furnace
gases to heat the water before it enters boiler. By increasing the temperature of water, chilling
of the boiler surface is prevented and then a less amount of sensible heat is required to
achieve saturation temperature, it reduces then the input heat to the boiler and increasing
efficiency.
There are two types of economizer
1. Independent type (not a part of boiler)
2. Integrated type (a part of boiler)

AIR PREHEATER
The function of an air preheater is to heat the inlet air before it is sent to the furnace. It is
placed after economizer, flue gases coming from economizer is being utilized to heat air.
Preheated air accelerates combustion and increasing the amount of heat produced.
Degree of preheating depends upon
1. Type of fuel
2. Type of fuel burning equipment
3. Rating of the boiler and furnace
Two types of preheaters commonly used,
1. Recuperative type (Both the fluids pass simultaneously)
2. Regenerative type (Fluids pass alternatively)

STEAM SEPARATOR
The basic work of steam separator is to ensure the quality of steam, steam from the boiler
may be in the form of wet steam, or incase of regenerative cycle, where condensate from
turbine is supplied back or being used by smaller capacity turbines to recover heat, steam
must be in the wet format. Steam separator removes water particles.
There are three types of steam separator
1. Impact or baffle type
2. Reverse current type
3. Centrifugal type
Baffle type steam separator

FEED PUMP
Feed pumps is the device required to supply water to the boiler. The quantity of feed water
should be at least equal to the amount of steam delivered to the turbine or required space. For
open cycle boiler in case of large plants, where there is no condenser or the amount of
feedback water is less, pumps are inevitable.
There are two types of feed pumps,
1. Reciprocating feed pump (Piston cylinder arrangement)
Single acting
Double acting
2. Rotary or centrifugal feed pump
Duplex reciprocating pump

INJECTOR
The basic work of an injector is to feed water to the
boiler on high pressure, it finds its application in such
places where there is no space to install feed pumps. It
works by the help of steam pressure in a way that the
pressure of steam is being utilised to increase the kinetic
energy of feed water.
Advantages
1. Low initial cost
2. Simplicity
3. Compactness
4. No dynamic parts
5. High thermal efficiency
Disadvantages
1. Low pumping efficiency
2. Can’t work for very hot steam
3. Irregularity in the operation when steam pressure
varies considerably
Injector

STUDY OF BOILER
• Fire tube boiler (Cochran boiler)
• Water tube boiler (Babcock and Wilcox boiler)
FIRE TUBE BOILER (COCHRAN BOILER)
Introduction
It’s a multi tube internally fired, fire tube boiler, as an
improvement of traditional vertical boiler, providing
more heating space relatively.
Construction
It consists of a vertical cylindrical shell with a
hemispherical top and hemispherical shaped furnace.
Furnace has ash pit at the bottom end above which fire
grate lies. Combustion chamber of the furnace widens to
connect the flue passage pipes, lined with fir bricks and
insulated with refractory materials on the shell side.
Smoke box is fitted in the hinged door for easy access to
tubes, and allow easy cleaning.
A number of horizontal fire tubes of equal lengths and
diameter connects the combustion chamber with smoke
box. Furnace is surrounded by water on all the side
except the opening for fire door and combustion
chamber.

WATER TUBE BOILER
(BABCOK & WILCOX BOILER)
This is a type of water tube boiler used
when pressure exceeds 10 bar and
capacity 7000 kg per hour.
Construction
It consists of a horizontal high pressure
drum, from each end of it connections are
made with an uptake and a down take
header. Headers are joined to each other
by large number of water tubes inclined at
an angle of 15 degree to provide water
circulation. Hand holes are provided for
the maintenance of tubes.
The entire assembly of water tubes is hung in a room made of masonry work, lined with fire
bricks to resist thermal expansion. As super heater additional U-shaped tubes are arranged
between drums and water tubes.
Furnace is arranged below the uptake header. Baffles are provided across the water tubes to
guide flue gases. A Chimney is provided for exit of gases and a damper at the opening of
chimney to provide draught. Cleaning doors are provided to access the tubes for cleaning and
removal of soot and various mountings for successful operation.

Advantage of Babcock and Wilcox boiler
 Suitable for all types of fuels and hand stokers for firing.
 Draught loss is small.
 All components are accessible for inspection during operation.
 Expansion and contraction has no harm on masonry work(construction).
 Replacement of defective tubes are easy.
Applications of Cochran boiler and Babcock and Wilcox boiler
 Cochran boiler finds its application in cranes, excavators and in small factories for general
contract work where temporary supply of steam is required.
 Babcock and Wilcox boiler finds its application in sugar mills and textile industries for
power generations and processing works.

COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS
Particulars Fire-tube boilers Water-tube boilers
Position of water and hot
gases
Hot gases inside the tubes
and water outside the tubes
Water inside the tubes and
hot gases outside the tubes
Mode of firing Generally internally fired Externally fired
Operation pressure Limited to 16 bar Can go up to 100 bar
Rate of steam production Lower Higher

Suitability
Not suitable for large power
plants
Suitable for large power
plants
Risk on bursting
Involves lesser risk of
explosion due to lower
pressure
More risk on bursting due to
high pressure
Floor area
For a given power it
occupies more floor area
For a given power it
occupies less floor area
Construction Difficult Simple

Transportation Difficult Simple
Shell diameter Large for same power Small for same power
Chances of explosion Less More
Treatment of water Not so necessary More necessary
Accessibility of various
parts
Various parts not so easily
accessible for cleaning,
repair and inspection
More accessible

Basics of steam boilers section b

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