boilers lectuer

BOILERSBOILERS

PREPARED FOR THE 3PREPARED FOR THE 3RDRD SEMESTER STUDENTSSEMESTER STUDENTS

MAEC. MECHANICALMAEC. MECHANICAL 11

Steam GeneratorsSteam Generators

• Generates steam at desired rate, desired pressure, and temperature by burning fuel

• It is a complex integration of furnace, super-heater, reheater , evaporator, economizer, air pre-heater, along with various auxiliaries such as pulverizes, burners, stokers, dust collectors, ash-handling equipment, chimney or stack

• Boiler is part of steam generator where phase change occurs from liquid to vapor at constant pressure and temperature

22MAEC. MECHANICALMAEC. MECHANICAL

Industrial Boilers

• Steam generators• Closed vessels• made of steel and • used for generation of steam• vaporizing water• combustion of fuels• eg. coal,coke,oil,wood,saw dust and begesse etc.• Steam used for producing power,for industrial process

work or for heating proposes.


Requirements of a Perfect Requirements of a Perfect Steam BoilersSteam Boilers• Should be absolutely reliable, capable of producing maximum

steam at required pressure for minimum of fuel consumption with minimum attention and minimum initial cost and operating cost.

• Simple constructions• Quick starting from cold• Strong enough against temp. stress & strains• Well design combustion chamber• Less floor area, space & light weight• Best gauges, safety valves and other mountings• Easily accessible for inspection, cleaning and repairs


ClassificationClassification of Steam Boilers of Steam Boilers

Horizontal and Vertical boiler

Fire tube and water tube Boiler

Externally and Internally fired boiler

Forced and Natural Circulation boiler

High, Medium and Low pressure boiler

Stationary and Portable boiler

Single and Multi tube boiler


VERTICAL BOILERVERTICAL BOILER

If the axis of the boiler is vertical it is called vertical boiler. e.g.:- Cochran boiler 66MAEC. MECHANICALMAEC. MECHANICAL

HORIZONTAL BOILER

If the axis of the boiler is horizontal ,It is called the Horizontal boiler. e.g. :- Lancashire boiler ,Locomotive boiler.


FIRE TUBE BOILERFIRE TUBE BOILER

The boiler in which the hot gases are inside the tubes and water surrounding the tubes is called fire tube boiler.e.g. : Lancashire boiler, Locomotive boiler.


WATER TUBE BOILER

The boiler in which the water is inside the tube and the hot gases surround them is called water tube boiler.

e.g.:-Babcock and Wilcox Boiler.


Externally fired and Internally fired tube Boilers.


DIFFERENCE

Externally fired boiler:

In the boiler if the fired is outside the shell, that boiler is known as externally fire boiler.

e.g.: Babcock and Wilcox boiler.

Internally fired boiler:

In the boiler in which the furnace is located inside the boiler shell it is known as internally fired boiler.

e.g.: Cochran boiler, Lancashire boiler.

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DIFFERENCE IN CIRCULATION

Forced circulation boiler:-

In the boilers if the circulation of water is done by a pump then they are known as forced circulation boiler.

e.g.:-Bension boiler, Lamount boiler

Natural circulation boiler:-

In the boiler if the circulation of water takes place due to diff. in density resulting from diff. in temperature, it is known as natural circulation boiler.

e.g.:-Lancashire boiler12MAEC. MECHANICAL

High, Medium and Low pressure Boilers. High pressure steam boiler:- It is one in which the working pressure of the boiler is

more than 25 bar. e.g.:-Babcock and Wilcox boiler.

Medium pressure boiler:- It is that boiler whose working pressure is between 10-25 bar. e.g.:-Lancashire boiler and Locomotive boiler.

Low pressure steam boiler:- It is that boiler whose working pressure is between 3.5-10

bar. e.g.:-Cochran boiler.

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Stationary and portable boilersStationary and portable boilers

Stationary boilers:-The boiler which can

not be transported easily from one place to another are called stationary boilers.

e.g.:-Lancashire boiler, Babcock and Wilcox boiler.

Portable boilers:-Portable boilers:-The boilers which can The boilers which can

be easily transported be easily transported from one place to from one place to another are called another are called portable boilers.portable boilers.

e.g.:-e.g.:-Locomotive Locomotive boiler.boiler.


Single and Multi tube Boiler

Single tube boilers:-The boilers having

only one fired tube or water tube are called single tube boilers.

e.g.:-Cornish boiler.

Multi tube Boilers:-The boilers having two

or more fire or water tubes for the circulation of hot gases or water are called multi tube boiler.

e.g.:-Lancashire boiler,Babcock and Wilcox boiler.

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Cochran Boiler It is well design of a

Vertical multitubular fire tube boiler

Improvement over the simple vertical boiler as it provides greater heating surface.

Total heating surface area is about 10-25 times the grate area

Efficiency 70-75% Ranges from 1m

dia.X2m high, evaporation 20kg/hr to 3m dia X6m high, evaporation 3000kg/hr.


Cochran Boiler

• Consists of an external cylindrical shell and a fire box• fuel is fed into the grate through the fuel door and lighted• fuel is burnt on the grate and hot gases go to the combustion chamber through a short flue tube• hot gases pass through fire tubes and heat the surrounding water and convert it into steam• Since the steam is lighter, it goes up the steam space as crown of the boiler and the grate are both hemispherical in shape• waste gases enter the smoke box and are released through the chimney• During shut down , the boiler attendant can enter the boiler through the man hole


Lancashire BoilerLancashire Boiler


Lancashire Boiler contd……


Lancashire Boiler• Is horizontal smoke tube boiler size range from a

shell 2m diaX6m long to 3m diaX10m long

• Working pressure range are up to 20kgf/cm2

• Ratio of heating surface to grate area is 24-30

• Efficiency is about 56% without economizer and 75% with economizer.

• Similar to Cornish boiler, in Lancashire Boiler two flue tubes but in Cornish boiler only one flue tubes are there.


Locomotive Boiler


Locomotive Boiler

• Horizontal• Multi-tubular• Natural circulation• Internally fired• Fire tube• Portable

• Mostly used for railways• Designed to capable of meeting

sudden and fluctuating demands of steam

• Fuel is fed into the firebox through the fuel door

• Air enters through the damper and the slots in the grate plate

• The hot gases pass through large fire tubes and enter the smoke box

• Circulation of air and hot gases is improved by means of induced draft produced in the smoke box


Babcock & Wilcox boiler


Babcock & Wilcox boiler

Water tube boiler having lager pressure ranges and larger sizes.

Three main parts, steam and water drum , water tubes and furnace


• Horizontal, externally fired, water tube, natural circulation type of stationary boiler• It consists of a welded steel high pressure drum mounted at the top• From each end of the drum connections are made with the uptake header and down take header• The headers are joined to each other by a large number of water tubes which are kept inclined at an angle of about 150 to the horizontal• The water tubes are straight, solid drawn steel tubes about 10 cm in diameter and are expanded into the bored holes o f the headers• The heating surface of the unit, forms the outer surface of the tubes and half of the cylindrical surface of the water drum which is exposed to flue gases• The furnace is arranged below the uptake header• The coal is fed to the chain gate stoker through the fire door• Baffles are provided across the water tubes to act as deflectors to the flue gases and to provide them with gas passes• The circulation of water is maintained by convective currents


Stirling three drum boilerStirling three drum boiler


Stirling Boiler

Example of water tube boiler where bent tube are used instead of straight tube which are used in Babcock and Wilcox boiler


Fitting and devices which are necessary for the safety and control are knows as boiler mountings


• • A pressure gauge is an instrument by means of which theA pressure gauge is an instrument by means of which thepressure exerted inside a vessel can be measured.pressure exerted inside a vessel can be measured.• There are two types of pressure gauges, one is • There are two types of pressure gauges, one is Bourdon tubeBourdon tubepressure gauge and the other is diaphragm type gauge.pressure gauge and the other is diaphragm type gauge.


Fitting or devices which are provided to increase the efficiency of the boiler and help in the smooth working of the plant are knows as boiler accessories.


Air PreheaterAir Preheater• • Used to recover heat from the exhaust Used to recover heat from the exhaust

flue gasesflue gases

• • Installed in between the economizer and Installed in between the economizer and chimney.chimney.

• • Temperature of the required air for Temperature of the required air for combustion is passed through the air combustion is passed through the air preheater where its temperature is preheater where its temperature is raised.raised.

• • Then hot air passed to the furnace.Then hot air passed to the furnace.

ADVANTAGESADVANTAGES

• • Gives higher furnace temperature which Gives higher furnace temperature which results more heat transfer thus increase results more heat transfer thus increase evaporative capacityevaporative capacity

• • Approximately boiler efficiency increases Approximately boiler efficiency increases about 2% for each 30-35oC rise in air about 2% for each 30-35oC rise in air temperature.temperature.


Main Features of Modern High Pressure boilersMain Features of Modern High Pressure boilers

• Method of water circulationMethod of water circulation– Natural circulationNatural circulation– Forced circulationForced circulation

• Type of tubingType of tubing– Friction is there in large length tubesFriction is there in large length tubes– Sets of tubes parallel to each other are usedSets of tubes parallel to each other are used– Give better control over steam qualityGive better control over steam quality

• Improved methods of heatingImproved methods of heating– Heating water by using super-heated steamHeating water by using super-heated steam– Evaporating water above critical pressure( saves Evaporating water above critical pressure( saves

latent heat)latent heat)– Increasing the velocity to sonic to improve heat Increasing the velocity to sonic to improve heat

transfer rate thorough tube wallstransfer rate thorough tube walls– Recovering waste heat by using economizer and air Recovering waste heat by using economizer and air

preheaterspreheaters4040MAEC. MECHANICALMAEC. MECHANICAL

Modern High Pressure boilers Once through Modern High Pressure boilers Once through BoilersBoilers

• Suitable for Supercritical pressure operation as there Suitable for Supercritical pressure operation as there is no latent heat of evaporation is requiredis no latent heat of evaporation is required

• Water flashes to vapor in case of super critical boilers Water flashes to vapor in case of super critical boilers directlydirectly

• Flashed high pressure steam is heated to desired Flashed high pressure steam is heated to desired super heated temperatures in super heaterssuper heated temperatures in super heaters

• Water enters the boilers at one end of a number of Water enters the boilers at one end of a number of parallel paths and emerges at the outlet as parallel paths and emerges at the outlet as superheated steam superheated steam

• Movement of the water is forced by the boiler feed Movement of the water is forced by the boiler feed pump pump

• The design is well suited to quick starts and rapid load The design is well suited to quick starts and rapid load changes changes


ADVANTAGESADVANTAGES• The tendency The tendency of of scale formation is eliminatedscale formation is eliminated

• Light weight tubes with better heating surface Light weight tubes with better heating surface arrangement can be used & less spacearrangement can be used & less space

• Use of forced circulation, there is more freedom in the Use of forced circulation, there is more freedom in the arrangement of furnace, tubes and boiler componentsarrangement of furnace, tubes and boiler components

• All the parts are uniformly heated reducing the thermal All the parts are uniformly heated reducing the thermal stress problemsstress problems

• The steam can be raised quickly The steam can be raised quickly to to meet the variable meet the variable load load

• The efficiency of plant is increased upto 40 to 42% The efficiency of plant is increased upto 40 to 42%

• A very rapid start from cold is possible A very rapid start from cold is possible


LA MONT BOILERLA MONT BOILER


• Introduced in 1925 by La Mont Introduced in 1925 by La Mont

• The feed water from hot well is supplied to a storage The feed water from hot well is supplied to a storage and separating drum (boiler) through the economizer and separating drum (boiler) through the economizer

• The most of the sensible heat is supplied to the feed The most of the sensible heat is supplied to the feed water passing through the economizer water passing through the economizer

• A centrifugal pump circulates the water equal to 8 to A centrifugal pump circulates the water equal to 8 to 10 times the weight of steam evaporated 10 times the weight of steam evaporated

• The distribution headers distribute the water through The distribution headers distribute the water through the nozzles into the evaporator the nozzles into the evaporator

• The steam separated in the boiler is further passed The steam separated in the boiler is further passed through the super-heater through the super-heater

• These boilers have been built to generate 45 to 50 These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar tons of superheated steam at a pressure of 120 bar and at a temperature of 500°C and at a temperature of 500°C


Draw backsDraw backs

• Formation and attachment of bubbles on the Formation and attachment of bubbles on the inner surfaces of the heating tubes inner surfaces of the heating tubes

• Attached bubbles to the tube surfaces reduced Attached bubbles to the tube surfaces reduced the heat flow and steam generation as it offers the heat flow and steam generation as it offers high thermal resistance than water high thermal resistance than water

• Sediment layer formation in the evaporatorsSediment layer formation in the evaporators

• Scaling of super heatersScaling of super heaters


BENSON BOILERBENSON BOILER


• Benson in 1922 raised boiler pressure to critical Benson in 1922 raised boiler pressure to critical pressure (225 bar)pressure (225 bar)

• The steam and water have the same density and The steam and water have the same density and therefore the danger of bubble formation can be easily therefore the danger of bubble formation can be easily eliminatedeliminated

• The water as passed through the economizer into the The water as passed through the economizer into the radiant evaporator (Majority of water converted into radiant evaporator (Majority of water converted into steam)steam)

• The remaining water is evaporated in the final The remaining water is evaporated in the final evaporator absorbing the heat from hot gases by evaporator absorbing the heat from hot gases by convection convection

• The saturated high-pressure steam (at 225 bar) is The saturated high-pressure steam (at 225 bar) is further passed through the super-heater further passed through the super-heater

• The maximum working pressure obtained in Benson The maximum working pressure obtained in Benson boiler is 500 barboiler is 500 bar

• The Benson boilers of 150 tones/hr generating The Benson boilers of 150 tones/hr generating capacity are in use capacity are in use


Advantages1.1.The total weight of Benson boiler is 20% less The total weight of Benson boiler is 20% less

than other boilers ( No Drum), reduces the cost than other boilers ( No Drum), reduces the cost of boilerof boiler

2.2.The erection of Benson boiler is easier and The erection of Benson boiler is easier and quicker and in smaller floor areaquicker and in smaller floor area

3.3.The furnace walls of the boiler can be more The furnace walls of the boiler can be more efficiently protected by using smaller diameter efficiently protected by using smaller diameter and closed pitched tubesand closed pitched tubes

4.4.The super-heater in the Benson boiler is an The super-heater in the Benson boiler is an integral part of forced circulation systemintegral part of forced circulation system

5.5.The Benson boiler can be started very quicklyThe Benson boiler can be started very quickly


8.8. The Benson boiler can be operated most The Benson boiler can be operated most economically by varying the temperature economically by varying the temperature and pressure at partial loads and over and pressure at partial loads and over loadsloads

9.9. Insensitiveness to load fluctuations Insensitiveness to load fluctuations makes it more suitable for grid power makes it more suitable for grid power station as it has better adaptive capacity station as it has better adaptive capacity to meet sudden load fluctuationsto meet sudden load fluctuations

10.10.The blow-down losses of Benson boiler The blow-down losses of Benson boiler are hardly 4% of natural circulation are hardly 4% of natural circulation boilers of same capacity.boilers of same capacity.

11.11.Explosion hazards are not that severe as Explosion hazards are not that severe as it consists of only tubes of small diameterit consists of only tubes of small diameter


Schmidt Hartman BoilerSchmidt Hartman Boiler


• Consists of two circuits– Primary ( Closed )– Secondary

• Primary circuit is closed one and having high pressure with distilled water

• High pressure steam of distilled water heats evaporator or steam generator content which is to be supplied to the prime mover via super heater

• High pressure condensate on its way back heats the low pressure feed heater

• Natural circulation is used in primary circuit5151MAEC. MECHANICALMAEC. MECHANICAL

AdvantagesAdvantages

• There is a rare chance of overheating

• Wide fluctuations of load can be catered

• Cleaning or maintenance is easy


Velox boilerVelox boiler


• When gas velocity exceeds the sound velocity the When gas velocity exceeds the sound velocity the heat is transferred from the gas at a much higher rate heat is transferred from the gas at a much higher rate than rates achieved with sub sonic flow than rates achieved with sub sonic flow

• Air has to be compressed to 2.5 bar to get supersonic Air has to be compressed to 2.5 bar to get supersonic velocity in burnersvelocity in burners

• Burnt gases pass through the initially in the annulus of Burnt gases pass through the initially in the annulus of the tubes as shown in diagramthe tubes as shown in diagram

• The mixture of water and steam formed sent to The mixture of water and steam formed sent to separator where the steam is separated from waterseparator where the steam is separated from water

• Separated steam is further sent to super-heater where Separated steam is further sent to super-heater where the flue gas coming out of the annulus tubes heat it to the flue gas coming out of the annulus tubes heat it to higher temperatureshigher temperatures

• The flue gas coming out of the super heater are used The flue gas coming out of the super heater are used to run the gas turbine which operates compressorto run the gas turbine which operates compressor

• Again flue gas of the gas turbine are used to heat the Again flue gas of the gas turbine are used to heat the economizer water and exhausted to chimney economizer water and exhausted to chimney


AdvantagesAdvantages

• High heat release rate 8-10 million Kcal / High heat release rate 8-10 million Kcal / m3 of combustion chamberm3 of combustion chamber

• Low excess air and draughtLow excess air and draught

• Compact and flexible unitCompact and flexible unit

• Can be started quicklyCan be started quickly


Supercharged boilerSupercharged boiler

• Combustion is carried out under pressure in Combustion is carried out under pressure in the combustion chamber the combustion chamber

• Supplying the compressed air (Minimum of Supplying the compressed air (Minimum of 5 bar )5 bar )

• The exhaust gasses from the combustion The exhaust gasses from the combustion chamber are used to run the gas turbinechamber are used to run the gas turbine

• The gas turbine runs the air compressor to The gas turbine runs the air compressor to supply the compressed air to the supply the compressed air to the combustion chamber combustion chamber


Boiler Performance

• As the function of a boiler is to generate steam, so the amount of water evaporated may be considered as a performance of the boiler.• The quantity of steam which is actually produced in boiler at observed conditions is known as an actual evaporation.• The observed conditions are the pressure, quality of steam and temperature of feed water.• As the steam is usually generated in a boiler at constant pressure, so heat supplied by the fuel is equal to the enthalpy absorbed by the steam.

• m s be the mass of steam formed actually in a given time in kg (or sometimes per kg of fuel), then

Q=m s (h2—h1)

where Q denotes the amount of heat utilized and h2—h1, denotes theamount of enthalpy absorbed per kg of steam.


6060MAECMAEC. . MECHANICALMECHANICAL

boilers lectuer

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