Boilers(Steam Generators)

Definition of a boiler• A boiler is a device used to generate

steam at a desired pressure and temperature by transferring heat energy produced by burning fuel to water to change it to steam.

• It is a combination of apparatus used for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vaporized.

• The fluid is contained in the boiler drum called shell and the thermal energy released during combustion of fuel is transferred to water and this converts water into steam at the desired temperature and pressure.

Function of a boiler

Applications of boilersPower generation: Mechanical or electrical power

may be generated by expanding steam in the steam engine or steam turbine.

Heating: The steam can be used for heating residential and industrial buildings in cold weather and for producing hot waters for hot water supply.

Industrial processes: Steam can also be used for industrial processes such as for sizing and bleaching etc. in textile industries and other applications like sugar mills, cement, agricultural and chemical industries.

Factors to be considered for selection of good boiler

1) The working pressure and quality of steam required2) Steam generation rate3) Floor area available4) Accessibility for repair and inspection5) Comparative initial cost6) Erection facilities7) The portable load factor8) The fuel and water available9) Operating and maintenance costs

Requirements of an efficient boiler1. The boiler should generate maximum amount of steam at a required pressure and

temperature and quality with minimum fuel consumption and expenses2. Steam production rate should be as per requirements3. It should be absolutely reliable 4. It should be light in weight5. It should not occupy large space.6. It should be capable of quick starting7. It should conform to safety regulations.8. The boiler components should be transportable without difficulty9. The installation of the boiler should be simple10. It should have low initial cost, installation cost and maintenance cost.11. It should be able to cope with fluctuating demands of steam supply.12. All parts and components should be easily accessible for inspection, repair and

replacement.13. The tubes of the boiler should not accumulate soot or water deposits and should be

sufficiently strong to allow for wear and corrosion14. The water and gas circuits should be such as to allow minimum fluid velocity (for

low frictional losses)

Classification of boilersRelative position of hot gases and water

•Fire tube boilers (Cochran, Lancashire, Cornish, Locomotive)•Water tube boilers (Babcock and Wilcox boiler, Stirling boiler)

Method of firing•Internally fired boilers ( Lancashire, Locomotive)•Externally fired boilers (Babcock and Wilcox boiler)

Pressure of steam•High pressure boilers(>80 bars-Cochran,Lancashire,Cornish, Locomotive)•Low pressure boilers (<=80 bars-Babcock and Wilcox boiler, Lamont

boiler)Method of circulation of water

•Natural circulation boilers (Lancashire, Locomotive, Babcock & Wilcox boilers)

•Forced circulation boilers (Two large fire tubes Lancashire boiler, Single large fire tube Cornish boiler, Cochran boiler, Many small tubes Locomotive boiler, Babcock Wilcox water tube boiler)

Nature of service to be performed•Land boilers•Mobile boilers (or) Portable boilers

Once through boilers Position and number of drums

• Single drum boilers• Multi-drum boilers(Longitudinal or crosswise)

Design of gas passages• Single pass boilers• Return pass boilers• Multi-pass boilers

Nature of draught• Natural draught boilers• Artificial draught boilers

Heat source• Combustion of solid, liquid or gaseous fuels• Electrical and nuclear energy• Hot waste gases of other chemical reactions

Fluid used• Steam boilers• Mercury boilers• Special boilers for heating special chemicals

Material of construction of boiler shell• Cast iron boilers• Steel boilers

Particulars Fire tube boiler Water tube boilerPosition of water and hot gases

Hot gases inside the tubes and water outside the tube

Water inside the tube and hot gases outside the tubes

Mode of firing Generally internally fired Externally firedOperating pressure

Operating pressure limited to 16 bar

Can work under as high pressures as 100 bar

Rate of steam consumption Lower Higher

Suitability for large power plants

Not suitable Suitable

Risk on bursting/explosion

Involves lesser risk on explosion due to lower pressure

Involves more risk on bursting due to high pressure

Floor area For a given power, occupies more

For a given power,occupies less

Differences between Water-tube and Fire-tube boilers

Particulars Fire tube boiler Water tube boilerConstruction Difficult SimpleTransportation Difficult SimpleShell diameter Large for same power Small for same powerChances of explosion Less More

Treatment of water Not so necessary More necessary

Accessibility of various parts

Various parts are not so easily accessible for cleaning, repair and inspection

Various parts are more accessible

Requirement of skill

Require less skill for efficient and economic working

Require more skill and careful attention

Differences between Water-tube and Fire-tube boilersContd…..

COCHRAN BOILERFeatures of Cochran boiler:1)Vertical 2)Multi-tubular3)Internally fired4)Natural circulation5)Fire tube boiler6)Up to maximum steam pressure of 15 bar 7)Maximum evaporative capacity of 4000 kg of steam per hour.

Construction of COCHRAN BOILERCochran 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 BOILERThe Cochran boiler is filled with water to the specified level and maintained

at that level by charging with makeup water using a feed water pump and when the water level drops below its specified level. The entire surface of the furnace except the openings for the fire door and the combustion chamber will be surrounded by water. The flue tubes will also be completely submerged in the water.

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. The steam stop valve allows the steam from the boiler to the steam supply pipe.

The Cochran boiler is mounted with the essential mountings and accessories like steam stop valve, safety valve, pressure gauge, water level indicator, fusible plug, blow off valve, feed check valve. The working pressure and steam capacity of Cochran boiler are 6.5 bar and 3500 kg /hr respectively.

Advantages1)Cochran Boiler occupies less floor space.2)Construction cost of Cochran Boiler is Low.3)Cochran boiler is semi-portable and hence easy to install and transport.4)Because of self contained furnace no brick work setting is necessary.Disadvantages1)The capacity of the Cochran boiler is less because of the vertical design.2)Cochran Boiler requires high head room space.3)Because of the vertical design, it often presents difficulty in cleaning and inspection.

Advantages & Disadvantages of COCHRAN BOILER

BABCOCK & WILCOX BOILERFeatures of Babcock & Wilcox boiler:1)Horizontal, Straight & Stationary2)Externally fired3)Natural circulation4)Water tube boiler5)Minimum steam pressure of 10 bar 6)Minimum evaporative capacity of 7000 kg of steam per hour.

Construction of BABCOCK & WILCOX BOILERBabcock and Wilcox boiler is a horizontal, externally fired, natural circulation, stationary, and water tube

boiler. The Babcock and Wilcox water tube boiler consists mainly four parts such as water and steam drum, water tubes, chain grate stoker, superheater tubes.

The water and steam drum is suspended from iron girders resting on the iron columns, and is independent of the brick work setting. This arrangement prevents unequal expansion troubles and facilitates repair of the brick work. A number of inclined water tubes at a very low inclination are connected at right angles to the end boxes called headers. The water tubes will be arranged in a number of vertical rows, each row consisting of 40 to 5 tubes. In each vertical row the tubes will be arranged one below the other in a serpentine form. There will be a number of such vertical rows one behind the other. Each one such vertical row of inclined water tubes are connected to one set of two headers. The header at the right end of the water tubes is called down take header and the other at the left end of the water tubes is called uptake header. Each of the vertical rows of water tubes which are arranged one behind the other are connected to one set of headers which are also arranged one behind the other. Each set of the headers are inturn connected to the boiler drum by one set of two tubes, on eat the uptake end and the other at the downtake end. A mud box is provided just below the downtake header. Any sediment in the water, due to its heavier specific gravity will settle down in the mud box and is blown off from time to time through the blow off pipe.

The grate is provided at the front end below the uptake header. The boilers of higher capacity are usually provided with a chain grate stoker, which consists of a slowly moving endless chain of grate bars. The coal fed on at the front end of the grate is burnt on the moving grate in the furnace and the residual ash falls at the outer end of the grate into the ash pit. The boiler is fitted with a superheater. The superheater consists of number of U-tubes secured at each end to the horizontal connecting boxes and placed in the combustion chamber below the boiler drum. The upper box of the superheater tube is connected to a T-tube, the upper branches of the T-tube being situated in the steam space in the drum. The lower box of the superheater tubes is connected to the steam stop valve mounted over the drum through a vertical tube passing outside the drum.

Working of BABCOCK & WILCOX BOILERThe water is introduced into the boiler drum through a feed valve. A constant water level is

maintained in the boiler drum. The water descends at the rear end into the downtake headers and passes up in the inclined water tubes, uptake headers and in the tubes connecting the uptake header and the drum. Thus a circuit is established between the drum and the water tubes for the flow of water.

The hot gases from the furnace grate are compelled by the baffle plate to pass upwards around the water tubes lying in between the combustion chamber under the water drum, then downwards around the water tubes in between the baffle plates, then once again upwards between the baffle plate and the downtake header, and finally passes out of the boiler through the exit door and the chimney.

During this path of the hot gases, the hottest gases emerging directly from the grate come in contact with the hottest portions of the water tubes. The water in these portions of the water tubes gets evaporated. The water and the steam mixture from this portion of the water tubes ascend through the uptake headers and reach the boiler drum.

The steam from the steam space in the boiler drum is led into the branches of T-tube, and then it passes into the upper connecting box of the superheater, then through its U-tubes. Since the superheater tubes are fitted in the combustion chamber and exposed to the hot gases, the steam passing in it will be superheated. The superheated steam from the superheater tubes are passed to the steam stop valve through the lower connecting box and the vertical tube fitted outside the drum. From the steam stop valve the superheated steam is passed to the prime-mover. When the superheated steam is not required the steam from the steam space directly passes out to the prime-mover through the steam stop valve.

BOILER MOUNTINGS1) Pressure gauge2) Fusible plug3) Steam stop valve4) Feed check valve5) Blow off cock6) Man and mud(sight)holes7) Two safety valves8) Two water level Indicators

Man and mud(sight)holes

These are used to allow men to enter inside the boiler for inspection and repair.

Two safety valves

The commonly used safety valves are:1)Dead weight safety valve2)Lever safety valve3)Spring loaded safety valve4)High steam and low water safety valve

Loading arrangement for Lever Safety Valve

VFapVFWGFWAFW vl Taking moments about the fulcrum F, we get

Where2

4da

BOILER ACCESSORIES1) Economiser2) Air Preheater3) Superheater4) Feed Pump5) Steam Separator6) Steam Trap

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.