Boiler-Definition, Fuel used, Types or Classification in details, Parts, Boiler Mounting, Accessories, Function, Application [PDF]


The history of boilers began with steam boilers that were used for power transportation. Early models like the “Scotch Marine” and similar early fire tube boilers were made from a steel shell with rounded tube sheets that were welded at both ends. The boiler is like a closed vessel from which steam is produced using several methods. They are generally classified into two main types:

  • Fire tube: Water surrounded by a tube and Hot gas flowing in the tube and
  • Water tube: Hot gases surrounded by tube and Water flowing in the tube.

Boiler Meaning

This is a closed vessel in which fluid (generally water) is heated up to high-pressure steam. The boilers are used in power plants, for the generation of steam. They are mostly used in power plants where steam turbines are used for the generation of electricity.

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Fuel applications

The fuel used in a boiler can be classified into the following 3 types

  • Solid fuels : Wood, Coal, Briquettes (a block of compressed coal dust ), Pet Coke, Rice Husk.
  • Liquid Fuels : LDO (Light Diesel Oil), Furnace oil.
  • Gaseous Fuels : LPG (Liquified Petroleum Gas), LNG (Liquified Natural Gas), PNG (Piped Natural Gas) can be used to carry out the combustion for a specific purpose.

Read More: Locomotive Boilers

Classification of Boilers

Boilers are classified into various types based on a number of factors which are as follows:

The Position of water and hot gasses

  • Fire Tube
  • Water Tube

Axis of Shell

  • Horizontal
  • Vertical

The position of the boiler

  • External Fired
  • Internally Fired


  • Low-Pressure
  • High-Pressure

Use of boilers

  • Mobile
  • Stationary

Method of circulation

  • Natural Circulation
  • forced Circulation


  • Single drum
  • Multi drum

Nature of drought

  • Forced drought
  • Natural drought

Fuel firing

  • Solid fuel fired
  • Liquid fuel-fired
  • Gaseous fuel-fired

According to furnace

  • Single furnace
  • Dual furnace

Fire tube boiler

Fire tube boilers are those in which fire or hot gas is present inside the tubes and water surrounds these fire tubes. The heat from the hot gases is conducted through the walls of the tube to the water.

Examples of the fire tube boiler

  • Simple vertical boilers
  • Cochran boilers
  • Lancashire boilers
  • Cornish boilers
  • Locomotive boilers
  • Scotch marine boilers
  • Velcon boilers boilers

Read More: Scotch Marine Boilers

Cochran Boiler

This system was made by Cochran & Co. of Annan. It is a multi-tubular vertical fire tube boiler having several horizontal fire tubes. It is the modification of a simple vertical boiler where the heating surface has been increased using several fire tubes. In this, the fire tubes are placed horizontally. The efficiency is much better than the simple vertical boiler.

Main parts of Cochran boiler

Shell: The main body of the boiler is known as a shell. It is hemispherical on the top, where space is provided for steam. This hemispherical top gives a higher volume-to-area ratio which increases the steam capacity.

Grate: The area where the fire is placed is known as a grate. It is placed at the bottom of the furnace where coal is burnt.

Combustion Chamber: It is lined with fire bricks on the side of the shell to prevent overheating of the boiler. Hot gases enter the fire tubes from the flue pipe through the combustion chamber. The combustion chamber is connected to the furnace.

Fire Tubes: There are various fire tubes whose one end is connected to the furnace and the other to the chimney. Several horizontal fire tubes are provided, thereby the heating surface is increased.

Fire Hole: The small hole provided at the bottom of the combustion chamber to place fuel is known as the fire hole.

Fir Box(Furnace): It works as a mediator of fire tubes and combustion chambers. It is a dome-shaped shell so that the gases can be deflected back till they are passed out through the flue pipe to the combustion chamber.

Chimney: It is provided for the exit of flue gases to the atmosphere from the smoke box.

Manhole: It is provided for the inspection and repair of the interior of the boiler shell.

Flue Pipe: It is a short passage connecting the firebox with the combustion chamber.

Working of Cochran Boiler

First, The coal is placed at the grate through the fire hole. Then the air is allowed to enter into the combustion chamber through the atmosphere and fuel is sparked through a fire hole. The flue gases start flowing into the hemispherical dome-shaped combustion chamber. These flue gases further move into the fire pipes. Heat is exchanged from flue gases to the water into the fire tubes.

Cochran Boiler Diagram
Cochran Boiler construction and working
Cochran Boiler parts
Cochran Boiler working
Cochran Boiler mountings
Cochran Boiler applications
Cochran Boiler advantages
Cochran Boiler disadvantages

The steam produced is collected in the upper side of the shell and taken out when the required pressure is generated. The flue gases are now sent to the chimney through the firebox from where it leaves to the atmosphere. This cycle repeats continuously. The steam generated is used in small industrial processes.

Cochran boiler mountings
  • Pressure Gauge: It measures the pressure of steam inside the boiler.
  • Safety Valve: It blows off the extra steam when the steam pressure inside the boiler reaches above the safety level .
  • Water level Indicator: The position of the water level in the Cochran boiler is indicated by the water level indicator.
  • Stop Valve: The stop valve is used to transfer steam to the desired location when it is required. Otherwise, it stops the steam in the boiler.
  • Blow off Valve: It is used to blow off impurities, mud, and sediments present in the boiler water.
  • Variety of process applications in industries
  • Chemical processing divisions
  • Pulp and Paper manufacturing plants
  • Refining units

Besides these applications, they are frequently employed in power generation plants where large quantities of steam (ranging up to 500 kg/s) having high pressures of approximately 16 megapascals (160 bar) and high temperatures reaching up to 550 °C are generally required.


In Cochran, any type of fuel can be used. It is best suitable for small capacity requirements. It gives about 70% thermal efficiency with coal firing and about 75% thermal efficiency with oil firing. The ratio of gratethe area to the heating surface area varies from 10: 1 to 25: 1.

  • Less floor area required.
  • Low initialization cost.
  • It is easy to operate.
  • Transport from one place to another is very easy.
  • It has a high volume to area ratio.
  • Low steam generation rate.
  • Limited pressure handling capacity.
  • It is difficult to inspect and maintain.

Locomotive Boiler

This is a type of fire tube boiler. It is used to create steam from water by using heat energy. This is a horizontal drum axis, multi-tubular, natural circulation, artificial draft, forced circulation, medium pressure, solid fuel-fired fire tube boiler that has an internal fire furnace. It is capable of producing a high steam rate and hence it is used for railway locomotive engines and in marines.

Locomotive boiler
boiler types
boilers engineering
boiler uses

Using a grate we insert the fuel into the boiler and then we provide fire to ignite the fuel. When fuel starts burning, hot flue gases are produced. These hot flue gases are passed through the pipe continuously until the surrounding water gets heated. So the water changes its phase into saturated steam. This steam is further used in the steam engine. (Locomotive Boiler)

Lancashire Boiler

Lancashire Boiler is a horizontal type and stationary fire tube boiler. This boiler was invented in the year 1844, by William Fairbairn. This boiler generates low-pressure steam and it is a natural circulation boiler. It has higha thermal efficiency of about 80 to 90 percent. The size ranges approximately between 7-9 meters in length and 2-3 meters in diameter. It is mostly used in locomotive engines and marines etc.

Working Principle of Lancashire Boiler
Lancashire Boiler
Lancashire Boiler working
water tube boiler

This Lancashire boiler works on the basic principle of the heat exchanger. It is a shell and tube type heat exchanger in which the flue gases flow through the tubes and the water flows through a shell. The heat is transferred from flue gases to the water through convection. It is a natural circulation boiler that uses the natural current to flow the water inside the boiler. Read More: Lancashire boiler

Water tube boiler

A water tube boiler is a boiler in which water is present inside the tubes and fire or hot gases surround these fire tubes. Examples of water tube boilers are La-Mont, Benson, Stirling, Yarrow, and Loeffler boilers.

Read More: Loeffler Boiler

La-Mont Boiler

It works on the principle of forced circulation of water within the boiler with the help of the centrifugal pump.

Working of La-Mont Boiler

Its working depends upon the pump. The centrifugal pump circulates the mixture of steam and water through the small diameter tubes of the boiler. A feed pump forces the water into the economizer where the temperature of the water increases. This water is forced into the evaporator tube by using a centrifugal pump driven by the steam turbine. Water passes 10 – 15 times into the evaporator tube.

La-Mont Boiler 
La-Mont Boiler working
La-Mont Boiler advantages
La-Mont Boiler disadvantages

The mixture of saturated steam and water is formed inside the tube. This mixture is sent to the steam separator drum which is outside the boiler. Steam from the separator is sent to the superheater, where the saturated steam converts into superheated steam. The water is again sent to the economizer where it again passes by the evaporator tubes.

The air from the air preheater enters the furnace where fuel burns. The flue gases first heat the evaporator tube and then pass by the superheater. These gases from the superheater are again used to preheat the air into the air preheater before exhausting it into the atmosphere. This working pressure of the boiler is above 170 bar and has a steam generation capacity of about 50000 kg/hour at a temperature of 773 K.

  • This boiler can generate a high amount of steam.
  • It is easy to start the operation.
  • The construction design of the Lamont boiler is very simple and easy to understand.
  • This boiler can reassemble with the natural circulation boiler.
  • It has a high heat transfer rate.
  • It is flexible in design.
  • Bubble formation on the surface of the tube reduces the heat transfer rate. This reduces the total quantity of steam generation.

Benson Boiler

It works on the pressure of the water which is increased to the supercritical pressure (i.e. above the critical pressure of 225 bar).

Working of Benson Boiler

When the pressure of water is increased to the supercritical level, the latent heat of water becomes zero and due to this, it directly changes into steam without boiling. This prevents the formation of bubbles at the tube surface. In Benson Boiler, the feed pump increases the pressure of the water to the supercritical pressure and then it enters into the economizer.

Benson Boiler
Benson Boiler working
Benson Boiler diagram
Benson Boiler

From the economizer, the water passes to the radiant heater. Here the water receives the heat through radiation and partly gets converted into steam. The temperature almost raises to the supercritical temperature. After that, a mixture of steam and water enters the convective evaporator where it is completely converted into steam and may superheat to some degree. Finally, it is passed through the superheater to obtain the desired superheated steam. This superheated steam is then used by turbines or engines to produce  electricity (Benson Boiler)

Babcock and Wilcox boiler

This was discovered by George Herman Babcock and Stephen Wilcox in the year 1967. This is a water tube, used in steam power plants. In this type of boiler, water is circulated inside the tubes and hot gases flow over the tubes. This is a Horizontal drum axis, natural draft, natural circulation, multitube, stationary, high pressure, solid fuel fired, externally fired Water tube boiler.

Babcock and Wilcox boiler
Difference Between Water Tube and Fire Tube Boiler
Boiler Mountings
Boiler Accessories
Boiler Function
Video on working of Boiler
Working of Babcock and Wilcox Boiler

The water in Babcock and Wilcox boiler starts to come in the water tubes from the drum through down the take header with the help of a boiler feed pump which continues to feed the water against the drum pressure. The water present in the inclined water tubes gets heated up by the hot flue gases produced by the burning of coal on a fire grate. These fuel gases uniformly heat the water tube with the help of a baffle a plate that works to deflect the flue gases uniformly throughout the tubes and absorbs the maximum heat from the flue gases. (Babcock and Wilcox boiler)

As the hot flue gases come in contact with water tubes, it exchanges the heat with the heater and converts it into steam. 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 current and is known as Natural Circulation. The Steam generated is moved upward, due to density difference, and through the up-take header, it gets collected at the upper side of the boiler drum. Anti-priming pipe inside the drum separates the moisture from the steam and sends it to the superheater.

The superheater receives water-free steam from an anti-priming pipe. It increases the temperature of the steam to the desired level and transfers it to the main steam stop valve of the boiler. The superheated steam stop valve is either collected in a steam drum or sends it inside the steam turbine for electricity generation.

  • The main application of a Babcock and Wilcox Boiler is to produce high-pressure steam in power generation industries.
  • The overall efficiency of the Babcock and Wilcox Boiler is high.
  • The steam generation rate is higher about 20 ton per hour at pressure 10 to 20 bars.
  • The tubes can be replaced easily.
  • The boiler can expand and contract freely.
  • It is easy to perform maintenance and clean.
  • It is less suitable for impure and sedimentary water, as a small deposit of scale may cause overheating and bursting of tubes. Hence, water treatment is very essential for water tube boilers.
  • Failure in the feed water supply even for a short period is liable to make the boiler overheat. Hence the water level must be watched very carefully during the operation of a water tube boiler.
  • Maintenance cost is very high. 

Difference Between Water Tube and Fire Tube Boiler

Fire Tube Boiler Water Tube Boiler
Hot flue gases flow inside the tube and the water outsides the tubes.Water flow inside the turbine and hot flue gases outside the tube.
These boilers are generally internally fired.They are externally fired.
Pressure is limited to 20 bar.The pressure is limited up to 100 bar.
This has a lower rate of steam production.This has a higher rate of steam production.
Not suitable for larger power plants.Suitable for larger power plants.
Involves lesser risk of explosion due to low pressure.Risk of the explosion is higher due to high boiler pressure.
For a given power, it occupies large floor space.For a given power, it occupies small floor space.
Robust in construction.Simple in construction.
Difficult in transportation.Simple in transportation.
They require less skill to operate, as compared to the water tube boiler.They required a skilled operator.
They are difficult to repair and cleaning as they are internally fired.They are easy to repair and clean as they are externally fired.
They require a large shell diameter because the firetube is situated inside the shell.They require a small shell diameter.
The efficiency is less as compared to the water tube boiler.The efficiency is more.
The maintenance is costly. It requires regular inspection.They are easy to maintain as they are externally fired.
Ex: Cornish, Lancashire Boiler.Ex: Babcock and  Wilcox Boiler.

Boiler Mountings

There are several mountings inside or outside the boiler which are as follows:

  • Water level indicator ( Water level gauge)
  • Safety valves
  • Stop valve
  • Pressure gauge
  • Feed check valve
  • Blow off Valve

Read More: Boiler Mountings

Boiler Accessories

Multiple boiler accessories are used to increase efficiency. The accessories may be installed either inside or outside the Boiler. They are as follows:

  • Feed Pump
  • Economizer
  • Air Preheater
  • Superheater

Boiler Function

The function of a boiler is to generate Steam from the Water supplied in the boiler. The fuels like furnace oil, baggage, coal, etc. are generally used in the boiler to generate and supply the required heat. After that, the steam is produced which can be used directly in process applications or it can be used to run steam turbines for producing electricity.

Boiler Applications

Boiler is used for a number of applications in industries and plants which are as follows:

  • Power Sector
  • Food processing industry
  • FMCG
  • Thermal Power plants
  • Sugar Plants
  • Plywood
  • Textiles

Read More: Cornish Boiler

Video on working of Boiler

Credits : The Engineer Portal T.E.P


In conclusion, a boiler is a closed vessel used for heating water or generating steam for various industrial and commercial applications. Boilers can be classified based on the fuel used, types, parts, mounting, and accessories. Understanding the different types and classifications of boilers is crucial in selecting the right one for a specific application. Proper maintenance and operation of boilers are essential for their efficient functioning and long service life. The different parts, mounting, and accessories of a boiler work together to ensure its proper functioning. Boilers play a vital role in various industries, including power generation, food processing, and chemical manufacturing, among others.

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