Home Thermal Engineering Boilers Steam Boilers: Parts, Types, Classification, Advantages, Application, and More

Steam Boilers: Parts, Types, Classification, Advantages, Application, and More

In this post, you’ll learn what is a steam boiler? types, parts, working, advantages, applications, selection of steam boilers and terms used for steam boilers.

Steam Boilers

The (steam generator) steam boiler is, usually, a closed vessel made up of steel. In steam generator, which the water is converted into steam by the application of heat. Fuels, which are commonly used in boilers are coal, oil, and gas for producing heat.

The steam generated by the boiler is used in various Industries for processing, heating and for running steam turbines in power plants. The steam produced may be supplied:

  1. To an external combustion engine, i.e. steam engines and turbines,
  2. At low pressure for industrial process work in cotton mills, sugar factories, breweries, etc., and
  3. For producing hot water, which can be used for heating installation at much lower pressures.
steam boilers

Important Terms for Steam Boilers

Following are important terms used in steam boilers:

1. Boiler Shell
2. Combustion Chamber
3. Grate
4. Furnace
5. Heating surface
6. Mountings
7. Accessories

Boiler Shell:

  • It is made up to steel plates bent into a cylindrical form and riveted or welded together.
  • The ends of the shell are closed through the endplates.
  • A boiler shell must have sufficient capacity to carry water and steam

Combustion Chamber:

  • It is the space, usually below the boiler shell, intended for burning fuel in order to produce steam from the water contained in the shell.


  • The grate is a platform, in the combustion chamber, on which fuel like coal or wood is burnt.
  • It consists of cast iron bars which are spaced apart so that air can pass through them.
  • The surface area of the grate, over which the fire takes place, is called the grate surface.


  • It is the space, above the grate and below the boiler shell, in which the fuel is actually burnt.
  • The furnace is also called firebox.

Heating Surface:

  • It is that part of the boiler surface, which is exposed to the fire or hot gases from the fire.


  • There are fittings which are mounted on the boiler for its proper functioning.
  • Mountings are water level indicator, safety valve, pressure gauge, etc.
  • It may be noted that a boiler cannot function safely without the mountings.


  • There are the devices, which form an integral part of a boiler but are not mounted on it.
  • They include superheater, economised, feed pump etc.
  • It may be noted the accessories help in controlling and running the boiler efficiently.

Essentials of a Good Steam Boiler

Followings are the essentials of a good steam boiler:

  1. The boiler should produce the maximum quantity of steam with the given fuel.
  2. It should be economical to initial and should require little attention during operation.
  3. It should rapidly meet the fluctuation of the load.
  4. The boiler should be capable of quick starting.
  5. It should be light in weight.
  6. It should occupy a small space.
  7. The joints should be few and accessible for inspection.
  8. The mad and other deposits should not collect on the heating plates.
  9. Refractory material should be reduced to a minimum. But it should be sufficient to secure easy ignition, and smokeless combustion oof the fuel on reduced load.
  10. Tubes should not accumulate soot or water deposits and should have a reasonable margin os strength to allow for water or corrosion.
  11. The water and flue gas circuits should be designed to allow a maximum fluid velocity without incurring heavy frictional losses.

Selection of Steam Boiler

Selection of steam boiler depends upon the following factors:

  1. Power required and working pressure.
  2. The geographical position of the powerhouse.
  3. Fuel and water are available.
  4. Probable permanency of the station.
  5. The probable load factor.

Classification of Boilers

The steam boilers classified as follows:

  1. According to the method of heating:
    1. Fire Tube boiler
    2. Water Tube Boiler
  2. According to the firing method:
    1. Internally fired boiler
    2. Externally fired boiler
  3. According to the axis of the shell:
    1. Vertical boiler
    2. Horizontal boiler
  4. According to the method of circulation of water and steam:
    1. Natural circulation boiler
    2. Forced circulation boiler
  5. According to the number of tubes:
    1. Single tube
    2. Multi tubular
  6. According to the use
    1. Stationary boiler
    2. Mobile boiler
  7. According to the source of heat

Fire Tube Boiler

  • In fire tube boilers the hot gases produced by the combustion of fuel passes through the tubes, which are surrounded by water.
  • The heat from the gases in conduction through the walls of the tubes to the surrounded water.

Ex: Simple Verical noiler, Cochran boiler, Lancashire boiler and Locomotive boiler.

Water Tube Boiler

  • In water tube boilers, the water flows inside the tubes (called water tubes) that are surrounded by flames and hot gases from outside.

Ex: Babcock and Wilcox boiler, La-Mount boiler, Benson boiler and Loeffler boiler.

Internally Fired Boiler

  • In an internally fired boiler, the furnace inside the boiler shell.
  • Most fire tube steam boilers are internally fired.

Externally Fired Boiler

  • In externally fired boilers, the furnace is arranged underneath in a brickwork setting.
  • Water tube steam boilers are externally fired.

Vertical Boiler

  • In vertical boilers, the axis of the shell is vertical.
  • Simple vertical boiler and Cochran boilers are vertical boilers.

Horizontal Boiler

  • In horizontal boilers, the axis of the shell is horizontal.
  • Babcock and Wilcox boiler, Locomotive boiler, Lancashire boiler are horizontal boilers.

Natural Circulation Boiler

  • In natural circulation boilers, the circulation of water is by natural convection currents, which are set up during the heating of water.
  • In most steam boilers, there is a natural circulation of water.

Forced Circulation Boiler

  • In forced circulation boilers, there is a forced circulation of water by a Centrifugal pump, which is driven by some external power.
  • Use of forced circular is made in high-pressure boilers such as La-Mount boiler, Benson boiler, and Loeffler boiler.

Single Tube Boiler

  • Single tube steam boilers have only one fire tube or water tube.
  • Simple vertical boiler and Cornish boilers are single tube boilers.

Multi Tubular Boiler

  • In a multi-tubular steam boiler, there are two or more fire tubes or water tubes.

Stationary Boiler

  • These boilers are mainly used in power plants, and in industrial process work.
  • These are called stationary because they do not move from one place to another.

Mobile Boiler

  • the mobile steam boilers are these which move from one place to another.
  • These boilers are locomotive and marine boilers.

Source of Heat Boiler

  • The steam boilers may also be classified according to, to the source of heat supplied for producing steam.
  • These sources may be the combustion of solid, liquid or gaseous fuel, electrical energy or nuclear energy, hot waste gases as by-products of other chemical processes etc.

Types of Boilers

Some of the Different types of boilers are as follows:

  1. Cochran Boiler
  2. Babcock and Wilcox Boiler
  3. Locomotive Boiler
  4. La Mount Boiler
  5. Benson Boiler

Advantage and disadvantage of water tube boiler over fire tube boilers:


  1. steam can be raised more quickly.
  2. steam at higher pressure can be produced.
  3. higher rate of evaporation.
  4. Sediment deposition is less.
  5. Suitable for any types of fuel and method of firing,
  6. More effective heat transfer.
  7. failure of water tubes will not affect the working of the boiler.
  8. Occupies less space.
  9. Easy maintenance.
  10. Easy transportation.


  1. Not suitable for ordinary water.
  2. Not suitable for mobile application.
  3. High initial cost and hence not economical.

That’s it, thanks for reading. If you have a question about the “steam boiler” tell us in the comments. If you like this article please share with your friends.

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Saif M
Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at theengineerspost.com