Classifications Of Boilers

Boiling is a convective heat transfer process which involves phase change from liquid to vapor. The boiler is used to vapor generation. Basically, a boiler is closed vessel in which steam is produced. The heat is providing above saturation temperature to liquid. Modern Boiler has widespread application for power plants. Selection of boiler for any power plant is important to process.Classification of boiler-

According to water and gas flow

1.Fire tube boiler

Fire tube boiler is a type of boiler in which hot gasses are pass through inside the tube. These tubes are surrounded by waters. The gasses of combustion produced by burning of fuel into the combustion chamber. And these gas are pass through flue tube. The heat of the gas is transferred through tube walls and converts water into the end of these tube provided hand holes.

Examples of fire tube boiler:

  • Cochran boiler
  • Lancashire boiler
  • Locomotive boiler

2. Water tube boiler

These are also a type of boiler in which water is pass through tubes and hot gasses are surrounded them. The fuel is burned inside the combustion chamber and produce flue gasses. The water tube boilers are used exclusively when superheated steam is required. For railway locomotive, water tube boiler is used.  These type of boiler is capable of working more than 10 bar pressure and able to generate high steam rate.

Examples of water tube boilers:

  •  Babcock
  •  Wilcox
  • Stirling
  • White-Forster

On the basis of pressure difference

1.High-pressure boilers

This boiler is used when the pressure requirement is more than 80 bar pressure. The modern high-pressure boiler is capable of working more than 150bar pressure and high temperature. Due to heating at high pressure, it saves a significant amount of heat energy. The heat transfer coefficient is also high for these types of boilers. The circulation of water is achieved by pumps.

Examples of high-pressure boilers

  • Lamont
  • Babcock and Wilcox
  • Velox
  • Benson boilers
  • Low pressure

2. low-pressure boilers


On the basis of axis

1.Vertical boiler

These types of the boiler are categorized by the axis of the boiler. If the axis is vertical so the boiler is known as a vertical boiler. The vertical boiler can be installed in the small area.


  • Cochran boiler

2.Horizontal boiler

the axis of this type of boiler is horizontal. It takes more area compare to vertical axis boiler. Some horizontal boiler contains special fire tube.

3.Inclined boiler

The tube installation makes some angle with the base, it’s known as an inclined boiler. Generally, the angle of inclination is less than 15 degrees. Through the fire door, fuel provides to the chamber.

On the basis of fire position

1.External boiler

in this type of boiler, the fuel is burned outside the shell. In externally fire boiler the water flows through tubes. And steam collected in space on the drum.


  • Babcock & Wilcox boiler
  • Stirling boiler

2.Internal boiler

In this type of boiler, the fuel is burned inside the shell.


  • Cochran
  • Lancashire boiler                  

On the basis of stability

1.Stationary boiler

These type of boiler are used for power large scale steam power plant. the power plant is established for a long time.

2.Portable power plant

This type of boilers is used in the small unit of industries. Mainly it was used for a temporary purpose.

See more:  Dimensionless Numbers And Their Importance

Selection of boiler

  • Selection of boiler depends upon power plant or industry requirements. Before selecting a boiler following factors should be considered.
  • Requirements: the requirement is changed according to power plants. Where resource available in large volume than high-capacity boilers preferred and for low resource availability small capacity boiler are preferred.
  • During operational condition, the working pressure also matters. Some power plant requires outlet steam with more than 80 bar pressure, for these types of plants high-pressure boiler suggested.
  • Area required for boiler installation rooms
  • Initial cost of boiler
  • The steam generation rate per hour.
  • The fuel and water capacity. Fuel requirements.


Requirements for a good boiler

  • The steam production rate should be as per requirements.
  • The boiler should produce the maximum weight of steam.
  • A simple construction and light in weight.
  • Sufficient steam and water capacity.
  • It should be capable of quick start.
  • Design for the easy transportation facility.
  • An arranged combustion chamber.




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