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Boilers: Introduction and Classification

Rishabh Pandey



  • The boiler system comprises a feed-water system, steam system, and fuel system. The feed-water system supplies treated water to the boiler and regulate it automatically to meet the steam demand. Various valves and controls are provided to access for maintenance and monitoring.
  • The steam system heats and vaporizes the feed water and controls steam produced in the boiler. Steam is directed through a piping system to the application. Throughout the system, steam pressure is regulated using valves and monitored with steam pressure gauges.
  • The fuel system consists of all equipment used to supply of fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system.

Boilers Classification:

There are a large number of boiler designs, but boilers can be classified according to the following criteria:

1. According to Relative Passage of water and hot gases:

  • Water Tube Boiler: A boiler in which the water flows through some small tubes which are surrounded by hot combustion gases, e.g., Babcock and Wilcox, Stirling, Benson boilers, etc.
  • Fire-tube Boiler: The hot combustion gases pass through the boiler tubes, which are surrounded by water, e.g., Lancashire, Cochran, locomotive boilers, etc.

2. According to Water Circulation Arrangement:

  • Natural Circulation: Water circulates in the boiler due to density difference of hot and water, e.g., Babcock and Wilcox boilers, Lancashire boilers, Cochran, locomotive boilers, etc.
  • Forced Circulation: A water pump forces the water along its path, therefore, the steam generation rate increases, Eg: Benson, La Mont, Velox boilers, etc.

3. According to the Use:

  • Stationary Boiler: These boilers are used for power plants or processes steam in plants.
  • Portable Boiler: These are small units of mobile and are used for temporary uses at the sites.
  • Locomotive: These are specially designed boilers. They produce steam to drive railway engines.
  • Marine Boiler: These are used on ships.

4. According to Position of the Boilers:

  • Horizontal, inclined or vertical boilers

5. According to the Position of Furnace

  • Internally fired: The furnace is located inside the shell, e.g., Cochran, Lancashire boilers, etc.
  • Externally fired: The furnace is located outside the boiler shell, e.g., Babcock and Wilcox, Stirling boilers, etc.

6. According to Pressure of steam generated

  • Low-pressure boiler: a boiler which produces steam at a pressure of 15-20 bar is called a low-pressure boiler. This steam is used for process heating.
  • Medium-pressure boiler: It has a working pressure of steam from 20 bars to 80 bars and is used for power generation or combined use of power generation and process heating.
  • High-pressure boiler: It produces steam at a pressure of more than 80 bars.
  • Sub-critical boiler: If a boiler produces steam at a pressure which is less than the critical pressure, it is called as a subcritical boiler.
  • Supercritical boiler: These boilers provide steam at a pressure greater than the critical pressure. These boilers do not have an evaporator and the water directly flashes into steam, and thus they are called once through boilers.

7. According to charge in the furnace.

  • Pulverized fuel,
  • Supercharged fuel and
  • Fluidized bed combustion boilers.


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