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Working of Boiler

To understand the working of the boiler it is important to understand what is a boiler, the classification of the boiler, what is the purpose of the boiler, and what happens in a boiler? Answers to these questions are key to the overall understanding of the concept of the boiler.

What is Boiler?

A boiler is defined as a closed vessel which is used to heat liquid usually water or to generate vapour or steam or any of such combination under pressure for external use by combustion of fossil fuels.

Parts of Boiler

The parts of a boiler can be classified into two divisions:

Boiler mounting

These are the essential parts that defined the performance of the boiler.

Water level indicator

This is used to check the water level inside the boiler which is fitted in the front of the boiler consisting of three cocks and a glass tube.

Pressure gauge

This is used to measure the pressure inside the boiler which is fitted in the front of the boiler.

Safety valve

This is fitted on the drum and is used to prevent the explosion of the boiler due to high pressure which blows off when the pressure inside exceeds the limit. These are always present two.

Stop valve

This is used to control the flow of steam from the boiler to the steam pipe.

Blow off cock

This is fitted at the bottom of the drum and is used to blow off mud and water and is used to empty the boiler.

Feed check valve

This is used as a Non-Return Valve which is fitted slightly below the normal water level on the drum. It is used to regulate the supply of water.


It is the space in the furnace used to burn the fuel.

Boiler accessories

These are the parts that are responsible for the efficiency of the boiler.

Feed pump

It is used to pump the water into the boiler for continuous working. Either electricity or steam turbine is used for the running of the feed pump.

Super heater

This is used to superheat the steam before it is passed in the turbine as saturated steam causes corrosion.


It is used to increase the efficiency of the boiler as the flue gases from the boiler is made to flow through the economizer before releasing it to the atmosphere.

Air preheater

It is also used to increase the efficiency of the boiler by preheating the air.

Working Principle of Boiler

Understanding the working of the boiler is very simple. The boiler is a closed vessel in which the water is stored. Hot gases are produced by burning fuel in the furnace. These hot gases are made to come in contact with the water vessel where the heat transfer takes place between the water and the steam. Therefore, the basic principle of the boiler is to convert water into steam by using heat energy. There are different types of boilers used for different purposes.

Efficiency of Boiler

The efficiency of the boiler is defined as the percentage of total heat exported by the outlet steam to the total supplied fuel.

Boiler efficiency (%)=(heat exported by outlet steam/heat supplied by the fuel)*100

Classification of Boiler

The classification of the boiler is based on the following criteria:

  • According to the relative passage of water and hot gases:
    • Water-tube boiler: A boiler in which the water flows through the tubes that are surrounded by hot combustion gases.
    • Fire-tube boiler: A boiler in which the hot combustion gases are surrounded by the water
  • According to water circulation arrangement:
    • Natural circulation: The circulation happens due to the difference in density.
    • Forced circulation: The circulation happens by pumping the water inside the boiler.
  • According to the use:
    • Stationary boiler: These boilers are used for power plants
    • Portable boiler: These are movable and used temporarily in sites.
    • Locomotive: These are used in the railway engines.
    • Marine boiler: These are used in ships.
  • According to the position of boilers:
    • Horizontal
    • Inclined
  • According to the pressure generated by steam:
    • Low-pressure boiler: The maximum pressure is 15-20 bar. It is used for heating purposes.
    • Medium pressure boiler: The maximum pressure is 20-80 bars. It is used for power generation.
    • High-pressure boiler: The maximum pressure is more than 80 bars.
    • Sub-critical boiler: If the boiler produces steam at a pressure that is less than the critical pressure it is known as sub-critical.
    • Supercritical boiler: These boilers are used to produce steam at a pressure which is greater than the critical pressure.
  • According to charge in the furnace:
    • Supercharged fuel
    • Pulverized fuel

Application of Boiler

The boiler has a diverse applications in the following industries:

  • Food processing industries
  • Chemical industries
  • Refineries and distilleries
  • Thermal power plants
  • Sugar plants
  • Textile industries
  • Health care industries
  • Paper industries
  • Pharmaceutical industries
  • FMCG (Fast Moving Consumer Goods)

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