Different Types of Boiler Used in Thermal Power Plant.

Experiment No.: 2

Experiment Name:

Study on the Different Types of Boiler Used in Coal Based Thermal Power Plant.


To study Water Tube Boiler and Fire Tube Boiler used in coal based thermal power plant.


  • What is boiler?
    It is an enclosed pressure vessel in which water is converted into steam by gaining heat from
    any source (coal, oil, gas etc).
    Boiler in thermal power plant accumulates the steam and build up a pressure to expend it in turbine and convert thermal energy to mechanical energy. The generator which is connected to turbine converts the mechanical energy into electric energy.
  • Types of Boiler:
    • Based on tube content:
      1. Fire Tube
      2. Water Tube
    • Based on operating pressure:
      1. Ultra-supercritical boiler: Pressure ≥ 27.0MPa or rated outlet temperature ≥ 590 ℃ boiler
      2. Supercritical boiler: 22.1MPa ≤ Pressure ≤ 27.0MPa
      3. Subcritical boiler: 16.7MPa ≤ Pressure ≤ 22.1MPa
      4. Ultra-high pressure boiler: 13.7MPa ≤ Pressure ≤16.7MPa
      5. High pressure boiler: 9.8MPa ≤ Pressure ≤ 13.7MPa
      6. Sub-high pressure boiler: 5.4MPa ≤ Pressure ≤ 9.8MPa
      7. Medium pressure boiler: 3.8MPa ≤ Pressure≤ 5.4MPa
    • Based on fuel used:
      1. Solid Fuel Fired
      2. Stoker Fired Boilers
      3. Pulverized Fuel Boilers
      4. Fluidized Bed Combustion (FBC) Boilers
      5. Oil Fired
      6. Gas Fired Boilers
    • Based on draught system:
      1. Natural Draught
      2. Mechanical Draught
      3. Forced Draught System
      4. Induced Draught System
      5. Balanced Draught System

Water Tube Boiler:

A high pressure water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

The heated water/steam mixture then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam pass through tubes in the hot gas path, (a superheater) to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore is typically used to drive turbines, since water droplets can severely damage turbine blades.

Saturated water at the bottom of the steam drum returns to the lower drum via large-bore ‘downcomer tubes’, where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase economy of the boiler, exhaust gases are also used to pre-heat combustion air blown into the burners, and to warm the feedwater supply in an economizer. Such watertube boilers in thermal power stations are also called steam generating units.

Schmatic diagram of water tube boiler

Fire Tube Boiler

A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.

In the locomotive-type boiler, fuel is burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water connected to the long, cylindrical boiler shell. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated (“wet”) steam. The steam rises to the highest point of the boiler, the steam dome, where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.

Schmatic diagram of (locomotive type) fire tube boiler

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