Boiler Definition, Types, Applications, Necessity, and Fuel Used [With PDF]

This article will cover a comprehensive overview of boilers, including their definition, types, applications, necessity, and the various fuels used for their operation. A PDF download link of the same has been provided at the end of this article. Let us first have an overview of boilers in general

overview of Boiler

A steam boiler, also known as a steam generator, is a closed container designed to heat water until it vaporizes and transforms into steam at a pressure that exceeds atmospheric pressure. It is the largest and most crucial component of a thermal power plant. As defined by the IBR Act 1923 (Indian Boiler Regulation), a steam boiler refers to any closed vessel with a capacity greater than 22.75 liters, which is used exclusively for generating steam under pressure.

boiler room with pipes valves etc.

some important terminologies used for boilers

To fully understand boilers, it is important to be familiar with some key terminologies commonly used in their context. This article will introduce and explain these important terminologies.

  • Boiler shell: A hollow cylindrical body made up of steel plates riveted or welded together.
  • Furnace: The part of the boiler where fuel is burned to produce heat that generates steam.
  • Grate: A space where fuel is burnt, consisting of cast-iron bars with spaces in between for airflow and ash disposal. Grates can be circular or rectangular.
  • Grate area: The surface area of the grate on which fuel burns, measured in square meters.
  • Heating surface: The boiler surface exposed to hot gases on one side and water on the other.
  • Water space and steam space: The volume of the boiler occupied by water is the water space, while the remaining space used for storing steam is the steam space.
  • Flue gases: Hot gases produced by fuel combustion in the furnace, containing water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), and complete/incomplete products of fuel combustion.

Classifications of Types of Boiler

There is a large number of boiler designs, but they may be classified according to 7 of the following ways:

According to the circulation of gases

  • Fire-tube boiler
  • Water-tube boiler
Fire-tube boiler

Fire tube boilers are a type of boiler where the hot gases generated by fuel combustion in the furnace pass through a series of tubes, known as smoke or fuel tubes, immersed in water before being released through the chimney. Heat is transferred from the hot gases to water through the tube walls. Some examples of fire tube boilers include the Cochran boiler, locomotive boiler, and others. Fire tube boilers are also referred to as smoke tube boilers.

Cochran boiler diagram
photo by mechanicaljunglee
Water-tube boiler

Water-tube boilers are a type of boiler where water circulates through a series of tubes, known as water tubes, while the hot gases generated by fuel combustion in the furnace pass around them on their way to the chimney. The heat from the hot gases is transferred to the water through the tube walls. Some examples of water-tube boilers include the Babcock and Wilcox boiler, the Benson boiler, and others.

According to Circulation of water

  • Free circulation
  • Forced circulation
Free circulation

In any water heating vessel, heat is transferred from one point to another via convection, rather than conduction, because water is a poor conductor of heat.

If a vessel containing water is heated from the bottom, the water in the lower portion will become less dense as it heats up, compared to the water in the upper portion. The less dense water rises to the top of the vessel, while the comparatively denser and cooler water from the upper portion of the vessel descends to take its place. This creates a convection current in the water, which continues until the temperature of all the water becomes the same. This method of water circulation is known as free circulation, and it occurs in boilers such as the Lancashire, Babcock, and Wilcox, among others.

Advantages of free circulation

  • Free circulation of water is essential for maintaining a uniform temperature throughout the boiler and preventing unequal expansion of different parts of the boiler.
  • Free circulation of water also enables steam to escape quickly from the heating surface as soon as it is formed.
  • If steam does not escape promptly after it forms, the boilerplates may become overheated because they are not constantly in contact with water.
Forced Circulation

Forced circulation is a method of water circulation in which pumps are utilized to maintain a continuous flow of water in the boiler. This type of circulation occurs as a result of the pressure created by the pump.

Forced circulation systems are generally used in high-pressure, high-capacity water-tube boilers.

Advantages of forced circulation

  • Higher rate of heat transfer from flue gases to water
  • Use of tubes with smaller diameters, reducing overall weight of the boiler
  • Fewer boiler drums required
  • Reduced scale formation
  • Quick steam generation
  • Easy management of load fluctuations without complicated control devices
  • Minimal chance of overheating boiler plates
  • Less weight per unit mass of steam generated

According to the number of tubes

  • Single tube boiler
  • Multi-tube boiler
Single tube boiler

The Cornish boiler is a type of boiler that is characterized as a single tumbler boiler due to the presence of only one flue tube in it.

Multi-tube boiler

The Cochran boiler can be classified as a multi-tube boiler due to the presence of multiple flue tubes within its structure.

According to the nature of use

  • Stationary boilers
  • locomotive boilers
  • Marine boilers.
locomotive boiler diagram
photo by marinersgalaxy
Stationary boilers: Stationary boilers are employed for the production of thermal power and for various industrial processes, such as in the chemical, paper, and textile industries.
Locomotive boilers: Boilers used in locomotive steam engines are called locomotive boilers.
Marine boilers: Boilers used in steamships are called marine boilers.

According to the nature of the fuel used by the boiler

  • Fuel-fired
  • Gas fired
  • Liquid fuel fired
  • Electrically fired
  • Nuclear fired

NOTE: Babcock and Wilcox boilers use solid or gaseous fuel. Volex boilers use oil fuel.

According to the pressure of the boiler:

  • High-pressure boiler
  • Medium-pressure boiler
  • Low-pressure boiler
velox boiler an example of high pressure boiler
photo by IQS Directory
High-pressure boiler: The pressure of the boiler above 80 bar.
Medium-pressure boiler: It has a working pressure of steam from 20 bar to 80 bar. It is used for power generation or process heating.
Low-pressure boiler: This type of boiler produces steam at 15-20 bar pressure. This is used for process heating.

According to the position of the axis of the boiler shell

  • Vertical boiler
  • Horizontal boiler
Vertical boiler: If the boiler axis is vertical, it is called a vertical boiler. For example, Cochran boiler.
Horizontal boiler: If the boiler axis is horizontal, it is called a horizontal boiler. For example, Lancashire boiler. So this are the classifications of the Boiler, now see the schematic diagram of a Boilers.

Schematic diagram of various types of boilers

Fire Tube Boiler Schematic Diagram

fire tube boiler diagram
photo by wikipedia

Water Tube Boiler Schematic Diagram

water tube boiler diagram
photo by wikipedia

Here is Video by Learn Engineering on How a Boiler Works? So let’s see:

three Types of Fuel Used in Boiler

It is categorized in three main types which are as follows

Solid Fuels: Wood, Coal, Briquettes (a block of compressed coal dust ), Pet Coke, Rice Husk.
Liquid Fuels: LDO (Light Diesel Oil), Furnace oil.
Gaseous Fuels: LPG (Liquified Petroleum Gas), LNG (Liquified Natural Gas), PNG (Piped Natural Gas) can be used to carry out the combustion for a specific purpose.

The Necessity of Boiler

  • Boilers serve as the central heating mechanism in industrial or residential settings.
  • Boilers operate with the same basic mechanisms that work together to create a heat-generating combustion process.
  • Boilers use natural gas from gas lines to fuel the combustion process for heat distribution in buildings.
  • The burner initiates the combustion process, and heat moves through the system using pumps, radiators, and heat exchangers.
  • Boiler manufacturers employ rapidly improving technology to build cost-efficient, eco-friendly, and powerful equipment.
  • Share your thoughts in the comments and help us grow by sharing this article on social media.

Applications of Boiler

  • Operating steam engines
  • Operating steam turbines
  • Operating reciprocating pumps
  • Industrial process work in chemical engineering
  • Producing hot water for room heating in very cold areas
  • Generating power in thermal power stations
  • Steam is suitable for process heating in various industries such as sugar mills, textile mills, dairy industry, and chemical industries due to its high heat content.


definition of boiler as per mechanical engineering ?

In mechanical engineering, a boiler can be defined as a closed vessel that is used to generate steam or hot water for various industrial, commercial, or residential heating applications through a controlled combustion process.

what are the types of boilers and their application ?

Boilers can be classified into various types based on their design, construction, fuel type, and application. Some common types of boilers include fire-tube boilers, water-tube boilers, electric boilers, and biomass boilers. Fire-tube boilers are suitable for small-scale applications, whereas water-tube boilers are suitable for high-pressure and high-temperature applications. Electric boilers are commonly used in residential and commercial settings where electricity is readily available. Biomass boilers use organic materials, such as wood pellets or agricultural waste, as fuel and are considered eco-friendly. Choosing the right type of boiler depends on factors such as the required capacity, fuel availability, cost, and environmental impact.

what are boiler safety precaution ?

Boiler safety precautions include regular maintenance and inspections, proper installation and operation, ensuring adequate ventilation and combustion air supply, monitoring and controlling boiler water level, pressure and temperature, and using appropriate safety devices such as pressure relief valves and flame safeguards.

application of boiler in everyday life ?

Boilers have various applications in everyday life, such as providing hot water for domestic use, heating buildings, and generating electricity. In homes, boilers are commonly used for space heating and hot water supply. They are also used in hospitals, schools, hotels, and other commercial buildings for heating and hot water supply. Industrial applications of boilers include process heating in various industries such as chemical, textile, and food processing. Power plants use boilers to generate steam for generating electricity. Boilers also have applications in the transportation sector, such as steam locomotives and ships powered by steam turbines.


In conclusion, boilers are an essential component of thermal power plants, used to generate steam under pressure, which is utilized for various industrial processes. This article provided a comprehensive overview of boilers, including their definition, types, applications, and the various fuels used for their operation. The article also covered some key terminologies commonly used in the context of boilers, and their classifications based on the circulation of gases, circulation of water, nature of use, the position of the axis of the boiler shell, and pressure of the boiler. Understanding the types and terminologies of boilers is crucial for ensuring their safe and efficient operation.


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1 thought on “Boiler Definition, Types, Applications, Necessity, and Fuel Used [With PDF]”

  1. Hello Anup,

    I had a question: why does my hot water heater only fully drain when I open the pressure relief valve? Without it open, it barely drains. Can you explain the physics behind this? Is it a simple case of the armospheric pressure pushing up on the spigot from outside is greater than the pressure inside the tank pushing down on spigot? Then for some reason opening the pressure relief valve makes the pressure inside greater than atmospheric pressure outside – and thus allows water to fully drain?


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