What is a boiler? its types, Fire tube, Water tube, uses with PDF

Welcome back to The Mechanical post! and today you will be reading about:
  • What is a boiler?
  • Working principle of boiler
  • Requirements of a  good boiler
  • Types of boiler
  • Fire tube boiler
  • Water tube boiler
  • Difference between fire tube and water tube boilers

In the end, you can download the PDF on Boilers and its types, Fire tube and Water tube boiler by clicking on the button provided.

What is a boiler? its types, Fire tube, Water tube, uses with PDF

Steam is used for many industrial purposes like power generation in thermal power plant, heating and drying purposes in textile and food processing industries etc.

Therefore, for producing steam in large quantity at a desired temperature and pressure a boiler is used. So let's see what is a boiler.

What is a boiler?

A boiler is a large closed vessel used to generate steam by evaporation of water when the heat is supplied to it. 

According to Indian Boiler Regulation Act 1923, a "Steam Boiler" is defined as a closed vessel exceeding 22.75 litres of capacity which is used exclusively for generating steam under pressure and includes any mountings and other fittings attached to such vessel which is fully or partly under pressure when steam is shut off.

In the early days, boiler's were made up of wrought iron. But as advancement took place, high pressure and temperature boilers were introduced.

Thus, wrought iron was then replaced by steel. Nowadays, modern-day boilers are made up of alloy steel which is capable of withstanding high pressures and temperatures.

Working principle of boiler

The working principle of the boiler is that water when supplied to the boiler, is converted into steam by the heat produced by the combustion of fuel.
Working prnciple of boiler
Usage of this image is allowed, however, attribution with a link is required

Before proceeding any further, let's get acquainted with some common terms related to the boiler.

Boiler shell

The boiler shell is a cylindrical outer casing of the boiler used within which mostly all the parts of the boiler are contained.


It is the place where the combustion of fuel takes place and the heat energy required to convert water into steam is liberated in the furnace.


The grate is placed below the furnace. It collects the ash or the byproducts produced by combustion of fuel, from where it can be emptied from time to time.                  


The water supplied to the boiler for producing steam is known as feedwater.

Requirements of a good boiler

A good boiler is required to have the following qualities :

  • The initial investment, installation and maintenance expenses must be as low as possible. 
  • The boiler should be capable of generating steam at the desired pressure and quality as quickly as possible with minimum fuel consumption. 
  • The boiler should occupy the lesser area. 
  • The boiler should be capable of keeping up with the fluctuating demands without any pressure loss.
  • All the parts of the boiler should be easily reachable for cleaning and inspection purposes. 
  • The boiler should have minimum joints as possible as to avoid any leakage, which may occur due to expansion and contraction of metal because of heating and cooling.
  • The boiler should be raised at the site within moderate time and labour. 
  • There should be no deposition of mud and foreign materials on the inside surface and on the outer surface of heat transferring parts.
  • The boiler should be in accordance with the safety regulations laid down by the Indian boiler act. (This is for boiler's in India, you may check out about boiler rules and regulations in your country)

Photo by CEphoto, Uwe Aranas | A steam locomotive which makes use of a boiler for woking.

Types of boiler

The following are the types or classification of boilers.

According to the position of water and hot gases (detailed explanation below)
  • Fire tube boiler 
  • Water tube boiler

According to the position of the furnace in the boiler
  • Externally fired - The furnace is placed outside the boiler.
  • Internally fired -  The furnace is placed within the boiler.

According to the position of the boiler itself
  • Mobile boiler -  The boiler is portable and can be shifted easily.
  • Stationary boiler - The boiler is not portable

According to the method of water circulation in the boiler
  • Natural circulation boiler - The circulation of feedwater and steam is natural.
  • Forced circulation boiler - Circulation of feedwater is with the help of pumps.

According to the axis of the boiler drum
  • Horizontal boiler
  • Inclined boiler
  • Vertical boiler

Now, let's learn about Fire tube and water tube boilers.

Fire tube boiler

A fire-tube boiler is a type of boiler within which hot gases pass through the tubes. These hot tubes are immersed or surrounded by water. The tubes cause transfer of heat and the temperature of water rises, eventually converting water into steam.

The figure below shows the Locomotive boiler which is a type of fire tube boiler.

Drawn using XaraXtreme by EmoscopesCC BY-SA 2.5, via Wikimedia Commons

Thus, in a Fire tube boiler, the heating area (ie. hot tubes) is less than the heated area (i.e the surrounding water) resulting in less heat transfer rates.

The Lancashire boiler, Cornish boiler, Cochran boiler, Locomotive boiler etc are the types of fire tube boilers.

Advantages of a fire tube boiler

Fire-tube boilers have the following advantages
  • Fire tube boilers are simple in design and rigid in construction.
  • Due to simplicity as compared to water tube boilers, its cost is less.
  • If the Fire tube boiler is constructed with a vertical drum, the floor area occupied is less.
  • In case of failure of feed water to the boiler, no major damage is caused as the boiler already contains a large quantity of water.

Limitations of fire tube boiler

The following are the limitations of fire tube boiler
  • Fire tube boilers take more time to reach the operating pressure and temperature. 
  • It has low operating pressure.
  • The maximum generating capacity of fire tube boilers is around 9000 kg/hr.
  • It has a slow steam generation rate, as a result, it cannot be used in thermal power plants.
  • Due to the large size of the shell, the transportation of the boiler is not easy.

Applications of Fire tube boiler

Fire-tube boilers, due to their simple construction and small capacity are used in Chemical industries, Sugar industries, Paper industries etc.

Water tube boiler

The water tube boiler is defined as the boiler in which the feed water flows through small diameter tubes and hot gases from the combustion chamber flow over it.

The figure below shows a water tube boiler:
JoojaCC BY-SA 4.0, via Wikimedia Commons

Due to the small diameter of the tubes, less amount of water is exposed to more amount of hot gases, because of which the water evaporates quickly.

Thus, in a water tube boiler, the heating area (ie. hot gases) is more than the heated area (i.e the tubes) resulting in high heat transfer rates.

The examples of water tube boilers include Babcock and Wilcox boiler, LaMont boiler, Velox boiler, Velox boiler etc.

We have a detailed post on LaMont boiler with PDF which you can read, and don't worry the link would open up in a new tab so that you can enjoy reading this article

Water tube boilers are usually high-pressure boilers operating at a critical pressure of 221 bar and above. At such high-pressure water directly converts into steam without consuming latent heat.

Advantages of Water tube boiler

The following are the advantages of water tube boiler
  • Water tube boilers can quickly reach operating pressure.
  • Generation of steam is quick.
  • The parts of the boiler can be dismantled easily, thus making it easy to transport.
  • All the parts of the boiler can be easily accessed for inspection, maintenance, cleaning and repair purposes.

Limitations of water tube boiler

The following are the limitations of water tube boiler
  • Water tube boiler needs careful observation.
  • It can be dangerous if impure or hard water is used as it can cause scale formation inside the tubes. This scale formation is can clog the small diameter tubes eventually leading to overheating and finally bursting.
  • Maintenance cost is high.
  • Continuous feedwater supply is required. In case of failure even for a short period, can cause overheating in the tubes.

Applications of water tube boiler

The main applications of the water tube boiler include high-pressure applications like power generation in a thermal power plant, petroleum industries, chemical industries, textile etc.

Difference between water tube boiler and Fire tube boiler

Given below is the difference between water tube and fire tube boilers:

Sr no

Water tube boiler

Fire tube boiler


In water tube boiler, water is circulated in tubes and hot gases are passed over it.

In Fire tube boiler, Hot  gas is circulated through the tube and water flows over it.


It can generate steam at higher pressure (more than 25 bar).


It can generate steam at a lower pressure (up to 25 bar).


Steam production rate is high.


Steam production rate is low.


It can produce steam up to 550˚C.


It can produce steam up to 300˚C


Water tube boilers are generally at the outer part fire type.


Fire tube boilers are generally at the outer part water type.


Water tube boilers are well suited for large power plant.


Fire tube boilers are not suitable for a large power plant.


Due to high operating pressure, there is more chance of explosion.


Due to low operating pressure less chance of explosion.


The diameter of the shell is small in a water tube boiler.

Larger shell diameter as compared to water tube for the same capacity of steam generation.



Various parts are accessible for inspection while the boiler is working.


Various parts are not accessible for cleaning and inspection when the boiler is working.


It requires more attention.


It requires less attention.


It occupies less floor area, as the rate of heat transfer is high.

It occupies more floor area, as the rate of heat transfer is less as compared to a water tube boiler if shell or drum is in a horizontal position.


Some FAQ

What are boilers used for?

Boilers are used to produce a large quantity of steam which has many industrial applications which include power generation in thermal power plant, ships which run on steam, textile industries, paper industries, chemical industries, food processing industries etc.

What is Fire tube boiler?

In a fire tube boiler, the hot gases from the combustion chamber are passed through the tubes, immersed in an enclosed vessel with water in it. This results in heat transfer by conduction and the conversion of water into steam. Hence, the name fire tube boiler.

What is a Water tube boiler?

A water tube boiler is a boiler in which feedwater is passed through the tubes and the hot gases from the furnace pass over the tubes. The hot gases heat the tubes, eventually heating water and converting it into steam.


That's an overview of Boilers and its types. If you like this post or have any suggestions do let us know in the comments we would love to hear it from you.

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We'll be back soon with another interesting article till then Keep learning and read The Mechanical post!


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