What is Boiler and Types of Boilers

In this article, we learn what is boiler, the types of boilers, requirement of good boiler and types of package boilers.

What is Boiler?

  • A boiler or steam generator is a closed-pressure vessel used for the generation of steam under high pressure.
  • This steam is mainly used for power generation, process heating, and space heating purposes.
  • A boiler is commonly made of steel in which the chemical energy of the fuel is converted into heat by the combustion process. This heat energy is transferred to water so as to produce steam.
  • The design of boilers is very complicated and depends on the type of fuel used and its power (capacity).

Types of Boilers

The types of boilers may be classified as follows:

1. According to the relative position of water and flue gases

Water tube boilers: In these boilers, the water passes through the tubes and flue gases pass through the external surface of the tubes.

For example: Babcock-Wilcox, La-mont, Benson and Package boilers.

Fire tube boilers: In these boilers, the flue gases are passed through the tubes and the tubes are surrounded by the water.

For example: Cochran, Lancashire and Locomotive boilers.

2. According to the method of furnace

Externally fired boilers: In these boilers, the furnace is placed outside the boiler shell.

For example: All water tube boilers.

Internally fired boilers: In these boilers, the furnace is placed inside the boiler shell.

For example: All fire tube boilers.

3. According to the method of water circulation

Natural circulation boilers: In these boilers, the water is circulated by natural convection which is set up due to heating of water. Due to temperature gradient water flows from high density region to low density region.

For example: Babcock and Wilcox boiler.

Forced circulation boilers: In these boilers, the water is circulated by using a pump driven by motor.

For example: La-mont and Benson boilers.

4. According to the use

Stationary boilers: These type of boilers are commonly used in power plants, etc. for power generation.

For example: Lancashire and Babcock-Wilcox boilers.

Mobile boilers: These boilers are used in ships, locomotives, etc. because they continuously move from one place to another place.

For example: Locomotive boilers,

5. According to the axis of shell

Vertical boilers: In these boilers the axis of shell is vertical.

For example Cochran boilers.

Horizontal boilers: In these boilers, the axis of shell is horizontal.

For example: Lancashire and Locomotive boilers.

6. According to the pressure of steam generated

Low pressure boilers: When the pressure of generated steam is below 25 bar, then it is called as low pressure boiler.

For example: Cochran, Lancashire and Locomotive boilers.

High pressure boilers: When the pressure of generated steam is above 25 bar and up to 160 bar then it is called as high pressure boiler.

For example: La-mont and Loeffler boilers.

7. According to the heat source

Heat is generated due to combustion of solid, liquid and gaseous fuels.

Waste heat from other processes or nuclear energy.

Requirements of a Good Boiler

A good boiler must possess the following qualities:

  • The boiler should be capable to generate steam at the desired pressure and quantity in minimum possible time with minimum fuel consumption.
  • The boiler should be light in weight and it should occupy less floor area.
  • The boiler must be able to meet the fluctuating demands without fluctuations in pressure.
  • All the boiler parts should be easily accessible or approachable for cleaning and inspection.
  • The boiler should be leak proof.
  • It should have simple installation with minimum time and manpower.
  • It should start quickly.
  • There should be no deposition of mud and foreign materials on heated surface because it affect the efficiency of boiler.
  • The design of boiler should allow high heat transfer rates with minimum pressure drop by providing high velocity of water and flue gases.
  • The initial cost, installation cost and maintenance cost of boiler should be minimum.

Package Boilers

  • Package boilers are widely used in the pharmaceutical industries, food industries and ceramic industries.
  • These boilers require less fuel and electricity for their operation.
  • If the oxygen concentration in the flue gases is reduced then these boilers operate more efficiently.
  • But, insufficient oxygen causes incomplete combustion which results in increased smoke emission.
  • The typical package boiler is either water tube or fire tube with a capacity of 5 to 25 tonns/hr steam generation capacity.
  • The most commonly used fuels for these boilers are heavy oil, light oil or gas.

Fire tube package boiler

Fire tube package boiler


  • A fire tube boiler is a boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water.
  • The heat of these gases is transferred through the walls of the tubes by thermal conduction hence heating the water and ultimately creating the steam.  
  • These type of boilers are commonly used in locomotives, marine applications, rockets, etc.
  • Cornish boiler, Lancashire boiler, Locomotive boiler, Vertical boiler are the types of fire tube boiler.
  • In these boilers, the 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 that penetrate the boiler and heat the water, in this way generating wet steam.
  • Sometimes, this steam is passed through the super heater to dry the steam or super heat the steam.

Water tube package boiler

Water tube package boiler


  • A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire.
  • In these boilers, fuel is burned inside the furnace, creating hot gas which heats water in the steam generating tubes.
  • In small boilers additional generating tubes are separate in the furnace while in large boilers water filled tubes make up the walls of the furnace to generate the steam.
  • The heated water then rises into the steam drum where saturated steam is drawn off the top of the drum.
  • In some cases, the steam will pass through the super heater to become super heated.
  • Cool water at the bottom of the steam drum returns to the feed water drum through down-comer tubes, where it pre-heats the feed water supply.
  • These boilers are used for power generation, sugar industry, textile industry, food processing plant, marine applications, locomotives, etc.
  • Babcock-Wilcox boiler, La-mont boiler, Benson boiler are the types of water tube boiler.
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