Boiler : Introduction and classification

Boiler introduction

A boiler is closed vessel in which water is heated to convert it into steam. Generally it is well known as a “steam generator”. It supplies necessary steam to the powerplant functional unit.

How it works?

The boiler consists of feed water system, steam system and fuel system.
The feed water system supplies the quality water to boiler and regulate it automatically to fulfill the system demand.
The steam system heats and vaporises the feed water and thus steam produced in the boiler. After that steam is directed through piping system for the use. 
The fuel system comprises of equipments used for the supply of fuel to generate the necessary heat.
All these three systems provided with various valves and control system which are access for maintenance and monitoring and thus complete the boiler process.

Classification of boiler on different parameters


1.) Fire tube boiler– In this type of boiler the hot combustion gasses passes through the boiler tubes and these tubes are surrounded by water.
e.g. Cochran, Cornish, Lancashire, Locomotive boilers.
2.) Water tube boiler- It is a type of boiler in which water flows through the boiler tubes and these tubes are surrounded by hot combustion gases.
e.g. Babcock and Wilcox boiler, LaMont boiler, Loeffler boiler, Stirling, Benson, yarrow, Velox boilers.


1.) Forced circulation- In this type of boiler a water pump forces the water through its path and hence the steam generation rate increases.
e.g. Velox, LaMont, Benson boilers.
2.) Natural circulation Here water circulates in the boiler because of difference in density of hot gases and water.
e.g. Babcock and Wilcox, Lancashire, Cochran boilers.


1.) Low pressure boiler- These type of boilers produces steam at a pressure of 5 to 20 bar. This steam is used for process heating.
e.g. Cochran, Cornish boiler.
2.) Medium pressure boiler – In this type of boiler steam generated at a pressure of 20 to 80 bar. This type of steam boiler is generally used for process heating as well as power generation.
e.g. Lancashire, Locomotive boiler.
3.) High pressure boiler This boiler produces steam at a pressure of more than 80 bar.
e.g. Babcock and Wilcox, LaMont boiler.
  * Supercritical boiler – In this type of boiler the operating parameters are above critical point. These boilers don’t have an evaporator. Here water directly converted in to steam, thus they are called ” once through boiler”. e.g. Benson boiler.
    Pressure at critical point- 221 bar
    Temperature at critical point- 374°C


These are generally differentiate in horizontal, vertical and inclined type of boiler. Although most of the boilers are horizontal type as of there position of tubes.
   But some boiler like Babcock and Wilcox have inclined tubes.
Let’s take an example and it will clear everything about this type of classification of boilers. See, Cochran boiler is a vertical boiler according to the position of overall boiler but here the tubes are in horizontal.
Thus one should always be careful of what an examiner is asking in exams, whether it is the position of whole unit or it is about position of tubes.
These are the major and important classification of boilers. There are also some different types of classification such according to use, charge in furnace etc. But generally all are covered under above classifications….

That was all about the Boiler introduction and classification. Hope you all like this article. If this was helpful to you then share this article with your well wishers. If you have any doubts regarding this or any mechanical engineering topics then let us know in comment section or contact us through email.
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