What is a Steam Boiler? Classification of Steam Boilers, Types of Steam Boilers

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What is steam boiler?

A boiler can be defined as a closed vessel in which water or other fluid is heated under pressure. This fluid is then circulated out of the boiler for use in various processes or power generation. In the case of power generation steam is taken out of the steam boiler at very high pressure and temperature.

In 200 B.C. a Greek named Hero designed a very simple machine which used the steam, generated in a vessel heated from below, to rotate a wheel as the steam escaped through two small pipes kept diametrically opposite, he called it as A_elopile_.


From 200 B.C. to date, many developments have taken place that today allow us to classify steam boilers in different ways. Hence steam generating boilers can be classified under various categories. The main purpose of steam boilers is to generate steam, and so the way in which the steam is generated and consumed forms the major category. The major two groups of boiler application are Industrial steam generators and power generation boilers. Boilers are also classified as fire tube and water tube boilers.

Fire tube boilers have almost become extinct; however this can be classified as

  • Locomotive boilers, which ruled rail transportation before diesel and electric engine came.
  • Industrial boilers, mainly used for green projects where initial steam is required
  • Domestic use boilers

Water tube boilers took over when size and capacity increased. This can be classified depending on type of circulation used to generate steam as

  • Natural circulation boiler
  • Forced circulation boilers
  • Super critical pressure boilers or zero circulation boilers

Depending on type of firing adopted in boilers they can be classified as

  • Stoker fired
  • Pulverized coal fired
  • Down shot fired
  • Fluidized bed boilers
  • Cyclone fired
  • Chemical recovery boilers
  • Incinerators

Of these the stokers which were predominantly used in early days of high pressure high capacity boilers are being replaced by pulverized coal fired boilers and fluidized bed boilers. Stoker boilers are still designed and used in few applications like sugar industries, etc. Fluidized boilers are also going through fast development and can be now sub classified as

  • Bubbling fluidized bed boilers
  • Pressurized fluidized bed boilers
  • Circulating fluidized bed boilers.

The higher capacity boilers are mainly circulating fluidized bed boilers due inherent limitations in bubbling bed boilers.

Boilers can be classified based on the type of fuel used as

  • Coal fired boilers
  • Oil fired boilers
  • Gas fired boilers
  • Multi-fuel fired
  • Industrial waste fired boilers
  • Biomass fired boilers

Various types of arrangement are used by designers in designing the boiler for meeting the end requirement. Hence boilers are classified based on the arrangement as

  • Top supported boilers
  • Bottom supported
  • Package boilers
  • Field erected boilers
  • Drum type boilers
    • Single drum
    • Bi drum
    • Three drums, but these are presently out of use
  • Tower type or single pass
  • Close coupled
  • Two pass boilers

Boilers therefore can be classified based on firing type, fuel used, construction type, circulation type, firing system design nature, and nature of steam application. Today’s steam generating systems owe their dependability and safety to more than 125 years of experience in the design, fabrication, and operation of water tube boilers.