Classification of Steam Boiler

Steam boilers are classified according to multiple conventions and design considerations as per needful engineering industrial requirements. Chiefly, they are grouped under the following criteria:

  1. Relative position of water and combustible gases
  2. Construction/Shell Axis
  3. Tube
  4. Furnace
  5. Fuel

The classification is shown graphically below.

classification of steam boiler

Relative position of water and combustible gases

This is the most fundamental criterion over which steam boilers are classified. From those eighteenth-century boilers to the modern high-pressure steam generators, the engineering yardstick for categorizing boilers is the same: What to keep in a steam shell and what in the boiler tubes?

Based on such engineering decisions, steam boilers are broadly classified as Fire tube and water tube boilers.

Fire Tube Boilers

They fall in the boiler class in which hot combustible gases (also called the flue gases) produced in the furnace of the combustion chamber pass in the boiler tubes (called flue tubes), whereas water is kept in the boiler shell. Their examples include the Cornish boiler, Lancashire boiler, Simple Vertical boiler, Corchan boiler, Locomotive boiler, and so on.

They are mostly used in small to slightly large industrial applications. The evaporative capacities of these boilers are lower compared with water-tube boilers. The steam final temperature and pressure are also low to moderate in a fire tube boiler when juxtaposed with its water tube counterpart. They occupy less space, are more efficient, consistent in operation, and sufficiently adaptable to the fluctuating steam demands.

Water Tube Boilers

They are the industrial boilers in which hot combustible burned gases produced in the furnace section of the combustion are allowed to pass over the tubes in which water runs.

So, in simple words, water runs in the boiler tubes which are known as steam-generating tubes, whereas, hot burned flue gases remain in the boiler shell and pass over the water tubes exit via. Chimney. Heat transfer between water and steam occurs through convective surfaces. They are mostly used in marine propulsion applications.

Their examples include the Thorneycroft boiler, Foster Wheeler Boiler (D, DSD, I, II, III, IV, ERSD), Babcock and Wilcox boiler, Yarrow boiler, Stirling boiler, Vgot boiler, Waste and Heat recovery bagasse boilers, and so forth.

They are used in large-scale industrial applications where high-quality superheated steam is required. The steam generation capacity which is also called the evaporative capacity of the boiler is much higher in the case of a water-tube type boiler compared with its fire-tube boiler. Their footprint is relatively larger. They are heavier and sturdier in design.

Shell Axis

Boilers are also categorized based on the axial orientation of the boiler’s main steam shell. Indexed on such classification, boilers are divided into Horizontal tube and Vertical tube boilers.

Horizontal Boiler

It is a boiler whose principal axis (or the main axis) of the shell is horizontal in the x-y plane. Their examples include Babcock and Wilcox boilers and small to large-size steam locomotive boilers among many others.

Vertical Boiler

It is a boiler whose main axis of the shell is vertical in the x-y plane. Their examples include simple vertical boilers, Cochran Boilers, and many others.


Based on this classification rule, boilers are divided into single-tube or multi-tubular boilers. But, what is this tube? Is it water or a flue gas tube? In the classification rule, this tube is a flue-gas tube which is commonly known as a flue tube or furnace tube. Its two classic types include Single-tube and multi-tubular boilers.

Single-tube Boiler

It is a tube boiler which has one furnace tube which carries hot burned gases. Its classic example is the Cornish Boiler.

Multi-tubular Boiler

It is a dual-tube boiler, that is, it carries more than two furnace tubes or flue tubes. Its typical example is the Lancashire Boiler which has two furnace tubes.


According to the relative position of the furnace with respect to the boiler body, the boilers are classified as internally-fired and externally-fired boilers.

Externally Fired Boilers

These boilers are designed in such a way that the furnace is situated inside the boiler’s main body. Their examples include the Cochran boiler, Lancashire boiler, Cornish boiler, and so on.

Internally Fired Boilers

These boilers are designed in such a way that the furnace is fitted outside the boiler’s main body. Their examples include Stirling boiler, B&W boiler, and so on.


Fuel does not define the criterion based on which boilers are specifically designed as such. It is not the case that the engineering structural design of a coal-fired boiler is radically different from that of an oil-fired boiler.

Rather, when boilers are classified based on their heating source, there are certain changes in the selection of construction materials for the boiler, and the placement of necessary instruments tooling, or equipment. Many examples of boilers that are grouped in this category include coal-fired boilers, oil-fired boilers, gas-fired boilers, and so on.