Locomotive Boiler: Parts, Operation, Merits, Demerits, Applications

Locomotive boiler falls in the category of modern boilers on account of their contemporary design features. It is a fire-tube boiler with a horizontal axis.

There are different designs of locomotive boilers for commercial use. They include the straight-top, the belpaire, the wagon top, the extended wagon top, and the wide firebox.

Main Parts of Locomotive Boiler

The principal parts of a typical locomotive boiler are mentioned as well as illustrated below:

schematic diagram of locomotive boiler

1. Shell

The shell of the locomotive boiler is horizontally oriented. It contains an internal fire-box. It has cylindrical geometry. It is illustrated in the figure above.

2. Fire-box

The fire-box is located at the one end of the Locomotive boiler. It is rectangular in geometry. It is illustrated in the figure above.

3. Smoke-Box

It is located at the trailing end of the boiler as shown in the figure.

In its construction, the fire-box is made of a front plate, a back plate, two side plates, and one crown. All these pieces are riveted together to form a rectangular geometry. The separation between these (inside) plates and the outer plates of the shell is shared by water space.

The inside plates of the fire-box and the outside plates of the shell stayed in place by means of copper stays. Whereas, its crown is designed to be well stayed in place by means of girder stays known as fire-box roof stays in order to keep the fire-box from collapsing under the immense pressure at its top.

4. Smoke-Tubes

There are a great many smoke-tubes for the purpose of having expanded heat surface area. The fire-box is connected to the smoke-box through the horizontal smoke-tubes.

The part of the smoke-tubes which is nearest to the fire-box is the hottest as it serves as the most effective heating surface of the boiler.

5. Steam-Jet

Though, it is not the structural part of the locomotive boiler as such. It is its integral operative need. A steam-jet produces a strong draught, that is, a heavy current of air by means of a steam jet in the boiler.

The needful draught is achieved from the steam exhausted by the engine cylinder. It is shown schematically below.

steam jet of locomotive boiler

6. Blast Pipe and Nozzle

They are provided in the smoke-box for the eventual withdrawal of the exhaust steam in the form of a powerful jet. It is shown schematically above.

7. Steam Dome

At the top of the shell near the fire-box is provided a dome-shaped chamber to collect steam. It is called the steam dome.

Steam, therefore, is taken to the engine cylinder, that is, from the elevated dome which contains a minimum amount of water particles or a nearly dry saturated steam.

8. Grate

It requires a large grate area to fire fuel at a rapid rate.


In its operation, the hot flue gases produced in the fire-box pass through the horizontal smoke-tubes and collect in the smoke-box for final discharge through the chimney. A necessary draught created by the exhaust steam of the engine cylinder causes a partial negative pressure in the smoke-box thereby pushing the flue gases to exit upward via the chimney, whereas, the exhaust steam is discharged in the form of a jet through a blast pipe and finally in a nozzle.


There are numerous industrial merits of locomotive boilers. A few of them are mentioned below:

  1. Increased compactness and rigidity.
  2. Higher steaming/evaporative capacity.
  3. Portability.
  4. Ease of installation.
  5. When proportioned with its work, it proves to be highly economical for factory use.


Despite its several benefits, there are quite a few demerits of locomotive boilers, such as:

  1. Large foot-print.
  2. Sufficient bracing (supporting) is required due to the large horizontal area.
  3. There is a higher propensity for corrosion in the water legs due to the sedimentary deposits.
  4. It is rather challenging to keep the water level always above the tubes and crown sheet.
  5. Difficult access to the boiler interior for cleaning and inspection.


The locomotive boilers have their historic application in locomotives for driving the shafts of steam engines. But, these are the gone days on the contemporary horizon of steam engineering. Modern steam locomotives do not use these boilers on account of their age-old technology.

On an industrial level, the applications of the locomotive boilers are not so rife. In the past, there were numerous applications of locomotive boilers, which include the following:

  1. It was used exclusively on railroads; that is, the steam generated by these boilers was subject to expansion in the locomotive engine cylinder to produce power.
  2. It was used in its prime time in marine-propulsion applications such as in early steam-ships and steam-boats.
  3. It was used as a static boiler in stationary applications such as those related to driving machinery in factories and industrial plants, agricultural and textile machinery, pumping stations, and so forth.
  4. It was used in the early days of electricity generation to run electricity generators.
  5. For heating large spaces such as industrial buildings.