A fire-tube boiler is a type of boiler in which hot gases from the fire pass through one or more tubes within the boiler. It is one of the two major types of boilers, the other being the water-tube boiler. A fire tube boiler can be either horizontal or vertical.

This type of boiler was used on virtually all steam locomotives in the horizontal “locomotive” form. It is also typical of early marine applications and small vessels, such as the small riverboat used in the movie The African Queen. It also has extensive use in the stationary engineering field, typically for low pressure steam use such as heating a building.

Schematic diagram of a “locomotive” type fire-tube boiler

Operation

In the locomotive type boiler, 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 tube. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated steam. The steam rises to the highest point of the boiler, the steam dome, where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.

In the locomotive boiler, the saturated steam is nearly always passed into a superheater, back through the larger flues at the top of the boiler, to dry the steam and heat it to superheated steam. The superheated steam is directed to the cylinders or a turbine to produce mechanical work. Exhaust gases are fed out through a chimney, and may be used to pre-heat the feed water to increase the efficiency of the boiler.

Draught for firetube boilers, particularly in marine applications, is usually provided by a tall smokestack. In all steam locomotives, since Stephenson’sRocket, additional draught was supplied by directing exhaust steam from the cylinders into the smokestack through a blastpipe, to provide a partical vacuum. Modern industrial boilers use fans to provide forced draughting of the boiler.

Another major advance in the Rocket was large numbers of small diameter firetubes instead of a single large flue (a multi-tubular boiler). This greatly increased the surface area for heat transfer, allowing steam to be produced at a much higher rate. Without this, steam locomotives could never have developed effectively as powerful prime movers.

Safety Considerations

Because the fire-tube boiler itself is the pressure vessel, it requires a number of safety features to prevent mechanical failure. Boiler explosion, which is a type of BLEVE (Boiling Liquid Expanding Vapor Explosion), can be devastating.

* Safety valves release steam before a dangerous pressure can be built up

* Fusible plugs over the firebox melt at a temperature lower than that of the firebox, therefore melting and dousing the fire in water should it overheat.

* Stays, or ties, physically link the firebox and boiler casing, preventing them warping

The fire-tube type boiler that was used in the Stanley Steamer automobile had several hundred tubes which were weaker than the outer shell of the boiler, making an explosion virtually impossible as the tubes would fail and leak long before the boiler exploded. In nearly 100 years since the Stanleys were first produced, no Stanley boiler has ever exploded.

Types of fire-tube boiler

* Cornish boiler has a single large flue containing the fire

* Lancashire boiler has two large flues containing the fires

* Locomotive boiler has a double-walled firebox and a large number of small flue-tubes. Larger flue-tubes carry the superheater elements, where present. Forced draught is provided in the locomotive boiler by injecting exhausted steam back into the exhaust via a blast pipe.