A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or (many) more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or “haystack” boilers, flued boilers with one or two large flues, fire-tube boilers with many small tubes, and high-pressure water-tube boilers.Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water penetrated by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part—being the strongest practical shape for a pressurized container—and this cylindrical tank may be either horizontal or vertical.
The design of these boilers is based on the maximum pressure, which is 1.1 times as much as operation pressure. Boiler at the pressure of 1.5 times bigger in comparison to the design pressure is investigated under hydraulic test with water at the least temperature of 7 °C. The exit temperature of steam at the operation pressure of 10 bar, is 184 °C. The required electricity for the initiation of boiler is three phase (50 Hz, 380 V). This kind of boiler is designed to work at the least possible pressure loss in combustion chamber so any type of standard burner can be utilized in order to run it.Economizers, recuperators and turbulators are some examples of energy consumption reducer systems which are implemented according to the capacity and operation pressure of system, and economical issues.
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 shell. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated (“wet”) 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 very often passed into a super heater, 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 steam engine’s cylinders or very rarely to 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 fire tube boilers, particularly in marine applications, is usually provided by a tall smokestack. In all steam locomotives since Stephenson’s Rocket, additional draught is supplied by directing exhaust steam from the cylinders into the smokestack through a blast pipe, to provide a partial vacuum. Modern industrial boilers use fans to provide forced or induced draughting of the boiler.Another major advance in the Rocket was large numbers of small-diameter fire tubes (a multi-tubular boiler) instead of a single large flue. 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.