Fire Tube Boiler
In the late 18th century, various design configurations of fire-tube boilers began to be widely used for steam generation in industrial plants, railway locomotives and steamboats. Fire-tube boilers are so named because the fuel combustion product gases (flue gas) flow through tubes surrounded by water contained in an outer cylindrical drum (see Figure 2). Today, steam-driven locomotives and river boats have virtually disappeared and fire-tube boilers are not used for steam generation in modern utility power plants. 
However, they are still used in some industrial plants to generate saturated steam at pressures of up to about 18 bar and at rates ranging up to about 25,000 kg/hour.[2] In that range, fire-tube boilers offer low capital cost, operational reliability, rapid response to load changes and no need for highly skilled labor. 
The major shortcoming of fire-tube boilers is that the water and steam are contained within the outer cylindrical shell and that shell is subject to size and pressure limitations. The tensile stress (or hoop stress) on the cylindrical shell walls is a function of the shell diameter and the internal steam pressure:[3]