A basic boiler design for hot water and steam generation consists of a sealed tank chamber that pumps cold water into the chamber and high-temperature water or steam to the desired application. Near the chamber will consist of a heating mechanism that also has piping that directs a heat source into the chamber. Using thermal conduction and heat transfer, the heat passes through the walls of the piping and raises the temperature of the nearby water to the desired controlled point for boiler efficiency.

A boiler may operate using natural gas, propane, electric, wood, or oil. For a fire tube boiler, the main heat source is fire generated from a furnace. The boiler tank is only filled halfway with water leaving about a 1/5 gap of empty space at the top. In the water portion of the tank is a large cylinder. Inside the large cylinder is a main center cylinder called a Morrison tube and smaller hollow cylinders around it. A large fire plume becomes sent down the Morrison tube that creates both heat and gasses. These gasses go down the boiler’s entire length as the entire surface becomes superheated. The heat transfers through the metal and into the water surrounding the cylinders until turning into steam.

The generated gases travel to the end of the boiler and then enter the smaller hollow cylinder tubes. The gases now move back up the tank chamber as it continues heating the water in this secondary passing phase. For some fire tube boiler designs, the gases and heat may travel a third time back down the boiler. Once the heat becomes fully transferred into the water through thermal conduction, the flue gases leave the chamber through an exhaust stack.

The main benefit of a fire tube boiler is that they are more economical than other types of boilers. They come in smaller compact designs while holding a larger water volume when compared to similar heat exchanger boilers. The numerous tubes and large center flue provide more heating surfaces for the flame and gasses to perform the heat transfer process. Fire boilers come in various configurations including horizontal return tubulars (HRT), vertical boilers, one-pass boilers, and three-pass boilers.