A firetube boiler is a cylindrical vessel, with the flame in the furnace and the combustion gases inside the tubes.
Generally, boiler efficiencies increase with the number of passes.Mechanical energy is the energy which is applied to an object due to its motion or due to its position.Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position).Combustion occurs within the furnace and the flue gases are roughed through the tubes to the stack outlet.Firetube boilers are available in two, three and four pass designs.The wetback boiler design has a water cooled turn around chamber used to direct the flue gases form the furnace to the tube banks.
The wetback design requires less refractory maintenance; however, internal pressure vessel maintenance, such as cleaning, is more difficult and costly.
For this reason, a watertube boiler is generally used if pressure above 350 psig design is needed.
Firetube boilers are usually built similar to a shell and tube heat exchanger.
Firetube boilers are available in either dryback or wetback design.
In the dryback boiler, a refractory-lined chamber, outside of the vessel, is used to direct the combustion gases from the furnace to the tube banks.
However it's time as passed, most ships nowadays use the more economical diesel burning heavy fuels.