A boiler is defined as "a closed vessel in which water or other liquid is heated, steam or vapor is generated, steam is superheated, or any combination thereof, under pressure or vacuum, for use external to itself, by the direct application of energy from the combustion of fuels, from electricity or nuclear energy."
Also included are fired units for heating or vaporizing liquids other than water where these units are separate from processing systems and are complete within themselves. This definition includes water heaters that exceed 200,000 Btu/hr heat input, 200 degrees Fahrenheit at the outlet, or 120 gallons nominal water containing capacity.
Boilers are one of the essensial equipments onboard ships. It's purpose is to provide heating to the the main deisel propulsion engine, to the bunker F.O. tanks(to make it less viscous for transfer purposes as well as easy ignition). The steam generated by the boiler can also be use for cleaning, and heating of the seawater in the freshwater generator thus evaporating it to make it potable water.
There are two types of boiler mainly use in the Marine Industry, The water-tube boilers and the fire-tube boilers. The most common of which is the water-tube. In the water-tube type, water passes though lines of tube where it is heated by the used of a burner and a furnace.