The marine boiler works on the simple principle of changing the state of water from liquid to vapor using heat energy. The water is boiled in enclosed pressure vessels so that the steam produced is not lost to the atmosphere.
The heating source is enclosed in insulated furnaces to ensure that heat energy is primarily transferred to the water and not to the surrounding area. Different types of boilers are used on ships depending on their size and type.
In a fuel-fired boiler, chemical energy in the fuel is converted into heat in the furnace and the function of the boiler is to transfer this heat to the water in the most efficient manner.
Once a boiler is designed to absorb the maximum amount of heat released in the process of combustion. This heat is transmitted to the water in the boiler by radiation, conduction, and convection.
“Radiant” heat is heat radiated from a hot medium, i.e. the flame, to a cold body, i.e. the boiler tube, and it depends on the difference in absolute temperature to the fourth power and the colour of the body which receives heat.
“Conduction” heat is heat that passes from the hot medium, i.e. the boiler gas, to a cold body, i.e. the tube by physical contact. The heat then passes from molecule to molecule of the metal with no displacement of molecules and it depends on the temperature difference across the tube thickness and the tube material.
“Convection” heat is heat transmitted from the hot medium, i.e. the boiler gas, to the cold body, i.e. the boiler tube, by the movement of the hot medium and it depends on the heat transfer coefficient and the temperature difference.
In the boiler furnace, the radiant heat waves are transmitted from the hot flame and gases to the boiler heating surfaces such as the plates and tubes. On striking these surfaces, most of the radiant heat is absorbed and passed by conduction to the water inside the tube.
The convection current in the water itself completes the transfer of the heat from the burning fuel to the water and steam. Hence all three forms of heat transmission occur simultaneously.
The amount and arrangement of heating surfaces determine the size, shape, and form of the boiler. The gases can be made to flow through flues or tubes surrounded by water, hence the name fire-tube boiler or they can be arranged to flow over the tubes with water circulating through them, hence the name water-tube boiler.
Watch this Video Lesson – Steam Boiler Fundamentals