The marine boiler produces steam and has been used since the days of steamships, and is still used today. It uses the heat generated by the ship’s engines and machinery to boil water at extremely high temperatures and pressures to produce water vapor. This steam is used in various facilities on board ships that use heating systems.
For tankers carrying various heavy fuel oils and bunker fuels, complex heating systems provide constant heat to the cargo to keep it at the low viscosity level required for transportation and distribution.
What are Marine boilers?
In cold climates, this steam is also used in seawater tanks and their associated seawater cooling systems, cooling and heating lines, ballast systems, fresh water storage, and other areas that freeze due to extremely low temperatures.
In addition, steam is used for other purposes, such as operating many pumps, turbines, engines, and other equipment and machines that rely on steam. Steam is also used for various consumption purposes on board ships, such as kitchens, air conditioning and heating systems, steam boiler systems, etc.
In the age of steam engines, with these facilities, the steam produced by the boilers was directly delivered to the main and auxiliary engines, driven by a continuous steam supply, which was another continuous supplement in addition to the main supply.
Principles adopted by boilers
Technically, ship boilers rely on the waste heat recovery system principle. Although the term seems new and groundbreaking, this simple concept has been used for a long time, as mentioned earlier, since the age of steam engines. The heat index extracted from all the relevant machines is used to boil water, produce steam with a very high temperature gradient, and then distribute it appropriately.
How is the heat extracted or utilized? The answer is simple: the exhaust or waste gases produced by engines and equipment. All heavy machinery that runs on a power source, whether diesel or electricity (formerly steam itself), releases hot gases that must be exhausted.
Therefore, these extremely hot gases are an ideal heat source that can be further utilized. In practical applications, the main and auxiliary machinery on board are the main source of this heat, so the working principle of marine boilers is also called the exhaust gas recovery mechanism.
The steering mechanism can be simply imagined as connecting the outlet of the car exhaust muffler to the stove through a pipe and then boiling a kettle of water. Looking through the car’s exhaust manifold indicator, you will find that the water boils into steam.
(Warning: Do not try this method at home without any regulatory guidance, instructions or precision, as it may cause the risk of carbon monoxide poisoning, accidental explosion, fire, and other serious consequences.)
Boiler installation
The same theory applies to ships, although in a more complicated way. The temperature of the gas discharged from the vessel is between 350 and 400°C.
The amount of heat (thermodynamic parameter) extracted from the gas recovered from the exhaust pipe or discharge pipe depends on the following factors:
- Gas flow rate
- Gas density
- Specific heat (a latent physical property of the gas)
- Temperature gradient at the outlet
The gases then pass through a series of pipes and are forced to circulate in an area where there is a lot of water to evaporate. The gases lose heat as they move, and their temperature drops significantly when they come into contact with the water medium. However, the average temperature of the residual gas remains above 100 degrees, which is the boiling point of water.
In theory, heat exchange occurs between the gas in the container and the working water medium, using three heat transfer methods: conduction, convection, and radiation.
Heat transfer occurs through contact with the hot surface of the pipe by heat conduction, while the high energy of the gas transfers a large amount of heat in the form of radiation. Finally, water is an ideal fluid medium. It also has the function of continuous and uniform internal heat transfer due to convection.
Water is boiled into steam and then transported to the consumption point. After the steam is consumed, the cooled steam is transformed back into a liquid called condensate. Condensate is used as a new supply source of working fluid in the boiler system, and water is often added to the water tank by a feed pump.
The water evaporates and becomes steam, which is sent to the point of consumption. After the steam is consumed, the cooled steam returns to a liquid state and is called condensate. Condensate is used as a supplementary source of working fluid in the boiler system, usually by injecting circulating water into the boiler drum tank using a feedwater pump.
For practical purposes, heat is extracted from the exhaust gas from two sources: the main engine and the auxiliary engine. When the engine is off, additional burners can be used to heat the boiler like a conventional burner if there is a demand.
Structure and types of boilers
The boiler is usually cylindrical or dome-shaped, and its size and complexity depend on the vessel’s size, type, capacity, and purpose. Tankers that require continuous cargo heating mechanisms, ships sailing in cold climates, and passenger ships with high steam consumption are important ships with large, complex, and usually multiple boiler designs.
A boiler is a closed furnace structure, in a broad sense, mainly consisting of a main drum or shell containing water, heat exchange tubes, water inlet (water), water outlet (steam), pumps, valves, plugs, monitoring instruments, brackets, and accessories. In the past, boilers were made of iron.
Over time, these materials were replaced by high-quality steels that met the requirements for strength, durability, and corrosion resistance. Carbon steel, creep-resistant alloy steel, high-quality ferritic stainless steel, and certain types of high-strength steel were used as construction materials.
All boiler structures comply with building codes and regulations. Material testing is crucial, as defects, cracks, or leaks can be dangerous in a high-pressure environment such as a boiler, and there is a risk of explosion. Destructive and non-destructive tests are carried out at various stages of formation and construction. Basically, ships have two main types of boilers depending on their configuration:
- Fire tube boilers
- Water tube boilers
In fire tube boilers (also known as water tube boilers), hot gases circulate in a network of pipes with water as the working fluid. They produce pressures between 10 and 15 bar and are connected to a large water supply system. They are relatively simple to build and are mainly used in situations where there is a high demand for steam.
However, it has the disadvantage that the hot water at a very high temperature is in direct contact with the shell, which poses a risk of explosion in large quantities. Therefore, they require high maintenance. In addition, they are inefficient and produce low steam pressure.
Water tube boilers
Water tube boilers work in the opposite way. Water passes through the tubes inside, while a large amount of hot gas is suspended in the chamber. Thanks to this design, the water evaporates much faster than in a smoke tube, which results in greater heat transfer and higher efficiency.
In addition, its composition is safer because the excess hot water does not come into direct contact with the paint. More and more ships are using water tubes instead of smoke tubes. However, the increased steam demand requires more complex configurations and powerful boiler pumping systems to achieve faster water supply and steam circulation.
They are also more expensive and complicated to operate. Since the water supply pumps must be running all the time, coupled with the circulation effect, it also increases the electrical energy consumption on board.
