Working of Marine Boilers Explained

Boilers generate steam and have been used since the era of steamships, and are used to date. They utilise the heat index generated from engines and machinery onboard to produce steam vapour by boiling water at very high temperatures and pressures, and this steam is used for various utilities on the vessel that use heating systems.

For tanker vessels carrying various forms of heavy fuel and bunker fuels, complex heating systems supply constant heat to the cargo so that they remain at the desired low viscosity levels necessary for transfer and distribution.

In colder climates, this steam content is also used for areas like the sea chest and associated seawater cooling systems, cooling and heating lines, ballasting systems, freshwater reserves, and so on, frozen due to very low temperatures.

Moreover, this steam is used for other purposes, such as driving several pumps, turbines, motors, equipment, and other machinery dependent on steam. Furthermore, steam is also used for various purposes for onboard consumption, like galley, air conditioning and heater systems, steam geyser systems, and so on.

In the era of steamships, the steam from boilers was essentially routed, along with these utilities, to main and auxiliary engines that were powered by constant steam supply, and this was a means of constant replenishment in addition to the mainstream supply.

The principle used in Boilers

From a technical point of view, boilers used in ships are based on what is known as the principle of a waste heat recovery system. Though the terminology may seem new and pioneering, this simple concept has been used for ages, right from the era of steamships, as mentioned above. The heat index extracted from all relevant machinery is tapped to be utilised in boiling the water, generating steam at a very high-temperature gradient, which is then circulated suitably.

How do you tap or make use of the heat content? The answer is simple: the exhaust gases or blowoffs from engines and equipment. All heavy machinery running on a power source, diesel or electric (steam itself in earlier days) releases hot gases that need to be expelled.

Hence, these hot gases, having very high temperatures, are ideal sources of heat that can be further utilised. For all practical purposes, the main and auxiliary machinery on a vessel are the main sources of this heat, and hence, the working principle of marine boilers is also essentially known as the exhaust gas recovery mechanism.

The guiding mechanism can be most simply visualised as connecting your car’s exhaust silencer outlet to a burner through a pipe and boiling a container of water. Thanks to the head index of the exhaust release of your vehicle, you will find the water boiling off into steam vapours.

(Warning: Please do not try this at home as a layman’s casual approach without any regulatory guidance, instruction, or precision, as this can pose risks of carbon monoxide poisoning, accidental explosions, fires, and other serious consequences).

The same theory is used in vessels, though in a much-complicated manner. The temperature of the exhaust gases from a vessel is of the order of 350 to 400 degrees Celsius.

The heat content, which is a thermodynamic parameter, extracted from the gases recovered from the exhaust or release lines is dependent on the following factors:

• Flow rate of the gas
• The density of the gas
• Specific heat (the gas’s inherent physical property)
• Temperature gradients at the outlet

These gases are then made to pass through a bundle of tubes and piping and made to circulate in the region where there is a volume of water to be evaporated. The gases lose heat on the go, and the temperature decreases significantly in contact with the water medium. Nonetheless, the average temperatures of the gas residues remain more than 100 degrees, the boiling point of water.

From a theoretical point of view, a heat exchange occurs between the gases and the working water medium in the container using all three types of heat transfer: conduction, convection, and radiation.

The heat transfer through the hot surface contact of the tubes and piping takes care of the conduction, whereas the high energy of the gases transfers a significant amount of heat in the form of radiation. Lastly, water is an ideal fluid medium. It also has a great deal of heat transfer internally within itself continuously and uniformly, thanks to convection.

The water boils off into steam, which is routed to the points of consumption. After the steam is consumed, the cooled-off vapours further convert back into a liquid state, known as the condensate. The condensate water is used as the replenishment supply for again the working fluid of the boiler system, and a feeder pump is often used to fill up the tank of the boiler drum with the recycled water.

For all practical purposes, the heat is extracted from two the exhausts of two main sources: the main engine and the auxiliary engine. When the engines are shut off, additional burners are used to heat the boilers like an ordinary stove if there is a demand.

Construction and Types of Boilers

Boilers are mostly cylindrical or dome-shaped, and their size and complexity depend on the size, type, capacity, and utility of the vessel. Tankers with a requirement for constant cargo heating mechanisms, vessels operating in cold climates, and passenger ships where there is a high requirement for steam consumption are some important vessels with large and complicated boiler designs, often multiple.

In a broad sense, boilers have a closed-chamber construction mainly composed of the main drum or shell containing the water, tubes and piping for heat exchange, inlet (for water) and outlet (steam), pumps, valves and stoppers, monitoring gauges, mountings, and fittings. Previously, boilers were made up of iron.

With time, the materials have shifted to high-grade steels that cater to the requirements of strength, durability, and anti-corrosive properties. Carbon steels, creep-resistant alloy steels, ferritic high-grade stainless steels, and some high-tensile steels are used for construction.

All boiler constructions adhere to codes and regulations for construction. As any faults, cracks, or leaks can be dangerous for a high-pressure environment like a boiler, posing risks of blasts or explosions, testing is a very important part of materials. Both destructive and non-destructive testing are performed at different stages of forming and construction.

Primarily, vessels have two main types of boilers based on configuration:

• Fire tube Boilers
• Water tube boilers

In fire tube boilers, also known as smoke tube boilers, hot gases circulate within a network of tubes and pipes surrounded by water serving as the working liquid. They produce pressures in the range of 10-15 bar and are associated with large volumes of water supply. They are simpler in construction and are mostly used when there is a large requirement for steam.

However, they are disadvantageous in that the heated water, at very high temperatures, is in direct contact with the outer shell, which, too, in large quantities, poses a risk of explosions. Thus, they require high maintenance. Moreover, they are inefficient and can generate less steam pressure.

Water tube boilers are just the other way around. The water is made to pass through the internal tubes and pipes, whereas the hot gas is suspended in the chamber in more quantities. Because of this design, the rate of evaporation of the water is much higher than that of the fire tube, and along with higher levels of heat transmission, the efficiency is higher.

Moreover, they are safer in the configuration as excess amounts of hot water are not in direct contact with the shell. More and more vessels are using water tubes over fire tubes these days. However, larger steam demands require more complicated configurations and a strong pumping system to and from the boiler, catering to expedited water supply and steam circulation.

They also have higher costs and are complicated to operate. As the water supply pumps need to be functional all the time, along with the circulation, they also account for higher electrical load consumption onboard.

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• What is Deck Plating on Ships?
• What are Triple Hull Vessels?
• What are Freeing Ports On Ships?
• What is Freeboard on Ships?

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About Author

Subhodeep is a Graduate of Naval Architecture and Ocean Engineering. He has deep interest in marine structures and goal-based design aspects, and is dedicated to sharing and propagating technical knowledge of the industry.

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