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The Boiler Series (Part 2 of 3): Source of Heat

Marine boilers can also be classified by their heat source. Marine boilers are usually fired by burner with fuel oil or waste gas (diesel engine exhaust) or a combination of the two. In general, ships are equipped with both fuel oil fired and waste-gas-powered boilers to maximize efficiency.

What are waste heat boilers?

Waste heat boilers (also known as Exhaust Gas Economizers) are part of a vessel’s Waste Heat Recovery System (WHRS). Waste heat in the form of exhaust gas from a vessel’s diesel engine is utilized to generate steam for a variety of purposes such as heating of heavy fuel for use in the main engine. This is accomplished by passing the exhaust gas from the main engine (s) through the boiler at various flow rates. This means that waste heat boilers can only be operated when ships are traveling at cruising speeds during which sufficient flow of exhaust gas is available.

What are fuel oil boilers?

On the other hand, fuel oil fired boilers generate their own heat using dedicated fuel oil burners for the sole purpose of steam generation, independent of the main engine. Ships usually rely on fuel oil fired boilers, in place of the main waste heat boilers, when the ships are docked with their main engine turned off.

What are composite boilers?

The last type of boiler is called the Composite boiler and as its name suggests, uses a combination of fuel oil fired burner and exhaust gas from the ship’s engine as its source of heat.

                             

Water-tube waste heat boiler diagram              Diesel-powered boiler

Image Source: ResearchGate, Ali Amiri             Image source: Sitong Boiler

Differences in application: Waste heat boilers are often found on larger vessels which typically use bigger diesel engines that provide a sufficient amount of waste heat. Although the installation of a WHRS increases upfront costs, it is a time-tested system that maximizes the vessel’s efficiency when cruising, leading to vast reductions in fuel consumption and hence lower operating costs. Together with the marine industry’s move in recent years towards being eco-friendlier, the waste heat boiler has become a popular type of boiler amongst ship owners.

Fuel oil fired boilers are often used on smaller steam or diesel engine powered ships as they do not produce a sufficient amount of waste heat. With its own dedicated burner, they can produce a larger amount of steam at higher pressures compared to waste heat boilers. Hence, all auxiliary boilers are fuel oil fired boilers as a larger amount of steam, beyond the capacity of waste heat boilers, are needed for auxiliary purposes such as heating of cargo and driving of cargo pumps.

Composite boilers are also often used on smaller vessels as an economical and space-saving alternative to having both fuel oil fired and waste-gas-powered boilers.