1. Tube Design
For starters, marine boilers can be classified by their tube design – Water-tube and Fire-tube (a
In Water-tube boilers, water is warmed inside the boiler tubes with hot gases surrounding them. On the contrary, Fire-tube boilers contain hot flue gases (smoke) inside the boiler tubes with water surrounding them.
Image source: Midwest Machinery
Differences in application: Water-tube boilers are chosen for applications that require large quantities of high-pressure steam. This is because water-tube boilers can generate higher pressures of steam of up to 5,000 psig and produce up to several million pounds-per-hour of steam. Most main propulsion boilers are water-tube boilers due to the larger quantities and pressures of steam required to drive the propellers of vessels.
On the other hand, Fire-tube boilers are best suited for applications that only require small quantities of low-pressure steam, such as for auxiliary machinery. Water-tube boilers are also chosen for applications that require steam on demand as steam generation occurs in a much shorter period of time compared to Fire-tube boilers.
Due their safer operation and lower maintenance requirements, Water-tube boilers are gaining popularity in the modern age.
2. Source of Heat
Marine boilers can also be classified by their heat source. Marine boilers are usually powered by diesel, waste gas (flue gas) or a combination of the two. In general, ships are equipped with both diesel-powered and waste-gas-powered boilers in order to maximise efficiency.
Waste heat boilers (also known as Exhaust Gas Economisers) 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 utilised 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 through the boiler at various flow rates. This means that waste heat boilers can only be operated when ships are travelling at cruising speeds during which sufficient flow of exhaust gas is available.
Image Source: ResearchGate, Ali Amiri
On the other hand, diesel-powered boilers generate their own heat using dedicated diesel burners for the sole purpose of steam generation, independent of the main engine. Ships usually rely on diesel-power boilers, in place of the main waste heat boilers, when ships are docked with their main engine turned off.
The last type of boiler is called the Composite boiler and as its name suggests, uses a combination of diesel burner and exhaust gas from the ship’s engine as its source of heat.
Image source: Sitong Boiler
Differences in application: Waste heat boilers are often found on larger vessels which typically use bigger diesel engines that provide sufficient amount of waste heat. Although the installation of a WHRS increases upfront costs, it is a time-tested system that maximises 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 more eco-friendly, the waste heat boiler has become a popular type of boiler amongst ship owners.
Diesel-powered boilers are often used on smaller steam or diesel engine powered ships as they do not produce sufficient amount of waste heat. With its own dedicated burner, they can produce larger amount of steam at higher pressures compared to waste heat boilers. Hence, all auxiliary boilers are diesel-powered boilers as 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 diesel-powered and waste-gas-powered boilers.
3. Gas flow
The number of passes in the gas flow is another way to classify marine boilers. This is applicable for fire-tube boilers as the flue gases are travelling in the tubes of the boiler. In general, fire-tube boilers are either single-pass or multipass type.
In a single-pass boiler, flue gases pass through the boiler once before exiting as exhaust while in a multipass boiler, flue gases pass through the boiler multiple times, usually twice or thrice, by means of winding the tube within the boiler.
Single-pass boiler Multipass boiler
Image source: ResearchGate Image source: PT Indiradwi Mitra
Differences in application: As hot gases only pass through the boiler once, a single-pass boiler has a much lower efficiency by design since the hot gases have less time and surface area to transfer its heat to the surrounding water around the tube. Multipass boilers are more efficient as hot gases are forced to pass through the boiler multiple times, increasing time and surface area for a more complete heat transfer. For this reason, most fire-tube marine boilers today are multipass boilers.
About the author
Michael Brecker is the President of Instruments & Controls, Inc. (ICI) and the co-founder of GIANTECH Industries. He specializes in the design, manufacture, repair and installation of diesel engine automation and monitoring systems, boiler burner management systems, combustion control systems, inert gas generator systems, and control valve repairs for both marine and land-based industries. His expertise in boilers has earned him a reputable position, making him one of the leading opinion makers in the industry.