A boiler is a pressure vessel that generates steam or hot water by boiling water in an enclosed chamber. The heat required to boil the water is generated by burning a fuel source in the boiler furnace, which can be diesel, fossil fuel, coal, etc.
Purpose Of Boilers On Ships
When used on ships, the main purpose of the boiler is to generate steam, which can be further utilised for several heat transfer processes. Modern boilers now come integrated with a heat recovery system, which maximizes energy usage and increases overall efficiency.
To understand the usage of boilers on ships, it is essential to understand the broad classification of Marine boilers used on ships:
- Main Boiler
- Auxiliary Boiler
- Main Boiler
The main boiler generates high-pressure steam for the ship’s propulsion. This propulsion is typically a steam turbine system that requires continuous steam input. Ships generally use heavy fuels, but with new stringent environmental regulations, they can also operate using Marine Gas Oil (MGO) or LNG fuel, which has high energy density and low carbon emissions.
Ship boilers are large enough to generate high-capacity steam and continuously operate at high pressure and loads.
- Auxiliary Boiler
The auxiliary boilers are low-capacity boilers, smaller in size than the main boilers. They generate low-pressure steam, which is used to heat cargo and fuel, for domestic use, and sometimes electricity generation. They are equipped with a fuel system that uses lighter fuels like marine gas oil or marine diesel oil. Since they are small, their start-up and shutdown time is less than that of the main boiler, allowing for quick steam production.
| Feature | Main Boiler | Auxiliary Boiler |
| Primary Function | Propulsion (in steam turbine ships) | Various onboard services (heating, cleaning, power generation, etc.) |
| Size and Capacity | Large, high-capacity | Smaller, lower capacity |
| Steam Pressure | High pressure (for turbines) | Lower pressure (suitable for auxiliary services) |
| Boiler Type | Typically water-tube boilers for higher efficiency and pressure | Fire-tube or water-tube, depending on application and ship size |
| Fuel | Heavy fuel oil, sometimes with diesel oil or natural gas | Heavy fuel oil, diesel oil, or sometimes exhaust gas from the main engine |
| Examples | Ocean liners (historical), some naval vessels, icebreakers | Tankers, cargo ships, cruise ships, all types of vessels for auxiliary services |
Boiler Applications on Ships
Steam Propulsion: Many naval ships, icebreakers, LNG ships, and old ocean liners still use a steam propulsion system, which consists of a mega steam turbine propulsion system connected to the propeller, which drives the massive ships.
The main boiler provides high-pressure steam, varying from 20 to 60 bars depending on the type of vessel and propulsion system.
Power Generation: A steam generator that uses steam turbines can generate electricity on the ship. The Auxiliary boiler supplies low-pressure steam to the steam turbine generator. In some boats, emergency generators can also be run on steam.
The auxiliary boiler provides steam in a pressure range of 6-15 bars depending on the type of vessel and propulsion system.
Cargo Heating: Tanker ships are built to carry huge amounts of oil in their cargo hold. It is crucial to heat this oil cargo as the ship travels through various regions with temperature fluctuations; otherwise, the flow of this cargo will become difficult during the loading or discharging process due to changes in viscosity. Auxiliary boilers are utilized to heat cargo tanks carrying crude oil, asphalt, or chemicals to maintain desired viscosity.
Fuel Heating: Many ships operate on heavy fuel oil on open seas, which makes it essential for a vessel to have a reliable steam generation system to heat this viscous fuel continuously.
Auxiliary boiler steam is used to heat the heavy fuel oil tanks to maintain the correct pumping viscosity.
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Source: MarineInsight
