How does a marine boiler work?

A marine boiler generates steam by transferring heat from a fuel or exhaust gas source into water contained in a closed pressure vessel until it boils, and that steam is then used throughout the ship for heating heavy fuel oil to a workable viscosity, heating cargo on tankers carrying viscous or temperature sensitive product, domestic hot water and accommodation heating, and on the relatively few remaining steam turbine propelled ships, for propulsion itself.

In an oil fired auxiliary boiler, a burner atomises heated fuel oil into a fine spray and burns it in a refractory lined furnace, and the resulting hot combustion gases pass across the heating surfaces, the furnace walls and banks of tubes, transferring their heat into the surrounding or internal water before exiting through the uptake and funnel. Two main mechanical arrangements are used: the water tube boiler, where water circulates inside the tubes with hot gas passing around the outside, generally preferred for higher operating pressures and faster steam raising; and the fire tube, or smoke tube, boiler, where hot combustion gas instead passes through tubes that are surrounded by water, a simpler, more compact design common on smaller auxiliary units. Steam generated collects in the upper part of the boiler drum, above the water level, and is drawn off through a main steam stop valve at a controlled pressure for distribution around the ship, while a feed water system continuously replaces the water lost to evaporation, monitored and controlled by water level gauge glasses and automatic level control to keep the level within safe limits, since running dry even briefly can severely damage or destroy the boiler.

Alongside the standalone oil fired boiler, most motor ships also carry an exhaust gas economiser, sometimes called a composite boiler when it shares a drum with an oil fired section, which recovers otherwise wasted heat from the main engine's exhaust gas to raise steam economically whenever the main engine is running at sea, reducing how often the oil fired boiler needs to be lit and saving fuel.

Because a boiler is fundamentally a pressure vessel containing very hot water and steam, it is fitted with essential safety devices: spring loaded safety valves set to lift automatically if pressure exceeds the safe working limit and tested regularly, low water level alarms and cutouts that shut off fuel if the water level falls dangerously low, and pressure gauges and level indicators the watchkeeper checks routinely. Feed water quality is treated with chemicals, commonly to control alkalinity and oxygen scavenging and to prevent scale formation, and the boiler water is periodically blown down to remove accumulated solids, since scale on the heating surfaces reduces heat transfer and can lead to local overheating and tube failure. Given the genuine danger a poorly maintained boiler represents, from both explosion risk and the corrosive damage that neglected water treatment causes over time, careful, routine attention to water chemistry, level control and safety devices is a core, non negotiable part of a marine engineer's job.