Auxiliary Boiler (Oil-Fired)

What defines this type

Unlike an exhaust gas economiser, which only makes steam from waste heat while the main engine runs, an oil-fired auxiliary boiler burns its own fuel and can raise steam at any time, in port, at anchor, or as a backup if the economiser cannot meet demand at sea. This independence is the entire point of the type: it is sized and operated as the ship's guaranteed steam source, with the economiser treated as a fuel-saving supplement when the main engine is running rather than the primary source.

Main components

• Furnace and combustion chamber – where atomised fuel burns; refractory lining protects the shell from direct flame radiation.
• Burner – a rotary cup or pressure jet burner atomising the fuel oil, paired with a forced draft fan supplying combustion air.
• Fuel oil system – heater, pump and atomiser conditioning heavy fuel oil to the viscosity the burner needs, or handling MDO/MGO directly at lower viscosity.
• Heating surface – smoke tube (fire tube) construction for smaller, simpler boilers, or water tube construction where higher pressure or capacity is needed.
• Superheater – fitted where the plant needs superheated rather than saturated steam, common on boilers also supplying turbine-driven auxiliaries.
• Safety valves – set to lift at design pressure and prevent overpressure of the shell.
• Water level control and gauge glasses – automatic feedwater control plus independent visual gauge glasses, backed by low water level alarms and burner cutout.
• Soot blowers – steam or air-driven lances that clear soot from the fire side of the heating surface without shutting the boiler down.

Selection and sizing

Sizing is set by the ship's steam demand in port, since that is usually the condition with no exhaust gas heat available at all:

• Steam capacity in t/h and design pressure in bar against the combined demand of heating, cargo heating on tankers, and hotel load.
• Fuel flexibility between HFO and distillate, since a boiler that cannot burn MGO cleanly becomes a liability in emission control areas.
• Smoke tube versus water tube construction, generally a function of required pressure and capacity rather than free choice.
• Burner turndown ratio, which decides how well the boiler can hold a low, steady load in port without constantly cycling on and off.

Regulations and class

Boiler safety valve capacity, low water level protection and burner management interlocks are checked against SOLAS Chapter II-2 machinery space safety requirements, and class rules set the periodic internal and external survey cycle including hydrostatic testing at boiler renewal survey. Fuel sulphur content burned in the auxiliary boiler falls under the same MARPOL Annex VI fuel sulphur limits that apply to the main engine, so a boiler burning HFO inside an emission control area needs the same fuel changeover discipline as the propulsion plant.