Exhaust Gas Economizer

What sets this type apart

An exhaust gas economizer is a heat exchanger, not a combustion boiler: exhaust gas from the main engine passes over a bank of finned tubes, and the heat it would otherwise vent to atmosphere raises steam or heats thermal oil instead. It has no burner of its own, so its steam output tracks main engine load directly. At low load or manoeuvring, exhaust temperature and flow drop and steam production falls with them, which is why most installations still keep an oil-fired auxiliary boiler for port stays and low-load conditions.

Main components

Tube bank

Finned steel tubes arranged for cross-flow of exhaust gas, sized to recover heat without imposing excessive backpressure on the main engine.

Steam drum

Collects generated steam and separates it from circulating water in a forced-circulation design, feeding the ship's steam services.

Circulation pump

Forced-circulation economizers use a pump to move water through the tube bank rather than relying on natural convection, giving more even heat transfer and less risk of local dry-out.

Soot blowers

Steam or compressed-air lances that periodically clear soot deposits from the tube surfaces; deposits are the single biggest threat to this equipment.

Water washing arrangement

Fixed piping for periodic fresh water washing of the tube bank, used at low load or in port when soot fires would otherwise be more likely.

Selection and sizing

Sizing is driven by main engine exhaust flow and temperature at normal sea speed, the steam demand the vessel actually needs to cover (cargo heating on tankers, fuel oil heating, accommodation services), and the allowable exhaust backpressure the engine maker permits. Oversizing the tube bank for heat recovery without checking backpressure can quietly cost fuel efficiency on the main engine itself.

Regulations and class

Class rules require periodic internal and external inspection of the pressure parts as a boiler, including the tube bank, drum and safety valves, on the same survey cycle as other steam-generating equipment. Fire risk from soot accumulation is addressed through class guidance on soot blower and water washing frequency rather than a fixed statutory interval, and MARPOL Annex VI exhaust temperature and backpressure limits from the main engine maker constrain how much fouling can be tolerated before cleaning is mandatory.

Typical faults

• Soot fire in the tube bank -- unburned fuel deposits accumulate and ignite from a hot spot; the consequence ranges from tube damage to a serious exhaust uptake fire if soot blowing has been skipped.
• Tube fouling from irregular soot blowing -- deposits build steadily; the consequence is falling steam output and rising exhaust backpressure that eats into main engine efficiency.
• Tube corrosion from sulphuric acid dew point -- running with cold feedwater below the acid dew point of the exhaust gas; the consequence is accelerated tube wall thinning and eventual leaks.
• Circulation pump failure -- loss of forced flow through the tube bank; the consequence is local overheating and tube damage if the unit is not shut down promptly.

What to look for in a supplier

• Tube bank material and fin design suited to the fuel sulphur content the vessel actually burns, not a generic low-sulphur assumption.
• Soot blower and water washing systems sized for the tube geometry supplied, with accessible lances for maintenance.
• Documented backpressure figures at the tube bank so the impact on main engine performance can be checked against the engine maker's limit.
• Spare tube sections or repair sleeves available without a long lead time, since tube leaks are the most common failure needing replacement parts.

Keep feedwater temperature above the acid dew point whenever the engine is running on higher-sulphur fuel -- cold feedwater is the quiet cause behind most premature tube corrosion, long before soot fires get the blame.