Composite boilers

Composite boilers are combination of oil-fired boilers and exhaust gas economizers. When the diesel engine is at full load the fuel oil burner only starts if the steam demand exceeds the steam production achieved from the diesel engines exhaust gases.
Most composite boilers nowadays have separated sections for the diesel engines exhaust gases and the flue gases from the fuel oil burners.
Composite boilers also works as a silencer for the diesel engine, but often an additional silencer is required.

Composite boilers that mixes the diesel engines exhaust gases and the flue gases from the fuel oil burner have existed, and may still exist.
Using the diesel engines exhaust gases as combustion air for the fuel oil burner is quite economically although it makes a rather complicated unit.

Starting, stopping and modulating the burner normally controls the steam pressure in the composite boiler. In the old days a huge damper was used to by-pass the composite boiler when the steam demand was less than the quantity produced by the diesel engines exhaust gases. Nowadays a pressure governed control valve dumps excess steam into a dump condenser.