Composite boilers are provided on ships where space is a constraint for providing two separate auxiliary boiler and exhaust gas economisers. The composite boiler consists of two separate sections of tubes (one for fuel oil firing and another for exhaust gas firing).
- Oil fired section is fired by an oil burner.
- Exhaust gas section is heated by exhaust gases from the main engine. To control the capacity of the boiler, a bypass valve is provided in the exhaust gas pipe which regulates the flow of gases to the boiler.