A single-furnace three-pass design boiler is shown in Figure above. The first pass is through the partly corrugated furnace and into the cylindrical wetback combustion chamber. The second pass is back over the furnace through small-bore smoke tubes and then the flow divides at the front central smoke box. The third pass is through outer smoke tubes to the gas exit at the back of the boiler.
There is no combustion chamber refractory lining other than a lining to the combustion chamber access door and the primary and secondary quart. Fully automatic controls are provided and located in a control panel at the side of the fire tube boiler.
A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs. The ZG boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.