The Thornycroft marine steam boiler has an upper steam drum
and a lower waterdrum connected together by banks of tubes. The tubes are very
curved and all of them discharge into the steamdrum above the water level. In
later models some of the tubes discharge below the water level. A down comer
between the steam drum and the water drum is located behind the boiler.
The flue gas pass through the banks of tubes and violent circulation arise in
the tubes and the evaporated steam conveys water to the steam drum. The heavy
water circulation keeps the tubes free from deposits.
These boilers have an upward narrowing shape and call for, especially if they
are placed side by side, a larger space than other square-shaped boilers.