Yarrow boilers

A Yarrow boiler consist of an upper steam drum connected by banks of inclined water-tubes to two lower water drums

All the water tubes discharge below the water level in the upper drum. The outer water-tubes function as down comer and the inner tubs function as evaporating tubes. This causes violent circulation of the water in the boiler and separate downcomers are not needed. To maintain such a circulation, the water level in the steam drum must always be kept well over the external tube-ends.
The furnace is placed between the lower drums and the flue gases flows between the tubes on both sides of the boiler steam drum.

Yarrow boilers were widely used in the navy of many countries. Later Yarrow boilers had bended tubes to improve the elasticity of the boiler construction.

Small Yarrow boilers were used in a navy launches. These longboats are used to transport passengers and luggage to the navy ships.

In some Yarrow boilers one of the water drum is replaced by two and superheating elements are arranged between the two banks of tubes.