Thornycroft boiler

Three-drum boilers are a class of water-tube boiler used to generate steam, typically to power ships. They are compact and of high evaporative power, factors that encourage this use. Other boiler designs may be more efficient, although bulkier, and so the three-drum pattern was rare as a land-based stationary boiler.

The fundamental characteristic of the "three-drum" design is the arrangement of a steam drum above two water drums, in a triangular layout. Water tubes fill in the two sides of this triangle between the drums, and the furnace is in the centre. The whole assembly is then enclosed in a casing, leading to the exhaust flue.

Firing can be by either coal or oil. Many coal-fired boilers used multiple firedoors and teams of stokers, often from both ends.

Development of the three-drum boiler began in the late 19th century, with the demand from naval ships that required high power and a compact boiler. The move to water-tube boilers had already begun, with designs such as the Babcock & Wilcox or the Belleville. The three-drum arrangement was lighter and more compact for the same power.

The new generation of "small-tube" water-tube boilers used water-tubes of around 2 inches (5 cm) diameter, compared to older designs of 3 or 4 inches. This gave a greater ratio of tube surface heating area to the tube volume, thus more rapid steaming. These small-tube boilers also became known as "express" boilers. Although not all of these were three-drum designs (notably the Thornycroft), most were some variation of this. As the tubes of the three-drum are close to vertical (compared to the Babcock & Wilcox), this encourages strong circulation by the thermosyphon effect, further encouraging steaming.

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