Thermic siphon

As they are directly exposed to the radiant heat of combustion, they have a high evaporative capacity relative to their size.

By arranging them near-vertically, they also have good water circulation by means of the thermosyphon effect.

The concept of a self-circulating thermic syphon began with stationary boilers and relatively simple Galloway tubes.

They reached their peak in steam locomotive boilers, where the complexity of a syphon was justified by the need for a compact and lightweight means of increasing boiler capacity.

[1][2][3] The Nicholson form combined a complex shape that provided more heating area in a given space than did the earlier tubes and funnels, yet was simple to make, being folded from a single sheet of steel.

A Nicholson syphon, before installation in the firebox
Thermic syphons in the boiler of Bulleid 's Leader class . They are particularly visible in the Leader boiler, as the firebox is dry-walled, rather than water-jacketed.
Czechoslovak State Railways 498.1 featured thermic siphons in the firebox
Galloway tubes in a Lancashire boiler