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.