steam-chimney

from The Century Dictionary.

• noun An annular chamber around the chimney of a boilerfurnace for superheating steam.
• noun The type is much used on river-boats, having the advantage that the part of the boiler near to the deck-structures is no hotter than the steam in the dome, while the gases in the central portion may be much hotter. The height of the dome removes the steam-outlet a considerable distance above the water-line, diminishing the dangers from priming in rough weather, or the mechanical entrainment of water into the engine. Slight superheating may result with low-pressure steam.