Courrgated furnace was a tube (initially iron, later steel) heated and swaged (rolled) under pressure to form corrugations. This increased the surface area of the tube, allowing more energy to transfer from the fire to the boiler. The corrugations also provided additional strength to resist boiler pressure..

Corrugated furnace	combustion chamber	corrugated furnace shell
corrugated furnace tube	stainless steel corrugated furnace	corrugated boiler flue tube