Definition - What does Firetube mean?
Firetube is a type of boiler design that operates on the concept of heat transfer by flowing combustible gas through inner tubes and water through outer tubes. Both set of tubes are in physical contact with each other. The corrosion susceptibility of firetubes must be regularly monitored due to high moisture levels present in the system.
Corrosionpedia explains Firetube
Firetubes can be used to generate steam. The heat generated in the combustible gas tubes is contacted and transferred to the water tubes via thermal conduction. Ultimately, the water begins to boil and becomes steam. Firetubes can be used with a wide variety of liquid substances to exchange heat.