The shell is a large water-filled pressure vessel, and the furnace and tubes inside it carry the fire and flue gas. The big water volume rides through load swings well and keeps steam pressure steady, but it takes longer to warm up and limits practical pressure and size. Firetubes are the workhorse of commercial and light industrial steam and hot water, from packaged Scotch marine units to vertical tubeless designs.
It's a 200 horse firetube boiler, so give it a slow warm-up, don't slam it to high fire from cold.
Firetube operators judge tube condition by stack temperature trends; a creeping stack temperature usually means it is time to brush.
Also heard as
- fire tube
- shell boiler
- smoke tube boiler
Do not mix it up with
- Watertube boiler: In a watertube boiler the water is inside the tubes and the fire is outside; a firetube is the reverse.
Related terms
Watertube boiler
A boiler in which water and steam circulate inside tubes while the fire and hot gases pass around the outside.
Scotch marine boiler
A horizontal packaged firetube boiler with a large internal furnace tube and two to four gas passes.
Tube sheet
The thick flat plate at each end of a firetube boiler shell that the tube ends are rolled or welded into.
Pass
One run of combustion gases from end to end through a boiler before they turn or leave.
Furnace
The part of a boiler where the fuel actually burns, such as the big flame tube of a firetube or the water-walled box of a watertube.