The burner fires into a big cylindrical furnace, often corrugated for strength, inside the shell, and the gases turn at the rear and travel back and forth through banks of smaller tubes. Designs are named by passes and by whether the rear turnaround is water-cooled, called wetback, or refractory-lined, called dryback. It is the most common packaged commercial steam boiler.
Scotch marine with the rear door open, you can see the tube sheet, we'll brush all three passes tonight.
Also heard as
- Scotch
- Scotch boiler
Related terms
Firetube boiler
A boiler in which hot combustion gases run through tubes that are surrounded by the water being heated.
Wetback
Describes a Scotch marine boiler whose rear gas turnaround chamber is surrounded by water instead of lined with refractory.
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.
Punch the tubes
To clean soot from firetube boiler tubes by running a brush on a long rod through each one.