In a firetube boiler, the furnace is the first pass and each bank of tubes the gases travel through afterwards is another pass, so boilers are sold as two-, three- or four-pass. More passes lengthen the gas path and keep velocities up for better heat transfer, at the cost of more draft resistance. Baffles and turbulators inside tubes are another way to wring heat out of each pass.
Third pass tubes are clean but the second pass is packed with soot, that's where the heat's going up the stack.
Also heard as
- gas pass
Related terms
Firetube boiler
A boiler in which hot combustion gases run through tubes that are surrounded by the water being heated.
Scotch marine boiler
A horizontal packaged firetube boiler with a large internal furnace tube and two to four gas passes.
Stack temperature
The temperature of the flue gas as it leaves the boiler, a direct measure of how much heat is going up the chimney.
Punch the tubes
To clean soot from firetube boiler tubes by running a brush on a long rod through each one.