Three-pass steam boilers are so-called because the combustion gases that are produced inside them in the burner go round a circuit that has three parts before leaving it:
• The first pass of gases takes place when they leave the burner for the combustion chamber.
• The second pass of gases is through the inversion chamber, which is completely cooled by water.
• The third pass of gases sends them back in the opposite direction towards the rear gas chamber.
Boilers that have this internal structure tend to use larger volumes of water due to their design, compared to other boilers that have two gas passes.
This means they can absorb specific production peaks due to having larger volumes of water that can be evaporated and transformed into steam.
The animated graphic below shows the sequence in which the gases pass through the ATTSU HH model three-pass gas fire tube steam boiler.