Belpaire firebox

The flat firebox top would make supporting it against pressure more difficult (e.g. by means of girders, or stays) compared to a round-top.

Due to the increased expense involved in manufacturing this boiler shell, just two major US railroads adopted the Belpaire firebox, the Pennsylvania and the Great Northern.

The advantage was a greater surface area for evaporation, and less susceptibility to priming (foaming), involving water getting into the cylinders, compared with the narrowing upper space of a classic cylindrical boiler.

The shape of the Belpaire firebox also allows easier placement of the boiler stays, because they are at right angles to the sheets.

Sanderson was an Englishman (later naturalized as an American citizen) who had attained his engineering degree from Cassel in Germany in 1875.

Diagramatic cross section of the Belpaire fire box showing the increased area for evaporation and larger volume of water contained in the square section above the box. The hatched circles show the outline of the barrel to which the firebox was attached.
A Round-topped firebox cross section shown for comparison. Note the angling of the stays.
Pacific-type flat-topped inner firebox