Belpaire firebox

The Belpaire firebox is a type of firebox used on steam locomotives. It was invented by Alfred Belpaire of Belgium in 1864. Today it generally refers to the shape of the outer shell of the firebox which is approximately flat at the top and square in cross-section. However it is the similar square cross-section inner firebox which provides the main advantages of this design i.e. it has a greater surface area at the top of the firebox where the heat is greatest, improving heat transfer and steam production, compared with a round-top shape.

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. However the use of a similarly shaped square outer boiler shell allows simpler perpendicular stays to be used between the shells. The Belpaire outer firebox is, nevertheless, more complicated and expensive to manufacture than a round-top version.

In steam boilers, the firebox is encased in a water jacket on five sides, (front, back, left, right and top) to ensure maximum heat transfer to the water and keep the firebox wall temperature well below the temperature at which the firebox material (e.g. copper or steel) weakens. Stays are used to support the surfaces against the high pressure between the outside wall and the interior firebox wall, and partially to conduct heat into the boiler interior.

In some boiler designs, the top of the boiler is cylindrical above the firebox, matching the contour of the rest of the boiler and naturally resisting boiler pressure more easily. However, in that case many of the boiler stays connecting the cylindrical outer boiler sheet and the square firebox sheet cannot be placed at right angles, which necessitates angling of the stays and even the fitting of flexible joints to compensate for heat expansion. Those features are difficult to build and weaken the boiler's structure.

In the Belpaire design, the outer boiler wall sheets are roughly parallel with the flat firebox sheets to allow easier placement of the stays although the thermal performance of the boiler is not improved. This arrangement gives the firebox end of the boiler a squarer shape.

Nevertheless, other locomotive boilers, such as the LNER pacifics, had flat-top inner fireboxes with round-top outer shells retaining all the thermal performance of the Belpaire type and without suffering problems of staying between shells.

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Wikipedia