The Belpaire firebox is a type of firebox used on steam locomotives. It was invented by Alfred Belpaire of Belgium in 1864. It has a greater surface area at the top of the firebox, improving heat transfer and steam production. Its rectangular shape makes attaching the firebox to the boiler more difficult, but this is offset by simpler interior bracing of the firebox. 
In boiler designs the firebox is encased in a water jacket on five sides, (front, back, left, right and top) to keep the firebox wall temperature well below the temperature at which steel weakens. Stays are used to space and strengthen the interior gap between the high pressure boiler outside wall and the interior firebox wall and to conduct heat into the boiler interior. 
In conventional designs the top of the boiler is cylindrical over the firebox to match the contour of the rest of the boiler; however, this causes a problem of placing stays at right angles to both the square firebox wrapper sheet and the cylindrical crown boiler sheet. This necessitated the angling of the stays and even the fitting of flexible joints to compensate for heat expansion.These features are difficult to build and weaken the structure.
In the Belpaire design, the boiler wall sheets are roughly parallel with the firebox sheets to allow better placement of the stays. This arrangement gives the firebox end of the boiler a more square shape and is usually made as large as possible within the loading gauge, to offer the greatest heating surface where the fire is hottest.
In the USA the Pennsylvania Railroad used Belpaire fireboxes on nearly all of its steam locomotives. The distinctive square shape practically became a "Pennsy" trademark, as no other American railroad except the Great Northern used Belpaire fireboxes in significant numbers.
In Britain, the Belpaire design was a standard feature on most Great Western Railway locomotives, and a significant number of the London, Midland and Scottish Railway's locomotives also employed the design. Some other British railway companies used the Belpaire firebox on a handful of locomotives, but not to any major extent.
An Australian Victorian Railways J class Belpaire firebox and boiler, in storage.
British GWR Castle Class locomotive Earl Bathurst. The Belpaire firebox is the square shape in front of the cab.
A round top boiler and firebox shown for comparison.
|Wikimedia Commons has media related to Belpaire fireboxes.|