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 that is offset by simpler interior bracing of the firebox. 
In steam boilers, 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 boiler designs, the top of the boiler is cylindrical above the firebox, matching the contour of the rest of the boiler. However, that means many of the boiler stays connecting the cylindrical crown boiler sheet and the square firebox wrapper sheet cannot be placed at right angles, a problem which necessitates the 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 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 squarer 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 Belpaire firebox was introduced in about 1882 or 83 by R. P. C. Sanderson, who at the time was working for the Shenandoah Valley Railway (essentially a subsidiary of the Pennsylvania Railroad, since they shared the same financial backing from E. W. Clark & Co.). Sanderson was an Englishman (later naturalized as an American citizen) who had attained his engineering degree from Cassel in Germany in 1875.
Having obtained knowledge of a special form of locomotive boiler (the Belpaire), Sanderson wrote to an old acquaintance from his college days who was working at the Henschel locomotive factory at Cassel. He sent Sanderson a tracing of Henschel's latest Belpaire boiler. When shown the design, Charles Blackwell, Superintendent of Motive Power for the Shenandoah Valley Railway, was very pleased with the design and placed an order for two passenger engines, afterwards numbered 94 and 95, and five freight engines, afterwards numbered, 56, 57, 58, 59, and 60, with the Baldwin and Grant locomotive companies. This was the beginning of the use of this type of locomotive boilers in the United States. The Pennsylvania Railroad used Belpaire fireboxes on nearly all of its steam locomotives. The distinctive square shape of the boiler cladding at the firebox end of locomotives practically became a "Pennsy" trademark, as no other American railroad, except for 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. A second firebox can be seen behind.
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.
- "The Belpaire Firebox". The Railway Engineer,. Volume 45: 237. April 1924. Retrieved 2014-08-22.
- By American Railway Master Mechanics' Association. "Dictionary of Terms". Locomotive Cyclopedia of American Practice. p. Page 18. Retrieved 2014-08-22.
- HAPS AND MISHAPS: The Autobiography of R. P. C. Sanderson, 1940, Philadelphia
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