Round-topped boiler
type of boiler used for steam engines
A round-topped boiler is a type of boiler used for some designs of steam locomotive and portable engine. It was an early form of locomotive boiler, although continuing to be used for new locomotives through to the end of steam locomotive manufacture in the 1960s.
Firebox
They use the early form of firebox, where the outer wrapper of the firebox is a semi-circular continuation of the cylindrical boiler barrel. They are relatively simple in shape and manufacture, but their design and service is complicated by the difference in shape between the outer and the flat-topped inner wrapper of the firebox. This requires complex staying to support it.
The first boilers of this form had raised fireboxes that were considerably larger, particularly higher, than the boiler barrel. In the extreme case, this gave rise to the haycock boiler, where the firebox was raised into a vaulted dome. In most cases though, the firebox was merely larger by the width of the internal iron framing section. Around 1850, at the instigation of Crampton, the firebox adopted the flush-topped firebox casing, where it was of the same diameter as the boiler barrel and joined to it by riveted lap strips.
The early boiler fireboxes had flat parallel sides (see figure 1).
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