BR Standard Class 9F 92020-9

British Railways Standard Class 9F steam locomotives Nos 92020-9 were experimentally built with Franco-Crosti boilers, thus forming a subclass. All ten were built in 1955 at Crewe Works.

Crosti 9F 92021 at Wellingborough shed in 1959, showing the unique layout on the right hand side of the loco

9F 92022 in 1964; the preheater drum had been sealed off in June 1962

9F 92029 in 1964

The Franco-Crosti boiler took the form of a single cylindrical water drum running along the underside of the main boiler barrel; the standard chimney at the front was only used during lighting-up, in normal working the gases went through firetubes inside the preheater drum that led to a second smokebox situated beneath the boiler from which there emerged a chimney on the right-hand side (fireman's), just forward of the firebox.

In the event, the experiment did not deliver the hoped-for benefits, and efficiency was not increased sufficiently to justify the cost and complexity. Moreover, conditions were unpleasant on the footplate in a cross-wind, this in spite the later provision of a small deflector plate forward of the chimney. These problems led to the subsequent sealing off of the preheater drum, over the period 1959-1961, and the locomotives were then worked conventionally. As a result of this, there was a reduced ability to generate steam, and so their power classification was reduced from 9F to 8F.[1]

All were fitted with the BR1B-type 4,275 imperial gallons (19,430 l) tender.