The Franco-Crosti pre-heating system used the exhaust gases to pre-heat the water feeding the boiler.
It was applied to a several hundred Italian locomotives, Priairies (Group 685, then 683), Moguls (Group 625, then 623) and two different models of Consolidations (Group 740): the first batch had two side pre-heaters (Group 743) and was the one in the image posted by Magickmaker and in the movie. A second batch of Consolidatios received a single pre-heater located under the boiler and became Group 741.
The trials carried out by the Italian Railways in 1941 between a 683 and an unmodified 685 showed great improvement in performance and in water and coal consumption. The modification, however, was not limited to the installation of the Franco-Crosti pre-heaters, but also included a series of accessories that were not present on the original locomotives and the effects of those accessories were grossly underestimated.
In fact, a series of trials carried out by MAV (Hungarian Railways) in 1942 showed no great improvement when the 683 was compared with modern steam locos.
After the war, the last 10 locomotives of the British 9F Class were equipped with a Franco-Crosti pre-heater under the boiler and the comparison with a regular locomotive of the same class showed that this system could only obtain little improvement in performance when applied to modern steam.
In Germany, a small series of 50-series Decapods was also equipped with this system, but I have no information about the results obtained.
In Italy, the Franco-Crosti locomotives were withdrawn some years before the end of steam, because they required more maintenance than a normal locomotive and the benefits of these equipment was minimal in local services.
One of the last Franco-Crosti locomotives, the 741.120, has been recently restored and returned to service, hauling steam specials in Tuscany
, and there are also plans to restore a 743
.
The 741.120 hauling a steam special on October 13th, 2007.
Claude