THE FRANCO-CROSTI PRINCIPLE.
The purpose of the system was to make better use of the heat remaining in the exhaust gases. The firebox gases go through a normal boiler and then through a preheater, which heats the feedwater before it reaches the boiler. Feedwater is delivered to the preheater at full boiler pressure and then passes directly into the main boiler through clack (non-return) valves.
To further increase efficiency, the exhaust steam heats a jacket around the outside of the preheater.
It is central to the concept that the preheater does not boil water, but remains at a substantially lower temperature than the boiler, so it can absorb heat from combustion gases cooled thereby. Attilio Franco and Dr Piero Crosti, two engineers of the FS, (the Italian state railway) began in Italy in 1939 with a much modified Gr 671. This took the Class number Gr 672 and number 001; there was only one. However, the first Franco-Crosti machine to be built appears to be the Belgian version, which was really two locomotives in one, joined back-to-back.
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155460- Bent Boiler hexaplex
Probably the oddest locomotive you will ever see, the wheel arrangement is 2-4-4-2 + 2-8-8-2 + 2-4-4-2 yes She is indeed a hexaplex This was an ambitious project intended for the Russian market, making full use of the enormous Russian loading gauge, and the 5-foot gauge track. Total weight would have been 410 tonnes, of which about 320 Tonnes would have been available for adhesion. She would have carried 60 m3 of water and 15 tonnes of coal. The idea was proven by a design which was only one set of wheels smaller to have been not just feasible but to have actually worked.
This locomotive has the wheel arrangement, 0-6-2 + 2-4-2-4-2 + 2-6-0
This is not a wheel arrangement you see every day...
She was 31 metres long and developed about 3000 horsepower.
The boilers were slewed sideways so the fireboxes overlapped, presumably to meet some limit on overall length despite the articulation. Two firemen were needed; in fact their use seems inescapable if only because of the firebox layout. This extra labour at a time when manpower costs were increasing must have cancelled some or all of the economies stemming from the Franco-Crosti system.
The effectiveness of the preheaters was demonstrated by exhaust gases that did not exceed 190 - 240 deg F. Problems with inward air leakage were expected with the joints between boilers and preheaters; these were built as spherical joints with labyrinth seals and leakage proved negligible.
0601, 0602 -German
At the end of 1943, at the request of the Sanger company of Bremen, two separate locomotives with new boilers were made, each using one of the existing preheater sections to make up an articulated 0-6-2+2-6-2T. The boiler was mounted on the rear section, with a flexible gas connection to the preheater at the front. The two locomotives departed for Germany in the summer of 1944, having been delayed by sabotage of the cylinders. They were used by Deutsche Werft, a German naval constructor at Kiel. The two locos came into Russian hands in May 1945, and were renumbered as 0601 and 0602. (which do not seem to fit any known Polish scheme) The first was based at Poznan (Posen), and the second at Gdansk (Danzig). 0602 is known to have been still extant at Cracow as late as 1955.