Een "Franco-Crosti" ketel is een gewone locketel met een nageschakelde tweede ketel. Deze tweede ketel verwarmt het voedingswater met behulp van de langs stromende rookgassen. De tweede ketel wordt daarom uitlaatvoorverwarmer genoemd. De Italiaanse constructeurs Franco en Crosti bouwden al in de jaren dertig de eerste testmachines met deze techniek. Door het verbeterde rendement daalde het kolenverbruik met ongeveer 20%. Om deze reden keerde de Deutsche Bundesbahn in het begin van de jaren vijftig weer terug naar dit principe en liet door Henschel twee locomotieven bouwen. Hoewel ze uit delen van de serie 52 ontstonden, werden de beide "Franco-Crosti"-locomotieven als 429000 en 429001 ingezet. Bij deze machines ligt de uitlaatvoorverwarmer onder de langketel. Aan beide kanten van de ketel steken de vlakke schoorstenen uit, wat een tamelijk markante indruk maakt. De normale schoorsteen dient tijdens het bedrijf niet meer voor het afvoeren van de rookgassen, maar wordt alleen nog maar voor het opwarmen gebruikt. Ondanks het verhoogde rendement waren de bedrijfskosten relatief hoog, omdat de voorverwarmketels snel roestten. De beide machines werden in 1959 resp. in 1960 afgekeurd.
The first Franco-Crosti boilered locomotive was built for the Belgian State Railways as No. 2096 in 1932. This massive 0-6-2 + 2-4-2-4-2 + 2-6-0 locomotive weighed 248 tons and developed around 3,000 horsepower. It was essentially two Franco-Crosti boilered locomotives joined back-to-back and was one of the most powerful articulated locomotives ever built.
The first Italian Franco-Crosti boilered locomotive was a modified Class 671 locomotive built for the Ferrovie dello Stato in 1939. This became the only member of the cab forward Class 672.
Following the experimental conversion of 672.001 a new class of Franco-Crosti boilered locomotives were built in 1940 and 1941. These were the FS Gr. 683, of which five were built as rebuilds of Class 685 locomotives. These were the first Franco-Crosti locomotives to also use Caprotti valve gear. These locomotives survived the war and continued in use until the last was withdrawn in 1962.
In 1940 the first of the FS Gr. 743 class were built. These were entirely new locomotives with two feedwater heaters, one on each side of the boiler. In all 94 of this class were built for FS.
Finally in 1954 the last Italian Franco-Crosti locomotives were built, the FS Gr. 741. Unlike the other Italian classes, these had a single feedwater heater mounted directly below the main boiler. In total 81 of these locomotives were built, again surviving into the 1970s.
In 1951 the Deutsche Bundesbahn (DB) rebuilt two Franco-Crosti boilered locomotives with two feedwater heaters located below the main boiler. These were variants of the standard DB class 52 design but were registered as class 42.90 due to the higher axle load.
These locomotives were followed in 1954–1958 by the conversion of 31 class 50 locomotives in a 2-10-0 (1'D) tender configuration with single feedwater heater located under the boiler. These thirty locos formed the new class 50.40 and proved quite successful economically but had problems with corrosion. All locomotives were withdrawn by 1967.
9F 92022 in 1964
Ten of the BR Standard Class 9F locomotives, Nos 92020 - 92029, were built with Franco-Crosti boilers in 1955. Like the German 50.40 these were 2-10-0 tender locomotives with a single feedwater heater under the main boiler.
In 1951 Oliver Bulleid was the chief mechanical engineer of the Córas Iompair Éireann and was experimenting with new forms of steam locomotive. He converted a 1907 Coey locomotive to use a Franco-Crosti boiler similar to the original Italian designs and to burn peat. The locomotive was a poor steamer and eventually was fitted with a forced-draught fan powered by a diesel engine carried on a wagon behind the tender.
The former Southern and Western Railway class K3 2-6-0 locomotive number 356, built by the North British Locomotive Company as an 0-6-0 in 1903 and subsequently rebuilt by the GS&WR, was converted into a testbed for the turf-burning project. Modifications to 356 included a new firebox, fitted with tuyeres, and two Crosti-type feedwater heaters, one on either side of the main boiler, which used heat from exhaust gases to heat boiler feedwater. Preheating coils were also located in the tender's water tank, and the chimney was positioned at the rear of the tender. Turf was fed to the firebox via an auger; there was no means of regulating this process. Steaming trials with the modified 356 began in 1952. Difficulties with steaming led to the addition of a forced-draught fan, which was mounted on a wagon behind the tender and driven by a Leyland bus engine. The results of trials with 356 were mixed, and the locomotive was broken up in 1957; however, some of the concepts were incorporated into CC1.