The value of the Franco-Crosti pre-heater is still a hotly debated question among Italian railfans. Basically, it should have increased the efficiency of the boiler by making use of exhaust gases.
Both the Belgian locomotive and the Italian 672 class had problems with the flexible joints conveying the exhaust gases to the pre-heater(s). Subsequent designs mounted one or two pre-heaters on the locomotive frame.
In Italy, the Franco-Crosti system was seen as an economical way to get more power from old steam locomotives and to prolong their useful lifetime: Italy electrified most main lines before World War II, and that the last steam locomotives for the Italian Railways were delivered in 1927. On the other hand, main line Diesel locomotives were only introduced around 1960.
The Italian Railways performed a test campaign in 1940 comparing a 685 class 1-3-1 (a 2-6-2 or Priairie for our American friends) with a 683 (i.e. a 685 class, modified with two side Franco-Crosti pre-heaters and a
substantially rebuilt boiler). The results were sensational, with the 683 delivering 10-15% more power while consuming less coal and water.
In 1942, other tests were performed, comparing the 683 class with an Hungarian 1-3-1 as the Franco-Crosti company was hoping to sell its system to the Hungarian Railways. The tests showed no advantage of the Franco-Crosti system, so Hungary never bought them. The Hungarian locomotive used for the tests was a quite recent design, unlike the class 685, which were designed around 1910.
The Franco-Crosti system appeared to work when applied to Italian locomotives designed before World War I, but when it was applied on modern steam locomotives showed all its faults (increased maintenance, corrosion effects on pre-heaters and chimneys) and none of its supposed virtues: the 10 class 9F British locomotives were quickly converted back to the standard configuration after a very short time when comparative tests proved a total failure.
All the locomotives mentioned in the linked article (623, 683, 741 and 743 classes) were taken out of service several years before their unmodified counterparts (625, 685 and 740 classes): their greatest power was not needed for the light duties, their greatest weight prevented them from circulating on the light rail of secondary branches and their need for increased maintenance condemned them. Two 743 class and one 741 class 1-4-0 (2-8-0 or Consolidation) still exist: I have seen all of them and I must admit they are quite "interesting", despite being technical failures. The 741 is operational and I believe it is the only Franco-Crosti locomotive still running. In the picture below she is hauling an excursion train in the mountains north of Florence.
P.S. Franco-Crosti locomotives are fitted with superheaters, as most locomotives after 1900.
) were the largest users of Franco-Crosti locomotives, so there is a vast literature about their strengths and weaknesses.