The Design Principle by Engineer Franco and Professor Crosti.
A Franco-Crosti boiler is a conventional locomotive boiler with an additional boiler for heating feed water with the assistance of the passing flue gases. This second boiler is therefore called an exhaust gas pre-heater. The Italian designers Franco and Crosti built their first test machines with this technology as early as the 1930s. The savings in coal amounted to approximately 20% with the improved efficiency. The German Federal Railroad took advantage of this idea and contracted Henschel to build two locomotives. Although they originated from class 52 parts, the two Franco-Crosti locomotives were assigned the road numbers 429000 and 429001. The exhaust gas pre-heater is situated below the regular boiler in these locomotives. The flat smokestacks for operation project from both sides of the boiler, which results in a very striking appearance. The standard smokestack is no longer used to discharge the exhaust gases during operation; it is only required for firing up the locomotive. Despite the increased efficiency, the operating costs were rather high and pre-heater boilers were rather subject to corrosion. The two locomotives were taken out of service in 1959 and 1960, respectively.