The introduction of superheating was the single most important development for the steam locomotive. Superheating increases the power output of a locomotive by up to 25%, with equivalent savings in coal and water, over non-superheated engines. Its widespread use from 1910 coincided with the needs from the railway operators for heavier trains to be hauled at higher speeds.
The first design for a locomotive superheater were put forward in 1850. Previous ideas utilised a steam drying process which raised the temperature of the steam by a few degrees to overcome the moisture within the steam. Contact with the metal surfaces of the pipes and cylinders cools the steam, resulting in the formation of water droplets. This condition also causes frictional resistance in the movement of the pistons and a fall in pressure. The use of a superheater, however, was not advanced for another 50 years when, after developments in metallurgy and lubricating oils capable of withstanding the severe cutting action of highly superheated steam, made superheating a practical proposition. Success was achieved largely due to the work of Dr. Wilhelm Schmidt, assisted by Dr. Robert Garbe, Chief Mechanical Engineer of the Berlin division of the Prussian State Railways and Jean Baptiste Flamme, Chief Mechanical Engineer of the Belgian Railways.
Steam generated in a boiler is known as saturated steam due to a high moisture content since it is in contact with the water. In a superheated boiler, this steam is passed through the regulator valve and main steam pipe to the superheater header and into the superheater elements within the large flue tubes. This superheated steam is then returned to the superheater header to be sent to the cylinders. The moisture that was present in the saturated steam is turned into additional steam and if its temperature is raised high enough, the steam approaches the condition of a perfect gas, progressively expanding as more heat is absorbed.