Superheater

However, saturated ("wet") steam at boiling point may contain, or condense into, liquid water droplets, which can cause damage to turbine blades.

That takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet in the smokebox.

The steam is then passed through a number of superheater elements, which are long pipes placed inside the larger diameter fire tubes, called flues.

At the end of its journey through the elements, the superheated steam passes into a separate compartment of the superheater header and then to the cylinders of the engine.

Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox.

[4] Douglas Earle Marsh carried out a series of comparative tests between members of his I3 class using saturated steam and those fitted with the Schmidt superheater between October 1907 and March 1910, proving the advantages of the latter in terms of performance and efficiency.

[citation needed] Robert Urie's design of superheater for the LSWR was the product of experience with his H15 class 4-6-0 locomotives.

In anticipation of performance trials, eight examples were fitted with Schmidt and Robinson superheaters, and two others remained saturated.

[6] However, World War I intervened before the trials could take place, although an LSWR Locomotive Committee report from late 1915 noted that the Robinson version returned the best fuel efficiency.

[7] The main advantages of using a superheater are reduced fuel and water consumption but there is a price to pay in increased maintenance costs.

For example, the North Eastern Railway fitted superheaters to some of its NER Class P mineral locomotives but later began to remove them.