On a steam locomotive, a method of improving efficiency. On a later steam locomotive, with a boiler pressure of 200 pounds per square inch or more, steam produced from the boiler is at a temperature around 400 degrees Fahrenheit. It was found that by heating this steam further, to approx 800 degrees Fahrenheit, much greater efficiency was obtained.

This superheating is achieved by exposing the steam to heat from the combustion gases again. Steam is collected in the boiler in the steam dome (or in a domeless locomotive, at the highest point in the boiler) and passes through the dry pipe to the smokebox. In a non-superheated locomotive, the saturated steam then goes straight into the cylinders.

In a superheated locomotive, however, the dry pipe is connected to a device known as the superheater header, positioned at the top of the smokebox. This sends the steam on a journey back the way it came. On a superheated locomotive, some of the boiler tubes are enlarged and known as flues. Their greater size allows a loop of thinner pipe, called a superheater element, to be fitted within them. Steam passes from the superheater header back down the superheater elements, and then back up the loop in the pipe towards the superheater header again. During this journey, it is exposed to the intense heat of the fire, and its temperature approximately doubles. It therefore contains much more energy.

From the superheater header, steam then passes to the valve chests and thus the cylinders in the normal fashion. Some locomotives, instead of having the throttle at the very beginning of all this apparatus, in the steam dome, had a front-end throttle fitted, after the superheater header, for increased responsiveness.

Su"per*heat`er (?), n. Steam Engine

An apparatus for superheating steam.


© Webster 1913.

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