The Reason For High Pressure
Maximising the efficiency of a heat engine depends fundamentally upon getting the temperature at which heat is accepted (i.e. raising steam in the boiler) as far as possible from the temperature at which it is rejected (i.e. the steam when it leaves the cylinder). This was quantified by Nicolas Léonard Sadi Carnot.
There are two options: raise the acceptance temperature or lower the rejection temperature. For a steam engine, the former means raising steam at higher pressure and temperature, which is in engineering terms fairly straightforward. The latter means bigger cylinders to allow the exhaust steam to expand further - and going this direction is limited by the loading gauge - and possibly condensing the exhaust to further lower the rejection temperature. This tends to be self-defeating because of frictional losses in the greatly increased volumes of exhaust steam to be handled.
Thus it has often been considered that high-pressure is the way to go to improve locomotive fuel efficiency. However, experiments in this direction were always defeated by much increased purchase and maintenance costs.
High-pressure locomotives were much more complicated than conventional designs. It was not simply a matter of building a normal fire-tube boiler with suitably increased strength and stoking harder. Structural strength requirements in the boiler shell make this impractical; it becomes impossibly thick and heavy. For high steam pressures the water-tube boiler is universally used. The steam drums and their interconnecting tubes are of relatively small diameter with thick walls and therefore much stronger.
The next difficulty is that of scale deposition and corrosion in the boiler tubes. Scale deposited inside the tubes is invisible, usually inaccessible, and a deadly danger, as it leads to local overheating and failure of the tube. This was a major drawback with the early water-tube boilers, such as the Du Temple design, tested on the French Nord network in 1907 and 1910. Water tubes in Royal Navy boilers were checked for blockage by carefully dropping numbered balls down the curved tubes.
A sudden steam leak into the firebox is perilous enough with a conventional boiler- the fire is likely to be blasted out of the firebox door, with unhappy results for anyone in the way. With a high-pressure boiler the results are even more dangerous because of the greater release of energy. This was demonstrated by the Fury tragedy, though the reason for the tube failure in that case was concluded to be overheating due to lack of steam flow rather than scaling.
Read more about this topic: High-pressure Steam Locomotive
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