Efficiency in a heat engine depends fundamentally upon getting the temperature at which heat is accepted (ie in the boiler) as far as possible from the temperature at which it is rejected. (ie steam when it leaves the cylinder)

There are two options; raise the acceptance temperature or lower the rejection temperature. 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. However, experiments in this direction were always defeated by much increased purchase and maintenance costs.

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You may be wondering why HP locos were so complicated. Why not just build a normal boiler with suitably increased strength and stoke harder?

Structural strength requirements in the boiler shell make this impractical; it becomes impossibly thick and heavy. For high 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 can lead 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 dropping numbered balls down the curved tubes, and being very careful that you had them all accounted for)

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 loco 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.

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THE SCHMIDT SYSTEM.
The only certain way to avoid corrosion and scale problems is to use distilled water, as they do in power stations. In fact you need to go further; dissolved gases such as oxygen and carbon dioxide also cause corrosion at high temperatures and pressures, and must be kept out. Most locomotives did not lug a condenser around with them, so there was no source of pure feed water. One solution was the Schmidt system; this used a sealed ultra-high-pressure circuit that simply transferred heat to a high-pressure circuit, by means of heating coils inside a high-pressure boiler. If this latter is fed with ordinary water, scale may form on the outside of the heating coils, but it cannot cause overheating, as the ultra-HP tubes are quite capable of withstanding their internal steam temperature, though not the firebox flame temperature.

Above: The Schmidt High-Pressure System.

The sealed ultra-high-pressure circuit ran at between 1200 and 1600 psi, depending on the rate of firing. The HP boiler worked at approx 850 psi, and the LP boiler at 200 - 250 psi. The LP cylinders were driven with a mixture of the HP cylinder exhaust and the LP boiler output. Note that both boilers had superheaters.

The French 241P, the German H17-206 and the British LMS Fury all used the Schmidt system, and were of basically similiar design. The New York Central HS-1a and the Canadian 8000 also used the Schmidt system but were a size larger altogether- the 8000 weighed more than twice the Fury.

None of them provided worthwhile economies.

Some details of the various Schmidt locos:

Railway	German State	LMS	NYC	CPR	PLM
Date	1925	1929	1931	1931	1929
Name	H17	Fury	HS-1a	8000	PL241B
Wheels	4-6-0	4-6-0	4-8-4	2-10-4	4-8-2
Fuel	Coal	Coal	Coal	Oil	Coal
HP boiler pressure psi	850	900	850	850	850
LP boiler pressure psi	205	250	250	250	200
HP cylinders diam in	11.4	11.5	13	15.5	9.5
HP cylinders stroke in	24.8	26	30	28	25.6
LP cylinders diam in	19.7	18	23	24	22
LP cylinders stroke in	24.8	26	30	30	27.5
Weight tons	91	89.3	181	206	115
Tractive effort lb	?	33,200	66,000	83,300	?

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OTHER HIGH-PRESSURE LOCOMOTIVES
Here is a guide to high-pressure locomotives that did not use the Schmidt system, neatly tabled in order of boiler pressure. (Over here Dr Asperger! Quick!) For several of them, for example the turbine designs, the operating pressure was not the most salient feature, so they do not appear in the high pressure section of the Unusual Locomotives page.

Locomotive Name	Country	Pressure psi	Boiler type
Velox	France	290	Velox water-tube
C&O Turbine	USA	310	Fire-tube
T18-1002 Turbine	Germany	323	Fire-tube?
Horatio Allen	USA	350	Water-tube
J B Jervis	USA	400	Water-tube
James Archbold	USA	500	Water-tube
L F Loree	USA	500	Water-tube
H45	GDR	750	Water-tube
232.P.1	France	840	Water & fire-tube
Eb3/5	Switzerland	850	Water-tube
UP Turbine	USA	1500	Water-tube
H02	Germany	1750	Schwarzkopff-Löffler

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