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High-pressure locomotive
















AT first sight the locomotive illustrated above does not, judged by modern standards, look anything out of the ordinary, but it is, nevertheless, an interesting and unique engine. The running parts are, in the main, those of a 4-6-0 express locomotive of the German State Railways, but there are three cylinders and three distinct boilers. All the boilers work at different pressures. The locomotive is an experimental one, built for the Schmidt’sche Heissdampf Gesellschaft, by Henschel und Sohn, Cassel, Germany.


It is well known that greater fuel economy is obtained as boiler pressures are increased; but the use of exceptionally high pressures involves, first, difficulties in construction and, secondly, trouble from scale deposited by the feed water on the heating surfaces. In this engine the steam from the boiler working at the highest pressure is not used directly as motive-power, and no feed water is introduced into that boiler.


The boiler consists of a number of tubes arranged in the form of a rectangular box and constituting the walls of the furnace. The tubes are, virtually, closed rings, part of which pass through the water of the second boiler. Steam at a pressure of anything between 1,100 lb and 1,300 lb per sq in is generated in the furnace part of the tubes and thence passing into the upper part immersed in the water of the second boiler. Here the steam, in giving up heat to that water, condenses and returns to the furnace part of the tubes to be evaporated again; the cycle is continuously repeated. The water in the first boiler is chemically pure and does not need to be added to unless some part is lost through the safety valve  or by leakage. There is thus no chance of scale being deposited in the tubes.


Steam is generated in the second boiler at a pressure of 853 lb per sq in and this steam is drawn off in the usual way. The third boiler is virtually the barrel of the ordinary type of locomotive boiler, that is, it is a drum in the water space of which are a number of longitudinal horizontal tubes forming the passages between the furnace and the smoke box.


The steam from the second boiler passes, by way of a superheater in the upper tubes of the third boiler, to the single high-pressure cylinder. The exhaust from this cylinder, because the inlet steam is at 853 lb per sq in, is naturally at a comparatively high pressure and, instead of being discharged up the funnel in the usual manner, it is led into a header which also receives steam from the third boiler at a pressure of 199 lb per sq in.


The mixed steam is passed through a superheater in the lower tubes of the third boiler and is then taken to the two low-pressure cylinders. The exhaust steam from the low-pressure cylinders is led through a feed water heater situated on top of the smokebox in front of the funnel. A large part of the scale is deposited in this heater, the water being drawn from the tender, and the heated, and largely purified, feed water is delivered to the third boiler. The feed water for the second boiler is taken from the third boiler, the transfer being made by a high-pressure pump.


In this way it was expected that the difficulties due to scale would be eliminated and that the economies arising from the use of exceptionally high pressures would be obtained. The locomotive has a wheelbase of 30 feet. The driving wheels are 6 ft 6 in in diameter, and the bogie wheels are of 3 ft 3⅜ in diameter. The grate area is 27 sq ft. The high-pressure cylinder is 11 7/16 in and the two low-pressure cylinders are 19 11/16 in. in diameter. All three cylinders have a stroke of 24 13/16 in. The weight of the locomotive in working order is 89.2 tons.


The engine was tested on eleven runs varying between 70 miles and 101 miles, the track being virtually level. The trains hauled varied in weight between 454 tons and 760 tons and the speeds between 55 miles and 62 miles an hour. The object of the tests was to find out how far the performance of this type of locomotive could be improved by the new design of boiler. A saving of about 20 per cent in the fuel consumption was effected. After four months' running the only scale present in the second boiler was a fine loose dust easily removed.


It may seem strange that an old engine should have been used for this experiment instead of a new one, but this procedure embodies the scientific principle that, in making an experiment, only one factor should be varied at a time. If the running gear of the engine had been altered as well as the boiler it would have been impossible to say which of the two was responsible for the improvement.

[From part 14, published 1 June 1937]

High-Pressure Locomotive