Steam Engine

Enough has been said to show that the invention of the steam turbine is by far the most important step in steam engineering since the time of Watt. It solved the problem of using steam ef ficiently in an engine without reciprocating parts.

Early Theory.

In the early development of the steam engine inventors had little in the way of theory to guide them. Watt had the advantage of a knowledge of Joseph Black's doctrine of latent heat ; but there was no philosophy of the relation of work to heat until long after the inventions of Watt were complete. The theory of the steam engine as a heat engine may be said to date from 1824, when N. L. Sadi Carnot published his Re flexions sur la puissance motrice du feu, a remarkable essay in which he showed that heat does work only by being let down from a higher to a lower temperature. But Carnot was not then aware that any of the heat disappears in the process, and it was not until the doc trine of the conservation of energy was established in 1843 by the experiments of James Prescott Joule that the theory of heat en gines began a vigorous growth. From 1849 onwards the science of thermodynamics was developed with extraordinary rapidity by Rudolf Clausius, W. J. Macquorn Rankine and William Thomson (Lord Kelvin) and was applied, especially by Rankine, to prac tical problems in the use of steam. Rankine's Manual of the

Steam Engine, published in 1859, was the first attempt at a sys tematic treatment of steam-engine theory. It involved the sim plifying assumption that the cylinder and piston might be treated as behaving to the steam like non-conducting bodies, in other words, that the transfer of heat between the steam and the metal might generally be disregarded. One effect of this was to treat the volume of steam consumed per stroke as corresponding to the volume of the cylinder up to the point of cut-off. When steam enters the engine cylinder it finds the metal chilled by the previous exhaust, and a portion of it is at once condensed. This has the effect of increasing, often very largely, the volume of boiler steam required per stroke. As expansion goes on, the water that was condensed during admission begins to be re-evaporated, and this action is often prolonged into the exhaust. It is now recognized that exchanges of heat between the steam and its metal envelope cannot be ignored. They cause the actual performance to fall short, in some cases very much short, of the ideal limit. They may be reduced by suitable design.