Page:The steam turbine, the Rede lecture 1911 (IA steamturbinerede00parsrich).pdf/20

8 THE PRINCIPLE OF THE essential if the turbine motor was to receive general acceptance as a prime mover. I therefore decided to split up the fall in pressure of the steam into small fractional expansions over a large number of turbines in series, so that the velocity of the steam nowhere should be great. Consequently, as we shall see later, a moderate speed of turbine suffices for the highest economy. This principle of compounding turbines in series is now universally used in all except very small engines, where economy in steam is of secondary importance. The arrangement of small falls in pressure at each turbine also appeared to me to be surer to give a high efficiency, because the steam flowed practically in a non-expansive manner through each individual turbine, and consequently in an analogous way to water in hydraulic turbines whose high efficiency at that date had been proved by accurate tests. I was also anxious to avoid the wellknown cutting action on metal of steam at high velocity.