Page:Model steam turbines; how to design and build them (IA modelsteamturbin00harrrich).pdf/27

Rh nozzle 1600 feet per second; whilst at 80 lbs. with a ratio of expansion of $(80+15)/15$=6$1/3$, the velocity is 2720, or 1⋅7 times as great. From the very close agreement of the throat velocities we should expect that up to the throat the ratio of expansion is the same whatever the initial pressure may be and this is so, it being a well-known fact that steam discharging through a converging nozzle expands to 58 per cent, of the initial pressure. This peculiarity

Fig. 11. -- De Laval Nozzle.

(which will be useful for calculating the size of the orifice for passing a given weight of steam) may be explained shortly as follows:--

In expanding to any extent, the volume of steam increases directly as the ratio of expansion. Thus at 20 lbs. absolute the volume of a pound of steam is 19⋅73 cubic feet, and at 10 lbs. absolute (ratio of expansion 2) 37⋅83 cubic feet. With a fixed discharge orifice, if the volume of steam is doubled, say, by increasing the ratio of expansion, the velocity required to discharge it in the same time must be