In entering the turbine, steam acts in two ways, and turbines are accordingly constructed on two plans. The more important type and the only one to be described here, is the impulse turbine, in which the steam from the boiler is completely or almost completely expanded into an expanding nozzle. As the steam forcibly strikes the vanes of the wheel, the turbine wheel rotates at a very high velocity. This is illustrated in the De Laval turbine which is used in place of the ordinary steam engine in the generation of electric power, or in the transmission of any other form of energy derived from steam.
Fig. 171. - Steam Turbine.
The working of the De Laval turbine is as follows: The steam is blown through stationary divergent nozzles where it is allowed to expand to the pressure of the exhaust chamber. Each particle of steam, which moves very rapidly, strikes against a concave vane or plate which projects from the drum like a spoke. This causes the wheel to move rapidly. The outer end of the buckets are covered by a ring which prevents the centrifugal escape of the steam. The nozzles vary in number and can be closed independently of each other, so that the number in use may be made to suit conditions of running.
As the material composing the turbine machine limits the speed at which it can safely be run, it is necessary to have some form of reducing gear in the transmission. The smaller types of De Laval turbines run at about 30,000 R. P. M., and are geared down to about 3000. The larger sizes run at about 10,000 R. P. M. under gear. Even with all the disadvantages of gearing, the turbine is used extensively in units ranging from 1 1/2 to 200 H. P.
Its principal parts are the shaft, drum, cylindrical case inside of which the drum revolves, vanes on the drum and cylindrical part, balance pistons.