204 HYDRAULIC EQUIPMENT 



than 90 and the angle a of the water leaving the guides is also 

 small. 



Fig. 105 represents a medium-speed runner, the angle /3 being 

 approximately 90 and the angle a larger than in the previous 

 type. 



Fig. 106 represents a high-speed runner for low heads and rel- 

 atively large quantities of water. It is seen that the angle is 

 larger than 90 and angle a also considerably larger than before, 

 thus giving a very high peripheral velocity. 



Comparing the above types it is also seen the width of the 

 buckets varies from a comparatively narrow to a wide size, while, 

 on the other hand, the shape changes from a forward to a back- 

 ward curved bucket as the speed increases. 



Impulse Turbines. The impulse or tangential turbine is 

 generally known as the Pelton turbine. It is a kinetic energy 

 wheel, the water being discharged from one or more nozzles against 

 a number of buckets attached to the periphery of the runner, 

 and the momentum of the mass of water in its impulse upon the 

 runner buckets is, therefore, the main principle utilized in the 

 energy transformation. When the water leaves the buckets it is 

 moving at so slow an absolute velocity that practically its entire 

 energy has been imparted to the runner. 



Since usually the number of nozzles is small as compared with 

 the number of buckets, the latter are in active use only during 

 part of a revolution, and hence this type of prime mover is some- 

 times called a water wheel instead of a turbine. This distinction 

 seems, however, rather arbitrary, ancl can probably be traced to 

 an attempt to show that the impulse turbine is derived from an 

 undershot water wheel. It seems, therefore, better to consider 

 both types as turbines, since they both really involve the same 

 principles and action. As a matter of fact, the term water wheel 

 is loosely applied to all sorts of turbines. 



The description of the tangential type of impulse turbine is 

 given on page 242. 



The speed of an impulse turbine of a given diameter is variable 

 only within very small limits. The speed is practically deter- 

 mined by the head, and can be varied only by variation of the 

 runner diameter. 



Selection of Turbines. In deciding upon the number, capacity 

 and speed of the units in a water-power station, the combination 



