I TURBINES 211 



In selecting a type of wheel, the number of runners or nozzles, 

 their capacity and speed must be chosen with a view of obtaining 

 not only the highest efficiency, but also the most economical com- 

 bination of the prime mover and generator. For example, a 

 wheel is to be operated under a head of 400 feet and develop 1500 

 H.P. How many nozzles should it have and at what speed may it 

 operate? Assume that a specific speed of 4 will give a good effi- 

 ciency for the wheel, then the actual speed of the unit will be 



This speed, however, may be entirely too low for the generator, 

 and, by providing two nozzles, each supplying 750 H.P. the 

 speed would be increased to 



R.P.M. = 185Xv / 2 = 260. 



and with four nozzles 



Let us also see what the result would be if we tried to apply a 

 reaction turbine running at 720 R.P.M. The specific speed would 

 then be 



and, consequently, this type would undoubtedly be the most 

 advantageous to use for our case. 



The efficiencies, especially at partial load, are related to the 

 specific speed, the curves of high specific speed runners being more 

 pointed than with the low specific speed type, thus allowing a 

 narrower margin for operation under the best conditions. This 

 is clearly shown in the curves in Fig. 108. 



The maximum full-load capacity of a turbine is that point 

 beyond which the output decreases with an increase in gate open- 

 ing. The margin between the point of maximum efficiency and of 

 maximum capacity depends upon the specific speed of the runner, 

 and is smaller the higher the specific speed. This is illustrated 

 in Fig. 108, which shows that as the specific speed is increased the 

 point at which maximum efficiency occurs approaches nearer to 

 the power delivered at full gate opening. The specific speed may 

 thus be increased to such an extent that the point of maximum 



