TURBINES 



209 



By comparison of the specific speeds it is possible to judge the 

 characteristics of water-wheel runners without considering their 

 pctual speed, power or head. Other things being equal, a high 

 specific speed means a high actual speed, and a low specific speed, 

 a low actual speed in revolutions per minute. With low-head 

 developments the speed must be selected as high as is good en- 

 gineering practice in order to keep down the weight and conse- 

 quently the cost of the generators. With very high heads it is 

 mostly a question of keeping the speed reasonably low so as to 

 avoid the use of costly high-speed generators. The limit of high 

 speed for low-head developments is fixed by the progress of the art 

 of designing high-speed runners, and the limit of low speeds under 

 high heads is fixed by the risks involved in designing runners for 

 operation with very low coefficient of specific speed. 



High speed under low heads also means large discharge capacity 

 per unit diameter, resulting in a large power capacity, while low 

 speeds under high heads mean a small discharge capacity per unit 

 diameter resulting in a small width of runner. 



Reaction turbines have been built for specific speeds as low as 7, 

 but 12 is probably as low as should be used in normal practice. 

 Starting at this speed, the efficiency will increase as the specific 

 speed approaches more normal values, the efficiency reaching 

 the highest values between specific speeds of 25 and 75. Above 90 

 it drops again at a rapidly increasing rate, and above 100 specific 

 speed results are much more problematic, at least for the present, 

 but efficiencies over 90 per cent have been obtained with specific 

 speeds all the way from 25 to 90. 



Pfau in his paper before the International Engineering Con- 

 gress in San Francisco classifies tentatively Francis reaction tur- 

 bines as follows, the types being claimed to operate successfully 

 under the heads given. 



TABLE XXXVIII 



