TURBINES 207 



design, but built IH various sizes and operated under the same 

 head: 



Q varies as D 2 ; 



H.P. varies as Z) 2 ; 



R.P.M. varies as yr. 



Hence, the speeds -of a set of similar runners, operating under 

 the same head, will vary inversely as the square roots of their 

 horse-powers, and if one runner gives a speed of R.P.M. with a 

 power H.P., it follows that the speed of a 1 H.P. turbine will be 

 R.P.M. XVILR Thus, if the head be 1 foot, the speed of the 

 1 H.P. runner or its specific speed, N s , will be 



R.P.M /H.P. 

 A'/i K \ /i 3 /* 



or 



-V, = R.P.M. X 



If it is desired to obtain the specific speed according to the metric 

 system with English units (Ft. and H.P.) used in the formula, tnul- 

 tiply the values obtained from the above formula by 4.45. In 



transferring we have 1 foot equal to ;r- meter and 1 English H.P. 



o.Zo 



equal to 0.986 metric H.P. Thus 





The value /i 6 / 4 may readily be figured out as follows: 



The diagram in Fig. 107 supplies a convenient graphic method 

 of deducing the specific speed of a runner from any given set of 

 conditions without the use of the formula. 



In figuring the specific speed of a turbine with more than one 

 runner or nozzle, the H.P. used should, of course, be the output 

 from each runner or nozzle. Furthermore, as the above formula 

 applies to single-runner turbines, it follows that in the case of a 

 turbine of the same capacity having n runners of ie same spe- 

 cific speed, it is seen that the R.P.M. would be \fn times the 



