206 HYDRAULIC EQUIPMENT 



quently the R.P.M. corresponding to the peripheral speed may 

 .vary widely according to type, make, and number of runners or 

 jets. 



Specific Speed. Turbine runners of different makes are best 

 compared on the basis of their specific speeds, this being the 

 number of revolutions per minute at the point of maximum 

 efficiency that a homologous or geometrically similar wheel would 

 give if it were to deliver 1 horse-power under unit head, usually 

 1 foot. With the same specific speeds the different designs vary 

 comparatively little, it being the aim of manufacturers to produce 

 a line of turbines covering all specific speeds with the highest effi- 

 ciencies possible at each specific speed, and turbines for use under 

 low heads should have as high a specific speed as possible with- 

 out sacrificing efficiency or other desirable characteristics. After 

 a certain design has been adopted for a certain specific speed, a 

 full series of such turbines can be laid out, all of identical design 

 with the original, each being an enlargement or reduction of 

 another. 



If 



Q = quantity of water; 



h = head; 



D = diameter of runner; 



then for any ,.ven turbine: 



Q varies as /i 1/2 ; 



H.P varies as QXh or fc 3/2 ; 



R.P.M. varies as h l/z . 



Hence, the horse-power delivered under 1 foot-head will be 

 HP l = 5^i a nd the speed will be R.P.M. 1 = R '^'f 1 ' 



If now the head is kept constant and it is assumed that all 

 dimensions of the runner are reduced proportionally, then the 

 dimensions will all remain in fixed ratio to the diameter, Z), and 

 all areas of passages through the runner will vary in proportion to 

 Z) 2 ; the velocities remaining constant on account of the constant 

 head. 



Therefore, for turbines of homologous or geometrically similar 



