480 



HYDRAULICS AND ITS APPLICATIONS 



avoid undue shock at entrance, the radial width of the buckets must then 

 be small compared with the radius of the wheel, and in general should 

 not exceed one-fifth of the latter. 



Governing is usually performed by cutting off the supply to one or 

 more guide passages by a circular slide (Fig. 224), or by a scroll gate, 

 and this greatly reduces the efficiency at part gate. Thus a turbine 

 which has an efficiency of, say, 82 per cent, at full gate, will probably not 

 exceed 66 per cent, at half gate. 



The most important improvement in the design of the Jonval turbine 



FIG. 224. Section through Runner and Guides of Downward Flow Jonval Turbine. 



consisted in the subdivision of the wheel into concentric compartments, 

 each forming a complete turbine. 



Then by regulating the water supply to each compartment in turn, and, 

 if necessary, completely cutting off the supply to one or more compart- 

 ments, the wheel may be utilized with fairly high efficiencies, and, by 

 supplying compartments at different distances from the axis, may be 

 given a constant speed under varying heads. Thus with a double turbine 

 the outer compartment, having a higher velocity, would be used with a 

 minimum supply and a maximum head, while the outer and inner com- 

 partments together would be designed to give the same power with a 

 reduced head and an increased supply. The vane angles should be 

 different in the two compartments. 



