TURBINES 



463 



this change from pressure head to velocity head necessitates an equiva- 

 lent reaction on the moving vanes, and work is thus done on the turbine 

 shaft. This turbine, as well as the impulse type, therefore owes its 

 propelling force not to statical pressure, but to changes of momentum, 

 the pressure difference over the concave and convex faces of a bucket 

 being produced, as in the impulse wheel, by the change of momentum in 

 the stream passing the bucket. In the case of a wheel having vertical 

 downward flow the weight of the water also adds to this propelling 

 force. 



Since the turbine works under pressure the buckets should always 





Ventilating 

 Holes 



Regulating 

 Slide -* 



Wheel n 



Vanes" 



K 2 5b to 3 Ob >| 



FIG. 210. Axial Flow Girard Turbine. 



remain full of water, and to this end admission should take place con- 

 tinuously all around the circumference of the wheel. If not, those 

 buckets which happen to be idle will either be empty, having discharged 

 their contents into the discharge pipe, or will contain dead water. In 

 the first case the buckets must be refilled before the pressure at the 

 circumference can be utilised, while in the second case the necessity for 

 imparting momentum to this dead water causes loss of energy by impact. 

 In either case the loss of energy may be considerable. In the impulse 

 wheel, on the other hand, the supply may be admitted either wholly or 

 partially around the circumference without loss of energy. 



These two main types of turbine may be subdivided, according as the 

 general direction of flow through the wheel is radial and perpendicular 



