GIRARD TURBINE 



465 



area, and are in consequence liable to be choked by any floating matter 

 which may escape the strainers in the head-race. 



For very high falls, then, the most suitable turbine is one in which the 

 wheel itself is not submerged ; in which the supply may be admitted to 

 as much of the circumference as is necessary to develop the required 

 power ; and such that any particular diameter may be adopted which 

 will best suit the desired speed of rotation. 



The impulse turbine in the form of the tangential water-wheel or of 

 the Girard turbine satisfies these conditions within wide limits, and for 

 heads between 100 and 300 feet will often, and for heads above 300 feet 

 will generally be the most suitable type of prime mover for all but the 

 largest powers. Since with suitable means of regulation the jet velocity 



PIG. 211. Girard Turbine with Outward Radial Flow. 



is constant under a constant supply head for all loads, under these 

 circumstances the efficiency of an impulse turbine is approximately 

 independent of the load, the variation in efficiency being chiefly due to 

 the proportionately greater effect of mechanical friction and of air resist- 

 ance or windage at low loads. 



ART. 129. THE GIRARD TURBINE. 



This turbine may be constructed either as an axial flow machine 

 (Fig. 210), in which case the axis is usually vertical, or with inward or 

 more commonly outward radial flow (Figs. 211 and 212). With radial 

 flow the axis may be either vertical or horizontal. The axial flow type 

 is more suitable for large powers under comparatively low heads, where 

 full circumferential injection is required. 



H.A.. 



H H 



