THEORY OF TURBINE DESIGN 



563 



are prevented from following the path PA by the action of the outer 

 walls of the bucket, but at points nearer the centre of rotation this con- 

 straint is absent and the particles 

 tend to foilow their natural paths. 

 The relative motion which then takes 

 place may be prevented by designing 

 the buckets so that the actual path 

 of each particle, in plan, is a straight 

 line perpendicular to the radius. In 

 this case, if Q is the middle point of 

 the bucket at inlet, R will be its 

 middle point at outlet, and the bucket 

 will be splayed out symmetrically 

 about R instead of B'. 



A second method which has been 

 adopted consists in making the 

 outlet edge at P, parallel to P S, 

 instead of being radial, where 



S P O = A P B. The inlet edge C D is then made parallel to this, 

 with the result that the length of path traversed by the various particles 

 becomes more nearly equal and the relative motion is largely prevented. 



FIG. 264. 



AKT. 151. GENERAL COMPARISON OF IMPULSE AND PRESSURE 

 TURBINES. 



The impulse wheel, having a peripheral velocity of approximately 



2 g H for the pressure turbine, is well 



\ f 2 g H, as against about '7 

 fitted for very high falls. For the same reason its velocity becomes low 

 under low heads, and this renders it unsuitable for driving electric 

 generating machinery under such conditions. Further, with low falls 

 the percentage variation in head is generally comparatively large, and 

 the difficulty of maintaining the speed approximately constant, and 

 of maintaining the efficiency under such head variations, is great. 

 On the other hand, the part gate efficiency is high where the head 

 is constant and where the load or supply is variable. Either type 

 may be used for any head up to 500 feet, though the impulse type is 

 preferable for heads above about 200 feet, except for very large power e 

 Also with either type full load efficiencies of about 85 per cent, may be 

 obtained under favourable circumstances, the more modern type of 

 Francis turbines with swivel guides having a slight advantage in this 



n n 9. 



