476 



HYDRAULICS AND ITS APPLICATIONS 



moving in the same direction as the wheel buckets, impinges on the 

 curved vanes B, has its direction of motion changed, and finally escapes 

 around the periphery of the wheel. 



This type of turbine has been used with heads from 1 foot to 350 feet, 

 and with moderate heads is capable of an efficiency of about 75 per cent. 

 at full power. With high heads the speed is, however, inconveniently 

 high, and the size of buckets and guide passages consequently small, 

 rendering these very apt to be choked. Speed regulation is usually 



performed either by 

 throttling the supply at 

 its entrance to the wheel 

 by means of a sliding 

 cylindrical or ring gate, 

 or sluice, fitting between 

 the fixed and moving 

 vanes (Fig. 220) and 

 actuated by means of a 

 governor or by thrott- 

 ling the discharge by 

 means of a similar ring 

 gate fitting outside the 

 moving vanes (Fig. 221). 

 This, by increasing the 

 pressure in the wheel 

 reduces the effective head 

 producing rotation. 

 The former method has 



,, -,. -, 



the Disadvantage that 



the entering streams of 

 water, after their contraction in escaping past the edge of the ring gate, 

 re-expand to fill the moving buckets, with consequent loss of energy. On 

 account of this the efficiency at part gate is low. It may be improved if 

 the turbine wheel be divided by parallel diaphragms as indicated in 

 Figs. 220 and 221, into what is in effect a series of wheels in parallel. 



In such a wheel this enlargement of section after cut off can only affect 

 one chamber, and the part gate efficiency is in consequence increased. 

 The method has the drawback, however, that the areas of the apertures are 

 reduced and also that frictional resistances to flow are increased by the dia- 

 phragms, so that the full gate efficiency suffers. In spite of this, it forma 

 the most general and satisfactory method of governing turbines of this type. 



FlG. 219. Section through Guides and Runner of 

 Fourneyron Turbine. 



