FRANCIS TURBINE 



489 



(5) In this type of turbine the supply water when moving with its maxi- 

 mum velocity (at entrance) is admitted to the wheel at its outer circumfer- 

 ence, and consequently at its most rapidly moving part. It thus becomes 

 practicable to design the guide passages and inlet vanes so that even for the 

 highest heads water may be admitted to the wheel without shock, while 

 since the water leaves 

 at the centre, which is 

 the most slowly moving 

 part of the wheel, it is 

 more easily discharged 

 without excessive loss 

 of kinetic energy. 



The wheel was im- 

 proved by constructing 

 its vanes so as to give 

 a combined radial and 

 axial discharge, and to 

 this end these vanes at 

 exit are given a curva- 

 ture in a direction paral- 

 lel to that of the disc 

 (see Fig. 280). This 

 largely increases the 

 available discharge area 

 and enables a greater 

 volume of water to be 

 handled with a given 

 size of wheel. 



Regulation is usually 

 performed by throttling 

 the supply of water to 

 the wheel either by slid- 

 ing cylindrical gates, register gates, or wicket gates. The cylindrical gate 

 usually consists of a plain cylinder throttling the supply at the entrance to 

 the wheel buckets (Fig. 230). In some instances these gates have been fitted 

 with fingers fitting between the guide vanes, as indicated in dotted lines 

 in Fig. 230, with the idea of diminishing the contraction in section of the 

 entrant stream, and thus the loss by shock at part gate. While this object is 

 realized to a limited extent, yet the introduction of the fingers, as will be 

 explained later (p. 525), renders close speed regulation almost impossible. 



FIG. 232. Inside Register Gate for Francis Turbine. 



