FRANCIS TURBINE 



501 



Strictly speaking, the spiral casing forms an essential part of the 

 Thomson wheel, though this may be replaced by an ordinary cylindrical 

 casing (Fig. 237), or the turbine be submerged in the open penstock with 

 free approach from all sides (Fig. 238), without seriously affecting the 

 efficiency. 



Where, however, it is required to take off power below head-water level 

 a casing is essential, while in any case it is advisable for heads exceed- 

 ing 10 feet. For medium falls above 10 feet, the horizontal shaft machine 

 is, in general, to be preferred. 



As modified by Fink and others, the pivoted guide vane devised by 

 Thomson is largely fitted to modern turbines of the Francis type, these 



Regulator 



FIG. 238. Arrangement of Vertical Shaft Single Vortex or Combined Flow Turbine 

 working under Low Head in Open Forebay. 



turbines only differing from the former type in that the guide vanes are 

 shorter, while their number is increased until approximately equal to the 

 number of wheel vanes. 



The machine as thus constructed is better fitted for dealing with large 

 volumes of water, and may be taken as being at present the most perfect 

 type of pressure turbine. 



Figs. 239 A, B, c, D, E, 1 illustrate^with some detail what is one of the 

 largest single wheel turbines of this" type yet built. This wheel, which 

 has inward radial flow and combined radial and axial discharge, is 

 designed to give 10,000 B.H.P. at 300 revolutions per minute, under an 



i At Snoqualmie Falls; reproduced by courtesy of the makers, the Platt Iron Work? 

 Davton, Ohio. 



