TURBINES 



239 



Where the intake openings are large it has become general 

 practice to divide the openings by means of vertical piers in a 

 number of channels (Fig. 130). This insures a more uniform dis- 



FIG. 130. Sectional Plan of Cedar Rapids Wheel Chambers. 



tribution of the water around the runner, while, on the other hand, 

 it strengthens the casing by subdividing the span. It also greatly 

 facilitates the application of the gates, which otherwise would be 

 of a size hardly possible to manipulate. 



Draft Tubes. A correct draft-tube design is absolutely es- 

 sential in order to obtain the maximum efficiency of a turbine 

 as a whole. It is an integral part of the design of the turbine and 

 should be furnished by the turbine builder. The fundamental 

 principles underlying their design and construction are that the 

 water shall leave the draft tube with as small velocity as possible 

 so that the maximum amount of kinetic energy is abstracted from 

 the water. The velocity of the water in the tailrace must, fur- 

 thermore, be sufficient to prevent it from backing up and it is, 

 therefore, necessary that the water emerging from the draft tube 

 must have a velocity at least equal to that in the tailrace. In 

 order to accomplish this the draft tube should be constructed on a 



