FRANCIS TURBINE 



505 



The greater part of the pressure thrust is eliminated by venting the 

 space behind the wheel into the draught tube through six holes in the 

 wheel disc, while a series of radial vanes cast on the back head of the 

 wheel casing and almost touching the wheel prevent the formation of a 

 forced vortex behind the wheel. The increase in pressure outwards, due 



Thrust ff/ocfi 

 4 rings - 



FIG. 239D. Longitudinal Section of Single Wheel Horizontal Shaft Francis Turbine 

 developing 10,000 H.P. at 300 R.P.M. under 260 ft. effective head. Outer diameter of 

 runner 66 ins. 



to centrifugal action, is thus prevented to a large extent, and the mean 

 pressure reduced. There is, however, a resultant thrust towards the 

 draught tube, which increases with the gate opening, and a closer balance 

 is obtained by means of a balancing piston behind the back head of the 

 wheel casing. This is a forged enlargement of the shaft, 17 inches in 

 diameter, and works in a water-packed brass sleeve as shown in Fig. 239o. 

 The chamber in front of the piston is supplied with pressure water 



