FRANCIS TURBINE 487 



more particularly, for mechanical reasons, to the inward flow type. It 

 is not commonly used with impulse turbines, though it may be fitted if 

 desired. 



The pressure turbine thus fitted possesses many advantages over. the 

 submerged type. The turbine is dry when the head water is cut off, 

 and is easily accessible : a horizontal shaft machine may frequently be 

 adopted where otherwise this would be impossible, and the general 

 accessibility for examination and repairs is. greatly increased, while the 

 risk of damage to machinery by floods is diminished. 



With a single horizontal shaft turbine, or with two horizontal turbines 

 on the same shaft if these discharge outwards, it is necessary for the 

 wheel shaft to pass through the draught tube as in 

 Fig. 228, arid a stuffing box becomes necessary to pre- 

 vent leakage of air into the tube. Tightness is com- 

 monly assured by means of a water seal, consisting of 

 a chamber C surrounding the shaft (Fig. 229), and sup- 

 plied with pressure water from the penstock by means 

 of a small pipe P. Any slackness at the gland then 

 allows this pressure water to escape outwards, or into FIG 929 ^L W ater Seal 

 the draught tube, and does not lead to air leakage. for Draught Tube 



While the draught tube often discharges vertically 



into the tail-race, it is advantageous to fit a right-angled bend beneath 

 the water level at exit, and to discharge in the direction of flow of the tail- 

 race. By so doing the kinetic energy of discharge is not entirely wasted, 

 a fair proportion being utilized in producing this flow. A still further 

 proportion may be utilized if the section of the tube be gradually 

 increased towards its exit. 



ART. 135. THE FRANCIS TURBINE. 



The next important step in turbine development was due to J. B. Francis 

 (1849), who, placing the guide vanes outside the wheel, and reversing the 

 direction of flow of the Fourneyron turbine so as to discharge at the 

 centre, obtained the inward radial flow turbine bearing his name 

 (Fig. 230). 



Several important advantages accompanied this change in design. 



(1) The inlet ports and wheel passages now being convergent, steady 

 flow became possible throughout the wheel. 



(2) The increased accessibility of the guide passages and vanes made 

 it possible to use improved methods of regulating the flow of water to the 

 runner. 



