484 



HYDRAULICS AND ITS APPLICATIONS 



charges below the surface level in the tail-race, thus forming what is 



termed a Suction or Draught tube. By this means the turbine may be 

 placed at any level, up to about 25 feet 1 above 

 the tail-race without any loss of head. The truth 

 of this statement may be seen if it be considered 

 that since the pressure in the draught tube at 

 the tail-race level is approximately equal to that 

 of the atmosphere (neglecting the kinetic head 

 in the tube), the pressure at the turbine will be 

 less than this by an amount equivalent to the 

 difference of statical head at turbine and tail- 

 race, so that the available head at the turbine 

 is equal to the difference of level between tur- 

 bine and head-race, together with the difference 

 between turbine and tail-race, i.e., to the differ- 

 ence between head-race and tail-race. Expressed 



symbolically, if suffixes (1) and (2) refer respectively to turbine and tail- 



race (Fig. 227), 



W=W -(fc-/n)feet. 

 .*. (Pressure + potential) head at exit from turbine, which equals 



FIG. 227. 



+ ht - fo feet. 



w 



/, /2! o 



-th- Jr - 0. 



.*. Available head for driving turbine = h^ , feet. 



= li% feet. 



In order that on starting a turbine the air may be carried out of the 

 draught tube, the velocity of flow through the tube should be greater 



1 The maximum elevation depends largely on the diameter of the draught tube, the 

 following values, adapted from those given by Meissner, showing the maximum values to be 

 used with a given diameter of tube. 



Krom these values, however, should be subtracted the head , corresponding to the velocity 

 of flow v feet per second, down the tube. 



