CHAPTER XIV 



Turbines Types Impulse Pressure Girard Haenel Barker's Mill Fourneyron Jonval 

 The Suction Tube Francis Thomson Compound Turbines American Mixed Flow 

 Type Governing of Turbines Head and Tail Races. 



AET. 127. TURBINES. 



IN general, by a turbine is meant a water-wheel which is so arranged 

 as to allow of water being admitted simultaneously at all points on its 

 circumference, thus enabling a greatly increased power to be obtained 

 with the same wheel diameter. 



Turbines may be divided into two main classes, known respectively 

 as Impulse turbines and as Pressure or Reaction turbines, according to the 

 manner in which they abstract energy from the supply water. 



In an impulse turbine, the whole head of the supply water is converted 

 into kinetic energy before the wheel is reached, the water issuing from 

 the nozzles or guide passages in a series of streams or jets moving with 

 high velocity and exposed to the pressure (usually atmospheric) obtaining 

 in the turbine casing. It then enters a series of buckets formed by 

 curved vanes in the turbine wheel, and in virtue of the change of 

 direction, and hence of tangential momentum produced by these vanes, 

 exerts a driving force, and so does work on the turbine shaft. Its surface 

 pressure remains uniform throughout the turbine if this is correctly 

 designed, and its direction is freely deviated by the vanes. For this 

 reason, this is sometimes termed a turbine of free deviation. 



For the pressure to remain uniform throughout the wheel it is essential 

 that the stream should not fill the space between any two moving vanes, 

 and to prevent this occurring the buckets are usually ventilated as shown 

 in Fig. 210, which represents a part section through the wheel and 

 guides of a Girard turbine. 



In a Pressure or Reaction turbine, the water on leaving the guide vanes 

 and entering the wheel is under pressure, and thus supplies energy partly 

 in the kinetic and partly in the pressure form. In its passage through 

 the wheel this pressure energy is gradually converted into kinetic energy, 

 and the water finally leaves the wheel at a pressure not sensibly greater 

 than that of the atmosphere. The change of momentum accompanying 



