4G4 HYDRAULICS AND ITS APPLICATIONS 



to the axis, parallel to the axis, or is a combination of these, these sub- 

 types being designated respectively as 



Radial flow \ 



Axial or parallel flow L turbines. 

 Mixed flow 



Radial flow turbines may again be subdivided into inward flow 

 machines, when flow takes place from the circumference to the centre of 

 the wheel, and outward flow when the flow is in the opposite direction. 

 The chief turbines in the various classes are : 



(1) Impulse Turbines. 



Axial or radial flow . . Girard turbine. 

 Mixed flow .... Pelton wheel. 



(2) Pressure or Reaction Turbines. 



{(a) Inward. Thomson vortex turbine and the original 

 Francis turbine. 

 (b) Outward. Fourneyron turbine. 



Axial flow . . Borda turbine, Henschel-Jonval turbine. 

 Mixed flow . . Hercules, Victor, and other turbines of the Ameri- 

 can type. All modern Francis turbines are to 

 a certain extent of the mixed flow type. 



One special type of turbine, the Haenel, may work either as an impulse 

 or pressure wheel. 



AKT. 128. THE IMPULSE TUKEINE. 



All impulse turbines may be considered as modifications of the 

 tangential or Pelton wheel, in which the jet is unconfined laterally. 



Where the supply head is very great the necessary peripheral speed of 

 a turbine to take full advantage of this is also great, while, as will be 

 shown later, the necessary speed is greater in the case of a pressure 

 turbine than with one of the impulse type. This peripheral speed may 

 be obtained either by having a wheel of large diameter with low angular 

 velocity, or with a small wheel making a large number of revolutions per 

 minute, and for a pressure turbine, where it is necessary to admit water 

 all around the circumference, it is imperative that the periphery, and 

 therefore the diameter, be comparatively small, and the angular velocity 

 in consequence high. For many purposes the necessary speed of rotation 

 under very high heads then becomes too great, while the hydraulic resist- 

 ances inside the turbine casing become excessive. 



In such a case the pressure turbine suffers from the further disadvan- 

 tage that the ports and passages are of necessity small and constricted in 



