THEORY OF TURBINE DESIGN 



557 



ART. 148. AMERICAN TYPE : COMBINED RADIAL INWARD FLOW AND 

 AXIAL DISCHARGE PRESSURE TURBINES. 



Here, as in the inward flow type of wheel, the turbine is to a certain 

 extent self-regulating. The design of the vanes at inlet is regulated by 

 he laws governing the design of the inward flow wheel and at outlet 

 >y the laws governing that of the axial flow type. In general, a higher 

 >eripheral velocity is adopted for this type, this varying from '7 to '75 

 ^/ 2 g h for maximum efficiency at full gate. Where regulated by 

 jylinder gates, the speed for maxi- 

 num efficiency falls to from *57 

 ,o '61 V 2g H at half gate, the 

 efficiency under these conditions 

 varying from about 80 per cent. 

 it full gate to 65 per cent, at half 

 'ate. 



With the better makes of this 

 ;lass of machine, having inlet 

 ingles of about the same magni- 

 tude as those of the Francis and 

 Thomson turbines, the full load 

 efficiency would appear to be 

 almost, if not quite, as high as is 

 attainable with either of the latter 

 types. 



The correct inclination of the 

 tangent plane to the vane at dis- 

 charge, at any radius, may be 



determined on the assumption FIG. 260. 



that when the wheel runs full 



there is no relative radial interchange of the particles of water. The 

 turbine may then be imagined as subdivided into a series of n elements 

 each having a depth at inlet = b -7- n, and a width at outlet =l--n, 

 where b is the depth of the wheel at inlet and I is the length of the dis- 

 charging periphery of each bucket (Fig. 260). It then becomes easy to 

 calculate the respective angles of inclination to the horizontal and the 

 vertical at the mid point of each element at outlet in order that the dis- 

 charge may be as nearly as possible axial, and so to obtain the contours 

 of the vane. 



This method cannot, however, be relied upon to give very accurate 



