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full load. For this reason it is desirable to run low-head wheels 

 under practically full load conditions. With high-head wheels 

 this is not so important, as the efficiency is still high at partial 

 loads. With wheels as represented by curve C, it is also neces- 

 sary to allow some margin above the normal full load for govern- 

 ing, as it is desirable to operate the turbine at its point of max- 

 imum efficiency. With high-head wheels, curve A, such a margin 

 need not be allowed. 



The curves plotted in Fig. 108 represent operating conditions 

 under constant head. This, however, is not always realized, 

 especially in low-head plants where floods and dry seasons some- 

 times cause quite a variation in the head, and this has, as pre- 

 viously mentioned, quite a bearing on the selection of the water 

 wheel, and should, therefore, be given careful consideration. 



If the speed of the unit could be allowed to vary at all times the 

 square root of the ratio of the heads, the shape of the performance 

 curve for any head other than normal would be the same as that 

 secured at normal head, but the output would vary as the f 

 power of the ratio of the heads. In the case of wheels driving 

 alternating-current generators a speed variation is not permissible 

 and the speed must be kept constant irrespective of any varia- 

 tion in head which may occur, and this will still further lower the 

 output due to the reduced efficiency when operating at the best 

 head and speed. 



In Fig. 109 is plotted a set of curves illustrating the effect of 

 a varying head. A 10,000-H.P. turbine is assumed to operate 

 normally under a 32-foot head, the speed to be constant for a 

 range of heads from 26 to 38 feet. As the head goes up to 38 feet 

 the shape of the curve approaches more closely curve R in Fig. 108, 

 while, when the head falls to 26 feet, the speed being constant, it 

 approaches more closely to curve C. In other words, when oper- 

 ating under a 38-foot head, the speed is lower than the best speed 

 for the runner under that head, while, when operating under the 

 26-foot head, the speed of the wheel is higher than the best speed. 

 Under 38-foot head the point of maximum efficiency is, further- 

 more, considerably below the normal full load at that head, while, 

 under 26-foot head, the power at which maximum efficiency occurs 

 is the actual full load, illustrating the points discussed above in 

 reference to the relation of the power at which maximum efficiency 

 occurs and the normal full-load power for various specific speeds. 



