COMPOUND TURBINE 



509 



supply pipe is 5 feet 3 inches and the discharge 5 feet 6 inches 

 diameter. A test of this wheel when driving 10,000 H.P. is stated to 

 have given 84 per cent, efficiency, though the figure is subject to the 

 inaccuracy of the method used for measuring the water consumption. 

 This was deduced from the fall in pressure across the head gate, whose 

 opening was accurately known. 



A number of test results of typical modern Francis turbines are given 

 in the table opposite, 1 the general arrangement of each turbine being 

 shown in Fig. 240. 



ART. 137. COMPOUND TURBINES. 



One possible method of getting over the difficulty of adapting pressure 

 turbines of small power to very high heads consists in compounding two 



Revs, per Minute. 

 FlQ. 241. Efficiency Curves for Single and for Quadruple Compound Turbine. 



or more runners in series on the same shaft, the fall in pressure and the 

 work done then taking place in stages. As will be shown later, the peri- 

 pheral speed of a pressure turbine for maximum efficiency is proportional 

 to *J H, where H is the working head, so that by doubling the number of 

 runners the rotative speed is reduced in the ratio 1 : *f~Z = '707. 



This reduction in speed has the effect of reducing disc friction, and the 



1 By Professor F. Frazil, "Proc. Inst. Mech. Engineers," 1910-11. 



