JONVAL TURBINE 481 



In practice, the turbine is regulated by opening the necessary number 

 of guide buckets in one compartment after another, and while causing 

 loss by shock, this is probably the best method of regulation. 



In the case of a Jonval turbine, built for the Zurich Waterworks, to 

 work under a head which varies from 4' 9" to 10' 6", the number of com- 

 partments was three. The turbine was designed for 90 H.P., and the 

 outer compartment alone gave this power under the full head, when 

 using 6,300 cubic feet per minute. The outer and middle compart- 

 ments together gave 90 H.P., under a head of 7' 10", using 8,400 cubic 

 feet per minute, while the three together gave 90 H.P. with a head of 

 4' 9" using ] 0,000 cubic feet per minute. The speed throughout was 

 25 revolutions, and official tests gave the following results : 



Head. Efficiency. 



Outer compartment alone 10' 6" 73*71 per cent. 

 Outer and middle together T 0" 75*39 per cent. 

 All three compartments 4' 9" 80*70 per cent. 



This type of turbine combines the advantages of fairly high efficiency 

 at part gate and of constant speed under variable head in a manner 

 which is unequalled by any but the radial inward flow or Francis turbine, 

 and is therefore suitable for variable conditions of working. It may be 

 applied to heads from 2 feet to 180 feet, and for speeds from 20 to 400 

 revolutions per minute, but is more particularly fitted for low and medium 

 heads with large quantities of water, the large area of the water passages 

 enabling it to pass large volumes of water. Since European users are in 

 general compelled to take advantage of such sources of power, this largely 

 accounts for the favour with which the Jonval turbine was received and 

 for its large development in Europe. Of late years, however, it has been 

 largely displaced by the inward radial flow turbine, and is not at the 

 present time manufactured to any large extent. 



One of its chief disadvantages is, that with a downward flow machine 

 the whole water pressure is transmitted to the step bearing unless pre- 

 vented by some special device. One method of doing this is to allow 

 the water to flow upwards through the wheel, while in a second method 

 a balance piston is fitted to the turbine shaft as shown in Fig. 226, one 

 side of this piston being exposed to the supply pressure, and any leakage 

 past it being drained to the tail-race. By suitably proportioning the 

 area of this piston, it then becomes possible to balance not only the water 

 pressure on the runner, but also the weight of the rotating parts. 



H.A, I T 



