THOMSON TURBINE 



499 



the part-load efficiency is also high, half-load efficiencies of as much as 

 82 per cent, being on record. The turbine may be constructed with either 

 horizontal or vertical shaft, mechanical considerations rendering the 

 former construction preferable. 



In the single vaned wheel the vanes are fixed on one side of the rotat- 

 ing disc, and discharge takes place on one side only. The disc is there- 

 fore subject to end thrust, which must either be hydraulically balanced or 

 taken up by means of a suitable thrust bearing. This disadvantage, 

 which is common to all single discharge pressure turbines, may be over- 



Air Pipe 



FIG. 237. Arrangement of Vortex Turbine with Horizontal Shaft and Draught Tube. 



come by mounting a pair of similar wheels on the same shaft, the flow 

 being in opposite directions through the two wheels. The two end thrusts 

 thus balance each other, while the arrangement has the further advantage 

 for electric driving, that with a given head, since two smaller wheels are 

 used instead by a single larger one, and since the peripheral speed of the 

 wheel depends solely on the head, the speed of rotation is higher with the 

 twin wheel. The single vaned wheel with vertical shaft is well fitted for 

 low or medium falls and where the quantity of water available is large 

 and the head is variable. Its efficiency is practically the same as that of 

 the double wheel. 



