THEORY OF TURBINE DESIGN 549 



(b) The disc friction due to rotation of the turbine crowns has been 

 considered in detail in Art. 62, p. 179. 



Its magnitude depends considerably on the type of turbine. In a 

 single wheel radial flow turbine the whole of the rear face of the runner 

 will, in general, be subject to this resistance, as will that portion of 

 the front face which lies between the shaft and the inner tips of the 

 vanes, while in the case of a double discharge turbine of the Thomson 

 vortex type, this rubbing area is almost doubled. A large increase in 

 disc friction is also experienced in the case of a turbine balanced by the 

 addition of a rotating balance piston. 



In a wheel of the parallel flow type the design may be such as to cause 

 a similar resistance at the outer circumference whose radius is r 2 and 

 breadth b%, the loss of energy in this case being given by/ w 3 r 2 4 b 2 foot Ibs. 

 per second. 



In any case this loss of energy per second is proportional to co 3 , and the 

 loss per Ib. to o> 2 , since Q is proportional to oo. The magnitude of the loss 

 may be from 3 to 6 per cent. 



(5) and (6) Eddy Losses. H G . These losses, due to eddy formation at 

 changes of curvature and to shock at entrance at part gate, do not admit 

 of even approximate calculation. They may, however, be minimised by 

 designing all passages to have as easy a curvature as possible, and by the 

 adoption of gates of the swivel type. With this type of gate these losses 

 may account for between 1 and 8 per cent, of the total head, while with 

 cylinder regulation they may amount to as much as 20 per cent, at 

 half gate. 



(7) Loss due to Rejection of Kinetic Energy in the Discharge, H K . 

 Assuming the whole of the kinetic energy of discharge from the buckets 

 to be lost, this loss is given by 



f * + W * foot Ibs. per Ib. 

 2 # 



In an inward radial flow turbine, where n = , we have 



?*3 



w 3 = u s f a cot y = /a cot y, and if / 3 = / 2 , the loss becomes 



ff 



Expressed in terms of - 2 , this becomes 



foot Ibs. per Ib. (2) 



f. ( /., tan a\ tan a) 2 ~] ., 



tan 2 a -f- ] n [ I r - ] 7- foot Ibs. 



L V tan /?/ tan y J 



