TURBINES 



241 



The residual velocity at the point where the discharge is 

 released to the atmosphere is an irreclaimable loss and should be 

 made as small as possible. The fact that this loss is not charge- 

 able to the turbine should always be taken into account in making 

 efficiency tests. 



It used to be common practice to make all draft tubes of steel 

 plate, but of late years they are usually like the wheel casings 

 molded in the concrete foundation of the power-house, except in 

 the case of small turbines (Figs. 131 and 132). It is not feasible 

 to build large draft tubes of plate, nor is it possible to obtain the 



FIG. 132. Lower End of a Molded Concrete Draft Tube. 



smooth curves and efficient design characteristic of concrete 

 tubes. 



Bearings. Most bearings of horizontal turbines are of the 

 ordinary babbitted generator type, except where submerged, in 

 which case lignum vitae bearings are used. Where water thrust is 

 to be taken care of thrust bearings must also be provided. 



For vertical units the thrust bearing is almost always located 

 above the generator on a cast-iron supporting truss, which at the 

 same time forms the generator head cover. The upper guide 

 bearing, which is located immediately below the thrust bearing, is 

 usually of the oil-lubricated babbitted type, while the lower one is a 



