470 



HYDRAULICS AND ITS APPLICATIONS 



then acts as a pressure turbine. It is fairly good for low falls with a head 

 which does not vary greatly, while its efficiency varies from about 60 per 

 cent, at half gate to a maximum of about 72 per cent. It is seldom 

 met with in modern practice. 



Whether a horizontal or a vertical shaft machine is to be .preferred 



depends largely on the location of 

 the plant. 



The horizontal shaft design gives 

 a motor which is very accessible, 

 and which is conveniently situated 

 for gearing by means of belting to 

 other machinery, and where the 

 power-house is situated near the 

 tail-race level, the supply being con- 

 veyed from the head-race by pipes, 

 this design will in general be 

 adopted, the turbines being placed 

 directly on the floor of the power- 

 house. Where the more convenient 

 site for the power-house is near the 

 head-race, a well must be sunk down 

 to tail-race level, connected to the 

 tail-race by means of a tunnel or 

 ..... pipe, and the turbines erected at the 

 jj|''| r bottom of this well. A vertical shaft 

 machine is now almost essential, this 

 shaft being carried vertically up- 

 wards into the power-house and 

 supported at intervals by suitable 

 bearings. When driving an electrical 

 generator this forms a convenient 

 arrangement. The armature is 

 mounted directly on the rotating 

 shaft, and the arrangement has the 

 further advantage that since all the electrical machinery may be placed 

 well above the head-race level it is not likely to be affected by floods. 

 This arrangement is, however, in general not to be recommended where 

 it is possible to place the power station near the tail-race level Not 

 only is the turbine well costly to construct, but the cost of construction 

 of the discharge tunnel or pipe line is much greater than that of the 



FIG. 214. Barker's Mill. 



