228 



HYDRAULIC EQUIPMENT 



the single or double discharge, both admitting of an exposed gate 

 mechanism. The double discharge has some advantages over the 

 single in that it is hydraulically balanced against end thrust. On 

 the other hand, if it has a central discharge, i.e., both runners dis- 

 charging into a common-draft tube (Fig. 118), the draft-tube con- 

 ditions are not so favorable, unless the runners are spaced well 

 apart. 



Horizontal turbines for very low heads are necessarily set in 

 open flumes or wheel pits. For high heads, the volute or spiral 



FIG. 119. Single-runner Horizontal Turbine with Cast-iron Spiral Case and 

 Single Discharge Tube. (Built by I. P. Morris Company.) 



casing is the preferable type, the question of central, double or 

 single discharge depending on the conditions to be met (see Figs. 

 119, 120, and 121). For intermediate heads, the cylindrical plate 

 steel casing, Fig. 122, has been commonly used in the past. It is 

 not as efficient, hydraulically, as the spiral casing, but is some- 

 times cheaper in first cost. In order to avoid prohibitive losses, it 

 is necessary to make the plate steel cylindrical casing much larger 

 than a volute casing, and the additional space and material would 

 tend to neutralize or reverse the reduction in cost. If the pen- 



