BORDA TURBINE 



475 



One form of this wheel, Whitelaw's mill, was constructed with arms in 

 the form of an equiangular spiral (Fig. 217), the idea being that when in 

 motion the water would flow outwards from the centre to the jet in radial 

 lines, and that any loss due to eddy formation would be avoided. A little 

 consideration will, however, show that this can only be the case when 

 v = u, and when in consequence no work is being done by the wheel. 



ART. 131. 



The Borda wheel was in all probability the first practical pressure 

 turbine to be constructed. This consists of a couple of concentric 

 cylindrical casings (Fig. 218) mounted 

 on a vertical shaft, the space between 

 these being provided with a series of in- 

 clined vanes. The casing is usually of 

 some considerable depth and the mean 

 radius of the vanes large. Water is led 

 into the casing in a direction almost 

 normal to the vanes and acts partly by 

 impact. The pressure produced by its 

 weight is, however, the chief factor in 

 producing rotation. Strictly speaking, 

 the wheel is a pressure or impulse tur- 

 bine according as the buckets run wholly 

 or partially full, this depending on the 

 distance between consecutive vanes. In 

 the Borda, in common with all other 

 axial flow turbines, since the direction of 

 the flow is parallel to the axis, the effect of centrifugal force on the flow 

 may be neglected. In spite of its crudity, efficiencies of up to 70 per cent, 

 have been obtained with this, which is the origin of all modern turbines 

 of the parallel flow type. 



ART. 132. THE FOURNEYRON TURBINE. 



The first ' aighly efficient pressure turbine was, however, a development 

 of the reaction wheel by Fourneyron (1827). The Fourneyron turbine is an 

 outward radial flow reaction wheel (Fig. 219). Water is supplied through 

 the pipe S which is closed at the bottom and terminates in an inverted 

 cone, the ^outside of the pipe forming the entrance to a series of guide 

 P. Guided by these passages, the water enters the wheel 



FIG. 218. Borda Turbine. 



