BREAST WHEEL 



431 



The following results of experiments by Smeaton indicate to what extent 

 the efficiency is lowered by increasing the proportion of the total head 

 absorbed in giving velocity energy to the supply water. 



Regulation is usually performed by a sluice governing the discharge 

 from the penstock. 



The overshot wheel is well suited for small powers and heads ranging 

 from about 15 to 50 feet, and when working under suitable conditions 

 gives efficiencies up to about 80 per cent. As the head diminishes, the 

 larger proportional loss of head necessitated by the depth of the buckets 

 and by the clearances BI and 8% renders the wheel less efficient, and for 

 heads between 15 feet and 6 feet the breast wheel becomes more suitable. 



AKT. 121. THE BREAST WHEEL (FiG. 189). 



Here the wheel itself is almost identical with the overshot. The 

 principles of its construction and of the design of its buckets are the 

 same, but water is now admitted to the buckets at some point in the 

 breast of the wheel. 



As before, the supply may be brought to the wheel either in an open 

 supply channel under a comparatively low head not exceeding 1 foot, in 

 which case the supply is led on below the wheel centre, or by means of a 

 closed supply pipe under a greater head, when the supply is led on above 

 the centre. The general arrangement in each case is indicated in Fig. 189 

 a and 6. The water is prevented from escaping from the buckets before 

 reaching the bottom of the wheel by means of a breastwork of masonry, 

 the clearance between the wheel and masonry being reduced to the mini- 

 mum possible, usually about T 3 B inch. The necessity for this breast-work 

 renders the wheel more expensive than the overshot. Regulation Is per- 

 formed by throttling the supply by means of sluices arranged as indicated 

 in Figs. 189 a and b. 



