MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

138(more than one water lifter is often driven off of the same watermill shaft).Table 5.4 summarizes some typical watermill specifications and applicationsdiscussed below.When essentially no head is available, except for that which producesa flow (e.g. stream, river, canal), it is practical only to use an undershotwheel. The simplest to construct is the floating mill version (Bradley,1912, p. 78), shown schematically in Figure 5.5a. It is installed sothat its peripheral paddles (at least two at all times) are immersed inthe driving flow. Figures 3.21a and b show such mills combined into asingle unit with norias.Because the efficiency of floating mills is rather low, anotherversion of the undershot wheel, the Poncelet wheel, was developed (about1800) which, when a suitable site is available, yields better efficiencies.As shown in Figure S.Sb, this mill requires the flow (or at least a portionof it) to be restricted to a channel, the width of the Poncelet wheel,and that a sluice gate be provided just upstream. In this way, a portionof the available flow depth is converted to a velocity head which canincrease the power applied to the mill. Poncelet wheels can be expectedto have efficiencies between 60 percent to rotate the mill. Figure 5.6aand b illustrate medium and high breast wheels, respectively. A low breastwheel is essentiallythe same as a Poncelet wheel.A major loss in efficiency of a breast wheel design is that, unlikeunder and overshot wheels, the wheel is moving toward the incoming waterrather than in the same direction, thus creating energy losses (e.g.,for any breast higher than the wheel axle, a moment, counter to the falling-water moment, is produced).This counter moment can be reduced to somedegree by the use of a sluice gate, as in Figure 5.6b. Breast wheels