YOUR OWN WATER-POWER PLANT
(Page 4 of 8)
One final point to keep in mind in making your
calculations: head is defined as the vertical distance
between the water surface behind the dam and the tail-water
surface at the wheel. For an impulse wheel, however, which
cannot operate submerged, the available head is measured
from headwater to the center line of the nozzle. As shown
in the construction drawing, there is only 5" difference
between the two definitions, but this can make some
difference in output when working with the moderate heads
for which this wheel is designed.
While many details can be altered, the reader should beware
of any that will affect operating characteristics. Thus
stainless steel buckets and antifriction bearings would
improve performance, involving only some extra work in
building the wheel. Changes in the nozzle diameter, wheel
radius, or effective head, however, should be undertaken
only after careful consideration of the probable effect on
performance.
RELATED CONTENT
PARTFIVE: BUILDING All OVERSHOT
WHEEL
Often seen beside a picturesque rural mill, an overshot
water wheel possesses two excellent
characteristics—considerable mechanical efficiency
and easy maintenance. Many have remained in service for
decades, and now lend a nostalgic charm to their
surroundings.
Operated by gravity, the overshot wheel derives its name
from the manner in which water enters the buckets set
around its periphery. Pouring from a flume above the wheel,
the water shoots into buckets on the down-moving side,
overbalancing the empty ones opposite and keeping the wheel
in slow rotation.
Since such a wheel may be located near but not actually in
the stream, it offers endless landscaping possibilities for
a country home where a stream with sufficient flow is
available. If a site on dry ground is chosen, the
foundation may be constructed dry and the water led to the
wheel and a tailrace excavated. With very little effort,
the scene may be turned into an attractive garden spot, the
wheel becoming both a landscaping feature and a source of
power.
It should be noted, however, that an overshot wheel is
practical only for a small-capacity output. How much power
it will produce depends upon the weight of water the
buckets hold and its radius, or lever arm. Expressed in
another way, the output depends upon the weight of water
transported and the height, or head, through which it falls
while in the buckets. For maximum efficiency, the wheel
must use the weight of the water through as much of the
head as possible. Therefore, the buckets should not spill
or sling water until very near tail water.
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