Renewable Energy Solutions

The modern impulse, or tangential wheel (so called because the driving
stream of water strikes the wheel at a tangent) is best adapted to
situations where the amount of water is limited, and the head is
large. Thus, a mountain brook supplying only seven cubic feet of water
a minute–a stream less than two-and-a-half inches deep flowing over a
weir with an opening three inches wide–would develop two actual
horsepower, under a head of 200 feet–not an unusual head to be found
in the hill country. Under a head of one thousand feet, a stream
furnishing 352.6 cubic feet of water a minute would develop 534.01
horsepower at the nozzle.

Ordinarily these wheels are not used under heads of less than 20 feet.

The Efficient Water Wheel

A wheel of this type, six feet in diameter, would develop six
horsepower, with 188 cubic feet of water a minute and 20-foot head.
The great majority of impulse wheels are used under heads of 100 feet
and over. In this country the greatest head in use is slightly over
2,100 feet, although in Switzerland there is one plant utilizing a
head of over 5,000 feet.

Efficient Modern Adaptations of the Archaic Undershot and Overshot Water Wheels

The old-fashioned impulse wheels were inefficient because of the fact
that their buckets were not constructed scientifically, and much of
the force of the water was lost at the moment of impact. The impulse
wheel of to-day, however, has buckets which so completely absorb the
momentum of water issuing from a nozzle, that the water falls into the
tailrace with practically no velocity. When it is remembered that the
nozzle pressure under a 2,250-foot head is nearly 1,000 pounds to the
square inch, and that water issues from this nozzle with a velocity of
23,000 feet a minute, the scientific precision of this type of bucket
can be appreciated.

A typical bucket for such a wheel is shaped like an open clam shell,
the central line which cuts the stream of water into halves being
ground to a sharp edge. The curves which absorb the momentum of the
water are figured mathematically and in practice become polished like
mirrors. So great is the eroding action of water, under great
heads–especially when it contains sand or silt–that it is
occasionally necessary to replace these buckets. For this reason the
larger wheels consist merely of a spider of iron or steel, with each
bucket bolted separately to its circumference, so that it can be
removed and replaced easily. Usually only one nozzle is provided; but
in order to use this wheel under low heads–down to 10 feet–a number
of nozzles are used, sometimes five, where the water supply is
plentiful.

The wheel is keyed to a horizontal shaft running in babbited bearings,
and this same shaft is used for driving the generator, either by
direct connection, or by means of pulleys and a belt. The wheel may be
mounted on a home-made timber base, or on an iron frame. It takes up
very little room, especially when it is so set that the nozzle can be
mounted under the flooring. The wheel itself is enclosed, above the
floor, in a wooden box, or a casing made of cast or sheet iron, which
should be water-tight.

Since these wheels are usually operated under great heads, the problem
of regulating their water supply requires special consideration. A
gate is always provided at the upper, or intake end, where the water
pipe leaves the flume. Since the pressure reaches 1,000 pounds the
square inch and more, there would be danger of bursting the pipe if
the water were suddenly shut off at the nozzle itself. For this reason
it is necessary to use a needle valve, similar to that in an ordinary
garden hose nozzle; and by such a valve the amount of water may be
regulated to a nicety. Where the head is so great that even such a
valve could not be used safely, provision is made to deflect the
nozzle. These wheels have a speed variation amounting to as much as 25
per cent from no-load to full load, in generating electricity, and
since the speed of the prime mover–the water wheel–is reflected
directly in the voltage or pressure of electricity delivered, the
wheel must be provided with some form of automatic governor. This
consists usually of two centrifugal balls, similar to those used in
governing steam engines; these are connected by means of gears to the
needle valve or the deflector.

As the demand for farm water-powers in our hill sections becomes more
general, the tangential type of water wheel will come into common use
for small plants. At present it is most familiar in the great
commercial installations of the Far West, working under enormous
heads. These wheels are to be had in the market ranging in size from
six inches to six feet and over. Wheels ranging in size from six
inches to twenty-four inches are called water motors, and are to be
had in the market, new, for $300 for the smallest size, and $2750 for
the largest. Above three feet in diameter, the list prices will run
from $2000 for a 3-foot wheel to $8000 for a 6-foot wheel. Where one has
a surplus of water, it is possible to install a multiple nozzle wheel,
under heads of from 10 to 100 feet, the cost for 18-inch wheels of
this pattern running from $1500 to $1800 list, and for 24-inch wheels
from $2000 to $2500. A 24-inch wheel, with a 10-foot head would give
1.19 horsepower, enough for lighting the home, and using an electric
iron. Under a 100-foot head this same wheel would provide 25.9
horsepower, to meet the requirements of a bigger-than-average farm
plant.