Though one of man’s oldest prime movers, a water wheel generator is
still a fascinating piece of machinery. Perhaps this is
because it appears comprehensible at a glance (although an
efficient wheel is actually a product of subtle and
inconspicuous design refinements), and because it seems to
be a way of getting power for nothing. The wheel
we describe here, with accompanying technical diagram, was especially designed for this series on
harnessing small streams,
and will reward a careful craftsman by delivering years of
constant service. It’s particularly suited for an
installation having a moderate head (25′ to 60′) and
relatively small flow (0.45 to 0.75 cubic feet per second).
Subsequent installments will describe the construction of
wheels suited for lesser heads of water and other varied
conditions.
The wheel is an impulse wheel,
driven by the impulses produced as water strikes revolving
blades or buckets. In a perfectly designed wheel, the water
strikes at high speed, exhausts its energy in driving the
wheel to which the bucket is attached, and then falls free
of the wheel.
Known as a Pelton wheel, this type developed from the
“hurdy gurdy”, a paddle wheel used in California by the
forty-niners. The hurdy-gurdy was a wheel that rotated in a
vertical plane, had flat vanes fixed around its
circumference, and was driven by the force of water
striking the vanes. It was not an efficient machine, but it
was simple to construct. Then an engineer named Lester
Pelton substituted a cup-shaped, divided bucket for each of
the vanes, and by that step added a high degree of
efficiency to the wheel’s other virtues.
No single wheel will meet all operating requirements, but
some will perform under a reasonably wide range of
conditions. The following table indicates the r.p.m. and
horsepower output that will be delivered by this wheel
under given conditions of head and flow. The latter is
measured in cubic feet per second:
Head: 25′
– Flow: 0.43; RPM: 350; HP: 1.0
Head: 30′
– Flow: 0.51; RPM: 390; HP: 1.3
Head: 40′
– Flow: 0.59; RPM: 450; HP: 2.0
Head: 50′
– Flow: 0.66; RPM: 500; HP: 2.8
Head: 60′
– Flow: 0.73; RPM: 550; HP: 3.75
Thus, if a survey of your stream indicates a head and flow
close to these values, our Pelton wheel will fit neatly
into your plans.
Strictly speaking a water wheel is an engine powered by
water, just as an automobile engine is powered by gasoline.
The important power-producing elements of the wheel are the
buckets and the nozzle, and considerable care should be
exercised to see that these parts are made correctly. The
nozzle meters the correct amount of water to the wheel, and
forms and directs the jet against the buckets. Both the
inside diameter and the location of the nozzle with respect
to the wheel are very important, since the jet must impinge
upon each bucket at the correct wheel radius or lever arm.
It must also be divided equally by the center ridge of each
bucket.
The function of the bucket is to convert the energy of the
jet, represented by its high speed, into mechanical energy
at the wheel shaft. To do this it must slow the water from
its high speed in the jet to practically zero speed when it
drops into the tail water. Maximum efficiency with this
wheel will be obtained if the buckets have the proper form and
size. This shape acts to slow the jet
by turning it smoothly through 180 deg. The surface of each
bucket must be as smooth as possible. A mirror finish is
desirable on the inside, and even the back of each bucket
should be ground and polished to minimize spray.
Important also is the correct orientation of the bucket to
the jet. When the full jet strikes, the bucket should be
perpendicular to it. Both the nozzle and the buckets will
wear under the action of the high-speed water, at a rate
determined by the silt content and should therefore be made
easily removable for replacement.
Above all, buckets must be uniform. If you can get access
to a metal-cutting handsaw, cut the blanks according to a
single pattern. This pattern can be shaped so as to form
the end bevels automatically when the blanks are bent, and
the bending itself can be done in a jig or hammering form.
This jig may be made of a piece of pipe of about 2″ outside
diameter mounted in hardwood endplates. Also provide a
holding fixture that will slide in the table groove of the
handsaw to assure that the slots for the end lugs are cut
and spaced uniformly. A holding jig should also be made to
line up the lugs and buckets for welding. On completion,
balance the wheel by laying weld beads along the backs of
any light buckets. Beads should be laid carefully and
ground smooth.
Ball bearings may be employed, but are not necessary since
the wheel turns at comparatively low speeds. If the builder
prefers to use plain bearings, it will simplify machining
the shaft, which should present a shoulder to the inside of
the bearing so that the wheel may be positioned. If plain
bearings are employed, babbitted linings are satisfactory,
provided provision is made for proper lubrication.
One vital job that the foundation must do is hold the wheel
and the nozzle in correct relative positions. It should be
placed on firm ground or piling so that it will not settle
unevenly, and must, of course, take advantage of all the
head possible. The penstock from the dam should have easy
access to the nozzle, and the tail water easy escape to the
stream. If possible use 4″ or larger pipe for the penstock
and lay it out to hold frictional losses to a minimum. The
width of the foundation is such as to allow the water to
fly clear of the buckets. The removable cover over the
upper half of the wheel may fit more closely, since no
water sprays from the buckets through this half of the
revolution.
The foundation may be made of such materials as timbers in
a framework, masonry, or concrete, so long as it fulfills
the above requirements. The wheel and the machinery being
driven may then be housed in any suitable, inexpensive
shed.
It’s not wise to dispense with a gate valve, which is used
to cut off or to throttle the water supply to the wheel.
Since a gate valve cannot be operated rapidly, it is the
best type, eliminating the risk of dangerous water hammer
in the penstock. It is also well suited for throttling
because fine adjustment is obtainable through the long
operating screw. In throttling, the gate valve should be
used together with a tachometer or revolution counter
connected to the wheel shaft to secure the optimum speed
and horsepower for the stream condition and load. Either
fasten a tachometer permanently to the shaft, or keep a
revolution counter handy in the wheel shed.
Generally the head and volume of water flowing to the wheel
will remain constant, resulting in a constant output. If
the machinery driven by the wheel has a level power demand
there will be little need for constant adjustment of the
valve.
The requisite piping, pipe fittings, steel sheet and rod
bolts and nuts, and gaskets are available at
building-supply houses houses or steel distributors.
Machine screws, lock nuts bearings, and the like may be
purchased from good-sized hardware distributors or
mail-order houses.
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. 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.
Reprinted courtesy of Popular Science Monthly, Popular Science Publishing Co. Inc. 1947
