The breastwheel is also similar in construction but is in reality an
improvement upon the overshot and undershot types. It depends for its
power on a combination of the action of gravity and the impulse of
the water striking the blades, or buckets. The water is fed into the
wheel a little below the height of the axle and usually enters with
considerable velocity, a part of which is transformed into useful work
by the wheel.
The turbine is a type of wheel which is very extensively used. It is
usually constructed of metal and consists primarily of a series of
curved vanes, or runners, whose arrangement is similar to a screw. The
action of the water flowing through these curved vanes causes the vanes
and shaft to revolve, the vanes being solidly connected to the shaft,
which may be either horizontal or vertical.
The fundamental working principle of an impulse waterwheel is the
turning into useful work of the impulse due to the velocity of a jet of
water issuing from a contracted orifice. This is accomplished usually
by conveying the water from the dam or other source of supply to the
waterwheel in a pipe of comparatively large size and then gradually
reducing the size of the pipe immediately in front of the wheel to a
comparatively small size by means of a reducer section, which is fitted
with a nozzle the opening of which may or may not be regulated in size.
This contraction of the stream of flowing water causes a spouting of
the water under pressure and the water issues in a jet with very high
velocity. The jet thus issuing from the nozzle strikes the cups of
the impulse wheel which are arranged at regular intervals around the
circumference of a metallic disc which is centered on an axle. The cups
transfer the velocity of the jet to the wheel, and the water drops from
them with very little velocity left in it.
[Illustration: Impulse Type of Waterwheel
Showing jet of water striking cups. Wheel illustrated is very powerful,
but principle of small wheels is the same]
In general, the turbine type of wheel is best adapted to low heads, or
falls, and the use of comparatively large volumes of water, and the
impulse wheel is best adapted to the use of a comparatively high head,
or fall, and a comparatively small amount of water. There are certain
intermediate conditions for which the manufacturers of each type claim
their wheel is best suited and in such instance a study of local
conditions is always necessary to determine which type of wheel is best
adapted.
The development of a water power by means of any kind of a waterwheel
results in the conversion of the energy of the falling water into
mechanical power which is exerted in a more or less rapidly revolving
shaft. In order to apply this power of the revolving shaft to some
useful purpose, there are several methods which may be used. The shaft
may be directly connected to the shaft of an electric generator, or
dynamo, to generate electric current, or it may be directly connected
to a machine which it is desired to operate, provided the machine,
or dynamo, is required to operate at the same speed as that of the
wheel shaft. This is frequently not the case, so that under ordinary
conditions the shaft of the wheel is fitted with a pulley, which in
turn is connected by belt to another pulley on the machine which is to
be driven.
[Illustration: Motor-driven Mangle]
By using pulleys of different diameters on the shaft of the waterwheel
and the shaft of the machinery to be driven, the speed of the machine
may be several times more or less than the speed of the waterwheel.
For instance, if the waterwheel revolves 200 revolutions per minute
and it is desired to operate a machine, connected by belt, at a speed
of 1000 revolutions per minute, a pulley of comparatively small size,
say four inches in diameter, is placed on the shaft to be driven, and
a pulley of five times the diameter, or twenty inches, is placed on
the shaft of the waterwheel. This causes the shaft of the machine to
revolve at a speed five times as great as the waterwheel. If the speed
of the waterwheel is greater than that required for the machinery to
be operated, then the reverse operation is followed out, placing a
small pulley on the shaft of the waterwheel and a larger one on the
shaft of the machinery to be driven. If the speed of the waterwheel
is to be magnified more than about six times, it usually requires the
installation of a countershaft and another series of pulleys in order
to avoid the use of very large and very small pulleys. A pulley which
has a very small diameter does not operate satisfactorily without
considerable loss of power, and a very large pulley is objectionable on
account of the space which it requires.
When a water power is once developed it may be applied to practical use
either near the place of development or at a considerable distance.
If it is to be used for power only, and not for lighting, and can be
used where it is developed, there is no need of converting it into
electricity. But if it is to be used for lighting, or for power to
be applied at a considerable distance from the water-power site,
then it becomes necessary to convert the power into electricity, in
which form it may be most conveniently transmitted from one place to
another. This requires an electric generator, or dynamo, to be driven
by the waterwheel, and a transmission line, preferably of copper or
aluminum wire, to carry the current where it is to be used. In order to
reconvert the current into power at the end of the transmission line,
where the power is to be used, it is necessary to run the current into
an electric motor, the shaft of which is made to revolve by the action
of the electric current. This motor may then be connected directly, or
by belt, gears or chain drive, to the machine to be driven.
It should be borne in mind that in each of these steps of changing from
water power to electric current, in transmitting the current over the
wires, in reconverting it into power, and in transferring this power
from a motor to a power-operated machine, there are losses of energy.
These losses vary considerably in different instances. Assuming, for
illustration, that a water power, whose theoretical power is ten
horsepower, is required to drive a power machine at a distance, the
efficiencies and losses will be somewhat as follows:
Waterwheel, efficiency 80%, Loss 20%, generates 8.0 horsepower.
Connections, “ 95%, “ 5%, transfers 7.6 “
Dynamo, “ 90%, “ 10%, generates 6.8 “
Transmission, “ 90%, “ 10%, transmits 6.2 “
Motor, “ 90%, “ 10%, develops 5.5 “
Connections, “ 95%, “ 5%, delivers 5.0 “
Therefore, only five horsepower would be actually delivered to the
machine to be driven. This amounts to only half of the theoretical
power of the falling water which is actually realized in useful work
of the machine being driven. If the power from the waterwheel is to
be applied directly without generating electricity a much higher
efficiency will be realized.