_WATER-POWER._
Water-wheels, next to steam-engines, are the most common motive
agents. For centuries water-wheels remained without much
improvement or change down to the period of turbine wheels, when
it was discovered that instead of being a very simple matter, the
science of hydraulics and water-wheels involved some very intricate
conditions, giving rise to many problems of scientific interest,
that in the end have produced the class known as turbine wheels.
A modern turbine water-wheel, one of the best construction,
operating under favourable conditions, gives a percentage of the
power of the water which, after deducting the friction of the
wheel, almost reaches the theoretical coefficient or equals the
gravity of the water; it may therefore be assumed that there will
in the future be but little improvement made in such water-wheels
except in the way of simplifying and cheapening their construction.
There is, in fact, no other class of machines which seem to
have reached the same state of improvement as water-wheels, nor
any other class of machinery that is constructed with as much
uniformity of design and arrangement, in different countries, and
by different makers.
Water-wheels, or water-power, as a mechanical subject, is
apparently quite disconnected with shop manipulation, but will
serve as an example for conveying general ideas of force and
motion, and, on these grounds, will warrant a more extended notice
than the seeming connection with the general subject calls for.
In the remarks upon steam-engines it was explained that power is
derived from heat, and that the water and the engine were both to
be regarded as agents through which power was applied, and further,
that power is always a product of heat. There is, perhaps, no
problem in the whole range of mechanics more interesting than to
trace the application of this principle in machinery; one that is
not only interesting but instructive, and may suggest to the mind
of an apprentice a course of investigation that will apply to many
other matters connected with power and mechanics.
Power derived from water by means of wheels is due to the gravity
of the water in descending from a higher to a lower level; but
the question arises, What has heat to do with this? If heat is
the source of power, and power a product of heat, there must be a
connection somewhere between heat and the descent of the water.
Water, in descending from one level to another, can give out no
more power than was consumed in raising it to the higher level, and
this power employed to raise the water is found to be heat. Water
is evaporated by heat of the sun, expanded until it is lighter
than the atmosphere, rises through the air, and by condensation
falls in the form of rain over the earth's surface; then drains
into the ocean through streams and rivers, to again resume its
round by another course of evaporation, giving out in its descent
power that we turn to useful account by means of water-wheels. This
principle of evaporation is continually going on; the fall of rain
is likewise quite constant, so that streams are maintained within a
sufficient regularity to be available for operating machinery.
The analogy between steam-power and water-power is therefore quite
complete. Water is in both cases the medium through which power is
obtained; evaporation is also the leading principle in both, the
main difference being that in the case of steam-power the force
employed is directly from the expansion of water by heat, and in
water-power the force is an indirect result of expansion of water
by heat.
Every one remembers the classification of water-wheels met with in
the older school-books on natural philosophy, where we are informed
that there are three kinds of wheels, as there were "three kinds
of levers"--namely, overshot, undershot, and breast wheels--with
a brief notice of Barker's mill, which ran apparently without any
sufficient cause for doing so. Without finding fault with the plan
of describing water-power commonly adopted in elementary books,
farther than to say that some explanation of the principles by
which power is derived from the water would have been more useful,
I will venture upon a different classification of water-wheels,
more in accord with modern practice, but without reference to the
special mechanism of the different wheels, except when unavoidable.
Water-wheels can be divided into four general types.
_First._ Gravity wheels, acting directly from the weight of the
water which is loaded upon a wheel revolving in a vertical plane,
the weight resting upon the descending side until the water has
reached the lowest point, where it is discharged.
_Second._ Impact wheels, driven by the force of spouting water
that expends its percussive force or momentum against the vanes
tangental to the course of rotation, and at a right angle to the
face of the vanes or floats.
_Third._ Reaction wheels, that are "enclosed," as it is termed,
and filled with water, which is allowed to escape under pressure
through tangental orifices, the propelling force being derived from
the unbalanced pressure within the wheel, or from the reaction due
to the weight and force of the water thrown off from the periphery.
_Fourth._ Pressure wheels, acting in every respect upon the
principle of a rotary steam-engine, except in the differences that
arise from operating with an elastic and a non-elastic fluid; the
pressure of the water resting continually against the vanes and
"abutment," without means of escape except by the rotation of the
wheel.
To this classification may be added combination wheels, acting
partly by the gravity and partly by the percussion force of
the water, by impact combined with reaction, or by impact and
maintained pressure.
Gravity, or "overshot" wheels, as they are called, for some reasons
will seem to be the most effective, and capable of utilising the
whole effect due to the gravity of the water; but in practice this
is not the case, and it is only under peculiar conditions that
wheels of this class are preferable to turbine wheels, and in no
case will they give out a greater per cent. of power than turbine
wheels of the best class. The reasons for this will be apparent by
examining the conditions of their operation.
A gravity wheel must have a diameter equal to the fall of water,
or, to use the technical name, the height of the head. The speed
at the periphery of the wheel cannot well exceed sixteen feet
per second without losing a part of the effect by the wheel
anticipating or overrunning the water. This, from the large
diameter of the wheels, produces a very slow axial speed, and a
train of multiplying gearing becomes necessary in order to reach
the speed required in most operations where power is applied.
This train of gearing, besides being liable to wear and accident,
and costing usually a large amount as an investment, consumes a
considerable part of the power by frictional resistance, especially
when such gearing consists of tooth wheels. Gravity wheels, from
their large size and their necessarily exposed situation, are
subject to be frozen up in cold climates; and as the parts are
liable to be first wet and then dry, or warm and cold by exposure
to the air and the water alternately, the tendency to corrosion
if constructed of iron, or to decay if of wood, is much greater
than in submerged wheels. Gravity wheels, to realise the highest
measure of effect from the water, require a diameter so great that
they must drag in the water at the bottom or delivering side, and
are for this reason especially affected by back-water, to which
all wheels are more or less liable from the reflux of tides or by
freshets. These disadvantages are among the most notable pertaining
to gravity wheels, and have, with other reasons--such as the
inconvenience of construction, greater cost, and so on--driven such
wheels out of use by the force of circumstances, rather than by
actual tests or theoretical deductions.
Impact wheels, or those driven by the percussive force of water,
including the class termed turbine water-wheels, are at this time
generally employed for heads of all heights.
The general theory of their action may be explained in the
following propositions:--
1. The spouting force of water is theoretically equal to its
gravity.
2. The percussive force of spouting water can be fully utilised if
its motion is altogether arrested by the vanes of a wheel.
3. The force of the water is greatest by its striking against
planes at right angles to its course.
4. Any force resulting from water rebounding from the vanes
parallel to their face, or at any angle not reverse to the motion
of the wheel, is lost.
5. This rebounding action becomes less as the columns of water
projected upon the wheel are increased in number and diminished in
size.
6. To meet the conditions of rotation in the wheel, and to
facilitate the escape of the water without dragging, after it has
expended its force upon the vanes, the reversed curves of the
turbine is the best-known arrangement.
It is, of course, very difficult to deal with so complex a subject
as the present one with words alone, and the reader is recommended
to examine drawings, or, what is better, water-wheels themselves,
keeping the above propositions in view.
Modern turbine wheels have been the subject of the most careful
investigation by able engineers, and there is no lack of
mathematical data to be referred to and studied after the general
principles are understood. The subject, as said, is one of great
complicity if followed to detail, and perhaps less useful to a
mechanical engineer who does not intend to confine his practice to
water-wheels, than other subjects that may be studied with greater
advantage. The subject of water-wheels may, indeed, be called an
exhausted one that can promise but little return for labour spent
upon it--with a view to improvements, at least. The efforts of the
ablest hydraulic engineers have not added much to the percentage of
useful effect realised by turbine wheels during many years past.
Reaction wheels are employed to a limited extent only, and will
soon, no doubt, be extinct as a class of water-wheels. In speaking
of reaction wheels, I will select what is called Barker's mill for
an example, because of the familiarity with which it is known,
although its construction is greatly at variance with modern
reaction wheels.
There is a problem as to the principle of action in a Barker wheel,
which although it may be very clear in a scientific sense, remains
a puzzle to the minds of many who are well versed in mechanics,
some contending that the power is directly from pressure, others
that it is from the dynamic effect due to reaction. It is one of
the problems so difficult to determine by ordinary standards, that
it serves as a matter of endless debate between those who hold
different views; and considering the advantage usually derived from
such controversies, perhaps the best manner of disposing of the
problem here is to state the two sides as clearly as possible, and
leave the reader to determine for himself which he thinks right.
Presuming the vertical shaft and the horizontal arms of a Barker
wheel to be filled with water under a head of sixteen feet, there
would be a pressure of about seven pounds upon each superficial
inch of surface within the cross arm, exerting an equal force in
every direction. By opening an orifice at the sides of these arms
equal to one inch of area, the pressure would at that point be
relieved by the escape of the water, and the internal pressure be
unbalanced to that extent. In other words, opposite this orifice,
and on the other side of the arm, there would be a force of seven
pounds, which being unbalanced, acts as a propelling power to drive
the wheel.
This is one theory of the principle upon which the Barker wheel
operates, which has been laid down in Vogdes' "Mensuration," and
perhaps elsewhere. The other theory alluded to is that, direct
action and reaction being equal, ponderable matter discharged
tangentally from the periphery of a wheel must create a reactive
force equal to the direct force with which the weight is thrown
off. To state it more plainly, the spouting water that issues from
the arm of a Barker wheel must react in the opposite course in
proportion to its weight.
The two propositions may be consistent with each other or even
identical, but there still remains an apparent difference.
The latter seems a plausible theory, and perhaps a correct one; but
there are two facts in connection with the operation of reaction
water-wheels which seem to controvert the latter and favour the
first theory, namely, that reaction wheels in actual practice
seldom utilise more than forty per cent. of useful effect from the
water, and that their speed may _exceed the initial velocity of
the water_. With this the subject is left as one for argument or
investigation on the part of the reader.
Pressure wheels, like gravity wheels, should, from theoretical
inference, be expected to give a high per cent. of power. The
water resting with the whole of its weight against the vanes or
abutments, and without chance of escape except by turning the
wheel, seems to meet the conditions of realising the whole effect
due to the gravity of the water, and such wheels would no doubt
be economical if they had not to contend with certain mechanical
difficulties that render them impracticable in most cases.
A pressure wheel, like a steam-engine, must include running contact
between water-tight surfaces, and like a rotary steam-engine, this
contact is between surfaces which move at different rates of speed
in the same joint, so that the wear is unequal, and increases as
the speed or the distance from the axis. When it is considered that
the most careful workmanship has never produced rotary engines that
would surmount these difficulties in working steam, it can hardly
be expected they can be overcome in using water, which is not only
liable to be filled with grit and sediment, but lacks the peculiar
lubricating properties of steam. A rotary steam-engine is in effect
the same as a pressure water-wheel, and the apprentice in studying
one will fully understand the principles of the other.
(1.) What analogy may be found between steam and water
power?--(2.) What is the derivation of the name turbine?--(3.)
To what class of water-wheels is this name applicable?--(4.)
How may water-wheels be classified?--(5.) Upon what principle
does a reaction water-wheel operate?--(6.) Can ponderable weight
and pressure be independently considered in the case?--(7.) Why
cannot radial running joints be maintained in machines?--(8.)
Describe the mechanism in common use for sustaining the weight of
turbine wheels, and the thrust of propeller shafts.