PUTTING RIVERS TO WORK
FLOWING water exerts a strange fascination upon mankind, even to the
present day. Tourists travel hundreds of miles to view the glorious
spectacle of a riotous tumbling cataract. Is it strange, then, that in
the olden times, when the world was peopled with gods and genii and
strange spirits, the ancients looked upon ceaselessly flowing rivers as
the symbol of life? It was most natural for them to covet the endless
power of a river and eventually, despite their superstitions, to try to
utilize some of its energy.
It may be that sailboats antedate the first water wheel, but it seems
much more probable that flowing water was the first inanimate power
harnessed by man. Windmills were certainly a later development. They
possessed the advantage that they could be located anywhere while the
water mill had of necessity to be built along the bank of a stream.
However, the power of the wind is so unreliable and fluctuates so
widely that it was little used, except in flat countries, where there
was little if any available water power.
Water power predominated until the steam engine was introduced, when
it had to give way to an even more reliable power and one which could
be located at any place to which fuel could be transported. Now,
however, we are going back to our first power, seeking it out in the
most inaccessible mountainous regions, because we have discovered the
means of taking the power it yields and transmitting it hundreds of
miles, over hills and plains to the point where we can put it to useful
service. Hydroelectric power has very aptly been termed “white coal.”
The first prime motor was the current wheel, that is, a wheel fitted
with paddles, which was journaled over a stream with the paddles
projecting into the water. This was a very inefficient machine; it
converted very little of the energy of a stream into useful mechanical
power. The idea of damming the stream and letting the waters flow
over the dam through a raceway upon a water wheel was a much later
development.
WATER WHEELS
Three types of water wheel which were in universal use before the
advent of the steam engine were the undershot wheel, the overshot
wheel, and the breast wheel (Figure 30). In the undershot wheel the
water stored back of a dam is let out near the bottom of the dam and
strikes the under side of the wheel, so that the top of the wheel turns
toward the dam. In the overshot wheel, the water flows over the wheel
striking the paddles or buckets on top and on the forward side, so that
the wheel turns forward. In the breast wheel, the water strikes the
paddles half way up the wheel on the rear side and drives the wheel in
the same direction as that of the undershot wheel.
When we speak of water power we are apt to think of the water as
actually furnishing the energy. As a matter of fact, it is not water
but gravity that drives the wheel, the water being merely the medium
that gravity acts upon. By having the water drop from a great height,
its velocity is greatly increased and the power it imparts to the wheel
is much higher. In mountainous regions it is easy to obtain a high head
of water and thus generate a great deal of power from a relatively
small stream. However, the ancient type of wheel with its paddles or
buckets has now practically passed out of existence, being superseded
by the Pelton wheel for high heads and the turbine for low heads of
water.
[Illustration: FIG. 30.--UNDERSHOT, OVERSHOT, AND BREAST WHEELS]
INVENTION OF THE PELTON WHEEL
[Illustration: FIG. 31.--SECTIONAL VIEW OF A PAIR OF PELTON WHEEL
BUCKETS SHOWING HOW THE WATER JET IS DIVIDED AND FOLLOWS THE CONTOUR OF
THE BUCKETS]
In California, a number of years ago, they made use of what was known
as the hurdy-gurdy wheel. This consisted of an ordinary wheel with
bucket-shaped paddles against which was directed a stream of water
at high velocity through a nozzle. There was a carpenter, named L.
A. Pelton, who used to make a business of building and repairing
such wheels and the flumes that carried the water to them. Although
uneducated, he was possessed of considerable native ingenuity and was a
very observant man. One day, when he was called in to repair a wheel,
he noticed that one of the buckets which had been misplaced received
the water from the nozzle without any splashing. The water struck the
edge of the bucket with practically no shock, whereas the other buckets
produced a great deal of splashing. Pelton had enough knowledge of
the principles of mechanics to realize that a splash means a waste of
energy, and that here was a bucket which, although out of plumb and
apparently defective, was really more efficient than any of the others
in the wheel. It occurred to him then that instead of having the jet of
water strike the middle of the buckets it ought to strike the edge, so
that all its power would be absorbed without any wasteful splashing. He
might have displaced the jet laterally so as to accomplish this result,
but he realized that that would have produced a considerable side
thrust on the wheel, which, of course, would have been objectionable,
and so he hit upon the plan of using double buckets and letting the
stream of water strike the pair of buckets along their dividing line.
(See Figure 31.) This would split the stream in two and let each half
strike the slanting face of the bucket, and follow the surface around
in the same way that it did on the single misplaced bucket, but the
reaction or side thrust on one bucket would be counteracted by that
on the other. This idea proved successful and out of it has grown
the Pelton wheel which is now universally used in all power plants
employing high heads of water.
A 4,000-FOOT HEAD OF WATER
A notable illustration of such a plant is the great installation at
Big Creek, Cal. Big Creek, despite its name, used to be a small stream
flowing down the mountains into a canyon. One would hardly suppose that
it was capable of yielding much power, but it had its source high up in
the Sierras and was fed mainly by melting snows. In the springtime, it
swelled to a good-sized torrent. By building three dams near the top of
the mountain, a lake was formed in which the water of the melting snow
was impounded, so that a steady stream of water could be supplied the
year round for power purposes. But even so, the stream hardly amounts
to very much if we consider only the quantity of water that passes
through it. The particular advantage of this installation is the fact
that in a distance of six miles from the dam the creek falls 4,000 feet.
An inhabitant of the Eastern States who is unused to mountain
heights may gain some conception of the meaning of this elevation by
gazing up to the pinnacle of the Woolworth Tower, which rises 795
feet above street level, then mentally multiplying its altitude by
five. Evidently even a small stream of water dropping from such an
elevation would develop an enormous amount of power. In fact, it was
considered inexpedient to use the entire fall at a single drop and so
it was divided into two stages. The water is carried through a tunnel
three-quarters of a mile long and then through a flow pipe along the
face of the mountain to a point where it may drop 2,000 feet to the
first power plant. After passing through this plant the water is
discharged into the creek and is then diverted into a second tunnel
four miles long and a series of steel conduits to a point from which it
may drop 2,000 feet more to the second power plant. In each power house
there are two electric generators, each fitted with a pair of Pelton
wheels. These wheels are a little less than eight feet in diameter and
each one develops 23,000 horsepower.
The water is directed into the buckets of the Pelton wheel in a stream
six inches in diameter, and it issues from the nozzle with a velocity
of 300 feet per second or about 210 miles per hour. A jet of water is
almost like a solid bar of wood. In fact, it is impossible to chop
through it with an ax. The water would swing the ax out of one’s hand
before it got part way through the jet. Traveling at such a high speed
the friction is so great that it would tear the skin off one’s hands,
if it did not actually tear the hand off the arm, and yet it strikes
the buckets of the wheels with no shock at all, for the first part of
the bucket it touches is nearly parallel to the jet, and as the water
sweeps around the curved face of the bucket it loses practically all of
its pressure and velocity and falls into the tail race. The electric
power generated by the two plants is stepped up to 150,000 volts and
sent out over transmission lines to points of service. The street cars
of Los Angeles are connected by a 240-mile electric harness to the
hydraulic horses of Big Creek.
Powerful as this stream is, a still higher head is used in Switzerland,
at Lake Fully, where there is a drop of over a mile in a distance of
2.8 miles. The water is carried by a short tunnel through the mountain,
and then makes a drop of over 5,000 feet to the power plant, where it
strikes the Pelton wheels at a velocity of 400 miles per hour, or about
seven times the speed of a fast express train.
HARNESSING THE MISSISSIPPI
In contrast to such high heads, we have the low-head power plants
which are employed where a large volume of water is available. The
most notable installation of this type, and the largest in the world,
is that at Keokuk, Iowa, where a dam has been thrown across the
Mississippi River. For many years it was thought impossible to make
any use of the vast volume of water that flows through this great
river. But above Keokuk there used to be a rapid extending back about
twelve miles. By building a dam across the river just below the rapid
it was possible to obtain a working head of about thirty-two feet, and
with the enormous volume of water available this provided sufficient
energy to make the development worth while. In marked contrast to
the installation at Big Creek, it is volume rather than velocity
that is employed, and hence turbines rather than Pelton wheels are
used. More water goes through a single turbine than is used in the
whole of the city of New York with all its elaborate aqueduct system.
Enormous turbines are used, fifteen feet in diameter, and when the
installation is complete there will be thirty units, each yielding
10,000 horsepower, or a total of 300,000 horsepower. A turbine, it may
be explained, differs from the ordinary water wheel in the fact that
the water runs through the wheel instead of around it (Figure 32). The
water may enter at the center and then flow out at the periphery, or
it may enter at the periphery and then be discharged from the center
of the wheel, or it may run axially through the wheel. In a Pelton
wheel there is a single jet which strikes but one pair of buckets at a
time, but in a turbine there are many jets distributed all around the
circumference of the wheel. The water is divided into a series of jets
by being forced through a stationary set of curved vanes. The blades of
the rotor or revolving part of the turbine are oppositely curved. If
the rotor were immovable the jets would have to change their direction
in passing through the rotor, but as the rotor is free to turn, the
jets react against these blades and set the wheel to revolving. The
turbine may be designed to run either on a horizontal axis or on a
vertical one.
[Illustration: FIG. 32.--TURBINE WHEELS; INFLOW TYPE SHOWN ON THE LEFT
AND OUTFLOW TYPE ON THE RIGHT]
The turbines used at the Keokuk plant are of the inflow type. The rotor
is mounted on a vertical shaft in a scroll-shaped concrete chamber,
something like a snail shell. Water pouring into this chamber is thus
given a swirling motion in the direction of rotation of the wheel. As
it flows into the wheel it passes first through a ring of fixed vanes,
which divide it into the jets.
The highest velocity of a wheel is naturally at the periphery and the
advantage of an inflowing turbine such as this is that the water is
traveling at its highest velocity when it strikes the periphery of the
rotor. As it loses its velocity it flows in toward the slower-moving
portions of the rotor. Finally it reaches the center, after giving up
practically all its energy, and falls into the tail pool through a
draft tube at the center of the rotor.
The scroll chambers at Keokuk are thirty-nine feet in diameter and the
draft tubes are eighteen feet in diameter. Water enters the scroll
chambers with a velocity of fourteen feet per second and comes out of
the draft tubes into the tail pool with its velocity cut down to but
four feet per second. Compare this with the velocity of the water jets
at Big Creek!
The current generated at Keokuk goes to St. Louis and surrounding towns
and serves a population of 1,120,000.
Now that we have learned how to transmit electrical power without
serious loss over enormous distances, it is only a question of time
before all the water power in the world is harnessed and put to the
service of man. The power costs nothing after once the plant has been
built; the only expense is that of maintaining the machinery and
keeping it in repair. It is estimated that there is some 200,000,000
horsepower available in this country, but this includes all flowing
water, much of which it would be impracticable, if not almost
impossible, to utilize. However, there is about 60,000,000 horsepower
commercially available, according to the figures of the U. S.
Geological Survey, of which we have developed so far only 6,000,000
horsepower.
The ancients used flowing streams not so much for power purposes as
to lift water to a higher level so that it would flow into their
irrigating ditches. Nowadays, electricity, steam, or air is used for
elevating water, but we have a very ingenious machine which makes
the stream lift a part of itself. This machine is very different in
principle from the old Egyptian noria. It depends upon the kinetic
energy of water in motion. You cannot push a nail into a piece of wood
with a hammer but you can easily drive it in by striking it with the
hammer. As the hammer is swung it acquires what we term kinetic energy
or energy of motion.
SETTING KITCHEN FAUCETS TO WORK
It is not generally realized that water in motion also acquires kinetic
energy. Whenever a faucet is turned off very quickly, there is a
hammering sound which is due to the fact that the moving water in the
water pipe is brought to an abrupt stop. This puts a severe strain on
the piping. A great deal of trouble was experienced from this source in
the early days of plumbing. At a hospital in Bristol, England, there
was a lead pipe leading from a cistern in one of the upper stories
to the kitchen. Every time the faucets were turned off abruptly the
momentum of the water caused the lead pipe to expand, and every now and
then the pipe was burst. In order to relieve the situation, a plumber
connected a pipe to the faucet and carried it up the side of the
building to the level of the cistern. His idea was that whenever the
water was turned off suddenly it would have a vent leading up to the
level of the water reservoir. Much to his surprise, the water issued
from the pipe in a jet of considerable height. To prevent the escape
of the water, he extended the pipe considerably, and still a jet of
water would issue from it. Eventually the relief pipe was carried up
twice the height of the cistern and even then the water would squirt
out occasionally when the faucets in the kitchen were turned off very
suddenly. Then the idea was conceived of placing a reservoir on one of
the upper floors of the hospital and letting the jet of water fill this
reservoir. Every time the faucet was operated in the kitchen a certain
amount of water flowed into the new cistern, and in this way it was
kept supplied with enough water to furnish that which was required for
the upper floors of the hospital.
[Illustration: FIG. 33.--SECTIONAL VIEW OF A WATER RAM]
It is on this principle that the hydraulic ram operates. Water from a
stream is made to flow down a pipe, and as it gains velocity a check
valve suddenly stops the flow which produces enough pressure to force
open a valve in an air chamber and let some of the water enter the
chamber. As soon as the pressure is relieved the check valve opens
and the valve into the chamber closes automatically until a moment
later the stream of water has gained sufficient velocity to repeat the
performance. Thus an intermittent jet of water is forced into the air
chamber and thence through a pipe to a reservoir. The height to which
the water will rise depends entirely upon the velocity of the water
flowing through the system. The air chamber is necessary to cushion
the action of the hydraulic ram and provide a fairly steady pressure
upon the water that flows up through the vent pipe. The check valve is
entirely automatic. It is held open against the pressure of the water
by a spring or a weight, but when the water is in motion is dragged
shut, only to spring open again when the pressure is reduced by the
escape of the water into the air chamber.
[Illustration: FIG. 34.--THE GYRATING WATER METER]
There is a very ingenious water-driven motor which is employed merely
to record the amount of water flowing through it. This is the Thomson
water meter which is illustrated in Figure 34. It consists of a
circular chamber with inwardly dished or conical top and bottom walls.
In the chamber is a flat disk with a ball and socket bearing. At one
side there is a vertical diaphragm in the chamber which passes through
a slot in the disk. This prevents the disk from revolving, but it
is free to oscillate. It has a motion similar to the gyrations of a
top when it is beginning to lose speed and die down, except that the
disk does not revolve. When the disk is in contact with the bottom
wall of the chamber on one side it contacts with the top wall on the
other so that the chamber is virtually divided into two compartments
by the disk, but by gyrating the disk these compartments are made to
revolve. Water enters at one side and discharges at the other side of
the vertical diaphragm. Now, if the disk is in the position shown in
Figure 34, the water, on entering, bears upon the upper face of the
inclined disk and wedges its way between the disk and the upper wall
of the chamber, making the disk oscillate on its ball center. As the
edge of the disk rises across the face of inlet port the water entering
the chamber bears against the under side of the disk, continuing the
gyratory motion. The water cut off on the upper side of the disk is
carried around to the outlet and discharges, while a fresh supply flows
in on the other side of the vertical partition and at the next half
turn the water in the lower compartment discharges at the outlet side
of the partition, while the compartment is filling on the other side of
the partition. A measured amount of water flows through the chamber at
each gyratory oscillation of the disk. A train of gearing is driven by
the gyrating disk which operates a set of dial pointers and a measure
of the amount of water passing through the meter is indicated.
DIGGING WITH WATER JETS
We have referred to the enormous velocity of the jets used to drive
Pelton wheels. Where high heads of water are obtainable water jets are
used very effectively for excavating purposes, particularly in mining
plants for washing down gold-bearing gravel banks. If water is not
found near such banks expensive canals, flumes, and pipe lines are
constructed and even tunnels are bored to bring the water to the point
where it can be utilized. Some of the giant nozzles spout streams from
2 to 8 inches in diameter with a pressure of from 50 to 200 pounds per
square inch. The powerful streams tear into the gravel banks, washing
them away into sluices in which riffle boxes are placed to catch the
precious metal. The back pressure of these nozzles is very heavy and
the larger ones have to be provided with strong anchorages. Water in
motion resists any change of direction and long levers have to be
provided to permit the miners to guide the nozzles.
The hydraulic jet is also used for general excavating wherever water
power is available. Sometimes it is employed under water when clearing
a channel to level down piles of stones that are too large to be picked
up by a suction dredge. Hollow iron piles are driven into a sandy
bottom by means of hydraulic jets. No hammer is needed. Water is pumped
into the pile and on issuing from the bottom of the pile it carries
sand with it, making a hole into which the pile sinks. Wooden piles
are driven in the same way by loosely attaching a water pipe to them
so that the pipe may be withdrawn when the pile has been driven far
enough. The pile is grooved at the lower end so that the pipe outlet
may be centered at the bottom of the pile.
COMPRESSING AIR WITH WATER
[Illustration: FIG. 35.--HOW THE VENA CONTRACTA RAISES WATER]
A very ingenious apparatus for compressing air was invented in the
earliest years of the iron age to furnish a continuous blast of air for
the Catalan forges. This compressor, known as a “trompe,” can hardly
be termed a machine because it contains no moving parts except water,
which is the motive power, and the air which it traps and compresses.
To understand its operation we must look into a peculiar property of
water flowing out of a reservoir into a pipe or nozzle. There is a
converging motion that tends to contract the jet of water just after it
leaves the pipe. This is known as the _vena contracta_. It produces
a partial vacuum in the pipe. If air ports are opened into the pipe
at this point, air will be sucked in to fill the vacuum and will be
carried out of the pipe by the friction of the water. In Figure 35 a
pipe is shown running from the _vena contracta_ to a water tank below.
The rise of water in this pipe indicates the degree of vacuum produced
by the jet.
[Illustration: FIG. 36.--THE “TROMPE” BY WHICH COMPRESSED AIR WAS
FURNISHED FOR CATALAN FORGES]
This principle is used in the hydraulic-air compressor or “trompe” as
it is called. Water flows out of one reservoir through a pipe into
another reservoir lower down. (See Figure 36.) Air enters the pipe
through ports at the point where the vein of water contracts and is
carried down into the second reservoir. This reservoir is sealed so
that the air is trapped in it. The water passes out through a pipe
which is carried high enough to keep a certain pressure of air in the
reservoir and prevent it from blowing out.
[Illustration: FIG. 37.--DIAGRAMMATIC SECTION OF A LARGE HYDRAULIC AIR
COMPRESSOR IN MICHIGAN]
There is a compressor of this type, constructed on an enormous scale,
in the northern part of Michigan. A sketch of the compressor is given
in Figure 37. The air reservoir in this case is a huge underground
rock-walled chamber nearly 350 feet below the surface, 8 feet wide, 26
feet high, and about 280 feet long. There are three intake pipes, 5
feet in diameter, each filled with an annular funnel-shaped head, which
sucks air into the water and carries it down into the chamber. At the
bottom of each intake pipe there is a concrete block with a conical
top projecting up into the pipe. The column of water flowing down the
pipe is spread out into an annular stream by the conical block and the
bubbles of air escape into the chamber. The water outlet of the chamber
is an inclined shaft which leads up about 270 feet to the surface of
the ground where it discharges into the tail race. The water is forced
up this inclined shaft by the pressure of the air trapped in the
chamber. The mouth of the shaft is, of course, below the level of the
water in the chamber so that there is no chance for the air to escape
unless the pressure becomes excessively high, when it will force the
water level below the mouth of the shaft and blow out. The discharge
sometimes forms a geyser 700 feet high. The air will continue to blow
until the pressure is reduced enough for the water level to rise and
cut off access of the air to the mouth of the shaft. Under normal
conditions there is a fall of 343 feet from the water level at the top
of the intake pipe to the water level in the chamber, and a vertical
rise of 271 feet from the water level in the chamber to the tail-water
level. The difference, or 72 feet, represents the working head. With
all three intakes operating, a total of 5,000 horsepower is developed.
Each intake delivers 11,930 cubic feet of air per minute at a pressure
of 128 pounds. Air enters the intake heads through tubes ⅜ inch in
diameter and there are 1,800 of them to each head. The air is employed
to operate machinery and tools in an adjacent mine.
One advantage of this type of air compressor is that it cools the air
while compressing it. This was hardly an advantage in the Catalan
forges, but when the air is used to drive machinery it is important
that it be precooled. When air is compressed by mechanical means
a great deal of heat is generated and the machines must be water
jacketed to extract this heat, but in the hydraulic compressor the
air bubbles are compressed as they pass down with the water to the
reservoir and the water absorbs the heat, delivering cool compressed
air at the bottom of the intake pipe.