POWER FROM HEAT
THE TERM “water power” is a misnomer. There is no power inherent in
water. It is gravity that makes water fall in cataracts or flow down
a river bed, and hence it is the force of gravity that is responsible
for the turning of our turbines and Pelton wheels. Water is merely the
medium through which the force of gravity acts.
In the steam engine, water, again, serves as a medium and it is the
energy of heat that is actually the working force. A closed vessel is
partly filled with water and heat is applied to it. The water grows
hotter and hotter until at a temperature of 212 degrees it begins to
vaporize. The temperature of the water remains stationary until the
space above the water is completely filled with steam at a pressure
equal to that of normal air pressure. The steam gauge then registers
zero. Continued application of heat raises the pressure of the steam
and also raises the temperature of the water. When the steam gauge
shows a pressure of 25 pounds per square inch, the water temperature
is nearly 267 degrees; at 100 pounds it is over 337 degrees; at 200
pounds it is 387 degrees; at 500 pounds it is about 467 degrees. As
steam is drawn off from the boiler a proportionate amount of water
turns immediately into steam to take its place. If the boiler should
burst, the entire mass of water would instantly flash into steam
because its temperature is far above the boiling point at normal air
pressure. That is why the explosion of a steam boiler is so violent. If
the boiler were entirely filled with steam the effect of an explosion
would not begin to be so destructive as if the boiler were half full
of water. It is really the explosion of water that does such serious
damage.
When the steam pressure in a boiler corresponds to the temperature of
the water it is said to be _saturated_. The water in a boiler boils
violently when the steam is drawn off rapidly and tiny droplets of
water are carried off in the steam, producing what is known as _wet
steam_. Steam that carries no water particles in suspension is called
_dry steam_. When the steam is heated above the temperature of the
water by means of an auxiliary heating device or by some peculiar
construction of the boiler, it is called _superheated steam_.
In order to economize fuel it is highly important that as much water
surface be exposed to the heat as possible. This may be done either by
passing the heat in tubes through the water or by passing the water
in tubes through the fire. Locomotive boilers are of the fire-tube
type. The flaming gases of the furnace pass through the boiler through
a series of tubes that lead to the stack. In the water-tube boilers
it is highly important that a good and rapid circulation of water be
maintained, otherwise there might be local generation of steam with
serious consequences.
THE GIFFARD INJECTOR
[Illustration: FIG. 45.--INJECTOR FOR INTRODUCING WATER INTO A STEAM
BOILER]
As steam is used from a boiler the water is slowly exhausted and it
must be replenished with a fresh supply. In early days of the steam
engine, water was pumped in against the boiler pressure by the use
of powerful pumps, but in 1858 a man named Giffard invented a most
ingenious apparatus by which steam of the boiler was used to force
water in directly against its own pressure. This seems like lifting
oneself by one’s boot straps. When the injector was first invented it
seemed so impossible for it to work that engineers would not accept it
until it had repeatedly demonstrated its operativeness. Even after it
was accepted and in common use its mysterious operation was a subject
of discussion for years. A sectional view of a Giffard injector is
shown in Figure 45. Steam from the boiler comes down the tube A and
passes out in a jet from the nozzle B. A needle valve C may be moved
into the nozzle to reduce or shut off the jet of steam. The jet enters
a conical chamber D which has a tube E that runs down into the water
reservoir. The steam jet blows the air out of chamber D, producing a
partial vacuum which draws water up the tube E and into the chamber D.
When the water reaches the steam jet it is driven out of the chamber
across a short open space into a slightly diverging tube or receiving
cone F and through a check valve G into the boiler. At H there is a
glass window through which the action of the water jet as it rushes
into the receiving tube may be watched. I is an overflow pipe leading
back to the reservoir. When the steam flows into the cone B it gathers
momentum and issues from the nozzle in a jet of high velocity. On
striking the water it combines with the water and condenses, but at the
same time it imparts its momentum to the water so that the water is
given more than enough momentum to drive it into the boiler against the
pressure in the boiler.
The pressure of steam is utilized to drive the piston of an engine
or the steam may be set in motion by letting it issue from a nozzle,
when its momentum may be employed to drive a steam turbine in the
same way that a water jet drives a Pelton wheel but in either case it
is heat that does the work. The steam expands as it moves the piston
and as it passes through the nozzle its expansion is accompanied by a
corresponding loss of heat.
As we have already noted, it was for the purpose of raising water that
inanimate powers were first set to work. It was with the same object in
view that steam was first employed.
HERO’S STEAM ENGINE
To be sure the first steam engine was invented by Hero, the disciple
of Ctesibius, in the second century before Christ, and it was not a
water-raising machine. It consisted of a hollow sphere (see Figure
46), mounted to rotate and fed with steam that entered it through the
journals. The steam issued from opposite sides of the sphere through
two bent tubes. The steam in issuing from the tubes reacted against the
tubes, pushing them back and causing the sphere to revolve.
Every action is accompanied by an equal and opposite reaction. A bullet
fired from a gun kicks the gun back. In order to push the bullet out of
the barrel, the powder must have something to push against and the push
against the gun is equal to the push against the bullet. Even if there
were no bullet to be pushed the discharge of the powder would react
against the gun. In order to push its own gases out of the gun, the
powder must push back against the gun.
[Illustration: FIG. 46.--HERO’S STEAM ENGINE]
In Hero’s engine, the steam, in order to push itself out of the sphere,
must push against the bent tubes. We have a similar reaction motor in
the revolving lawn sprinkler. The water issuing from the bent arms of
the sprinkler forces them back and causes them to revolve. Were the
arms radial and not bent the reaction would be there just the same, but
it would be exerted against the center of the wheel and hence there
would be no rotation.
Reaction has nothing to do with the pressure of the atmosphere as
so many people imagine. A gun would kick just as hard and the lawn
sprinkler or Hero’s engine would operate just as well and as fast in
a perfect vacuum. It was to demonstrate the principle of reaction that
Hero built his steam engine. As far as we know it was never put to
useful work and remained merely a scientific toy.
NEWCOMEN’S ATMOSPHERIC ENGINE
The first practical use of steam power was applied to the pumping
of water from the deep mines of Cornwall. Newcomen’s engine, which
antedated Watt’s by several decades, was known as an atmospheric
engine. It consisted of a cylinder fitted with a piston, but the top of
the cylinder was open. The piston was connected to one end of a lever
or walking beam, the other end of which was connected by a chain with
the plunger of a pump situated at the bottom of the mine shaft. Little
power was required to lift the piston because it was counterbalanced by
the chain and plunger attached to the opposite end of the walking beam.
The real work was done in pushing the piston down and thereby raising
the pump plunger. Steam under low pressure was let into the cylinder
under the piston to raise it and then a jet of water was sprayed into
the steam-filled cylinder. This condensed the steam, producing vacuum
and the atmospheric pressure acting on the upper surface of the piston
forced the piston down, raising the pump plunger. The engine ran very
slowly, making only about 15 strokes per minute. Later it was improved,
producing about 30 strokes per minute.
WATT’S STEAM ENGINE
Watt was the first man to build an engine in which the real work was
done by the pressure of steam as in modern steam engines. As long as
the steam engine was used for pumping water the reciprocating piston
could be connected directly to the pump plunger and it was unnecessary
to exert power on both sides of the piston, but in 1782 Watt obtained a
patent on a double-acting engine. Steam was admitted first on one side
of the piston and then on the other. The walking beam was connected
by means of a connecting rod to a crank on a shaft that carried a
flywheel, and the seesaw motion of the beam was converted into rotary
motion of the wheel. This opened up new industrial opportunities for
the steam engine.
The steam was exhausted from the cylinder into a condenser, i.e., a
chamber, in which a spray of water converted the steam into water,
producing a vacuum, thereby relieving the piston of back pressure and
virtually adding that much more power to the steam operating on the
opposite side of the piston. A pump driven by the engine drew the water
and the air liberated from the condensed steam out of the condenser.
Watt invented an ingenious governor to control the flow of steam to
the engine and insure a uniform motion. (See Figure 47.) This governor
consisted of a pair of levers hinged to a revolving shaft and each
provided with a ball weight at its free end. The shaft was revolved
by the engine and if the engine tended to run too fast the balls were
thrown out by centrifugal action. In so doing they operated a throttle
valve cutting down the steam supply. When the engine slowed down the
balls would drop, admitting more steam. In this way the speed of the
engine was kept within close limits. The ball governor in improved form
is still widely used in stationary engines.
USING THE HEAT IN THE STEAM
[Illustration: FIG. 47.--WATT’S BALL GOVERNOR]
Another of Watt’s inventions which has proved of highest importance
in steam engineering was the cut off. In his first engine the piston
was subjected to the full boiler pressure throughout its stroke, and
the steam that poured out of the exhaust port of the cylinder at the
end of the piston stroke was almost as hot as that which entered the
cylinder at the beginning of the stroke. The heat that goes out in the
exhaust represents just so much wasted energy. Watt realized this and
so he invented a valve which would cut off the flow from the boiler
before the piston had completed its stroke. Then the steam back of
the piston would continue to expand because of the heat within it
and would keep on pushing the piston. Of course, the pressure would
gradually diminish and there would be less power in the stroke than if
the full boiler pressure were pushing the piston all the way, but this
loss of power is offset by the saving in steam and in the fuel used
to heat the steam. The point at which the cut-off takes place depends
largely upon the pressure of the steam. If the steam is cut off when
the piston has made only one-fifth of its stroke, one-fifth as much
steam will be used at each stroke as would be the case if the steam
were used nonexpansively. However, in actual practice the expansion of
the cut-off steam instead of being five times would be only about four
times, because of the clearance that must be allowed between the piston
and the end of the cylinder. If steam of 100 pounds absolute pressure
is used, the average pressure throughout the piston will be only about
57 per cent of the full pressure of the steam in the boiler, but each
pound of steam will actually do .57 × 4 = 2.28 times as much work as
it would if used nonexpansively. All sorts of valve gear have been
invented to admit steam quickly and cut it off at the proper point to
produce the most efficient result.
In low-pressure cylinders in order to prevent loss of heat through the
wall of the cylinder, the latter is steam-jacketed. In other words,
there is an outer casing surrounding the cylinder and between this
casing and the cylinder steam is admitted to keep the cylinder walls
hot.
In order to make full use of the heat in steam it is, in some engines,
sent through a series of two, three, and even four cylinders. The
exhaust from one cylinder goes into a second larger cylinder. From here
after doing work on a piston it discharges into a third still larger
cylinder and from that may be led into a fourth cylinder. The cylinders
must be progressively larger to allow for the expansion of the steam.
The ordinary steam engine labors under the disadvantage of having
to start and stop its pistons at the end of each stroke. Every body
possesses inertia, whether it be moving or at rest. If it be at rest,
it takes much more energy to set it in motion than to keep it moving.
In fact it would keep on moving without further expenditure of energy
were there no friction and no forces acting against it. In order to
stop the body energy must be expended to overcome its inertia. The
more rapidly a body is started and the more quickly it is stopped, the
more work must be done in overcoming its inertia. In a steam engine
not only the piston but other parts connected to it may be required to
reciprocate several hundred times per minute. A great deal of energy is
uselessly expended in starting and stopping these parts.
Many efforts have been made to produce a rotary engine in which the
piston rotates instead of reciprocating, thus doing away with the
work of overcoming inertia. However, there are serious obstacles to
the construction of such an engine, and as yet no truly efficient and
practical rotary engine has been built.
DE LAVAL’S STEAM TURBINE
However, there is another type of engine in which the steam is applied
continuously and all the parts revolve. Such an engine was the reaction
turbine invented by Hero, to which reference has already been made.
Modern turbines, however, are of very different construction. They
resemble the Pelton wheels and turbines used in developing water
power, differing from them mainly in the fact that use is made of the
expansive energy of steam which is lacking in water. In the De Laval
steam turbine a wheel is used which has a series of curved buckets all
around its periphery that are closed at the outer end by a circular
rim. (See Figure 48.) Steam is directed against this bucket, not
tangentially as in a Pelton wheel, but from the side. Several steam
nozzles are employed and as the steam jets strike the buckets and sweep
around their curved surfaces they react against the buckets and drive
the wheel around. In order to operate efficiently the velocity of the
steam must be very high and the wheel must also turn at high speed.
When steam flows through a diverging nozzle its velocity is greatly
accelerated by its expansive effort. Such nozzles are used in the De
Laval turbines and the steam issues from them with a velocity which
may be higher than that of a rifle bullet. The buckets are forged and
the hard-scale surface is left on them; otherwise they would wear away
quickly under the action of the powerful jets of steam.
[Illustration: FIG. 48.--THE DE LAVAL STEAM TURBINE]
The turbine wheel may revolve at a speed of 30,000 revolutions per
minute. Such a tremendous speed has its disadvantages. If a wheel is
to run smoothly it must revolve on its center of gravity. A lopsided
wheel, or one that is mounted a little off center, produces a pounding
action which imposes a serious strain upon the bearings and the
revolving parts. The wheel tries to turn on its own center of gravity
and will do so if permitted to. It is impossible to balance a wheel so
perfectly that the axis it turns on passes exactly through its center
of gravity. At ordinary speeds this slight eccentricity is so slight
that it is practically negligible, but when we have to deal with 30,000
revolutions per minute the least divergence between the center of
rotation and the center of gravity will produce dangerous strains. For
this reason the wheel of the De Laval turbine is mounted on a flexible
shaft and on floating bearings, so that it will automatically find
and turn on its own center of gravity. In order to utilize the power
developed in the wheel, gearing must be used to step down the speed.
PARSONS EXPANSION TURBINE
The Parsons steam turbine resembles a water turbine in its action.
Instead of having a few nozzles directing steam against the buckets
there is a nozzle for each bucket, and instead of a single wheel there
is a series of them through which the steam passes, successively
passing through a set of nozzles between each wheel. The nozzles are
formed of blades on the periphery of stationary wheels. These blades
are curved in the opposite direction to the blades on the revolving
wheels, as shown in Figure 49. Steam in passing through the ring of
curved stationary blades is divided up into a series of jets which
strike the curved blades of the first wheel. In passing through this
wheel the direction of the stream is reversed, and it enters between
the second set of stationary blades, which turn it back again and
direct it against the next wheel. Thus the steam pursues a sinuous
course through the series of wheels. To allow for the expansion of
the steam the blades are made progressively longer and the wheels of
progressively larger diameter from the inlet to the exhaust end of the
engine.
[Illustration: FIG. 49.--PERIPHERAL VIEW OF THE BLADES OF A PARSONS
TURBINE]
The Curtis turbine combines the De Laval and the Parsons principles.
The steam enters through a series of nozzles which are of the expanding
type, then it goes through a series of moving and stationary blades, as
in the Parsons turbine, from which it enters another set of expanding
nozzles and gains velocity and momentum before passing through the
second series or stage of moving and fixed blades. (See Figure 50.)
Steam turbines are particularly adapted for use in electric power
plants. The speed of rotation of the Parsons and Curtis types is much
lower than that of the De Laval and hence the electric generators may
be directly driven by them without the interposition of any gearing.
They can be built of larger power than the reciprocating engines
because they are so economical of space.
[Illustration: FIG. 50.--PERIPHERAL VIEW OF THE BLADES AND NOZZLES OF A
CURTIS TURBINE]
A good comparison of turbine versus reciprocating engines is offered
by the 74th Street power station of the Manhattan Elevated Railway,
New York. This station, which was completed in 1901, was equipped with
eight huge reciprocating engines, each developing 8,000 horsepower
normally, and capable of delivering a maximum of 12,500 horsepower. The
whole plant, therefore, had a maximum capacity of 100,000 horsepower.
Gradually these units have been giving way to steam turbines of much
higher power, and in 1919 there was installed one powerful turbine
which alone was capable of developing as much power as the entire plant
of 1901. This is a triple-compound turbine comprising one high-pressure
turbine and a low-pressure turbine at each side. Steam enters the
high-pressure turbine at 205 pounds pressure to the square inch and
then exhausts into the low-pressure turbines, passing from them into
condensers which operate under 29 inches vacuum. Each turbine drives
a separate generator and the combined horsepower of the whole unit is
about 100,000, while the floor space occupied is only 50 by 52 feet.
The economy of space and of fuel offered by the steam turbine is of
great value in the power plants of ships, and this form of prime
mover has been installed on modern high-speed passenger liners and
also on high-speed war vessels. While in certain respects the turbine
is ideal for such service, there are two handicaps which must be
overcome. In the first place, the most efficient speed for the turbine
is considerably higher than the efficient speed of the propeller
and some means must be provided for stepping down the speed. In
the second place, the turbine cannot be as economically controlled
as a reciprocating engine and its direction of rotation cannot be
reversed, so that difficulties are encountered in maneuvering the ship
in harbors. It is no simple matter to gear down the high speed and
enormous power of a turbine, However, an elaborate system of gearing
has been provided for this purpose which has proved satisfactory even
in powerful battle cruisers. The British battle cruisers with a power
plant of 134,000 horsepower are driven by geared turbines. To reverse
the propellers separate low-power turbines are used.
A more attractive system of control is to have the turbines drive
electric generators and then use the electric power to drive the
propellers through motors mounted on the propeller shafts. The electric
power can easily be controlled from the bridge and the propellers may
be reversed by reversing the motors. However, the disadvantage of the
electric system is that it occupies a great deal of space, particularly
in plants running over 100,000 horsepower.