STEAM ENGINES
The first steam engine was an exceedingly simple affair. It had neither
eccentric, cylinder, crank, nor valves, and it did not depend upon the
pressure of the steam acting against a piston to drive it back and
forth, because it had no piston.
It is one of the remarkable things in the history and development of
mechanism, that in this day of perfected steam engines, the inventors of
our time should go back and utilize the principles employed in the first
recorded steam engine, namely, the turbine. Instead of pressure exerting
a force against a piston, as in the reciprocating engine, the steam
acted by impacting against a moving surface, and by obtaining more or
less reaction from air-resistance against a freely discharging steam jet
or jets.
The original engine, so far as we have any knowledge, had but one moving
part, namely, a vertical tubular stem, to which was attached a cross or
a horizontal tube.
The Original Engine.--Figure 8 is a side view of the original engine.
The vertical stem A is pivoted to a frame B, and has a bore C which
leads up to a cross tube D. The ends of the tube D are bent in opposite
directions, as shown in the horizontal section, Fig. 9.
[Illustration: _Fig. 8. The Original Engine._]
[Illustration: _Fig. 9. Horizontal Section of Tube._]
Steam enters the vertical stem by means of a pipe, and as it rushes up
and out through the lateral tubes D, it strikes the angles E at the
discharge ends, so that an impulse is given which drives the ends of the
tube in opposite directions. As the fluid emerges from the ends of the
tubes, it expands, and on contacting with the air, the latter, to a
certain extent, resists the expansion, and this reacts on the tube.
Thus, both forces, namely, impact and reaction, serve to give a turning
motion to the turbine.
The Reciprocating Engine.--The invention of this type of engine is
wrapped in mystery. It has been attributed to several. The English
maintain that it was the invention of the Marquis of Worcester, who
published an account of such an engine about 1650. The French claim is
that Papin discovered and applied the principle before the year 1680.
In fact, the first actual working steam engine was invented and
constructed by an Englishman, Captain Savery, who obtained a patent for
it in 1698. This engine was so constructed as to raise water by the
expansion and condensation of steam, and most engines of early times
were devoted solely to the task of raising water, or were employed in
mines.
Atmospheric Engines.--When we examine them it is difficult to see how we
can designate them as steam engines. The steam did not do the actual
work, but a vacuum was depended on for the energy developed by the
atmospheric pressure.
A diagram is given, Fig. 10, showing how engines of this character were
made and operated. A working beam A was mounted on a standard B, and one
end had a chain C on which was placed heavy weights D. Near this end was
also attached the upper end of a rod E, which extended down to a pump.
[Illustration: Fig. 10. Steam-Atmospheric Engine.]
The other end of the working beam had a chain F, which supported a
piston G working within a vertically-disposed cylinder H. This cylinder
was located directly above a boiler I, and a pipe J, with a valve
therein, was designed to supply steam to the lower end of the cylinder.
A water tank K was also mounted at a point above the cylinder, and this
was supplied with water from the pump through a pipe L. Another pipe M
from the tank conducted water from the tank to the bottom of the
cylinder.
The operation of the mechanism was as follows: The steam cock N, in the
short pipe J, was opened to admit steam to the cylinder, below the
piston. The stem of the steam cock also turned the cock in the water
pipe M, so that during the time the steam was admitted the water was
shut off.
When the steam was admitted so that it filled the space below the
piston, the cock N was turned to shut off the steam, and in shutting off
the steam, water was also admitted. The injection of water at once
condensed the steam within the cylinder so a partial vacuum was formed.
It will be remembered that as steam expanded 1700 times, the
condensation back into water made a very rarified area within the
cylinder, and the result was that the piston was drawn down, thus
raising both the weight D and also the pump rod E. This operation was
repeated over and over, so long as the cock N was turned.
The turning of the stem of this cock was performed manually,--that is,
it had to be done by hand, and boys were usually employed for doing
this. When, later on, some bright genius discovered that the valve
could be turned by the machinery itself, it was regarded as a most
wonderful advance.
The discovery of this useful function has been attributed to Watt. Of
this there is no conclusive proof. The great addition and improvements
made by Watt, and which so greatly simplified and perfected the engine,
were through the addition of a separate condenser and air pump, and on
these improvements his fame rests.
From the foregoing it will be seen that the weight D caused the piston
to travel upwardly, and not the force of the steam, and the suction
produced by the vacuum within the cylinder did the work of actuating the
pump piston, so that it drew up the water.
The Piston.--From this crude attempt to use steam came the next step, in
which the steam was actually used to move the piston back and forth and
thus actually do the work. In doing so the ponderous walking beam was
dispensed with, and while, for a long period the pistons were
vertically-placed, in time a single cylinder was used, and a crank
employed to convert the reciprocating into a circular motion.
Fig. 11 shows a simple diagram of a steam engine, so arranged that the
operation of the valves may be readily understood. The cylinder A has a
steam chest B, which contains therein a slide valve C to cover the ports
at the ends of the cylinder. This figure shows the crank turning to the
right, and the eccentric D on the engine shaft is so placed, that while
the crank E is turning past the dead center, from 1 to 2, the slide
valve C is moved to the position shown in Fig. 12, thereby covering port
F and opening port G.
[Illustration: _Fig. 11. Simple Valve Motion. First position._]
[Illustration: _Fig. 12. Simple Valve Motion. Second position._]
It will be seen that the slide valve is hollowed within, as at H, and
that the exhaust port I leads from this hollowed portion while the live
steam from the boiler enters through pipe J and fills the space K of
the chest.
In Fig. 11 live steam has been entering port F, thus driving the piston
to the right. At the same time the exhaust steam at the right side of
the piston is discharging through the port G and entering the hollow
space within the slide valve. In Fig. 12 the conditions are reversed,
and now live steam enters port G, and the exhaust passes out through
port F.
When the engine crank reaches the point 3, which is directly opposite 1,
the reverse action takes place with the slide valve, and it is again
moved to its original position, shown in Fig. 12.
Importance of the Valve.--Every improvement which has been made in the
engine has been directed to the valve. The importance of this should be
fully understood. As the eccentric is constantly turning it is a
difficult matter to so arrange the valve as to open or close it at the
correct time, absolutely, and many devices have been resorted to to
accomplish this.
Expanding the Steam.--As all improvements were in the direction of
economizing the use of steam, it was early appreciated that it would be
a waste to permit the steam to enter the cylinder during the entire
period that the engine traveled from end to end, so that the valve had
to be constructed in such a way that while it would cut off the
admission of steam at half or three-quarters stroke, the exhaust would
remain on until the entire stroke was completed.
Some engines do this with a fair degree of accuracy, but many of them
were too complicated for general use. In the form of slide valve shown
the pressure of the steam on the upper side, which is constant at all
times, produces a great wearing action on its seat. This necessitated
the designing of a type of valve which would have a firm bearing and be
steam tight without grinding.
Balanced Valve.--One of the inventions for this purpose is a valve so
balanced by the steam pressure that but little wear results. This has
been the subject of many patents. Another type also largely used in
engines is known as the _oscillating_ valve, which is cylindrical or
conical in its structure, and which revolves through less than a
complete revolution in opening and closing the ports.
Rotary Valve.--The rotary valve, which constantly turns, is employed
where low pressures are used, but it is not effectual with high
pressures. This is also cylindrical in its structure, and has one or
more ports through it, which coincide with the ports through the walls
of the engine, as it turns, and thus opens the port for admitting live
steam and closing the discharge port at the same time or at a later
period in its rotation.
Engine Accessories.--While the steam engine is merely a device for
utilizing the expansive force of steam, and thus push a cylinder back
and forth, its successful operation, from the standpoint of economy,
depends on a number of things, which are rarely ever heard of except by
users and engineers.
Many of these devices are understood only by those who have given the
matter thorough study and application. To the layman, or the ordinary
user, they are, apparently, worth but little consideration. They are the
things, however, which have more than doubled the value of the steam
engine as a motor.
Efficiency of Engines.--When it is understood that with all the
refinements referred to the actual efficiency of a steam engine is less
than 30 per cent. some idea may be gained of the value which the various
improvements have added to the motor.
Efficiency refers to the relative amount of power which is obtained from
the burning fuel. For instance, in burning petroleum about 14,000 heat
units are developed from each pound. If this is used to evaporate water,
and the steam therefrom drives an engine, less than 4200 heat units are
actually utilized, the remaining 9800 heat units being lost in the
transformation from the fuel to power.
[Illustration: _Fig. 13. Effective pressure in a Cylinder._]
The value of considering and providing for condensation, compression,
superheating, re-heating, compounding, and radiation, and to properly
arrange the clearance spaces, the steam jackets, the valve adjustments,
the sizes of the ports and passages, and the governor, all form parts of
the knowledge which must be gained and utilized.
How Steam Acts in a Cylinder.--Reference has been made to the practice
of cutting off steam before the piston has made a full stroke, and
permitting the expansive power of the steam to drive the piston the rest
of the way, needs some explanation.
As stated in a preceding chapter the work done is estimated in foot
pounds. For the purpose of more easily comprehending the manner in which
the steam acts, and the value obtained by expansion, let us take a
cylinder, such as is shown in Fig. 13, and assume that it has a stroke
of four feet. Let the cylinder have a diameter of a little less than one
foot, so that by using steam at fifty pounds pressure on every square
inch of surface, we shall have a pressure of about 5000 pounds on the
piston with live steam from the boiler.
In the diagram the piston moves forwardly to the right from 0 to 1,
which represents a distance of one foot, so that the full pressure of
the steam of the boiler, representing 5000 pounds, is exerted on the
piston. At 1 the steam is cut off, and the piston is now permitted to
continue the stroke through the remaining three feet by the action of
the steam within the cylinder, the expansive force alone being depended
on.
As the pressure of the steam within the cylinder is now much less and
decreases as the piston moves along, we have taken a theoretical
indication of the combined pressure at each six inch of the travel of
the piston. The result is that we have the following figures, namely,
4000, 2700, 1750, 1000, 450 and 100. The sum of these figures is 10,000
pounds.
The piston, in moving from 0 to 1, moved one foot, we will say, in one
second of time, hence the work done by the direct boiler pressure was
5000 _foot pounds_; and since the piston was moved three feet more by
the expansion of the steam only, after the steam pressure was shut off,
the work done in the three seconds required to move the piston, was an
additional 5000 foot pounds, making a total of 10,000 foot pounds for
four seconds, 150,000 foot pounds per minute, or about 45 horse power.
[Illustration: _Fig. 14. Indicating pressure Line._]
This movement of the piston to the right, represented only a half
revolution of the crank, and the same thing occurs when the piston moves
back, to complete the entire revolution.
Indicating the Engine.--We now come to the important part of engine
testing, namely, to ascertain how much power we have obtained from the
engine. To do this an indicator card must be furnished. A card to
indicate the pressure, as we have shown it in the foregoing diagram
would look like Fig. 14.
The essential thing, however, is to learn how to take a card from a
steam engine cylinder, and we shall attempt to make this plain, by a
diagram of the mechanism so simplified as to be readily understood.
[Illustration: _Fig. 15. Indicating the Engine._]
In Fig. 15 we have shown a cylinder A, having within a piston B, and a
steam inlet pipe C. Above the cylinder is a drum D, mounted on a
vertical axis, and so geared up with the engine shaft that it makes one
complete turn with each shaft revolution. A sheet of paper E, ruled with
cross lines, is fixed around the drum.
The cylinder A has a small vertical cylinder F connected therewith by a
pipe A, and in this cylinder is a piston H, the stem I of which extends
up alongside of the drum, and has a pointed or pencil J which presses
against the paper E.
Now, when the engine is set in motion the drum turns in unison with the
engine shaft, and the pressure of the steam in the cylinder A, as it
pushes piston B along, also pushes the piston H upwardly, so that the
pencil point J traces a line on the ruled paper.
It will be understood that a spring is arranged on the stem I in such a
manner that it will always force the piston H downwardly against the
pressure of the steam.
Mean Efficiency.--We must now use a term which expresses the thing that
is at the bottom of all calculations in determining how much power is
developed. You will note that the pressure on the piston during the
first foot of its movement was 10,000 pounds, but that from the point 1,
Fig. 13, to the end of the cylinder, the pressure constantly decreased,
so that the pressure was not a uniform one, but varied.
Suppose we divide the cylinder into six inch spaces, as shown in Fig.
13, then the pressure of the steam at the end of each six inches will be
the figures given at bottom of diagram, the sum total of which is
30,000, and the figures at the lower side show that there are eight
factors.
The figure 10,000 represents, of course, two six inch spaces in the
first foot of travel.
The result is, that, if we divide the sum total of the pressures at the
eight points by 8, we will get 3750, as the mean pressure of the steam
on the piston during the full stroke of the piston.
In referring to the foot pounds in a previous paragraph, it was assumed
that the piston moved along each foot in one second of time. That was
done to simplify the statement concerning the use of foot pounds, and
not to indicate the time that the piston actually travels.
Calculating Horse Power.--We now have the first and most important
factor in the problem,--that is, how much pressure is exerted against
the piston at every half revolution of the crank shaft. The next factor
to be determined is the distance that the piston travels in one minute
of time.
This must be calculated in feet. Let us assume that the engine turns the
crank shaft at a speed of 50 revolutions a minute. As the piston travels
8 feet at each revolution, the total distance traveled is 400 feet.
If, now, we have a constant pressure of 3750 pounds on the piston, and
it moves along at the rate of 400 feet per minute, it is obvious that
by multiplying these two together, we will get the figure which will
indicate how many pounds the steam has lifted in that time.
This figure is found to be 1,500,000, which means foot pounds, as we
have by this means measured pressure by feet, or pounds lifted at each
foot of the movement of the piston.
As heretofore stated, we must now use the value of a horse power, so
that we may measure the foot pounds by it. If we had a lot of wheat in
bulk, and we wanted to determine how much we had, a bushel measure would
be used. So with power. The measure, as we have explained, is 33,000,
and 1,500,000 foot pounds should give as a result a little over 45 horse
power.
Condensation.--We now come to the refinements in engine
construction,--that which adds so greatly to the economy of operation.
The first of these is condensation. The first reciprocating engine
depended on this to do the actual work. In this age it is depended upon
simply as an aid.
The first thing however that the engineer tries to do is to prevent
condensation. This is done by jacketing the outside of the cylinder with
some material which will prevent radiation of heat, or protect the steam
within from being turned back into water by the cool air striking the
outside of the cylinder.
Atmospheric Pressure.--On the other hand, there is a time when
condensation can be made available. The pressure of air on every square
inch of surface is 14-3/4 pounds. When a piston moves along and steam is
being exhausted from the cylinder, it must act against a pressure of
14-3/4 pounds on every square inch of its surface.
The problem now is to get rid of that back pressure, and the old type
engines give a hint how it may be done. Why not condense the steam
discharged from the engine cylinder? In doing so a vacuum is produced on
the exhaust side of the piston, at the same time a pressure is exerted
on its other side.
The Condenser.--Thus the condenser is brought into existence, as an aid.
By jacketing condensation is prevented; it is fought as an enemy. It is
also utilized as a friend. It is so with many of the forces of nature,
where man for years vainly fought some principle, only to find, later
on, that a friend is more valuable than a foe, and to utilize a material
agency in nature is more economical than to fight it.
Pre-heating.--The condenser does two things, both of which are of great
value to the economical operation of the engine. For the purpose of
rapidly converting the steam back into water as it issues from the
engine cylinder, water is used. The steam from the cylinder has a
temperature of 212 degrees and upwards, dependent on its pressure.
Water, ordinarily, has a temperature of 70 degrees, or less, so that
when the steam strikes a surface which is cooled down by the water, it
is converted back into liquid form, but at a temperature less than
boiling water. The water thus converted back from the steam gives up
part of its heat to the water which cools the condenser, and the water
from the condenser, as well as the water used to cool the condenser, are
thus made available to be fed into the boiler, and thus assist in again
converting it into a steam.
The economy thus lies in helping the coal, or other fuel, do its work,
or, to put it more specifically, it conserves the heat previously put
out by the coal, and thus saves by using part of the heat over again.
Superheaters.--Another refinement, and one which goes to the very
essence of a heat motor, is the method of superheating the steam. This
is a device located between the boiler and the engine, so that the
steam, in its transit from the boiler to the engine, will be heated up
to a high degree, and in the doing of which the pressure may be
doubled, or wonderfully increased.
This may be done in an economical manner in various ways, but the usual
practice is to take advantage of the exhaust gases of the boiler, in the
doing of which none of the heat is taken from the water in the boiler.
The products of combustion escaping from the stacks of boilers vary.
Sometimes the temperature will be 800 degrees and over, so that if pipes
are placed within the path of the heated gases, and the supply steam
from the boiler permitted to pass through them a large amount of heat is
imparted to the steam from a source which is of no further use to the
water being generated in the boiler.
Compounding.--When reference was made to the condensation of steam as it
issued from the boiler, no allusion was made to the pressure at which it
emerged. If the cylinder was well jacketed, so that the amount of
condensation in the cylinder was small, then the pressure would still be
considerable at the exhaust. Or, the steam might be cut off before the
piston had traveled very far at each stroke, in which case the exhaust
would be very weak.
In practice it has been found to be most economical to provide a high
boiler pressure, and also to superheat the steam, but where it is not
superheated, and a comparatively high boiler pressure is provided,
compounding is resorted to.
To compound steam means to use the exhaust to drive a piston. In such a
case two cylinders are placed side by side, one, called the high
pressure cylinder, being smaller than the low pressure cylinder, which
takes the exhaust from the high pressure.
The exhaust from the second, or low pressure cylinder may then be
supplied to a condenser, and in that case the mechanism would be termed
a compound condensing engine. If a condenser is not used, then it is
simply a compound engine.
Triple and Quadruple Expansion Engines.--Instead of using two cylinders,
three, or four, are employed, each succeeding cylinder being larger than
the last. As steam expands it loses its pressure, or, stated in another
way, whenever it loses pressure it increases in volume. For that reason
when steam enters the first cylinder at a pressure of say 250 pounds, it
may exhaust therefrom into the next cylinder at a pressure of 175
pounds, with a corresponding increase in volume.
To receive this increased volume, without causing a sensible back
pressure on the first cylinder, the second cylinder must be larger in
area than the first; in like manner when it issues from the exhaust of
the second cylinder at 125 pounds pressure, there is again an increase
in volume, and so on.
[Illustration: _Fig. 16. Compound Engine._]
Examine Fig. 16, which shows a pair of cylinders, A being the high, and
B the low pressure cylinders, the exhausts of the high pressure being
connected up with the inlets of the low pressure, as indicated by the
pipes, C D.
The diagram does not show the valve operations in detail, it being
sufficient to explain that when the valve E in the pipe C is closed, the
valve F, at the other end of the cylinders, in the pipe D, is closed.
The same principle is employed in the triple and quadruple expansion
engines, whereby the force of the steam at each exhaust is put to work
immediately in the next cylinder, until it reaches such a low pressure
that condensation is more effective than its pressure.
The diagram, as given, is merely theoretical, and it shows the following
factors:
First: The diameter of each piston.
Second: The area of each piston in square inches.
[Illustration: Fig. 16a. Relative Piston Pressures.]
Third: The steam pressure in each cylinder.
Fourth: The piston pressure of each cylinder.
It will be seen that an engine so arranged is able to get substantially
the same pressure in each of the second, third and fourth cylinders, as
in the first (see Fig. 16a), and by condensing the discharge from the
fourth cylinder a most economical use of steam is provided for. The
Steam Turbine.--We must now consider an entirely new use of steam as a
motive power. Heretofore we have been considering steam as a matter of
pressure only, in the development of power. It has been observed that
when the pressure of steam decreases at the same temperature it is
because it has a greater volume, or a greater volume results.
[Illustration: Fig. 17. Changing Pressure into Velocity]
When steam issues from the end of a pipe its velocity depends on its
pressure. The higher the pressure the greater its velocity. The elastic
character of steam is shown by its action when ejected from the end of a
pipe, by the gradually enlarging area of the discharging column.
In a reciprocating engine the power is derived from the pressure of the
steam; in a turbine the power results from the impact force of the steam
jet. Such being the case velocity in the movement of the steam is of
first importance.
Pressure and Velocity.--To show the effectiveness of velocity, as
compared with pressure, examine Fig. 17. A is a pipe discharging steam
at a pressure of 100 pounds. To hold the steam in the pipe would
require a pressure of 100 pounds against the disk B, when held at 1, the
first position.
Suppose, now, the disk is moved away from the end of the pipe to
position 2. The steam, in issuing forth, strikes the disk over a larger
area, and in escaping it expands, with the result that its velocity from
1 to 2 is greater than the movement of the steam within the pipe that
same distance.
[Illustration: _Fig. 18. Reaction against Air._]
[Illustration: _Fig. 19. Reaction against Surface._]
The disk is now moved successively to positions 3, 4, 5, and so on. If
we had a measuring device to determine the push against the disk at the
various positions, it would be found that there is a point at some
distance from the end of the pipe, at which the steam has the greatest
striking force, which might be called the focal point.
A blow pipe exhibits this same phase; the hottest point is not at the
end of the pipe, but at an area some distance away, called the focal
point of heat.
The first feature of value, therefore, is to understand that pressure
can be converted into velocity, and that to get a great impact force,
the steam must be made to strike the hardest and most effective blow.
When a jet of steam strikes a surface it is diverted or it glances in a
direction opposite the angle at which it strikes the object. In
directing a jet against the blades of a turbine it is impossible to make
it strike squarely against the surface.
[Illustration: Fig. 20. Turbine Straight Blades.]
Let us assume that a wheel A, Fig. 20, has a set of blades B, and a
steam jet is directed against it by the pipe C. It will be seen that
after the first impact the steam is forced across the blades, and no
further force is transferred to them.
Form of Blades.--The blades are therefore so curved, that the steam
after the first impact cannot freely pass along the blade, as it does on
a straight blade, but imparts on every element of the curved-back blade,
thereby giving up continually part of its speed to the blade.
This is clearly shown in Fig. 21, where the pipe D ejects the stream of
steam against the concaved blades E. Many modifications have been made
in the shapes of these blades, all designed to take advantage of this
action.
[Illustration: _Fig. 21. Curved Blades._]
[Illustration: _Fig. 22. Compound Turbine._]
Compounding the Jet.--We may extend the advantages gained by this form
of blades, and diverting the course of the jet, so that it will be
directed through a series of wheels, each of which will get the benefit
of the moving mass from the pipes.
Such a structure is shown in Fig. 22, in which three bladed wheels A, B,
C, are caused to rotate, a set of stationary blades D, E, being placed
between the three moving wheels, but the stationary blades are disposed
in reverse directions. When the steam from pipes F, F, impinges against
the blades of the first wheel A, it is directed by the stationary blade
D to the next wheel B, and from the stationary blade E to the blades of
the next wheel C, thus, in a manner somewhat similar to the compounding
effect of the steam engine, utilizes the pressure which is not used at
the first impulse.