CHAPTER XXV.
STEAM AND MOTIVE POWER.
IN the previous chapter we have mentioned that the heat
value of coal is measured by the number of heat-units it contains,
and that each heat-unit represents 772 foot-pounds of work, or
the energy required to raise 772 pounds one foot. According to
the figures given, each pound of coal contains an enormous amount
of possible work energy. The operating of the locomotive, and
of all other steam-engines, is a process of transforming the heat
energy of coal into mechanical work. In some kinds of engines
driven by hot air or gas, the operation of converting heat into
work is done without the use of steam. A greater proportion of
the heat energy can be utilized in that way; but there are mechanical
obstacles which prevent such systems from being used where much
power is required.
CONVENIENCE OF STEAM FOR CONVERTING HEAT
INTO WORK.
Steam, the vapor of water, has been found the most
convenient medium for transforming the energy of coal into the
useful work of pulling railroad trains, and of driving other kinds
of machinery. Water has the greatest heat-absorbing capacity of
any known substance, which makes it an excellent means of converting
heat into work; but it has some peculiarities which readily lead
to great loss of energy if not carefully controlled. If we follow
the circle of operations which the burning of coal for steam-making
purposes sets going, we shall meet at every move heat-losses which
show us why so small a portion of the entire heat energy of coal
reaches the crank-pins that turn the wheels of the engine. But
an intelligent study of the losses will also help an engineer
to restrain them to the lowest possible limit.
HEAT USED IN EVAPORATING WATER.
Suppose we take one pound of water at a temperature
of 40 degrees Fah., and apply heat to it in an open vessel. If
we put a thermometer in the water, we shall find that the temperature
will rise rapidly till it reaches 212 degrees, the boiling-point
at the pressure of the atmosphere (at sea level). Then the mercury
stops rising, but the water keeps absorbing the heat and turning
into steam. It takes rather more than 5½ times the quantity
of heat to evaporate the whole of the pound of water into steam
that it took to raise the temperature from the tank temperature
to the boiling-point; for, although it is not shown by the thermometer,
the converting of the pound of water from the boiling-point into
steam uses up 965.7 heat-units, that being called the latent heat
of steam at atmospheric pressure. In raising the water to the
boiling-pointfrom 40 degrees to 212 degrees172 heat-units
were used, and in vaporizing the water 965.7 units, making in
all 1137.7 heat-units, which are expended in evaporating one pound
of water under the pressure of the atmosphere alone, which is
14.7 pounds to the square inch. Steam formed under this light
pressure fills 1644 times the space occupied by the water it was
made from. The volume of steam varies nearly inversely as the
pressure, so that when the steam is generated under the pressure
of two atmospheres, it fills only 822 times the space that the
water did. Every step in the increase of pressure reduces the
volume of the steam in like proportion. Steam at 150 pounds per
square inch gauge pressure is only 173 times the volume of the
water. Steam gauge pressure is the pressure above the atmosphere;
absolute pressure is reckoned from the vacuum line.
LITTLE EXTRA HEAT NEEDED FOR MAKING HIGH-PRESSURE
STEAM.
If the pound of water, instead of being left to boil
in an open vessel, had been put into a boiler where a pressure
of 165 pounds absolute was put upon it, that being equal to a
gauge pressure of 150 pounds, the result would have been different.
When heat was now applied, the mercury would keep rising till
the temperature of 365.7 degrees was reached before the water
would begin to boil. To raise it to the boiling-point under this
pressure, 330.4 heat-units would be put in the water, and then
the addition of 855.1 more heat-units would convert the whole
pound of water into steam, the total expenditure of heat being
1185.5 heat-units. From this it will be seen that while the generating
of steam at atmospheric pressure, which gives no capacity to speak
of for doing work, calls for an expenditure of 1137.7 heat-units,
raising the steam to the high gauge pressure of 150 pounds takes
only 1185.5 heat-units. Steam of 100 pounds gauge pressure uses
up 1177 heat-units, so that it takes very little more heat to
raise the steam to the higher pressure where it has the power
of doing much more work than to the lower pressures. A study of
these facts will show why it is most economical to use steam of
high pressure.
CONDITIONS OF STEAM.
Steam formed in ordinary boilers, where only sufficient
heat is applied to evaporate the water, is called saturated steam.
It is also sometimes spoken of as dry steam, or anhydrous steam.
Saturated steam contains only just sufficient heat to maintain
it in a gaseous condition, and the least abstraction of heat causes
a portion of the steam to fall back into water when it loses its
power of doing work. This is why it is important that steam cylinders
and passages should be well protected from cold. The condensation
of steam that goes on in badly lagged cylinders wastes a great
deal of fuel.
When heat is applied to steam that is not in contact with water,
the steam absorbs more heat and is said to be superheated. Superheated
steam has a greater energy than saturated steam in proportion
to the amount of heat added. The practical advantage of superheated
steam is, that it does not turn into water in the cylinder so
readily as saturated steam.
METHODS OF USING STEAM.
Having got steam raised to 150 pounds gauge pressure,
which is almost 165 pounds absolute, the next move is to use it
to the best advantage, so that the greatest possible amount of
work will be got out of every pound of steam generated. In ordinary
circumstances, the higher the temperature of steam admitted into
the cylinders of a steam-engine, and the lower the temperature
at which it is passed out by the exhaust, the greater will be
the economy, if the reduction of temperature has been due to the
conversion of heat into mechanical work.
That the steam passed into the cylinders may be used to the
best possible advantage, the ordinary practice is to cause the
expansive force of the steam to do all the work practicable. As
has been already mentioned in a former chapter, high-pressure
steam is like a powerful spring put under compression, and is
ever ready to stretch out when its force is directed against anything
movable. In that way it pushes the piston when the valve is cutting
off admission of steam before the end of the stroke is reached.
We shall try to show how such practice is economical.
THE STEAM-ENGINE INDICATOR.
To find
out what is going on in the inside of the cylinders of an engine,
to show accurately how the steam is distributed, the use of the
steam-engine indicator is necessary. The indicator consists essentially
of a small steam-cylinder, whose under side is connected by pipes
to the main cylinder of the engine under inspection. Inside the
indicator cylinder is a nicely fitting piston, whose upper movement
is resisted by a spring of known strength. The piston-rod passes
up through the top of the indicator cylinder; and its extremity
is connected with mechanism for operating a pencil, and marking
on a card a diagram whose lines coincide with the movement of
the indicatur piston.
Fig. 39 gives perspective and sectional views of the Tabor
indicator, an instrument well adapted for application to locomotives.
The card to be marked is fastened in the paper drum attached to
the indicator.This drum receives a circular motion from a cord
which is operated by the cross-head of the locomotive, and the
connection is so arranged that the drum will begin to move round
just as the main piston begins its stroke. The circular motion
of the drum is continued till the piston reaches the end of its
stroke, when the drum reverses its movement, and returns to the
exact point from which it started. Now the indicator cylinder
being in communication with the main cylinder, when the latter
begins to take steam, the pressure will be applied to the indicator
piston, which was pushed upward, at the same time transmitting
its movement to the pencil. The indicator piston will rise and
fall in accordance with the steam pressure in the cylinder; and
the circular movement of the drum coinciding with the cross-head
movement, the pencil will describe a diagram which represents
the pressure inside the main cylinder at the various points of
the stroke.
THE INDICATOR DIAGRAM.
Fig. 40 is a very good diagram taken from a locomotive
cutting off at about 37 per cent of the stroke and running at
150 revolutions per minute. A is the atmospheric line traced
before steam is admitted to the indicator. V is the vacuum
line traced according to measurement, 14.7 pounds below the atmospheric
line. D E is the admission-line, D being the point
where the valve opens to admit steam. E F is the steam-line,
beginning at the point of change in direction of the admission-line.
The steam-line in this diagram drops down before the point of
cut-off is reached, through the steam admission not being rapid
enough to keep it up. F G is the expansion-line traced
after the steam is cut off. At the point G the exhaust
takes place, and the exhaust-line is from G to the end
of the stroke. H I is the line of counter-pressure, and
is high or low according to the quantity of steam left in the
cylinder by the exhaust. The use of small nozzles always causes
a high counter-pressure line. The compression line begins at I,
the point where the value closes, and runs up to D, the
pressure rising as the steam left in the cylinder, after the valve
closes, gets pressed by the piston into small space.
For an exhaustive and easily understood treatise on the indicator,
our readers are referred to Hemenway's "Indicator Practice
and Steam-engine Economy," published by John Wiley and Sons,
New York.
PRACTICAL ILLUSTRATION OF STEAM-USING.
Suppose
the steam in our boiler is raised to 165 pounds absolute pressure,
and we apply it under different conditions to do work in the cylinder
Z Z shown in Fig. 41, which is 16 inches diameter and has
a stroke of 24 inches. The diagram above the cylinder represents
the action of steam in the cylinder. The vertical lines represent
the steam at different points of the. piston's stroke. If the
cylinder were filled with steam at boiler pressure during the
entire stroke of the piston, the diagram of work would resemble
the rectangle A C E B Using the steam in this way is impracticable,
but an approximation to it is possible, and it will serve to illustrate
the subject. Ignoring the quantity needed to fill the clearance
spaces, the steam from one pound of water, which is called a pound
of steam, would just be sufficient to fill the cylinder once.
CURVE OF EXPANDING STEAM.
Instead of permitting the steam to follow the piston
unimpeded during the whole stroke, we will cut it off at 6 inches
or one-quarter stroke, as shown in the illustration Fig. 41, where
the valve Y is closing the port y, just as the piston
X has moved one quarter the stroke. The piston will now
be pushed the remainder of the stroke by the expansive force of
the steam, the latter falling in pressure as the space to be filled
increases, and obeying what is called Mariotte's law, the pressure
varying inversely as the volume. By the time the piston has moved
to half-stroke, the steam is filling twice the space it was in
when cut-off took place, and accordingly its pressure has fallen
to the point b which represents 82.5 pounds to the square
inch. At the end of the stroke when release takes place, the pressure
has fallen to 41.25 pounds. We find by calculation that the average
pressure on the piston when the steam was cut off at quarter-stroke
was 98.42 pounds to the square inch. In this case just one quarter
the quantity of steam was drawn from the boiler that was taken
when steam followed full stroke, yet with the small quantity of
steam, the average pressure on the piston was considerably more
than half of what it was when four times the volume of steam was
used.
The description of the action of the steam does not represent
with any degree of accuracy what actually takes place; but it
gives the facts closely enough to indicate how steam can be saved
or wasted.
EFFECTS OF HIGH INITIAL AND LOW TERMINAL
PRESSURE.
All engineers who have given the economical use of
steam intelligent study agree that the proper way to use steam
in a cylinder is to get it in as near boiler pressure as possible,
so that the greatest possible ratio of expansion may be obtained
while doing the necessary work. Where this practice is not followed,
the steam is used wastefully. Locomotives that are run with the
throttle partly closed, when, by notching the links back it could
be used full open, are throwing away part of the fuel-saving advantages
that high pressure offers.
For this practice the engineers are not in every case to blame,
for many locomotives are constructed with valve motion so imperfectly
designed that the engines will not run freely when they are linked
close up. With the small nozzles made necessary to force the steam-making
in small boilers, the back cylinder pressure is so great that
the high compression, resulting from an early valve-closure, prevents
the engine from running at the speed required.
From whatever cause it originates, the practice of running
with the throttle partly closed causes much waste of fuel. A few
examples will be given:
The diagram shown
in Fig. 42 was taken from a locomotive running at 192 revolutions
per minute. The boiler pressure was 145 pounds, and the initial
pressure on this card is 136 pounds. This high cylinder pressure
was obtained by keeping the throttle-valve full open. The driving-wheels
were 68 inches diameter, and the engine was running close on forty
miles an hour and was developing, with 18 x 24-inch cylinders,
sufficient power to haul a train weighing 300 tons at the rate
of fifty miles an hour. Steam was cut off at about seven inches
of the stroke, expanded down to 25 pounds above the atmospheric
line, and showed an average back pressure of 4 pounds. The work
was done using at the rate of 21.5 pounds per horse-power per
hourvery economical work.
Diagram Fig. 43 shows about the same power as the other one;
but it was taken with the steam partly throttled, and cutting
off at 10½ inches. In this case it will be noted that the
initial pressure is only 102 pounds, that the terminal pressure
is 31 pounds above the atmosphere, and that the counter-pressure
is 7 pounds. In this case the work is done by using steam at the
rate of 25.8 pounds per horse-power per hour, which is 16.6 per
cent more steam than was used with the other way of working. There
was no reason whatever for working the engine in this manner,
except the careless practice that some runners get into.
A still worse
case is shown by the diagram Fig. 44. Here the engine, which was
running at 176 revolutions per minute, was worked cutting off
at half stroke, and the average steam pressure kept down by throttling.
Consequently the initial pressure is low, the terminal pressure
and the back pressure high. This condition of working calls for
the use of a large volume of steam to perform the work. The initial
pressure is 109 pounds, the terminal pressure 45 pounds, and the
back pressure 11 pounds. The engine while working this way used
steam at the rate of 32 pounds per horse-power per hour, or 33
per cent more than was used in the first case. These are examples
taken from the ordinary working of locomotives. They are no mere
theories. They are the record of accurate measurements, and are
as trustworthy as the indications of the steam-gauge. Using 33
per cent more steam than what is absolutely necessary is just
throwing away one-third of the coal put into the fire-box.
To put the
matter in a more concrete form: If the engine from which diagram
Fig. 42 was taken was running 33.3 miles to the ton of coal, only
27.7 miles to the ton would be made when using the steam shown
in diagram Fig. 43 and only 22.3 miles when diagram Fig. 44 was
the record of the steam consumed.
COMPOUND LOCOMOTIVES.
There are some disadvantages to working with wide extremes
of pressure in a cylinder. The temperature tends to change with
changes of pressure, and this leads to loss through condensation
of the steam in the cylinder. In the working of the simple engine
we have been dealing with, where steam of 165 pounds absolute
pressure was used, the steam enters the cylinder at about 365
degrees Fah., and escapes close to atmospheric pressure at a temperature
of about 220 degrees. The metal of the cylinder inclines to maintain
an even temperature at some average point between the high admission
and the low exhaust temperatures. When the steam enters the cylinder
it goes into a comparatively cool chamber, and the metal of the
cylinder walls and heads draws some heat from the incoming steam.
The portion of the steam robbed of its heat becomes spray, and
helps to dampen the steam that continues to pass into the cylinder.
As the events of the stroke go on, and release of pressure takes
place after the opening of the exhaust port, the steam which became
condensed in the beginning of the stroke is ready to flash back
into steam under the release of pressure. If this happens as the
steam is passing into the exhaust port, it draws heat from the
cylinder metal to aid in the act of vaporization, the whole of
this heat being carried up the chimney. The heat thus carried
away from the cylinder metal has to be returned by the incoming
steam of next stroke, and causes the initial condensation spoken
of. Compression helps to prevent condensation by heating the cylinder
at the end where steam is about to enter.
Another disadvantage of the locomotive cylinder is that the
opportunities for using the steam expansively are very limited.
To provide a remedy for the losses due to cylinder condensation,
and to provide better means of using the steam expansively, compound
locomotives have been brought into use. A compound locomotive,
while expanding the steam more than can be done with a simple
engine, has a much more even temperature throughout the two strokes
in which the steam is used. If there is condensation and revaporization
of steam in the high-pressure cylinder, it passes into the low-pressure
cylinder and is there used to do useful work. In a compound engine
the work is more evenly distributed throughout the stroke than
in a simple engine, consequently the strains and shocks given
to the machinery are less. This ought to make the compound a durable
machine.
The most common compound locomotive has two cylinders, high
pressure on one side and low pressure on the other. The capacity
of the low-pressure cylinder is about twice that of the other.
Steam passes from the boiler into the high-pressure steam-chest,
and is, by the slide-valve, admitted to the cylinder in the ordinary
way. When the exhaust port of this cylinder opens, the steam passes
into a receiver which carries it to the low-pressure cylinder
steam-chest. From thence it is admitted into the cylinder by the
slide-valve at the proper time. The initial pressure of the low-pressure
cylinder is always less than the terminal pressure of the high-pressure
cylinder. In other words, the high-pressure cylinder has at least
as much back pressure as the low-pressure cylinder has positive
pressure.
An intercepting valve or other mechanism is generally provided
with compound locomotives to admit steam of low pressure direct
from the boiler to the low-pressure cylinder to help in starting.
Table of Contents
| Contents Page
|