A Catechism of the Steam Engine
by
John Bourne
Part 2 out of 8
equal to the versed sine of the arc described at _g_, or, in other words,
the line described by the point _g_ becomes a straight line instead of a
curve.
[Illustration: Fig. 21.]
107. _Q._--Does the air pump rod move vertically as well as the piston rod?
_A._--It does. The air pump rod is suspended from a cross head, passing
from the centre of one of the links _b d_ to the centre of the other link,
on the opposite side of the beam. Now, as the distance from the central
axis of the great beam to the point _b_ is equal to the length of the rod
_c d_, it will follow that the upper end of the link will follow one arc,
and the lower end an equal and opposite arc. A point in the centre of the
link, therefore, where these opposite motions meet, will follow no arc at
all, but will move up and down vertically in a straight line.
108. _Q._--The use of the crank is to obtain a circular motion from a
reciprocating motion?
_A._--That is the object of it, and it accomplishes its object in a very
perfect manner, as it gradually arrests the velocity of the piston towards
the end of the stroke, and thus obviates what would otherwise be an
injurious shock upon the machine. When the crank approaches the lowest part
of its throw, and at the same time the piston is approaching the top of the
cylinder, the motion of the crank becomes nearly horizontal, or, in other
words, the piston is only advanced through a very short distance, for any
given distance measured on the circle described by the crank pin. Since,
then, the velocity of rotation of the crank is nearly uniform, it will
follow that the piston will move very slowly as it approaches the end of
the stroke; and the piston is brought to a state of rest by this gradually
retarded motion, both at the top and the bottom of the stroke.
109. _Q._--What causes the crank to revolve at a uniform velocity?
_A._--The momentum of the machinery moved by the piston, but more
especially of the fly wheel, which by its operation redresses the unequal
pressures communicated by the crank, and compels the crank shaft to revolve
at a nearly uniform velocity. Everyone knows that a heavy wheel if put into
rapid rotation cannot be immediately stopped. At the beginning and end of
the stroke when the crank is vertical, no force of torsion can be exerted
on the crank shaft by the crank, but this force is at its maximum when the
crank is horizontal. From the vertical point, where this force is nothing,
to the horizontal point, where it is at its maximum, the force of torsion
exerted on the crank shaft is constantly varying; and the fly wheel by its
momentum redresses these irregularities, and carries the crank through that
"dead point," as it is termed, where the piston cannot impart any rotative
force.
110. _Q._--Are the configuration and structure of the steam engine, as it
left the hand of Watt, materially different from those of modern engines?
_A._--There is not much difference. In modern rotative land engines, the
valves for admitting the steam to the cylinder or condenser, instead of
being clack or pot-lid valves moved by tappets on the air pump rod, are
usually sluice or sliding valves, moved by an eccentric wheel on the crank
shaft. Sometimes the beam is discarded altogether, and malleable iron is
more largely used in the construction of engines instead of the cast iron,
which formerly so largely prevailed. But upon the whole the steam engine of
the present day is substantially the engine of Watt; and he who perfectly
understands the operation of Watt's engine, will have no difficulty in
understanding the operation of any of the numerous varieties of engines
since introduced.
THE MARINE ENGINE.
111. _Q._--Will you describe the principal features of the kind of steam
engine employed for the propulsion of vessels?
_A._--Marine engines are of two kinds,--paddle engines and screw engines.
In the one case the propelling instrument is paddle wheels kept in rotation
at each side of the ship: in the other case, the propelling instrument is a
screw, consisting of two or more twisted vanes, revolving beneath the water
at the stern. Of each class of engines there are many distinct varieties.
112. _Q._--What are the principal varieties of the paddle engine?
[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
_A._--There is the side lever engine (fig. 26), and the oscillating engine
(fig. 27), besides numerous other forms of engine which are less known or
employed, such as the trunk (fig. 22), double cylinder (fig. 23), annular,
Gorgon (fig. 24), steeple (fig. 25), and many others. The side lever
engine, however, and the oscillating engine, are the only kinds of paddle
engines which have been received with wide or general favor.
[Illustration: Fig. 24.]
113. _Q._--Will you explain the main distinctive features of the side lever
engine?
_A._--In all paddle vessels, whatever be their subordinate characteristics,
a great shaft of wrought iron, s, turned round by the engine, has to be
carried from side to side of the vessel, on which shaft are fixed the
paddle wheels. The paddle wheels may either be formed with fixed float
boards for engaging the water, like the boards of a common undershot water
wheel, or they may be formed with _feathering_ float boards as they are
termed, which is float boards movable on a centre, and so governed by
appropriate mechanism that they enter and leave the water in a nearly
vertical position. The common fixed or radial floats, however, are the kind
most widely employed, and they are attached to the arms of two or more
rings of malleable iron which are fixed by appropriate centres on the
paddle shaft. It is usual in steam vessels to employ two engines, the
cranks of which are set at right angles with one another. When the paddle
wheels are turned by the engines, the float boards engaging the water cause
a forward thrust to be imparted to the shaft, which propels forward the
vessel on the same principle that a boat is propelled by the action of
oars.
[Illustration: Fig. 25.]
114. _Q._--These remarks apply to all paddle vessels?
_A._--They do. With respect to the side lever engine, it may be described
to be such a modification of the land beam engine already described, as
will enable it to be got below the deck of a vessel. With this view,
instead of a single beam being placed overhead, two beams are used, one of
which is set on each side of the engine as low down as possible. The cross
head which engages the piston rod is made somewhat longer than the diameter
of the cylinder, and two great links or rods proceed one from each end of
the cross head to one of the side levers or beams. A similar cross bar at
the other end of the beams serves to connect them together and to the
connecting rod which, proceeding from thence upwards, engages the crank,
and thereby turns round the paddle wheels.
115. _Q._--Will you further illustrate this general description by an
example?
[Illustration: Fig. 26.]
_Q._--Fig. 26 is a side elevation of a side lever engine; x x represent the
beams or keelsons to which the engines are attached, and on which the
boilers rest. The engines are tied down by strong bolts passing through the
bottom of the vessel, but the boiler keeps its position by its weight
alone. The condenser and air pump are worked off the side levers by means
of side rods and a cross head. A strong gudgeon, called the _main centre_,
passes through the condenser at K, the projecting ends of which serve to
support the side levers or beams. L is the piston rod, which, by means of
the cross head and side rods, is connected to the side levers or beams, one
of which is shown at H H. The line M represents the connecting rod, to
which motion is imparted by the beams, through the medium of the cross tail
extending between the beams, and which by means of the crank turns the
paddle shaft S. The eccentric which works the slide valve is placed upon
the paddle shaft. It consists of a disc of metal encircled by a hoop, to
which a rod is attached, and the disc is perforated with a hole for the
shaft, not in the centre, but near one edge. When, therefore, the shaft
revolves, carrying the eccentric with it, the rod attached to the
encircling hoop receives a reciprocating motion, just as it would do if
attached to a crank in the shaft.
116. _Q._--Will you describe the mode of starting the engine?
_A._--I may first mention that when the engine is at rest, the connection
between the eccentric and the slide valve is broken, by lifting the end of
the eccentric rod out of a notch which engages a pin on the valve shaft,
and the valve is at such times free to be moved by hand by a bar of iron,
applied to a proper part of the valve gear for that purpose. This being so,
the engineer, when he wishes to start the engine, first opens a small valve
called the _blow through valve_, which permits steam from the boiler to
enter the engine both above and below the piston, and also to fill the
condenser and air pump. This steam expels the air from the interior of the
engine, and also any water which may have accumulated there; and when this
has been done, the blow through valve is shut, and a vacuum very soon forms
within the engine, by the condensation of the steam. If now the slide valve
be moved by hand, the steam from the boiler will be admitted on one side of
the piston, while there is a vacuum on the other side, and the piston will,
therefore, be moved in the desired direction. When the piston reaches the
end of the stroke, the valve has to be moved in the reverse direction, when
the piston will return, and after being moved thus by hand, once or twice,
the connection of the valve with the eccentric is to be restored by
allowing the notch on the end of the eccentric rod to engage the pin on the
valve lever, when the valve will be thereafter moved by the engine in the
proper manner. It will, of course, be necessary, when the engine begins to
move, to open the injection cock a little, to enable water to enter for the
condensation of the steam. In the most recent marine engines, a somewhat
different mechanism from this is used for giving motion to the valves, but
that mechanism will be afterwards described.
117. _Q._--Are all marine engines condensing engines?
_A._--Nearly all of them are so; but recently a number of gunboats have
been constructed, with high pressure engines. In general, however, marine
engines are low pressure or condensing engines.
118. _Q._--Will you now describe the chief features of the oscillating
paddle marine engine?
_A._--In the oscillating paddle marine engine, the arrangement of the
paddle shaft and paddle wheels is the same as in the case already
described, but the whole of the side levers, side rods, cross head, cross
tail, and connecting rod are discarded. The cylinder is set immediately
under the crank; the top of the piston rod is connected immediately to the
crank pin; and, to enable the piston rod to accommodate itself to the
movement of the crank, the cylinder is so constructed as to be susceptible
of vibrating or oscillating upon two external axes or trunnions. These
trunnions are generally placed about half way up on the sides of the
cylinder; and through one of them steam is received from the boiler, while
through the other the steam escapes to the condenser. The air pump is
usually worked by means of a crank in the shaft, which crank moves the air
pump bucket up and down as the shaft revolves.
119. _Q._--Will you give an example of a paddle oscillating engine?
_A._--I will take as an example the oscillating engines constructed by
Messrs. Ravenhill & Salked, for the Holyhead Packets. Fig. 27 is a
longitudinal section of this vessel, showing an engine and boiler; and fig.
28 is a transverse section of one of the engines, showing also one of the
wheels. There are two cylinders in this vessel, and one air pump, which
lies in an inclined position, and is worked by a crank in the shaft which
stretches between the cylinders, and which is called the _intermediate
shaft_. A A, is one of the cylinders, B B the piston rod, and C C the
crank. D is the crank in the intermediate shaft, which works the air pump
E. There are double eccentrics fixed on the shaft, whereby the movement of
the slide valves is regulated. The purpose of the double eccentrics is to
enable an improved arrangement of valve gear to be employed, which is
denominated the _link motion_, and which will be described hereafter. I I
are the steam pipes leading to the steam trunnions K K, on which, and on
the eduction trunnions connected with the pipe M, the cylinders oscillate.
120. _Q._--By what species of mechanism are the positions of the paddle
floats of feathering wheels governed?
_A._--The floats are supported by spurs projecting from the rim of the
wheel, and they may be moved upon the points of the spurs, to which they
are attached by pins, by means of short levers proceeding from the backs of
the floats, and connected to rods which proceed towards the centre of the
wheel. The centre, however, to which these rods proceed is not concentric
with the wheel, and the rods, therefore, are moved in and out as the wheel
revolves, and impart a corresponding motion to the floats. In some
feathering wheels the proper motion is given to the rods by means of an
eccentric on the ship's side. The action of paddle wheels, whether radial
or feathering, will be more fully described in the chapter on Steam
Navigation.
SCREW ENGINES.
121. _Q._--What are the principal varieties of screw engines?
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
_A._--The engines employed for the propulsion of screw vessels are divided
into two great classes,--geared engines and direct acting engines; and each
of these classes again has many varieties. In screw vessels, the shaft on
which the screw is set requires to revolve at a much greater velocity than
is required in the case of the paddle shaft of a paddle vessel; and in
geared engines this necessary velocity of rotation is obtained by the
intervention of toothed wheels,--the engines themselves moving with the
usual velocity of paddle engines; whereas in direct acting engines the
required velocity of rotation is obtained by accelerating the speed of the
engines, and which are connected immediately to the screw shaft.
122. _Q._--Will you describe some of the principal varieties of geared
engines?
_A._--A good many of the geared engines for screw vessels are made in the
same manner as land engines, with a beam overhead, which by means of a
connecting rod extending downwards, gives motion to the crank shaft, on
which are set the cog wheels which give motion to pinions on the screw
shaft,--the teeth of the wheels being generally of wood and the teeth of
the pinions of iron. There are usually several wheels on the crank shaft
and several pinions on the screw shaft; but the teeth of each do not run in
the same line, but are set a little in advance of one another, so as to
divide the thickness of the tooth into as many parts as there are
independent wheels or pinions. By this arrangement the wheels work more
smoothly than they would otherwise do.
123. _Q._--What other forms are there of geared screw engines?
_A._--In some cases the cylinders lie on their sides in the manner of the
cylinders of a locomotive engine. In other cases vertical trunk engines are
employed; and in other cases vertical oscillating engines.
124. _Q._--Will you give an example of a geared vertical oscillating
engine?
_A._--The engines of a geared oscillating engine are similar to the paddle
wheel engines (figs. 27 and 28), but the engines are placed lengthways of
the ship, and instead of a paddle wheel on the main shaft, there is a
geared wheel which connects with a pinion on the screw shaft. The engines
of the Great Britain are made off the same patterns as the paddle engines
constructed by Messrs. John Penn & Son, for H.M.S. Sphinx. The diameter of
each cylinder is 82-1/2 inches, the length of travel or stroke of the
piston is 6 feet, and the nominal power is 500 horses. The Great Britain is
of 3,500 tons burden, and her displacement at 16 feet draught of water is
2,970 tons. The diameter of the screw is 15-1/2 feet, length of screw in
the line of the shaft, 3 feet 2 inches, and the pitch of the screw, 19
feet.
125. _Q._--What do you mean by the pitch of the screw?
_A._--A screw propeller may be supposed to be a short piece cut off a screw
of large diameter like a spiral stair, and the pitch of a spiral stair is
the vertical height from any given step to the step immediately overhead.
126. _Q._--What is the usual number of arms?
_A._--Generally a screw has two arms, but sometimes it has three or more.
The Great Britain had three arms or twisted blades resembling the vanes of
a windmill. The multiple of the gearing in the Great Britain is 3 to 1, and
there are 17-1/2 square feet of heating surface in the boiler for each
nominal horse power. The crank shaft being put into motion by the engine,
carries round with it the great cog wheel, or aggregation of cog wheels,
affixed to its extremity; and these wheels acting on suitable pinions on
the screw shaft, cause the screw to make three revolutions for every
revolution made by the engine.
127. _Q._--What are the principal varieties of direct acting screw engines?
_A._--In some cases four engines have been employed instead of two, and the
cylinders have been laid on their sides on each side of the screw shaft.
This multiplication of engines, however, introduces needless complication,
and is now but little used. In other cases two inverted cylinders are set
above the screw shaft on appropriate framing; and connecting rods attached
to the ends of the piston rods turn round cranks in the screw shaft.
128. _Q._--What is the kind of direct acting screw engine employed by
Messrs. Penn.
_A._--It is a horizontal trunk engine. In this engine a round pipe called a
trunk penetrates the piston, to which it is fixed, being in fact cast in
one piece with it; and the trunk also penetrates the top and bottom of the
cylinder, through which it moves, and is made tight therein by means of
stuffing boxes. The connecting rod is attached at one end to a pin fixed in
the middle of the trunk, while the other end engages the crank in the usual
manner. The air pump is set within the condenser, and is wrought by a rod
which is fixed to the piston and derives its motion therefrom. The air pump
is of that species which is called double-acting. The piston or bucket is
formed without valves in it, but an inlet and outlet valve is fixed to each
end of the pump, through the one of which the water is drawn into the pump
barrel, and through the other of which it is expelled into the hot well.
THE LOCOMOTIVE ENGINE.
129. _Q._--Will you describe the more important features of the locomotive
engine?
_A._--The locomotive employed to draw carriages upon railways, consists of
a cylindrical boiler filled with brass tubes, through which the hot air
passes on its progress from the furnace to the chimney, and attached to the
boiler are two horizontal cylinders fitted with pistons, valves, connecting
rods, and other necessary apparatus to enable the power exerted by the
pistons to turn round the cranked axle to which the driving wheels are
attached. There are, therefore, two independent engines entering into the
composition of a locomotive, the cranks of which are set at right angles
with one another, so that when one crank is at its dead point, the other
crank is in a position to act with its maximum efficacy. The driving
wheels, which are fixed on the crank shaft and turn round with it, propel
the locomotive forward on the rails by the mere adhesion of friction, and
this is found sufficient not merely to move the locomotive, but to draw a
long train of carriages behind it.
130. _Q._--Are locomotive engines condensing or high pressure engines.
_A._--They are invariably high pressure engines, and it would be impossible
or at least highly inconvenient, to carry the water necessary for the
purpose of condensation. The steam, therefore, after it has urged the
piston to the end of the stroke, escapes into the atmosphere. In locomotive
engines the waste steam is always discharged into the chimney through a
vertical pipe, and by its rapid passage it greatly increases the intensity
of the draught in the chimney, whereby a smaller fire grate suffices for
the combustion of the fuel, and the evaporative power of the boiler is much
increased.
131. _Q._--Can you give an example of a good locomotive engine of the usual
form?
_A._--To do this I will take the example of one of Hawthorn's locomotive
engines with six wheels represented in fig. 29; not one of the most modern
construction now in use, nor yet one of the most antiquated. M is the
cylinder, R the connecting rod, C C the eccentrics by which the slide valve
is moved; J J is the steam pipe by which the steam is conducted from the
steam dome of the boiler to the cylinder. Near the smoke stack end of this
pipe is a valve K or regulator moved by a handle _p_ at the front of the
boiler, and of which the purpose is to regulate the admission of the steam
to the cylinder; _f_ is a safety valve kept closed by springs; N is the
eduction pipe, or, as it is commonly termed in locomotives, the _blast
pipe_, by which the steam, escaping from the cylinder after the stroke has
been performed, is projected up the chimney H. The water in the boiler of
course covers the tubes and also the top of the furnace or fire box. It
will be understood that there are two engines in each locomotive, though,
from the figure being given in section, only one engine can be shown. The
cylinders of this engine are each 14 inches diameter; the length of the
stroke of the piston is 21 inches. There are two sets of driving wheels, 5
feet diameter, with outside connections.
[Illustration: Fig. 29.]
132. _Q._--What is the tender of a locomotive?
_A._--It is a carriage attached to the locomotive, of which the purpose is
to contain coke for feeding the furnace, and water for replenishing the
boiler.
133. _Q._--Can you give examples of modern locomotives?
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
_A._--The most recent locomotives resemble in their material features the
locomotive represented in fig. 29. I can, however, give examples of some of
the most powerful engines of recent construction. Fig. 30 represents
Gooch's express engine, adapted for the wide gauge of the Great Western
Railway; and fig. 31 represents Crampton's express engine, adapted for the
ordinary or narrow gauge railways. The cylinders of Gooch's engine are each
18 inches diameter, and 24 inches stroke; the driving wheels are 8 feet in
diameter; the fire grate contains 21 square feet of area; and the heating
surface of the fire box is 153 square feet. There are in all 305 tubes in
the boiler, each of 2 inches diameter, giving a heating surface in the
tubes of 1799 square feet. The total heating surface, therefore, is 1952
square feet. Mr. Gooch states that an engine of this class will evaporate
from 300 to 360 cubic feet of water in the hour, and will convey a load of
236 tons at a speed of 40 miles an hour, or a load of 181 tons at a speed
of 60 miles an hour. The weight of this engine empty is 31 tons; of the
tender 8-1/2 tons; and the total weight of the engine when loaded is 50
tons. In one of Crampton's locomotives, the Liverpool, with one set more of
carrying wheels than the fig., the cylinders are of 24 inches diameter and
18 inches stroke; the driving wheels are 8 feet in diameter; the fire grate
contains 21-1/2 square feet of area; and the heating surface of the fire
box is 154 square feet. There are in all 300 tubes in the boiler of 2-3/16
inches external diameter, giving a surface in the tubes of 2136 square
feet, and a total heating surface of 2290 square feet. The weight of this
engine is stated to be 35 tons when ready to proceed on a journey. Both
engines were displayed at the Great Exhibition in 1851, as examples of the
most powerful locomotive engines then made. The weight of such engines is
very injurious to the railway; bending, crushing, and disturbing the rails,
and trying very severely the whole of the railway works. No doubt the
weight may be distributed upon a greater number of wheels, but if the
weight resting on the driving wheels be much reduced, they will not have
sufficient bite upon the rails to propel the train without slipping. This,
however, is only one of the evils which the demand for high rates of speed
has produced. The width of the railway, or, as it is termed, the _gauge_ of
the rails, being in most of the railways in this kingdom limited to 4 feet
8-1/2 inches, a corresponding limitation is imposed on the diameter of the
boiler; which in its turn restricts the number of the tubes which can be
employed. As, however, the attainment of a high rate of speed requires much
power, and consequently much heating surface in the boiler, and as the
number of tubes cannot be increased without reducing their diameter, it has
become necessary, in the case of powerful engines, to employ tubes of a
small diameter, and of a great length, to obtain the necessary quantity of
heating surface; and such tubes require a very strong draught in the
chimney to make them effective. With a draught of the usual intensity the
whole of the heat will be absorbed in the portion of the tube nearest the
fire box, leaving that portion nearest the smoke box nothing to do but to
transmit the smoke; and with long tubes of small diameter, therefore, a
very strong draught is indispensable. To obtain such a draught in
locomotives, it is necessary to contract the mouth of the blast pipe,
whereby the waste steam will be projected into the chimney with greater
force; but this contraction involves an increase of the pressure on the
eduction side of the piston, and consequently causes a diminution in the
power of the engine. Locomotives with small and long tubes, therefore, will
require more coke to do the same work than locomotives in which larger and
shorter tubes may be employed.
CHAPTER II.
HEAT, COMBUSTION, AND STEAM.
HEAT.
134. _Q._--What is meant by latent heat?
_A._--By latent heat is meant the heat existing in bodies which is not
discoverable by the touch or by the thermometer, but which manifests its
existence by producing a change of state. Heat is absorbed in the
liquefaction of ice, and in the vaporization of water, yet the temperature
does not rise during either process, and the heat absorbed is therefore
said to become latent. The term is somewhat objectionable, as the effect
proper to the absorption of heat has in each case been made visible; and it
would be as reasonable to call hot water latent steam. Latent heat, in the
present acceptation of the term, means sensible liquefaction or
vaporization; but to produce these changes heat is as necessary as to
produce the expansion of mercury in a thermometer tube, which is taken as
the measure of temperature; and it is hard to see on what ground heat can
be said to be latent when its presence is made manifest by changes which
only heat can effect. It is the _temperature_ only that is latent, and
latent temperature means sensible vaporization or liquefaction.
135. _Q._--But when you talk of the latent heat of steam, what do you mean
to express?
_A._--I mean to express the heat consumed in accomplishing the vaporization
compared with that necessary for producing the temperature. The latent heat
of steam is usually reckoned at about 1000 degrees, by which it is meant
that there is as much heat in any given weight of steam as would raise its
constituent water 1000 degrees if the expansion of the water could be
prevented, or as would raise 1000 times that quantity of water one degree.
The boiling point of water, being 212 degrees, is 180 degrees above the
freezing point of water--the freezing point being 32 degrees; so that it
requires 1180 times as much heat to raise 1 lb. of water into steam, as to
raise 1180 lbs. of water one degree; or it requires about as much heat to
raise a pound of boiling water into steam, as would raise 5-1/2 lbs. of
water from the freezing to the boiling point; 5-1/2 multiplied by 180 being
990 or 1000 nearly.
136. _Q._--When it is stated that the latent heat of steam is 1000 degrees,
it is only meant that this is a rough approximation to the truth?
_A._--Precisely so. The latent heat, in point of fact, is not uniform at
all temperatures, neither is the total amount of heat the same at all
temperatures. M. Regnault has shown, by a very elaborate series of
experiments on steam, which he has lately concluded, that the total heat in
steam increases somewhat with the pressure, and that the latent heat
diminishes somewhat with the pressure. This will be made obvious by the
following numbers:
Pressure. Temperature. Total Heat. Latent Heat.
15 lbs. 213.1 deg. 1178.9 deg. 965.8 deg.
50 281.0 1199.6 918.6
100 327.8 1213.9 886.1
If, then, steam of 100 lbs. be expanded down to steam of 15 lbs., it will
have 35 degrees of heat over that which is required for the maintenance of
the vaporous state, or, in other words, it will be surcharged with heat.
137. _Q._--What do you understand by specific heat?
_A._--By specific heat, I understand the relative quantities of heat in
bodies at the same temperature, just as by specific gravity I understand
the relative quantities of matter in bodies of the same bulk. Equal weights
of quicksilver and water at the same temperature do not contain the same
quantities of heat, any more than equal bulks of those liquids contain the
same quantity of matter. The absolute quantity of heat in any body is not
known; but the relative heat of bodies at the same temperature, or in other
words their specific heats, have been ascertained and arranged in tables,--
the specific heat of water being taken as unity.
138. _Q._--In what way does the specific heat of a body enable the quantity
of heat in it to be determined?
_A._--If any body has only half the specific heat of water, then a pound of
that body will, at any given temperature, have only half the heat in it
that is in a pound of water at the same temperature. The specific heat of
air is .2669, that of water being 1; or it is 3.75 times less than that of
water. An amount of heat, therefore, which would raise a pound of water 1
degree would raise a pound of air 3.75 degrees.
COMBUSTION.
139. _Q._--What is the nature of combustion?
_A._--Combustion is nothing more than an energetic chemical combination,
or, in other words, it is the mutual neutralization of opposing
electricities. When coal is brought to a high temperature it acquires a
strong affinity for oxygen, and combination with oxygen will produce more
than sufficient heat to maintain the original temperature; so that part of
the heat is rendered applicable to other purposes.
140. _Q._--Does air consist of oxygen?
_A._--Air consists of oxygen and nitrogen mixed together in the proportion
of 3.29 lbs. of nitrogen to 1 lb. of oxygen. Every pound of coal requires
about 2.66 lbs. of oxygen for its saturation, and therefore for every pound
of coal burned, 8.75 pounds of nitrogen must pass through the fire,
supposing all the oxygen to enter into combination. In practice, however,
this perfection of combination does not exist; from one-third to one-half
of the oxygen will pass through the fire without entering into combination
at all; so that from 16 to 18 lbs. of air are required for every pound of
coal burned. 18 lbs. of air are about 240 cubic feet, which may be taken as
the quantity of air required for the combustion of a pound of coal in
practice.
141. _Q._--What are the constituents of coal?
_A._--The chief constituent of coal is carbon or pure charcoal, which is
associated in various proportions with volatile and earthy matters. English
coal contains 80 to 90 per cent. of carbon, and from 8 to 18 per cent. of
volatile and earthy matters, but sometimes more than this. The volatile
matters are hydrogen, nitrogen, oxygen, and sulphur.
142. _Q._--What is the difference between anthracite and bituminous coal?
_A._--Anthracite consists almost entirely of carbon, having 91 per cent. of
carbon, with about 7 per cent. of volatile matter and 2 per cent. of ashes.
Newcastle coal contains about 83 per cent. of carbon, 14 per cent. of
volatile matter, and 3 per cent. of ashes.
143. _Q._--Will you recapitulate the steps by which you determine the
quantity of air required for the combustion of coal?
_A._--Looking to the quantity of oxygen required to unite chemically with
the various constituents of the coal, we find for example that in 100 lbs.
of anthracite coal, consisting of 91.44 lbs. of carbon, and 3.46 lbs. of
hydrogen, we shall for the 91.44 lbs. of carbon require 243.84 lbs. of
oxygen--since to saturate a pound of carbon by the formation of carbonic
acid, requires 2-2/3 lbs. of oxygen. To saturate a pound of hydrogen in the
formation of water, requires 8 lbs. of oxygen; hence 3.46 Fibs. of hydrogen
will take 27.68 lbs. of oxygen for its saturation. If then we add 243.84
lbs. to 27.68 lbs. we have 271.52 lbs. of oxygen required for the
combustion of 100 lbs. of coal. A given weight of air contains nearly 23.32
per cent of oxygen; hence to obtain 271.52 lbs. of oxygen, we must have
about four times that quantity of atmospheric air, or more accurately, 1164
lbs. of air for the combustion of 100 lbs. of coal. A cubic foot of air at
ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal
require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic
feet of air, supposing every atom of the oxygen to enter into combination.
If, then, from one-third to one-half of the air passes unconsumed through
the fire, an allowance of 240 cubic feet of air for each pound of coal will
be a small enough allowance to answer the requirements of practice, and in
some cases as much as 300 cubic feet will be required,--the difference
depending mainly on the peculiar configuration of the furnace.
144. _Q._--Can you state the evaporative efficacy of a pound of coal?
_A._--The evaporative efficacy of a pound of carbon has been found
experimentally to be equivalent to that necessary to raise 14,000 lbs. of
water through 1 degree, or 14 lbs. of water through 1000 degrees, supposing
the whole heat generated to be absorbed by the water. Now, if the water be
raised into steam from a temperature of 60 deg., then 1118.9 deg. of heat will have
to be imparted to it to convert it into steam of 15 lbs. pressure per
square inch. 14,000 / 1118.9 = 12.512 Lbs. will be the number of pounds of
water, therefore, which a pound of carbon can raise into steam of 15 lbs.
pressure from a temperature of 60 deg.. This, however, is a considerably larger
result than can be expected in practice.
145. _Q._--Then what is the result that may be expected in practice?
_A._--The evaporative powers of different coals appear to be nearly
proportional to the quantity of carbon in them; and bituminous coal is,
therefore, less efficacious than coal consisting chiefly of pure carbon. A
pound of the best Welsh or anthracite coal is capable of raising from 9-1/2
to 10 lbs. of water from 212 deg. into steam, whereas a pound of the best
Newcastle is not capable of raising more than about 8-1/2 lbs. of water
from 212 deg. into steam; and inferior coals will not raise more than 6-1/2
lbs. of water into steam. In America it has been found that 1 lb. of the
best coal is equal to 2-1/2 lbs. of pine wood, or, in some cases to 3 lbs.;
and a pound of pine wood will not usually evaporate more than about 2 1/2
lbs. of water, though, by careful management, it may be made to evaporate 4
1/2 lbs. Turf will generate rather more steam than wood. Coke is equal or
somewhat superior to the best coal in evaporative effect.
146. _Q._--How much water will a pound of coal raise into steam in ordinary
boilers?
_A._--From 6 to 8 lbs. of water in the generality of land boilers of medium
quality, the difference depending on the kind of boiler, the kind of coal,
and other circumstances. Mr. Watt reckoned his boilers as capable of
evaporating 10.08 cubic feet of water with a bushel or 84 lbs. of Newcastle
coal, which is equivalent to 7 1/2 lbs. of water evaporated by 1 lb. of
coal, and this may be taken as the performance of common land boilers at
the present time. In some of the Cornish boilers, however, a pound of coal
raises 11.8 lbs. of boiling water into steam, or a cwt. of coal evaporates
about 21 cubic feet of water from 212 deg..
147. _Q._--What method of firing ordinary furnaces is the best?
_A._--The coals should be broken up into small pieces, and sprinkled thinly
and evenly over the fire a little at a time. The thickness of the stratum
of coal upon the grate should depend upon the intensity of the draught: in
ordinary land or marine boilers it should be thin, whereas in locomotive
boilers it requires to be much thicker. If the stratum of coal be thick
while the draught is sluggish, the carbonic acid resulting from combustion
combines with an additional atom of carbon in passing through the fire, and
is converted into carbonic oxide, which may be defined to be invisible
smoke, as it carries off a portion of the fuel: if, on the contrary, the
stratum of coal be thin while the draught is very rapid, an injurious
refrigeration is occasioned by the excess of air passing through the
furnace. The fire should always be spread of uniform thickness over the
bars of the grate, and should be without any holes or uncovered places,
which greatly diminish the effect of the fuel by the refrigeratory action
of the stream of cold air which enters thereby. A wood fire requires to be
about 6 inches thicker than a coal one, and a turf fire requires to be 3 or
4 inches thicker than a wood one, so that the furnace bars must be placed
lower where wood or turf is burned, to enable the surface of the fire to be
at the same distance from the bottom of the boiler.
148. _Q._--Is a slow or a rapid combustion the most beneficial?
_A._--A slow combustion is found by experiment to give the best results as
regards economy of fuel, and theory tells us that the largest advantage
will necessarily be obtained where adequate time has been afforded for a
complete combination of the constituent atoms of the combustible, and the
supporter of combustion. In many of the cases, however, which occur in
practice, a slow combustion is not attainable; but the tendencies of slow
combustion are both to save the fuel, and to burn the smoke.
149. _Q._--Is not the combustion in the furnaces of the Cornish boilers
very slow?
A.--Yes, very slow; and there is in consequence very little smoke evolved.
The coal used in Cornwall is Welsh coal, which evolves but little smoke,
and is therefore more favorable for the success of a smokeless furnace; but
in the manufacturing districts, where the coal is more bituminous, it is
found that smoke may be almost wholly prevented by careful firing and by
the use of a large capacity of furnace.
150. _Q._--Do you consider slow combustion to be an advisable thing to
practise in steam vessels?
_A._--No, I do not. When the combustion is slow, the heat in the furnaces
and flues is less intense, and a larger amount of heating surface
consequently becomes necessary to absorb the heat. In locomotives, where
the heat of the furnace is very intense, there will be the same economy of
fuel with an allowance of 5 or 6 square feet of surface to evaporate a
cubic foot of water as in common marine boilers with 10 or 12.
151. _Q._--What is the method of consuming smoke pursued in the
manufacturing districts?
_A._--In Manchester, where some stringent regulations for the prevention of
smoke have for some time been in force, it is found that the readiest way
of burning the smoke is to have a very large proportion of furnace room,
whereby slow combustion may be carried on. In some cases, too, a favourable
result is arrived at by raising a ridge of coal across the furnace lying
against the bridge, and of the same height: this ridge speedily becomes a
mass of incandescent coke, which promotes the combustion of the smoke
passing over it.
152. _Q._--Is the method of admitting a stream of air into the flues to
burn the smoke regarded favorably?
_A._--No; it is found to be productive of injury to the boiler by the
violent alternations of temperature it occasions, as at some times cold air
impinges on the iron of the boiler, and at other times flame,--just as
there happens to be smoke or no smoke emitted by the furnace. Boilers,
therefore, operating upon this principle, speedily become leaky, and are
much worn by oxidation, so that, if the pressure is considerable, they are
liable to explode. It is very difficult to apportion the quantity of air
admitted, to the varying wants of the fire; and as air may at some times be
rushing in when there is no smoke to consume, a loss of heat, and an
increased consumption of fuel may be the result of the arrangement; and,
indeed, such is the result in practice, though a carefully performed
experiment usually demonstrates a saving in fuel of 10 or 12 per cent.
153. _Q._--What other plans have been contrived for obviating the nuisance
of smoke?
_A._--They are too various for enumeration, but most of them either operate
upon the principle of admitting air into the flues to accomplish the
combustion of the uninflammable parts of the smoke, or seek to attain the
same object by passing the smoke over or through the fire or other
incandescent material. Some of the plans, indeed, profess to burn the
inflammable gases as they are evolved from the coal, without permitting the
admixture of any of the uninflammable products of combustion which enter
into the composition of smoke; but this object has been very imperfectly
fulfilled in any of the contrivances yet brought under the notice of the
public, and in some cases these contrivances have been found to create
weightier evils than they professed to relieve.
154. _Q._--You refer, I suppose, to Mr. Charles Wye Williams' Argand
furnace?
_A._--I chiefly refer to it, though I also comprehend all other schemes in
which there is a continuous admission of air into the flues, with an
intermittent generation of smoke.
155. _Q._--This is not so in Prideaux's furnace?
_A._--No; in that furnace the air is admitted only during a certain
interval, or for so long, in fact, as there is smoke to be consumed.
156. _Q._--Will you explain the chief peculiarities of that furnace?
_A._--The whole peculiarity is in the furnace door. The front of the door
consists of metal Venetians, which are opened when the top lever is lifted
up, and shut when that lever descends to its lowest position. When the
furnace door is opened to replenish the fire with coals, the top lever is
raised up, and with it the piston of the small cylinder attached to the
side of the furnace. The Venetians are thereby opened, and a stream of air
enters the furnace, which, being heated in its passage among the numerous
heated plates attached to the back of the furnace door, is in a favorable
condition for effecting the combustion of the inflammable parts of the
smoke. The piston in the small cylinder gradually subsides and closes the
Venetians; and the rate of the subsidence of the piston may obviously be
regulated by a cock, or, as in this case, a small screw valve, so that the
Venetians shall just close when there is no more smoke to be consumed;--the
air or other fluid within the cylinder being forced out by the piston in
its descent.
157. _Q._--Had Mr. Watt any method of consuming smoke?
_A._--He tried various methods, but eventually fixed upon the method of
coking the coal on a dead plate at the furnace door, before pushing it into
the fire. That method is perfectly effectual where the combustion is so
slow that the requisite time for coking is allowed, and it is much
preferable to any of the methods of admitting air at the bridge or
elsewhere, to accomplish the combustion of the inflammable parts of the
smoke.
158. _Q._--What are the details of Mr. Watt's arrangement as now employed?
_A._--The fire bars and the dead plate are both set at a considerable
inclination, to facilitate the advance of the fuel into the furnace. In
Boulton and Watt's 30 horse power land boiler, the dead plate and the
furnace bars are both about 4 feet long, and they are set at the angle of
30 degrees with the horizon.
159. _Q._--Is the use of the dead plate universally adopted in Boulton and
Watt's land boilers?
_A._--It is generally adopted, but in some cases Boulton and Watt have
substituted the plan of a revolving grate for consuming the smoke, and the
dead plate then becomes both superfluous and inapplicable. In this
contrivance the fire is replenished with coals by a self-acting mechanism.
160. _Q._--Will you explain the arrangement of the revolving grate?
_A._--The fire grate is made like a round table capable of turning
horizontally upon a centre; a shower of coal is precipitated upon the grate
through a slit in the boiler near the furnace mouth, and the smoke evolved
from the coal dropped at the front part of the fire is consumed by passing
over the incandescent fuel at the back part, from which all the smoke must
have been expelled in the revolution of the grate before it can have
reached that position.
161. _Q._--Is a furnace with a revolving grate applicable to a steam
vessel?
_A._--I see nothing to prevent its application. But the arrangement of the
boiler would perhaps require to be changed, and it might be preferable to
combine its use with the employment of vertical tubes, for the transmission
of the smoke. The introduction of any effectual automatic contrivance for
feeding the fire in steam vessels, would bring about an important economy,
at the same time that it would give the assurance of the work being better
done. It is very difficult to fire furnaces by hand effectually at sea,
especially in rough weather and in tropical climates; whereas machinery
would be unaffected by any such disturbing causes, and would perform with
little expense the work of many men.
162. _Q._--The introduction of some mechanical method of feeding the fire
with coals would enable a double tier of furnaces to be adopted in steam
vessels without inconvenience?
_A._--Yes, it would have at least that tendency; and as the space available
for area of grate is limited in a steam vessel by the width of the vessel,
it would be a great convenience if a double tier of furnaces could be
employed without a diminished effect. It appears to me, however, that the
objection would still remain of the steam raised by the lower furnace being
cooled and deadened by the air entering the ash-pit of the upper fire, for
it would strike upon the metal of the ash-pit bottom.
163. _Q._--Have any other plans been devised for feeding the fire by
self-acting means besides that of a revolving grate?
_A._--Yes, many plans, but none of them, perhaps, are free from an
objectionable complication. In some arrangements the bars are made like
screws, which being turned round slowly, gradually carry forward the coal;
while in other arrangements the same object is sought to be attained by
alternately lifting and depressing every second bar at the end nearest the
mouth of the furnace. In Juckes' furnace, the fire bars are arranged in the
manner of rows of endless chains working over a roller at the mouth of the
furnace, and another roller at the farther end of the furnace. These
rollers are put into slow revolution, and the coal which is deposited at
the mouth of the furnace is gradually carried forward by the motion of the
chains, which act like an endless web. The clinkers and ashes left after
the combustion of the coal, are precipitated into the ash-pit, where the
chain turns down over the roller at the extremity of the furnace. In
Messrs. Maudslays' plan of a self-feeding furnace the fire bars are formed
of round tubes, and are placed transversely across the furnace. The ends of
the bars gear into endless screws running the whole length of the furnace,
whereby motion is given to the bars, and the coal is thus carried gradually
forward. It is very doubtful whether any of these contrivances satisfy all
the conditions required in a plan for feeding furnaces of the ordinary form
by self-acting means, but the problem of providing a suitable contrivance,
does not seem difficult of accomplishment, and will no doubt be effected
under adequate temptation.
164. _Q._--Have not many plans been already contrived which consume the
smoke of furnaces very effectually?
_A._--Yes, many plans; and besides those already mentioned there are
Hall's, Coupland's, Godson's, Robinson's, Stevens's, Hazeldine's, Indie's,
Bristow and Attwood's, and a great number of others. One plan, which
promises well, consists in making the flame descend through the fire bars,
and the fire bars are formed of tubes set on an incline and filled with
water, which water will circulate with a rapidity proportionate to the
intensity of the heat. After all, however, the best remedy for smoke
appears to consist in removing from it those portions which form the smoke
before the coal is brought into use. Many valuable products may be got from
the coal by subjecting it to this treatment; and the residuum will be more
valuable than before for the production of steam.
STEAM.
165. _Q._--Have experiments been made to determine the elasticity of steam
at different temperatures?
_A._--Yes; very careful experiments. The following rule expresses the
results obtained by Mr. Southern:--To the given temperature in degrees of
Fahrenheit add 51.3 degrees; from the logarithm of the sum, subtract the
logarithm of 135.767, which is 2.1327940; multiply the remainder by 5.13,
and to the natural number answering to the sum, add the constant
fraction .1, which will give the elastic force in inches of mercury. If the
elastic force be known, and it is wanted to determine the corresponding
temperature, the rule must be modified thus:--From the elastic force, in
inches of mercury, subtract the decimal .1, divide the logarithm of the
remainder by 5.13, and to the quotient add the logarithm 2.1327940; find
the natural number answering to the sum, and subtract therefrom the
constant 51.3; the remainder will be the temperature sought. The French
Academy, and the Franklin Institute, have repeated Mr. Southern's
experiments on a larger scale; the results obtained by them are not widely
different, and are perhaps nearer the truth, but Mr. Southern's results are
generally adopted by engineers, as sufficiently accurate for practical
purposes.
166. _Q._--Have not some superior experiments upon this subject been lately
made in France?
_A._--Yes, the experiments of M. Regnault upon this subject have been very
elaborate and very carefully conducted, and the results are probably more
accurate than have been heretofore obtained. Nevertheless, it is
questionable how far it is advisable to disturb the rules of Watt and
Southern, with which the practice of engineers is very much identified, for
the sake of emendations which are not of such magnitude as to influence
materially the practical result. M. Regnault has shown that the total
amount of heat, existing in a given weight of steam, increases slightly
with the pressure, so that the sum of the latent and sensible heats do not
form a constant quantity. Thus, in steam of the atmospheric pressure, or
with 14.7 Lbs. upon the square inch, the sensible heat of the steam is 212
degrees, the latent heat 966.6 degrees, and the sum of the latent and
sensible heats 1178.6 degrees; whereas in steam of 90 pounds upon the
square inch the sensible heat is 320.2 degrees, the latent heat 891.4
degrees, and the sum of the latent and sensible heats 1211.0 degrees. There
is, therefore, 33 degrees less of heat in any given weight of water, raised
into steam of the atmospheric pressure, than if raised into steam of 90
Lbs.[1] pressure.
167. _Q._--What expansion does water undergo in its conversion into steam?
_A._--A cubic inch of water makes about a cubic foot of steam of the
atmospheric pressure.
168. _Q._--And how much at a higher pressure?
_A._--That depends upon what the pressure is. But the proportion is easily
ascertained, for the pressure and the bulk of a given quantity of steam, as
of air or any other elastic fluid, are always inversely proportional to one
another. Thus if a cubic inch of water makes a cubic foot of steam, with
the pressure of one atmosphere, it will make half a cubic foot with the
pressure of two atmospheres, a third of a cubic foot with the pressure of
three atmospheres, and so on in all other proportions. High pressure steam
indeed is just low pressure steam forced into a less space, and the
pressure will always be great in the proportion in which the space is
contracted.
169. _Q._--If this be so, the quantity of heat in a given weight of steam
must be nearly the same, whether the steam is high or low pressure?
_A._--Yes; the heat in steam is nearly a constant quantity, at all
pressures, but not so precisely. Steam to which an additional quantity of
heat has been imparted after leaving the boiler, or as it is called
"surcharged steam," comes under a different law, for the elasticity of such
steam may be increased without any addition being made to its weight; but
surcharged steam is not at present employed for working engines, and it may
therefore be considered in practice that a pound of steam contains very
nearly the same quantity of heat at all pressures.
170. _Q._--Does not the quantity of heat in any body vary with the
temperature?
_A._--Other circumstances remaining the same the quantity of heat in a body
increases with the temperatures.
171. _Q._--And is not high pressure steam hotter than low pressure steam?
_A._--Yes, the temperature of steam rises with the pressure.
172. _Q._--How then comes it, that there is the same quantity of heat in
the same weight of high and low pressure steam, when the high pressure
steam has the highest temperature?
_A._--Because although the temperature or sensible heat rises with the
pressure, the latent heat becomes less in about the same proportion. And as
has been already explained, the latent and sensible heats taken together
make up nearly the same amount at all temperatures; but the amount is
somewhat greater at the higher temperatures. As a damp sponge becomes wet
when subjected to pressure, so warm vapor becomes hot when forced into less
bulk, but in neither case does the quantity of moisture or the quantity of
heat sustain any alteration. Common air becomes so hot by compression that
tinder may be inflamed by it, as is seen in the instrument for producing
instantaneous light by suddenly forcing air into a syringe.
173. _Q._--What law is followed by surcharged steam on the application of
heat?
_A._--The same as that followed by air, in which the increments in volume
are very nearly in the same proportion as the increments in temperature;
and the increment in volume for each degree of increased temperature is
1/490th part of the volume at 32 deg.. A volume of air which, at the
temperature of 32 deg., occupies 100 cubic feet, will at 212 deg. fill a space of
136.73 cubic feet. The volume which air or steam--out of contact with
water--of a given temperature acquires by being heated to a higher
temperature, the pressure remaining the same, may be found by the following
rule:--To each of the temperatures before and after expansion, add the
constant number 458: divide the greater sum by the less, and multiply the
quotient by the volume at the lower temperature; the product will give the
expanded volume.
174. _Q._--If the relative volumes of steam and water are known, is it
possible to tell the quantity of water which should be supplied to a
boiler, when the quantity of steam expended is specified?
_A._--Yes; at the atmospheric pressure, about a cubic inch of water has to
be supplied to the boiler for every cubic foot of steam abstracted; at
other pressures, the relative bulk of water and steam may be determined as
follows:--To the temperature of steam in degrees of Fahrenheit, add the
constant number 458, multiply the sum by 37.3, and divide the product by
the elastic force of the steam in pounds per square inch; the quotient will
give the volume required.
175. _Q._--Will this rule give the proper dimensions of the pump for
feeding the boiler with water?
_A._--No; it is necessary in practice that
the feed pump should be able to supply the boiler with a much larger
quantity of water than what is indicated by these proportions, from the
risk of leaks, priming, or other disarrangements, and the feed pump is
usually made capable of raising 3-1/2 times the water evaporated by the
boiler. About 1/240th of the capacity of the cylinder answers very well for
the capacity of the feed pump in the case of low pressure engines,
supposing the cylinder to be double acting, and the pump single acting; but
it is better to exceed this size.
176. _Q._--Is this rule for the size of the feed pump applicable to the
case of high pressure engines?
_A._--Clearly not; for since a cylinder full of high pressure steam,
contains more water than the same cylinder full of low pressure steam, the
size of the feed must vary in the same proportion as the density of the
steam. In all pumps a good deal of the effect is lost from the imperfect
action of the valves; and in engines travelling at a high rate of speed, in
particular, a large part of the water is apt to return, through the suction
valve of the pump, especially if much lift be permitted to that valve. In
steam vessels moreover, where the boiler is fed with salt water, and where
a certain quantity of supersalted water has to be blown out of the boiler
from time to time, to prevent the water from reaching too high a degree of
concentration, the feed pump requires to be of additional size to supply
the extra quantity of water thus rendered necessary. When the feed water is
boiling or very hot, as in some engines is the case, the feed pump will not
draw from a depth, and will altogether act less efficiently, so that an
extra size of pump has to be provided in consequence. These and other
considerations which might be mentioned, show the propriety of making the
feed pump very much larger than theory requires. The proper proportions of
pumps, however, forms part of a subsequent chapter.
[1] A table containing the results arrived at by M. Regnault is given in
the Key.
CHAPTER III.
EXPANSION OF STEAM AND ACTION OF THE VALVES.
177. _Q._--What is meant by working engines expansively?
_A._--Adjusting the valves, so that the steam is shut off from the cylinder
before the end of the stroke, whereby the residue of the stroke is left to
be completed by the expanding steam.
178. _Q._--And what is the benefit of that practice?
_A._--It accomplishes an important saving of steam, or, what is the same
thing, of fuel; but it diminishes the power of the engine, while increasing
the power of the steam. A larger engine will be required to do the same
work, but the work will be done with a smaller consumption of fuel. If, for
example, the steam be shut off when only half the stroke is completed,
there will only be half the quantity of steam used. But there will be more
than half the power exerted; for although the pressure of the steam
decreases after the supply entering from the boiler is shut off, yet it
imparts, during its expansion, _some_ power, and that power, it is clear,
is obtained without any expenditure of steam or fuel whatever.
179. _Q._--What will be the pressure of the steam, under such
circumstances, at the end of the stroke?
_A._--If the steam be shut off at half stroke, the pressure of the steam,
reckoning the total pressure both below and above the atmosphere, will just
be one-half of what it was at the beginning of the stroke. It is a well
known law of pneumatics, that the pressure of elastic fluids varies
inversely as the spaces into which they are expanded or compressed. For
example, if a cubic foot of air of the atmospheric density be compressed
into the compass of half a cubic foot, its elasticity will be increased
from 15 lbs. on the square inch to 30 lbs. on the square inch; whereas, if
its volume be enlarged to two cubic feet, its elasticity will be reduced to
7-1/2 lbs. on the square inch, being just half its original pressure. The
same law holds in all other proportions, and with all other gases and
vapors, provided their temperature remains unchanged; and if the steam
valve of an engine be closed, when the piston has descended through
one- fourth of the stroke, the steam within the cylinder will, at the end
of the stroke, just exert one-fourth of its initial pressure.
180. _Q._--Then by computing the varying pressure at a number of stages,
the average or mean pressure throughout the stroke may be approximately
determined?
[Illustration: Fig. 32. Diagram showing law of expansion of steam in a
cylinder.]
_A._--Precisely so. Thus in the accompanying figure, (fig. 32), let E be a
cylinder, J the piston, _a_ the steam pipe, _c_ the upper port, _f_ the
lower port, _d_ the steam pipe, prolonged to _e_ the equilibrium valve, _g_
the eduction valve, M the steam jacket, N the cylinder cover, O stuffing
box, _n_ piston rod, P cylinder bottom; let the cylinder be supposed to be
divided in the direction of its length into any number of equal parts, say
twenty, and let the diameter of the cylinder represent the pressure of the
steam, which, for the sake of simplicity, we may take at 10 lbs., so that
we may divide the cylinder, in the direction of its diameter, into ten
equal parts. If now the piston be supposed to descend through five of the
divisions, and the steam valve then be shut, the pressure at each
subsequent position of the piston will be represented by a series, computed
according to the laws of pneumatics, and which, if the initial pressure be
represented by 1, will give a pressure of .5 at the middle of the stroke,
and .25 at the end of it.
If this series be set off on the horizontal lines, it will mark out a
hyperbolic curve--the area of the part exterior to which represents the
total efficacy of the stroke, and the interior area, therefore, represents
the diminution in the power of a stroke, when the steam is cut off at
one- fourth of the descent. If the squares above the point, where the steam
is cut off, be counted, they will be found to amount to 50; and if those
beneath that point be counted or estimated, they will be found to amount to
about 69. These squares are representative of the power exerted; so that
while an amount of power represented by 50 has been obtained by the
expenditure of a quarter of a cylinder full of steam, we get an amount of
power represented by 69, without any expenditure of steam at all, merely by
permitting the steam first used to expand into four times its original
volume.
181. _Q._--Then by working an engine expansively, the power of the steam is
increased, but the power of the engine is diminished?
_A._--Yes. The efficacy of a given quantity of steam is more than doubled
by expanding the steam four times, while the efficacy of each stroke is
made nearly one-half less. And, therefore, to carry out the expansive
principle in practice, the cylinder requires to be larger than usual, or
the piston faster than usual, in the proportion in which the expansion is
carried out. Every one who is acquainted with simple arithmetic, can
compute the terminal pressure of steam in a cylinder, when he knows the
initial pressure and the point at which the steam is cut off; and he can
also find, by the same process, any pressure intermediate between the first
and the last. By setting down these pressures in a table, and taking their
mean, he can determine the effect, with tolerable accuracy, of any
particular measure of expansion. It is necessary to remark, that it is the
total pressure of the steam that he must take; not the pressure above the
atmosphere, but the pressure above a perfect vacuum.
182. _Q._--Can you give any rule for ascertaining at one operation the
amount of benefit derivable from expansion?
_A._--Divide the length of stroke through which the steam expands, by the
length of stroke performed with full pressure, which last call 1; the
hyperbolic logarithm of the quotient is the increase of efficiency due to
expansion. According to this rule it will be found, that if a given
quantity of steam, the power of which working at full pressure is
represented by 1, be admitted into a cylinder of such a size that its
ingress is concluded when one-half the stroke has been performed, its
efficacy will be raised by expansion to 1.69; if the admission of the steam
be stopped at one-third of the stroke, the efficacy will be 2.10; at
one- fourth, 2.39; at one-fifth, 2.61; at one-sixth, 2.79; at one-seventh,
2.95; at one-eighth, 3.08. The expansion, however, cannot be carried
beneficially so far as one-eighth, unless the pressure of the steam in the
boiler be very considerable, on account of the inconvenient size of
cylinder or speed of piston which would require to be adopted, the
friction of the engine, and the resistance of vapor in the condenser, which
all become relatively greater with a smaller urging force.
183. _Q._--Is this amount of benefit actually realized in practice?
_A._--Only in some cases. It appears to be indispensable to the realization
of any large amount of benefit by expansion, that the cylinder should be
enclosed in a steam jacket, or should in some other way be effectually
protected from refrigeration. In some engines not so protected, it has been
found experimentally that less benefit was obtained from the fuel by
working expansively than by working without expansion--the whole benefit
due to expansion being more than counteracted by the increased
refrigeration due to the larger surface of the cylinder required to develop
the power. In locomotive engines, with outside cylinders, this condition of
the advantageous use of expansion has been made very conspicuous, as has
also been the case in screw steamers with four cylinders, and in which the
refrigerating surface of the cylinders was consequently large.
184. _Q._--The steam is admitted to and from the cylinder by means of a
slide or sluice valve?
[Illustration: Fig. 33.]
_A._--Yes; and of the slide valve there are many varieties; but the kinds
most in use are the D valve,--so called from its resemblance to a half
cylinder or D in its cross section--and the three ported valve, shown in
fig. 33, which consists of a brass or iron box set over the two ports or
openings into the cylinder, and a central port which conducts away the
steam to the atmosphere or condenser; but the length of the box is so
adjusted that it can only cover one of the cylinder ports and the central
or eduction port at the same time. The effect, therefore, of moving the
valve up and down, as is done by the eccentric, is to establish a
connection alternately between each cylinder port and the central passage
whereby the steam escapes; and while the steam is escaping from beneath the
piston, the position of the valve is such, that a free communication exists
between the space above the piston and the steam in the boiler. The piston
is thus urged alternately up and down--the valve so changing its position
before the piston arrives at the end of the stroke, that the pressure is by
that time thrown on the reverse side of the piston, so as to urge it into
motion in the opposite direction.
185. _Q._--Is the motion of the valve, then, the reverse of that of the
piston?
_A._--No. The valve does not move down when the piston moves down, nor does
it move down when the piston moves up; but it moves from its mid position,
to the extremity of its throw, and back again to its mid position, while
the piston makes an upward or downward movement, so that the motion is as
it were at right angles to the motion of the piston; or it is the same
motion that the piston of another engine, the crank of which is set at
right angles with that of the first engine, would acquire.
186. _Q._--Then in a steam vessel the valve of one engine may be worked
from the piston of the other?
_A._--Yes, it may; or it may be worked from its own connecting rod; and in
the case of locomotive engines, this has sometimes been done.
187. _Q._--What is meant by the lead of the valve?
_A._--The amount of opening which the valve presents for the admission of
the steam, when the piston is just beginning its stroke. It is found
expedient that the valve should have opened a little to admit steam on the
reverse side of the piston before the stroke terminates; and the amount of
this opening, which is given by turning the eccentric more or less round
upon the shaft, is what is termed the lead.
188. _Q._--And what is meant by the lap of the valve?
_A._--It is an elongation of the valve face to a certain extent over the
port, whereby the port is closed sooner than would otherwise be the case.
This extension is chiefly effected at that part of the valve where the
steam is admitted, or upon the _steam side_ of the valve, as the technical
phrase is; and the intent of the extension is to close the steam passage
before the end of the stroke, whereby the engine is made to operate to a
certain extent expansively. In some cases, however, there is also a certain
amount of lap given to the escape or eduction side, to prevent the eduction
from being performed too soon when the lead is great; but in all cases
there is far less lap on the eduction than on the steam side, very often
there is none, and sometimes less than none, so that the valve is incapable
of covering both the ports at once.
189. _Q._--What is the usual proportional length of stroke of the valve?
_A._--The common stroke of the valve in rotative engines is twice the
breadth or depth of the port, and the length of the valve face will then be
just the breadth of the port when there is lap on neither the steam nor
eduction side. Whatever lap is given, therefore, makes the valve face just
so much longer. In some engines, however, the stroke of the valve is a good
deal more than twice the breadth of the port; and it is to the stroke of
the valve that the amount of lap should properly be referred.
190. _Q._--Can you tell what amount of lap will accomplish any given amount
of expansion?
_A._--Yes, when the stroke of the valve is known. From the length of the
stroke of the piston subtract that part of the stroke which is intended to
be accomplished before the steam is cut off; divide the remainder by the
length of the stroke of the piston, and extract the square root of the
quotient, which multiply by half the stroke of the valve, and from the
product take half the lead; the remainder will be the lap required.
191. _Q._--Can you state how we may discover at what point of the stroke
the eduction passage will be closed?
_A._--To find how much before the end of the stroke the eduction passage
will be closed:--to the lap on the steam side add the lead, and divide the
sum by half the stroke of the valve; find the arc whose sine is equal to
the quotient, and add 90 deg. to it.; divide the lap on the eduction side by
half the stroke of the valve, and find the arc whose cosine is equal to the
quotient; subtract this arc from the one last obtained, and find the cosine
of the remainder; subtract this cosine from 2, and multiply the remainder
by half the stroke of the piston; the product is the distance of the piston
from the end of the stroke when the eduction passage is closed.
192. _Q._--Can you explain how we may determine the distance of the piston
from the end of the stroke, before the steam urging it onward is allowed to
escape?
_A._--To find how far the piston is from the end of its stroke when the
steam that is propelling it by expansion is allowed to escape to the
atmosphere or condenser--to the lap on the steam side add the lead; divide
the sum by half the stroke of the valve, and find the arc whose sine is
equal to the quotient; find the arc whose sine is equal to the lap on the
eduction side, divided by half the stroke of the valve; add these two arcs
together and subtract 90 deg.; find the cosine of the residue, subtract it from
1, and multiply the remainder by half the stroke of the piston; the product
is the distance of the piston from the end of its stroke when the steam
that is propelling it is allowed to escape into the atmosphere or
condenser. In using these rules, all the dimensions are to be taken in
inches, and the answers will be found in inches also.
193. _Q._--Is it a benefit or a detriment to open the eduction passage
before the end of the stroke?
_A._--In engines working at a high rate of speed, such as locomotive
engines, it is very important to open the exhaust passage for the escape of
the steam before the end of the stroke, as an injurious amount of back
pressure is thus prevented. In the earlier locomotives a great loss of
effect was produced from inattention to this condition; and when lap was
applied to the valves to enable the steam to be worked expansively, it was
found that a still greater benefit was collaterally obtained by the earlier
escape of the steam from the eduction passages, and which was incidental to
the application of lap to the valves. The average consumption of coke per
mile was reduced by Mr. Woods from 40 lbs. per mile to 15 lbs. per mile,
chiefly by giving a free outlet to the escaping steam.
194. _Q._--To what extent can expansion be carried beneficially by means of
lap upon the valve?
_A._--To about one-third of the stroke; that is, the valve may be made with
so much lap, that the steam will be cut off when two thirds of the stroke
have been performed, leaving the residue to be accomplished by the agency
of the expanding steam; but if more lap be put on than answers to this
amount of expansion, a very distorted action of the valve will be produced,
which may impair the efficiency of the engine. If a further amount of
expansion than this is wanted, it may be accomplished by wire drawing the
steam, or by so contracting the steam passage that the pressure within the
cylinder must decline when the speed of the piston is accelerated, as it is
about the middle of the stroke.
195. _Q._--Will you explain how this result ensues?
_A._--If the valve be so made as to shut off the steam by the time two
thirds of the stroke have been performed, and the steam be at the same time
throttled in the steam pipe, the full pressure of the steam within the
cylinder cannot be maintained except near the beginning of the stroke where
the piston travels slowly; for, as the speed of the piston increases, the
pressure necessarily subsides, until the piston approaches the other end of
the cylinder, where the pressure would rise again but that the operation of
the lap on the valve by this time has had the effect of closing the
communication between the cylinder and steam pipe, so as to prevent more
steam from entering. By throttling the steam, therefore, in the manner here
indicated, the amount of expansion due to the lap may be doubled, so that
an engine with lap enough upon the valve to cut off the steam at two-thirds
of the stroke, may, by the aid of wire drawing, be virtually rendered
capable of cutting off the steam at one-third of the stroke.
196. _Q._--Is this the usual way of cutting off the steam?
_A._--No; the usual way of cutting off the steam is by means of a separate
valve, termed an expansion valve; but such a device appears to be hardly
necessary in ordinary engines. In the Cornish engines, where the steam is
cut off in some cases at one-twelfth of the stroke, a separate valve for
the admission of steam, other than that which permits its escape, is of
course indispensable; but in common rotative engines, which may realize
expansive efficacy by throttling, a separate expansion valve does not
appear to be required.
197. _Q._--That is, where much expansion is required, an expansion valve is
a proper appendage, but where not much is required, a separate expansion
valve may be dispensed with?
_A._--Precisely so. The wire drawing of the steam causes a loss of part of
its power, and the result will not be quite so advantageous by throttling
as by cutting off. But for moderate amounts of expansion it will suffice,
provided there be lap upon the slide valve.
198. _Q._--Will you explain the structure or configuration of expansion
apparatus of the usual construction?
[Illustration: Fig 34.]
_A._--The structure of expansion apparatus is very various; but all the
kinds operate either on the principle of giving such a motion to the slide
valve as will enable it to cut off the steam, at the desired point, or on
the principle of shutting off the steam by a separate valve in the steam
pipe or valve casing. The first class of apparatus has not been found so
manageable, and is not in extensive use, except in that form known as the
link motion. Of the second class, the most simple probably is the
application of a cam giving motion to the throttle valve, or to a valve of
the same construction, which either accurately fits the steam pipe, or
which comes round to a face, which, however, it is restrained from touching
by a suitable construction of the cam. A kind of expansion valve, often
employed in marine engines of low speed, is the kind used in the Cornish
engines, and known as the equilibrium valve. This valve is represented in
fig. 34. It consists substantially of an annulus or bulging cylinder of
brass, with a steam-tight face both at its upper and lower edges, at which
points it fits accurately upon a stationary seat. This annulus may be
raised or lowered without being resisted by the pressure of the steam, and
in rotative engines it is usually worked by a cam on the shaft. The
expansion cam is put on the shaft in two pieces, which are fastened to each
other by means of four bolts passing through lugs, and is fixed to the
shaft by keys. A roller at one end of a bell-crank lever, which is
connected with the expansion valve, presses against the cam, so that the
motion of the lever will work the valve. The roller is kept against the cam
by a weight on a lever attached to the same shaft, but a spring is
necessary for high speeds. If the cam were concentric with the shaft, the
lever which presses upon it would remain stationary, and also the expansion
valve; but by the projection of the cam, the end of the lever receives a
reciprocating motion, which is communicated to the valve.
199. _Q._--The cam then works the valve?
_A._--Yes. The position of the projection of the cam determines the point
in relation to the stroke at which the valve is opened, and its
circumferential length determines the length of the time during which the
valve continues open. The time at which the valve should begin to open is
the same under all circumstances, but the duration of its opening varies
with the amount of expansion desired. In order to obtain this variable
extent of expansion, there are several projections made upon the cam, each
of which gives a different degree, or _grade_ as it is usually called, of
expansion. These grades all begin at the same point on the cam, but are of
different lengths, so that they begin to move the lever at the same time,
but differ in the time of returning it to its original position.
200. _Q._--How is the degree of expansion changed?
_A._--The change of expansion is effected by moving the roller on to the
desired grade; which is done by slipping the lever carrying the roller
endways on the shaft or pin sustaining it.
201. _Q._--Are such cams applicable in all cases?
_A._--In engines moving at a high rate of speed the roller will be thrown
back from the cam by its momentum, unless it be kept against it by means of
springs. In some cases I have employed a spring formed of a great number of
discs of India rubber to keep the roller against the cam, but a few brass
discs require to be interposed to prevent the India rubber discs from being
worn in the central hole.
202. _Q._--May not the percussion incident to the action of a cam at a high
speed, when the roller is not kept up to the face by springs, be obviated
by giving a suitable configuration to the cam itself?
_A._--It may at all events be reduced. The outline of the cam should be a
parabola, so that the valve may be set in motion precisely as a falling
body would be; but it will, nevertheless, be necessary that the roller on
which the cam presses should be forced upward by a spring rather than by a
counterweight, as there will thus be less inertia or momentum in the mass
that has to be moved.
203. _Q._--An additional slide valve is sometimes used for cutting off the
steam?
_A._--Yes, very frequently; and the slide valve is sometimes on the side or
back of the valve casing, and sometimes on the back of the main or
distributing valve, and moving with it.
204. _Q._--Are cams used in locomotive engines?
_A._--In locomotive engines the use of cams is inadmissible, and other
expedients are employed, of which those contrived by Stephenson and by
Cabrey operate on the principle of accomplishing the requisite variations
of expansion by altering the throw of the slide valve.
205. _Q._--What is Stephenson's arrangement?
[Illustration: Fig. 35.]
_A._--Stephenson connects the ends of the forward and backward eccentric
rods by a link with a curved slot in which a pin upon the end of the valve
rod works. By moving this link so as to bring the forward eccentric rod in
the same line with the valve rod, the valve receives the motion due to that
eccentric; whereas if the backward eccentric rod is brought in a line with
the valve rod, the valve gets the motion proper for reversing, and if the
link be so placed that the valve rod is midway between the two eccentric
rods, the valve will remain nearly stationary. This arrangement, which is
now employed extensively, is what is termed "the link motion." It is
represented in the annexed figure, fig. 35, where _e_ is the valve rod,
which is attached by a pin to an open curved link susceptible of being
moved up and down by the bell-crank lever _f''_ _f''_, supported on the
centre _g_, and acting on the links _f_, while the valve rod _e_ remains in
the same horizontal plane; _d d'_ are the eccentric rods, and the link is
represented in its lowest position. The dotted lines _h' h''_ show the
position of the eccentric rods when the link is in its highest position,
and _l l'_ when in mid position.
206. _Q._--What is Cabrey's arrangement?
_A._--Mr. Cabrey makes his eccentric rod terminate in a pin which works
into a straight slotted lever, furnished with jaws similar to the jaws on
the eccentric rods of locomotives. By raising the pin of the eccentric rod
in this slot, the travel of the valve will be varied, and expansive action
will be the result.
207. _Q._--What other forms of apparatus are there for working steam
expansively?
_A._--They are too numerous for description here, but a few of them may be
enumerated. Fenton seeks to accomplish the desired object by introducing a
spiral feather on the crank axle, by moving the eccentric laterally against
which the eccentric is partially turned round so as to cut off the steam at
a different part of the stroke. Dodds seeks to attain the same end by
corresponding mechanical arrangements. Farcot, Edwards, and Lavagrian cut
off the steam by the application of a supplementary valve at the back of
the ordinary valve, which supplementary valve is moved by tappets fixed to
the valve casing. Bodmer, in 1841, and Meyer, in 1842, employed two slides
or blocks fitted over apertures in the ordinary slide valve, and which
blocks were approximated or set apart by a right and left handed screw
passing through both.[1] Hawthorn, in 1843, employed as an expansion valve
a species of frame lying on the ordinary cylinder face upon the outside of
the valve, and working up against the steam side of the valve at each end
so as to cut off the steam. In the same year Gonzenbach patented an
arrangement which consists of an additional slide valve and valve casing
placed on the back of the ordinary slide valve casing, and through this
supplementary valve the steam must first pass. This supplementary valve is
worked by a double ended lever, slotted at one end for the reception of a
pin on the valve link, the position of which in the slot determines the
throw of the supplementary valve, and the consequent degree of expansion.
208. _Q._--What is the arrangement of expansion valve used in the most
approved modern engines?
_A._--In modern engines, either marine or locomotive, it is found that if
they are fitted with the link motion, as they nearly all are, a very good
expansive action can be obtained by giving a suitable adjustment to it,
without employing an expansion valve at all. Diagrams taken from engines
worked in this manner show a very excellent result, and most of the modern
engines trust for their expansive working to the link motion and the
throttle valve.
[1] In 1838 I patented an arrangement of expansion valve, consisting of two
movable plates set upon the ordinary slide valve, and which might be drawn
together or asunder by means of a right and left handed screw passing
through both plates. The valve spindle was hollow, and a prolongation of
the screw passed up through it, and was armed on the top with a small
wheel, by means of which the plates might be adjusted while the engine was
at work. In 1839 I fitted an expansion valve in a steam vessel, consisting
of two plates, connected by a rod, and moved by tappets up against the
steam edges of the valve. In another steam vessel I fitted the same species
of valve, but the motion was not derived from tappets, but from a moving
part of the engine, though at the moderate speed at which these engines
worked I found tappets to operate well and make little noise. In 1837 I
employed, as an expansion valve, a rectangular throttle valve, accurately
fitting a bored out seat, in which it might be made to revolve, though it
did not revolve in working. This valve was moved by a pin in a pinion,
making two revolutions for every revolution of the engine, and the
configuration of the seat determined the amount of the expansion. In 1855 I
have again used expansion valves of this construction in engines making one
hundred revolutions per minute, and with perfectly satisfactory results.--
J.B.
CHAPTER IV.
MODES OF ESTIMATING THE POWER AND PERFORMANCE OF ENGINES AND BOILERS.
HORSES POWER.
209. _Q._--What do you understand by a horse power?
_A._--An amount of mechanical force that will raise 33,000 lbs. one foot
high in a minute. This standard was adopted by Mr. Watt, as the average
force exerted by the strongest London horses; the object of his
investigation being to enable him to determine the relation between the
power of a certain size of engine and the power of a horse, so that when it
was desired to supersede the use of horses by the erection of an engine, he
might, from the number of horses employed, determine the size of engine
that would be suitable for the work.
210. _Q._--Then when we talk of an engine of 200 horse power, it is meant
that the impelling efficacy is equal to that of 200 horses, each lifting
33,000 lbs. one foot high in a minute?
_A._--No, not now; such was the case in Watt's engines, but the capacity of
cylinder answerable to a horse power has been increased by most engineers
since his time, and the pressure on the piston has been increased also, so
that what is now called a 200 horse power engine exerts, almost in every
case, a greater power than was exerted in Watt's time, and a horse power,
in the popular sense of the term, has become a mere conventional unit for
expressing a certain size of engine, without reference to the power
exerted.
211. _Q._--Then, each nominal horse power of a modern engine may raise much
more than 33,000 lbs. one foot high in a minute?
_A._--Yes; some raise 52,000 lbs., others 60,000 lbs., and others 66,000
lbs., one foot high in a minute by each nominal horse power. Some engines
indeed work as high as five times above the nominal power, and therefore no
comparison can be made between the performances of different engines,
unless the power actually exerted be first discovered.
212. _Q._--How is the power actually exerted by engines ascertained?
_A._--By means of an instrument called the indicator, which is a miniature
cylinder and piston attached to the cylinder cover of the main engine, and
which indicates, by the pressure exerted on a spring, the amount of
pressure or vacuum existing within the cylinder. From this pressure,
expressed in pounds per square inch, deduct a pound and a half of pressure
for friction, the loss of power in working the air pump, &c.; multiply the
area of the piston in square inches by this residual pressure, and by the
motion of the piston, in feet per minute, and divide by 33,000; the
quotient is the actual number of horses power of the engine. The same
result is attained by squaring the diameter of the cylinder, multiplying by
the pressure per square inch, as shown by the indicator, less a pound and a
half, and by the motion of the piston, in feet per minute, and dividing by
42,017.
213. _Q._ How is the nominal power of an engine ascertained?
_A._--Since the nominal power is a mere conventional expression, it is
clear that it must be determined by a merely conventional process. The
nominal power of ordinary condensing engines may be ascertained by the
following rule: multiply the square of the diameter of the cylinder in
inches, by the velocity of the piston in feet per minute, and divide the
product by 6,000; the quotient is the number of nominal horses power. In
using this rule, however, it is necessary to adopt the speed of piston
prescribed by Mr. Watt, which varies with the length of the stroke. The
speed of piston with a 2 feet stroke is, according to his system, 160 per
minute; with a 2 ft. 6 in. stroke, 170; 3 ft., 180; 3 ft. 6 in., 189; 4
ft., 200; 5 ft., 215; 6 ft., 228; 7 ft., 245; 8 ft., 256 ft.
214. _Q._--Does not the speed of the piston increase with the length of the
stroke?
_A._--It does: the speed of the piston varies nearly as the cube root of
the length of the stroke.
215. _Q._--And may not therefore some multiple of the cube root of the
length of the stroke be substituted for the velocity of the piston in
determining the nominal power?
_A._--The substitution is quite practicable, and will accomplish some
simplification, as the speed of piston proper for the different lengths of
stroke cannot always be remembered. The rule for the nominal power of
condensing engines when thus arranged, will be as follows: multiply the
square of the diameter of the cylinder in inches by the cube root of the
stroke in feet, and divide the product by 47; the quotient is the number of
nominal horses power of the engine, supposing it to be of the ordinary
condensing description. This rule assumes the existence of a uniform
effective pressure upon the piston of 7 lbs. per square inch; Mr. Watt
estimated the effective pressure upon the piston of his 4 horse power
engines at 6-8 lbs. per square inch, and the pressure increased slightly
with the power, and became 6.94 lbs. per square inch in engines of 100
horse power; but it appears to be more convenient to take a uniform
pressure of 7 lbs. for all powers. Small engines, indeed, are somewhat less
effective in proportion than large ones, but the difference can be made up
by slightly increasing the pressure in the boiler; and small boilers will
bear such an increase without inconvenience.
216. _Q._--How do you ascertain the power of high pressure engines?
_A._--The actual power is readily ascertained by the indicator, by the same
process by which the actual power of low pressure engines is ascertained.
The friction of a locomotive engine when unloaded is found by experiment to
be about 1 lb. per square inch on the surface of the pistons, and the
additional friction caused by any additional resistance is estimated at
about .14 of that resistance; but it will be a sufficiently near
approximation to the power consumed by friction in high pressure engines,
if we make a deduction of a pound and a half from the pressure on that
account, as in the case of low pressure engines. High pressure engines, it
is true, have no air pump to work; but the deduction of a pound and a half
of pressure is relatively a much smaller one where the pressure is high,
than where it does not much exceed the pressure of the atmosphere. The
rule, therefore, for the actual horse power of a high pressure engine will
stand thus: square the diameter of the cylinder in inches, multiply by the
pressure of the steam in the cylinder per square inch less 1-1/2 lb., and
by the speed of the piston in feet per minute, and divide by 42,017; the
quotient is the actual horse power.
217. _Q._--But how do you ascertain the nominal horse power of high
pressure engines?
_A._--The nominal horse power of a high pressure engine has never been
defined; but it should obviously hold the same relation to the actual power
as that which obtains in the case of condensing engines, so that an engine
of a given nominal power may be capable of performing the same work,
whether high pressure or condensing. This relation is maintained in the
following rule, which expresses the nominal horse power of high pressure
engines: multiply the square of the diameter of the cylinder in inches by
the cube root of the length of stroke in feet, and divide the product by
15.6. This rule gives the nominal power of a high pressure engine three
times greater than that of a low pressure engine of the same dimensions;
the average effective pressure being taken at 21 lbs. per square inch
instead of 7 lbs., and the speed of the piston in feet per minute being in
both rules 128 times the cube root of the length of stroke.[1]
218. _Q._--Is 128 times the cube root of the stroke in feet per minute the
ordinary speed of all engines?
_A._--Locomotive engines travel at a quicker speed--an innovation brought
about not by any process of scientific deduction, but by the accidents and
exigencies of railway transit. Most other engines, however, travel at about
the speed of 128 times the cube root of the stroke in feet; but some marine
condensing engines of recent construction travel at as high a rate as 700
feet per minute. To mitigate the shock of the air pump valves in cases in
which a high speed has been desirable, as in the case of marine engines
employed to drive the screw propeller without intermediate gearing, India
rubber discs, resting on a perforated metal plate, are now generally
adopted; but the India rubber should be very thick, and the guards employed
to keep the discs down should be of the same diameter as the discs
themselves.
219. _Q._--Can you suggest any eligible method of enabling condensing
engines to work satisfactorily at a high rate of speed?
_A._--The most feasible way of enabling condensing engines to work
satisfactorily at a high speed, appears to lie in the application of
balance weights to the engine, so as to balance the momentum of its moving
parts, and the engine must also be made very strong and rigid. It appears
to be advisable to perform the condensation partly in the air pump, instead
of altogether in the condenser, as a better vacuum and a superior action of
the air pump valves will thus be obtained. Engines constructed upon this
plan may be driven at four times the speed of common engines, whereby an
engine of large power may be purchased for a very moderate price, and be
capable of being put into a very small compass; while the motion, from
being more equable, will be better adapted for most purposes for which a
rotary motion is required. Even for pumping mines and blowing iron
furnaces, engines of this kind appear likely to come into use, for they are
more suitable than other engines for driving the centrifugal pump, which in
many cases appears likely to supersede other kinds of pumps for lifting
water; and they are also conveniently applicable to the driving of fans,
which, when so arranged that the air condensed by one fan is employed to
feed another, and so on through a series of 4 or 5, have succeeded in
forcing air into a furnace with a pressure of 2-1/2 lbs. on the square
inch, and with a far steadier flow than can be obtained by a blast engine
with any conceivable kind of compensating apparatus. They are equally
applicable if blast cylinders be employed.
220. _Q._--Then, if by this modification of the engine you enable it to
work at four times the speed, you also enable it to exert four times the
power?
_A._--Yes; always supposing it to be fully supplied with steam. The nominal
power of this new species of engine can readily be ascertained by taking
into account the speed of the piston, and this is taken into account by the
Admiralty rule for power.
221. _Q._--What is the Admiralty rule for determining the power of an
engine?
_A._--Square the diameter of the cylinder in inches, which multiply by the
speed of the piston in feet per minute, and divide by 6,000; the quotient
is the power of the engine by the Admiralty rule.[2]
222. _Q._--The high speed engine does not require so heavy a fly wheel as
common engines?
_A._--No; the fly wheel will be lighter, both by virtue of its greater
velocity of rotation, and because the impulse communicated by the piston is
less in amount and more frequently repeated, so as to approach more nearly
to the condition of a uniform pressure.
223. _Q._--Can nominal be transformed into actual horse power?
_A._--No; that is not possible in the case of common condensing engines.
The actual power exerted by an engine cannot be deduced from its nominal
power, neither can the nominal power be deduced from the power actually
exerted, or from anything else than the dimensions of the cylinder. The
actual horse power being a dynamical unit, and the nominal horse power a
measure of capacity of the cylinder, are obviously incomparable things.
224. _Q._--That is, the _nominal_ power is a commercial unit by which
engines are bought and sold, and the _actual_ power a scientific unit by
which the quality of their performance is determined?
_A._--Yes; the nominal power is as much a commercial measure as a yard or a
bushel, and is not a thing to be ascertained by any process of science, but
to be fixed by authority in the same manner as other measures. The actual
power, on the contrary, is a mechanical force or dynamical effort capable
of raising a given weight through a given distance in a given time, and of
which the amount is ascertainable by scientific investigation.
225. _Q._--Is there any other measure of an actual horse power than 33,000
lbs. raised one foot high in the minute?
_A._--There cannot be any _different_ measure, but there are several
equivalent measures. Thus the evaporation of a cubic foot of water in the
hour, or the expenditure of 33 cubic feet of low pressure steam per minute,
is reckoned equivalent to an actual horse power, or 528 cubic feet of water
raised one foot high in the minute involves the same result.
[1] Tables of the horse power of both high and low pressure
engines are given in the Key.
[2] Example.--What is the power of an engine of 42 inches
diameter, 3-1/2 feet stroke, and making 85 strokes per minute? The
speed of the piston will be 7 (the length of a double stroke) x 85 =
595 feet per minute. Now 42 x 42 = 1,764 x 595 = 1,049,580 / 6,000 =
175 horses power.
DUTY OF ENGINES AND BOILERS.
226. _Q._--What is meant by the duty of a engine?
_A._--The work done in relation to the fuel consumed.
227. _Q._--And how is the duty ascertained?
_A._--In ordinary mill or marine engines it can only be ascertained by the
indicator, as the load upon such engines is variable, and cannot readily be
determined; but in the case of engines pumping water, where the load is
constant, the number of strokes performed by the engine will represent the
work done, and the amount of work done by a given quantity of coal
represents the duty. In Cornwall the duty of an engine is expressed by the
number of millions of pounds raised one foot high by a bushel, or 94 lbs.
of Welsh coal. A bushel of Newcastle coal will only weigh 84 Lbs.; and in
comparing the duty of a Cornish engine with the performance of an engine in
some locality where a different kind of coal is used, it is necessary to
pay regard to such variations.
228. _Q._--Can you tell the duty of an engine when you know its consumption
of coal per horse power per hour?
_A._--Yes, if the power given be the actual, and not the nominal, power.
Divide 166.32 by the number of pounds of coal consumed per actual horse
power per hour; the quotient is the duty in millions of pounds. If you
already have the duty in millions of pounds, and wish to know the
equivalent consumption in pounds per actual horse power per hour, divide
166.32 by the duty in millions of pounds; the quotient is the consumption
per actual horse power per hour. The duty of a locomotive engine is
expressed by the weight of coke it consumes in transporting a ton through
the distance of one mile upon a railway; but this is a very imperfect
method of representing the duty, as the tractive efficacy of a pound of
coke becomes less as the speed of the locomotive becomes greater; and the
law of variation is not accurately known.
229. _Q._--What amount of power is generated in good engines of the
ordinary kind by a given weight of coal?
_A._--The duty of different kinds of engines varies very much, and there
are also great differences in the performance of different engines of the
same class. In ordinary rotative condensing engines of good construction,
10 lbs. of coal per nominal horse power per hour is a common consumption;
but such engines exert nearly twice their nominal power, so that the
consumption per actual horse power per hour may be taken at from 5 to 6
lbs. Engines working very expansively, however, attain an economy much
superior to this. The average duty of the pumping engines in Cornwall is
about 60,000,000 lbs. raised 1 ft. high by a bushel of Welsh coals, which
weighs 94 lbs. This is equivalent to a consumption of 3.1 lbs. of coal per
actual horse power per hour; but some engines reach a duty of above
100,000,000, or 1.74 lbs. of coal per actual horse power per hour.
Locomotives consume from 8 to 10 lbs. of coke in evaporating a cubic foot
of water, and the evaporation of a cubic foot of water per hour may be set
down as representing an actual horse power in locomotives as well as in
condensing engines, if expansion be not employed. When the locomotive is
worked expansively, however, there is of course a less consumption of water
and fuel per horse power, or per ton per mile, than when the full pressure
is used throughout the stroke; and most locomotives now operate with as
much expansion as can be conveniently given by the slide valves.
230. _Q._--But is not the evaporative power of locomotives affected
materially by the proportions of the boiler?
_A._--Yes, but this may be said of all boilers; but in locomotive boilers,
perhaps, the effect of any misproportion becomes more speedily manifest. A
high temperature of the fire box is found to be conducive to economy of
fuel; and this condition, in its turn, involves a small area of grate bars.
The heating surface of locomotive boilers should be about 80 square feet
for each square foot of grate bars, and upon each foot of grate bars about
1 cwt. of coke should be burnt in the hour.
231. _Q._--Probably the heat is more rapidly absorbed when the temperature
of the furnace is high?
_A._--That seems to be the explanation. The rapidity with which a hot body
imparts heat to a colder, varies as the square of the difference of
temperature; so that if the temperature of the furnace be very high, the
larger part of the heat passes into the water at the furnace, thereby
leaving little to be transmitted by the tubes. If, on the contrary, the
temperature of the furnace be low, a large part of the heat will pass into
the tubes, and more tube surface will be required to absorb it. About 16
cubic feet of water should be evaporated by a locomotive boiler for each,
square foot of fire grate, which, with the proportion of heating surface
already mentioned, leaves 5 square feet of heating surface to evaporate a
cubic foot of water in the hour. This is only about half the amount of
surface usual in land and marine boilers per cubic foot evaporated, and its
small amount is due altogether to the high temperature of the furnace,
which, by the rapidity of transmission it causes, is tantamount to an
additional amount of heating surface.
232. _Q._--You have stated that the steam and vacuum gauges are generally
glass tubes, up which mercury is forced by the steam or sucked by the
vacuum?
_A._--Vacuum gauges are very often of this construction, but steam
gauges more frequently consist of a small iron tube, bent like the letter
U, and into which mercury is poured. The one end of this tube communicates
with the boiler, and the other end with the atmosphere; and when the
pressure of the steam rises in the boiler, the mercury is forced down in
the leg communicating with the boiler and rises in the other leg, and the
difference of level in the legs denotes the pressure of the steam. In this
gauge a rise of the mercury one inch in the one leg involves a difference
of the level between the two legs of two inches, and an inch of rise is,
therefore, equivalent to two inches of mercury, or a pound of pressure. A
small float of wood is placed in the open leg to show the rise or fall of
the mercury, and this leg is surmounted by a brass scale, graduated in
inches, to the marks of which the float points.
233. _Q._--What other kinds of steam and vacuum gauges are there?
_A._--There are many other kinds; but probably Bourdon's gauges are now in
more extended use than, any other, and their operation has been found to be
satisfactory in practice. The principle of their action may be explained to
be, that a thin elliptical metal tube, if bent into a ring, will seek to
coil or uncoil itself if subjected to external or internal pressure, and to
an extent proportional to the pressure applied. The end of the tube is
sharpened into an index, and moves to an extent corresponding to the
pressure applied to the tube; but in the more recent forms of this
apparatus, a dial and a hand, like those of a clock, are employed, and the
hand is moved round by a toothed sector connected to the tube, and which
sector acts on a pinion attached to the hand. Mr. Shank, of Paisley, has
lately introduced a form of steam gauge like a thermometer, with a
flattened bulb; and the pressure of the steam, by compressing the bulb,
causes the mercury to rise to a point proportional to the pressure applied.
THE INDICATOR.
234. _Q._--You have already stated that the actual power of an engine is
ascertained by an instrument called the indicator, which consists of a
small cylinder with a piston moving against a spring, and compressing it to
an extent answerable to the pressure of the steam. Will you explain further
the structure and mode of using that instrument?
[Illustration: Fig. 36]
_A._--The structure of the common form of indicator will be most readily
apprehended by a reference to fig. 36, which is a McNaught's indicator.
Upon a movable barrel A, a piece of paper is wound, the ends of which are
secured by the slight brass clamps shown in the drawing. The barrel is
supported by the bracket _b_, proceeding from the body of the indicator,
and at the bottom of the barrel a watch spring is coiled with one end
attached to the barrel and the other end to the bracket, so that when the
barrel is drawn round by a string wound upon its lower end like a roller
blind, the spring returns the barrel to its original position, when the
string is relaxed. The string is attached to some suitable part of the
engine, and at every stroke the string is drawn out, turning round the
barrel, and the barrel is returned again by the spring on the return
stroke.
235. _Q_--But in what way can these reciprocations of the barrel determine
the power of the engine?
_A._--They do not determine it of themselves, but are only part of the
operation. In the inside of the cylinder _c_ there is a small piston moving
steam tight in a cylinder of which _d_ is the piston rod, and _e_ a spiral
spring of steel, which the piston, when forced upwards by the steam or
sucked downwards by the vacuum, either compresses or extends; _f_ is a cock
attached to the cylinder of the indicator, and which is screwed into the
cylinder cover. It is obvious that, so soon as this cock is opened, the
piston will be forced up when the space above the piston of the engine is
opened to the boiler, and sucked down when that space is opened to the
condenser--in each case to an extent proportionate to the pressure of the
steam or the perfection of the vacuum, the top of the piston _c_ being open
to the atmosphere. A pencil, _p_, with a knife hinge, is inserted into the
piston rod, at _e_, and the point of the pencil bears upon the surface of
the paper wound upon the drum A. If the drum A did not revolve, this pencil
would merely trace on the paper a vertical line; but as the drum A moves
round and back again every stroke of the engine, and as the pencil moves up
and down again every stroke of the engine, the combined movements trace
upon the paper a species of rectangle, which is called an indicator
diagram; and the nature of this diagram determines the nature of the
engine's performance.
236. _Q._--How does it do this?
_A._--It is clear that if the pencil was moved up instantaneously to the
top of its stroke, and was also moved down instantaneously to the bottom of
its stroke, and if it remained without fluctuation while at the top and
bottom, the figure described by the pencil would be a perfect rectangle, of
which the vertical height would represent the total pressure of the steam
and vacuum, and therefore the total pressure urging the piston of the
engine. But in practice the pencil will neither rise nor fall
instantaneously, nor will it remain at a uniform height throughout the
stroke. If the steam be worked expansively the pressure will begin to fall
so soon as the steam is cut off; and at the end of the stroke, when the
steam comes to be discharged, the subsidence of pressure will not be
instantaneous, but will occupy an appreciable time. It is clear, therefore,
that in no engine can the diagram described by an indicator be a complete
rectangle; but the more nearly it approaches to a rectangle, the larger
will be the power produced at every stroke with any given pressure, and the
area of the space included within the diagram will in every case accurately
represent the power exerted by the engine during that stroke.
237. _Q._--And how is this area ascertained?
_A._--It may be ascertained in various ways; but the usual mode is to take
the vertical height of the diagram at a number of equidistant points on a
base line, and then to take the mean of these several heights as
representative of the mean pressure actually urging the piston. Now if you
have the pressure on the piston per square inch, and if you know the number
of square inches in its area, and the velocity with which it moves in feet
per minute, you have obviously the dynamical effort of the engine, or, in
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