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HOW IT WORKS
AUTHOR'S NOTE.
I beg to thank the following gentlemen and firms for the help they have
given me in connection with the letterpress and illustrations of "How It
Works"--
Messrs. F.J.C. Pole and M.G. Tweedie (for revision of MS.); W. Lineham;
J.F. Kendall; E. Edser; A.D. Helps; J. Limb; The Edison Bell Phonograph
Co.; Messrs. Holmes and Co.; The Pelton Wheel Co.; Messrs. Babcock and
Wilcox; Messrs. Siebe, Gorman, and Co.; Messrs. Negretti and Zambra;
Messrs. Chubb; The Yale Lock Co.; The Micrometer Engineering Co.;
Messrs. Marshall and Sons; The Maignen Filter Co.; Messrs. Broadwood and
Co.
[Illustration: ON THE FOOTPLATE OF A LOCOMOTIVE.]
How It Works
Dealing in Simple Language with Steam, Electricity,
Light, Heat, Sound, Hydraulics, Optics, etc.
and with their applications to Apparatus
in Common Use
By
ARCHIBALD WILLIAMS
Author of "The Romance of Modern Invention,"
"The Romance of Mining," etc., etc.
THOMAS NELSON AND SONS
London, Edinburgh, Dublin, and New York
PREFACE.
How does it work? This question has been put to me so often by persons
young and old that I have at last decided to answer it in such a manner
that a much larger public than that with which I have personal
acquaintance may be able to satisfy themselves as to the principles
underlying many of the mechanisms met with in everyday life.
In order to include steam, electricity, optics, hydraulics, thermics,
light, and a variety of detached mechanisms which cannot be classified
under any one of these heads, within the compass of about 450 pages, I
have to be content with a comparatively brief treatment of each subject.
This brevity has in turn compelled me to deal with principles rather
than with detailed descriptions of individual devices--though in several
cases recognized types are examined. The reader will look in vain for
accounts of the Yerkes telescope, of the latest thing in motor cars, and
of the largest locomotive. But he will be put in the way of
understanding the essential nature of _all_ telescopes, motors, and
steam-engines so far as they are at present developed, which I think may
be of greater ultimate profit to the uninitiated.
While careful to avoid puzzling the reader by the use of mysterious
phraseology I consider that the parts of a machine should be given their
technical names wherever possible. To prevent misconception, many of
the diagrams accompanying the letterpress have words as well as letters
written on them. This course also obviates the wearisome reference from
text to diagram necessitated by the use of solitary letters or figures.
I may add, with regard to the diagrams of this book, that they are
purposely somewhat unconventional, not being drawn to scale nor
conforming to the canons of professional draughtsmanship. Where
advisable, a part of a machine has been exaggerated to show its details.
As a rule solid black has been preferred to fine shading in sectional
drawings, and all unnecessary lines are omitted. I would here
acknowledge my indebtedness to my draughtsman, Mr. Frank Hodgson, for
his care and industry in preparing the two hundred or more diagrams for
which he was responsible.
Four organs of the body--the eye, the ear, the larynx, and the
heart--are noticed in appropriate places. The eye is compared with the
camera, the larynx with a reed pipe, the heart with a pump, while the
ear fitly opens the chapter on acoustics. The reader who is unacquainted
with physiology will thus be enabled to appreciate the better these
marvellous devices, far more marvellous, by reason of their absolutely
automatic action, than any creation of human hands.
A.W.
UPLANDS, STOKE POGES, BUCKS.
CONTENTS.
Chapter I.--THE STEAM-ENGINE.
What is steam?--The mechanical energy of steam--The boiler--The
circulation of water in a boiler--The enclosed furnace--The
multitubular boiler--Fire-tube boilers--Other types of boilers--Aids
to combustion--Boiler fittings--The safety-valve--The
water-gauge--The steam-gauge--The water supply to a
boiler 13
Chapter II.--THE CONVERSION OF HEAT ENERGY
INTO MECHANICAL MOTION.
Reciprocating engines--Double-cylinder engines--The function of
the fly-wheel--The cylinder--The slide-valve--The eccentric--"Lap"
of the valve: expansion of steam--How the cut-off is
managed--Limit of expansive working--Compound engines--Arrangement
of expansion engines--Compound locomotives--Reversing
gears--"Linking-up"--Piston-valves--Speed governors--Marine-speed
governors--The condenser 44
Chapter III.--THE STEAM TURBINE.
How a turbine works--The De Laval turbine--The Parsons turbine--Description
of the Parsons turbine--The expansive action of
steam in a Parsons turbine--Balancing the thrust--Advantages
of the marine turbine 74
Chapter IV.--THE INTERNAL-COMBUSTION ENGINE.
The meaning of the term--Action of the internal-combustion engine--The
motor car--The starting-handle--The engine--The carburetter--Ignition
of the charge--Advancing the spark--Governing
the engine--The clutch--The gear-box--The compensating
gear--The silencer--The brakes--Speed of cars 87
Chapter V.--ELECTRICAL APPARATUS.
What is electricity?--Forms of electricity--Magnetism--The permanent
magnet--Lines of force--Electro-magnets--The electric
bell--The induction coil--The condenser--Transformation of
current--Uses of the induction coil 112
Chapter VI.--THE ELECTRIC TELEGRAPH.
Needle instruments--Influence of current on the magnetic needle--Method
of reversing the current--Sounding instruments--Telegraphic
relays--Recording telegraphs--High-speed telegraphy 127
Chapter VII.--WIRELESS TELEGRAPHY.
The transmitting apparatus--The receiving apparatus--Syntonic
transmission--The advance of wireless telegraphy 137
Chapter VIII.--THE TELEPHONE.
The Bell telephone--The Edison transmitter--The granular carbon
transmitter--General arrangement of a telephone circuit--Double-line
circuits--Telephone exchanges--Submarine telephony 147
Chapter IX.--DYNAMOS AND ELECTRIC MOTORS.
A simple dynamo--Continuous-current dynamos--Multipolar
dynamos--Exciting the field magnets--Alternating current dynamos--The
transmission of power--The electric motor--Electric lighting--The
incandescent lamp--Arc lamps--"Series" and "parallel" arrangement of
lamps--Current for electric lamps--Electroplating 159
Chapter X.--RAILWAY BRAKES.
The Vacuum Automatic brake--The Westinghouse air-brake 187
Chapter XI.--RAILWAY SIGNALLING.
The block system--Position of signals--Interlocking the signals--Locking
gear--Points--Points and signals in combination--Working
the block system--Series of signalling operations--Single
line signals--The train staff--Train staff and ticket--Electric
train staff system--Interlocking--Signalling operations--Power
signalling--Pneumatic signalling--Automatic
signalling 200
Chapter XII.--OPTICS.
Lenses--The image cast by a convex lens--Focus--Relative position
of object and lens--Correction of lenses for colour--Spherical
aberration--Distortion of image--The human eye--The use of
spectacles--The blind spot 230
Chapter XIII.--THE MICROSCOPE, THE TELESCOPE,
AND THE MAGIC-LANTERN.
The simple microscope--Use of the simple microscope in the telescope--The
terrestrial telescope--The Galilean telescope--The
prismatic telescope--The reflecting telescope--The parabolic
mirror--The compound microscope--The magic-lantern--The
bioscope--The plane mirror 253
Chapter XIV.--SOUND AND MUSICAL INSTRUMENTS.
Nature of sound--The ear--Musical instruments--The vibration of
strings--The sounding-board and the frame of a piano--The
strings--The striking mechanism--The quality of a note 270
Chapter XV.--WIND INSTRUMENTS.
Longitudinal vibration--Columns of air--Resonance of columns of
air--Length and tone--The open pipe--The overtones of an
open pipe--Where overtones are used--The arrangement of the
pipes and pedals--Separate sound-boards--Varieties of stops--Tuning
pipes and reeds--The bellows--Electric and pneumatic
actions--The largest organ in the world--Human reeds 287
Chapter XVI.--TALKING-MACHINES.
The phonograph--The recorder--The reproducer--The gramophone--The
making of records--Cylinder records--Gramophone
records 310
Chapter XVII.--WHY THE WIND BLOWS.
Why the wind blows--Land and sea breezes--Light air and moisture--The
barometer--The column barometer--The wheel barometer--A
very simple barometer--The aneroid barometer--Barometers
and weather--The diving-bell--The diving-dress--Air-pumps--Pneumatic
tyres--The air-gun--The self-closing door-stop--The
action of wind on oblique surfaces--The balloon--The
flying-machine 322
Chapter XVIII.--HYDRAULIC MACHINERY.
The siphon--The bucket pump--The force-pump--The most marvellous
pump--The blood channels--The course of the blood--The
hydraulic press--Household water-supply fittings--The
ball-cock--The water-meter--Water-supply systems--The household
filter--Gas traps--Water engines--The cream separator--The
"hydro" 350
Chapter XIX.--HEATING AND LIGHTING.
The hot-water supply--The tank system--The cylinder system--How
a lamp works--Gas and gasworks--Automatic stoking--A
gas governor--The gas meter--Incandescent gas lighting 386
Chapter XX.--VARIOUS MECHANISMS.
CLOCKS AND WATCHES:--A short history of timepieces--The construction
of timepieces--The driving power--The escapement--Compensating
pendulums--The spring balance--The cylinder
escapement--The lever escapement--Compensated balance-wheels--Keyless
winding mechanism for watches--The hour hand
train. LOCKS:--The Chubb lock--The Yale lock. THE CYCLE:--The
gearing of a cycle--The free wheel--The change-speed gear.
AGRICULTURAL MACHINES:--The threshing-machine--Mowing-machines.
SOME NATURAL PHENOMENA:--Why sun-heat varies
in intensity--The tides--Why high tide varies daily 410
HOW IT WORKS.
Chapter I.
THE STEAM-ENGINE.
What is steam?--The mechanical energy of steam--The boiler--The
circulation of water in a boiler--The enclosed furnace--The
multitubular boiler--Fire-tube boilers--Other types of
boilers--Aids to combustion--Boiler fittings--The safety-valve--The
water-gauge--The steam-gauge--The water supply to a boiler.
WHAT IS STEAM?
If ice be heated above 32° Fahrenheit, its molecules lose their
cohesion, and move freely round one another--the ice is turned into
water. Heat water above 212° Fahrenheit, and the molecules exhibit a
violent mutual repulsion, and, like dormant bees revived by spring
sunshine, separate and dart to and fro. If confined in an air-tight
vessel, the molecules have their flights curtailed, and beat more and
more violently against their prison walls, so that every square inch of
the vessel is subjected to a rising pressure. We may compare the action
of the steam molecules to that of bullets fired from a machine-gun at a
plate mounted on a spring. The faster the bullets came, the greater
would be the continuous compression of the spring.
THE MECHANICAL ENERGY OF STEAM.
If steam is let into one end of a cylinder behind an air-tight but
freely-moving piston, it will bombard the walls of the cylinder and the
piston; and if the united push of the molecules on the one side of the
latter is greater than the resistance on the other side opposing its
motion, the piston must move. Having thus partly got their liberty, the
molecules become less active, and do not rush about so vigorously. The
pressure on the piston decreases as it moves. But if the piston were
driven back to its original position against the force of the steam, the
molecular activity--that is, pressure--would be restored. We are here
assuming that no heat has passed through the cylinder or piston and been
radiated into the air; for any loss of heat means loss of energy, since
heat _is_ energy.
THE BOILER.
The combustion of fuel in a furnace causes the walls of the furnace to
become _hot_, which means that the molecules of the substance forming
the walls are thrown into violent agitation. If the walls are what are
called "good conductors" of heat, they will transmit the agitation
through them to any surrounding substance. In the case of the ordinary
house stove this is the air, which itself is agitated, or grows warm. A
steam-boiler has the furnace walls surrounded by water, and its function
is to transmit molecular movement (heat, or energy) through the furnace
plates to the water until the point is reached when steam generates. At
atmospheric pressure--that is, if not confined in any way--steam would
fill 1,610 times the space which its molecules occupied in their watery
formation. If we seal up the boiler so that no escape is possible for
the steam molecules, their motion becomes more and more rapid, and
_pressure_ is developed by their beating on the walls of the boiler.
There is theoretically no limit to which the pressure may be raised,
provided that sufficient fuel-combustion energy is transmitted to the
vaporizing water.
To raise steam in large quantities we must employ a fuel which develops
great heat in proportion to its weight, is readily procured, and cheap.
Coal fulfils all these conditions. Of the 800 million tons mined
annually throughout the world, 400 million tons are burnt in the
furnaces of steam-boilers.
A good boiler must be--(1) Strong enough to withstand much higher
pressures than that at which it is worked; (2) so designed as to burn
its fuel to the greatest advantage.
Even in the best-designed boilers a large part of the combustion heat
passes through the chimney, while a further proportion is radiated from
the boiler. Professor John Perry[1] considers that this waste amounts,
under the best conditions at present obtainable, to eleven-twelfths of
the whole. We have to burn a shillingsworth of coal to capture the
energy stored in a pennyworth. Yet the steam-engine of to-day is three
or four times as efficient as the engine of fifty years ago. This is due
to radical improvements in the design of boilers and of the machinery
which converts the heat energy of steam into mechanical motion.
CIRCULATION OF WATER IN A BOILER.
If you place a pot filled with water on an open fire, and watch it when
it boils, you will notice that the water heaves up at the sides and
plunges down at the centre. This is due to the water being heated most
at the sides, and therefore being lightest there. The rising
steam-bubbles also carry it up. On reaching the surface, the bubbles
burst, the steam escapes, and the water loses some of its heat, and
rushes down again to take the place of steam-laden water rising.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
If the fire is very fierce, steam-bubbles may rise from all points at
the bottom, and impede downward currents (Fig. 1). The pot then "boils
over."
Fig. 2 shows a method of preventing this trouble. We lower into our pot
a vessel of somewhat smaller diameter, with a hole in the bottom,
arranged in such a manner as to leave a space between it and the pot
all round. The upward currents are then separated entirely from the
downward, and the fire can be forced to a very much greater extent than
before without the water boiling over. This very simple arrangement is
the basis of many devices for producing free circulation of the water in
steam-boilers.
We can easily follow out the process of development. In Fig. 3 we see a
simple U-tube depending from a vessel of water. Heat is applied to the
left leg, and a steady circulation at once commences. In order to
increase the heating surface we can extend the heated leg into a long
incline (Fig. 4), beneath which three lamps instead of only one are
placed. The direction of the circulation is the same, but its rate is
increased.
[Illustration: FIG. 3.]
A further improvement results from increasing the number of tubes (Fig.
5), keeping them all on the slant, so that the heated water and steam
may rise freely.
THE ENCLOSED FURNACE.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
Still, a lot of the heat gets away. In a steam-boiler the burning fuel
is enclosed either by fire-brick or a "water-jacket," forming part of
the boiler. A water-jacket signifies a double coating of metal plates
with a space between, which is filled with water (see Fig. 6). The fire
is now enclosed much as it is in a kitchen range. But our boiler must
not be so wasteful of the heat as is that useful household fixture. On
their way to the funnel the flames and hot gases should act on a very
large metal or other surface in contact with the water of the boiler, in
order to give up a due proportion of their heat.
[Illustration: FIG. 6.--Diagrammatic sketch of a locomotive type of
boiler. Water indicated by dotted lines. The arrows show the direction
taken by the air and hot gases from the air-door to the funnel.]
THE MULTITUBULAR BOILER.
[Illustration: FIG. 7.--The Babcock and Wilcox water-tube boiler. One
side of the brick seating has been removed to show the arrangement of
the water-tubes and furnace.]
To save room, boilers which have to make steam very quickly and at high
pressures are largely composed of pipes. Such boilers we call
multitubular. They are of two kinds--(1) _Water_-tube boilers; in which
the water circulates through tubes exposed to the furnace heat. The
Babcock and Wilcox boiler (Fig. 7) is typical of this variety. (2)
_Fire_-tube boilers; in which the hot gases pass through tubes
surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates
this form.
The Babcock and Wilcox boiler is widely used in mines, power stations,
and, in a modified form, on shipboard. It consists of two main
parts--(1) A drum, H, in the upper part of which the steam collects; (2)
a group of pipes arranged on the principle illustrated by Fig. 5. The
boiler is seated on a rectangular frame of fire-bricks. At one end is
the furnace door; at the other the exit to the chimney. From the furnace
F the flames and hot gases rise round the upper end of the sloping tubes
TT into the space A, where they play upon the under surface of H before
plunging downward again among the tubes into the space B. Here the
temperature is lower. The arrows indicate further journeys upwards into
the space C on the right of a fire-brick division, and past the down
tubes SS into D, whence the hot gases find an escape into the chimney
through the opening E. It will be noticed that the greatest heat is
brought to bear on TT near their junction with UU, the "uptake" tubes;
and that every succeeding passage of the pipes brings the gradually
cooling gases nearer to the "downtake" tubes SS.
The pipes TT are easily brushed and scraped after the removal of plugs
from the "headers" into which the tube ends are expanded.
Other well-known water-tube boilers are the Yarrow, Belleville,
Stirling, and Thorneycroft, all used for driving marine engines.
FIRE-TUBE BOILERS.
Fig. 6 shows a locomotive boiler in section. To the right is the
fire-box, surrounded on all sides by a water-jacket in direct
communication with the barrel of the boiler. The inner shell of the
fire-box is often made of copper, which withstands the fierce heat
better than steel; the outer, like the rest of the boiler, is of steel
plates from 1/2 to 3/4 inch thick. The shells of the jacket are braced
together by a large number of rivets, RR; and the top, or crown, is
strengthened by heavy longitudinal girders riveted to it, or is braced
to the top of the boiler by long bolts. A large number of fire-tubes
(only three are shown in the diagram for the sake of simplicity) extend
from the fire-box to the smoke-box. The most powerful "mammoth" American
locomotives have 350 or more tubes, which, with the fire-box, give 4,000
square feet of surface for the furnace heat to act upon. These tubes
are expanded at their ends by a special tool into the tube-plates of the
fire-box and boiler front. George Stephenson and his predecessors
experienced great difficulty in rendering the tube-end joints quite
water-tight, but the invention of the "expander" has removed this
trouble.
The _fire-brick arch_ shown (Fig. 6) in the fire-box is used to deflect
the flames towards the back of the fire-box, so that the hot gases may
be retarded somewhat, and their combustion rendered more perfect. It
also helps to distribute the heat more evenly over the whole of the
inside of the box, and prevents cold air from flying directly from the
firing door to the tubes. In some American and Continental locomotives
the fire-brick arch is replaced by a "water bridge," which serves the
same purpose, while giving additional heating surface.
The water circulation in a locomotive boiler is--upwards at the fire-box
end, where the heat is most intense; forward along the surface;
downwards at the smoke-box end; backwards along the bottom of the
barrel.
OTHER TYPES OF BOILERS.
For small stationary land engines the _vertical_ boiler is much used.
In Fig. 8 we have three forms of this type--A and B with cross
water-tubes; C with vertical fire-tubes. The furnace in every case is
surrounded by water, and fed through a door at one side.
[Illustration: FIG. 8.--Diagrammatic representation of three types of
vertical boilers.]
The _Lancashire_ boiler is of large size. It has a cylindrical shell,
measuring up to 30 feet in length and 7 feet in diameter, traversed from
end to end by two large flues, in the rear part of which are situated
the furnaces. The boiler is fixed on a seating of fire-bricks, so built
up as to form three flues, A and BB, shown in cross section in Fig. 9.
The furnace gases, after leaving the two furnace flues, are deflected
downwards into the channel A, by which they pass underneath the boiler
to a point almost under the furnace, where they divide right and left
and travel through cross passages into the side channels BB, to be led
along the boiler's flanks to the chimney exit C. By this arrangement the
effective heating surface is greatly increased; and the passages being
large, natural draught generally suffices to maintain proper combustion.
The Lancashire boiler is much used in factories and (in a modified form)
on ships, since it is a steady steamer and is easily kept in order.
[Illustration: FIG. 9.--Cross and longitudinal sections of a Lancashire
boiler.]
In marine boilers of cylindrical shape cross water-tubes and fire-tubes
are often employed to increase the heating surface. Return tubes are
also led through the water to the funnels, situated at the same end as
the furnace.
AIDS TO COMBUSTION.
We may now turn our attention more particularly to the chemical process
called _combustion_, upon which a boiler depends for its heat. Ordinary
steam coal contains about 85 per cent. of carbon, 7 per cent. of oxygen,
and 4 per cent. of hydrogen, besides traces of nitrogen and sulphur and
a small incombustible residue. When the coal burns, the nitrogen is
released and passes away without combining with any of the other
elements. The sulphur unites with hydrogen and forms sulphuretted
hydrogen (also named sulphurous acid), which is injurious to steel
plates, and is largely responsible for the decay of tubes and funnels.
More of the hydrogen unites with the oxygen as steam.
The most important element in coal is the carbon (known chemically by
the symbol C). Its combination with oxygen, called combustion, is the
act which heats the boiler. Only when the carbon present has combined
with the greatest possible amount of oxygen that it will take into
partnership is the combustion complete and the full heat-value (fixed by
scientific experiment at 14,500 thermal units per pound of carbon)
developed.
Now, carbon may unite with oxygen, atom for atom, and form _carbon
monoxide_ (CO); or in the proportion of one atom of carbon to _two_ of
oxygen, and form _carbon dioxide_ (CO_2). The former gas is
combustible--that is, will admit another atom of carbon to the
molecule--but the latter is saturated with oxygen, and will not burn,
or, to put it otherwise, is the product of _perfect_ combustion. A
properly designed furnace, supplied with a due amount of air, will cause
nearly all the carbon in the coal burnt to combine with the full amount
of oxygen. On the other hand, if the oxygen supply is inefficient, CO as
well as CO_2 will form, and there will be a heat loss, equal in
extreme cases to two-thirds of the whole. It is therefore necessary that
a furnace which has to eat up fuel at a great pace should be
artificially fed with air in the proportion of from 12 to 20 _pounds_ of
air for every pound of fuel. There are two methods of creating a violent
draught through the furnace. The first is--
The _forced draught_; very simply exemplified by the ordinary bellows
used in every house. On a ship (Fig. 10) the principle is developed as
follows:--The boilers are situated in a compartment or compartments
having no communication with the outer air, except for the passages down
which air is forced by powerful fans at a pressure considerably greater
than that of the atmosphere. There is only one "way out"--namely,
through the furnace and tubes (or gas-ways) of the boiler, and the
funnel. So through these it rushes, raising the fuel to white heat. As
may easily be imagined, the temperature of a stokehold, especially in
the tropics, is far from pleasant. In the Red Sea the thermometer
sometimes rises to 170° Fahrenheit or more, and the poor stokers have a
very bad time of it.
[Illustration: FIG. 10.--Sketch showing how the "forced draught" is
produced in a stokehold and how it affects the furnaces.]
[Illustration: SCENE IN THE STOKEHOLD OF A BATTLE-SHIP.]
The second system is that of the _induced draught_. Here air is
_sucked_ through the furnace by creating a vacuum in the funnel and in a
chamber opening into it. Turning to Fig. 6, we see a pipe through which
the exhaust steam from the locomotive's cylinders is shot upwards into
the funnel, in which, and in the smoke-box beneath it, a strong vacuum
is formed while the engine is running. Now, "nature abhors a vacuum," so
air will get into the smoke-box if there be a way open. There
is--through the air-doors at the bottom of the furnace, the furnace
itself, and the fire-tubes; and on the way oxygen combines with the
carbon of the fuel, to form carbon dioxide. The power of the draught is
so great that, as one often notices when a train passes during the
night, red-hot cinders, plucked from the fire-box, and dragged through
the tubes, are hurled far into the air. It might be mentioned in
parenthesis that the so-called "smoke" which pours from the funnel of a
moving engine is mainly condensing steam. A steamship, on the other
hand, belches smoke only from its funnels, as fresh water is far too
precious to waste as steam. We shall refer to this later on (p. 72).
BOILER FITTINGS.
The most important fittings on a boiler are:--(1) the safety-valve; (2)
the water-gauge; (3) the steam-gauge; (4) the mechanisms for feeding it
with water.
THE SAFETY-VALVE.
Professor Thurston, an eminent authority on the steam-engine, has
estimated that a plain cylindrical boiler carrying 100 lbs. pressure to
the square inch contains sufficient stored energy to project it into the
air a vertical distance of 3-1/2 miles. In the case of a Lancashire
boiler at equal pressure the distance would be 2-1/2 miles; of a
locomotive boiler, at 125 lbs., 1-1/2 miles; of a steam tubular boiler,
at 75 lbs., 1 mile. According to the same writer, a cubic foot of heated
water under a pressure of from 60 to 70 lbs. per square inch has _about
the same energy as one pound of gunpowder_.
Steam is a good servant, but a terrible master. It must be kept under
strict control. However strong a boiler may be, it will burst if the
steam pressure in it be raised to a certain point; and some device must
therefore be fitted on it which will give the steam free egress before
that point is reached. A device of this kind is called a _safety-valve_.
It usually blows off at less than half the greatest pressure that the
boiler has been proved by experiment to be capable of withstanding.
In principle the safety-valve denotes an orifice closed by an
accurately-fitting plug, which is pressed against its seat on the boiler
top by a weighted lever, or by a spring. As soon as the steam pressure
on the face of the plug exceeds the counteracting force of the weight
or spring, the plug rises, and steam escapes until equilibrium of the
opposing forces is restored.
On stationary engines a lever safety-valve is commonly employed (Fig.
11). The blowing-off point can be varied by shifting the weight along
the arm so as to give it a greater or less leverage. On locomotive and
marine boilers, where shocks and movements have to be reckoned with,
weights are replaced by springs, set to a certain tension, and locked up
so that they cannot be tampered with.
[Illustration: FIG. 11.--A LEVER SAFETY-VALVE. V, valve; S, seating; P,
pin; L, lever; F, fulcrum; W, weight. The figures indicate the positions
at which the weight should be placed for the valve to act when the
pressure rises to that number of pounds per square inch.]
Boilers are tested by filling the boilers quite full and (1) by heating
the water, which expands slightly, but with great pressure; (2) by
forcing in additional water with a powerful pump. In either case a
rupture would not be attended by an explosion, as water is very
inelastic.
The days when an engineer could "sit on the valves"--that is, screw them
down--to obtain greater pressure, are now past, and with them a
considerable proportion of the dangers of high-pressure steam. The
Factory Act of 1895, in force throughout the British Isles, provides
that every boiler for generating steam in a factory or workshop where
the Act applies must have a proper safety-valve, steam-gauge, and
water-gauge; and that boilers and fittings must be examined by a
competent person at least once in every fourteen months. Neglect of
these provisions renders the owner of a boiler liable to heavy penalties
if an explosion occurs.
One of the most disastrous explosions on record took place at the Redcar
Iron Works, Yorkshire, in June 1895. In this case, twelve out of fifteen
boilers ranged side by side burst, through one proving too weak for its
work. The flying fragments of this boiler, striking the sides of other
boilers, exploded them, and so the damage was transmitted down the line.
Twenty men were killed and injured; while masses of metal, weighing
several tons each, were hurled 250 yards, and caused widespread damage.
The following is taken from a journal, dated December 22, 1895:
"_Providence_ (_Rhode Island_).--A recent prophecy that a boiler would
explode between December 16 and 24 in a store has seriously affected the
Christmas trade. Shoppers are incredibly nervous. One store advertises,
'No boilers are being used; lifts running electrically.' All stores have
had their boilers inspected."
THE WATER-GAUGE.
No fitting of a boiler is more important than the _water-gauge_, which
shows the level at which the water stands. The engineer must continually
consult his gauge, for if the water gets too low, pipes and other
surfaces exposed to the furnace flames may burn through, with disastrous
results; while, on the other hand, too much water will cause bad
steaming. A section of an ordinary gauge is seen in Fig. 12. It consists
of two parts, each furnished with a gland, G, to make a steam-tight
joint round the glass tube, which is inserted through the hole covered
by the plug P^1. The cocks T^1 T^2 are normally open, allowing the
ingress of steam and water respectively to the tube. Cock T^3 is kept
closed unless for any reason it is necessary to blow steam or water
through the gauge. The holes C C can be cleaned out if the plugs P^2
P^3 are removed.
Most gauges on high-pressure boilers have a thick glass screen in front,
so that in the event of the tube breaking, the steam and water may not
blow directly on to the attendants. A further precaution is to include
two ball-valves near the ends of the gauge-glass. Under ordinary
conditions the balls lie in depressions clear of the ways; but when a
rush of steam or water occurs they are sucked into their seatings and
block all egress.
[Illustration: FIG. 12.--Section of a water-gauge.]
On many boilers two water-gauges are fitted, since any gauge may work
badly at times. The glasses are tested to a pressure of 3,000 lbs. or
more to the square inch before use.
THE STEAM-GAUGE.
It is of the utmost importance that a person in charge of a boiler
should know what pressure the steam has reached. Every boiler is
therefore fitted with one _steam-gauge_; many with two, lest one might
be unreliable. There are two principal types of steam-gauge:--(1) The
Bourdon; (2) the Schäffer-Budenberg. The principle of the Bourdon is
illustrated by Fig. 13, in which A is a piece of rubber tubing closed at
one end, and at the other drawn over the nozzle of a cycle tyre
inflator. If bent in a curve, as shown, the section of the tube is an
oval. When air is pumped in, the rubber walls endeavour to assume a
circular section, because this shape encloses a larger area than an oval
of equal circumference, and therefore makes room for a larger volume of
air. In doing so the tube straightens itself, and assumes the position
indicated by the dotted lines. Hang an empty "inner tube" of a pneumatic
tyre over a nail and inflate it, and you will get a good illustration of
the principle.
[Illustration: FIG. 13.--Showing the principle of the steam-gauge.]
[Illustration: FIG. 14.--Bourdon steam-gauge. Part of dial removed to
show mechanism.]
In Fig. 14 we have a Bourdon gauge, with part of the dial face broken
away to show the internal mechanism. T is a flattened metal tube
soldered at one end into a hollow casting, into which screws a tap
connected with the boiler. The other end (closed) is attached to a link,
L, which works an arm of a quadrant rack, R, engaging with a small
pinion, P, actuating the pointer. As the steam pressure rises, the tube
T moves its free end outwards towards the position shown by the dotted
lines, and traverses the arm of the rack, so shifting the pointer round
the scale. As the pressure falls, the tube gradually returns to its zero
position.
The Schäffer-Budenberg gauge depends for its action on the elasticity of
a thin corrugated metal plate, on one side of which steam presses. As
the plate bulges upwards it pushes up a small rod resting on it, which
operates a quadrant and rack similar to that of the Bourdon gauge. The
principle is employed in another form for the aneroid barometer (p.
329).
THE WATER SUPPLY TO A BOILER.
The water inside a boiler is kept at a proper level by (1) pumps or (2)
injectors. The former are most commonly used on stationary and marine
boilers. As their mechanism is much the same as that of ordinary force
pumps, which will be described in a later chapter, we may pass at once
to the _injector_, now almost universally used on locomotive, and
sometimes on stationary boilers. At first sight the injector is a
mechanical paradox, since it employs the steam from a boiler to blow
water into the boiler. In Fig. 15 we have an illustration of the
principle of an injector. Steam is led from the boiler through pipe A,
which terminates in a nozzle surrounded by a cone, E, connected by the
pipe B with the water tank. When steam is turned on it rushes with
immense velocity from the nozzle, and creates a partial vacuum in cone
E, which soon fills with water. On meeting the water the steam
condenses, but not before it has imparted some of its _velocity_ to the
water, which thus gains sufficient momentum to force down the valve and
find its way to the boiler. The overflow space O O between E and C
allows steam and water to escape until the water has gathered the
requisite momentum.
[Illustration: FIG. 15.--Diagram illustrating the principle of a
steam-injector.]
[Illustration: FIG. 16.--The Giffard injector.]
A form of injector very commonly used is Giffard's (Fig. 16). Steam is
allowed to enter by screwing up the valve V. As it rushes through the
nozzle of the cone A it takes up water and projects it into the "mixing
cone" B, which can be raised or lowered by the pinion D (worked by the
hand-wheel wheel shown) so as to regulate the amount of water admitted
to B. At the centre of B is an aperture, O, communicating with the
overflow. The water passes to the boiler through the valve on the left.
It will be noticed that the cone A and the part of B above the orifice O
contract downward. This is to convert the _pressure_ of the steam into
_velocity_. Below O is a cone, the diameter of which increases
downwards. Here the _velocity_ of the water is converted back into
_pressure_ in obedience to a well-known hydromechanic law.
An injector does not work well if the feed-water be too hot to condense
the steam quickly; and it may be taken as a rule that the warmer the
water, the smaller is the amount of it injected by a given weight of
steam.[2] Some injectors have flap-valves covering the overflow orifice,
to prevent air being sucked in and carried to the boiler.
When an injector receives a sudden shock, such as that produced by the
passing of a locomotive over points, it is liable to "fly off"--that is,
stop momentarily--and then send the steam and water through the
overflow. If this happens, both steam and water must be turned off, and
the injector be restarted; unless it be of the _self-starting_ variety,
which automatically controls the admission of water to the
"mixing-cone," and allows the injector to "pick up" of itself.
For economy's sake part of the steam expelled from the cylinders of a
locomotive is sometimes used to work an injector, which passes the water
on, at a pressure of 70 lbs. to the square inch, to a second injector
operated by high-pressure steam coming direct from the boiler, which
increases its velocity sufficiently to overcome the boiler pressure. In
this case only a fraction of the weight of high-pressure steam is
required to inject a given weight of water, as compared with that used
in a single-stage injector.
[1] "The Steam-Engine," p. 3.
[2] By "weight of steam" is meant the steam produced by boiling a
certain weight of water. A pound of steam, if condensed, would form a
pound of water.