represented at O in _fig._ 71., called the _man-hole_, is provided
to admit persons into the boiler for the purpose of cleaning or
repairing its interior.
(165.) The manner in which the governor regulates the supply of
steam from the boiler to the cylinder, proportioning the quantity
to the work to be done, and thereby sustaining a uniform motion,
has been already explained (p. 125.). Since then the _consumption_
of steam in the engine is subject to variation, owing to the
various quantities of work it may have to perform, it is evident
that the _production_ of steam in the boiler should be subject to
a proportional variation. For otherwise, one of two effects would
ensue: the boiler would either fail to supply the engine with
steam, or steam would accumulate in the boiler from being produced
in too great abundance, and would escape at the safety-valve, and
thus be wasted.
In order to vary the production of steam in proportion to the
demands of the engine, it is necessary to stimulate or mitigate
the furnace, as the evaporation is to be augmented or diminished.
The activity of the furnace must depend on the current of air
which is drawn through the grate-bars, and this will depend on the
magnitude of the space afforded for the passage of that current
through the flues. A plate called a _damper_ is accordingly placed
with its plane at right angles to the flue, so that by raising and
lowering it in the same manner as the sash of a window is raised
or lowered, the space allowed for the passage of air through the
flue may be regulated. This plate might be regulated by the hand,
so that by raising or lowering it the draught might be increased
or diminished, and a corresponding effect produced on the [Pg282]
evaporation in the boiler: but the force of the fire is rendered
uniformly proportional to the rate of evaporation by the following
arrangement, without the intervention of the engineer. The column
of water sustained in the feed-pipe (_figs._ 71, 72.) represents
by its weight the difference between the pressure of steam within
the boiler and that of the atmosphere. If the engine consumes
steam faster than the boiler produces it, the steam contained in
the boiler acquires a diminished pressure, and consequently the
column of water in the feed-pipe will fall. If, on the other hand,
the boiler produce steam faster than the engine consumes it, the
accumulation of steam in the boiler will cause an increased
pressure on the water it contains, and thereby increase the height
of the column of water sustained in the feed-pipe. This column
therefore necessarily rises and falls with every variation in the
rate of evaporation in the boiler. A hollow float P is placed upon
the surface of the water of this column; a chain connected with
this float is carried upwards, and passed over two pulleys, after
which it is carried downwards through an aperture leading to the
flue which passes beside the boiler: to this chain is attached the
damper. By such an arrangement it is evident that the damper will
rise when the float P falls, and will fall when the float P rises,
since the weight of the damper is so adjusted, that it will only
balance the float P when the latter rests on the surface of the
water.
Whenever the evaporation of the boiler is insufficient, it is
evident from what has been stated, that the float P will fall and
the damper will rise, and will afford a greater passage for air
through the flue. This will stimulate the furnace, will augment
its heating power, and will therefore increase the rate of
evaporation in the boiler. If, on the other hand, the production
of steam in the boiler be more than is requisite for the supply of
the engine, the float will be raised and the damper let down, so
as to contract the flue, to diminish the draught, to mitigate the
fire, and therefore to check the evaporation. In this way the
excess, or defect, of evaporation in the boiler is made to act
upon the fire, so as to render the heat proceeding from the
combustion as nearly as possible proportional to the wants of the
engine. [Pg283]
(166.) The method of feeding the furnace by hand through the
fire-door being subject to the double objection of admitting more
cold air over the fuel than is necessary for its combustion, and
the impracticability of insuring that regular attendance on the
part of the stokers, directed the attention of engineers to the
construction of self-regulating furnaces. The most effectual of
these, and that which has come into most general use, was invented
by Mr. William Brunton of Birmingham.
The advantages proposed to be attained by him were those expressed
in his patent:—
"First, I put the coal upon the grate by small quantities, and at
very short intervals, say every two or three seconds. 2dly, I so
dispose of the coals upon the grate, that the smoke evolved must
pass over that part of the grate upon which the coal is in full
combustion, and is thereby consumed. 3dly, As the introduction of
coal is uniform in short spaces of time, the introduction of air
is also uniform, and requires no attention from the fireman.
"As it respects economy: 1st, The coal is put upon the fire by an
apparatus driven by the engine, and so contrived that the quantity
of coal is proportioned to the quantity of work which the engine
is performing; and the quantity of air admitted to consume the
smoke is regulated in the same manner. 2dly, The fire-door is
never opened, excepting to clean the fire; the boiler, of course,
is not exposed to that continual irregularity of temperature which
is unavoidable in the common furnace, and which is found
exceedingly injurious to boilers. 3dly, The only attention
required is to fill the coal-receiver every two or three hours,
and clean the fire when necessary. 4thly, The coal is more
completely consumed than by the common furnace, as all the effect
of what is termed stirring up the fire (by which no inconsiderable
quantity of coal is passed into the ash-pit), is attained without
moving the coal upon the grate."
A circular grate is placed on a vertical revolving shaft; on the
lower part of this shaft, under the ash-pit, is placed a toothed
wheel driven by a pinion. This pinion is placed on another vertical
shaft, which ascends above the boiler; and [Pg284] on the other end
of this is placed a bevelled wheel driven by a pinion. This pinion
is attached to a shaft, which takes its motion from the axis of the
fly-wheel, or any other revolving shaft connected with the engine. A
constant motion of revolution is therefore imparted to the circular
grate, and its velocity being proportional to that of the engine,
will necessarily be also proportional to the quantity of fuel which
ought to be consumed. Through that part of the boiler which is over
the fire-grate a vertical tube or opening is made directly over that
part of the furnace which is most distant from the flues. Over this
opening a hopper is placed, which contains the fuel by which the
boiler is to be fed; and in the bottom of this hopper is a sliding
valve, capable of being opened or closed, so as to regulate the
quantity of fuel supplied to the fire-grate. The fuel dropping in in
small quantities through this open valve falls on the grate, and is
carried round by it, so as to leave a fresh portion of the grate to
receive succeeding feeds. The coals admitted through the hopper are
previously broken to a proper size; and in some forms of this
apparatus there are two rollers, at a regulated distance asunder,
the surfaces of which are formed into blunt angular points, and
which are kept in slow revolution by the engine. Between these
rollers the coals must pass before they reach the valve through
which the furnace is fed, and they are thus broken and reduced to a
regulated size. The valve which regulates the opening through which
the feed is admitted, is connected by chains and pulleys with the
self-regulating damper already described, so that in proportion as
the damper is raised, the valve governing the feed may be opened.
Thus, while the quantity of air admitted by the damper is increased
according to the demands of the engine, the quantity of fuel
admitted for the feed is increased by opening the valve in the
bottom of the hopper in the same proportion. Apertures are also
provided in the front of the grate, governed by regulators, by which
the quantity of air necessary and sufficient to produce the
combustion of the gas evolved from the fuel is admitted, these
openings being also connected with the self-regulating damper.
A considerable portion of the heat imparted to the water [Pg285]
in the boiler escapes by radiation from the surface of the boiler,
steam-pipes, and other parts of the machinery in contact with the
steam and hot water. The effects of this are rendered very
apparent in marine engines, where a large quantity of water is
found to be condensed in the great steam-pipes leading from the
boiler to the cylinder. In stationary land boilers this loss of
heat is usually diminished, and in some cases in a great degree
removed, by surrounding the boiler with non-conducting substances.
In some cases the boiler is built round in brick work. In
Cornwall, where the economy is regarded perhaps to a greater
extent than elsewhere, the boiler and steam-pipes are surrounded
with a packing of sawdust, which being almost a non-conductor of
heat, is impervious to the heat proceeding from the surfaces with
which it is in contact, and consequently confines all the heat
within the boiler. In marine boilers it has been the practice
recently to clothe the boiler and steam-pipes with a coating of
felt, which is attended with a similar effect. When these remedies
are properly applied, the loss of heat proceeding from the
radiation of the boiler is reduced to an extremely small amount.
The engine-houses of some of the Cornish engines, where the boiler
generates steam at a very high temperature, are nevertheless
frequently maintained at a lower temperature than the external
air, and on entering them they have in a great degree the effect
of a cave.
(167.) All mechanical action is measured by the amount of force
exercised, or resistance overcome, and the space through which
that force has acted, or through which the resistance has been
moved.
The gross amount of mechanical action developed by the moving
power of an engine, is expended partly on moving the engine
itself, and partly on overcoming the resistance on which the
engine is intended to act. That part of the mechanical energy of
the moving power which is expended on the resistance or load which
the engine moves exclusively, and of the power expended on moving
the engine itself, is called _the useful effect_ of the machine.
The _gross effect_, therefore, exceeds the _useful effect_ by the
[Pg286] amount of power spent in moving the engine, or which may
be wasted or destroyed in any way by the engine.
It is usual to express and estimate all mechanical effect whatever
by nature of the resistance overcome, by an equivalent weight
raised a certain height. Thus, if an engine exerts a certain power
in driving a mill, in drawing a carriage on a road, or in
propelling a vessel on water, the resistance against which it has
to act must be equal to a definite amount of weight. If a carriage
be drawn, the traces are stretched by the tractive power, by the
same tension that would be given to them if a certain weight were
appended to them. If the paddle-wheels of a boat are made to
revolve, the water opposes to them a resistance equal to that
which would be produced, if instead of moving the water the wheel
had to raise some certain weight. In any case, therefore, weight
becomes the exponent of the energy of the resistance against which
the moving power acts.
But the amount of mechanical effect depends conjointly on the
amount of resistance, and the space through which that resistance
is moved. The quantity of this effect, therefore, will be
increased in the same proportion, whether the quantity of
resistance or the space through which that resistance is moved be
augmented. Thus, a resistance of one hundred pounds, moved through
two feet, is mechanically equivalent to a resistance of two
hundred pounds moved through one foot, or of four hundred pounds
moved through six inches. To simplify, therefore, the expression
of mechanical effect, it is usual to reduce it invariably to a
certain weight raised one foot. If the resistance under
consideration be equivalent to a certain weight raised through ten
feet, it is always expressed by ten times the amount of that
weight raised through one foot.
It has also been usual in the expression of mechanical effect, to
take the pound weight as the unit of weight, and the foot as the
unit of length, so that all mechanical effect whatsoever is
expressed by a certain number of pounds raised one foot.
(168.) The gross effect of the moving power in a steam-engine, is
the whole mechanical force developed by the evaporation [Pg287]
of water in the boiler. A part of this effect is lost by the
partial condensation of the steam before it acts upon the piston,
and by the imperfect condensation of it subsequently: another
portion is expended on overcoming the friction of the different
moving parts, and in acting against the resistance which the air
opposes to the machine. If the motion be subject to sudden shocks,
a portion of the power is then lost by the destruction of momentum
which such shocks produce. But if those parts of the machine which
have a reciprocating motion be, as they ought to be, brought
gradually to rest at each change of direction, then no power is
absorbed in this way.
(169.) The useful effect of an engine is variously denominated
according to the relation under which it is considered. If it be
referred to the time during which it is produced, it is called
POWER.
(170.) If it be referred to the fuel, by the combustion of which
the evaporation has been effected, it is called DUTY.
(171.) When steam-engines were first brought into use, they were
commonly applied to work pumps for mills which had been previously
worked or driven by horses. In forming their contracts, the first
steam-engine builders found themselves called upon to supply
engines capable of executing the same work as was previously
executed by some certain number of horses. It was therefore
convenient, and indeed necessary, to be able to express the
performance of these machines by comparison with the animal power
to which manufacturers, miners, and others, had been so long
accustomed. When an engine, therefore, was capable of performing
the same work in a given time as any given number of horses of
average strength usually performed, it was said to be an engine of
so many horses' power. Steam-engines had been in use for a
considerable time before this term had acquired any settled or
uniform meaning, and the nominal power of engines was accordingly
very arbitrary. At length, however, the use of steam-engines
became more extended, and the confusion and inconvenience arising
out of all questions respecting the performance of engines,
rendered it necessary that some fixed [Pg288] and definite
meaning should be assigned to the terms by which the powers of
this machine were expressed. To have abandoned the term
_horse-power_, which had been so long in use, would have been
obviously inconvenient; nor could there be any objection to its
continuance, provided all engine-makers, and all those who used
engines, could be brought to agree upon some standard by which the
unit of horse-power might be defined. The performance of a horse
of average strength working for eight hours a day was therefore
selected as a standard, or unit, of steam-engine power. Smeaton
estimated that such an animal, so working, was capable of
performing a quantity of work equal in its mechanical effect to
22,916 lbs. raised one foot per minute, while Desaguliers
estimated the same power at 27,500 lbs. raised through the same
height in the same time. The discrepancy between these estimates
probably arose from their being made from the performances of
different classes of horses. Messrs. Boulton and Watt caused
experiments to be made with the strong horses used in the
breweries in London, and from the result of these trials they
assigned 33,000 lbs. raised one foot per minute, as the value of a
horse's power. This is the unit of engine-power now universally
adopted; and when an engine is said to be of so many horses'
power, what is meant is, that that engine, in good working order
and properly managed, is capable of moving a resistance equal to
33,000 lbs. through one foot per minute. Thus an engine of ten
horse-power is one that would raise 330,000 lbs. weight one foot
per minute.
Whether this estimate of an average horse's power be correct or
not, in reference to the actual work which the animal is capable
of executing, is a matter of no present importance in its
application to steam-power. The steam-engine is no longer used to
replace the power of horses, and therefore no contracts are based
upon such a comparison. The term horse-power, therefore, as
applied to steam-engines, must be understood to have no reference
whatever to the actual animal power, but must be taken as a term
having no other meaning than the expression of the ability of the
[Pg289] machine to move the amount of resistance above mentioned
through one foot per minute.
(172.) It has been already explained (67.) that the conversion of
a given volume of water into steam is productive of a certain
definite amount of mechanical force, this amount depending on the
pressure under which the water is evaporated, and the extent to
which the expansive principle is used in working the steam. It is
evident that this amount of mechanical effect is a major limit,
which cannot be exceeded by the power of the engine.
If the steam be not worked expansively, then the whole power of
the water, transmitted in the form of steam from the boiler to the
working machinery, will be a matter of easy calculation, when the
pressure at which the steam is worked is known. A table,
exhibiting the mechanical power of a cubic foot of water converted
into steam at various pressures, expressed in an equivalent number
of pounds' weight raised one foot high, is given in the Appendix
to this volume. Where much accuracy is sought for, the pressure at
which the steam is used must be taken into account; but by
reference to the table it will be seen, that when steam is worked
without expansion, its mechanical effect varies very little with
the pressure. It may therefore be assumed, as has been already
stated, that for every cubic inch of water transmitted in the form
of steam to the cylinders, a force is produced, represented by a
ton weight raised a foot high. Now, as 33,000 lbs. is very nearly
15 tons, it follows that 15 cubic inches of water converted into
steam per minute, or 900 cubic inches per hour, will produce a
mechanical force equal to one horse. If, therefore, to 900 cubic
inches be added the quantity of water per hour necessary to move
the engine itself, independently of its load, we shall obtain the
quantity of water per hour which must be supplied by the boiler to
the engine for each horse-power, and this will be the same
whatever may be the magnitude or proportions of the cylinder.
(173.) The quantity of power expended in working the engine
itself, independently of that required to move its load, will be
less in proportion to the degree of perfection which [Pg290] may
be attained in the construction of the engine, and to the order in
which it is kept while working. Engines vary one from another so
much in these respects, that it is scarcely possible to lay down
any general rules for the quantity of power to be allowed over and
above what is necessary to move the load. The means whereby
mechanical power is expended in working the engine may be
enumerated as follows:—
_First._ Steam in passing from the boiler to the cylinder is
liable to lose its temperature by the radiation of the steam-pipes
and other passages through which it is conducted. Since the steam
produced in the boiler is in contact with water, it will be common
steam (94.), and consequently the least loss of heat will cause a
partial condensation. To whatever extent this condensation may be
carried, a proportional loss of power, in reference to the heat
obtained from the fuel, will be entailed upon the engine.
It has been said that the force necessary to move the steam from
the boiler to the cylinder through passages more or less
contracted, subject to the friction of the pipes and tubes through
which it moves, should be taken into account in estimating the
power, and a corresponding deduction made. This, however, is not
the case: the steam having passed into the cylinder remains common
steam, its pressure being diminished by reason of the force
expended in thus moving it from the boiler to the cylinder. But
its mechanical efficacy at the reduced pressure is not sensibly
different from the efficacy which it had in the boiler. If at the
reduced pressure its volume were the same, then a loss of effect
would be sustained equivalent to the difference of the pressures;
but its volume being augmented in very nearly the same proportion
as its pressure is diminished, the mechanical efficacy of a given
weight of steam in the cylinder will be sensibly the same as in
the boiler.
_Second._ The radiation of heat from the cylinder and its
appendages, will cause a partial condensation of steam, and
thereby produce a diminished mechanical effect.
_Third._ The steam, which at each stroke of the piston fills the
passages between the steam-valves and the piston, at the [Pg291]
moment the latter commences the stroke will be inefficient. If it
were possible for the piston to come into steam-tight contact with
each end of the cylinder, and that the steam-valve should be in
immediate contact with the side or top of the piston, then the
whole of the steam which would pass through the steam-valve would
be efficient; but as some space, however small, must remain
between the piston and the ends of the cylinder, and between the
side of the cylinder and the steam-valve, there will always be a
volume of steam bearing a sensible proportion to the magnitude of
the cylinder, which at each stroke of the piston will be
inefficient. This volume of steam is called the _clearance_.
_Fourth._ Since the piston must move in steam-tight contact with
the cylinder, it must have a definite amount of friction with the
sides of the cylinder by whatever means it may be packed. This
friction will produce a corresponding resistance to the moving
power.
_Fifth._ The various joints of the machinery where steam is
contained are subject to leakage, and whatever amount of steam
shall thus escape must be placed to the account of power lost.
_Sixth._ When the eduction-valve is opened to admit the steam to
the condenser, a certain force is required to expel the steam from
the cylinder. This force reacts upon the piston, and counteracts
to a proportional extent the moving power of the steam on the
other side. Besides this the water in the condenser cannot be
conveniently reduced below the temperature of about 100°, and at
this temperature steam has a pressure of about 1 lb. per square
inch. This vapour will continue to fill the cylinder, and will
resist the moving power which impels the piston.
_Seventh._ Power must be provided for opening and closing the
valves or slides, for working the air-pump, hot-water pump, and
cold-water pump, and finally to overcome the friction on the
journals and centres of the parts of the parallel motion, the main
axle of the beam, the connecting rod, crank, and fly-wheel axle.
It will be apparent how very much these sources of resistances
must vary in different engines, and how rough [Pg292] an
approximation any general estimate must be of their gross amount.