Fig. 56.

(137.)

Of the cocks or valves which are opened and closed by the motion of an axis passing through their centre, the throttle-valve, whether worked by hand or by the governor, is an example. But the most common form for cocks is that of a cylindrical or slightly conical plug (fig. 56.), inserted in an aperture of corresponding magnitude passing across the pipe or passage which the cock is intended to open or close. One or more holes are pierced transversely in the cock, and when the cock is turned so that these holes run in the direction of the tube, the passage through the tube is opened; but when the passage through the cock is placed at right angles to the tube, then the sides of the tube stop the ends of the passage in the cock, and the passage through the tube is obstructed. The simple cock is designed to open or close the passage through a single tube. When the cock is turned, as in fig. 57., so that the passage through the cock shall be at right angles to the length of the tube, then the passage through the tube is stopped; but when the cock is turned from that position through a quarter of a revolution, as in fig. 58., then the passage through the cock takes the direction of the passage through the tube, and the cock is opened, and the passage through the tube unobstructed. In such a cock the passage may be more or less throttled by [Pg239] adjusting the position of the cock, so that a part of the opening in it shall be covered by the side of the tube.
Figs. 57., 58.

(138.)

It is sometimes required to put one tube or passage alternately in communication with two others. This is accomplished by a two-way cock. In this cock the passage is curved, opening usually at points on the surface of the cock, at right angles to each other. Such a cock has already been described, and its use illustrated in the description of the Marquis of Worcester's engine (17.); the two-way cock, as represented at K and R (fig. 4.), being the means by which steam and water are alternately supplied to the two forcing vessels.
Figs. 59., 60.

(139.)

When it is required to put four passages alternately in communication by pairs, a four-way cock is used. Such a cock has two curved passages (fig. 59.), each similar to the curved passage in the two-way cock. Let S C B T be the four tubes which it is required to throw alternately into communication by pairs. When the cock is in the position fig. 59., the tube S communicates with T, and the tube C with B. By turning the cock through a quarter of a revolution, as in fig. 60., the tube S is made to communicate with B, and the tube C with T; and if the cock continue to be turned at intervals through a quarter of a revolution, these changes of communication will continue to be alternately made. It is evident that this may be accomplished by turning the cock continually in the same direction.

The four-way cock is sometimes used as a substitute for the valves or slides in a double-acting steam engine to conduct the steam to and from the cylinder. If S represent a pipe conducting steam from the boiler, C that which leads to the condenser, T the tube which leads to the top of the cylinder, and B that which leads to the bottom, then when the cock is in the position (fig. 59.), steam would flow from the boiler to [Pg240] the top of the piston, while the steam below it would be drawn off to the condenser; and in the position (fig. 60.), steam would flow from the boiler to the bottom of the piston, while the steam above it would be drawn off to the condenser. Thus by turning the cock through a quarter of a revolution towards the termination of each stroke, the operation of the machine would be continued.

One of the disadvantages which is inseparable from the use of a four-way cock for this purpose is the loss of the steam at each stroke, which fills the tubes between the cock and the ends of the cylinder. This disadvantage could only be avoided by the substitution of two two-way cocks (138.) instead of a four-way cock. A two-way cock at the top of the cylinder would open an alternate communication between the cylinder and steam pipe, and the cylinder and condenser, while a similar office would be performed by another two-way cock at the other end.

The friction on cocks of this description is more than on other valves; but this is in some degree compensated by the great simplicity of the instrument. When the cock is truly ground into its seat, being slightly conical in its form, the pressure of the steam has a tendency to keep the surfaces in contact; but this pressure also increases the friction, and has a tendency to wear the seat of the cock into an elliptical shape. Consequently, such cocks require to be occasionally ground and refitted.

(140.)

The four-way cock, as above described, admits the steam to one end of the piston at the same moment that it stops it at the other end. It would therefore be inapplicable where steam is worked expansively. A slight modification, however, analogous to that already described in the slides, will adapt it to expansive action. This will be accomplished by giving to one of the passages through the cock one aperture larger than the other, and working the cock so that this passage shall always be used to conduct steam to the cylinder; also by enlarging both apertures of the other passage, and using it always to conduct steam from the cylinder. The effect of such an arrangement will be readily understood.
Figs. 61., 62.

Let the position of the cock at the commencement of the [Pg241] descending stroke be represented in fig. 61. Steam flows from S through T to the top of the cylinder, while it escapes from B through C from the bottom of the cylinder. When the piston has arrived at that point at which the steam is to be cut off, let the cock be shifted to the position represented in fig. 62. The passage of steam from the boiler is now stopped, but the escape of steam from the bottom of the cylinder through C continues, and the cock is maintained in this position until the piston approaches the bottom of the cylinder, when it is further shifted to the position represented in fig. 63. Steam now flows from S through B to the bottom of the cylinder, while the steam from the top of the cylinder escapes through C to the condenser. When the piston has arrived at that point where the steam is to be cut off, the cock is shifted to the position represented in fig. 64. The communication between the steam and the bottom of the piston is now stopped, while the communication between the top of the cylinder and the condenser is still open. During the next double stroke of the piston the position of the cock is similarly changed, but in the contrary direction, and in the same way the motion is continued. Under these circumstances the cock, instead [Pg242] of being moved constantly in the same direction, as in the case of the common four-way cock, will require to be moved alternately in opposite directions.

Figs. 63., 64.

Pistons.

(141.)

The office of a piston being to divide a cylinder into two compartments by a movable partition which shall obstruct the passage of any fluid from one compartment to the other, it is evident that the two conditions which such an instrument ought to fulfil are, first, that the contact of its sides with the surface of the cylinder shall be so close and tight throughout its entire play that no steam or other fluid can pass between them; secondly, that it shall be so free from friction, notwithstanding this necessary tightness, that it shall not absorb any injurious quantity of the moving power.

Since, however accurately the surfaces of the piston and cylinder may be constructed, there will always be in practice more or less imperfection of form, it is evident that the contact of the surface of the piston with the cylinder throughout the stroke can only be maintained by giving to the circumference of the piston sufficient elasticity to accommodate itself to such inequalities of form. The substance, whatever it may be, used for this purpose, and by which the piston is surrounded, is called packing.

In steam pistons the material used for packing must be such as is capable of resisting the united effects of heat and moisture. Hence leather and other animal substances are inapplicable.

The packing used for steam pistons is therefore of two kinds, vegetable packing, usually hemp, or metallic packing.

The common hemp-packed piston has been already in part described (79.). The bottom of the piston is a circular plate just so much less in diameter than the cylinder as is sufficient to allow its free motion in ascending and descending. A little above its lowest point this plate begins gradually to diminish in thickness, until its diameter is reduced to from one to two inches less than that of the cylinder, leaving therefore around [Pg243] it a hollow space, as represented in fig. 65. The cover of the piston is a plate similarly formed, being in like manner gradually reduced in thickness downwards, so as to correspond with the lower plate. In the hollow space which thus surrounds the piston a packing of unspun hemp or soft rope, called gasket, is introduced by winding it round the piston so as to render it an even and compact mass. When the space is thus filled up, the top of the piston is attached to the bottom by screws. The curved form of the space within which the hempen packing is confined is such that when the screws are tightened, that part of the packing which is nearest to the top and bottom of the piston is forced against the cylinder, so as to produce upon the two parallel rings as much pressure as is necessary to render it steam-tight. When by use the packing is worn down so as to produce leakage, the cover of the cylinder must be removed, and the screws connecting the top and bottom of the piston tightened: this will force out the packing and render the piston steam-tight. This packing is lubricated by melted tallow let down upon the piston from the funnel inserted in the top of the cylinder, furnished with a stop-cock to prevent the escape of steam. The lower end of the piston-rod is formed slightly conical, the thickest part of the cone being downward. It is passed up through the piston, and a nut or wedge between the top and bottom is inserted so as to secure the piston in its position upon the rod.

Fig. 65.

The process of removing the top of the cylinder for the purpose of tightening the screws in the piston is one of so laborious a nature, that the men entrusted with the superintendence of these machines are tempted to allow the engine to work notwithstanding injurious leakage at the piston, rather than incur the labour of tightening the screws as often as it is necessary to do so.

To avoid this inconvenience, the following method of [Pg244] tightening the packing of the piston without removing the lid of the cylinder, was contrived by Woolf. The head of each of the screws was formed into a toothed pinion, and as these screws were placed at equal distances from the centre of the piston, these several pinions were driven by a large toothed wheel, revolving on the piston-rod as an axis. By such an arrangement it is evident that if any one of the screws be turned, a like motion will be imparted to all the others through the medium of the large central wheel. Woolf accordingly formed, on the head of one of the screws, a square end. When the piston was brought to the top of the cylinder, this square end entered an aperture made in the under side of the cover of the cylinder. This aperture was covered by a small circular piece screwed into the top of the cylinder, which was capable of being removed so as to render the square head of the screw accessible. When this was done, a proper key being applied to the square head of the screw, it was turned; and by being turned, all the other screws were in like manner moved. In this way, instead of having to remove the cover of the cylinder, which in large cylinders was attended with great labour and loss of time, the packing was tightened by merely unscrewing a piece in the top of the cylinder not much greater in magnitude than the head of one of the screws.

This method was further simplified by causing the great circular wheel already described to move upon the piston-rod, not as an axis, but as a screw, the thread being cut upon a part of the piston-rod which worked in a corresponding female screw cut upon the central plate. By such means, the screw whose head was let into the cover of the cylinder which turned, would cause this circular plate to be pressed downwards by the force of the screw constructed on the piston-rod. This circular plate thus pressed downwards, acted upon pins or plugs which pressed together the top and bottom of the cylinder in the same manner as they were pressed together by the screws connecting them as already described.

Metallic Pistons.

(142.)

The notion of constructing a piston so as to move steam-tight in the cylinder without the use of packing of vegetable [Pg245] matter was first suggested by the Rev. Mr. Cartwright, a gentleman well known for other mechanical inventions. A patent was granted in 1797 for a new form of steam engine, in which he proposed to use the vapour of alcohol to work the piston instead of the steam of water: and since the principle of the engine excluded the use of lubrication by oil or tallow, he substituted a piston formed of metallic rings pressed against the surface of the cylinder by springs, so as to be maintained in steam-tight contact with it, independently either of packing or lubrication. Although the engine for which this form of piston was intended never came into practical use, yet it is so simple and elegant in its structure, and forms a link so interesting in the history of the steam engine, that some explanation of it ought not to be omitted in this work.

The steam-pipe from the boiler is represented cut off at B (fig. 66.); T is a spindle-valve, for admitting steam above the piston, and R is a spindle-valve in the piston; D is a curved pipe forming a communication between the cylinder and the condenser, which is of very peculiar construction. Cartwright proposed effecting a condensation without a jet, by exposing the steam to contact with a very large quantity of cold surface. For this purpose, he formed his condenser by placing two cylinders nearly equal in size, one within the other, allowing the water of the cold cistern in which they were placed to flow through the inner cylinder, and to surround the outer one. Thus, the thin space between the two cylinders formed the condenser.

Fig. 66.

The air-pump is placed immediately under the cylinder, and the continuation of the piston-rod works its piston, which is solid and without a valve. F is the pipe from the condenser to the air-pump, through which the condensed steam is drawn off through the valve G on the ascent of the piston, and on the descent this is forced through a tube into a hot well H, for the purpose of feeding the boiler through the feed-pipe I. In the top of the hot well H is a valve which opens inwards, and is kept closed by a ball floating on the surface of the liquid. The pressure of the condensed air above the surface of the liquid in H forces it through I into the boiler. When the air accumulates in too great a degree [Pg246] in H, the surface of the liquid is pressed so low that the ball falls and opens the valve, and allows it to escape. The air in H is that which is pumped from the condenser with the liquid, and from which it was disengaged.

Let us suppose the piston at the top of the cylinder: it strikes the tail of the valve T, and raises it, while the stem of the piston-valve R strikes the top of the cylinder, and is pressed into its seat. A free communication is at the same time open between the cylinder, below the piston and the condenser, through the tube D. The pressure of the steam [Pg247] thus admitted above the piston acting against the vacuum below it, will cause its descent. On arriving at the bottom of the cylinder, the tail of the piston-valve R will strike the bottom, and it will be lifted from its seat, so that a communication will be opened through it with the condenser. At the same moment, a projecting spring K, attached to the piston-rod, strikes the stem of the steam-valve T, and presses it into its seat. Thus while the further admission of steam is cut off, the steam above the piston flows into the condenser, and the piston being relieved from all pressure, is drawn up by the momentum of the fly-wheel, which continues the motion it received from the descending force. On the arrival of the piston again at the top of the cylinder, the valve T is opened and R closed, and the piston descends as before, and so the process is continued.

The mechanism by which motion is communicated from the piston to the fly-wheel is peculiarly elegant. On the axis of the fly-wheel is a small wheel with teeth, which work in the teeth of another larger wheel L. This wheel is turned by a crank, which is worked by a cross-piece attached to the end of the piston-rod. Another equal-toothed wheel M is turned by a crank, which is worked by the other end of the cross-arm attached to the piston-rod.

One of the peculiarities of this engine is, that the liquid which is used for the production of steam in the boiler circulates through the machine without either diminution or admixture with any other fluid, so that the boiler never wants more feeding than what can be supplied from the hot well H. This circumstance forms an important feature in the machine, as it allows of ardent spirits being used in the boiler instead of water, which, since they boil at low heats, promised a saving of fuel. The inventor proposed that the engine should be used as a still, as well as a mechanical power, in which case the whole of the fuel would be saved.

Fig. 67.
Fig. 68.

(143.)

That part of Cartwright's piston which in the common piston is occupied by the packing of gasket, already explained (141.), was filled by a number of rings, one placed within and above another, and divided into three or four [Pg248] segments. Two rings of brass were made of the full size of the cylinder, and so ground as to fit the cylinder nearly steam-tight. These were cut into several segments A A A (fig. 67.), and were placed one above the other, so as to fill the space between the top and bottom plates of the piston. The divisions of the segments of the one ring were made to fit between the divisions of the other. Within these another series of rings, B B B, were placed, similarly constructed, so as to fit within the first series in the same manner as the first series were made to fit within the cylinder. The joints of the upper series of each set of rings are exhibited in the plan (fig. 67.); the places of the joints of the lower series are shown by dotted lines; the position of the rings of each series one above the other is shown in the section (fig. 68.). The joints of the inner series of rings are so placed as to lie between those of the outer series, to prevent the escape of steam which would take place by one continued joint from top to bottom of the packing. The segments into which the rings are divided are pressed outwards by steel springs in the form of the letter V, the springs which act upon the outer series of segments abutting upon the inner series, and those which act on the inner series abutting upon the solid centre of the piston: these springs are represented in fig. 67.
Fig. 69.

(144.)

An improved form was given to the metallic piston by Barton. Barton's piston consists of a solid cylinder of cast iron, represented at A in section in fig. 69., and in plan in [Pg249] fig. 70. In the centre of this is a conical hole, increasing in magnitude downwards, to receive the piston-rod, in which the latter is secured by a cross-pin B. A deep groove, square in its section, is formed around the piston, so that while the top and bottom of the piston form circles equal in magnitude to the section of the cylinder, the intermediate part of the body of the piston forms a circle less than the former by the depth of the groove. Let a ring of brass, cast iron, or cast steel, be made to correspond in magnitude and form with this groove, and let it be divided as represented in fig. 70., into four segments C C C C, and four corresponding angular pieces D D D D. Let the groove which surrounds the piston be filled by the four segments with the four wedge-like angular pieces within them, and let the latter be urged against the former by eight spiral springs, as represented in fig. 69. and fig. 70. These springs will abut against the solid centre by the piston, and will urge the segments C against the cylinder. The spiral springs which urge the wedges are confined in their action by steel pins which pass through their centre, and by being [Pg250] confined in cylindrical cavities worked into the wedges and into corresponding parts of the solid centre of the piston, as the segments C wear, the springs urge the wedges outwards, and the points of the latter protruding, are gradually worn down so as to fill up the spaces left between the segments, and thus to complete the outer surface of the piston.
Fig. 70.

Various other forms of metallic pistons have been proposed, but as they do not differ materially in principle from those we have just described, it will not be necessary here to describe them.

ENGINE AT THE CITY SAW MILLS.
FURNACE AT THE CITY SAW MILLS.

CHAP. IX.

[Pg251]
TOC  INX

CONSTITUENTS OF COAL.—PROCESS OF COMBUSTION.—HEAT EVOLVED IN IT.—FORM AND STRUCTURE OF BOILER.—WAGGON BOILER.—FURNACE.—METHOD OF FEEDING IT.—COMBUSTION OF GAS IN FLUES.—CONSTRUCTION OF GRATE AND ASH-PIT.—MAGNITUDE OF HEATING SURFACE OF BOILER.—STEAM-SPACE AND WATER-SPACE IN BOILER.—POSITION OF FLUES.—METHOD OF FEEDING BOILER.—LEVEL GAUGES.—SELF-REGULATING FEEDERS.—STEAM-GAUGE.—BAROMETER-GAUGE.— INDICATOR.—COUNTER.—SAFETY-VALVE.—FUSIBLE PLUGS.—SELF-REGULATING DAMPER.—SELF-REGULATING FURNACE.—POWER AND DUTY OF ENGINES.—HORSE-POWER OF STEAM ENGINES.—EVAPORATION PROPORTIONAL TO HORSE-POWER.—SOURCES OF LOSS OF POWER.—ABSENCE OF GOOD PRACTICAL RULES FOR POWER.—COMMON RULES FOLLOWED BY ENGINE MAKERS.—DUTY DISTINGUISHED FROM POWER.—DUTY OF BOILERS.—PROPORTION OF STROKE TO DIAMETER OF CYLINDER.—DUTY OF ENGINES.—CORNISH SYSTEM OF INSPECTION.—ITS GOOD EFFECTS.—HISTORICAL DETAIL OF THE DUTY OF CORNISH ENGINES.

(145.)

The machinery which has been explained in the preceding chapters, consisting of the cylinder with its passages and valves, the piston-rod, parallel motion, beam, connecting-rod and crank, together with the condenser, air-pump, and other appendages, having no source of moving power in themselves, must be regarded as mere instruments by which the mechanical effect developed by the furnace and the boiler is transmitted to the working point and so [Pg252] modified as to be adapted to the uses to which the machine is applied. The boiler is at once a magazine in which the moving power is stored in sufficient quantity to supply the demands of the engine and an apparatus in which that power is fabricated. The mechanical effect evolved in the conversion of water into steam by heat, is the process by which the power of the steam-engine is produced, and space is provided in the boiler, capacious enough to contain as much steam as is necessary for the engine, besides a sufficient quantity of water to continue that supply undiminished, notwithstanding the constant drafts made upon it by the cylinder: even the water itself, from the evaporation of which the mechanical power is produced, ought to be regarded as an instrument by which the effect of the heat of the combustible is rendered mechanically efficient, inasmuch as the same heat, applied not only to other liquids but even to solids, would likewise be productive of mechanical effects. The boiler and its furnace are therefore parts of the steam-engine, the construction and operation of which are entitled to especial attention.

(146.)

Coal, the combustible almost universally used in steam-engines, is a substance, the principal constituents of which are carbon and hydrogen, occasionally mixed with sulphur in a small proportion, and earthy incombustible matter. In different sorts of coal the proportions of these constituents vary, but in coal of good quality about three quarters of the whole weight of the combustible is carbon.

When carbon is heated to a temperature of about 700° in an atmosphere of pure oxygen, it will combine chemically with that gas, and the product will be the gas called carbonic acid. The volume of carbonic acid produced by this combination, will be exactly equal to that of the oxygen combined with the carbon, and therefore the weight of a given volume of the gas will be increased by the weight of carbon which enters the combination. It is found that two parts by weight of oxygen combined with three of carbon, form carbonic acid. The weight of the carbonic acid, therefore, produced in the combustion, will be greater than the weight of the oxygen, bulk for bulk, in the proportion of five to two, the volume being the same and the gases being [Pg253] compared at the same temperatures and under equal pressures. In this combination heat is evolved in very large quantities. This effect arises from the heat previously latent in the carbon and oxygen being rendered sensible in the process of combustion. The carbonic acid proceeding from the combustion is by such means raised to a very high temperature, and the carbon during the process acquires a heat so intense as to become luminous; no flame, however, is produced.

Hydrogen, heated to a temperature of about 1000°, in contact with oxygen will combine with the latter, and a great evolution of heat will attend the process; the gases will be rendered luminous, and flame will be produced. The product of this process will be water, which being exposed to the intense heat of combustion, will be immediately converted into steam. Hydrogen combines with eight times its own weight of oxygen, producing nine times its own weight of water.

Hydrogen gas is, however, not usually disengaged from coal in a simple form, but combined chemically with a certain portion of carbon, the combination being called carburetted hydrogen. Pure hydrogen burns with a very faintly luminous blue flame, but carburetted hydrogen gives that bright flame occasionally having an orange or reddish tinge, which is seen to issue from burning coals: this is the gas used for illumination, being expelled from the coal by the process of coking, and conducted to the various burners through proper pipes.

The sulphur, which in a very small proportion is contained in coals, is also combustible, and combines in the process of combustion with oxygen, forming sulphurous acid: it is also sometimes evolved in combination with hydrogen, forming sulphuretted hydrogen.

Atmospheric air consists of two gases, azote and oxygen, mixed together in the proportion of four to one; five cubic feet of atmospheric air consisting of four cubic feet of azote and one of oxygen. Any combustible will combine with the oxygen contained in atmospheric air, if raised to a temperature somewhat higher than that which is necessary to cause its combustion in an atmosphere of pure oxygen.

If coals, therefore, or other fuel exposed to atmospheric [Pg254] air, be raised to a sufficiently high temperature, their combustible constituents will combine with the oxygen of the atmospheric air, and all the phenomena of combustion will ensue. In order, however, that the combustion should be continued, and should be carried on with quickness and activity, it is necessary that the carbonic acid, and other products, should be removed from the combustible as they are produced, and fresh portions of atmospheric air brought into contact with it; otherwise the combustible would soon be surrounded by an atmosphere composed chiefly of carbonic acid to the exclusion of atmospheric air, and therefore of uncombined oxygen, and consequently the combustion would cease, and the fuel be extinguished. To maintain the combustion, therefore, a current of atmospheric air must be constantly carried through the fuel: the quantity and force of this current must depend on the quantity and quality of the fuel to be consumed. It must be such that it shall supply sufficient oxygen to the fuel to maintain the combustion, and not more than sufficient, since any excess would be attended with the effect of absorbing the heat of combustion, without contributing to the maintenance of that effect.

Heat is communicated from body to body in two ways, by radiation and by contact.

Rays of heat issue from a heated body, and are dispersed through the surrounding space in a manner, and according to laws, similar to those which govern the radiation of light. The heat thus radiated meeting other bodies is imparted to them, and penetrates them with more or less facility according to their physical qualities.

A heated body also brought into contact with another body of lower temperature, communicates heat to that other body, and will continue to do so until the temperature of the two bodies in contact shall be equalised. Heat proceeds from fuel in a state of combustion in both these ways: the heated fuel radiates heat in all directions around it, and the heat thus radiated will be imparted to all parts of the furnace which are exposed to the fuel.

The gases, which are the products of the combustion, escape from the fuel at a very high temperature, and consequently, in acquiring that temperature they absorb a considerable [Pg255] quantity of the heat of combustion. But besides the gases actually formed in the process of combustion, the azote forming four fifths of the air carried through the fuel to support the combustion, absorbs heat from the combustible, and rises into the upper part of the furnace at a high temperature. These various gases, if conducted directly to the chimney, would carry off with them a considerable quantity of the heat. Provision should therefore be made to keep them in contact with the boiler such a length of time as will enable them to impart such a portion of the heat which they have absorbed from the fuel, as will still leave them at a temperature sufficient, and not more than sufficient, to produce the necessary draft in the chimney.

(147.)

The forms of boiler which have been proposed as the most convenient for the attainment of all these requisite purposes have been very various. If strength alone were considered, the spherical form would be the best; and the early boilers were very nearly hemispheres, placed on a slightly concave base. The form adopted by Watt, called the waggon boiler, consists of a semi-cylindrical top, flat perpendicular sides, flat ends, and a slightly concave bottom. The steam intended to be used in boilers of this description did not exceed the pressure of the external atmosphere by more than from 3 to 5 lbs. per square inch; and the flat sides and ends, though unfavourable to strength, could be constructed sufficiently strong for this purpose. In a boiler of this sort, the air and smoke passing through the flues that are carried round it, are in contact at one side only with the boiler. The brickwork, or other materials forming the flue, must therefore be non-conductors of heat, that they may not absorb any considerable portion of heat from the air passing in contact with them. A boiler of this form is represented in fig. 71.
Fig. 71

The grate and a part of the flues are rendered visible by the removal of a portion of the surrounding masonry in which the boiler is set. The interior of the boiler is also shown by cutting off one half of the semi-cylindrical roof. A longitudinal vertical section is shown in fig. 72., and a cross section in fig. 73. A horizontal section taken above the level of the grate, and below the level of the water in the boiler, showing [Pg256] the course of the flues, is given in fig. 74. The corresponding parts in all the figures are marked by the same letters.

Fig. 72.

The door by which fuel is introduced upon the grate is represented at A, and the door leading to the ash-pit at B. The fire bars at C slope downwards from the front at an angle of about 25°, giving a tendency to the fuel to move from the front towards the back of the grate. The ash-pit D is constructed of such a magnitude, form, and depth, as to admit a current of atmospheric air to the grate-bars, sufficient to sustain the combustion. The form of the ash-pit is usually wide below, contracting towards the top.

The fuel when introduced at the fire-door A, should be laid on that part of the grate nearest to the fire-door, called the dead plates: there it is submitted to the process of coking, by which the gases and volatile matter which it contains are expelled, and being carried by a current of air, admitted [Pg257] through small apertures in the fire-door over the burning fuel in the hinder part of the grate, they are burnt. When the fuel in front of the grate has been thus coked, it is pushed back, and a fresh feed introduced in front. The coal thus pushed back soon becomes vividly ignited, and by continuing this process, the fuel spread over the grate is maintained in the most active state of combustion at the hinder part of the grate. By such an arrangement, the smoke produced by the combustion of the fuel may be burnt before it enters the flues. The flame and heated air proceeding from the burning fuel arising from the grate, and rushing towards the back of the furnace, passes over the fire-bridge E, and is carried through the flue F which passes under the boiler. This flue (the cross section of which is shown in fig. 73., by the dark shade put under the boiler) is very nearly equal in width to the bottom of the boiler, the space at the bottom of the boiler, near the corners, being only what is sufficient to give the weight of the boiler support on the masonry forming the [Pg258] sides of the flue. The bottom of the boiler being concave, the flame and heated air as they pass along the flue rise to the upper part by the effects of their high temperature, and lick the bottom of the boiler from the fire-bridge at E to the further end G.

Fig. 73.

At G the flue rises to H, and turning to the side of the boiler at I I, conducts the flame in contact with the side from the back to the front; it then passes through the flue K across the front, and returns to the back by the other side [Pg259] flue L. The side flue is represented, stripped of the masonry, in fig. 71., and also appears in the plan in fig. 74., and in the cross section in fig. 73. The course of the air is represented in fig. 74. by the arrows. From the flue L the air is conducted into the chimney at M.

Fig. 74.

By such an arrangement, the flame and heated air proceeding from the grate are made to circulate round the boiler, and the length and magnitude of the flues through which it is conducted should be such, that when it shall arrive at the chimney its temperature shall be reduced, as nearly as is consistent with the maintenance of draught in the chimney, to the temperature of the water with which it is in contact.

The method of feeding the furnace, which has been described above, is one which, if conducted with skill and care, would produce a much more perfect combustion of the fuel than would attend the common method of filling the grate from the back to the front with fresh fuel, whenever the furnace is fed. This method, however, is rarely observed in the management of the furnace. It requires the constant attention of the stokers (such is the name given to those who feed the furnaces). The fuel must be supplied, not in large quantities, and at distant intervals, but in small quantities and more frequently. On the other hand, the more common practice is to allow the fuel on the grate to be in a great degree burned away, and then to heap on a large quantity of fresh fuel, covering over with it the burning fuel from the back to the front of the grate. When this is done, the heat of the ignited coal acting upon the fresh fuel introduced, expels the gases combined with it and, mixed with these, a quantity of carbon, in a state of minute division, forming an opaque black smoke. This is carried through the flues and drawn up the chimney. The consequence is, that not only a quantity of solid fuel is sent out of the chimney unconsumed, but the hydrogen and other gases also escape unburned, and a proportional waste of the combustible is produced; besides which, the nuisance of an atmosphere filled with smoke ensues. Such effects are visible to all who observe the chimneys of steam-vessels, while the engine is in operation. When the furnaces are thus filled with fresh fuel, a large volume of [Pg260] dense black smoke is observed to issue from the chimney. This gradually subsides as the fuel on the grate is ignited, and does not reappear until a fresh feed is introduced.

This method of feeding, by which the furnace would be made to consume its own smoke, and the combustion of the fuel be rendered complete, is not however free from counteracting effects. In ordinary furnaces the feed can only be introduced by opening the fire-doors, and during the time the fire-doors are opened a volume of cold air rushes in, which passing through the furnace is carried through the flues to the chimney. Such is the effect of this in lowering the temperature of the flues, that in many cases the loss of heat occasioned is greater than any economy of fuel obtained by the complete consumption of smoke. Various methods, however, may be adopted by which fuel may be supplied to the grate without opening the fire-doors, and without disturbing the supply of air to the fire. A hopper built into the front of the furnace, with a moveable bottom or valve, by which coals may be allowed to drop in from time to time upon the front of the grate, would accomplish this.

(148.)

In order to secure the combustion of the gases evolved from the coals placed in the front of the grate, it is necessary that a supply of atmospheric air should be admitted with them over the burning fuel. This is effected by small apertures or regulators, provided in the fire-doors, governed by sliding-plates, by which they may be opened or closed to any required extent.

A patent has recently been granted to Mr. Williams, one of the directors of the City of Dublin Steam Navigation Company, for a method of consuming the unburned gases which escape from the grate, and are carried through the flues. This method consists in introducing into the flue tubes placed in a vertical position, the lower ends of which being inserted in the bottom of the flue are made to communicate with the ash-pit, and the upper ends of which are closed. The sides and tops of these tubes are pierced with small holes, through which atmospheric air drawn from the ash-pit issues in jets. The oxygen supplied by this air immediately combines with the carburetted hydrogen, which [Pg261] having escaped from the furnace unburned is carried through the flues at a sufficient temperature to enter into combination with the oxygen admitted through holes in the tubes. A number of jets of flame thus proceed from these holes, having an appearance similar to the flame of a gas-lamp.

It is evident that such tubes must be inefficient unless they are placed in the flues so near the furnace, that the temperature of the unburned gases shall be sufficiently high to produce their combustion.