The question is often asked, What is smoke? Dr Lardner answers, "That when coals are thrown on to a furnace a smoke will arise, which, passing into the flues over the burning coal, will be ignited,"—a definition which, as Mr Wye Williams has pointed out, would compel us to consider the gas we burn in our houses as smoke; otherwise, the word is employed for the whole of the products which rise from burning fuel; or sometimes, as when we speak of the consumption of smoke and the prevention of smoke, we mean only the visible products of combustion. At the time when the use of pit-coal became general, but before the properties of combustible gases were well understood, all visible vapours rising from heated bodies were denominated smoke; hence the terms red smoke, black smoke, and white smoke; and more recently parliamentary smoke, a term for that thin transparent smoke through which objects could be seen, and derived from a local act of parliament, compelling millowners to construct their furnaces so as to consume their smoke, or, as we should now more accurately say, prevent its production.
It will be well if for scientific purposes the meaning of the term can be restricted and defined. When a combustible undergoes ignition, its elements, separated from their original combinations, unite with oxygen, and pass off as usually invisible gases. For these new gaseous combinations, the term products of combustion is fittest. If, however, the supply of oxygen be deficient, or the temperature of combustion lowered by the presence of a colder body, the combustion will be imperfect, and a part of the combustible will pass off in a finely divided, but generally visible form. Usage has tended to the restriction of the term smoke to the visible products of imperfect combustion, or unconsumed fuel disengaged in a minutely-divided condition from a gaseous combination. We have therefore the expressions—
Combustible; for the fuel, usually free carbon, or a combination of hydrogen and carbon, or sulphur.
Supporter of combustion; oxygen.
Products of combustion; the resulting combinations of oxygen with the combustible.
Smoke; the visible product of imperfect combustion, arising from deficiency of heat or deficiency of oxygen, and is usually free carbon, liberated in the combustion of hydro-carbons, which deposits itself on coal surfaces as soot.
Ash; the inorganic residuum of some kinds of fuel.
Perfect and imperfect combustion of coal.
To take an instance, and observe the process:—With ordinary coal, the oxygen it contains passes off with a corresponding quantity of hydrogen, as water, with little or no heating effect. The free carbon burns yielding carbonic acid, which in turn, in some cases, combines with another atom of carbon, and, if there be a deficiency of oxygen, passes off without producing a due heating effect. The hydrogen of the hydro-carbons forms steam, liberating the carbon which has the less affinity for oxygen. If, however, the supply of oxygen be ample, the carbon in its nascent condition combines with it very readily, and forms invisible carbonic acid; but if the supply of oxygen be deficient, it is seized by the hydrogen, and the carbon condenses into a black cloud of smoke, which is then much less facile of combustion. Hence we speak of the prevention of smoke rather than of its combustion, because the true process is the supply of an adequate quantity of air, which, combining with the carbon in the condition in which it is most susceptible, prevents the formation of smoke. Practically, however, in furnaces this can be only imperfectly attained, and after admitting as nearly as possible a due quantity of air in the most effective manner, we must be content to consume it by double furnaces and similar means, so far as that is possible.
At the more early period of our history, there were few chimneys in England; but we read that in Henry VIII.'s reign most gentlemen's houses had at least one chimney. At that time pit-coal began to be used in private houses; and there is extant among the records of Elizabeth's reign, a motion to the effect, "That many dyers, brewers, smiths, and other artificers of London, having of late taken to the use of pit-coal for their fires, instead of wood, which filled the air with noxious vapour and smoke, very prejudicial to the health, especially of persons coming from the country, a law might pass to prohibit the use of such fuel (at least during the session of parliament) by those artificers." From this it will be seen what a serious nuisance smoke was then considered; and much as its abatement is desired at the present time, it is yet fortunate that the inhabitants of London got over their prejudices, and considered the advantages of pit-coal very greatly to outweigh its defects.
The subject of the prevention of smoke has assumed very considerable importance, from the numerous local acts by which it is now rendered compulsory upon mill-owners and others who employ furnaces; and in many of the larger towns the nuisance, if not got rid of, has been considerably lessened by legislative measures. The subject is still one of great difficulty and perplexity, to which the incredible number of patent grates and furnaces will bear testimony; and the engineering profession is not yet agreed upon the best forms for our furnaces and stoves, so as to secure the largest effect from a given quantity of fuel, and at the same time abate to the greatest extent the nuisance of smoke. The difficulties do not arise from our ignorance of the constitution of fuel, or of the laws of combustion, but from practical obstacles, such as necessarily arise from the variable production of gaseous and other volatile products, and the want of system and regularity, which attends the management of a furnace. Habits of economy, and attention to a few simple rules, have not been sufficiently enforced, and it is obvious that much may yet be done in establishing and carrying out a well-organized plan of operations. If this were accomplished, and the management of steam-engine furnaces consigned to men properly trained for their respective duties, many of these difficulties would vanish, and the atmosphere of our towns become more pure, whilst the owners of mills would be compensated by the saving of fuel which such a system would secure.
Of the various kinds of fuel and their elementary analysis, sufficient has already been said under the head of fuel-making, and elsewhere. For our present purpose very accurate analysis is not essential, the following values of those constituents of fuel which contribute to the heating effect will be sufficient:— To find from this table the quantity of oxygen necessary for the combustion of 1 lb. of each kind of fuel, we must remember that each equivalent of hydrogen requires one of oxygen; oxygen is eight times as heavy as hydrogen, and therefore one pound of hydrogen will require eight of oxygen. Similarly each pound of carbon will require 1½ lb. of oxygen for its conversion into carbonic oxide, and 2½ for its conversion into carbolic acid.
Hence we get for the quantities of oxygen and corresponding weight of air required for the combustion of 1 lb. of each of the above fuel:
| Description | Pounds of Oxygen to 1 lb. of fuel | Pounds of air to 1 lb. of fuel | |-------------|----------------------------------|-------------------------------| | Charcoal | 2-48 | 11-16 | | Lignite | 1-76 | 7-02 | | Peat | 1-80 | 8-10 | | Coal, Welsh | 2-57 | 11-56 | | Newcastle | 2-55 | 11-47 | | Lancashire | 2-08 | 9-36 | | Scotch | 2-09 | 9-40 | | Derbyshire | 2-41 | 10-84 | | Anthracite | 2-69 | 12-10 | | Coke | 2-40 | 10-80 |
Hence we may consider that 12 pounds or 150 cubic feet of atmospheric air are on the average required for the combustion of each pound of fuel. In practice, however, we cannot expect the whole of the entering air to be so economised, and it is necessary to admit from 1½ to 2 or more times the theoretical quantity of air necessary for combustion, and although the proportions have not been accurately ascertained, a larger surplus of air is required for bituminous coal than for coke, and less with excited than with slow combustion.
All bodies in chemically combining liberate heat, as in decomposing, they absorb it. The quantity of heat liberated has generally been estimated in pounds of water raised in temperature, or more recently by Mr Rankine, Mr Thomson, and those who have developed the dynamical theory of heat, as thermal units, or that amount of heat necessary to raise the temperature of 1 lb. of water 1 degree Fahrenheit. The most reliable experiments appear to give the following values for the heat liberated by combustion:
| Heat liberated by combustion | In pounds of water heated from 32° to 212° Fahr. | In thermal units. | |------------------------------|--------------------------------------------------|------------------| | 1 lb. hydrogen, combining with oxygen | 344-6 | 62,032 | | 1 lb. carbon combining with one equivalent of oxygen | 24-4 | 4,400 | | 1 lb. carbon combining with two equivalents of oxygen | 80-0 | 14,500 |
Hence, from these numbers, we may determine the heat which would be liberated by the combustion of each of the previous descriptions of fuel, of which we know the chemical composition:
With these values we may compare the following experimental determinations on similar fuels:
| Fuel | Pounds of water heated from 32° to 212° Fahr. | Authority | |---------------|----------------------------------------------|-----------| | Charcoal | 68 | Berthier | | Peat | 75-7 | Winkler | | Peat charcoal | 181 to 350 | Berthier | | Lignite | 40-1 to 58-9 | | | Coking-coal | 37 to 57 | | | Durham and Newcastle coal | 53 to 72 | | | Coke | 60-3 to 65-6 | | | Anthracite | 70-3 | |
Plans for the Prevention of Smoke in ordinary Grates.
The first and most general plan which has been proposed for the prevention of smoke in ordinary grates, for fires, domestic purposes, is the introduction of fresh fuel below instead of above that undergoing combustion; a plan which, when carried out by suitable mechanical arrangements, is sound in principle, and advantageous in practice. In 1785 James Watt took out the earliest patent for the prevention of smoke in furnaces on this principle, but as his patent applies to boiler-furnaces, it will be recurred to hereafter.
Mr Cutler, in 1815, took out a patent for a fire-place to be supplied with fuel from below. A metal chamber below the grate is filled with fuel, which, as it burns away at the top, is supplied by raising the bottom of the fuel-box, or chamber, by a system of pulleys and chains. Other patentees have sought to diminish the difficulty of raising the whole day's consumption, by introducing the fuel horizontally from behind; and a third party have sought to employ a feeding-shovel, which is filled with fuel, and then thrust under the fire along the bottom grating; in this position its contents are forced beneath the burning fuel by a piston worked through the handle.
Dr Arnott's stove, which has formed the basis of the Dr Arnott's greater number of smoke-consuming grates, is constructed, on the principle of Cutler's. The fuel-box is placed immediately beneath the grate, and a movable bottom is raised when the fire is to be replenished by means of the poker used as a lever. The aperture of the chimney is contracted by brickwork, or by a metallic hood lined with tile, somewhat in the shape of an inverted funnel, and a valve or damper, with a conspicuous handle moving over a graduated plate, is placed on the throat of the chimney, that the draught may be regulated with the greatest nicety. A channel communicating with the external air is introduced under the hearth, to diffuse into the room, and about the fire, a supply of air which becomes tempered in passing through the air-channel and under the fender.
In a patent founded on Arnott's stove (Tillet, 1846), it was proposed that the fire-grate should slide back into a
recess, to admit a fresh supply of fuel into the box. In another (Jeakes, 1854), the very convenient modification has been adopted of making the curtain or front of the fuel-box and grate-bars to descend, instead of raising the coal. By this arrangement a fresh portion of coal is secured for combustion between the open bars, whilst some of the objections to the mechanical arrangements necessary for raising a considerable weight of coal are obviated.
Mr Bachofner has proposed a somewhat different plan—viz., the use of a small supplementary grate for burning coke, anthracite, or other non-bituminous fuel, and which can be raised or lowered at pleasure. The ordinary coal having been supplied to a common grate, this supplementary fire is lowered upon it, and the smoke consumed as it passes through.
Rotatory grates have been employed in some cases, the grate being made to turn on an axis, so that on fresh fuel being supplied, the grate is inverted, and the smoke compelled to pass through the clear fire. In the earliest patent on this principle the grate was a revolving cylinder, divided longitudinally by gratings into three compartments; these were filled in succession with fresh fuel, and the grate revolved each time, so that the clear fire might be brought to the top. This plan was first patented in 1818 by Spencer, and with very slight modification, has been several times patented since. Though possessing many merits, the enclosure of the fire in a cylindrical grate creates a prejudice against its use, and the isolation of the grate would seem to prevent the conduction of the heat, and its distribution, over a large radiating metallic surface.
A downward current of air, instead of a supply of fuel from below, has been proposed in some patents for the prevention of smoke in fire-grates. Thus, in a patent by Mr Williamson, in 1855, the back of the fire is an open grating of iron or fire-clay, behind which is a flue communicating with the chimney. To this flue there is a perpendicular valve or register, which forms the back of the fire-place, over which is a horizontal flap or register. When the register of the flue is thrown back, the draught passes direct into the chimney; when the register is vertical, and the flue thrown open, the draught is downwards through the fire and back of the grate, into the flue behind. In this system the prejudices against a change in the form of grate are removed, and a nearly smokeless fire secured without any modification of the ordinary mode of firing. A partial alleviation of the smoke nuisance has also been secured in the plan of Mr Stevens, by deflecting the heated air rising from the grate downward, on the burning surface of the fuel.
In all these plans for the prevention of smoke in ordinary house-fires, it being always assumed that there is a copious supply of air, the principle is to secure a large increase of the temperature of the smoke at the point where the hydrocarbons, &c., distil from the fresh fuel so as to insure their perfect consumption as soon as formed. We have seen that the more perfect plans provide for this purpose, either (1.) a supply of fresh fuel from beneath, so that the upward draught carries the gaseous products directly through the clear burning fuel at top, with the further advantage of producing them slowly and uniformly; or (2.) a downward draught, which effects the same purpose. When these plans are conjoined with an adequate provision for the supply of air from the external atmosphere, they appear calculated to insure the almost complete prevention of smoke, with a decrease of dirt, and a great economy of fuel. That Arnott grates have sometimes failed is probably to be attributed to mismanagement or prejudice on the part of attendants, and it is beyond question that an increasing number of them are in use, with a considerable saving of fuel. They possess the further advantage of burning coal of any size, as it cakes before it is brought into use, and they remain alight almost without attention.
Plans for the Prevention of Smoke in Stationary and Marine-Boiler furnaces.
We may leave out of consideration those plans in which the smoke and products of combustion required to be forced through water, in order to dispel them, as inapplicable and founded on false principles.
Plans for the Prevention of Smoke depending upon Double Furnaces, with alternate Firing.
Double furnaces for the prevention of smoke, patented double by Mr Losh, were in use as early as 1815, and in various furnaces, ways have been employed since. At first a diaphram or Fairbairn's mid-feather was employed running the length of the grate-double bars; and afterwards the plan of two cylindrical flues, fitted boiler, within the same boiler, was introduced by Mr W. Fairbairn of Manchester, where a local act for the suppression of smoke has been long in operation. The boiler adopted by him is shown in figs. 1, 2, and 3, and is a mean between the Cornish single-flue and the tubular boiler. It is perfectly cylindrical, and contains two circular flues \(a\), varying from 2 feet 6 inches to 2 feet 9 inches in diameter, extending throughout its length. Towards the front end the flues receive the grate-bars, hearth-plates, &c.; beyond these is built a brick bridge, over which the products of combustion are deflected. After passing through the boiler, the two flues unite; and after again making a circuit outside and under the boiler, pass to the chimney. These two flues enable the stoker to feed the furnaces alternately, and so maintain a more uniform generation of steam than would
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1 The curious in this subject may compare patents 1818, No. 4316; 1825, No. 5257; 1854, Nos. 613, 1617, 1221; 1855, No. 648. be possible with a single flue; and the flame passing from the one flue assists in the consumption of the gases generated in the other. Another form of boiler on the same principle, erected at the works of Messrs Salt of Saltaire, will be found described under the head Steam-Engine.
Mr Fairbairn has long been of opinion, that this simple system of alternate firing, when conjoined with the three requisites of the generation of steam with economy, viz., plenty of capacity in the boiler, sufficient admission of air, and, what is quite as necessary, very careful and attentive stoking, will secure, without any costly apparatus, the prevention of smoke, so far as that is possible, with any given description of coal. Attempts have been made to save fuel, and effect perfect combustion, under boilers of a capacity insufficient to raise and maintain the required pressure of steam without forcing the fire. Now, it should be distinctly understood, that it is next to impossible to burn smoke, or effect perfect combustion where the fires have to be forced; the consideration of capacity should never be lost sight of in our attempts at boiler improvement, and the consequent prevention of smoke.
Mr Robert Armstrong constructs what he calls the universal argand furnace upon the same principle, but by a different arrangement. The fire-bars are laid in three rows, the first next the door, horizontal; the second sloping downwards towards the third, which is horizontal, but at a lower level than the first. A double bearing bar between the first and second rows, permits a stream of air to enter the furnace at that point. In charging, a thin layer of coal is laid on next the door, and maintained in active combustion; behind and up to the bridge the coal is thrown in as thickly as possible; thus the flame from the thin stratum in front mingling with the air rushing in at the double bearing bar, acts like a torch in effecting the gaseous products distilling from the thick layer of coal behind. From time to time the ignited embers from which the flame has been expelled are raked forwards, and fresh coal is thrown on behind. Very little smoke is evolved from this species of furnace, and it differs but little in form from those ordinarily employed.
There is, however, this advantage peculiar to two furnaces combined, viz., that the air required for combustion must to some extent vary in quantity in a single furnace; we cannot determine the time at which the maximum would be required, nor that at which the minimum would be sufficient, nor what amount of surplus is necessary, but that the quantity varies according to the state of the fire, the time of feeding, and the attention of the stoker seems to be beyond question. Now, a double furnace tends to equalise the supply; the two fires fed alternately will not require a maximum or a minimum supply of air at precisely the same time, and when the two currents of gaseous products mingle, the surplus of one furnace will supply the deficiencies of the other. In this way the tendency is to compensate the supply and demand, and to prevent waste from too great or too small an admission of air.
In marine boilers the system of double furnaces is to a large extent employed, and when sufficient air is admitted, proper care and attention bestowed on the furnaces, smoke may to a large extent be prevented. Fig. 4 shows the general form of a marine tubular boiler of the ordinary construction. \(a\), the furnace; \(b b b b\), a series of small tubes, through which, in a finely divided state, the products of combustion pass; \(c\), the uptake, in which the products of two or more furnaces mingle as they pass to the chimney.
Plans for the Prevention of Smoke depending on Mechanical contrivances for feeding the furnace and distributing the fuel.
Of this class is the earliest patent for smoke prevention, Watt's taken out by James Watt in 1785. By this plan the fire is patented supplied from above downwards, by a reservoir of fuel in contact with the burning mass, the combustion of which is supported by a strong lateral current of air, passing direct through the fire into a flue on the other side, aided by a slight downward current beside or through the fuel, which last descends by its own gravity as it is consumed by the fire. For the purpose of intercepting and completing the combustion of any of the gaseous products, a clear and separate fire is maintained near the entrance into the flues, so that the products of the first fire being subjected to the intense heat of the second, and mingled with atmospheric air, may be most effectually consumed.
Apart from the external reservoir, we owe to Watt the dead-plate often adopted in stationary boilers. The fire-bars and dead-plate at the furnace-doors being rather more than usually inclined, to facilitate the advance of the fuel.
The fresh coal is thrown on to the dead-plate, where it gradually cokes, the more volatile constituents distilling over, and being consumed by the bright fire beyond. The coked fuel is then pushed forwards on to the bars, and a new supply of coal introduced on the dead-plate. This plan, so far as it goes, seems right in principle and effective in practice, and where the combustion is slow and the supply of air properly proportioned, obviates as effectually as more complicated contrivances the consumption of the smoke.
The succeeding patents of the principle of mechanical feeding as a substitute for hand-labour, have followed two different arrangements. Some have made the grate itself carry forward the fuel, either (1.), by revolving horizontally, or (2.), by rolling forward longitudinally over fixed wheels, or (3.), by the rising and falling of the alternate bars, causing the thrusting forward of the fuel. Others have left the grate-bars stationary, and have used fans revolving either (1.) horizontally, or (2.) vertically, to distribute the fuel, or (3.) have depended in part on manual labour for that purpose.
In 1819 Brunton patented a furnace of a circular form, Brunton' with a grate revolving horizontally, and having the fuel revolving supplied from above by an external hopper. The circular grate, grate is supported on a vertical axis, and has on its periphery a rim moving in sand, to prevent the entrance of air. The feeder is similar to the hopper of a corn-mill, excepting that the bottom is oblong; the coal falls through an opening, the width of which is adjusted by a vertical shutter, according to the size of the coal, and the supply... is maintained by an inclined plate, to which a reciprocating motion is given by machinery. Mr Brunton proposes that the grate should revolve once in six minutes, to ensure a uniform spreading of the coal in a constantly renewed thin layer over the whole area of the grate.
In 1822, Brunton also patented what he terms the peristaltic furnace, in which the distribution of the coal is effected by the movement of the grate-bars. Each alternate bar is attached at the end next the furnace-door to an eccentric motion, by which it is raised above the level of the stationary bars, pushed forward a small distance, and then allowed to return and fall below the mean level. In this way, at every stroke the coal is shifted forwards a short distance from the door towards the bridge-end of the furnace, the bars being inclined at the rate of 1 in 7 to facilitate its progress. Near the door, the air-spaces between the bars are reduced as much as is consistent with their motion, so as to form a sort of dead-plate on which the coals may coke. The fuel is supplied at the upper end by a feeder as above.
Perhaps the most complete apparatus on the plan we are considering is that of Mr Juckes; but unfortunately it is, like those of Brunton, objectionable on the score of complication and liability to derangement. In this plan, as shown in fig. 5, endless chains supply the place of the ordinary fire-bars, and revolve over two fixed rollers, a a. These rollers are kept in slow revolution, carrying the chains along with them. The coals are thrown on the inclined plate, b, at the mouth of the furnace, and fall gradually upon the moving grate-bar surface, on which, as they coke, they are carried along towards the back of the furnace. Thus the combustion is maintained by a slow and uniform supply of fuel, which gradually heats and parts with its gases, as it is drawn forwards through the furnace; whilst the whole of the products of combustion, after mingling with air, have to pass through the hottest part of the furnace.
Of the plans for the distribution of the coal over stationary grate-bars by revolving fans, we may notice first that of Stanley in 1822. The coal is placed in a hopper external to the furnace, whence it falls on two fluted rollers, in passing between which it is reduced to a small size, and equalized in quantity. Below the rollers is placed a fan, with three vanes revolving vertically opposite the entrance to the furnace, and somewhat above the bars. This fan makes 200 revolutions per minute for a grate 5 feet 6 inches in length, and a greater number for a larger distance proportionally. The rollers above are worked from the fan axis by intermediate worms and wheels, so as to make but 1 revolution to 784 revolutions of the fan; hence the coal, falling in slow shower, is struck by the vanes of the fan, revolving with great rapidity, and projected forwards into the furnace, where it spreads uniformly over the surface of the burning fuel.
In 1834, Stanley and Walmsley patented a plan, in which the crushed coal fell upon a horizontal revolving plate or fan projecting into the furnace, and thence was scattered over the bars by centrifugal action. At the same time the bars were intended to rock, as described above, but by a different arrangement, and a siphon tube and float were attached for regulating the feed according to the pressure of steam in the boiler. There is no doubt that the uniform distribution of the coal, in small but unintermitting quantities, over the whole surface of the fire, so far as it is secured by the above systems, must be to a large extent advantageous in the prevention of smoke and economy of fuel; but the complication and expense of the apparatus have hitherto prevented their general use, although for several years before the question of the smoke nuisance was taken up by the public, Stanley's feeding machines were extensively employed. They do appear to effect the uniform distribution of the coal more perfectly than when it is trusted to hand-labour, and where used they have permitted an inferior quality of coal to be substituted for that previously required.
Mr Bourne and other engineers look to the application of mechanical apparatus for feeding as likely to prove of signal importance in marine boilers. They suggest that, with their aid, two tiers of furnaces might be employed, and the present very reduced area of grate-bar surface in marine boilers increased, and that the feeding would be more efficiently performed, especially in stormy weather, when the difficulty and labour of stoking night and day are very considerable.
Plans for the Prevention of Smoke depending upon increased admission of air.
Under this section we have to consider a large series of plans, some of which have gained a wide notoriety, and attained a considerable measure of success in their application. The earliest plans consisted of a simple additional opening at the bridge, by which a stream of air mingled with the gaseous products when at their highest temperature. A later plan was the introduction of a large plate, pierced with holes, by which it was conceived that a greater diffusion of the air in more finely divided streams was obtained. Then the necessity of a variable instead of a constant admission of air was advocated, and plans were introduced for augmenting the supply of oxygen at the time of feeding, and slowly diminishing it afterwards. Another class of inventors have sought, not to increase the aperture for air simply, but to increase the draught by exhausting-fans, or to induce excited combustion; and some have sought to effect this more completely, and at the same time to utilise the waste heat of the chimney, by attempts to imitate the hot blast of iron-furnaces introduced by Mr Neilson.
Numerous attempts have been made to abate the nuisance of smoke by the admission of air through split bridges, or through the furnace-doors. These have been more or less successful as the boilers have been of less or greater capacity. No one plan has, however, proved its superiority so strikingly as to justify an adhesion to it rather than to any other plan; nor has any project attained such a preference in general estimation as to have secured its application in a preponderating majority of cases.
Mr C. Wye Williams, the apostle of "nature" and "the chemical point of view," was one of the earliest, as he has been one of the most pertinacious and consistent, advocates of the introduction of a large volume of ordinary cold air into the furnace; and if he has not solved all the difficulties of the subject, we have at least to thank him for the labour he has expended in proving the necessity for air as one of the prime conditions of economy in the use of fuel, and success in the prevention of smoke. Probably Mr Williams has underrated the necessity for space in the furnace, and has unduly ridiculed those plans in which the consumption of smoke is sought by an increase of its temperature; that is, by passing it over or amongst incandescent fuel; but it is certain that those plans, even if they effect the consumption of visible smoke, will be most wasteful of fuel if the conditions on which Mr Williams insists are not at the same time complied with. If the supply of air be deficient, a large quantity of carbonic oxide must be formed—the carbonic acid decomposing and forming, after taking up an additional atom of carbon, a double volume of carbonic oxide, and passing off invisibly. Where this takes place there must be an equivalent loss of heat, the carbonic oxide carrying off 10,100 thermal units per pound of carbon, which a sufficient supply of oxygen would have liberated.
To the impossibility of regulating the supply of air, so that, on the one hand, there may be no carbonic oxide formed, and, on the other, that no unnecessary waste of heat be occasioned by the gases passing into the chimney, is probably to be attributed the fact, that in practice much economy of fuel from smoke-burning is not generally attained, although in carefully conducted experiments as much as 20 to 30 per cent. has sometimes been saved.
The earliest attempts at the introduction of an increased supply of air were by "split bridges" or openings from the external air through the bridge, behind the grate-bars. These were first used by Mr Wakefield and Mr Parkes in 1820—the former feeding thinly and carefully, and diffusing the air at the bridge by several apertures; the latter throwing in large quantities of coal next the door, the gases from which becoming intensely heated as they passed over the incandescent fuel behind, were ignited as they mingled with the air rushing in at the bridge.
Two different principles have guided the later patentees of methods for the admission of air. Mr Wye Williams, and those who have followed his views, have contended for a uniform supply, whatever be the condition of the furnace. Another party, of whom we may instance Mr Syme Prideaux, contend for a variable supply, greatest when the fuel is first thrown on, and lessening gradually as the coal becomes coked and burns clear. Mr Williams relies upon frequent firing, by which he thinks the needs of the furnace will be equalised and made uniform; whilst Mr Prideaux urges that, during the first violent distillation from the fresh coal, there is a need for a larger quantity of air than at any other time.
Mr Williams lays the greatest possible stress upon the mechanical division of the air, by causing it to enter the furnace, not in a single stream, but through what he terms a diffusion-plate, or perforated partition. There is probably some advantage in this mode of admission, which is also adopted by Mr Prideaux, but it is probably not the whole secret of smoke prevention. When we consider the laws of the diffusion of gases, it can scarcely be credited that a thin perforated plate should divide a current of air into streams remaining separate at any considerable distance from the apertures, or make so enormous a difference in the decompositions going on in the furnace, as Mr Williams seems to contend. Nevertheless, we owe much to Mr Williams for his advocacy of a convenient mode of introducing air at the point where it is necessary for perfecting the combustion of the gases; and there is no doubt that the use of his simple perforated plate, giving a large increase of aperture for the admission of air, and placed behind the bridge, so as to supply pure air at the point where the stream of gases enters the furnace at its highest temperature, will in many cases much reduce the production of smoke, and in experimental trials has realized an important saving of fuel.
Mr Williams makes the area of aperture in the diffusion-plate equal to \( \frac{1}{4} \)th of the area of the grate, supposing 25 lb. of coal to be burned per square foot per hour.
Mr Syme Prideaux, who contends for a variable supply of air, constructs the door of his furnaces with metal venetians (fig. 6), which are opened by a lever when fresh coal is introduced. The other end of the lever is connected with a piston working in a cylinder, which fills with water when the piston is raised and the venetians opened. This water then slowly escapes through a small orifice, the area of which can be regulated at pleasure with the greatest nicety; and as the cylinder empties, the piston falls, and the openings in the furnace-door are closed. Thus an opening of \( \frac{1}{4} \)th the area of the fire-grate is provided with an arrangement by which, when fresh coal is thrown on, the whole area may be opened and a maximum supply of air allowed to pass; then, without any attention from the stoker, this surplus opening gradually closes at a regulated rate, and by the time the coal is coked the whole of the air has to pass through the furnace-bars. Mr Prideaux further provides a series of vertical plates at the back of the furnace-door, by which he thinks the air is somewhat heated before entering the furnace. Lately Mr Prideaux has adopted a mechanical arrangement somewhat different from the piston and cylinder, which is self-acting on the opening of the door. The invention in its essential features remains, however, the same.
In 1822 and 1823, blast-tents were taken out for and heated applying a blast to boiler-furnaces, either for forcing air in at the bridge, or by exhausting it at the chimney.
Other inventors, as Hall in 1836 and Coad in 1835, have sought to save a portion of the waste heat, and also to prevent smoke by heating the air before it enters the furnace, in opposition to the views of Mr Wye Williams, who considers the colder air to be preferred. They effect their purpose by causing the air to enter through a series of pipes placed in the flue or chimney, and heated by the waste products of combustion. If their plans were successful they would at least remove the principal objection which has been urged against Mr Williams's system—viz., that the cold currents which he introduces, striking against the boiler bottom, cause unequal contraction and loosen the rivets. But they have given no data for judging of the amount of heat attained by the air, which would appear to be very inconsiderable.
Abroad an entirely new system has been introduced, under the name, "Système Beaufumé," and is said, on good authority, to possess very great advantages, both as regards economy and entire prevention of smoke. The process has already been described under the head Iron-Making, to which process it is peculiarly applicable. The combination of the fuel takes place in a closed brick retort, surrounded by a water-chamber, supplied by a fan with a very deficient amount of air; in fact, only sufficient for the production of carbonic oxide gas, and the volatilization of the hydrocarbons. The gases thus produced are conveyed in pipes to the combustion-chamber, where, by the introduction of air, they are completely burned.
On the Prevention of Smoke in Locomotive Boilers.
The locomotive boiler requires a separate consideration, from the peculiarity of the conditions under which it generates steam, and the necessity for special modification of the contrivances for the prevention of its smoke.
The stringent enactments by which smoke upon railways has been prevented hitherto have practically operated to prevent the use of any fuel except coke, with which, in the ordinary locomotive furnace, there is no difficulty. Of late, however, the great demand for coal, and its consequent high price, together with the expense of the operation of coking, have induced several experiments to be made, with a view of employing the coal in the raw state, so as to avoid the waste necessarily attendant upon the coking process.
Mr Beattie's plan.
On the London and South Western Railway, Mr Joseph Beattie has introduced a system of combining the use of coal and coke, by means of an auxiliary fire-box or furnace. This auxiliary furnace is attached to the back of the ordinary fire-box, at a little below the level of the foot-plate, and is surrounded by a water-space similar to and communicating with the waterspace of the ordinary fire-box. The flame and combustible gases thrown off by the coal pass into the coke fire-box through a series of small tubes, and, to promote combustion, they strike against and are diffused by a fire-tile curved bridge, forming a sort of combustion-chamber. The coal and coke fire-boxes are provided with separate ash-pans and close-fitting dampers, whereby the draught to each can be regulated with the greatest nicety, independently of one another. The damper to the coke fire-box is generally kept nearly closed, and is only opened about 1½ inch, with trains of 20 to 24 carriages; but the damper of the coal fire-box is generally kept quite open to admit the full draught, by which means the coal-fire being excited to the utmost, the gases and flame pass into the coke-fire, and with them air in a heated state; the high temperature of the coke-fire is maintained, and more perfect combustion is the result. The combustion of the smoke is completely effected, and a considerable economy of cost obtained. Mr Beattie combines with this plan an arrangement for heating the feed-water, so as to economize the waste heat of the exhaust-steam as it passes into the atmosphere.
Mr William Jenkins has patented a plan for the use of coal in locomotives, depending entirely upon the introduction, through a series of tubes in the fire-box, of a larger supply of air. These tubes answer as stays, and are placed on the tube-plate side, so as to admit air to the fuel above the fire-bars; the outside ends have a valve arrangement to regulate the quantity of air to be passed through them, or entirely shut it off; and in combining with these arrangements a partition of iron or other suitable material passing across the fire-box, the object of which is to prevent the air admitted through the tube-stays from passing directly to the flue-tubes of the boiler, this partition acting to cause the air admitted to mix with the combustible gases and ignite them, thus consuming the smoke. To obviate the smoke when the engine is standing, Mr Jenkins employs a steam-jet in the chimney, which can be turned on at pleasure, to supply the place of the blast from the cylinders.
Fig. 7 represents a longitudinal section of an ordinary fire-box, with Mr Jenkins's arrangements for burning coal.
The principal apertures for air are shown at \(a\), as arranged in rows across the tube-plate to the number of eighty or more; these apertures are formed by short tubes, which take the place of the ordinary fire-box stays. A plate \(c\), perforated with holes corresponding to those in the fire-box, is fitted on the outside, so as to slide in grooves. This plate is so arranged, that in one position it completely closes the air-apertures; but it may be drawn back by the levers \(d\), and handle indicated at \(h\), and any requisite amount of opening allowed for the admission of air. The partition \(k\) across the fire-box is supposed to be of iron, and is perforated with holes, the purpose of which is to distribute the air admitted through the tube-stays in numerous jets, so as to secure its commingling with the combustible gases. This partition is screwed to the tube-plate, and rests at the ends or flanges secured to the sides of the fire-box. There are also somewhat similar arrangements in the fire-door \(m\), by which, if necessary, an increased supply of air can be admitted.
This system is partially in use on the Great Northern Railway, on the Lancashire and Yorkshire, and on several other lines in this and foreign countries, and appears to act with great efficiency and economy. The Lancashire and Yorkshire Railway has dispensed with the use of coke altogether, at an estimated saving of £27,000 per annum.
| Year | Consumption of Coal (tons) | |------|--------------------------| | 1856 | 33,12 | | 1859 | 30,79 |
Saving: 2,33
showing an absolute saving in weight, apart from that arising from the expense and loss of the coking process. The plan possesses the further merit of great simplicity, and it requires no modification of the ordinary arrangements of a locomotive engine. In fact, it may be applied to old stock at an expense of £6 or £7 a piece. The following comparison of the cost of running on the Lancashire and Yorkshire Railway has been furnished me by Mr Jenkins:
| Month ending Sept. 17, 1857—Coke | Month ending Sept. 16, 1858—Coal | |----------------------------------|---------------------------------| | Average Consumption per mile run | Cost per mile run | | Goods | Passengers | Total | Goods | Passengers | Total | | 49.12 | 23.89 | 33.40 | 2.58 | 1.66 | 2.14 | | 37.17 | 23.76 | 30.72 | 1.19 | 0.76 | 0.98 |
Mr D. K. Clark has proposed a system of coal-burning in locomotives, which does not widely differ from that of Mr Jenkins. He agrees with him in believing that a plan, to be successful, must be simple in design, facile in execution, applicable to existing stock, and easy to manage. His apparatus is external to the fire-box, and therefore not exposed to heat, and is controlled by a single stop-cock. Air is admitted above the fuel by one or more rows of tubes similar to those of Mr Jenkins' patent, inserted in the walls of the fire-box, and jets of steam are projected through the tubes from nozzles \( \frac{1}{4} \) inch in diameter, in small steam-pipes, placed outside the fire-box. The jets of steam are used principally when the engine is standing, with the aid of a light draught from a ring-jet in the chimney to prevent the smoke, and they may be shut off when not required. The supply of air through the tubes may be further regulated by dampers. The grate-bars are placed close together, with narrow air-spaces, and the ash-pan and damper are tightly fitted. This system is said to be working with success on the Great North of Scotland Railway, and on the North London Railway.
On the Taff Vale Railway, Mr Tomlinson has also substituted coal for coke in the ordinary engines; the principal novelty of his plan being the introduction of a layer of broken fire-brick upon the grate-bars, to protect them from injury from the burning fuel.
It is alleged that smoke arising from domestic fires and steam-engine chimneys cannot be entirely prevented; but at least it has been shown that two-thirds of the immense volume of smoke which ordinarily pours into the atmosphere from steam-boilers may be prevented, and the atmosphere of our towns rendered much more clear and salubrious, where attempts are made to abate the nuisance. The local acts of large towns, such as Manchester, Glasgow, Liverpool, Leeds, &c., have enabled the authorities to inflict penalties on offenders of this kind, and have proved that dense smoke may be almost entirely removed with proper care and attention to the mode of firing, supply of air, &c., and that with economy in the consumption of fuel. It therefore follows that it is a duty that the owners of steam-engines should employ the proper means to relieve the public from an evil which admits of remedy.
On the River Thames, above London Bridge, the nuisance of smoke from the funnels of the small steamers became intolerable, until Lord Palmerston obtained an act to compel the working of their engines without smoke. Under this act the nuisance has been almost entirely removed; and it is to be regretted that it does not extend to the steamers below the bridge. Some persons maintain that the improvement is to be attributed to the use of Welsh smokeless coal; but, so far as the public are concerned, this question is of little moment, seeing that the evil is prevented. Be this as it may, it has been shown that the discharge of dense volumes of smoke can be prevented. If done once, it can be again; and, in order to abate the nuisance, the local authorities should have power to enforce a similar law in every case where it can be done without injury to individuals or to our industrial interests.
Local acts for the abatement of the nuisance of smoke are not of a stringent character; the penalty seldom exceeds a fine of 40s. It is probably desirable that it should not throw any impediment in the way of our commercial or manufacturing progress, as any legal interference with the industrial interests of the community is to be deprecated and avoided. Yet, since it has been proved that a more efficient process of combustion can be secured, even in the case of bituminous coal, so as to obviate the nuisance of smoke, and, at the same time, to be a positive advantage to the consumer, there is no hardship in compelling the adoption of the requisite means for that purpose, when it will confer so great a boon on the rest of the community.
It is on this ground that we advocate the enforcement of local acts of Parliament; and we believe that they have contributed much to the comfort of the inhabitants of large manufacturing towns where steam-engines are employed, and where, in general, a reckless expenditure of fuel is indulged in, with no useful result at all, unless enveloping the surrounding district in a dense canopy of smoke is to be considered as such.
It is now many years since tall chimneys were erected for the purpose of abating this nuisance; but these erections are only to be regarded as transferring the nuisance to greater distances, where the unburned carbonaceous matter is deposited in large flakes, to the annoyance of a larger circle of inhabitants, and more serious injury to vegetable life. These are considerations of much importance to the public; and we hope that the time is not far distant when we may calculate upon a more improved system of consumption of our mineral deposits. We are bound to urge on the public the necessity of adhering to a sounder and more economical principle of action than has hitherto been generally adopted.
The reader may consult Charles Wye Williams, On the Combustion of Coal and the Prevention of Smoke, 1841; and Prize Essay on the Prevention of Smoke, 1854; T. Syme Prideaux, Rudimentary Treatise on Fuel, 1853; Fairbairn, Useful Information for Engineers, 1857; Bourne and Armstrong, On the Modern Practice of Boiler Engineering, 1856; Report of Commissioners appointed to Inquire into the Warming and Ventilation of Dwellings, 1857. (w.f.)
SMOKE-SAIL is a small sail hoisted against the foremast of a ship, when she rides head to wind, to allow the smoke of the galley to ascend, and to prevent its falling back on the quarter-deck.