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The Philosophy of the Fire-place.
water. It requires two measures of hydrogen to combine with one measure of oxygen, and the resulting steam, if at the same temperature, would occupy just two measures, but of course it is produced at a very high degree of heat, and thus greatly expanded.
Having thus explained what is the nature of combustion, we turn to the philosophy of smoke.
Start not, gentle reader, nor imagine we are about summarily to introduce you to the cloud philosophy of Professor Teufelsdruck, or to pour forth a rhapsody on the virtues of the narcotic weed. No! the smoke with which we have to deal is more material than the first, perhaps more innocent than the second.
When sitting over a fire, every one has observed a coal which has just begun to feel the intense heat. From some fissure exudes a black pitchy mass, ever changing its shape, and from it starts a small jet of gas shooting out perhaps between the bars with considerable force, and then turning upwards towards the chimney. Now what is taking place here? The coal is a chemical compound, or rather a number of different chemical compounds, formed principally of carbon and hydrogen. There are also present oxygen and nitrogen, with small and very varying quantities of sulphur, and also of those mineral constituents which form the ash. With none of these latter substances have we anything to do now, as they do not affect the question at issue. The jet that springs from the roasting coal is composed of a number of gaseous compounds of carbon and hydrogen, formed by the destructive action of heat, and is identical with the gas with which our streets and dwellings are lighted. All these products are termed hydrocarbons, but some of them (especially those named light carburetted hydrogen, and olefiant gas) are perfectly gaseous at ordinary temperatures, while others condense on cooling to oily liquids, or sometimes to solid bodies. These constitute the tar of the gas works, and to these indeed it is due that we see the little jet, for its partial opacity and its brown or blue tint are owing to the fact that the slightly volatile bodies are already condensing. But as we watch the swelling coal, the jet suddenly catches fire, and it burns with all the whiteness and brilliancy of a gas flame. And now what is taking place? The gas has mixed with the air, and fire having communicated with it, the hydrogen has left its friendly carbon to join company with the oxygen, and fly off as vapour of water, while the carbon unites also with oxygen, and forms the invisible carbonic acid. Stay, we are proceeding too fast; the carbon deserted by its hydrogen, for an instant remains alone, not as a gas, but as extremely finely divided charcoal-charcoal indeed, but not black; so
heated is the flame that it becomes instantly white hot, and thus imparts the luminosity that we see. But white-hot charcoal will not long exist as such in our atmosphere: it combines with the first particle of oxygen which it encounters, and flies away again as an invisible gas. That the flame does contain actual charcoal may be proved by any one who will cut it through by a piece of white porcelain, card, or anything else which will momentarily cool it, when the white surface will be seen covered with carbonaceous soot. From this white brilliant flame no smoke proceeds, but presently the flame becomes red, and from its apex thin films of sooty smoke ascend. This is, because from some reason the temperature of the flame is reduced, the charcoal is heated only to redness, and by the time it reaches the tip of the flame is so lowered in temperature that it will not combine with the oxygen with which it may then come into contact, but ascends the chimney as finely divided charcoal, perhaps to be there deposited, perhaps to be borne aloft into the atmosphere, by the accompanying steam and invisible gases. Here then,' as says Mr. Williams, 'in an open fire-grate are represented the usual ' varieties in the stages of perfect and imperfect combustion:'white or red flame, with or without smoke. These varieties, however, are the result exclusively of one cause, namely, the varying quantities of gas evolved, and the extent to which in each case they have obtained contact with the air, before 'their temperature had been too much reduced. From these appearances we justly infer what takes place in the furnace.' For, indeed, the furnace is only a fireplace on a larger scale, and much more confined.
There is one other point to which we must allude in the coal fire. After the gas has been tolerably well expelled from the coals, they cake together and burn away with a bright red heat and no flame. In this condition they form no visible smoke, and the product of combustion is almost wholly carbonic acid, since the hydrogenous constituents had nearly all been consumed before the fuel was reduced to this coky state.
There is an unfortunate ambiguity prevailing with regard to the use of the word smoke; an ambiguity that has been rendered the more puzzling by one of its results, namely, that the Act of Parliament requires that the furnaces shall consume or burn the smoke,' and most of those who have treated on the subject speak without hesitation of smoke consumption,' while Mr. Williams and a few others maintain most stoutly, that to burn smoke is scientifically absurd, and practically impossible. Now Mr. Williams certainly has the advantage of most of his opponents in scientific knowledge; he understands the theory of
What is Meant by Smoke?
the matter well, and does not commit such self-contradictory blunders as a gentleman who lately read a most elaborate paper before the Society of Arts on the subject, and during the course of it made the following statements: Smoke, as it appears to 'the eye when issuing from a factory chimney, is a compound of 'soot, dust, steam, and gas, of the same description as is produced and distributed by the gas companies.' And, 'The smoke which affects the public is mainly composed of the hydrogen and such portions of carbon as are thrown off with it. Yet we have in vain sought for any clear indication of what meaning Mr. Williams attaches to the term smoke. He obviously repudiates the popular use of the term, for he refuses to apply it to the brown jet which issues from the gas coal, and which of course is combustible enough. With this we find no fault, but he writes'According to the views of scientific men, the term smoke is ' applied to the volatile produce of the furnace after the process of 'flame or combustion has taken place, in contradistinction to that 'which issues direct from the coal, and which is called gas or gaseous vapours. Now this volatile produce' may be either invisible or a black cloud, according to the completeness or incompleteness of the combustion. Do scientific men call it smoke in the first of these cases? They do not. We certainly remember that Professor Faraday, in lecturing at the Royal Institution on the ventilation of rooms heated by gas, used the term smoke in this manner; but he did so only because no other short word was at hand, and he apologized for the unusual application of the term. Nor does the essayist himself abide by this definition: by prevention of smoke, he means not putting out the fires, but producing invisible products of combustion, and further on he gives the following wiser description:- Smoke must be taken as it is namely, a compound cloud of all these three gaseous 'bodies (nitrogen, carbonic acid, and steam), together with the 'portion more or less of the solid, uncombined, visible, free car'bon.' But what of the dictum that smoke cannot be consumed? If smoke be the volatile products' of combustion, the oxydized gases, they of course can be neither consumed nor prevented; if smoke be the diffused carbon, that is combustible enough; if smoke be the carbon, together with the large volume of oxydized gases and nitrogen through which it is diffused, the gases certainly cannot be burnt, but that which renders the mass visible, and gives it a right to be called smoke, may be. If Mr. Williams means to affirm that when black carbon is once deposited in the midst of a mass of these oxydized gases, it cannot by any possible means be made to combine with oxygen, we doubt the correctness of the assertion. That prevention is
better than cure, however, we do not doubt, and we think Mr. Williams has done great service in fixing attention on this fact, even though his determination to maintain his favourite theory, that smoke cannot be consumed, has led him into several unguarded statements in Section VII. of his Treatise.
It was stated above that a furnace is a fireplace on a large scale and more confined, and it is just upon this confinement that the great difficulty hinges. The engineer's object is of course to heat the boiler as much as possible; thus he must leave as little space as he can for the heat to radiate away into and be lost. Besides, in many places, especially in steam-boats, the smallness of the whole machinery is a great desideratum. It must be remembered also, that if a very large amount of cold air sweeps through the burning fuel, or between it and the bottom of the boiler, it will most materially reduce the temperature of the surrounding iron, and will also tend to carry the hot gases up into the chimney. The fear of this seems to be continually before the minds of our engine-makers, and they sometimes will make a furnace of a cylindrical form seven or eight feet long, and only about two feet in diameter; and this long box the stoker will almost fill with coal, and then shut the door as tight as possible, just as though the object were to imitate a gas retort. If they could succeed perfectly in this endeavour, unignited gas would assuredly stream up the chimney, generating no heat, and eventually the fire itself would go out for want of the great supporter of combustion. But this of course is never attained in practice; air does find access, the hydrogen of the gas is consumed, but the widowed carbon has little chance of meeting with another partner, and so in a sad black stream the sooty particles are borne up the flue and into the surrounding air. Disgust of neighbours, prosecution, fine, follow; but in vain, the furnace does not feel the punishment, the tall chimney smokes ruthlessly as before; renewed disgust, and further prosecution and heavier fine threaten its unhappy owner, and he turns doubtfully to those who with clamorous voices profess their ability to cure the giant evil.
Sir Henry de la Beche and Dr. Lyon Playfair presented, in March, 1845, to the House of Commons a Report, in which they ascribed the nuisance complained of to the three following
1st. The want of proper construction and adjustment between the fireplaces and the boiler, and the disproportionate size of the latter to the amount of work which they are expected to perform.
2nd. The deficiency of draft and imperfect construction of the flues leading to a chimney of inadequate height or capacity.
Opposite Ideas of a Stoker's Duty.
'3rd. The carelessness of stoking and management by those intrusted with the charge of the fireplaces and boilers.'
To take the last of these first into consideration, an improvement in the mode of 'firing the furnaces' might be often easily effected; and on that subject Mr. Williams has many judicious remarks. It is impossible for a stoker to do credit to himself if he have to feed a furnace which is radically defective in its construction; but a man, by his inefficiency, will often impede the proper working of a good furnace. There are stokers who seem to think that their whole duty consists in shovelling coals into the flue as rapidly as they can, and heaping them up inside, just where it will give them least trouble; that is, immediately within the door, and midway between the sides. Now, this is just the wrong place to put the coals; the bars ought to be equally covered; and as the combustion goes on most rapidly (in ordinary cases) at the sides, and at the further end, attention should be especially paid to throw fresh fuel into these places. Then, again, no heaping up ought ever to be allowed, for it narrows the passage which the air can traverse, and so produces imperfect combustion. The throwing of much coal on at a time, or in great lumps, is another vicious practice. In Cornwall they manage these things well, as the following quotation from Tredgold will show.*
The mode of firing adopted in Cornwall is spreading the charge of fuel equally and thinly over the fire, and feeding the fire frequently with small quantities at a time, and with coal broken into small pieces. It is, in fact, merely a return to the method recommended by Smeaton and Watt. The former in his direction for working the York Waterworks engine, August 29, 1785, says: Break every coal that is bigger than a goose's egg, and the oftener you fire, and the thinner, the better. The fire should be kept an equal thickness, and free from open places or holes, which are extremely prejudicial, and should be filled up as soon as they appear.''
The following narrative by Mr. Lowe, as given in the Society of Arts' Journal, may be interesting in illustration of this point:
'He had often occasion to cross in the steamer from Holyhead to Kingstown, and he had remarked the splendid bow of carbon which the Columbia left in her wake from port to port. He had pointed it out one day to the captain of the vessel, and he had been permitted to try the experiment of stoking three of the furnaces every ten minutes instead of every twenty minutes, and by leaving the furnace-door on the latch instead of closed tight; the result was that the smoke all
• As quoted in Mr. Williams's Essay.