184 ON THE CAUSES OF INJURY TO BOILERS. With respect to this direct connexion I find but little notice has been taken. Professor Kelland, of Cambridge (now of Edinburgh), has however, in his "Theory of Heat," distinctly recognised it as demanding attention. Under the head " Convection," he observes, "A mode of loss of heat analogous to conduction, is that to which Dr. Prout has applied the term convection. When a hot body is in contact with the air, the part next the body becoming more elastic (rarefied), flies off and is supplied by colder portions; thus the heat of the body is conveyed away more rapidly than it would be if the air were not in motion. It is obvious that this circumstance will materially affect all experiments on the motion of heat, in which it is hardly possible to estimate the effects due to this cause." Now it is this very circumstance, as it affects the condition of the metals employed in evaporative processes, that I am desirous of examining; inasmuch as this " convection," or carrying away, as the term imports, will, in practice, be found to be the primary source of good or evil. Conduction being but the secondary, or induced cause. Professor Kelland, further on, in examining the mode by which heat is transferred from one part of a body to another, observes, "When speaking of solids, this is called conduction. It is clear, from the term itself, that we do not include, under this head, the transfer of heat by radiation; nor do we include that transfer which takes place amongst the particles themselves, carrying with them the caloric they have acquired, which we designate convection." It is manifest from this clearly defined distinction between conduction and convection, that as the former refers to solids, so the latter refers to fluidsaeriform or liquid; inasmuch as the carrying with them" the caloric they have received, involves a mobility among the particles which is inconsistent with the nature of solids, while it is a correct definition of that of fluids. BY C. W. WILLIAMS, ESQ. Practically speaking, then, this distinction is peculiarly applicable to our present inquiry, the whole turning on this point, whether conduction or convection be the prime mover in producing those fluctuations of temperature in the conductor which lead to useful or injurious results. Let us now examine, practically, the effect of this convection, or power of carrying away the heat, and the extent to which it influences the heat conducted, both as regards quantity and rapidity. Let us suppose that a given quantity of heat is passing through a metallic bar, with a rapidity due to its maximum power of conduction ; and that it is taken up, or absorbed, by the recipient, with a corresponding rapidity. The temperature of the conductor will then truly indicate that permanent state, referred to by Mr. Brande. If, however, this absorbing power on the part of the recipient be diminished, the rate at which the heat passes from the conductor will also be diminished in the same ratio; and as a necessary corollary, the rate of the current of heat through the conductor will be reduced in a corresponding degree. The practical question then is, to what extent will this " permanent state" or statical temperature of the conductor be affected by such diminished power of convection in the recipient; for this statical heat will ever influence the question whether such conductor be under or overheated. The following experiment will illustrate some of the relations which affect this inquiry. The annexed engraving, it will be seen, exhibits a state of things, corresponding in principle, to that referred to in your Number 967, in which the thermometer indicated the statical heat of the conductor bar. I have now extended the illustration by lengthening the bar and introducing three thermometers, thus, to indicate the heat at three different sections, and mark more accurately the varying temperatures. In this engraving, as in the former one, let A represent the conductor bar; B the vessel to contain the recipient, water, or air, or whatever it may be; C the cock for letting off the liquid, when employed as a recipient; D1, D2, D3, the three thermometers, indicating the statical heat at their respective portions of the bar; E the constant source of heat, being that from a powerful gas-burner, furnished with a metallic dome, on the 186 ON THE CAUSES OF INJURY TO BOILERS. principle of the "Solar Lamp ;" and F, the protecting shield. To these I have now added the chamber G, which was filled with charcoal powder, to act the part of a non-conductor. By this means, radiation from the bar was prevented; the thermometers received heat alone from their points of contact with the bar -the internal streams of conducted heat being confined, as though it were water in a canal or tube. By the following table, No. 1, we find that in fifty minutes the water in the vessel B was raised to the boiling point, by the issue of what I have called the stream of dynamical heat, from the end of the bar at A. A "permanent state" of temperature was then attained; the thermometers at their respective distances from the source of heat, in arithmetical progression, indicating the temperatures, in geometrical progression, (or sufficiently near for our purpose, for accuracy in this respect was not attempted), of 400°, 309, 242, -the extreme difference being 158°. Let us now suppose this to be the state of things in which the maximum power of conduction of the metal was brought into action. It is here manifest that the amount of statical heat in the conductor will bear some ratio to the joint power of conduction in the bar and convection in the recipient water. On the recipient being changed, by letting out the water and letting in the air, a new state of things is induced. The power of convection will be diminished, (air being a worse recipient than water); the quantity of heat received from the conductor in each unit of time will also be diminished; and the rate at which the stream of conducted heat passes through the bar will be influenced in a corresponding degree. The source of heat, however, remaining constant, the inevitable result is that accumulation will take place in the bar, and the thermometers instantly indicate an increase of statical heat. This I believe to be the rationale of the process, and this is exactly what we find confirmed in practice. Now, if the velocity of the convection of the new recipient, air, be ascertained, as well as the conductive power of the metal bar, and both taken as constants, we shall be enabled to approximate to the amount of statical heat in the conductor, and the degree in which the metal will be affected. It is worthy of notice here, that contrary to what might have been expected BY C. W. WILLIAMS, ESQ. on the water being withdrawn, the thermometer, nearest to the recipient and farthest from the heat, rose with the greatest rapidity, indicating a species of revulsion, or backing up of the heat; and proving that the increased heat of the bar was referable to the diminished power of the recipient, and not to the supply of heat. This is extremely instructive, proving the diminished power of convec tion in the recipient air, comparatively with that of water. Time in Therm. 0 5 10 15 20 25 30 35 By inspection of this table, No. 1, we find that between the periods, 55 and 60 minutes, (say in five minutes), the thermometer No. 3, (the farthest from the heat), had risen 42°, that is, from 242° to 284; whereas, No. 1, (that nearest to the heat), rose but 13° in the same five minutes, say from 400° to 413°; thus proving, that what may be called the wave of statical increase had flowed backwards towards the source of heat, and in a manner strictly analogous to what would take place if, instead of a stream of heat, it had been a stream of water in a tube or canal. The bearing, practically, of this illustration is, that the temperature of a conductor plate or bar will be influenced, not so much by the quantity of heat imparted to the conductor, as by the absorbing or convecting power of the recipient. This also shows the practical error of attributing injury to what is going on outside the boiler, or in the plates themselves, while we neglect what is taking place within the boiler, where the real source of injury exists. FURNACES AND BOILERS. This table further shows, that, by reason of the restricted power which the air possesses of carrying away the heat from the conductor bar, the temperature of the latter rapidly increases, until a second "permanent" state is established; and when the three therinometers stand, respectively, at 476, 415°, 362°, the extreme difference here is but 114, whereas with water as the recipient it was 158. This relation of 114° to 158°, then, indicates the ratio of the conductive powers of the bar brought into action by the influence of the respective recipients, air and water. Again, we see that this new statical heat of the bar is in the ratio of the convecting power of the recipient, the thermometer nearest to the source of heat rising from 46° to 476,° while that most removed rose from 242° to 362°. Many other instructive relations might here be noticed, if time and space permitted. The more accurately to observe the varying relations between the statical and dynamical heat, under a different state of things, I reversed the experiinent, and began the operation with air as the recipient; then changing it to water, so soon as the first permanent state of temperature was established, and which, by the Table No. II., we see took place in forty-five minutes. The results here are equally interesting and instructive. 187 mained stationary, such being the indicated amount of statical heat. (It should here be observed, that these numbers cannot be taken as giving the absolute temperature of the bar, although they approach sufficiently near to give the relative temperatures, under the influence of the recipients, air and water. It will hereafter be shown how near they approach to such absolute temperature.) The recipient being now changed from air to water-the latter being poured into the vessel B-we see the superior convecting power of the latter at once brought into action, by the immediate lowering of the temperature of the bar. On the water being again raised to the boiling point, a permanent state is again established, as in the preceding example, whatever variance has taken place being accidental. It is here important to observe how the analogy between the current of conducted heat and a current of water is maintained; for we see that, although thermometer No. 3 arrives at its minimum, 228°, in 60 minutes, as marked with an *, No. 2 does not reach it until after 70 minutes, and No. 1 until 75 minutes. These experiments were made with great care, although the adjustment of the apparatus required much exactness, to produce uniform results. Throughout the whole we see sufficient to justify the observation, that much remains yet to be done, before this complicated subject be exhausted. The practical inference I draw from the above experiments is, that they afford sufficient proof of the position before stated, namely, that it is not to the furnace or draught, or activity of the fire, we are to look for that accumulation of heat in the plates of ordinary boilers which produces overheating and rupture, but to the recipient, and its powers of carrying away and absorbing the heat which the conductor plate or bar is capable of imparting. Purposing to continue this subject on a future occasion, minutes. No. 1. No. 2. 0 54 54 54. 5 170 132 100 10 270 220 178 15 350 288 237 20 Although you declined to publish my last letter in reply to Mr. Dirck's attack on me last year, respecting this same "smokenuisance" controversy, I still confidently expect that your sense of justice and propriety will induce you to insert unmutilated the enclosed copy of my report on the failure of Mr. Williams's patent furnace at the works of Messrs. Hamnett and Co., of Manchester, and which report was first printed by me in July last, and since published in the Mining Journal and elsewhere.* After perusing this report, I trust your readers will be able to appreciate in a proper manner Mr. Williams's repeated assertion that I made that report before I had seen "a single furnace erected by him," or by his "directions." Why, this very furnace was erected by his advertised agents Dircks and Co., Mr. Williams himself being present when it was tried, and a witness to its failure. An alteration was afterwards made by Mr. Williams's direction, and then it failed again by the giving way of the boiler as it is demonstrable it always must do, whenever the engine is fully loaded, or a considerable supply of steam is wanted from the boiler, so as to require tolerably hard firing. The above facts have been verified to the satisfaction of every one who has chosen to inquire of the proprietors of the boiler in question; and the same consequences have ensued, many of them fatal to human life, and must inevitably ensue again, wherever * The truth of the report referred to is denied by Mr. Williams, and Mr. Armstrong afterwards admits that it is the subject of an action pending against him for libel We do not, under these circumstances, consider that we should be acting either with "justice," or "propriety," were we to comply with his request. When proved to be no libel, we shall be very ready to give it a place in our pages; but if it be one, it has had more than sufficient publicity already. We may here add, in explanation of the asterisks in the first paragraph of this letter, that they denote parts which we have left out, because we cannot allow our pages to be made, on any pretext, the medium of wanton insuit to any one, far less to a scientific inquirer of so original, so philosophical, so practically useful, and withal so sincere and candid a stamp as Mr. Williams.-ED. M. M. Mr. Williams's principle of letting in cold air at or beyond the bridge of the furnace has been carried out under the same circumstances. Another of several of the patent furnaces examined by me was on the premises of those same agents, Dircks and Co., engineers, &c., Vulcan-street, Liverpool, which the public were repeatedly invited to inspect by public advertisement, and which I described and condemned in the Liverpool Mercury twelve months ago, as well as in your Magazine for March 6th of last year. It is certainly true that Mr. W. threw the legal responsibility for the damage done to the boiler, in the first case above mentioned, on his agents. But, if Mr. Williams really thinks it necessary to endeavour to get rid of these facts on the ground of his not being responsible for his agents, then, indeed, his case is more hopeless than even I had supposed. The desperate case in which Mr. Williams finds himself is still, however, more clearly evinced by this last strange exhibition of himself in your pages. In page 88 of the Mechanics' Magazine, he gravely states that I wrote to his agent a letter of "recantation," which he professes to quote from, beginning as follows:-"I find that the opinions expressed in my report before named were formed on erroneous data," &c. &c. Now, for a reckless audacious assertion, such as I have before had occasion to give to its author its only proper name, this deliberately written one beats all that I believe was ever before recorded in the annals of mendacity, and really deserves a patent for its originality;* for I not only never wrote a single line or a word of what he charges me with, nor authorised any one to do so for me, nor have I ever expressed a single syllable, either verbally or otherwise, to the effect stated by him; but I have never even had the least communication with either himself, his agents, or his solicitors, in any way, or on any subject whatever, since the letter, from which the extract referred to in the above-quoted passage appears to have been taken, was written; that letter being written by himself, or his agents, and sent to me by his solicitors, annexed to one from them • Mr. Armstrong is aware that Mr. Williams recalled the statement in question of his own accord, the moment he saw it in print-for this is what he previously alludes to, as Mr. Williams's "recantation at page 111;" and to speak of an acknowledged mistake in such terms as these is not right. We must, in justice to Mr. Williams, add, that he wrote to us to make the necessary correction in the statement, even before it appeared in print; but, in consequence of the Number in which it was published happening to be printed off a day earlier in the week than usual, his letter came to hand a day too late for the purpose.-ED. M. M. |