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Engineers and Mechanics Encyclopedia 1839: Railways: Nicholas Wood's Report of the Rainhill Trials

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Rainhill Trials

"In examining the above, we find a very important effect in the economy of fuel, produced by the Rocket over the old engines, in the proportion of 10 to 18.34, supposing the heating powers of coke and coal be equal. The cause of this is very obvious, and is entirely attributable to the use of the tubes of small diameter, presenting such an area of surface to the water in the boiler. These tubes were used at the suggestion of Mr Booth, treasurer to the Liverpool and Manchester Railway Company, and nothing, since the introduction of those engines, has given such an impulse to their improvement.

"With a less area of fire-grate than the old engines, the surface exposed to the radiant beat of the fire is as 20:11.5, and the surface exposed to the communicative power of the heated air and flame, as 117.8:29.75, nearly four times as great.

"Nor is this the only difference; in the old engines, the area of the tube (of 22 inches diameter) for the passage of the flame and heated air to the chimney, was 380.13 inches; and of this large body of flame and air passing through the tube, only an extent of surface of 69.11 inches was exposed to the water in the boiler. In the Rocket engine, the area of heated air and flame in 25 tubes; 3 inches each in diameter, was 176.7 inches, while the surface exposed was 235.6 inches.

"It is not necessary, perhaps, to pursue the comparison further. The economy of fuel which must result from the exposure of so much greater surface to the water, cannot fail to insure a more perfect abstraction of the heat, and thus not only save the fuel, but prevent great part of the previous destruction of the chimney, by the intense heat of the wasted caloric.

"The same remarks apply to the Sans Pareil of Mr. Hackworth, as to the old engine, though in a less degree. In the Rocket, the surface exposed to the radiant heat of the fire, compared with the area of fire-grate, is as 3.3:1, while in the Sans Pareil, it is only 1.5:1; the some proportions as in the old engines. In the Rocket, the surface exposed to the heated air and flame, compared with the area of fire-grating, is as 19.6:1; while, in the Sans Pareil, the proportion is only 7.5:1. The bulk of air passing through the tube of the latter, will, at its exit into the chimney, be 176.7 square incises, the exposed surface being 47.12, or 25:1, nearly; while, as before stated, the bulk of air passing through the tubes of the Rocket, is 176.7 incises, or precisely that of the Sans Pareil, while the surface exposed, is 235.6 inches, or 1.3:1. These will sufficiently account for the great difference in the economy of fuel between the two engines, the Rocket requiring only 11.7 lbs. to convert a cubic foot of water into steam, while the Sans Pareil required 28.81bs.

"Some explanation is, perhaps, necessary, why the Sans Pareil should, in this respect, be more extravagant than the old engines, while the extent of surface, compared with the area of fire-grate, is much greater, and therefore should exhibit a more economical result; and this explanation is the more necessary, as, though not appearing at first sight, it involves a principle of the greatest importance in the economy of those engines; and which, if not acted upon, would render the use of the tubes, however otherwise valuable, considerably less effective.

"It will readily occur to any one, paying a little attention to the matter, that the system of tubes may be carried so far, as to reduce the temperature of the flame and heated air nearly equal to that of the water in the boiler; in which case, when it reaches the chimney, it will be incapable, from its reduced temperature, of producing a sufficient draught of air through the fire-grate. This would prevent all the advantages being taken of the refracting powers, which would otherwise result from the use of these tubes. It is stated in another part of this work, that on the introduction of those engines, it was necessary to resort to the application of waste steam thrown upwards into the chimney, to create a sufficient current of air through the fire; which was afterwards laid aside, or only partially used, when only slow rates of speed were required.

"Mr. Hackworth had, it appears, in his engine, resorted to the use of this in a more forcible manner than before used, throwing it up as a jet, and which, when the engine moved at a rapid rate, and the steam thereby almost constantly issuing from the pipe, had a most powerful effect.

"This, though effecting the object for which it was intended, being carried too far, partly in consequence of the rapid speed at which the engine was made to travel, was productive of another evil, which, though operating fatally so far as regarded that particular experiment, was capable of easy remedy.

“The consequence was, that when the engine began to travel at the rate of 12 or 13 miles an hour, the draught was so great, that it actually threw the cinders out of the chimney with considerable force, producing a destruction of fuel enormously great, so much so, that the consumption was at least 692 1bs. per hour.

"The area of fire-grate of the Sans Pareil was 10 feet; supposing that the area of the fire-grate of the Rocket had been the same, the consumption of the latter engine, with its power of exhaustion, would only have been 2611bs.; showing that the force of draught was so much greater in the Sans Pareil, as to consume nearly twice the quantity of fuel in the same time.

"This will satisfactorily account for the apparent anomaly in the consumption of fuel with this engine, compared with that of the old engines, having a single tube; otherwise, though not likely to come up to the Rocket in point of economy of fuel, we should have expected an effect considerably greater than in the old engines. The combustion of the fuel being so very rapid, and the abstracting surface so small, the heated air would pass off at a very high temperature; thus accounting for the loss of effect. The knowledge of this fact, - or rather, availing ourselves of this power for the purpose of creating a draught in the chimney, - leads us to an inquiry of great interest. By an extension in the use of these tubes of small diameter, there is little doubt of our being able (sup- posing we can force the necessary quantity of air through them), to reduce the temperature of the heated air, before its exit into the chimney, nearly equal to the water in the boiler. This would be abstracting all the useful heat, and probably effecting all the economy of which the fuel is susceptible.

"Perhaps it would not be advisable to carry it quite so far as this; for when the temperatures become nearly equal, the abstraction of heat would be so slow as to require a greater length of tube than it would be convenient to employ. We may, therefore, suppose that in all cases the temperature of heated air passing into the chimney will be greater than that of the water in the boiler. This heat will, however, be insufficient, in engines of this kind, to cause a sufficient quantity of air to pass through the fire for the purpose of combustion; and it becomes a question, whether we should allow a portion of the heat to escape for that purpose, or, by contracting the exit of the escape of the steam from the cylinders into the chimney, to erect the same object.

"Whether the last method is the most economical or not, though there is every reason to suppose it is, perhaps it is the only one with these engines that is suitable for their action upon railways, especially for quick travelling. The performance of those engines depends entirely upon the quantity of steam they can raise in a given time; and when travelling at the rate of fifteen miles an hour, or upwards, the production of steam is required to be very rapid indeed: the mode of producing a proper draught through the fire, by throwing the steam into tine chimney, after its passage through the cylinders, is, perhaps, therefore the best; as the quicker the engines travel, and when, consequently, the necessity for steam is the greatest, the then rapid and almost continuous exit of the steam into the chimney, increasing in proportion to the increased speed of the engine, produces at the same time a correspondingly greater quantity of steam.

"In the Rocket engine, this mode of increasing the draught of the chimney was but partially used; the steam was made to pass into the chimney by two pipes, one from each cylinder, and the size of the aperture was not, therefore, sufficiently small to cause the steam to pass into the chimney with adequate force, still, in that engine, we find it only required 11.7 lbs. to evaporate a cubic foot of water, - 36 per cent. less than with the old engines. We shall afterwards find that this has been considerably more reduced in the engines lately made.

"The Novelty engine is on a different principle from those previously considered, the necessary supply of air to the fire being produced by a bellows. In this case a chimney becomes unnecessary, and from the way in which the Novelty is constructed, the air was forced through the fire in a very condensed or compressed state. The area of fire-grate being little more than one-third of that of the Rocket, and the surface exposed to the radiant action of the fire less than one-half the temperature to which the fire was raised, must, of course, be considerably greater, to evaporate an equal quantity of water in the smile time. The abstraction of heat would be probably more perfect in the Novelty, for the tube through which the flame and heated air passed in its exit to the atmosphere was 36 feet in length in one tube; whereas in the Rocket there was the same length, though subdivided into six tubes. It is, however, extremely, questionable, whether one tube, 36 feet long, or 6 tubes, each 6 feet long, of the same sectional area, are more preferable; the latter would, of course, give a much greater exposure of surface. The area of exit of the heated air into the atmosphere of the Rocket, was 25 times that of the Novelty; from which we may imagine the degree of compression necessary to force the same quantity of air through the fire; though we do not say, that to raise an equal quantity of steam, an equal quantity of air, in that highly compressed state, is necessary.

"It was much to be regretted, that the experiment with the Novelty could not be continued sufficiently long to ascertain the power of raising steam by this method; the inquiry was of the utmost importance. Theoretically considered, we are of opinion, that this mode of generating steam is more economical in point of fuel, than in engines, the combustion of the fire of which is kept up by the rarefication in the chimney; but there are practical objections to set against this, of which the destruction of fire-bars, and the poster required to work the bellows, are not the least. We say theoretical, because, suppose two generators, the area of the grate-bars, extent of radiant, and communicative surface, are in both the same, except the area of exit pipe into the chimney, which, with the generator worked by the bellows, is one half of that by exhaustion of the chimney. If the same quantity of air pass through the grate-bars in each, that with the bellows will necessarily be in a more compressed state, to force the same quantity of heated air through the narrow exit; and this compressed state of the heated air will, of course, cause more of the caloric to be abstracted than in the other case; for we suppose the temperature reduced to the same, in both cases, in the exit pipe. For if the heated air, in both cases, pass into a chimney of the same area, and equal to that of the exit pipe from the generator, on the exhaustion principle; the temperature of the heated air being supposed to be the same, in both cases, in the exit pipe: the heated air from the generator with the bellows will, therefore, have to expand itself in the chimney, into twice its volume, which will, of course, reduce its temperature below that of the other; thus proving a more complete abstraction of the heat. The only question is, whether the disadvantage in practice, consequent upon the operation of such a principle, does not counterbalance any advantage gained in the economy of fuel; and this we must leave to experience to determine.

"The question between the two modes, however, assumes a new character since the application of the steam from the cylinder to create a current of air in the chimney; as in that case we can, by the use of a greater number of smaller tubes, reduce the temperature so low, until, if advisable to do so, it is equal to that of the water in the boiler. And it then becomes a subject of inquiry, which of the two modes occasions a greater loss of power in obtaining the necessary current of air; the stocking of the bellows in the one case, or the loss of power by the obstructed passage of the steam into the chimney, in the other.

"It is perhaps necessary, after the above disquisition, to explain, so far as we are able, the cause of the failure of the Novelty engine at the Liverpool experiments; to show that it arose from no defect in the principle, but only in the construction of that engine. It will be seen by the sketches of this engine that the flame and heated air, after leaving the fire, passed through the winding pipe of the horizontal generator. The generator was only 12 inches diameter, and there were three folds of the flue-tube within it, in diameter from 4 inches at one end, to 3 inches at the other; very little space was therefore left between the flue-tube and the top of the generator. The temperature of the flame within this tube, when the engine was running at a quick rate, would be very great, especially where it left the upright generator; and the evolution of heat would, therefore, be so rapid, that the passage of the steam out would prevent the water from flowing along this horizontal generator; and the consequence was, that the flue-tube got dry, and either collapsed with the heat and pressure, or gave way at the joint. This, it will be seen, however, arises from no defect of principle, and was easily remedied.

"The performance of the improved locomotive engines upon a level railway, has been estimated at from 30 to 40 tons, moved at the rate of 15 miles hour; according to which the following table, reduced from Mr. Wood's calculations, will show the quantity at different inclinations of plane.

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The following extracts from the newspapers (which we believe are substantially correct) afford an account of some interesting experiments, which show what the locomotives and the railway are capable of.

"On Saturday, the 4th of December, 1830, the Planet engine, Mr. Stevenson's, took the first load of merchandise which has passed along the railway from Liverpool to Manchester. The team consisted of 18 carriages, containing 135 bags and bales of American cotton, 200 barrels of flour, 63 sacks of oat-meal, and 34 sacks of malt, weighing altogether 51 tons, 11 cwt. 1 quarter. To this must be added the weight of the waggons and oil-cloths, 23 tons, 8 cwt. 3 quarters. Tender, water and fuel, 4 tons, and 15 persons on the team, 1 ton, making a total weight of 80 tons, exclusive of the engine, about 6 tons.

“The journey was performed in 2 hours and 54 minutes, including three stoppages of 5 minutes each (one only being necessary under ordinary circumstances), for oiling, watering, and taking in fuel; under the disadvantages, also, of an adverse wind, and of a great additional friction in the wheels and axles, owing to their being entirely new. The team was assisted up the Rainhill plane by other engines, at the rate of 9 miles ass hour, and descended the Sutton incline at the rate of 16.5 miles an hour. The average rate on the other parts of the road was 12.5 miles an hour, the greatest speed on the level being 15.5 miles an hour, which was maintained for a mile or two at different periods of the journey."—Liverpool Paper.

The annexed experiment shows the velocity of motion that had been attained on the railway at the period mentioned.

"The journey between the two places was on the 23d of November (1830) performed by the Planet engine in 60 minutes, including 2 minutes, the time employed in taking in water on the road, as usual.

"The motive for performing the journey was that the engine had been engaged to bring up from Manchester to Liverpool some voters for the election, and by some cause or other, the time of setting out was delayed, rendering it necessary to use extraordinary dispatch in order to convey the voters to Liverpool in time."—Liverpool Paper.

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