A Catechism of the Steam Engine

by C. E. John Bourne

A.--Yes, it would have at least that tendency; and as the space available for area of grate is limited in a steam vessel by the width of the vessel, it would be a great convenience if a double tier of furnaces could be employed without a diminished effect. It appears to me, however, that the objection would still remain of the steam raised by the lower furnace being cooled and deadened by the air entering the ash-pit of the upper fire, for it would strike upon the metal of the ash-pit bottom.

163. Q.--Have any other plans been devised for feeding the fire by self-acting means besides that of a revolving grate?

A.--Yes, many plans, but none of them, perhaps, are free from an objectionable complication. In some arrangements the bars are made like screws, which being turned round slowly, gradually carry forward the coal; while in other arrangements the same object is sought to be attained by alternately lifting and depressing every second bar at the end nearest the mouth of the furnace. In Juckes' furnace, the fire bars are arranged in the manner of rows of endless chains working over a roller at the mouth of the furnace, and another roller at the farther end of the furnace. These rollers are put into slow revolution, and the coal which is deposited at the mouth of the furnace is gradually carried forward by the motion of the chains, which act like an endless web. The clinkers and ashes left after the combustion of the coal, are precipitated into the ash-pit, where the chain turns down over the roller at the extremity of the furnace. In Messrs. Maudslays' plan of a self-feeding furnace the fire bars are formed of round tubes, and are placed transversely across the furnace. The ends of the bars gear into endless screws running the whole length of the furnace, whereby motion is given to the bars, and the coal is thus carried gradually forward. It is very doubtful whether any of these contrivances satisfy all the conditions required in a plan for feeding furnaces of the ordinary form by self-acting means, but the problem of providing a suitable contrivance, does not seem difficult of accomplishment, and will no doubt be effected under adequate temptation.

164. Q.--Have not many plans been already contrived which consume the smoke of furnaces very effectually?

A.--Yes, many plans; and besides those already mentioned there are Hall's, Coupland's, Godson's, Robinson's, Stevens's, Hazeldine's, Indie's, Bristow and Attwood's, and a great number of others. One plan, which promises well, consists in making the flame descend through the fire bars, and the fire bars are formed of tubes set on an incline and filled with water, which water will circulate with a rapidity proportionate to the intensity of the heat. After all, however, the best remedy for smoke appears to consist in removing from it those portions which form the smoke before the coal is brought into use. Many valuable products may be got from the coal by subjecting it to this treatment; and the residuum will be more valuable than before for the production of steam.

STEAM.

165. Q.--Have experiments been made to determine the elasticity of steam at different temperatures?

A.--Yes; very careful experiments. The following rule expresses the results obtained by Mr. Southern:--To the given temperature in degrees of Fahrenheit add 51.3 degrees; from the logarithm of the sum, subtract the logarithm of 135.767, which is 2.1327940; multiply the remainder by 5.13, and to the natural number answering to the sum, add the constant fraction .1, which will give the elastic force in inches of mercury. If the elastic force be known, and it is wanted to determine the corresponding temperature, the rule must be modified thus:--From the elastic force, in inches of mercury, subtract the decimal .1, divide the logarithm of the remainder by 5.13, and to the quotient add the logarithm 2.1327940; find the natural number answering to the sum, and subtract therefrom the constant 51.3; the remainder will be the temperature sought. The French Academy, and the Franklin Institute, have repeated Mr. Southern's experiments on a larger scale; the results obtained by them are not widely different, and are perhaps nearer the truth, but Mr. Southern's results are generally adopted by engineers, as sufficiently accurate for practical purposes.

166. Q.--Have not some superior experiments upon this subject been lately made in France?

A.--Yes, the experiments of M. Regnault upon this subject have been very elaborate and very carefully conducted, and the results are probably more accurate than have been heretofore obtained. Nevertheless, it is questionable how far it is advisable to disturb the rules of Watt and Southern, with which the practice of engineers is very much identified, for the sake of emendations which are not of such magnitude as to influence materially the practical result. M. Regnault has shown that the total amount of heat, existing in a given weight of steam, increases slightly with the pressure, so that the sum of the latent and sensible heats do not form a constant quantity. Thus, in steam of the atmospheric pressure, or with 14.7 Lbs. upon the square inch, the sensible heat of the steam is 212 degrees, the latent heat 966.6 degrees, and the sum of the latent and sensible heats 1178.6 degrees; whereas in steam of 90 pounds upon the square inch the sensible heat is 320.2 degrees, the latent heat 891.4 degrees, and the sum of the latent and sensible heats 1211.0 degrees. There is, therefore, 33 degrees less of heat in any given weight of water, raised into steam of the atmospheric pressure, than if raised into steam of 90 Lbs. [1] pressure.