MR. MALLET'S METHOD OF OBTAINING VACUUM FOR ATMOSPHERIC RAILWAYS BY DIRECT CONDENSATION OF STEAM-(CONTINUED FROM P. 199.) THE only remaining valves are the "snifting valves,"or large balanced valves at the ends of the vacuum vessels to give egress to the air on admitting steam, and similar but smaller ones to the condenser. These latter valves are self-acting. The six former valves spoken of, viz., the three belonging to each vacuum vessel, are moved either by hand, or by a motion derived from suitably formed "cams," actuated by the same very small steam-engine which is required to feed the boilers with water. It is proposed to condense the steam, so that the condensing water shall flow off at 70° Fahrenheit; at this temperature such quantity of it as is required is withdrawn by the feed pumps from the well at the lower end of the condenser syphon, and pumped into the hot-water tube in the boiler flue before spoken of. This constitutes the whole apparatus, its operation is as follows: Steam being up in the boilers, a little is blown off into the condenser, and the stop-valve shut; the condensing water is admitted, and a very slight vacuum is produced in the condenser. This is requisite to cause the steam from the first vacuum vessel to enter the condenser rapidly, when permitted. The steam valve is now opened to one vacuum vessel, say the left-hand one, the other two valves, viz., the condenser and railway valves, being shut. The air is expelled from it by the snifting valves, and as soon as steam "blows through," the valve from the boiler is shut, and the condenser valve opened; the contents of the vacuum vessel now rush into the condenser, and are condensed. The valve between the vacuum vessel and condenser is now shut, and that between the former and railway tube opened, when air from the latter rushes into the partial vacuum of the vessel. The moment the steam valve was shut to the left-hand vessel, it was opened to the right-hand one, which in the same way was filled, and by a precisely similar set of operations a partial vacuum was formed in it, and communication made between it and the railway tube just subsequent to the moment when communication was closed between it and the former vessel, and so on alternately, each vessel being filled with steam; this condensed, a vacuum produced, and air admitted therein from the railway tube until in equilibrium. These actions recur at regular intervals, and the cams moving with a uniform motion, are so arranged as to open and close the several valves at the proper times. These times and the general working of the machine, which is much more simple than it seems in words, are best seen by inspecting the following diagram or expression in signs of the whole motions. (For which see opposite page.) It will thus be observed that each of the vacuum vessels is alternately engaged in exhausting the railway tube and in forming its own vacuum. The valves are at the first moment moved by hand, and afterwards uniformly by the engine, which is worked by the same steam boilers as are employed in the apparatus at large. We have next to consider the proportions of the various parts of the apparatus. Let us suppose, as applicable to a length of six miles of 15-inch pipe. The capacity of the tube is ft. sq. ft. 5280 × 6 × 1·227=38872 cubic feet. The capacity of each condensing vessel we will assume to be one half this =19436 cubic feet. And as each consists of two cylinders— 19436 2 = =9718 cubic feet, capacity of one. These cylinders may be made of 10 feet diameter=78.54 square feet area. 9718 Hence the length= 78.54 =124 feet. The capacity of the condenser need not be more than one half that of either condensing vessel, for such a proportion has been found to give sufficiently rapid condensation in large steam-engines, and the same must hold here. Hence the condenser will be one cylinder of 10 feet diameter, and 124 feet long. The dimensions of the boilers are dependent upon the supply of steam demanded per minute. We shall presently see that a sufficient vacuum for starting will be produced in a 15-inch pipe of six miles long, by three Note. To make this Table completely intelligible, the reader should rule lines across at intervals of ten seconds. *Exhausting* *Exhausting* *Exhausting* *Exhausting* *Exhausting* 211 exhaustions of one vacuum vessel, and that three to four more exhaustions will discharge the tube as the train advances. Now if the speed of the train be 30 miles per hour, 6 miles will be passed over in 12 minutes, which gives an interval of 3 minutes between each exhaustion. This is the limit therefore for the supply of steam. We must have as much steam as will blow the air out of, and fill one vacuum vessel every three minutes at 212° Fah., or the power of this in the boilers. But for the more rapid and effectual blowing through of the vacuum vessels, as well as to save boiler room, it is proposed to generate steam in the boilers at 3 atmospheres = 45 lbs. above the atmospheric pressure. Hence required the full of one vacuum vessel of dense steam at 45 lbs. per three minutes. Now allowingth of the volume of steam to be lost, which we shall hereafter show to be an ample allowance, we require every three minutes 19436 + 19436 10 = 21379 steam at same pressure is that required in one minute. Now steam at this pressure gives about 620 volumes from one of water, hence 620: 1 :: 2376: 3.83-cubic feet of water required to be evaporated per minute;-say 4 cubic feet per minute. Hence we get the following dimensions for the boilers, supposing those of the Cornish principle adopted. If locomotive boilers be used, four boilers of the common large size would be sufficient; but then the consumption of fuel per cubic foot of water evaporated would rise from 5.9 lbs. of coal to about 11.0 or 11.5 lbs. of coke. Steam being generated at 45 lbs. in the boilers, we must (in order to obtain a rapid rush of steam into the vacuum steam space. The results of experience in Cornish boilers prove that five boilers, each 6 feet 6 inches diameter, by 110 feet long, with a fire-grate surface of 72 square feet each, in two fire-places, one at each end of each boiler, will generate the required volume of steam, consuming 5-9 lbs. of coal per cubic foot of water evaporated from 70° Fah. into steam at 45 lbs. per square inch. These five boilers will possess a steam space of 3500 cubic feet. The remainder of that required, viz., 3630, must be provided by steam chests placed upon the boilers. Let us next consider if the allowance of th of the whole volume of steam for waste is sufficient, which is 1943, say 2000, cubic feet of steam at each filling of the vacuum vessels; the sources of waste steam are: 1. Cooling by radiation from the pipes and vacuum vessels. 2. Heating the cold air entering the vacuum vessels from the railway tube. 3. Blowing off at safety valves of boilers. 4. Leakage and blowing through at snifting valves. The two last can only be guessed at ; the first and second may be calculated approximately. The surface of one pair of vacuum vessel cylinders=2 cylinders of iron, 10 feet 6 inches diameter, by 124 feet long, assumed half an inch thick, which is double their proposed scantling, and adding, as usual, th for angle and rivet iron. ATMOSPHERIC RAILWAYS BY DIRECT CONDENSATION OF STEAM. which, at 20 lbs. per square foot, and adding th, is=209704 lbs.; and as a cubic foot of wrought iron weighs 477 lbs. 209704 -417 cubic feet of iron. But 477 0051 lbs. of coal will heat 1 cubic foot of iron 1° of Fahr. And assuming the utmost possible waste, or that the whole mass of iron representing the vacuum vessel were heated at first starting from 50° to 200° or 150° Fahr., then 0.765 lbs. of coal will heat 1 cubic foot of iron 150°, and hence 417 × 0·765=319 lbs. of coal. But from the construction of these vessels no more than a thin film of pine timber of the lining will be heated to 212° at each filling with steam, so that probably theth of the above, or the loss of 3 lbs. of coal, will be the outside of the waste at each filling with steam by cooling from the vessel itself. Now 3 lbs. of coal is represented by 607 cubic feet of steam at 212°. But we have a further loss of heat in the air, which entering the vacuum vessel at each period or stroke, (as we shall call each filling with steam, and condensation thereof,) must be heated by radiation from the vessel, and by the next entering steam from the mean temperature 50° to say 200°=150° Fahr. The vacuum vessel holds 19436 cubic feet; let us assume this whole volume of air heated 150° at the first stroke, half of it at the second, and half of that at the third, &c. Now 00000184 lbs. of coal will heat 1 cubic foot of air 1 degree Fahr., hence 000276. of coal to heat 1 cubic foot 150°; and 000276 × 19436=5′364 lbs. of coal to heat the whole volume at the first stroke 150°; half this, or 2 682 lbs. for the second, and half that for the third, &c.; but suppose the waste of the first stroke to continue all through, on the previous data, 5.364 lbs. of coal=1092 cubic feet of steam. So that it appears, after the apparatus is got to work, that 1699 cubic feet of steam is more than enough to meet the principal sources of waste, leaving 301 cubic feet of steam at each stroke for leakage and blowing through. Hence our allowance of th the whole volume for waste is ample. We have next to consider what amount of vacuum we can obtain by a given number of exhaustions of the vacuum vessels and equilibrations with the air in the railway tube. It is obvious that a vacuum-producing apparatus of this sort 213 (b+r) sent the amount of rarefaction after n exhaustions, and that we may so consider the present case, applying to the results obtained suitable corrections for the three following sources of deduction from the amount of vacuum, víz. :— 1. For the tension of aqueous vapour due to the temperature of condensation, this being determined at 70° Fahr., is =0.726 inches of mercury. This amount of deduction for vapour is too great, for the limit of tension of vapour will be, after a few seconds, that due to the coolest part of the apparatus; but this is the railway tube, which is always in connexion with one vacuum vessel; and as the temperature in this would probably never be above 60°, and its average below 50°, the real tension of vapour to be calculated on would be due to those temperatures. 2. For the volume of combined air liberated from the condensing water, and carried in by the steam. Rain water contains from 2 to 3 per cent., say 5 cubic feet of air for every 100 feet of water introduced to the condenser. 3. For the expansion of the air from the railway tube entering the vacuum vessels and becoming heated, and hence practically less entering at each stroke than is due to the capacity of the vessel and difference of pressure. Air expands a of its volume for 1 degree Fahr. or more correctly according to Rudberg but take the larger expansion, and assume that the air entering the vacuum vessels from the tube gains 20° Fahr., ́ then 20-its expansion=2. But the effect of this is tantamount to diminishing the capacity of the vacuum vessel, or the value of b part; so that if the capacity of the railway tube be in this case represented by 200, that of the vacuum vessel will be say 100 – (21⁄2 say 1)=95. To this another small correction should be made for the air of the railway tube becoming saturated with vapour on entering the vacuum vessel; this may however be neglected. In order to obtain the value of our second correction, we must now determine the volume of condensing water required for each stroke or period. We 50° Fahr. The volume of condensing water, therefore, required for one stroke or period, at the following temperatures, is 32° 120 This air we may suppose introduced to the vacuum vessel dry, and as it will become saturated with vapour at 70°, from the moist sides of the vessel, its bulk will be enlarged; but inasmuch as we have taken 5 per cent, in place of 24 or 3 per cent. for the air evolved, and as in most cases the condensing water will not even contain this much, and may be used over again, and so evolve scarcely any, it is not worth while to apply this correction. Hence, taking the nearest whole numbers above the preceding and as the evolved air will always occupy one vacuum vessel and the condenser together, whose united capacity is=29154 cubic feet, we have at the first stroke, and with the above temperatures of condensing water, the following values for the air evolved in terms of the vessels' capacity :— It thus appears, where the capacity of the vessels is so large, that up to the eighth stroke this correction is quite unimportant, even with condensing water at 50° Fahr., and supposing the whole of the air evolved to remain in the vessels. But, as at each succeeding stroke the vacuum vessels are filled with steam, twothirds of the whole quantity of air evolved from the condensed water of the preceding stroke is blown out; and hence the actual depressions of the vacuum gauge would only be one-third of the values in the preceding table, or insensible until after about 80 strokes,when the condenser itself would have to be cleared of air by blowing through. And thus it appears, that the larger the vacuum vessels can be made in capacity the better; the limit of size being wholly dependent upon practical considerations of construction, and upon the power of rapid supply of steam to fill them. In the present case we have provided a capacity such that eleven trains may be passed before it becomes necessary to clear the condenser of air; it is therefore plain, that if this operation were performed at every fifth train, the vessels might be of only one-half the capacity here assigned. The generating power of the boilers being still the same, the rapidity of producing vacuum would be but little diminished. The question, however, of the most advantageous possible size of vessels under given conditions, is one requiring further investigation, based upon certain experiments, which require to be made. (To be concluded in our next.) |