The Boy's Playbook of Science Part 50

In locomotive steam-engine boilers, the princ.i.p.al object is to generate steam with the greatest rapidity; hence the boiler consists of two parts--viz., a square box containing the fire, and around which a thin stratum of water circulates, whilst the draught for the fire rushes through a number of copper tubes placed in the second or cylindrical part of the boiler. By the use of these tubes an immense _surface_ of water is exposed to the action of the fire, and the steam is not only generated with amazing rapidity, but is also maintained at a very high pressure.

Within the last few years "superheated steam" has been favourably mentioned, and employed economically for driving certain engines. The principle consists in first generating steam, and then pa.s.sing it through coils of strong wrought-iron pipe, by which it acquires additional heat, and we have therefore combined in steam the ordinary principle of evaporation of water with the heated-air principle of Stirling, described at p. 367. We give a drawing of Scott's patent generator and superheated steam engine. (Fig. 399.)

The apparatus is used as follows:--A fire is made in the furnace, and so soon as a pyrometer connected with that indicates about 800 degrees, [Page 435] a little water is pumped into the coils by hand, which is immediately converted into steam. The donkey engine is then started, which maintains the necessary feed of _air_ and water. The generator produces a copious supply of elastic mixed gaseous vapour, at a pressure of 250 pounds on the square inch; and it is stated that this engine works satisfactorily, and is started in the incredibly short time of from three to five minutes, so that for marine engines in war vessels, expecting to be ordered out suddenly, no fuel need be burnt till the moment required.

[Ill.u.s.tration: Fig. 399. Scott's patent generator, or new _versus_ old steam.]

Experiments with superheated steam have already been tried most successfully on board the Peninsular and Oriental Company's ship the _Valetta_, whereby it is stated that a saving of thirty per cent. in fuel [Page 436] is obtained. The engine to which the superheated steam was adapted was constructed by Penn and Sons, and the vessel attained a speed of nearly sixteen knots per hour, and under the most adverse circ.u.mstances had an abundance of steam to spare.

"A most important experimental improvement in steam machinery was on Thursday last tried for the first time down the river, on board the Peninsular and Oriental Company's ship, the _Valetta_. The actual nature of the improvement may be described in a few words as consisting of a simple apparatus for working marine engines by means of superheated steam; but it is not too much to say that in the success or failure of this experiment are involved results so important as to affect materially all ocean-going steamers, and, indeed, steam machinery of all kinds. To be able to work machinery with superheated steam, means to command increased power with a thirty per cent. reduction in the consumption of fuel. A principle which can effect such important changes in the universal application of steam has not remained undiscovered to the present day. The want of superheated steam has long been felt, and the enormous comparative advantages of working engines on such a plan have long been known. A simple and effective working of the principle, however, has been an engineering difficulty which various expedients--all, however, sufficiently successful to show the value of the improvement--have failed to obviate entirely. This obstacle has now, we believe, been effectually overcome by Mr. Penn, and the value of the improvement so clearly demonstrated, that the general application of the principle to steam machinery of every kind may now be regarded as certain.

"The idea of working engines by superheated steam, and the immense saving of fuel and increase of power it would effect, was, we believe, first started many years ago by Mr. Howard, and subsequently by Dr.

Haycraft. The difficulties, however, in the way of its adoption at that time, and the undue estimate of the importance of the principle, prevented those gentlemen from realizing very great practical results.

At a later period the matter was again taken up by an American engineer--Mr. Weatherhead--who, however, only superheated a portion of the steam and mixed it with common steam in its way to the cylinders.

The success which attended even this partial application of the process again revived the idea, and encouraged other engineers to turn their attention to the subject. The result of these renewed efforts is that several methods of securing the great economy to be effected by superheating the steam are now under trial, and there is no doubt that a most important step in the progress of steam, especially as applied to ocean navigation, is now at last on the point of being successfully accomplished.

"The value of the improvement on the score of economy in working may be best ill.u.s.trated by a single fact--namely, that the Peninsular and Oriental Company's bill for coal annually amounts to the enormous sum of 700,000_l._, and that by working their vessels with superheated steam properly applied, it is become almost certain that, without any [Page 437] detriment to the machinery, from 28 to 30 per cent. of this gigantic outlay can be saved. As to the various proposed methods of superheating steam, it may be briefly explained, that the conditions required to be fulfilled are perfect simplicity of arrangement with ready control over the apparatus; that it should be so placed as not to be liable to accidental injury in the engine-room; and that the heat employed for superheating the steam should be waste heat which has already done its duty in the boilers and is pa.s.sing away.

"All these conditions have been most satisfactorily fulfilled by Mr.

Penn in the new engines on board the _Valetta_, which were tried down the Thames for the first time on Thursday. The _Valetta_, as our readers may remember, was for many years the mail-boat between Ma.r.s.eilles, Malta, and Constantinople. While thus employed, she had Penn's engines of 400 horse-power, and to work these up to an average speed of 15 miles an hour required a consumption of fuel of from 70 to 75 tons of coal per day. At no time was it less than from 45 to 55 tons. These engines have now been removed to a vessel nearly double the tonnage of the _Valetta_, and the latter fitted with engines by Mr. Penn on the superheating principle. We may mention that, besides this alteration, the _Valetta_ has been considerably improved. A p.o.o.p and forecastle have been added, increased accommodation given to pa.s.sengers, and the whole vessel fitted up in the richest style. The saloon is one of the simplest and handsomest things of the kind we have seen, sufficiently lofty and capacious, and above all, admirably ventilated on the system which is now being adopted on all sea-going steamers, and the merit of devising which belongs to Mr. Robinson, of the Peninsular and Oriental Company.

"To return, however, to the engines. Mr. Penn, at the repeated request of Mr. Allen, the Managing Director of the Peninsular and Oriental Company, undertook to apply to them the principle of superheating, to which his attention had many years before been seriously directed by Dr.

Haycraft. His method of doing this is to place in the smoke-box of the boiler, through which the hot air from the furnace first pa.s.ses, as large a number of small pipes as is consistent with allowing a free draught from the furnaces. Through these all the steam from the boilers pa.s.ses in its way to the cylinders. By this plan an immense heating surface in the pipes is secured, the steam is in a subdivided form, so as to be readily acted on, and the waste heat from the furnace is utilized at the point where its intensity is greatest, and where the greatest conveniences exist for applying the apparatus. By means of three ordinary stop-valves, the whole contrivance can be shut in or off from the engines at pleasure. In ordinary engines steam leaves the boilers at about 250, but declines from this temperature in its way to the engines to 230, undergoing from condensation a still greater and more serious diminution of heat in the cylinders. From these causes, and also from the immense quant.i.ty of waste heat which escapes through the smoke-box and up the funnels, there has always been a theoretical loss of steam power amounting to forty per cent., as [Page 438] compared with the coal consumed. It is this loss of power and waste of heat which the superheating process is intended to prevent, and which will, of course, allow a reduction of from twenty-eight to thirty per cent. on the fuel now consumed. By the superheating process the steam is raised in pa.s.sing along the pipes in the smoke-box (where the heat is about 650) from a temperature of 250 to 350, and so enters the cylinders at 100 in excess of the temperature due to its pressure. This extra heat is, of course, rapidly communicated to the metals, and prevents the condensation in the cylinders or other parts of the engines, which would otherwise, of course, take place. Singularly enough, a smaller amount of cold water is required to condense the steam at this high temperature of 350 than when at the ordinary heat of common steam.

"The trial trip of the _Valetta_ on Thursday was most satisfactory, not only as regards the engines, but still more so as to the application for the superheating process. At the measured mile at the Lower Hope, near the Nore, the result of repeated runs gave an average speed of nearly 14 knots per hour, thus realizing with engines of 260 horse-power, and a small consumption of fuel, the same rate of speed as had been gained with her previous engines of 400 horse-power, and a consumption of seventy-five tons of coals per day. The superheating apparatus evidently effected a most important saving in fuel, but until an average of many days' working can be obtained, it would be difficult to estimate the exact amount economized. There seems, however, every reason to believe that an average of fourteen knots an hour can be obtained with a consumption of only from twenty-four to twenty-six tons per diem. The thermometer during the trial indicated in the steam pipes an addition to the ordinary temperature of 100, which Mr. Penn believes to be enough for all practical purposes of superheating.

Even when making from thirty-three to thirty-four revolutions per minute, and driving the vessel against a strong head wind and tide, it was impossible to consume all the steam generated, which was blowing off from both boilers all the trip. The engines are remarkable for the extraordinary beauty and simplicity of their proportions, qualities well known in all engines from Penn and Sons, and which, combined with the strength of the materials and perfection of the workmanship, make this firm the foremost in the world for machinery of this description. Both cylinders are oscillating, of sixty-two inches diameter, and with a stroke of four feet six inches. The paddles are on the feathering principle, and the boilers of Lamb and Co.'s patent. During the whole course of the trials, and when going at one time nearly sixteen knots, there was no perceptible vibration, even at the end of the saloon nearest to the engines. When it is remembered that the superheating process which can effect such important results is capable, as we have said, of application to steam machinery of every kind, including even locomotives, it cannot be doubted that the trial of Thursday and its great success is one of the most important events for the progress of steam which we have had to chronicle for many years." (_The Times_, April 23rd. 1859.)

[Page 439]

Whilst speaking of the application of this somewhat novel condition of steam, it may be observed that many inventors, who have paid little or no attention to _first principles_, have proposed to apply the vapours of alcohol, ether, or turpentine, instead of that of water; and they have founded their notions on the idea that in consequence of the less latent and sensible heat of alcohol, ether, and turpentine vapour, and of the small quant.i.ty of fuel required to boil them, that they would compete advantageously with steam. This view of the case, however, is soon proved to be a very shortsighted one, because the _amount_ of _expansion_ has been quite overlooked; and if it was desirable, by way of comparison, to produce a cubic foot of steam, alcohol, ether, or turpentine, the steam would stand first for cheapness, and would require the least quant.i.ty of fuel to produce it, so that if the more expensive of combustible liquids could be obtained for nothing, it would still be cheaper to employ water.

Latent heat, or equivalent for fuel.

A cubic foot of water yields 1700 cubic feet of steam = 1000 A cubic foot of alcohol produces 493 cubic feet = 457.

Then, by rule of proportion, 493 cubic inches : 457 :: 1700 : 1575 A cubic foot of ether yields only 212 cubic feet of vapour = 312, and 212 : 312 :: 1700 : 2500 A cubic foot of the oil of turpentine affords 192 cubic feet of vapour = 183, and 192 : 183 :: 1700 : 1620

It will therefore be seen that water, when converted into steam, expands eight times as much as sulphuric ether, and nearly three times and a half as much as alcohol.

The application of steam for the purpose of propelling vessels has already been mentioned in connexion with the Spanish inventor, Blasco de Garay, in the year 1543. The first patent in this kingdom granted for that purpose was that of Mr. Jonathan Hull in 1773. In 1787, Mr. Miller tried a number of important experiments in the propulsion of vessels by steam-engines, and it would appear that Lord Cullen advocated his ideas, and endeavoured to secure the co-operation of the great firm of Boulton and Watt, who, occupied with their land engines, could not pay attention to it; and twenty years elapsed after the reply of Watt to Lord Cullen's application, before the real novelty appeared of a first successful experiment with a steam-boat in "the open sea," by Henry Bell, in 1811.

A picture of this boat, called the _Comet_, which was afterwards wrecked, is shown at p. 418. Henry Bell's _novelty_ was _success_, and he is fairly ent.i.tled to the merit of first introducing steam navigation into Europe.

In 1811, the public stared with mingled astonishment and satisfaction at the realization of that which was called a fable. Only forty-seven years afterwards another generation spontaneously exhibits the liveliest interest in the gigantic private speculation of the _Great Eastern_.

[Page 440] Henry Bell's vessel of 1811 was 40 feet keel, 10 feet 6 inches beam, and 25 tons burthen! The _Great Eastern_ of 1859 is 692 feet long, 83 feet wide, 60 feet deep, and 24,000 tons burthen!! The whole nation with one voice wish her G.o.d speed in her projected voyage across the Atlantic, as the embodiment of that great goodwill which every generous-hearted Englishman feels towards the enlightened free-born people of the United States.

Should the author's little vessel, with its humble freight of science, meet with the approbation of his good friends, the boys and their advisers, another and another, if health permits, shall be launched for their benefit. _Vale._

[Ill.u.s.tration]

THE END.