Acetylene, the Principles of Its Generation and Use / A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power - F. H. Leeds; W. J. Atkinson Butterfield

ILLUMINATING POWER OF SELF-LUMINOUS ACETYLENE.

If the two streams of gas impinge at an angle of 90°, twin-injector burners for acetylene appear to work best when the gas enters them at a pressure of 2 to 2.5 inches; for a higher pressure the angle should be made a little acute. Large burners require to have a wider distance between the jets, to be supplied with acetylene at a higher pressure, and to be constructed with a smaller angle of impingement. Every burner, of whatever construction and size, must always be supplied with gas at its proper pressure; a pressure varying from time to time is fatal.

It is worth observing that although injector burners are satisfactory in practice, and are in fact almost the only jets yet found to give prolonged satisfaction, the method of injecting air below the point of combustion in a self-luminous burner is in some respects wrong in principle. If acetylene can be consumed without polymerisation in burners of the simple fish-tail or bat's-wing type, it should show a higher illuminating efficiency. In 1902 Javal stated that it was possible to burn thoroughly purified acetylene in twin non-injector burners, provided the two jets, made of steatite as usual, were arranged horizontally instead of obliquely, the two streams of gas then meeting at an angle of 180°, so as to yield an almost circular flame. According to Javal, whereas carbonaceous growths were always produced in non-injector acetylene burners with either oblique or horizontal jets, in the former case the growths eventually distorted the gas orifices, but in the latter the carbon was deposited in the form of a tube, and fell off from the burner by its own weight directly it had grown to a length of 1.2 or 1.5 millimetres, leaving the jets perfectly clear and smooth. Javal has had such a burner running for 10 or 12 hours per day for a total of 2071 hours; it did not need cleaning out on any occasion, and its consumption at the end of the period was the same as at first. He found that it was necessary that the tips should be of steatite, and not of metal or glass; that the orifices should be drilled in a flat surface rather than at the apex of a cone, and that the acetylene should be purified to the utmost possible extent. Subsequent experience has demonstrated the possibility of constructing non-injector burners such as that shown in Fig. 13, which behave satisfactorily even though the jets are oblique. But with such burners trouble will inevitably ensue unless the gas is always purified to a high degree and is tolerably dry and well filtered. Non-injector burners should not be used unless special care is taken to insure that the installation is consistently operated in an efficient manner in these respects.

GLOBES, &C.--It does not fall within the province of the present volume to treat at length of chimneys, globes, or the various glassware which may be placed round a source of light to modify its appearance. It should be remarked, however, that obedience to two rules is necessary for complete satisfaction in all forms of artificial illumination. First, no light much stronger in intensity than a single candle ought ever to be placed in such a position in an occupied room that its direct rays can reach the eye, or the vision will be temporarily, and may be permanently, injured. Secondly, unless economy is to be wholly ignored, no coloured or tinted globe or shade should ever be put round a source of artificial light. The best material for the construction of globes is that which possesses the maximum of translucency coupled with non-transparency, i.e., a material which passes the highest proportion of the light falling upon it, and yet disperses that light in such different directions that the glowing body cannot be seen through the globe. Very roughly speaking, plain white glass, such as that of which the chimneys of oil-lamps and incandescent gas-burners are composed, is quite transparent, and therefore affords no protection to the eyesight; a protective globe should be rather of ground or opal glass, or of plain glass to which a dispersive effect has been given by forming small prisms on its inner or outer surface, or both. Such opal, ground, or dispersive shades waste much light in terms of illuminating power, but waste comparatively little in illuminating effect well designed, they may actually increase the illuminating effect in certain positions; a tinted globe, even if quite plain in figure, wastes both illuminating power and effect, and is only to be tolerated for so-believed aesthetic reasons. Naturally no globe must be of such figure, or so narrow at either orifice, as to distort the shape of the unshaded acetylene flame--it is hardly necessary to say this now, but some years ago coal-gas globes were constructed with an apparent total disregard of this fundamental point.

[CHAPTER IX]

INCANDESCENT BURNERS--HEATING APPARATUS--MOTORS--AUTOGENOUS SOLDERING

MERITS OF LIGHTING BY INCANDESCENT MANTLES.--It has already been shown that acetylene bases its chief claim for adoption as an illuminant in country districts upon the fact that, when consumed in simple self- luminous burners, it gives a light comparable in all respects save that of cost to the light of incandescent coal-gas. The employment of a mantle is still accompanied by several objections which appear serious to the average householder, who is not always disposed either to devote sufficient attention to his burners to keep them in a high state of efficiency or to contract for their maintenance by the gas company or others. Coal-gas cannot be burnt satisfactorily on the incandescent system unless the glass chimneys and shades are kept clean, unless the mantles are renewed as soon as they show signs of deterioration, and, perhaps most important of all, unless the burners are frequently cleared of the dust which collects round the jets. For this reason luminous acetylene ranks with luminous coal-gas in convenience and simplicity, while ranking with incandescent coal-gas in hygienic value. Very similar remarks apply to paraffin, and, in certain countries, to denatured alcohol. Since those latter illuminants are also available in rural places where coal-gas is not laid on, luminous acetylene is a less advantageous means of procuring artificial light than paraffin (and on occasion than coal-gas and alcohol when the latter fuels are burnt under the mantle), if the pecuniary aspect of the question is the only one considered. Such a comparison, however, is by no means fair; for if coal- gas, paraffin, and alcohol can be consumed on the incandescent system, so can acetylene; and if acetylene is hygienically equal to incandescent coal-gas, it is superior thereto when also burnt under the mantle. Nevertheless there should be one minor but perfectly irremediable defect in incandescent acetylene, viz., a sacrifice of that characteristic property of the luminous gas to emit a light closely resembling that of the sun in tint, which was mentioned in Chapter 1. Self-luminous acetylene gives the whitest light hitherto procurable without special correction of the rays, because its light is derived from glowing particles of carbon which happen to be heated (because of the high flame temperature) to the best possible temperature for the emission of pure white light. The light of any combustible consumed on the "incandescent" system is derived from glowing particles of ceria, thoria, or similar metallic oxides; and the character or shade of the light they emit is a function, apart from the temperature to which they are raised, of their specific chemical nature. Still, the light of incandescent acetylene is sufficiently pleasant, and according to Caro is purer white than that of incandescent coal-gas; but lengthy tests carried out by one of the authors actually show it to be appreciably inferior to luminous acetylene for colour-matching, in which the latter is known almost to equal full daylight, and to excel every form of artificial light except that of the electric arc specially corrected by means of glass tinted with copper salts.

CONDITIONS FOR INCANDESCENT ACETYLENE LIGHTING.--For success in the combustion of acetylene on the incandescent system, however, several points have to be observed. First, the gas must be delivered at a strictly constant pressure to the burner, and at one which exceeds a certain limit, ranging with different types and different sizes of burner from 2 to 4 or 5 inches of water. (The authors examined, as long ago as 1903, an incandescent burner of German construction claimed to work at a pressure of 1.5 inches, which it was almost impossible to induce to fire back to the jets however slowly the cock was manipulated, provided the pressure of the gas was maintained well above the point specified. But ordinarily a pressure of about 4 inches is used with incandescent acetylene burners.) Secondly, it is necessary that the acetylene shall at all times be free from appreciable admixture with air, even 0.5 per cent, being highly objectionable according to Caro; so that generators introducing any noteworthy amount of air into the holder each time their decomposing chambers are opened for recharging are not suitable for employment when incandescent burners are contemplated. The reason for this will be more apparent later on, but it depends on the obvious fact that if the acetylene already contains an appreciable proportion of air, when a further quantity is admitted at the burner inlets, the gaseous mixture contains a higher percentage of oxygen than is suited to the size and design of the burner, so that flashing back to the injector jets is imminent at any moment, and may be determined by the slightest fluctuation in pressure--if, indeed, the flame will remain at the proper spot for combustion at all. Thirdly, the fact that the acetylene which is to be consumed under the mantle must be most rigorously purified from phosphorus compounds has been mentioned in Chapter V. Impure acetylene will often destroy a mantle in two or three hours; but with highly purified gas the average life of a mantle may be taken, according to Giro, at 500 or 600 hours. It is safer, however, to assume a rather shorter average life, say 300 to 400 burning hours. Fourthly, owing to the higher pressure at which acetylene must be delivered to an incandescent burner and to the higher temperature of the acetylene flame in comparison with coal-gas, a mantle good enough to give satisfactory results with the latter does not of necessity answer with acetylene; in fact, the authors have found that English Welsbach coal-gas mantles of the small sizes required by incandescent acetylene burners are not competent to last for more than a very few hours, although, in identical conditions, mantles prepared specially for use with acetylene have proved durable. The atmospheric acetylene flame, too, differs in shape from an atmospheric flame of coal-gas, and it does not always happen that a coal- gas mantle contracts to fit the former; although it usually emits a better light (because it fits better) after some 20 hours use than at first. Caro has stated that to derive the best results a mantle needs to contain a larger proportion of ceria than the 1 per cent. present in mantles made according to the Welsbach formula, that it should be somewhat coarser in mesh, and have a large orifice at the head. Other authorities hold that mantles for acetylene, should contain other rare earths besides the thoria and ceria of which the coal-gas mantles almost wholly consist. It seems probable, however, that the composition of the ordinary impregnating fluid need not be varied for acetylene mantles provided it is of the proper strength and the mantles are raised to a higher temperature in manufacture than coal-gas mantles by the use of either coal-gas at very high pressure or an acetylene flame. The thickness of the substance of the mantle cannot be greatly increased with a view to attaining greater stability without causing a reduction in the light afforded. But the shape should be such that the mantle conforms as closely as possible to the acetylene Bunsen flame, which differs slightly with different patterns of incandescent burner heads. According to L. Cadenel, the acetylene mantle should be cylindrical for the lower two- thirds of its length, and slightly conical above, with an opening of moderate size at the top. The head of the mantle should be of slighter construction than that of coal-gas mantles. Fifthly, generators belonging to the automatic variety, which in most forms inevitably add more or less air to the acetylene every time they are cleaned or charged, appear to have achieved most popularity in Great Britain; and these frequently do not yield a gas fit for use with the mantle. This state of affairs, added to what has just been said, makes it difficult to speak in very favourable terms of the incandescent acetylene light for use in Great Britain. But as the advantages of an acetylene not contaminated with air are becoming more generally recognised, and mantles of several different makes are procurable more cheaply, incandescent acetylene is now more practicable than hitherto. Carburetted acetylene or "carburylene," which is discussed later, is especially suitable for use with mantle burners.

ATMOSPHERIC ACETYLENE BURNERS.--The satisfactory employment of acetylene in incandescent burners, for boiling, warming, and cooking purposes, and also to some extent as a motive power in small engines, demands the production of a good atmospheric or non-luminous flame, i.e., the construction of a trustworthy burner of the Bunsen type. This has been exceedingly difficult to achieve for two reasons: first, the wide range over which mixtures of acetylene and air are explosive; secondly, the high speed at which the explosive wave travels through such a mixture. It has been pointed out in Chapter VIII. that a Bunsen burner is one in which a certain proportion of air is mixed with the gas before it arrives at the actual point of ignition; and as that proportion must be such that the mixture falls between the upper and lower limits of explosibility, there is a gaseous mixture in the burner tube between the air inlets and the outlet which, if the conditions are suitable, will burn with explosive force: that is to say, will fire back to the air jets when a light is applied to the proper place for combustion. Such an explosion, of course, is far too small in extent to constitute any danger to person or property; the objection to it is simply that the shock of the explosion is liable to fracture the fragile incandescent mantle, while the gas, continuing to burn within the burner tube (in the case of a warming or cooking stove), blocks up that tube with carbon, and exhibits the other well-known troubles of a coal-gas stove which has "fired back."