Therefore acetylene is said to have, according to Wolff, an illuminating power of about 280 candles, or according to other observers, whose results have been commonly quoted, of 240 candles. The same method of calculating the nominal illuminating power of a gas is applied within the United Kingdom in the case of all gases which cannot be advantageously burnt at the rate of 5 cubic feet per hour in the standard burner (usually an Argand). The rate of 5 cubic feet per hour is specified in most Acts of Parliament relating to gas-supply as that at which coal-gas is to be burnt in testings of its illuminating power; and the illuminating power of the gas is defined as the intensity, expressed in standard candles, of the light afforded when the gas is burnt at that rate. In order to make the values found for the light evolved at more advantageous rates of consumption by other descriptions of gas--such as oil-gas or acetylene--comparable with the "illuminating power" of coal- gas as defined above, the values found are corrected in the ratio of the actual rate of consumption to 5 cubic feet per hour.
In this way the illuminating power of 240 candles has been commonly assigned to acetylene, though it would be clearer to those unfamiliar with the definition of illuminating power in the Acts of Parliament which regulate the testing of coal-gas, if the same fact were conveyed by stating that acetylene affords a maximum illuminating power of 48 candles (i.e., 240 / 5) per cubic foot. Actually, by misunderstanding of the accepted though arbitrary nomenclature of gas photometry, it has not infrequently been assorted or implied that a cubic foot of acetylene yields a light of 240 candle-power instead of 48 candle-power. It should, moreover, be remembered that the ideal illuminating power of a gas is the highest realisable in any Argand or flat-flame burner, while the said burner may not be a practicable one for general use in house lighting. Thus, the burners recommended for general use in lighting by acetylene do not develop a light of 48 candles per cubic foot of gas consumed, but considerably less, as will appear from the data given later in this chapter.
It has been stated that in order to avoid loss of heat from a flame through the burner, that burner should present only a small mass of material (i.e., be as light in weight as possible), and should be constructed of a bad heat-conductor. But if a small mass of a material very deficient in heat-conducting properties comes in contact with a flame, its temperature rises seriously and may approach that of the base of the flame itself. In the case of coal-gas this phenomenon is not objectionable, is even advantageous, and it explains why a burner made of steatite, which conducts heat badly, in always more economical (of heat and therefore of light) than an iron one. In the case of acetylene the same rule should, and undoubtedly does, apply also; but it is complicated, and its effect sometimes neutralised, by a peculiarity of the gas itself. It has been shown in Chapters II. and VI. that acetylene polymerises under the influence of heat, being converted into other bodies of lower illuminating power, together with some elemental carbon. If, now, acetylene is fed into a burner which, being composed of some material like steatite possessed of low heat-conducting and radiating powers, is very hot, and if the burner comprises a tube of sensible length, the gas that actually arrives at the orifice may no longer be pure acetylene, but acetylene diluted with inferior illuminating agents, and accompanied by a certain proportion of carbon. Neglecting the effect of this carbon, which will be considered in the following paragraph, it is manifest that the acetylene issuing from a hot burner--assuming its temperature to exceed the minimum capable of determining polymerisation-- may emit less light per unit of volume than the acetylene escaping from a cold burner. Proof of this statement is to be found in some experiments described by Bullier, who observed that when a small "Manchester" or fish-tail burner was allowed to become naturally hot, the quantity of gas needed to give the light of one candle (uncorrected) was 1.32 litres, but when the burner was kept cool by providing it with a jacket in which water was constantly circulating, only 1.13 litres of acetylene were necessary to obtain the same illuminating value, this being an economy of 16 per cent.
EARLY BURNERS.--One of the chief difficulties encountered in the early days of the acetylene industry was the design of a satisfactory burner which should possess a life of reasonable length. The first burners tried were ordinary oil-gas jets, which resemble the fish-tails used with coal- gas, but made smaller in every part to allow for the higher illuminating power of the oil-gas or acetylene per unit of volume. Although the flames they gave were very brilliant, and indeed have never been surpassed, the light quickly fell off in intensity owing to the distortion of their orifices caused by the deposition of solid matter at the edges. Various explanations have been offered to account for the precipitation of solid matter at the jets. If the acetylene passes directly to the burner from a generator having carbide in excess without being washed or filtered in any way, the gas may carry with it particles of lime dust, which will collect in the pipes mainly at the points where they are constricted; and as the pipes will be of comparatively large bore until the actual burner is readied, it will be chiefly at the orifices where the deposition occurs. This cause, though trivial, is often overlooked. It will be obviated whenever the plant is intelligently designed. As the phosphoric anhydride, or pentoxide, which is produced when a gas containing phosphorus burns, is a solid body, it may be deposited at the burner jets. This cause may be removed, or at least minimised, by proper purification of the acetylene, which means the removal of phosphorus compounds. Should the gas contain hydrogen silicide siliciuretted hydrogen), solid silica will be produced similarly, and will play its part in causing obstruction. According to Lewes the main factor in the blocking of the burners is the presence of liquid polymerised products in the acetylene, benzene in particular; for he considers that these bodies will be absorbed by the porous steatite, and will be decomposed under the influence of heat in that substance, saturating the steatite with carbon which, by a "catalytic" action presumably, assists in the deposition of further quantities of carbon in the burner tube until distortion of the flame results. Some action of this character possibly occurs; but were it the sole cause of blockage, the trouble would disappear entirely if the gas were washed with some suitable heavy oil before entering the burners, or if the latter were constructed of a non-porous material. It is certainly true that the purer is the acetylene burnt, both as regards freedom from phosphorus and absence of products of polymerisation, the longer do the burners last; and it has been claimed that a burner constructed at its jets of some non-porous substance, e.g., "ruby," does not choke as quickly as do steatite ones. Nevertheless, stoppages at the burners cannot be wholly avoided by these refinements. Gaud has shown that when pure acetylene is burnt at the normal rate in 1-foot Bray jets, growths of carbon soon appear, but do not obstruct the orifices during 100 hours' use; if, however, the gas-supply is checked till the flame becomes thick, the growths appear more quickly, and become obstructive after some 60 hours' burning. On the assumption that acetylene begins to polymerise at a temperature of 100° C., Gaud calculates that polymerisation cannot cause blocking of the burners unless the speed of the passing gas is so far reduced that the burner is only delivering one- sixth of its proper volume. But during 1902 Javal demonstrated that on heating in a gas-flame one arm of a twin, non-injector burner which had been and still was behaving quite satisfactorily with highly purified acetylene, growths were formed at the jet of that arm almost instantaneously. There is thus little doubt that the principal cause of this phenomenon is the partial dissociation of the acetylene (i.e., decomposition into its elements) as it passes through the burner itself; and the extent of such dissociation will depend, not at all upon the purity of the gas, but upon the temperature of the burner, upon the readiness with which the heat of the burner is communicated to the gas, and upon the speed at which the acetylene travels through the burner.
Some experiments reported by R. Granjon and P. Mauricheau-Beaupré in 1906 indicate, however, that phosphine in the gas is the primary cause of the growths upon non-injector burners. According to these investigators the combustion of the phosphine causes a deposit at the burner orifices of phosphoric acid, which is raised by the flame to a temperature higher than that of the burner. This hot deposit then decomposes some acetylene, and the carbon deposited therefrom is rendered incombustible by the phosphoric acid which continues to be produced from the combustion of the phosphine in the gas. The incombustible deposit of carbon and phosphoric acid thus produced ultimately chokes the burner.
It will appear in Chapter XI. that some of the first endeavours to avoid burner troubles were based on the dilution of the acetylene with carbon dioxide or air before the gas reached the place of combustion; while the subsequent paragraphs will show that the same result is arrived at more satisfactorily by diluting the acetylene with air during its actual passage through the burner. It seems highly probable that the beneficial effect of the earliest methods was due simply or primarily to the dilution, the molecules of the acetylene being partially protected from the heat of the burner by the molecules of a gas which was not injured by the high temperature, and which attracted to itself part of the heat that would otherwise have been communicated to the hydrocarbon. The modern injector burner exhibits the same phenomenon of dilution, and is to the same extent efficacious in preventing polymerisation; but inasmuch as it permits a larger proportion of air to be introduced, and as the addition is made roughly half-way along the burner passage, the cold air is more effectual in keeping the former part of the tip cool, and in jacketing the acetylene during its travel through the latter part, the bore of which is larger than it otherwise would be.
INJECTOR AND TWIN-FLAME BURNERS.--In practice it is neither possible to cool an acetylene burner systematically, nor is it desirable to construct it of such a large mass of some good heat conductor that its temperature always remains below the dissociation point of the gas. The earliest direct attempts to keep the burner cool were directed to an avoidance of contact between the flame of the burning acetylene and the body of the jet, this being effected by causing the current of acetylene to inject a small proportion of air through lateral apertures in the burner below the point of ignition. Such air naturally carries along with it some of the heat which, in spite of all precautions, still reaches the burner; but it also apparently forms a temporary annular jacket round the stream of gas, preventing it from catching fire until it has arrived at an appreciable distance from the jet. Other attempts were made by placing two non- injector jets in such mutual positions that the two streams of gas met at an angle, there to spread fan-fashion into a flat flame. This is really nothing but the old fish-tail coal-gas burner--which yields its flat flame by identical impingement of two gas streams--modified in detail so that the bulk of the flame should be at a considerable distance from the burner instead of resting directly upon it. In the fish-tail the two orifices are bored in the one piece of steatite, and virtually join at their external ends; in the acetylene burner, two separate pieces of steatite, three-quarters of an inch or more apart, carried by completely separate supports, are each drilled with one hole, and the flame stands vertically midway between them. The two streams of gas are in one vertical plane, to which the vertical plane of the flame is at right angles. Neither of these devices singly gave a solution of the difficulty; but by combining the two--the injector and the twin-flame principle--the modern flat-flame acetylene burner has been evolved, and is now met with in two slightly different forms known as the Billwiller and the Naphey respectively. The latter apparently ought to be called the Dolan.
The essential feature of the Naphey burner is the tip, which is shown in longitudinal section at A in Fig. 8. It consists of a mushroom headed cylinder of steatite, drilled centrally with a gas passage, which at its point is of a diameter suited to pass half the quantity of acetylene that the entire burner is intended to consume. The cap is provided with four radial air passages, only two of which are represented in the drawing; these unite in the centre of the head, where they enter into the longitudinal channel, virtually a continuation of the gas-way, leading to the point of combustion by a tube wide enough to pass the introduced air as well as the gas. Being under some pressure, the acetylene issuing from the jet at the end of the cylindrical portion of the tip injects air through the four air passages, and the mixture is finally burnt at the top orifice. As pointed out in Chapter VII., the injector jet is so small in diameter that even if the service-pipes leading to the tip contain an explosive mixture of acetylene and air, the explosion produced locally if a light is applied to the burner cannot pass backwards through that jet, and all danger is obviated. One tip only of this description evidently produces a long, jet-like flame, or a "rat-tail," in which the latent illuminating power of the acetylene is not developed economically. In practice, therefore, two of these tips are employed in unison, one of the commonest methods of holding them being shown at B. From each tip issues a stream of acetylene mixed with air, and to some extent also surrounded by a jacket of air; and at a certain point, which forms the apex of an isosceles right-angled triangle having its other angles at the orifices of the tips, the gas streams impinge, yielding a flat flame, at right- angles, as mentioned before, to the plane of the triangle. If the two tips are three-quarters of an inch apart, and if the angle of impingement is exactly 90°, the distance of each tip from the base of the flame proper will be a trifle over half an inch; and although each stream of gas does take fire and burn somewhat before meeting its neighbour, comparatively little heat is generated near the body of the steatite. Nevertheless, sufficient heat is occasionally communicated to the metal stems of these burners to cause warping, followed by a want of alignment in the gas streams, and this produces distortion of the flame, and possibly smoking. Three methods of overcoming this defect have been used: in one the arms are constructed entirely of steatite, in another they are made of such soft metal as easily to be bent back again into position with the fingers or pliers, in the third each arm is in two portions, screwing the one into the other. The second type is represented by the original Phôs burner, in which the curved arms of B are replaced by a pair of straight divergent arms of thin, soft tubing, joined to a pair of convergent wider tubes carrying the two tips. The third type is met with in the Drake burner, where the divergent arms are wide and have an internal thread into which screws an external thread cut upon lateral prolongations of the convergent tubes. Thus both the Phôs and the Drake burner exhibit a pair of exposed elbows between the gas inlet and the two tips; and these elbows are utilised to carry a screwed wire fastened to an external milled head by means of which any deposit of carbon in the burner tubes can be pushed out. The present pattern of the Phôs burner is shown in Fig. 9, in which A is the burner tip, B the wire or needle, and C the milled head by which the wire is screwed in and out of the burner tube.
