MODERN ILLUMINANTS GAS FLAMES Illuminating gas is produced by destructive distillation of almost any organic compound, notably coal, and contains many different gaseous compounds, together with the elementary gas hydrogen and some inert nitrogen. Coal gas is manufactured from coals having a rather large percentage of volatile ingredients (17 to 35%), with a yield from 9,00o to 13,000 cu.ft. of gas per ton of coal. (See COAL AND COAL MINING.) Water gas is manufactured from anthracite coal, 3o lb. of coal yielding about i,000 cu.ft. of gas at typical burning pressure of 2 to 5 inches water gauge.
The lamp efficiency of a hydro-carbon fuel and the device in which it is burned, taken together as a source, depends on the relative number of carbon particles liberated on heating and on the temperature to which these particles are heated. To some extent the former factor is opposed to the latter. In any flame source there is always a great loss of energy through other means than temperature radiation, the principal ones being those of convection and con duction. Other things being the same, the greater the number of carbon particles, the greater will be the portion of the energy supply which will be radiated by the incandescent particles. This consideration points to a high carbon content as a desirable fuel characteristic. Other things being the same, the higher the flame temperature the greater will be that portion of the radiation which occurs within the visible limits. The temperature of a flame decreases with an increase in the number of carbon par ticles liberated per unit volume of the dissociated vapour. In that a greater number of carbon particles per given volume means a greater rate of loss of energy by radiation and consequently a reduced temperature, this consideration points to a low carbon content as a desirable fuel characteristic. In practice, some sort of medium carbon content is, therefore, desirable.
In the candle the wick is the burner. In addition to bringing the fuel to the flame by capillary action, it serves to keep the flame away from the large body of fuel to such an extent that only sufficient energy is lost to liquefy it and thus prepare it for its transfer by capillary action.
Kerosene.—In kerosene lamps the chimney and air vents control the air supply, protect from drafts and to some extent preheat the incoming air. The standard illuminating power for kerosene burned in a common flat flame is about i,ioo candle-hours per gallon, measured normal to the flame.
Acetylene.—The use of acetylene light now seems to be limited to isolated places, to miners' lamps and navigation buoys. The burners used are usually lava tips with air vents. The tip is provided with jet openings that distribute the gas streams at such an angle as to make one stream abut upon the other and thus produce a flat flame. Due to its very high temperature, the efficiency of the acetylene flame is relatively high.
Pintsch Gas.—Pintsch gas, obtained from the destructive distil lation of petroleum, contains largely methane, along with some heavier hydro-carbons. Compressed in tanks from 8 to 14 atmos pheres, it has been largely used for railway, lighthouse and buoy purposes. Used with a Welsbach mantle, its efficiency is greatly increased.
Carburetted Air Gas.—Carburetted air gas consists usually of a mixture of air with a very volatile gasolene and is commonly employed where the use of other artificial gas, natural gas or elec tric lights is not possible or convenient. When burned in open flame burners, coal and carburetted water gas of 450 to 65o B.T.U. value give varying candle-powers ranging from 2.4 to 4 per cubic foot of gas consumed. This light-giving power may be increased six or more times by burning the gas in a Bunsen burner and introducing solid substances, other than carbon, that give radiation selectively in the visible spectrum.
The open-flame burner using illuminating gas has practically disappeared as a light source, having yielded to the much more efficient and steadier Welsbach mantle. The burners for open gas flames, whether constructed of metal or of lava, are essentially limited to three types. In the bat-wing burner, the gas issues from a narrow slot and forms a thin sheet of flame. In the fish-tail burner, two circular streams of gas meet at an acute angle and on ignition likewise spread out into a thin sheet of flame. In the Argand burner, parallel cylindrical jets of gas issue from a number of openings arranged in a circle and on ignition form a cylindrical flame enclosed by a glass chimney. The air for combustion is supplied from the bottom of the burner. Gas mantles of to-day give about six times the light obtainable by burning gas in the open flame, and contain about 1% of
to 99% Th02. They radiate energy in the portion of the spectrum that includes wave lengths well suited to illumination. An ideal mantle would be one emitting luminous radiation only, and of spectral distribution best suited to the eye.
Means for preventing back-firing are far more necessary for the inverted lamp than for the upright lamp, because of the necessarily high temperature of the mixture. This is ac complished by a burner tip considerably widened at the top and provided with a gauze.
Higher gas pressures (2 lb. per square inch) give an efficiency about twice that of low-pressure burners, but this efficiency is given at the expense of complicated pressure appliances and short life of the mantle. However, high-pressure lamps are used ex tensively in some countries for street lighting. One of the recent valuable contributions to the gas-lighting art has been the de velopment of a burner whose Bunsen tube lies in a horizontal position and thus is adapted to operation in a suspended bowl. One of the hollow suspension arms serves as a gas distributor from the outlet on which the structure is suspended.