The figures in the last column refer to the dry acetylene in the gas, no correction having been made for the heat absorbed by the water vapour present. As will appear in Chapter X., the average of actual determinations of the calorific value of ordinary acetylene is 363 large calories or 1440 B.Th.U. per cubic foot. The temperature of ignition of acetylene has been generally stated to be about 480° C. V. Meyer and Münch in 1893 found that a mixture of acetylene and oxygen ignited between 509° and 515° C. Recent (1909) investigations by H. B. Dixon and H. F. Coward show, however, that the ignition temperature in neat oxygen is between 416° and 440° (mean 428° C.) and in air between 406° and 440°, with a mean of 429° C. The corresponding mean temperature of ignition found by the same investigators for other gases are: hydrogen, 585°; carbon monoxide, moist 664°, dry 692°; ethylene, in oxygen 510°, in air 543°; and methane, in oxygen between 550° and 700°, and in air, between 650° and 750° C.
Numerous experiments have been performed to determine the temperature of the acetylene flame. According to an exhaustive research by L. Nichols, when the gas burns in air it attains a maximum temperature of 1900° C. ± 20°, which is 120° higher than the temperature he found by a similar method of observation for the coal-gas flame (fish-tail burner). Le Chatelier had previously assigned to the acetylene flame a temperature between 2100° and 2400°, while Lewes had found for the dark zone 459°, for the luminous zone 1410°, and for the tip 1517° C, Féry and Mahler have also made measurements of the temperatures afforded by acetylene and other fuels, some of their results being quoted below. Féry employed his optical method of estimating the temperature, Mahler a process devised by Mallard and Le Chatelier. Mahler's figures all relate to flames supplied with air at a temperature of 0° C. and a constant pressure of 760 mm.
Hydrogen . . . . . . . . . . . 1900 1960
Carbon monoxide . . . . . . . . . -- 2100
Methane . . . . . . . . . . . -- _ 1850
Coal-gas (luminous) . . . . . . . . 1712 |
" (atmospheric, with deficient supply of air) . 1812 | 1950
" (atmospheric, with full supply of air) . . 1871 _|
Water-gas . . . . . . . . . . -- 2000
Oxy-coal-gas blowpipe . . . . . . . 2200 --
Oxy-hydrogen blowpipe . . . . . . . 2420 --
Acetylene . . . . . . . . . . 2548 2350
Alcohol . . . . . . . . . . . 1705 1700
Alcohol (in Denayrouze Bunsen) . . . . . 1862 --
Alcohol and petrol in equal parts . . . . 2053 --
Crude petroleum (American) . . . . . . -- 2000
Petroleum spirit " . . . . . . . -- 1920
Petroleum oil " . . . . . . . -- 1660
Catani has published the following determinations of the temperature yielded by acetylene when burnt with cold and hot air and also with oxygen:
Acetylene and cold air . . . . . . 2568° C.
" air at 500° C . . . . 2780° C.
" air at 1000° C . . . . 3000° C.
" oxygen . . . . . . 4160° C.
EXPLOSIVE LIMITS.--The range of explosibility of mixtures of acetylene and air has been determined by various observers. Eitner's figures for the lower and upper explosive limits, when the mixture, at 62.6° F., is in a tube 19 mm. in diameter, and contains 1.9 per cent. of aqueous vapour, are 3.35 and 52.3 per cent. of acetylene (cf. Chapter X.). In this case the mixture was fired by electric spark. In wider vessels, the upper explosive limit, when the mixture was fired by a Bunsen flame, was found to be as high as 75 per cent. of acetylene. Eitner also found that when 13 of the 21 volumes of oxygen in air are displaced by carbon dioxide, a mixture of such "carbon dioxide air" with acetylene is inexplosive in all proportions. Also that when carbon dioxide is added to a mixture of acetylene and air, an explosion no longer occurs when the carbon dioxide amounts to 46 volumes or more to every 54 volumes of air, whatever may be the proportion of acetylene in the mixture. [Footnote: According to Caro, if acetylene is added to a mixture composed of 55 per cent. by volume of air and 45 per cent. of carbon dioxide, the whole is only explosive when the proportion of acetylene lies between 5.0 and 5.8 per cent. Caro has also quoted the effect of various inflammable vapours upon the explosive limits of acetylene, his results being referred to in Chapter X.] These figures are valuable in connexion with the prevention of the formation of explosive mixtures of air and acetylene when new mains or plant are being brought into operation (cf. Chapter VII.). Eitner has also shown, by direct investigation on mixtures of other combustible gases and air, that the range of explosibility is greatly reduced by increase in the proportion of aqueous vapour present. As the proportion of aqueous vapour in gas standing over water increases with the temperature the range of explosibility of mixtures of a combustible gas and air is naturally and automatically reduced when the temperature rises, provided the mixture is in contact with water. Thus at 17.0° C., mixtures of hydrogen, air, and aqueous vapour containing from 9.3 to 65.0 per cent, of hydrogen are explosive, whereas at 78.1° C., provided the mixture is saturated with aqueous vapour, explosion occurs only when the percentage of hydrogen in the mixture is between 11.2 and 21.9. The range of explosibility of mixtures of acetylene and air is similarly reduced by the addition of aqueous vapour (though the exact figures have not been experimentally ascertained); and hence it follows that when the temperature in an acetylene generator in which water is in excess, or in a gasholder, rises, the risk of explosion, if air is mixed with the gas, is automatically reduced with the rise in temperature by reason of the higher proportion of aqueous vapour which the gas will retain at the higher temperature. This fact is alluded to in Chapter II. Acetone vapour also acts similarly in lowering the upper explosive limit of acetylene (cf. Chapter XI.).
It may perhaps be well to indicate briefly the practical significance of the range of explosibility of a mixture of air and a combustible gas, such as acetylene. The lower explosive limit is the lowest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in less than that percentage explosion is impossible. The upper explosive limit is the highest percentage of combustible gas in the mixture of it and air at which explosion will occur in the mixture if a light or spark is applied to it. If the combustible gas is present in the mixture with air in more than that percentage explosion is impossible. Mixtures, however, in which the percentage of combustible gas lies between these two limits will explode when a light or spark is applied to them; and the comprehensive term "range of explosibility" is used to cover all lying between the two explosive limits. If, then, a naked light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is below the lower limit of explosibility, the gas will not take fire, but the light will continue to burn, deriving its necessary oxygen from the excess of air present. On the other hand, if a light is applied to a vessel containing a mixture of a combustible gas and air, in which mixture the proportion of combustible gas is above the upper limit of explosibility, the light will be extinguished, and within the vessel the gaseous mixture will not burn; but it may burn at the open mouth of the vessel as it comes in contact with the surrounding air, until by diffusion, &c., sufficient air has entered the vessel to form, with the remaining gas, a mixture lying within the explosive limits, when an explosion will occur. Again, if a gaseous mixture containing less of its combustible constituent than is necessary to attain the lower explosive limit escapes from an open-ended pipe and a light is applied to it, the mixture will not burn as a useful compact flame (if, indeed, it fires at all); if the mixture contains more of its combustible constituent than is required to attain the upper explosive limit, that mixture will burn quietly at the mouth of the pipe and will be free from any tendency to fire back into the pipe--assuming, of course, that the gaseous mixture within the pipe is constantly travelling towards the open end. If, however, a gaseous mixture containing a proportion of its combustible constituent which lies between the lower and the upper explosive limit of that constituent escapes from an open- ended pipe and a light is applied, the mixture will fire and the flame will pass back into the pipe, there to produce an explosion, unless the orifice of the said pipe is so small as to prevent the explosive wave passing (as is the case with a proper acetylene burner), or unless the pipe itself is so narrow as appreciably to alter the range of explosibility by lowering the upper explosive limit from its normal value.
By far the most potent factor in altering the range of explosibility of any gas when mixed with air is the diameter of the vessel containing or delivering such mixture. Le Chatelier has investigated this point in the case of acetylene, and his values are reproduced overleaf; they are comparable among themselves, although it will be observed that his absolute results differ somewhat from those obtained by Eitner which are quoted later:
Explosive Limits of Acetylene mixed with Air.--(Le Chatelier.)
___________________________________________________________
| | | |
| | Explosive Limits. | |
| Diameter of Tube |_______________________| Range of |
| in Millimetres. | | | Explosibility. |
| | Lower. | Upper. | |
|__________________|___________|___________|________________|
| | | | |
| | Per Cent. | Per Cent. | Per Cent. |
| 40 | 2.9 | 64 | 61.1 |
| 30 | 3.1 | 62 | 58.9 |
| 20 | 3.5 | 55 | 51.5 |
| 6 | 4.0 | 40 | 36.0 |
| 4 | 4.5 | 25 | 20.5 |
| 2 | 5.0 | 15 | 10.0 |
| 0.8 | 7.7 | 10 | 2.3 |
| 0.5 | ... | ... | ... |
|__________________|___________|___________|________________|