Giving the Sizes of Pipe which should be used in practice for Acetylene Mains when the fall of pressure in the Main is not to exceed 1.0 inch. (Based on Morel's formula.)
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| | |
| Cubic feet | |
| of | |
| Acetylene |Diameters of Pipe to be used up to the lengths stated|
| which the | |
| Main is | |
| required |_____________________________________________________|
| to pass | | | | | | | | | |
| in One | 3/4 | 1 |1-1/4|1-1/2|1-3/4| 2 |2-1/2| 3 | 4 |
| Hour |inch.|inch.|inch.|inch.|inch.|inch.|inch.|inch.|inch.|
|____________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
| | | | | | | | | | |
| |Miles|Miles|Miles|Mile.|Miles|Miles|Miles|Miles|Miles|
| 10 | 2.40|10.13|30.90| ... | ... | ... | ... | ... | ... |
| 25 | 0.38| 1.62| 4.94|12.30| ... | ... | ... | ... | ... |
| 50 | 0.09| 0.40| 1.23| 3.07| 6.65|12.96| ... | ... | ... |
| 100 | 0.02| 0.10| 0.30| 0.77| 1.66| 3.24| 9.88| ... | ... |
| 200 | ... | 0.02| 0.07| 0.19| 0.41| 0.81| 2.47| 6.15| ... |
| 300 | ... | 0.01| 0.03| 0.08| 0.18| 0.36| 1.09| 2.73|11.52|
| 400 | ... | ... | 0.0 | 0.05| 0.10| 0.20| 0.61| 1.53| 6.48|
| 500 | ... | ... | 0.0 | 0.03| 0.06| 0.13| 0.39| 0.98| 4.14|
| 750 | ... | ... | ... | 0.01| 0.03| 0.05| 0.17| 0.43| 1.84|
| 1000 | ... | ... | ... | ... | 0.01| 0.03| 0.10| 0.24| 1.03|
| 1500 | ... | ... | ... | ... | ... | 0.01| 0.01| 0.11| 0.46|
| 2000 | ... | ... | ... | ... | ... | ... | 0.02| 0.06| 0.26|
| 2500 | ... | ... | ... | ... | ... | ... | 0.01| 0.04| 0.16|
| 5000 | ... | ... | ... | ... | ... | ... | ... | 0.01| 0.04|
|____________|_____|_____|_____|_____|_____|_____|_____|_____|_____|
[CHAPTER VIII]
COMBUSTION OF ACETYLENE IN LUMINOUS BURNERS--THEIR DISPOSITION
NATURE OF LUMINOUS FLAMES.--When referring to methods of obtaining artificial light by means of processes involving combustion or oxidation, the term "incandescence" is usually limited to those forms of burner in which some extraneous substance, such as a "mantle," is raised to a brilliant white heat. Though convenient, the phrase is a mere convention, for all artificial illuminants, even including the electric light, which exhibit a useful degree of intensity depend on the same principle of incandescence. Adopting the convention, however, an incandescent burner is one in which the fuel burns with a non-luminous or atmospheric flame, the light being produced by causing that flame to play upon some extraneous refractory body having the property of emitting much light when it is raised to a sufficiently high temperature; while a luminous burner is one in which the fuel is allowed to combine with atmospheric oxygen in such a way that one or more of the constituents in the gas evolves light as it suffers combustion. From the strictly chemical point of view the light-giving substance in the incandescent flame lasts indefinitely, for it experiences no change except in temperature; whereas the light-giving substance in a luminous flame lasts but for an instant, for it only evolves light during the act of its combination with the oxygen of the atmosphere. Any fluid combustible which burns with a flame can be made to give light on the incandescent system, for all such materials either burn naturally, or can be made to burn with a non- luminous flame, which can be employed to raise the temperature of some mantle; but only those fuels can be burnt on the self-luminous system which contain some ingredient that is liberated in the elemental state in the flame, the said ingredient being one which combines energetically with oxygen so as to liberate much local heat. In practice, just as there are only two or three substances which are suitable for the construction of an incandescent mantle, so there is only one which renders a flame usefully self-luminous, viz., carbon; and therefore only such fuels as contain carbon among their constituents can be burnt so as to produce light without the assistance of the mantle. But inasmuch as it is necessary for the evolution of light by the combustion of carbon that that carbon shall be in the free state, only those carbonaceous fuels yield light without the mantle in which the carbonaceous ingredient is dissociated into its elements before it is consumed. For instance, alcohol and carbon monoxide are both combustible, and both contain carbon; but they yield non-luminous flames, for the carbon burns to carbon dioxide in ordinary conditions without assuming the solid form; ether, petroleum, acetylene, and some of the hydrocarbons of coal-gas do emit light on combustion, for part of their carbon is so liberated. The quantity of light emitted by the glowing substance increases as the temperature of that substance rises: the gain in light being equal to the fifth or higher power of the gain in heat; [Footnote: Calculated from absolute zero.] therefore unnecessary dissipation of heat from a flame is one of the most important matters to be guarded against if that flame is to be an economical illuminant. But the amount of heat liberated when a certain weight (or volume) of a particular fuel combines with a sufficient quantity of oxygen to oxidise it wholly is absolutely fixed, and is exactly the same whether that fuel is made to give a luminous or a non-luminous flame. Nevertheless the atmospheric flame given by a certain fuel may be appreciably hotter than its luminous flame, because the former is usually smaller than the latter. Unless the luminous flame of a rich fuel is made to expose a wide surface to the air, part of its carbon may escape ultimate combustion; soot or smoke may be produced, and some of the most valuable heat-giving substance will be wasted. But if the flame is made to expose a large surface to the air, it becomes flat or hollow in shape instead of being cylindrical and solid, and therefore in proportion to its cubical capacity it presents to the cold air a larger superficies, from which loss of heat by radiation, &c., occurs. Being larger, too, the heat produced is less concentrated.
It does not fall within the province of the present book to discuss the relative merits of luminous and incandescent lighting; but it may be remarked that acetylene ranks with petroleum against coal-gas, carburetted or non-carburetted water-gas, and semi-water-gas, in showing a comparatively small degree of increased efficiency when burnt under the mantle. Any gas which is essentially composed of carbon monoxide or hydrogen alone (or both together) burns with a non-luminous flame, and can therefore only be used for illuminating purposes on the incandescent system; but, broadly speaking, the higher is the latent illuminating power of the gas itself when burnt in a non-atmospheric burner, the less marked is the superiority, both from the economical and the hygienic aspect, of its incandescent flame. It must be remembered also that only a gas yields a flame when it is burnt; the flame of a paraffin lamp and of a candle is due to the combustion of the vaporised fuel. Methods of burning acetylene under the mantle are discussed in Chapter IX.; here only self-luminous flames are being considered, but the theoretical question of heat economy applies to both processes.
Heat may be lost from a flame in three several ways: by direct radiation and conduction into the surrounding air, among the products of combustion, and by conduction into the body of the burner. Loss of heat by radiation and conduction to the air will be the greater as the flame exposes a larger surface, and as a more rapid current of cold air is brought into proximity with the flame. Loss of heat by conduction, into the burner will be the greater as the material of which the burner is constructed is a better conductor of heat, and as the mass of material in that burner is larger. Loss of heat by passage into the combustion products will also be greater as these products are more voluminous; but the volume of true combustion products from any particular gas is a fixed quantity, and since these products must leave the flame at the temperature of that flame--where the highest temperature possible is requisite--it would seem that no control can be had over the quantity of heat so lost. However, although it is not possible in practice to supply a flame with too little air, lest some of its carbon should escape consumption and prove a nuisance, it is very easy without conspicuous inconvenience to supply it with too much; and if the flame is supplied with too much, there is an unnecessary volume of air passing through it to dilute the true combustion products, which air absorbs its own proper proportion of heat. It is only the oxygen of the air which a flame needs, and this oxygen is mixed with approximately four times its volume of nitrogen; if, then, only a small excess of oxygen (too little to be noticeable of itself) is admitted to a flame, it is yet harmful, because it brings with it four times its volume of nitrogen, which has to be raised to the same temperature as the oxygen. Moreover, the nitrogen and the excess of oxygen occupy much space in the flame, making it larger, and distributing that fixed quantity of heat which it is capable of generating over an unnecessarily large area. It is for this reason that any gas gives so much brighter a light when burnt in pure oxygen than in air, (1) because the flame is smaller and its heat more concentrated, and (2) because part of its heat is not being wasted in raising the temperature of a large mass of inert nitrogen. Thus, if the flame of a gas which naturally gives a luminous flame is supplied with an excess of air, its illuminating value diminishes; and this is true whether that excess is introduced at the base of the actual flame, or is added to the gas prior to ignition. In fact the method of adding some air to a naturally luminous gas before it arrives at its place of combustion is the principle of the Bunsen burner, used for incandescent lighting and for most forms of warming and cooking stoves. A well-made modern atmospheric burner, however, does not add an excess of air to the flame, as might appear from what has been said; such a burner only adds part of the air before and the remainder of the necessary quantity after the point of first ignition--the function of the primary supply being merely to insure thorough admixture and to avoid the production of elemental carbon within the flame.
ILLUMINATING POWER.--It is very necessary to observe that, as the combined losses of heat from a flame must be smaller in proportion to the total heat produced by the flame as the flame itself becomes larger, the more powerful and intense any single unit of artificial light is, the more economical does it become, because economy of heat spells economy of light. Conversely, the more powerful and intense any single unit of light is, the more is it liable to injure the eyesight, the deeper and, by contrast, the more impenetrable are the shadows it yields, and the less pleasant and artistic is its effect in an occupied room. For economical reasons, therefore, one large central source of light is best in an apartment, but for physiological and æsthetic reasons a considerable number of correspondingly smaller units are preferable. Even in the street the economical advantage of the single unit is outweighed by the inconvenience of its shadows, and by the superiority of a number of evenly distributed small sources to one central large source of light whenever the natural transmission of light rays through the atmosphere is interfered with by mist or fog. The illuminating power of acetylene is commonly stated to be "240 candles" (though on the same basis Wolff has found it to be about 280 candles). This statement means that when acetylene is consumed in the most advantageous self-luminous burner at the most advantageous rate, that rate (expressed in cubic feet per hour) is to 5 in the same ratio as the intensity of the light evolved (expressed in standard candles) is to the said "illuminating power." Thus, Wolff found that when acetylene was burnt in the "0000 Bray" fish- tail burner at the rate of 1.377 cubic feet per hour, a light of 77 candle-power was obtained. Hence, putting x to represent the illuminating power of the acetylene in standard candles, we have:
1.377 / 5 = 77 / x hence x = 280.