Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/310

294 IRON matter on the carbon dioxide first formed. One of the most successful of these is the Siemens gas producer, which is applicable to the production of heat by means of gaseous fuel generated from all kinds of waste materials, such as shale and combustible rubbish of all sorts, and is repre sented in fig. 4 ; the air, being admitted only through the When steam is allowed to pass into the producer along with the air, it reacts on the hot carbon of the fuel, producing " water gas " in virtue of the reaction FIG. 4. Siemens Gas Producer. bars C, is converted into nitrogen and carbon oxide in its passage through the incandescent mass, whilst hydrocarbons and hydrogen are also evolved in the upper portion by the action of the heat on the organic substances used as fuel, passing off by the gas flue B. A is the charging hole for the introduction of fresh fuel ; the ashes are stoked out from time to time from between the bars, which may with advantage be made capable of rotation about their own axes when shale is burnt, so as to facilitate the extrac tion of the burnt residue. E is a pipe which allows water to drip down into the ash pit D, and so to keep it always wet. The ollowing analyses will give an idea of the com position by volume of the gas from such producers : Analyst . . Siemens. Snclus. Particuhirs J Mixture of 3 parts caking coal and 1 part non- caking. Dowlals. J V (Hydrogen 8-2 24-2 j-2 2 8-6 24-4 2-4 traces 35 1 25 fi 1-6 2-8 30 Combustible J Carbon oxide (_0ther hydrocarbons.. Incombustible (Carbon dioxide gases. (Xitrogen 4-2 61-2 G5 l 5-2 59-4 C1 6 4 3 66-7 70 100-0 100-0 100-0 The extent to which this can be safely done depends on the fuel burnt, the decomposition being attended by an absorption of heat ; if more steam be admitted than can be decomposed, the surplus passes on unchanged and dilutes the gases, serving no useful pur pose, but rather the contrary. Usually the small amount of steam requisite is produced by placing a water-tank underneath the grate, supplied continually with water as indicated in fig. 4, so that evaporation is set up by the radiating effect of the fire ; only a relatively small volume is thus drawn in with the air used for combustion, but enough to give several parts per cent, of additional hydrogen and carbon oxide in the gas, and sensibly to increase the heating power. Roughly speaking, the calorific value of a unit of weight of gas from a Siemens producer is about 650; for one part by weight of carbon oxide will develop 2400 units of heat, and average gas contains about 25 per cent, by weight of carbon oxide with a little hydrogen (some 5 per cent, by weight), and hydrocarbons equivalent to some few parts per cent, more of carbon oxide. The mean specific heat of the gases being about 24, an alteration in temperature of 300 would represent about 72 units of heat, or 11 per cent, of the heating power ; so that by conveying the hot gases from a Siemens producer such a distance that their temperature is reduced by 300, a considerable loss of effective heating power is experienced, amount ing to about one-ninth of the actual heat developed by combustion. Partly owing to this cause, and partly owing to radiation, absorption of heat by the brickwork of the fireplace, &c., it has been calcu lated that the heat actually producible by means of gaseous fuel is only about two-thirds of that due to the fuel actually employed ; but manifestly the latter sources of loss apply to solid fuel burnt in an ordinary firegrate just as much as to a gas producer. Experi ence shows that when the producers are near to the furnaces fed by them the fuel consumption is perceptibly lessened. Siemens Regenerative Furnace, The peculiar feature of this furnace is that the waste heat is employed to heat up both the gaseous fuel and the air requisite to burn it before they are introduced into the furnace or chamber in which they undergo combustion. This is effected by making the exit gases pass through " regenerators," consisting of piles of firebricks stacked loosely together so as to ex pose as much surface as possible. Figs. 5 and 6 represent such a regenerative furnace as arranged for melting steel on an open hearth (Journal of Chemical Society, 1873, p. GC1). Fia. 5. Open-Hearth Furnace Longitudinal Section. Four such piles are employed, two being heated up by the waste gases escaping from the melting furnace, whilst the other two are in use, the one for heating the gaseous fuel supplied from a Siemens gas producer, or from a gas main fed by several such producers, the other for heating he air requisite for the combustion of the gas. By suitable valves the waste gases are shunted from the first to the second pair of regenerators, whilst simultaneously the gas and air are changed from the second to the first pair ; as the temperature at which the gas and air enter is close to that at which the products of combustion leave the furnace, whilst the regenerators are being heated up, the temperature of the combustion chamber continually rises (when not reduced by the introduction of cold substances) with each reversal of the currents through the regenerators ; so that ultimately the only limit to the temperature attainable is the refrac toriness of the materials of which the furnace is constructed. Even Welsh Dinas brick, which perfectly resists the ordinary steel melting