Page:The American Cyclopædia (1879) Volume VII.djvu/648

 636 GAS the less will be the weight of coke or car- bonaceous residue in the retort, and there- fore the greater will be the quantity of car- bon which remains combined with hydrogen ; but the hydrocarbons so formed will be most- ly liquid and solid, and not gaseous. On the other hand, the higher the temperature the greater will be the weight of solid carbona- ceous residue ; and the proportion of perma- nent gases will be increased, and their levity also, in proportion to the heat, so that they may be almost entirely composed of hydrogen and carbonic oxide. A mean temperature has therefore to be employed by the gas manufac- turer, and this will also depend upon the qual- ity of the coal or other material used. Practi- cally it is impossible to subject the entire mass of coal to the most favorable conditions, be- cause the different portions contained in a re- tort are necessarily subjected to different de- grees of heat. The manufacture of gas from coal, as well as from wood, petroleum, or resin, consists of three processes : 1, the distillation of the crude gas ; 2, its separation from tarry and other condensible matters; 3, its purification from other deleterious and unpleasant gases. The distillation is performed in fire-clay retorts placed in furnaces, from five to ten retorts being placed in one furnace in large works, which may contain 100 furnaces. They are usually of a semi-cylindrical shape, somewhat like that of a sole drain tile, about 9 ft. hi length, and from 16 to 20 in. in diameter. They were formerly made of cast iron, as fire clay was too porous and allowed the gas to escape ; but since the introduction of appara- FIG. 7. Synoptical Plan of Gas Works. tus for relieving the pressure, which will be described further on, fire-clay retorts are found sufficiently tight, and are much more durable. Moreover, they may be glazed and rendered more impervious than iron. A longitudinal section of a furnace and two retorts is shown in fig. 7, and also a plan of the different parts of a gas works, so arranged as to give an idea of the process of manufacture, but not showing the parts in their actual position, as this would be impossible in one figure. The two retorts, of a set of five, are shown at &, #, the furnace at 5, the chimney at c. The retorts are first raised to a red heat, and then charged about two thirds full with coal by means of a kind of scoop having somewhat the form of the retort, and about the same length, which is introduced by two men, turned over, and withdrawn. The operation is called stoking, and the men who perform it stokers. Machi- nery is now being introduced in this country and in Europe, by which it will be performed by steam power. From 100 to 200 Ibs. are usually introduced at one charge. A lid is then fitted to the mouthpiece of the retort with bolts and a luting of gypsum mixed with iron filings, and a heat of about 2,200 continued for about five hours. The constituents of the coal are thus converted into the products given in the above table, and those portions, comprising all ex- cept the coke, which are volatile at that tem- perature, pass into the tube d, called the stand pipe, which ascends from the mouth of the re- tort and is inserted into the hydraulic main, a transverse section of which is shown at e. This is a long tube, usually semi-cylindrical, and from 18 to 24 in. in diameter, running the whole length of a row of furnaces, which may be 100 or 200 ft. When the operation of dis- tillation commences, the main is partly filled with water, into which the stand pipes lead- ing from the retort dip and discharge all the gases and vapors, a considerable portion of which, from the reduction of temperature, are there condensed, forming tar and ammoniacal liquor, which would soon fill the main were it not drawn off from time to time. In this way the fluid contents are kept at about the same level, and there is no necessity of adding water after the first supply. A large pipe, /, carries the still uncondensed gases and vapors to the cooler and condenser g, through which they