The second method consists in compressing the gas by the action of an Archimedes screw, to such a degree, before it is admitted into the purifier, as that it may overcome the pressure of the column of water in that vessel. [Fig. 487.] exhibits this apparatus in section. D D is the Archimedes worm, the axis of which revolves at bottom upon the gudgeon e; it possesses a three-fold spiral, and is turned in the opposite direction to that in which it scoops the water. The cistern which contains it has an air-tight cover. The gas to be purified passes through the pipe C into the space D, over the water level d; the upper cells of the worm, scoop in the gas at this point, and carry it downwards, where it enters at g into the cavity E of a second cistern. In order that the gas, after it escapes from the bottom of the worm, may not partially return through g into the cavity D, an annular plate g h is attached to its under edge, so as to turn over it. The compressed gas is conducted from the cavity E through the pipe G into the purifying machine; a is a manometer, to indicate the elastic tension of the gas in D. On the top of the worm a mechanism is fitted for keeping it in constant rotation.
A perfect purification of light-gas from sulphuretted hydrogen, either by milk of lime or a solution of the green sulphate of iron, is attended with some difficulty, when carried so far as to cause no precipitation of sulphuret in acetate of lead, because such a degree of washing is required as is apt to diminish its illuminating power, by abstracting the vapour of the rich oily hydrocarburet which it contains. Moreover, the coal gas obtained towards the end of the distillation contains some sulphuret of carbon, which affords sulphurous acid on being burned, and can be removed by no easy method hitherto known. The lime in the purifier disengages from the carbonate and hydrosulphuret of ammonia carried over with the gas, especially when it has been imperfectly cooled in the condenser, a portion of ammoniacal gas, which, however, is not injurious to its illuminating power. The best agent for purifying gas would be the pyrolignite of lead, were it not rather expensive, because it would save the trouble of stirring, and require a smaller and simpler apparatus.
The Gasometer.—The gasometer serves not merely as a magazine for receiving the gas when it is purified, and keeping it in store for use, but also for communicating to the gas in the act of burning such an uniform pressure as may secure a steady unflickering flame. It consists of two essential parts; 1. of an under cistern, open at top and filled with water; and 2. of the upper floating cylinder or chest, which is a similar cistern inverted, and of somewhat smaller dimensions, called the gas-holder: see F, [fig. 482.] The best form of this vessel is the round or cylindrical; both because under equal capacity it requires least surface of metal, and it is least liable to be warped by its own weight or accidents. Since a cylindrical body has the greatest capacity with a given surface when its height is equal to its semi-diameter, its dimensions ought to be such that when elevated to the highest point in the water, the height may be equal to the radius of the base. For example, let the capacity of the gas-holder in cubic feet be k, the semi-diameter of its base be x, the height out of the water be h; h is = x = ∛k3·14. This height may be increased by one or two feet, according to its magnitude, to prevent the chance of any gas escaping beneath its under edge, when it is raised to its highest elevation in the water.
The size of the gasometer should be proportional to the quantity of gas to be consumed in a certain time. If 120,000 cubic feet be required, for instance, in 10 hours for street illumination, and if the gas retorts be charged four times in 24 hours, 30,000 cubic feet of gas will be generated in 6 hours. Hence the gasometer should have a capacity of at least 70,000 cubic feet, supposing the remaining 50,000 cubic feet to be produced during the period of consumption. If the gasometer has a smaller capacity, it must be supplied from a greater number of retorts during the lighting period, which is not advantageous, as the first heating of the supernumerary retorts is wasteful of fuel. Some engineers consider that a capacity of 30,000 cubic feet is the largest which can with propriety be given to a gasometer; in which case, they make its diameter 42 feet, and its height 23. When the dimensions are greater, the sheet iron must be thicker and more expensive; and the hollow cylinder must be fortified by strong internal cross braces.
The water cistern is usually constructed in this country with cast-iron plates bolted together, and made tight with rust-cement.
In cases where the weight of water required to fill such a cistern might be inconvenient to sustain, it may be made in the form represented in [fig. 488.]; which, however, will cost nearly twice as much. Parallel with the side of the cistern, a second cylinder C, of the same shape but somewhat smaller, is fixed in an inverted position to the bottom of the first, so as to leave an annular space B B between them, which is filled with water, and in which the floating gasometer A plays up and down. The water must stand above the cover of the inverted cylinder. a and b are the pipes for leading the gas in and out. Through an opening in the masonry upon which the gasometer apparatus rests, the space C may be entered, in order to make any requisite repairs.
The water cistern may also be sunk in the ground, and the sides made tight with hydraulic mortar, as is shown in [fig. 489.], and to make it answer with less water, a concentric cylindrical mass of masonry may be built at a distance of 2 or 3 inches within it.