MANUFACTURE OF GAS.

GASOMETERS.

Very few of the readers of this book will remember the appearance presented by London streets before the introduction of gas; when all the thoroughfares were darker than even the commonest streets are now, and the only light emanated either from the shop windows or from dim oil lamps, which the rain or the wind would frequently put out, even when they did not burn out of themselves for want of being trimmed and replenished. A century ago these oil lamps were quite insufficient to light even the main streets, and people who walked out at night generallyhired a “link boy” to light them as they went, with a great flambeau of hemp and pitch, which smoked and smelt insufferably. The nobility and gentlefolks who rode in carriages were also attended by footmen with flambeaux of a better sort, and outside the doors of some of the oldest houses in London there may still be seen the great iron extinguishers attached to the railings, where the torch-bearers put out their lights till it was time to escort their masters and mistresses home. The poet Gay, who wrote the celebrated fables, describes the link boys, and gives them rather a bad character for so often being connected with the bands of thieves and footpads which infested London streets, and robbed people with impunity in the dark. He says:—

“Though thou art tempted by the link man’s call,Yet trust him not along the lonely wall;In the midway he’ll quench the flaming brand,And share the booty with the pilfering band.Still keep the public streets, where oily rays,Shot from the crystal lamp, o’erspread thy ways.”

“Though thou art tempted by the link man’s call,

Yet trust him not along the lonely wall;

In the midway he’ll quench the flaming brand,

And share the booty with the pilfering band.

Still keep the public streets, where oily rays,

Shot from the crystal lamp, o’erspread thy ways.”

Long before this, however, the inflammable nature of the vapour which streamed from burning coal had been observed by scientific men, and it was thought by many people that some method might be invented for making this gas useful for the purpose of lighting streets or houses. It had also been observed that the air suddenly escaping from the shafts of coal mines was often highly inflammable, and some experiments were made in the distillation of coal as early as 1726. In 1765 Lord Lonsdale proposed to the magistrates of Whitehaven to convey the gas from the neighbouring mines through pipes for lighting the town.

A number of eminent men afterwards made experiments with gas, but no decided practical result followed until aMr. Murdock, of Cornwall, began to manufacture gas for lighting his house and offices at Redruth. In 1798 the same gentleman used gas for lighting the Soho Foundry, where four years afterwards a public exhibition of the new invention was made by means of an illumination to celebrate the proclamation of peace.

In 1803 a gentleman named Winsor first publicly showed at the Lyceum Theatre, in London, a system of illumination by gas, which was the commencement of our present method of lighting our streets and houses; and after great difficulties and various experiments, a company was formed for the purpose of carrying out the undertaking of superseding the old oil lamps and making use of the new invention. The premises of the company and their factory were situated in Pall Mall, where the Carlton Club now stands, and the lights first appeared from the corner of St. James’s Street to the Haymarket, while several jets were placed in front of Carlton House, the residence of the Prince Regent, afterwards George the Fourth.

By slow degrees, and by the assistance of Mr. Clegg, an eminent engineer, the manufacture of gas improved, and in 1814, when the allied sovereigns visited this country, and a general illumination was ordered, a magnificent pagoda in St. James’s Park was erected for the exhibition of the new light. The following year Guildhall was fitted with gas burners; and, although it had been predicted that the new invention wouldruin the navyby superseding the use of the oil brought by the whalers, and though some people declared that it was only a scheme for blowing up London, the success of these experiments led to its rapid adoption in most of the large towns of Europe.

It will now be necessary to say something of coal, ofwhich more than 400,000 tons are used every year for the manufacture of gas in London alone.

The name coal was originally given to any substance used for fuel, and the use of mineral coal, or, as it was formerly termed,sea-coal, from the fact of its being brought by sea, has not prevailed for longer than 200 years. Coal is found in this country in extensive deposits called coal-fields, the most important of which are:—1. Those of the great northern district, including the coal-fields north of the river Trent. 2. Those of the central district, including the Leicester, Warwick, Stafford, and Shropshire coal-fields. 3. Those of the western districts in North Wales, South Wales, and Gloucester. The coal occurs in a number of layers, or beds, termedseams, and these are separated from each other by layers of slatey clay, calledshale, and coarse, hard sandstone, known asgrit.

The seams of coal are mostly comparatively thin, but varying from a few inches to six or eight feet in thickness, and underneath the layers is usually found a bed of mountain limestone extending beyond the coal-field, and rising to the surface of the ground around it.

The first process for finding coal is to bore with iron tools a perpendicular hole in the ground; then if coal is discovered, a shaft, or pit, from ten to fifteen feet in diameter, is sunk, and lined with brick, cast iron, or wood, to prevent the admission of water and the falling in of the sides. When the shaft reaches a seam of coal, passages are made, twelve to fourteen feet wide, and varying in height with the coal; from these proceed smaller ones, which are again crossed by large ones, enormous blocks of coal being left to support the earth above. As the minebecomes larger, a second shaft is sunk at some distance from the first, in order that air may be supplied to the mine, and that the gas that escapes from the coal may be carried away. A current of air is caused by burning a large fire in one shaft, and the draught is directed to the part of the mine where it is required, by doors, which close up some of the passages and leave others open.

There are several varieties of coal found in this country. The most important are: the common, or bituminous coal, which soils the fingers on handling; the cannel, or candle coal, which burns with a bright flame, but does not soil; the anthracite, or culm coal, which burns without flame, does not form cinder when half consumed, and is most useful for furnaces, in consequence of its intense heat and the absence of smoke. Although often classed amongst the minerals, coal is evidently of vegetable origin; many sorts of it being distinctly fibrous, and showing the grain of the wood from which it is composed. Upon examination with the microscope, coal shows vegetable remains; and these are frequently so perfect that they prove its formation from such plants as ferns and fir-trees, of a kind somewhat different from those which now exist.

The value of coal depends entirely upon its inflammable and combustible properties. It is used as the source of artificial warmth in our dwellings, and our manufactures are mainly dependent on it. Without coal there would scarcely be any working in iron, copper, lead, or other metals, as in populous countries the supply of wood is soon exhausted. Our potteries and our glass works are also carried on by its use, and the power of steam which moves all our great engines and machines is dependent on coal for its existence.

Great Britain produces more than one-half the coal that is consumed in the world, and about 180,000 persons are employed in obtaining it in our collieries.

The first process in the manufacture of gas is to subject the coal to a great heat, by which it iscarbonised, or burnt until only the cinder, or coke, remains after the gas has flown off. This is done by placing the coal in retorts of fire-clay or iron, which are previously heated. These retorts, which occupy a building called theretort house, having been charged with coal, are perfectly closed, and the doorluted, or stopped, with a sort of cement, so that the gas can only escape up theascension pipe, the coke being left in the retort.

Retort House.

The gas from the coal then passes through the ascension pipe by what is called thedip pipe, into a main, a large horizontal tube extending along the length of the furnaces. This main is about half filled with water or tar, in which the ends of the dip pipes are immersed, so that as the gas runs in, it ascends through the liquid into the space above, but cannot flow back again into the dip pipes. In this way it is all collected in the upper part of the main pipe, and is ready for purification.

Where double retorts are used, each end is worked with at least three stokers, and an extra man for preparing the lids of the mouth-pieces. Others are required for extinguishing the coke, wheeling the coal into the retort house, clinkering furnaces, and attending to fires. Three stokers, assisted by a man to extinguish the coke, will perform all the work of taking off the lids, raking out the coke, extinguishing and wheeling it away from a bench of seven retorts, in twelve or thirteen minutes; they will then put the proper charge for each retort in thescoop, deliver its contents, and be ready for charging another bench in a further space of seven minutes, while a fourth workman will in the meantime have put on the lids, so that the whole work of discharging and charging the seven retorts will occupy barely twenty minutes.

This extreme dexterity is of course only acquired by long practice, and it must be admitted the labour is very severe; but this is moderated by the time the men have for repose between the charges. The first process in discharging or drawing is for one or two of the men to relieve the screws of the mouth-pieces of the retorts about to be discharged, by giving three or four rapid turns; another man instantly gives a knock to each of the crossbars to disengage them from the ears of the lid, and at the same time strikes the lid a blow with a piece of iron or hammer, in order to break the luting, and a light is immediately applied to prevent explosion, which would be likely to crack the retort if of clay. For want of this precaution, many lamentable accidents have happened through the gas exploding when combined with atmospheric air. The men then lift off the cross bar and screw of each retort, placing them on the ground, and then each seizes hold of a lid in both hands, lifting it by the projecting ears, and placing it aside to cool, ready for luting for another charge.

Fender. Wheelbarrow. Bus.

Three of the stokers then take up theiriron rakes, which are simply rods of ¾-inch iron, about 12 feet long, having a handle at one end; the other end being turned at right angles is flat, about 6 inches long, 2 inches wide, and ½-inch thick. These are inserted in the retort, and the red-hot coke drawn to the mouth, whence it drops into thecoke vault, where there is a man ready to extinguish by throwing water on it; or when there is no vault the coke drops intoiron barrowsplaced ready to receive it, and wheeled rapidly away when the charge is withdrawn. If the coke were not immediately extinguished it would smoulder, and the surface become covered with earthy ash, and detract from its appearance and value.

Rake. Clinkering Spud. Scoop. Fire Shovel. Key. Auger.

Formerly, in charging retorts, the operation being comparatively very protracted, there was a considerable loss of gas, in addition to the time and extra fatigue to the men. In order to remedy these inconveniences, a method has been contrived for depositing the whole charge in the retort at once; for this purpose aniron scoopis used, thisbeing a semi-cylinder of sheet iron, from 8 to 10 feet long and 10 or 12 inches diameter, with a cross handle at the end to assist in lifting and turning it round to empty the coals in the retort.

Shovel. Clinking Bat.

The charge of coal is placed in the scoop while it rests on the ground, having a bent rod underneath for the purpose of lifting it: one man takes hold of the cross handle, and two others lift the other end by the bent rod, and introduce it into the mouth of the retort. The scoop with its contents is then pushed forward to the further end, turned completely over, and immediately withdrawn, leaving the coal in the retort, which is raked into a layer of uniform thickness, when the lid, previously luted and ready, is placed in its position and screwed up as quickly as possible. The operation of charging a retort with the scoop does not occupy more than thirty or forty seconds, so that very little escape of gas can take place. Theshovelis used for lifting the coal to the scoop, theclinking batfor breaking or removing the coke in the vault, the spud for a similar purpose.

The gas in the hydraulic main is of course very impure, having undergone no alteration since it came from the coal in the retort. It contains a quantity of tar and ammoniacal liquor in vapour, and these have to be separated from itand this can be effected by what is called condensation, the instrument used being known as acondenser. There are two or three forms of condensers in use, but a common one, which is represented in the engraving, is called the horizontal condenser, and is a rectangular box or chest formed of cast iron plates, put together with flanges, and perfectly tight joints. Its interior is provided with a series of iron trays, containing each about two inches in depth of water, and so arranged that the gas, entering at the bottom of the chest, passes in succession over the surface of the water in each tray, and traversing the whole length of trough ten or twelve times, passes off at the upper side. In the mean time a continuous stream of water enters at the top, and in its descent absorbs a portion of the ammonia, at the same time cooling and condensing the vapours in combination with the gas.

Condensers.

Horizontal condensers are sometimes composed of a series of pipes placed in a horizontal position, and immersed in water.

Wet Purifier.

The other form of condenser, which is very generally employed, consists of a series of vertical pipes, connected in pairs by semicircular bends at top, and attached to a cast-iron box or chest at bottom. This chest has a series of divisions, the ends of which are sealed by liquid placed therein, so that the gas in its passage has to pass through the whole series of pipes. The pipes by their contact with the atmosphere radiate the heat acquired from the gas in its passage, and it being in consequence cooled, deposits the vapours as liquid in the form of tar, and water saturated with ammonia, generally called ammoniacal liquor. This condenser is sometimes used with an application of cold water on its exterior, in order to increase the cooling effect.

The other part of the purification is by a chemical process, in which a solution of lime is used to remove other impurities in the gas. This is calledwet lime purification, or when the lime is only slackened (or moistened) it is calleddry lime purification, and the process takes the sulphuretted hydrogen, the carbonic acid, and other matters from the gas before it is stored for use.

Dry Purifier.

The wet lime purifier consists of a cast-iron cylinder entirely closed at top and bottom, except where the inlet and outlet pipes join it, and where an opening is required for charging it with lime-water, which same opening is also used for drawing off the charge. To the inside of the cover of this outer cylinder is bolted an inlet cylinder usually made of wrought-iron plate. This inlet cylinder is open at the lower part, and reaches to within a foot from the bottomof the outer cylinder, but has bolted to its lower flange a wide ring or dash plate of sheet iron, the outer diameter being only 8 or 9 inches less than that of the outer cylinder, so that a space of about 4 or 5 inches is left between the outside of the ring and the interior of the large cylinder.

The gas passes down through the inlet cylinder, and by its pressure forces its way up through the fluid lime, the surface of which is 8 or 9 inches above the dash plate.

The wet lime purifiers are variously worked; when four are used, two vessels are employed at one time, and when the lime in the first is incapable of absorbing the impurity, that purifier is put out of action, and the second and third are worked, and so on in succession.

When quick lime is slackened, reduced to powder, and slightly moistened with water, chemically this is called the hydrate of lime, and is often employed to absorb the sulphuretted hydrogen and carbonic acid from the gas. The process is termed dry lime purification.

Dry lime purifiers are generally rectangular cast-iron vessels, varying from 3 feet to 30 feet square, and from 3 feet to 4 feet 6 inches deep. Sometimes in small works they are made circular; this, however, is not very frequent, and is done for convenience or economy in construction. Each purifier contains a series of perforated shelves, trays, or sieves, supported by suitable bearers of wrought or cast iron, the ends of which are attached to “snuggs” cast on the purifier. In large apparatus there are also pillars placed at intermediate distances to carry the weight of the sieves and purifying material.

The upper part of the purifier is surrounded by a cistern or reservoir of from 6 inches to 24 inches deep, and from 3 inches to 6 inches wide, which is often cast with the purifier,and forms part of it, or at other times is attached thereto by bolts and cement, and is for the purpose of containing water to seal the cover. The cover of the purifier is of boiler plate or cast iron, the latter being preferable on account of its durability; but the increased weight is an impediment to its adoption. The rim or border of the cover is rather deeper than the cistern into which it is placed, and is effectually sealed by the water, so preventing the gas escaping from that point.

Often the purifier is divided into two compartments, so that the gas ascends through a set of sieves on the one side, and descends through another set on the other side, answering the purpose of two sets of apparatus. In all establishments, however small they may be, two distinct purifiers at least are necessary, to enable the impure lime to be removed from the one whilst the gas is being purified by the other.

The next subject for consideration is that of thegasholders, or vessels in which the gas is stored ready for delivery into the mains, which distribute it throughout the districts to be lighted. These vessels were originally termedgasometers, which name is sometimes even now applied to them; but as they have nothing whatever to do with the measurement of gas, but are mere vessels of capacity or stores, the simple name of gasholder is more expressive and appropriate.

The gasholder is composed of two distinct parts, one of which contains water, and is called the tank, the other is the vessel which contains the gas, being really the gasholder. On the Continent the former is very generally termed the “cistern,” and the latter the “bell.”

The tank is a large cylindrical vessel, constructed usually, for the sake of economy, of brickwork or masonry,but when the ground is marshy, or when water exists abundantly a short distance below the surface of the earth, which would prevent the construction in masonry at a moderate price, these tanks are made in cast-iron, and, indeed, in small works, are often of wrought iron. In the interior of the tank there are two vertical pipes for the admission and egress of the gas, called the inlet and outlet pipes; the former being in direct communication with the manufacturing apparatus, the latter with the mains which convey the gas to the town. These pipes rise a few inches above the level of the top of the tank, so that the water cannot overflow into them. A series of columns, generally of cast-iron, but sometimes of wood, or brick piers, are placed at equal distances around the tank for the purpose of guiding the holder.

The holder is a cylindrical vessel closed at the top, which is termed the roof, and open at the bottom, made of sheet iron, varying in thickness according to the dimensions of the apparatus, the smaller sizes being constructed of thin material in order to avoid an excess of pressure, whilst those of very large dimensions are made of stout plates for the purpose of obtaining sufficient pressure to expel the gas to the burners. The holder is somewhat less in diameter, but of the same depth as the tank in which it is placed, sometimes being partially suspended by chains which pass over grooved pulleys and counter-balance weights, but more frequently only guided by rollers attached around its lower and upper edges, which work against suitable guides in the tank and on the columns in such a manner as to permit the holder to ascend and descend in the tank with the greatest freedom.

The action of the gasholder is very simple. The tankbeing filled with water, and the holder immersed therein ready for use, there is a space between the surface of the water and the roof of the holder; the gas enters by the inlet pipe into this space, and with the force it acquires in being expelled from the coal, pressing on the surface of the water and underneath the roof, and over the whole area of both, causes the holder to rise. Thus, by its own force or pressure, the gas provides room for itself, and in proportion to the quantity entering so does the holder rise out of the water. For instance, a holder having 100 feet area, or about 11 feet 4 inches diameter, in rising 10 feet will receive 1,000 cubic feet of gas, and in descending, the same quantity would be expelled.

Gasholders, though often suspended, are never entirely counter-balanced, having always sufficient weight to give the necessary pressure for forcing the gas through the mains and smaller pipes to the burners, all through the neighbourhood which is supplied from it. The gasholder should be so constructed that, when it is full or at its greatest height, its lower edge will be so far under water as to prevent the gas from escaping.

The water in the tank serves three purposes; it is the means of resistance for the gas to lift the holder, it prevents the gas escaping or mixing with the atmosphere; and it is the means of expelling or forcing out the gas as the holder descends.

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