How it Works - Archibald Williams

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metal plates with a space between, which is filled with water (see Fig. 6). The fire is now enclosed much as it is in a kitchen range. But our boiler must not be so wasteful of the heat as is that useful household fixture. On their way to the funnel the flames and hot gases should act on a very large metal or other surface in contact with the water of the boiler, in order to give up a due proportion of their heat.

      

      THE MULTITUBULAR BOILER.

      Fig. 7. Fig. 7.—The Babcock and Wilcox water-tube boiler. One side of the brick seating has been removed to show the arrangement of the water-tubes and furnace.

      To save room, boilers which have to make steam very quickly and at high pressures are largely composed of pipes. Such boilers we call multitubular. They are of two kinds—(1) Water-tube boilers; in which the water circulates through tubes exposed to the furnace heat. The Babcock and Wilcox boiler (Fig. 7) is typical of this variety. (2) Fire-tube boilers; in which the hot gases pass through tubes surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates this form.

      The Babcock and Wilcox boiler is widely used in mines, power stations, and, in a modified form, on shipboard. It consists of two main parts—(1) A drum, H, in the upper part of which the steam collects; (2) a group of pipes arranged on the principle illustrated by Fig. 5. The boiler is seated on a rectangular frame of fire-bricks. At one end is the furnace door; at the other the exit to the chimney. From the furnace F the flames and hot gases rise round the upper end of the sloping tubes TT into the space A, where they play upon the under surface of H before plunging downward again among the tubes into the space B. Here the temperature is lower. The arrows indicate further journeys upwards into the space C on the right of a fire-brick division, and past the down tubes SS into D, whence the hot gases find an escape into the chimney through the opening E. It will be noticed that the greatest heat is brought to bear on TT near their junction with UU, the "uptake" tubes; and that every succeeding passage of the pipes brings the gradually cooling gases nearer to the "downtake" tubes SS.

      

      The pipes TT are easily brushed and scraped after the removal of plugs from the "headers" into which the tube ends are expanded.

      Other well-known water-tube boilers are the Yarrow, Belleville, Stirling, and Thorneycroft, all used for driving marine engines.

      FIRE-TUBE BOILERS.

      Fig. 6 shows a locomotive boiler in section. To the right is the fire-box, surrounded on all sides by a water-jacket in direct communication with the barrel of the boiler. The inner shell of the fire-box is often made of copper, which withstands the fierce heat better than steel; the outer, like the rest of the boiler, is of steel plates from ½ to ¾ inch thick. The shells of the jacket are braced together by a large number of rivets, RR; and the top, or crown, is strengthened by heavy longitudinal girders riveted to it, or is braced to the top of the boiler by long bolts. A large number of fire-tubes (only three are shown in the diagram for the sake of simplicity) extend from the fire-box to the smoke-box. The most powerful "mammoth" American locomotives have 350 or more tubes, which, with the fire-box, give 4,000 square feet of surface for the furnace heat to act upon. These tubes are expanded at their ends by a special tool into the tube-plates of the fire-box and boiler front. George Stephenson and his predecessors experienced great difficulty in rendering the tube-end joints quite water-tight, but the invention of the "expander" has removed this trouble.

      The fire-brick arch shown (Fig. 6) in the fire-box is used to deflect the flames towards the back of the fire-box, so that the hot gases may be retarded somewhat, and their combustion rendered more perfect. It also helps to distribute the heat more evenly over the whole of the inside of the box, and prevents cold air from flying directly from the firing door to the tubes. In some American and Continental locomotives the fire-brick arch is replaced by a "water bridge," which serves the same purpose, while giving additional heating surface.

      The water circulation in a locomotive boiler is—upwards at the fire-box end, where the heat is most intense; forward along the surface; downwards at the smoke-box end; backwards along the bottom of the barrel.

      OTHER TYPES OF BOILERS.

      For small stationary land engines the vertical boiler is much used. In Fig. 8 we have three forms of this type—A and B with cross water-tubes; C with vertical fire-tubes. The furnace in every case is surrounded by water, and fed through a door at one side.

      Fig. 8. Fig. 8.—Diagrammatic representation of three types of vertical boilers.

      The Lancashire boiler is of large size. It has a cylindrical shell, measuring up to 30 feet in length and 7 feet in diameter, traversed from end to end by two large flues, in the rear part of which are situated the furnaces. The boiler is fixed on a seating of fire-bricks, so built up as to form three flues, A and BB, shown in cross section in Fig. 9. The furnace gases, after leaving the two furnace flues, are deflected downwards into the channel A, by which they pass underneath the boiler to a point almost under the furnace, where they divide right and left and travel through cross passages into the side channels BB, to be led along the boiler's flanks to the chimney exit C. By this arrangement the effective heating surface is greatly increased; and the passages being large, natural draught generally suffices to maintain proper combustion. The Lancashire boiler is much used in factories and (in a modified form) on ships, since it is a steady steamer and is easily kept in order.

      Fig. 9. Fig. 9.—Cross and longitudinal sections of a Lancashire boiler.

      In marine boilers of cylindrical shape cross water-tubes and fire-tubes are often employed to increase the heating surface. Return tubes are also led through the water to the funnels, situated at the same end as the furnace.

      AIDS TO COMBUSTION.

      We may now turn our attention more particularly to the chemical process called combustion, upon which a boiler depends for its heat. Ordinary steam coal contains about 85 per cent. of carbon, 7 per cent. of oxygen, and 4 per cent. of hydrogen, besides traces of nitrogen and sulphur and a small incombustible residue. When the coal burns, the nitrogen is released and passes away without combining with any of the other elements. The sulphur unites with hydrogen and forms sulphuretted hydrogen (also named sulphurous acid), which is injurious to steel plates, and is largely responsible for the decay of tubes and funnels. More of the hydrogen unites with the oxygen as steam.

      The most important element in coal is the carbon (known chemically by the symbol C). Its combination with oxygen, called combustion, is the act which heats the boiler. Only when the carbon present has combined with the greatest possible amount of oxygen that it will take into partnership is the combustion complete and the full heat-value (fixed by scientific experiment at 14,500 thermal units per pound of carbon) developed.

      Now, carbon may unite with oxygen, atom for atom, and form carbon monoxide (CO); or in the proportion of one atom of carbon to two of oxygen, and form carbon dioxide (CO2). The former gas is combustible—that is, will admit another atom of carbon to the molecule—but the latter is saturated with oxygen, and will not burn, or, to put it otherwise, is the product of perfect combustion. A properly designed furnace, supplied with a due amount of air, will cause nearly all the carbon in the coal burnt to combine with the full amount of oxygen. On the other hand, if the oxygen supply is inefficient, CO as well as CO2 will form, and there will be a heat loss, equal in extreme cases to two-thirds of the whole. It is therefore necessary that a furnace which has to eat up fuel at a great pace should

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