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822
IRON AND STEEL


ports of the furnace, which itself must be 'shallow in order to hold the flame down close to the charge. It is in large part because of this shallowness, which contrasts so strongly with the height and roominess of the Bessemer converter, that the process lasts hours where the Bessemer process lasts minutes, though there is the further difference that in the open-hearth process the transfer of heat from flame to charge through the intervening layer of slag is necessarily slow, whereas in the Bessemer process the heat, generated as it is in and by the metallic bath itself, raises the temperature very rapidly. The slowness of this rise of the temperature compels us to make the removal of the carbon slow for a very simple reason. That removal progressively raises the melting-point of the metal, after line Aa of fig. 1, i.e. makes the charge more and more in fusible; and this progressive rise of the melting-point of the charge must not be allowed to outrun the actual rise of temperature, or in other words the charge must always be kept molten, because once solidified it is very hard to remelt. Thus the necessary slowness of the heating up of the molten charge would compel us to make the removal of the carbon slow, even if this slowness were not already forced on us by the danger of having the charge froth so much as to run out of the furnace. The general plan of the open-hearth process was certainly conceived by ]osiah Marshall Heath in 1845, if not indeed by Réaurnur in 1722, butfor lack of a furnace in which a high enough temperature could be generated it could not'be carried out- until the development of the Siemens regenerative gas furnace about 1860. It was in large part through the efforts of Le Chatelier that this process, so long conceived, was at last, in.1864, put, into actual use by the brothers Martin, of Sireuil in France. . V

99. Siemens Open-Hearth Furnace.-These furnaces are usually stationary, but in that shown in figs. 19 to 22 the working chamber or furnace body, G of fig. 22, rotates about its own axis, rolling on the rollers M shown in fig. 21. In this working chamber, a long quasi-cylindrical vessel of brickwork, heated by burning within it pre-heated gas with pre-heated air, the charge is melted and brought to the desired composition and temperature. The working chamber indeed is the furnace proper, in which the whole of the open-hearth process is carried out, and the function of all the rest of the apparatus, apart from the tilting mechanism, is simply to pre-heat the air and gas, and to lead them to the furnace proper and thence to the chimney. How this is done may be understood more easily if figs. I9 and 20 are regarded for a moment as forming a single diagrammatic figure instead of sections in different planes. The unbroken arrows show the direction of the incoming gas and air, the broken ones the direction of the escaping product's of their combustion. The air and gas, the latter coming from the gas producers or other source, arrive through H and I respectively, and their path thence is determined by the position of the reversing valves K and K'. In the position shown in solid lines, these valves deflect the air and gas into the left-hand pair of " re generators ” or spacious heat-transferring chambers. In these, bricks in great numbers are piled loosely, in such a way that, while they leave ample passage for the gas and air, yet they offer to them a very great extent of surface, and therefore readily transfer to them the heat which they have as readily sucked out of the escaping products of combustion in the last preceding phase. The gas and air thus separately pre-heated to about IIOO° C. (2oI2° F.) rise thence as two separate streams through the u takes (fig. 22), and first mix at the moment of entering the working cliamber through the ports L and L' (fig. 19). As they are so hot at starting. their combustion of course yields a very much higher temperature than if they had been cold before burning, and they form an enormous flame, which fills the great working chamber. The products of combustion are sucked by the pull of the chimney through the farther or right-hand end of this chamber, out through the exit ports, as shown by the dotted arrows, down through the right<hand pair of re generators, heating to perhaps If§ 00 ° C.. the upper part of the loosely-piled masses of brickwork within them, and thence past the valves K and K' to the chimneyrflue O. During this phase the incoming gas and air have been withdrawing heat from the left-hand re generators, which have thus been coolingxlown, while the escaping products of combustion haye been depositing heat in the right-hand pair of regencrators, which have thus been heating up. After some thirty minutes this condition of thmgs is reversed by turning the valves K and K' QQ? into the positions shown in dotted lines, when they deflect the incoming gas and air into the right-hand re generators, so that they may absorb in passing the heat which hasjust been stored thertfy il1l'f1C€ Tlley pass up through the right-hand nptakes and ports into the working chamber, -here ns hryforv they mix, burn and heat' the charge. Thu-nce thvy are sucked out by the chimney-draught through the left-hand ports, down through the uptakes and re generators, here again meeting and heating the loose mass of “ re generator ” brickwork, and finally escape by the chimney-flue O. After another thirty minutes the FIG. 19.-Section on EF through Furnace and Port Ends. ~ I

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FIG. 22.-Section on AB-through Uptake, Slag Pocket and Regenerator.

FIGS. ro to 22.-Diagrammatic Sections of Tilting Siemens Furnace. Hydraulic cylinder for tilting

the furnace.

Flue leading to chimney.

Slag pockets.

Charging boxes.

Water-cooled joints between

gurxiace proper, G, andgports

(1, I* urnace body., ,-

H, Air supply.

], Gas supply.

K, Air reversing valve.

K', Gas reversing valve.

l., Air port.

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M, Rollers on which the furnace

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tilts.

current is again reversed to its' initialzdirection, and so on. » These rcg|:nerat'ors are the cssciice or the § iCTHl'I'S or regenerative

luruace"; they are heat-traps, catching and storing by their