The Sheffield coke-holes or shaft-melting furnaces hold only two crucibles each, and they are not run at night. The furnace itself con sists of an oval chamber about three feet deep to the grate bars and a foot and a half to two feet in section. These holes are arranged around one or more sides of the melting-house.
Their cover-bricks are just above the floor level. The melting holes are connected either to sepa rate flues, some half dozen of them being united to form one large chimney, or else the indi vidual flues may be united in a large common flue and stack. The draft is roughly regulated by inserting bricks in the flues. Beneath the furnaces there is always a large ash pit and cel lar. In Sheffield the crucibles, after air-drying and annealing, are put into the furnace and well packed around with coke, and the materials to be melted are charged into the red hot crucible through a sheet iron funnel. The crucible rests on a stand and this in turn is supported on the grate bars. The charging being ended, a cover is placed on the crucible, the fire is urged and during the operation the coke must be replen ished at least twice. Three heats from a cru cible constitute a day's work and require about 12 hours' time, and three heats, also, is about the life of the crucible.
The regenerative gas furnace, which is used exclusively in the United States, has never been popular in England, although it is much more economical as. regards fuel consumption. Nu merous reasons have been given to explain why it is not used in Strangely enough it has been stated that gas fires are not under such ipaod control as coke fires, or again, that the,eptire crucible and its contents are not heated to a uniform temperature. Whatever weight these objections may have, it is interest ing to note that whenever English capital has been invested in the crucible steel business in America, graphite crucibles and gas-fired regen erative, furnaces have been adopted without apparent hesitation, The American furnace consists' of from 2 to 10 melting holes in a row, and each hole will accommodate six crucibles at a time.. Each hole is provided with three ports on either side through which the gas and air and the products of their combustion pass. The direction of flow of the gas and air is reversed about every 15 minutes, in the way common to all Siemens regenerative furnaces. Beneath and at either side of the melting holes are the regenerative chambers, one for gas and one for air on each side. The chambers are filled with so-called checker work of brick, which takes up heat from the outcoming gases or imparts it to the entering gas and air. The furnace of Dawson,
Robinson and Pope, which is said to be giving very good results at Jessop and Sons, Sheffield, differs from the American furnace in that all four regenerative chambers are on the same side of the Melting holes, which have four ports eath. The melting hole is curved or horse shoe shaped on the side opposite the ports. The gas and air thus enter by two of the ports and rush around the curved outer edge of the fur nace and down the other two ports. The six crucibles break and impede the passage of the flame and absorb its heat as it passes.
The melting holes have a layer of coke from six to eight inches deep in the bottom and on this rest the crucibles. The crucibles are usu ally filled carefully by hand, while cold, and are set into the furnace. The crucibles are well adapted to stand this sudden change of tem perature if they have been well baked and have been kept in a dry place prior to use. Unlike the coke furnace, the gas furnace is run con tinuously, day and night, from Monday morning to Saturday afternoon, from 33 to 36 heats con stituting a week's work. These furnaces will last from eight months to a year or even longer before the holes need relining or the checker work needs replacing. Coke furnaces require relining about once a month. The first cost of a gas furnace is much greater than that of the coke furnace, but the crucibles are more readily accessible in them and they are much more economical of fuel. A ton of ordinary bitumi nous coal, burned in a producer, will melt a ton of steel, while the coke shaft furnace requires about three tons of the more expensive coke to melt a ton of steel. Commenting on this phase of the subject, Harbord, the distinguished Eng lish authority, says: "The economy in fuel, case with which gas can be regulated, freedom from clinkers on pots, offer so many advan tages over the coke-hole, that notwithstanding past failures of gas furnaces, there seems every prospect of this or some similar gas-fired fur nace being very largely adopted in crucible steel melting.° By the "past failure of gas furnaces° Professor Harbord must refer to their failure of general adoption in England rather than in any real difficulty with the gas furnace itself as compared with coke-holes or other furnaces. More crucible steel is melted in gas furnaces than in all other forms combined.