This is a field in which we have little or no theory to guide us. Our information comes solely from tedious and expensive empirical methods, better described as "trial and error." Our knowledge of what constitutes correct filling comes solely from our observation as to how furnaces work with different types of distribution, and while the whole history of this vast subject abounds in contradictory experiences, such as the failure of apparatus and methods of operation at one plant which have made a brilliant success at another, and other facts difficult or impossible to reconcile with one another, certain laws stand out as established by experience so nearly universal that I believe no one will question them.

These laws are:

First

The distribution of each kind of stock in the furnace must be symmetrical, about not less than two axes making equal angles with each other.

Second

The same proportion of lump and fine must obtain in each of the segments into which the circle of the top is cut by these equally spaced axes.

Third

The distribution of fine and coarse material radially must be such as to produce as nearly as possible an equal ascent of gas through each portion of the furnace stack.

To these must be added in some cases, if not in all, a fourth - The stock should be distributed not in uniform annular rings around the hopper, but in heaps, equally spaced around it.

In further explanation of the first law it may be said, on the authority of many trials, that it is not sufficient to dump all the stock symmetrically on one diameter of the furnace. No matter how carefully it may be spaced along that diameter, or even if it be dumped by tipping to first one side and then the other of the diameter, the results are unsatisfactory. This is for the reason that the tendency of this method of filling is to make two or perhaps three flat columns of coarse material separated by one or two flat columns of fines as indicated in Figs. 26 and 27, and the gas travels so much more freely through the coarse material that this descends much more rapidly than the fine, and reaches the hearth in an unreduced condition, or perhaps the opposite action takes place, and the gas being unable to penetrate the columns of fine material, does not reduce it adequately, so that it reaches the hearth improperly reduced. At all events one portion of the stock is reduced ahead of the other and the quantity of fuel which must be burned in the hearth is that required by the least reduced portion, which therefore limits the economy of the furnace. We must, therefore, dump the stock on two axes at right angles, three at 00 deg. apart, or four at 45 deg. We can use some angle not commensurable with 360 deg., but the angle in that case also must be constant; this produces the spiral effect described later.

Fig. 26. Sketch of distribution when charging along one diameter, one wall of fires.

Fig. 27. Sketch of distribution when charging along one diameter, two walls of fires.

When hand-filling is used it is quite easy to secure compliance with all four of the laws above stated. By dumping at four or six symmetrically placed points around the hopper, with barrows of the same sort of material, we comply with the first two laws. By the proper adjustment of the size of the bell to the diameter of the stock-line we comply with the third, and the essence of the system involves compliance with the fourth, that is, dumping in heaps symmetrically placed around the bell.

Hand-filling requires that top-fillers be carefully trained to their work, and that some pains be taken to see that they do not dump all the material on the side next to the hoist or incorrectly in other respects. This is a grave temptation to them when the furnace is driving rapidly, and they are pushed to keep up. Their succumbing to this temptation has been the ruination of more than one furnace manager who could not catch them at this trick, or stop them from it; because nothing will more quickly and completely ruin the successful operation of the furnace than filling on one side or in any other irregular way.

Even though the top fillers are well trained and faithful, there remain certain precautions to be taken even with this simple and easy system of filling. The necessity for these arises from two causes. First, the distribution and shape of the four heaps on the bell, if they are dumped consecutively, are not all alike by any means. Second, the charges are practically never made up of a number of burrows of each material divisible by the number of dumping points. That is to say, we may easily have four or six barrows of coke to the charge, to agree with the number of dumping points, but this having been determined, we must have a number of barrows of different kinds of ore and of limestone, depending upon the weight of the coke charge so established, and these numbers will not commonly, in fact seldom or never, be similarly divisible by four or six as the case may be.

The lack of symmetry of the different heaps in the hopper when dumped consecutively is due to the fact that the first barrow dumped, striking the hopper when it is perfectly empty, spreads out laterally in both directions. The second one dumped on the diameter at right angles to it, endeavors to do the same, but the pile which it tends to make is stopped from spreading toward the first barrow dumped by the pile formed by the contents of the latter. Consequently the contents of the second barrow spread freely on one side, but much less freely on the other. Thus the center of gravity of its contents does not lie in its dumping plane; but is deflected away from the first dumping point. The action in the case of the third barrow dumped is exactly the same, it in turn is forced around by the pre-existing heap from the second barrow. When we come to the fourth barrow, the space into which it would naturally spread out laterally, is cut off on one side by the heap from the first barrow, and on the other side by the heap from the third, with the result that this heap is very generally higher and always much narrower than that from any of the other barrows.