One of the most important factors in the whole subject of furnace operation was overlooked in the early designs of mechanically filled furnaces. This was the question of breakage of the coke. In some plants this seemed to have been almost totally disregarded. Coke was dumped from hopper cars on high trestles into bins perhaps almost empty, involving a drop of 15 to 35 feet, dropped again from the gate of the bins to the bottom of the scale car or bucket, 6, 8 or 10 feet more, dropped from the scale car into the skip, a distance about the same as the last, from the skip into the receiving hopper some 7 or 8 feet, from the upper bell on to the main bell, about an equal distance again, finally from the main bell on to the stock line, some 8 or 10 feet, if the furnace were full, and almost any distance if it were empty.

The detrimental action of fine coke in the furnace, and its much reduced fuel value, particularly for heat in the hearth, are well known, and need not be further stressed here though apparently they were overlooked by some of the furnace designers of fifteen years ago. Just how much the coke was broken up, how much the average size of the individual pieces was reduced, and what percentage of detrimental fines were made by this treatment in extreme cases, we shall never know now; perhaps it is just as well that we should not. But the general result was seen in the fuel consumption, and eventually in the cost sheet of many of these plants, while the furnace manager, who in most cases was not responsible for the design, spent many unhappy hours over these conditions, for which he had in many cases to take the blame.

It will be noted that with the Duquesne system, presently to be described, the buckets were so designed as to come close up under the bins with only a short spout beneath the gates of the latter. This reduced the drop into the bucket to the minimum, and eliminated two dumpings of the coke as compared with those plants where it was transferred first to the scale car, and then to the skip before reaching the upper bell.

It is altogether possible that this feature had an appreciable influence in bringing about the early success of Duquesne as compared with many of the skip-filled furnaces built soon afterward. Now furnace managers and engineers understand this matter of coke breakage better, and it is recognized that no top should be permitted to give the coke an inch more drop than is absolutely necessary to accomplish the transfer of the stock.

In more recent years the "shatter test" of coke has been invented and attention is paid to the strength of the coke, so that more is known as to the injury done it by any given drop, but with this advance has gone the increasing realization that every unnecessary foot of drop was highly objectionable and could not fail to cause some proportionate share of increased coke breakage.

As a consequence the enormously long throats which were used on some furnaces fifteen years ago have disappeared, and one of the major considerations of the present day in regard to any system of filling is its probable effect on the coke. Considering all these important points which were largely if not wholly ignored or neglected, the only wonder is not that furnaces did bad work in the early day of mechanical filling, but that they ran at all.

Rotating Tops

If the location of a given irregularity be rotated through an angle increased by a definite amount at each charge, we shall obviously obtain the equivalent of spiral hand-filling and so by evening up over a number of charges the kind of material delivered to each portion of the furnace circumference, shall secure the equivalent of uniform filling in each charge. Furnace tops which operate on this principle are called rotating or revolving tops.

There are in use three principal types of rotating tops - the Brown, the Baker and the McKee. All of these are patented, and the whole subject has been one of much complexity, not only technical, but to some extent legal. The merits and demerits of the different types of apparatus have been the subject of acute and heated discussion. Each of them has been thrown out of use at one or more plants, and it is therefore necessary to admit that there are objections to all of them. To state these objections as well as their advantages fearlessly and firmly is the only course open for a satisfactory treatment of this subject; this will be my aim however far short of this object I may come.