In the days when furnaces were hand-filled and practically the only mechanism on the top was the single bell with its lever and bell cylinder, considerations of accessibility played no part, there being but little apparatus and that all in plain sight. When a bell had to be taken down from the top of the furnace or sent up it was hoisted out with some simple but powerful hoist; set on rails and skidded over to the platform of the hoist, commonly known as the cage, on which it was lowered to the bottom and a new one to replace it was brought up in the same way.

With the mechanical top all that is changed. There is in all cases a considerable quantity of machinery and in many designs a vast amount. This is erected when the furnace is built and is never expected to be removed until the furnace itself is torn down. The main bell is not only at the bottom of a hopper in many cases deeper than the height of a man, but is covered over by an air-tight gas seal so that access to it is most difficult even for inspection, while before any repairs can be made a considerable portion of the top must be dismantled.

This is a point which has been too little considered in the design of furnace tops. Some engineer in a cool and pleasant drafting room designs some form of apparatus which pleases his fancy, and which may then be built easily enough because its construction is fundamentally no different from that of any other mechanical apparatus. But when the repairs become necessary they are turned over to the operating man, and must be made under circumstances which may be conservatively described as follows:

The temperature of every part that needs to be handled is high enough to burn the hand, in many cases so hot that the radiation makes it disagreeable to approach. Every crack and cranny is filtered full of dust that at the best is packed tight, and if it has been rained on is cemented almost as hard as iron. The air is not only heated almost to the point of suffocation in many cases, but in practically every case is mixed with furnace gas containing 25 per cent. carbon-monoxide, of which 1 per cent. in the atmosphere is quickly fatal, and a small fraction of a per cent. causes violent sickness. The nut of every bolt is either burned or rusted fast, or so jammed with dust that it will move but one "flat" at a time, and often will not start at all except with a sledge hammer and chisel. The room available on the top platform is generally just sufficient to permit bare access around the top of the furnace when everything is in place. The little space in which one might move with safety or speed is seized for the storage of parts as soon as the top is dismantled for access to the bell and hopper.

Fig. 56. Bell hoist - crank end view.

Under these circumstances it would seem as though some consideration might be given to accessibility and to designing some of the parts, notably the gas seal, so that they would go to place and function properly without the necessity of coming to an exact position. It is almost impossible to bring these parts to exact positions after they have been subjected to the heat which is unavoidable and the flame which often occurs at the top of the furnace, but these points appear to have been given very little consideration by designers.

In quite a number of types the gas-seal hood which rests on the lip of the main hopper supports the distributor; this means that in order to change bells the whole distributor must be taken off and the gas seal completely dismantled before the real work of removing the bell can be started.

It will perhaps illustrate the operating man's point of view to explain the former method of holding the lip ring down to its seat in the hopper. This was almost universally done with square bars resting against the lip ring at the bottom and passing through a socket near the top of the hopper, a key in a key-way through the bar just below the socket was driven by a sledge so as to force the bar down against the upper edge of the lip ring, thus holding it to place. It is obvious that when the lip ring became loosened by the constant jar of the bell in closing, these keys could only be tightened by putting a man into the gas seal through a manhole such, for instance, as that shown in Fig. 36, and having him drive up the keys.

It will be seen that he had to stand in a V-shaped space without a particle of footing. The temperature under the seal in many furnaces is seldom much less than two hundred degrees, even if means have been taken to cool it off, while there is almost certain to be a certain amount of gas leakage through the main bell and no chance of obtaining adequate ventilation through the small manholes in the gas seal. I have known of at least one case where a man was killed by gas on going in under the gas seal to do some such work. The gas was not in sufficient quantity to be visible, and several men were rendered unconscious by it before the danger was realized, but by quick work on the part of the master mechanic, all were taken out alive except the one.

In course of time the operating men determined that the holding bars for the lip ring must be tightened from outside the gas seal, and changed the construction to permit this, a practice which is now almost universally followed. The general principle of the method by which it is done is shown on the left-hand side of the hopper in Fig. 42.

Handling The Bells For Repairs

Of course, neither the hoisting apparatus for the skip nor that for the bucket system of filling affords any possibility of handling the bells, and therefore some other provision must always be made for letting down old bells and bringing up new ones. This is very generally in the form of a runway extending well out over the side of the furnace, as shown in Fig. 36. In this case the runway is the same as that which carries the adjustable receiving hopper. When a bell is to be handled the hopper is removed and the trolley shown at the extreme left-hand end of the runway is brought into action. This trolley runs far enough for the bell when hanging from it in its outmost position to clear the furnace entirely and to be brought up vertically; then when it is above the hand-rail the trolley is traversed over to the center of the furnace and the bell lowered into place. Sometimes a jib crane is used for this purpose, but in some cases within my experience these have been forced out of plumb by distortion of the furnace top and have been extremely hard to swing around, especially when under load. For this reason the runway with the traveler is much to be preferred.

Ordinary hoisting arrangements cannot be used on the trolley on account of the great height of lift involved, and it is well to provide not only a trolley with its sheave, but to have some means of doing the hoisting quickly and safely.

Bells are certain to give out at intervals, probably their life does not average much over a year, and to have a good top design and good arrangements for hoisting and a top accessible without too much dismantling means the difference between a shut down of twelve and one of forty-eight hours.