A reverberatory, or air furnace, is a furnace in which ore, metal, or other material is , exposed to the action of flame, but not to the contact of burning fuel. The flame passes over a bridge and then downward upon the material spread upon the hearth. Such furnaces are extensively used in shops where heavy work is being executed. They are also used for melting iron or other metals. For this purpose, however, they are not economical, since they require about twice as much fuel as that used in the cupola for the production of good hot iron. To be effective the flame must be made to reverberate from the low roof of the furnace down upon the hearth and work. The form of the roof and the velocity of the currents determine the hottest part of the furnace.
A common form of reverberatory furnace is shown in Fig. 7. The whole is lined with fire brick from the top of the grates to the top of the stack. The fuel is burned in a fire box separated from the heating portion of the furnace by a low bridge wall D. Access to the grate 13 obtained by suitable doors both above and below. When in service, both doors are tightly closed and a strong forced draft is admitted to the ash pit. Beyond the bridge wall is the furnace proper. This usually consists of a low chamber with a level floor. Like the fire box it is completely lined with a thick wall of fire brick. Access is obtained to this chamber through a vertically sliding door. These doors are also lined with fire brick and are usually suspended from chains. These pass over pulleys, and have counterbalancing weights at the other end.
Fig. 7. Section of Reverberatory Furnace.
The operation of the furnace is exceedingly simple. After the fuel has been charged upon the grates, the ash-pit and furnace doors are closed; the material to be heated is put upon the floor of the chamber; the doors are closed and the draft admitted to the ash pit. The thick walls which surround the furnace prevent radiation of its heat. The fire brick are, therefore, heated to incandescence and the hot gases sweep through the chamber. The flow of the gases is usually checked by a choke damper on top of the stack.
The outer form of these furnaces is usually rectangular. The brick walls are tied together by stay rods to prevent bulging, and the corners are protected by angle irons.
The selection of the fuel is an important matter in the operation of these furnaces. Experiments have been made with almost every kind of fuel. That now universally used is a soft bituminous coal that will not cake.