The cupola is a very basic melting device(Furnace) in which molten iron from iron ore or scrap can be produced. Sometimes it may be used for melting bronze too. So from this it is quite evident that the cupola is subjected to temperatures in excess of 1000 °C. For those already acquainted with metallurgy, it can be termed of as a trimmed down version of the well known blast furnace with a very limited capacity of production, which can range anywhere from 1 ton to 35 ton.
The name “Cupola” has been derived from the very common cupolas found on roof tops which aid in exhaust of gases from houses.
The cupola does not make use of electricity for its operation. Hence all the energy needed for generating the enormous amount of heat is produced right in the furnace itself. Heat is generated from chemical reaction taking place inside the shaft of the furnace. The furnace is designed in such a way so as to transfer maximum amount of heat to charge and not to lose heat from the shaft. If heat is lost to the walls, then the furnace may melt and collapse.
The cupola is basically a hollow cylindrical structure having internal diameters of 450mm to 2000mm. The cylinder has an outer shell made up of steel which is lined with refractory bricks in the inner side. The bottom too is refractory lined and is collapsible as and when required. The bottom is flat, circular and is made up of two matching, interlocking and open able hinged doors which are locked during furnace operation. The half doors when open should leave a clearance of at least 6 inches from the bottom surface. The free edges of the doors have wedges which facilitate the interlocking when closed. A prop is provided at the bottom to keep the doors from falling. Height of the props can be adjusted with the help of a pair of wedges.
While the Cupolas remain same in operation, they vary in size and capacity only. Hence, the classification is based on the production capacity and the inner diameter of the furnace. The melting is purely stoichiometric, which means only a fixed ratio of coal and ore/scrap can be used in all types of cupolas. This implies that the production capacity is a direct function of the volume or the diameter of the furnace.
For a listing of cupola sizes one can refer to “Modern Shop Practice” by H M Raymond.