Bell Furnace

a batch-operated heat-treatment furnace in which products are heated under a movable dome. It is used for heat treatment of sheet and light-section rolled products in a controlled gaseous medium. Bell furnaces are classified according to use (treatment of coils of narrow strip, sheets, rods, and so on).

The most common bell furnaces are the single-stack and multistack types used for annealing coils of cold-rolled steel strip. In multistack bell furnaces, three to eight stacks, each of which has its own muffle for protection against the effects of the hot air and products of combustion, are mounted on a rectangular stand under the heating dome. Each stack is 3–5 m high and contains three to five coils with a total weight of up to 180 tons. The dome is heated by gas or electrical resistance heaters. Upon completion of the heating process, the dome is transferred by crane to another stand and the products are left to cool under the muffles on the first stand. Heat exchange under the muffle is intensified by forced circulation of controlled gas. Cooling is accelerated by wetting the muffle with water or blowing cold air over it.

During treatment in a bell furnace the coils of strip are loose, with spaces between the coil loops so that the gas circulates between the loops and flows over the entire surface of the strip, which makes possible thermochemical treatment and acceleration of heating and cooling.