BELL TYPE FURNACES
Bell Annealing is a type of annealing that derives its name from the shape of the furnace used during the process. Bell Annealing heats batches of metal which are placed on a base assembly, enclosed by an inner cover, and covered by the furnace. An overhead crane is used to load the base and move the equipment—when the furnace is suspended from the crane, it looks like a bell. The base assembly is the source of convection and the main method of heat transfer to the charge. The inner cover seals in the desired atmosphere and protects the charge from the burners direct heat. Keeping contaminants out of the annealing atmosphere prevents chemical changes as well as eliminating the formation of oxides and soot on the metal. The furnace brings the charge to the desired temperature to allow for the metallurgical changes to occur. Direct fired, tangentially fired, radiant tube, and electrical resistance are furnace types related to the method used to heat the charge. After heat treatment, cooling is performed by removing the furnace-leaving the inner cover in place to maintain the protective atmosphere. If a bright finish is desired, the metal must be cooled to near ambient temperature before exposing the metal to air. In this case, another piece of equipmentis utilized: a forced-cooler. The forced-cooler replaces the furnace at the end of the heating cycle and uses air and sometimes spray water to accelerate the cooling of the outside of the inner cover.
The main advantages of a Bell-Type annealing furnace are:
• Consistent product quality
• Good production rates
• Low operating costs
• Efficient use of furnace asset by cooling
with inner cover
• Savings in shop floor space requiring less capital
investment and reducing material handling
Bell furnaces are used to anneal both strip and wire coils. Furnaces designed for strip are generally of a single-stack configuration. The base diameter accommodates on coil centered over the base fan. The strip coils are stacked on top of one another, separated by convector plates. The circulated atmosphere flows up the sides and back down to the fan through the center of the coil.
To anneal wire, the coils must first be placed on a carrier, which is then loaded onto the annealing base. Furnaces designed for wire are generally of a multi-stack configuration. This requires a larger base, making the wire annealing furnaces short and wide compared to the strip counterparts. Like a strip furnace, the convection flow goes up on the sides and down the center. For multi-stack wire systems, a plenum is used above the diffuser to direct all the flow to the inner cover wall and provide a return path for flow to the large convection fan.