Stack Annealing Furnaces

Stack annealing furnaces are bell-type furnaces in which stacked coils of steel wire or strip are heated to about 1250 F (680 C), copper heat treated at 500 to 900 F (2.60 to 480 C) (see figure 3.12). They may be direct fired or indirect fired, depending on the materials being annealed. Cover annealing furnaces have a gas-tight inner cover or bell within the bell furnace in which a prepared atmosphere is circulated by a base fan. Radiant tubes may be used instead of an inner cover. (See fig. 3.22.) 

If the properties of the material being heated could be adversely affected by slight overheating, the difference between furnace gas temperature and final load temperature must be kept small, especially if the heated material has poor thermal conductance. This combination of two requirements is encountered in the annealing of thick coils of thin strip steel. 

Most cover annealers are single stack furnaces, but there are some multistack annealers with three, four, six, or eight stacks, each with a bell cover, all within one rectangular furnace. (Radiant tubes were used in addition to the inner covers in the past because of poor heating between the inner covers.) Now, type H high-velocity burners are fired down or up between the inner covers. 

Although the strip is coiled under tension, successive wraps do not have continuous contact with one another because the apparently smooth surface of the strip has microscopic irregularities. These thin spaces are filled with trapped air, which has very poor thermal conductivity. The result is that the heating time may be more than 2 hr per inch of coil radial thickness. 

For annealing commercial-quality steel strip, the goal is no more variation than 70 F (39 C) for deep-drawing quality, no more than 34 F (19 C). Cooling times under the inner cover may be almost as long as the heating cycle. With wider and 

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Intermediate and low-grade clay refractories Annealing furnaces, runner brick, sleeves and nozzles, ladles, oil refineries, stills, incinerators, vaults, flues, furnace stacks, (lig( sters, bake ovens, dryers, carburetors, etc. 

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