Front-End-Fired Continuous Furnaces

Many believe that for greatest uniformity of temperature in top- and bottom-fired continuous furnaces, it is desirable to favor almost constant temperature from furnace end to end plus a soak zone for the ultimate heat flow rate per unit of time. This is not true if reflecting scale forms in the charge or preheat zone at temperatures above 2320 F (1270 C). Such scale will reduce heat transfer so that the product will be colder and productivity will be lower than if the charge zone had been limited to between 2250 F and 2300 F (1232 C and 1260 C). Reflecting scale develops when scale softens and becomes very smooth and the steel temperature under the scale has relatively low conductivity, preventing the steel from absorbing heat from the scale. 

An example of this problem was in the operation of a large rotary furnace heating large rounds. All flve fired zones were operated above 6.F. At the end of the first heating zone, the scale was soft and reflective while the bottom of the rounds were very cold black. 

After the first piercer, the maximum surface temperature was 2100 F, and when the round was rolled down into the discharge conveyor, distinctive barber poling was seen. Maximum furnace production was 110 tph. 

When charge Zones 2 and 3 were reduced to 2(XX) F and 2350 F, respectively, the temperature after the first piercer increased to 2200 F and the furnace averaged 125 tons/hr for several days. The scale was very thin and dull black without a reflective layer. (See discussions of scale formation and decarburization in chap. 8.) 

Front-end-fired furnaces should have soak zones to allow equalization independently of the heating zones. Otherwise, (see fig. 4.18) the heating zones must be limited to maximum soak-zone temperatures when the heating zone temperature could be higher for maximum productivity. 

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