Three-zone reheat furnace

Fig. 6.24. Three-zone reheat furnace temperature control for best productivity, least fuel rate. This control system minimizes scale formation by preventing overheating. Scale accumulation forces bottom zone gases to top zone, reducing bottom side heating. PV = process variable SP = setpoint T/s = temperature sensor.

Fig. 5.27. Continuous steel reheat furnace with nine pairs of regenerative burners in three top control zones and four pairs in a bottom zone. The sweep of hot poc from side burners can alternately proceed all the way across the furnace width, avoiding the former uneven heating when opposed burners created a hot spot pileup of heat in the center when on high fire, and a cool stripe down the middle on low fire.

Fig. 6.30. Heating curve for a three-zone steel reheat furnace top curve) and loads lower curve) In normal operation (without any delay). The billet discharge temperature is 2220 F (1215 C).

Fig. 6. 32. Heating curve for a three-zone steel reheat furnace top curve) and of the third billet to enter the furnace at the end of a 30-min delay lower curve). Discharge temperature of this third load piece is only 2000 F (1093 C)—too cold to roll. Note that the furnace temperature at the charging entrance has cooled from 1360 F (738 C) in figure 6.30 to 920 F (493 C) and furnace temperature at the entrance to the heat zone has dropped from 2140 F (1171 C) in figure 6.30 to 1450 F (788 C) in this figure 6.32.

Fig. 6. 33. Heating curve for a three-zone steel reheat furnace top curve) and of third billet lower curve) to enter the furnace after a 30-mln delay and with coauthor Shannon s system of T-sensor locations (nearer hearth for load temperature sensing and control, Instead of furnace or flame). Steel discharge temperature is 2240 F (1227 C)—good for rolling, and the furnace can resume its usual productivity more promptly after the delay.

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