Soaking Pit Heating Control

Heat-Soaking Ingots—Evolution of One-Way-Fired Pits 

The steel industry has been using soaking pits for at least 125 years. Originally, they were simply refractory boxes in the earth with no combustion systems. From these simple units, the industry graduated to regenerative pits which had no instrumentation to the bottom-fired pits with ceramic recuperators to one-way top-fired pits with or without metallic recuperators. With the one-way top-fired pits, more pit area is under the crane per unit of real estate, so they became the universally accepted standard. Typical size 22 ft (6.7 m) long, 8.5 to 10 ft (2.6 to 3.0 m) wide, and 10 to 17 ft (3.0 to 5.2 m) deep. The combustion system has one or two burners located high on one end of the pit with the flue directly beneath them. 

These one-way-fired pits were fired with blast furnace gas, coke oven gas, natural gas, or heavy oil. With the number of these pits in operation, it is a wonder that more data are not available concerning their deficiencies. They were built to supply primary mills which rolled ingots into slabs, rounds, and bars, all to be reheated and rolled into finished products, but they had temperature differences longitudinally and top to bottom. 

Many operators felt that this improvement was all that would ever be needed, but they were not aware that the bottom longitudinal temperatures, when the ingots were judged reliable, were 150°F to 200°F (83°C to 111°C) colder at the burner wall than the ingots at the opposite wall, and the top-to-bottom temperature difference at the burner wall was 40°F to 100°F (22°C to 56°C). A few individuals knew of these problems, but there were no solutions at that time except to raise the control temperatures until product quality was tolerable. 

In summary, the major slab (instead of ingot) soak-pit problems are  

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Example A 25 long x 10 wide soaking pit heating 36" x 36" x 90" high ingots (33 000 pounds each) can be heated from cold to ready to roll in 10 hr, with a cutback time of 2.2 hr with burners and controls for spin condol. Without spin-control burners... 

With this soaking pit control system, ingots are all heated alike in much shorter time, and with no greater temperature differential (AT) from top to bottom of the ingots than 40 °F (22 °C) with a hearth coverage of 35%. Greater density of hearth coverage increases the AT. 

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