Top-fired soak pits

In one-way, top-fired soaking pits, complications stem from large temperature differentials from burner wall to wall opposite the burner. With burners that produce straight ahead poc gas flow lines, the temperature differential in the space above the ingots can be 140 to 300 °F (78 to 167 °C),with the highest temperature near the wall opposite the burner. 

Example (a) In a one-way, top-fired soaking pit without spin, control of its poc will have an end-to end temperature difference of about 175°F (97°C) at the time when the load is expected to be reliable, but with oxy-fuel firing and its lower mass circulation, the corresponding end-to-end temperature difference might be 400°F (222°C) or more. 

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. 

Some engineers attempt to counter this problem with increased recirculation. They could spin the combustion products to reduce temperature differentials along the length of the pit, but the top-to-bottom temperature differentials would remain approximately three times as great as those with ambient air firing (120°F or 67°C). Even this possibility is unlikely because the volume of poc is so small and because convection heat transfer is proportional to velocity to the 0.7 power. The result is that oxygen combustion in soaking pits is not a wise choice when the quality of rolled material is temperature-uniformity-sensitive. 

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