Problems with One-Way, Top-Fired Soak Pits

burner manufacturer North American Mfg. Company of Ohio produced a burner that controlled the temperature to 10°F (5.6°C) by a lot of spin or no spin (on/off control). The result was that the high-fire period was lengthened and the cutback period was reduced. A hot heat was ready in about 5 hr instead of 8 hr. Temperature measurements were taken with five thermocouples along the length of the pit bottom. When the pit temperature was thought to be uniform and the ingots ready to be rolled, the front-to-back temperature difference was 175°F (97°C). 

To correct this temperature differential, a proportionally controlled spin of the poc was needed to automatically control temperature in the sidewalls, front, and back of the pit. 

At minimum fuel and air input, the ingot top-to-bottom temperature differential was again about 40°F (22°C). This difference was caused by the heat losses of the pit bottom. The basic reasoning for this is that with a smaller mass of gas flowing, the temperature drop of the gas must be greater to supply the bottom heat loss. Example 6.6 below illustrates this. 

Example 6.6 A pit fiirnace is being fired with natural gas and 10% excess air, and has a 2400 F (1589 C) flue gas exit temperature. The wall, hearth, and roof losses are calculated to be 1.55 kkBtu/hr. With cold air firing, there is a40°F (22°C) temperature difference from top to bottom of the ingots. Predict the corresponding temperature difference when using 1300 F (704 C) combustion air, and when using oxy-fuel firing. 

If cold air firing has a 40°F (22°C) temperature drop from top to bottom of the pit, the same pit with 1300 F combustion air would have a temperature drop of 

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