Effect of Load Thickness

Many charts have been developed for predicting the time it takes to heat steel. (See figs. 3.12 and 4.21a.) The industry now has better methods for predicting required heating times (e.g., the Shannon Method, in chap. 8). It combines (a) the radiation heat transfer equation for the time it takes to transfer the required heat to the load, with (b) lag time theory. Together, (a) and (b) predict how fast and how uniformly a product can be heated, knowing the size and nature of the pieces to be heated and their location relative to the furnace gases and the refractory. 

The lag time theory uses the following equations and factors to determine the extra time required for the center of a load piece to catch up with its surface temperature. The time necessary for a piece to reach a required temperature with uniformity throughout depends on the conductivity, density, and thickness of the material, and the number of sides exposed for heat transfer. Equations 3.1 and 3.2, for heating steel, show that the lag time increases as the square of the thickness. (See fig. 3.8.) 

Fi = 1.25 for three-side heating, Fi = 1 for four-side heating. 

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. 

Spinning the products of combustion helps greatly. Sometimes there is too much spin, but more often there is not enough. Even with the degree of spin controlled to give a flat temperature profile in the combustion chamber, the pit bottom temperature may be 100 to 200 °F (55 to 110 °C) hotter at the opposite end than at the burner end. 

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HEATING CAPACITY OF BATCH FURNACES 3.5. EFFECT OF LOAD THICKNESS... 

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