Loads combustion zone heat transfer

In practical combustion systems, the predominant mode of heat transfer is usually not molecular conduction, but turbulent diffusion, except at the boundaries and the flame front. Conduction is the only mode of heat transfer through refractory walls, and it determines ignition and extinction behaviors of the flame. Turbulent diffusion, an apparent or pseudo conduction mechanism arising from turbulent eddy motions, will be discussed in Section 4.4. The relations from the theory of conduction heat transfer15-17 can be used to evaluate heat losses through furnace walls and load zones, and through the pipe walls inside boilers and heat exchangers, etc. 

In furnaces with top and bottom heat and preheat zones, there is greater resistance to poc gas flow below the loads and their conveyor. That resistance causes the bulk of the bottom gases to flow into the top zones, reducing the effective heat transfer exposure areas significantly. This movement of combustion gases into the top zones reduces productivity and lowers available heat, increasing fuel use per ton of product. 

Generally, the rate of heat transfer is low near the burner wall because the temperature differences are very small. (Load movement is counterflow to flame movement thus, the flame reactants are coolest as they leave any one zone whereas the load pieces are hottest as they leave any one zone.) As the distance from the burner wall increases, the load surface is colder and the flame temperature is hotter because the combustion reaction rate accelerates. However, a control T-sensor 15 ft (4.6 m) from the burner wall limits the furnace temperature at that point. (This temperature is held to a setpoint determined by the operator or by a model.) With high-spin burners, as one follows the temperature profile away from its maximum and in the direction of flame reactant flow, the furnace temperature declines quickly to the setpoint, and thereafter drops rapidly to the exit. 

With these two baffles, furnace pressure can be controlled, and practically all the hot combustion gases from the last zone would be forced to move to the first zone via all the other zones in the circle. In so doing, these gases would be forced to transfer more heat to the loads. 

As the gases move away from the burner wall, their reaction accelerates, providing more and more energy for transfer to walls, roof, and load. As the temperature rises, more and more heat is transmitted to the product directly, and indirectly by way of the refractory. The temperature profile begins at the burner wall 100°F to 150°F (56°C to 83°C) below the zone temperature as typically measured from the roof 15 ft (4.6 m) beyond the burner wall. Depending on the type of burner, the rate of temperature rise to the location of the control T-sensor may or may not be rapid. If the burner has a lot of combustion spin, the temperature will rise rapidly, beginning at the burner wall. 

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