Zone Temperature in Car Furnaces

The lengthwise division of the furnaee into three top and three bottom zones is necessary because of the differences in heat loss and in heat transfer between the center and the ends. Similarly, because of the difference between the two ends, usually only one end has a door (high loss) whereas the other end does not (low loss). 

The reason for dividing the longitudinal zones into top and bottom zones is beeause there are usually considerable differenees in the losses and the heat transfer rates at different levels. Door seals may leak more outward at top than inward at bottom. Car seals may leak more at front than at baek, and more at front and back than at the sides. In some cases, the flow pattern of the flames poc may completely upset the predictions of the previous two statements beeause of different impacts or suctions caused by the jet effects and heat transfer patterns of the many flames. Another reason for separate top and bottom zones is that eost and practieal reasons often result in as much as 25% less clearance spaee below the loads than above them. 

In furnaces loaded with pieees of very different front-to-back dimensions, three or more lengthwise zones are neeessary for uniform heating. In furnaces loaded with pieces having very different thieknesses (vertically), two or more vertical zones should be used to achieve uniform heating. 

All variations of the previous paragraph are reasons for careful attention to (a) zoning for temperature uniformity control (this chapter) and (b) burner locations, burner flame types, and furnace flow patterns (chap. 7). (See fig. 6.8 showing soak temperature variations between three and five lengthwise zones at minimum firing rates (top set of curves) and at moderate firing rates ([bottom set of curves]). 

To limit temperature differences to 15°F ( 8.3°C), the top and bottom end zones (door and backwall) should be as short as possible. The minimum practical number of burners in these four end zones is one burner each. To limit the length of the temperature slope in each of these zones to the end zone itself, the temperature control sensors in each of these end zones should be located at the junction between the door or back-end zone and the adjacent zones, top and bottom. 

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