Dampers for Furnace Pressure Control

The air control valve and its drive motor, controller, and transmitter can be located in any cool (but not freezing) environment away from the flue and not on top of the furnace. 

All dampers and control valves have their most difficult sensitivity problems at low-firing rates (high-turndown), where they tend to bump, hump, and overjump. For better sensitivity, a constant-pressure air-jet damper can be combined with a sliding-guillotine refractory damper, or a hinged clapper damper. (See fig. 6.16.) Dampers tend to lose usefulness with wear and lack of maintenance. 

Multiple flues were once popular as a means of distributing the gas flows along the furnace length. That idea works only if there is a near-equal number of burners 

TABLE 6.3 Benefits of automatic furnace pressure controi—A case history. Batch forging furnace heating 5200 ib (2364 kg) of 3.9 in. (0.1 m) diameter steel parts to 2400 F (1316 C) with natural gas. Ceramic fiber walls 8 (0.2 m) thick. 

With any kind of individual vertical flue controls, a flue that happens to carry more hot gas will get hotter and natural convection will create more draft or pull, causing that flue to get even hotter—a true snowball in hell. If scale or refractory crumbs accumulate unevenly on the floor near multiple bottom flues, this same sort of acceleration will happen in the least-plugged flue. These sorts of problems have led many engineers to favor one flue per zone, or per furnace, and to use wise engineering in burner placement, and best control of furnace circulation. (See chap. 7.) This is more easily accomplished in continuous furnaces where the pieces march through several zones and past a number of burners. 

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Figure 3.5 shows a 40 ft (12.2 m) long car-hearth in a 17.5 ft (5.3 m) high fiber-lined furnace with high-velocity burners at top and between the piers. Automatic furnace pressure control makes it possible to use top flues. Drilled square air manifolds shoot curtains of air across the flue exits as throttleable air curtain dampers for furnace pressure control. 

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