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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 54 TECHNICAL FEATURE Manufacturers are always under pressure to provide equipment for the lowest price. Consequently it is a good idea for the furnace owner and the designer to mutually agree the refractory selection. In the end, lining life and energy used for tons melted is better served by being smart with refractory choice and design! DOOR SEALING There is a natural draft in a furnace which is a function of temperatures and stack height. If there is no pressure control in the furnace, then any gaps around the door seal will result in the natural draft bringing in cold dilution air causing the furnace to have to work harder just to maintain temperature. With this air comes the potential increase in oxidation of the metal. Our example furnace has a door 6 meters long. Let's say it does not close fully but has a 6mm gap on the bottom edge only and the automatic pressure control has been disconnected. We could expect the standby energy losses to go up by 170 kW. This gap could be due to poor design, distortion, poor maintenance or simply a lump of scrap under the door seal. This unnecessary extra loss is more than three times the conduction losses mentioned above. It is not hard to get a 6mm gap. Not only is this using up fuel, but also allowing the metal to oxidize at a higher rate than it otherwise would. A 0.1% loss in yield due to oxidation in 20,000 ton per year of production amounts to 20 ton per year of metal. Door sealing is a very real and important issue requiring good design and manufacture. Door seal design measures: 1. Heavy duty heat resistant alloy castings to seal against. This is more expensive but is more robust to resist damage from charging, forklift trucks, etc, which can then give rise to the kind of seal losses mentioned earlier. 2. A door made out of cast refractory panel segments bolted to a steel frame. This allows the door to flex and absorb the thermal stresses without distortion. The doors are clamped against the frame by hydraulic cylinders FURNACE PRESSURE CONTROL Since some air leakage is almost inevitable, the furnace pressure is regulated in order to prevent cold air from entering the furnace and limit hot air escaping. We can't over pressurize as we do not want hot gases escaping and overheating door components or potentially allowing fumes into the operators working environment. Pressure control is usually chosen so as to be neutral at the base of the door. This requires a pressure controlled damper in the flue in order to maintain furnace pressure. A decrease in fuel consumption of 10% is quite possible. Figure 7. Furnace door with steel frame and cast sections Figure 8. Heat Losses from opening the door Figure 6. Power Losses from a Door Leak