is extracted from the flue gases rising up from the melting process. The heat is transferred by convection, guaran- teeing an exchange of thermal energy at low temperatures. When the metal reaches the melting bridge at the base of the shaft, it is through-heated well enough to enable high melting rates. Consequently, the metal is subjected to the high temperature zone and the direct impact of the burner gases for a short time, which reduces melting loss. A furnace design incorporating a
Fig. 2. The thermal counter-flow principle of the shaft melting furnace provides optimal energy utilization and high metal quality.
cover to save energy during holding by minimizing heat loss due to radiation from the bath surfaces (Fig. 1). Tiltable electric resistance furnaces typically come with a hydraulic system and tilt- ing device. In fuel-heated furnaces, energy consumption per ton of molten metal depends on the design of the furnace and size of the crucible. Other impor- tant factors are the exact adjustment of the crucible to the size of the furnace chamber, correct tuning of the burners and the aging condition of the crucible. Depending on the crucible size, cold- air burners consume 4,590-5,287 cu. ft. (130-150 cu. m) of gas to melt 1.12 tons (1 metric ton) of aluminum to 1,382F (720C). For the same task, electrically heated crucible furnaces require an energy input of 400 kWh. The time needed to melt a complete
crucible charge varies. A preheated gas- fired 771-lb. (350-kg) crucible with a 20% hot heel needs 85 minutes to melt down the charge, whereas a 1,764-lb. (800-kg) crucible needs 130 minutes. Using a cold crucible may prolong the melting time by more than 50%. Electrically heated furnaces need about double the time required by gas-fired furnaces.
Shaft Furnaces For High Volume For larger production volumes re-
quiring a large amount of metal and fast melting rates, crucible furnaces may not be the most economic option. Shaft melting furnaces provide a better option, with melting rates start at 661 lbs./hour (300 kg/hour), up to 15,432 lbs./hour (7,000 kg/hour). According to the melting rate, holding capacities can range from 1,102-44,092 lbs. (500-20,000 kg) in the same unit.
MODERN CASTING / August 2010 The most important criteria for as-
sessing the quality of a shaft melting furnace are: • high metal quality, such as low gas pickup and low non-metallic con- tamination of the melt;
• low melting loss due to oxidation in the furnace atmosphere;
• high thermal efficiency and low energy consumption per ton of molten metal;
• high temperature uniformity of the melt ready for tapping;
• simple and safe operation during charging and tapping;
• compliance with environmental and safety regulations, specifically concern- ing noise emissions, exhaust gas emis- sions and pollutant concentrations. In a fuel-heated shaft melting fur-
nace, metal charged in the cold condi- tion at the top of the shaft is heated up while moving downwards inside the shaft (Fig. 2). In the process, heat
separate burner system for the melting shaft and the holding area can provide continuous production of molten metal within a temperature tolerance of +/-9F (5C). The two-chamber furnace design ensures high quality tapped metal. The melt is characterized by a low concentration of suspended, insoluble contaminant, and a hydrogen content running below equilibrium solubility. The efficiency of two-chamber fur-
naces is limited in melting small-piece returns, such as chips, in operating conditions calling for tapping rates below 150 kg/hour and for frequent alloy changes.
Low on Energy The specific energy consumption
of a shaft melting furnace differs among the various furnace concepts available on the market. Indepen- dent studies give consumption rates ranging from 580 to 900 kWh/ton of aluminum. As a general rule, energy consumption is influenced by the furnace size, temperature of the melt in the bath and input material (alloy,
Fig. 3. Roller tables can be used as a buffer for charging containers in fully automatic charging. 37
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