The specific energy consumption of shaft melting furnaces depends very much on different, marketable furnace concepts and is numbered with 580 to 900 kWh/t of aluminum in independent researches. In general, energy consumption depends on the furnace size, melt temperature in the bath as well as the charge material (alloy, unit size, etc.).
Regarding the StrikoMelter® shaft melting furnace with a StrikoWestofen GmbH ETAMAX® shaft, the manufacturer guarantees an energy consumption in continuous operation of 600 kWh/t at a melting temperature of 720 °C. This applies to the use of pigs and lumpy circulation material.
Measure for energy consumption reduction:
The following measure for a energy consumption reduction at the shaft melting furnaces should be present or implemented:
In general, a continuous melting process is an advantage for low energy consumption. Interruptions result in loss of thermal energy. When the burner process is interrupted, the molten metal solidifies again. When restarting the production, energy for the transition of the metal into a liquid state has to be added again. This double melting process also causes an extended oxidation on the metal surface and has negative effects on metal quality.
Optimum furnace efficiency cannot always be achieved due to changing purchase amounts by the foundry. For this reason, a stored furnace bath quantity should be used in case of low furnace efficiency. The required molten metal can be taken from this while the melting process is interrupted. The furnace operates in the holding mode and thermal losses can be minimized by closing the shaft cover. If the bath is emptied to the amount of 50 percent, a new melting process is started which then - depending on the bath size - continues for several hours. This example illustrates two basic aspects: on the one hand the importance of a correct furnace dimensioning where the melting performance-bath content ratio is correctly set, on the other hand the importance of staff training. The operator influences the furnace energy consumption to a large extent by the charging type and the melting process. This fact represents an important aspect of company resource management.
Material pre-heating is another important factor of ehergy consumption. The spectrum of furnace type energy consumptions described in books, summarized under the collective term “shaft melting furnace”, to a large extent result from the shaft geometry and the related percentage of material pre-heating in the shaft. For this reason, an efficient shaft melting furnace disposes of “cold” activities where the good is added, a “warm” activity for pre-heating and a melting zone where a high energy density is constructively realized. In general, for a high heat utilization, an even and high shaft filling with a low voids fraction is essential.
For measuring the melting progress and maintaining the shaft filled, the shaft cross-section underneath the feeding area can be scanned by a laser beam. If the sensor notices that the shaft is free for the next charging process, the furnace control automatically starts the feeding process. The laser control immediately detects the shaft filling level and enables feeding as early as possible, independent of material form, size and bulk density. This way, the shaft is ideally used and the energetic efficiency is much better than with indirect procedures such as through furnace temperature measurements or even through time control.
Often, furnace surface temperature is used for measuring radiation losses of the furnace. However, this approach disregards door and charging losses which also have to be taken into account. All this losses are integrated in the furnace efficiency. Due to insufficiently closing doors, the high amount of heat escapes through convection. If both doors in the hot melting room area have to be opened for charging, each charging process causes massive heat radiation into the furnace hall. Melting furnaces, on the contrary, where metal is added from the top to the cold shaft area, basically have no charging losses. In the end, only the total furnace efficiency as a result from multiplying fuelling and furnace efficiency represent the degree of furnace energy quality.
Shaft melting furnaces of high quality achieve efficiencies of more than 50 percent.
Bayerisches Landesamt für Umweltschutz (Editor): Effiziente Energienutzung in Nicht-Eisen-Metall-Schmelzbetrieben, Augsburg 2005