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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 56 fourfold increase in holding power consumption. In a year, assuming we are holding 20% of the time, over and above the conduction losses, we would be completely wasting some 1,000,000 kWh or 3,600 GJ and generating 212 ton of CO2 in the process, needing its own 22 ha of forest to convert that CO2. This does not take into consideration the effect this excess air may have had on the metal and dross formation and the cost, fuel and greenhouse gas emissions used to try to re-process that dross. STIRRING Without stirring the bath, the surface aluminium is hotter than the average bath temperature. Consequently the surface metal is much more prone to oxidise as the rate of oxidation is rapid above 775oC. The rate of heat transfer is also lower due to the lower temperature between bath and roof. Stir the bath and the surface metal temperature is lowered, oxidation is reduced, the roof to metal differential is greater and so the heat transfer is improved. Stirring the bath by opening the door and using a fork truck and paddle is contrary to our objective of keeping the door closed as much as possible. That is where electromagnetic stirring of the bath and metal pumps can provide an environmentally friendly, well engineered solution. Being able to achieve this stirring without opening the door is a great saving of energy and improvement in productivity. An electromagnetic stirrer is really a linear motor, rather like a squirrel cage induction motor with its stator winding opened out and laid flat under the metal bath. The moving field couples with the molten Aluminium and pulls it along in just the same way as it couples with the solid aluminium rotor bars in a motor and pulls them around. The power supply is really a variable speed drive running at very low speed (typically less than 1 Hz). This is proven technology to the likes of ABB who know how to make variable speed drives and motors. The power consumption is relatively low. One important feature of this type of stirrer is that the "motor" winding needs to couple with the molten aluminium through a low loss stainless steel window. It is possible to install this stainless plate during manufacture or during a reline. The power supply and coil can be installed later provided the customer has the foresight to make provision in advance. Other methods of stirring are possible such as the EMP electromagnetic pump off to the side of the furnace, or a mechanical pump. These two methods provide a very effective way of charging smaller material such as swarf without having to open the furnace door. These benefits of stirring have been covered in more detail by EMP  and ABB  in the past. The savings come from reduced melt times, less door opening, uniformity of metal temperature, quicker alloying and reduced maintenance. All these features are entirely appropriate for environmentally friendly furnace design. Add the introduction of carbon trading to the payback calculation factoring in the above issues and it should be given serious consideration. BURNER IMPROVEMENTS In the discussion so far, our example furnace has used ambient air burners. There is a lot that can be done with the burners to improve efficiency. Flame shape, velocity, burner position, are all factors. There is also the significant benefit of preheating the incoming combustion air using the hot exhaust gases. RECUPERATION Pre-heating by passing the hot exhaust past the incoming air, i.e., a gas to gas heat exchanger can improve efficiency 15% to 20% , a substantial improvement. Furnace efficiencies of up to 60% are possible. REGENERATION Preheating the incoming air using heat storage is referred to as regeneration. These can be 40% to 50% better than normal ambient air burners . Overall efficiencies in the region of Figure 9. Illustration of compounding losses Figure 10. Temperature equalisation from stirring