Evaluating Aluminum Melting Furnace Transient Energy Efficiency

"Recent increases in energy cost have lead to a renewed focus on energy efficiency during aluminum melting operations to reduce fuel usage. The goals of a batch melting operation are to melt the charge using the minimum required energy, to melt the metal to the required temperature within the required timeframe, and to avoid generating unwanted contaminants by overheating the charge.The heat transfer efficiency of a conventional hydrocarbon fired aluminum melting furnace varies over the course of the melt cycle, depending on the conditions in the furnace. The purpose of this work was to develop a method of determining the transient heat transfer efficiency throughout the furnace cycle and to take advantage of this knowledge to optimize the melting process through furnace controls and proper production operations. This furnace survey methodology has been developed and used to evaluate a number of furnaces.Description of a Typical Batch Melting Furnace CycleA typical batch melting furnace cycle will consist of three stages:1. Charging – Solid metal charged through an open roof, solid or molten charged through an open door, and/or molten poured through a furnace blister2. Melting – From the time the door or roof is closed and the burners are turned to high fire until the charge is all liquid and has reached a temperature suitable for alloying (typically ~1350 °F / 730 °C)3. Holding – From the end of the melting stage until transfer or casting is completeIn some cases, further charging can take place after the melting cycle has started. This can happen when the entire solid charge will not fit in the furnace in one load, or if additional molten metal is added while the charge is melting. The bulk of the energy used in the melter is consumed during the melting cycle. The thermodynamic minimum energy requirements for melting pure aluminum are:• Energy required to heat pure aluminum from room temperature to the melting point at 1220°F (660°C) is 288 BTU/lb (670 kJ/kg)• Energy required to change state of pure aluminum from solid to liquid (heat of fusion) is 171 BTU/lb (397 kJ/kg)• Energy required to heat molten from melting point at 1220°F (660°C) to 1350°F (730°C) is 36 BTU/lb (83 kJ/kg)• Total energy required to melt a solid charge from room temperature to 1350°F (730°C ) is 495 BTU/lb (1151 kJ/kg)"