Choosing the Right Furnace for Your Operation
A furnace is not a small investment. Its selection should be a careful and well thought process. A thorough understanding of its intended application will ensure proper selection, where the value should be more important than cost. It is critical during the selection process of melting technology that an aluminum foundry integrates seven areas (Fig. 1) into the evaluation:
- Furnace efficiency.
- Cost of energy.
- Molten quality.
- Melt losses.
- Molten aluminum handling practices.
- Environmental requirements.
It is recognized that each foundry differs on the importance and prioritization of these factors.
Fundamentally, these furnaces are divided in two different styles: melting and holding furnaces. Furnaces may be heated with fossil fuels, or electrical energy. Furnaces can be grouped in five basic types: crucible, reverberatory (wet-bath or dry-hearth), stack melter, dosing, and rotary furnace.
Know that there are many variables for aluminum foundries to consider when the need for a new furnace arises to the ordering and/or building of the furnace. Foundries could eliminate future operational issues and cost-related hardships in the melting operation by properly addressing, evaluating, and documenting at least 15 critical steps (Fig. 2), that are relevant in the determination and selection of a new furnace.
It is also important that all personnel directly involved in the melting area have a basic knowledge and understanding of the interrelated aspects of combustion, heat transfer, furnace attributes, furnace efficiency, and molten metal processing techniques, so that energy and melt losses are minimized.
Throughout the entire evaluation and selection process, the foundry should seek the expertise and technical help of the equipment manufacturer. Ultimately, the foundry will determine the best furnace choice for its facility, but the managers involved need to be equipped with all relevant technical information before making that decision.
Need for a Furnace
The need for a new furnace must be seen as a capital improvement consideration for upgrading, replacing, or installing a new furnace. While the first decision should be whether to have a melting or holding furnace, it is important to consider that both holding and melting can be accomplished in a melting furnace. At the same time, foundry presonnel should decide whether the furnace will be used for a high tonnage single alloy in a central melting area, or if it will be used as a dedicated furnace for low-volume production runs. Capital investment decision parameters include fabrication, purchasing, installation, startup, and depreciation. Opportunity cost factors include energy, maintenance, consumables, labor, operation, and space availability.
Once the need for the new furnace is established, determining the melting capacity requirement is of vital importance. The actual day-to-day molten aluminum needs must be a fraction of the theoretical maximum melting capacity. The melting requirement must be based on the number of pounds per hour of molten metal required, assuming no downtime. Since the supply of molten metal will depend on the number (and melt rate capacity) of melting furnaces available, there will be a compromise between installing several furnaces of small capacity (greater flexibility) versus installing one (or a few) of larger capacity (greater efficiency).
Other considerations to keep in mind that would affect melting capacity are:
- Batch versus continuous melting.
- Melting furnace versus holding furnace.
- The furnace maximum theoretical melting rate does not determine the necessary available melting volume, due to furnace cleaning, maintenance and/or holding periods, which reduce the realistic melting rate.
- Charging practices greatly influence melting rates.
- Do not overbuy a furnace. Buying a furnace that melts 30% more than needed will greatly increase energy costs, especially in a tower or reverberatory furnace.
- A tower melting furnace must have a full stack to be efficient. If this furnace will not be in constant use or refilled on a timely basis, it would be similar to having a dry hearth melter, which melts at much higher temperature.
- The same is true of a reverberatory melter. In an underutilized furnace, the radiant roof or thermal head will cool down in between charges and use more energy to return the thermal head back to operating temperature before it continues to melt efficiently. In reverberatory furnaces, the thermal head temperature must be at least 300−400F (166.7-222.2C) hotter than the melt temperature to be able to melt at the rated capacity.
The type of charge materials (i.e., scrap castings; gates; runners; small pieces such as chips, turnings, borings; large pieces such as riser cutoffs, sows, ingots, chips; molten metal; etc.) all greatly influence the following:
- Amount of melting capacity that will be available.
- Design of the furnace charging ramps and wells, with respect to location, shape, and size.
- Design of drag-out ramps and furnace doors for cleaning.
- Submerged arches, which are more critical to consider if peripheral pump equipment or chip melting is going to be utilized.
Discharge and Transfer Methods
Another important design feature to establish during the selection and customization of a new melting furnace is how the molten metal will be removed from the furnace and delivered to the next process operation. Options are manual tap out cone, hydraulic tap out cone, tilting furnace, traditional transfer pump with piping, electromagnetic pump, pressure-pouring devices, and transfer pump with a trough. Any of these options affect the amount of turbulence in the molten flow. In addition, all furnaces must have tap outs for emergency draining, and full take-down for maintenance and re-lining.
All the discharge and transfer methods have advantages and disadvantages that must be carefully evaluated. Factors such as safety, capital cost, operational running costs, maintenance, cleanliness of molten metal, amount of dross generated in the transferring process, molten metal temperature loss, and type of flow generated should be analyzed and evaluated. Turbulent flow should be avoided. It tends to accelerate the formation of metal oxides that become entrapped in the molten bath. Turbulence is characterized by erratic variations in the speed and direction of flow throughout the stream of molten aluminum being discharged and transferred.
Distinct Design Features
Operational design features to consider include refractory specifications, side-wells for charging, cleaning and/or recirculation, positions and locations of charging and access doors, hearths, and burner capacities and types.
If the molten bath is going to be in excess of 30,000 lbs. (15 tons), it is almost certain the furnace may require a circulation pump to improve molten metal temperature and chemistry homogeneity, to minimize sludging in the furnace floor, to melt faster, to increase overall furnace efficiency, and to increase furnace refractory life.
If chlorine injection is going to be needed in the process, then a circulation pump to inject chlorine is necessary.
The choice of energy to be used in the furnace must be based on evaluating the most cost effective—not necessarily the most efficient—energy source. While energy cost is, without a doubt, a very important factor, there are other equally significant factors to evaluate: availability of fossil fuel and electricity, casting methods, production demands, materials melted, and process parameters. The objective in ideal energy selection is to attempt to produce aluminum at the lowest cost per pound of metal poured. Thus, there is a compromise between optimizing energy efficiency, minimizing metal loss, and producing the required metal quality for the application.
Physical Plant Restraints
Careful examination of the proposed furnace location in the plant is critical to properly maintain and operate the furnace. Unfortunately, this area is commonly overlooked by foundries. Factors such as floor space availability and aisles are critical for cleaning and servicing the furnace (charge, discharge). The selection of the furnace may be greatly influenced by the layout and operation of the foundry, especially how it integrates with the pouring and casting equipment.
Establishing the operating temperature range is also critical, since lower and higher temperatures than the required pouring temperature range will yield scrap castings, negatively affect molten metal quality, and damage the furnace refractory due to oxide buildup and/or sludging. Molten metal furnace temperatures vary depending on the alloy used, the furnace design, and other proprietary factors.
Providing the operating temperature range needed to the furnace manufacturer will lead to determining the correct refractory and proper temperature control components. Additional considerations for temperature controls are influenced by whether the furnace is a breakdown (melting) furnace and/or a holding furnace. Ultimately, the furnace must have the ability to produce and deliver quality molten aluminum, at the needed rate, while maintaining strict temperature gradients.
Although in the majority of the cases the melt department personnel have a clear idea of the required operating temperature range, in many instances they do not recognize (intentionally or unintentionally) that the actual control of the molten temperature during operation is significantly influenced by their melting practices, despite the furnace design capability.
One of the critical factors for controlling molten temperature is the physical condition and position (depth in the bath) of the furnace thermocouple. Unfortunately, this is a common poorly-controlled variable until molten temperature issues arise. The depth of the thermocouple should be no more than a couple of inches below the midpoint between the metal line (at full capacity) and the furnace floor, or at the same depth of the bottom of the tap out block. Problems with thermocouple failures become more critical in larger melting furnaces that are subject to stagnant baths, because of the lack of molten metal circulation. Temperature stratification throughout a deep bath can be significant. Oxide buildup and dross adherence can retard thermocouple response, and yield incorrect temperature readings. Careless cleaning practices to remove such material may result in thermocouple breakage.