Introduction

Over the years, the aluminum casting industry has been searching for an energy efficient rapid melting device with reduced losses from oxidation and contamination. To accomplish these goals, along with energy efficiency, the furnace design must incorporate heating systems, which allow directing highly concentrated heat on the aluminum ingots, sprues, or scrap in order to provide rapid and efficient melting.   Alternately a retrofit is required for existing aluminum furnaces which may assist with the rapidity and associated energy saving.  Only recently, a new mid temperature range (1200K–1600 K) convective plasma device has been patented, and has become available commercially  known as the Plasma AirTorch™  shown in figure 1. 

Non-plasma convective heating is used in materials processing industry for heat treatment and melting applications.  Plasma enhancement has often been attempted.  The research in industrial applications of Plasma has largely been concentrated on two types of systems, viz., thermal plasma at atmospheric or near atmospheric pressures and low pressure plasmas.  Thermal plasmas are used extensively in applications such as plasma spray coatings and arc welding.  Typical temperature in such applications may be in the range of 5000-15000K and pressure is atmospheric.  In the second case, plasmas at low pressures are used for applications such as chemical vapor deposition and polymer processing.  These are generally cold plasmas as due to low collision coupling between electrons and heavy particles, the temperature of ions and neutral remains at room temperature.  Thermal plasmas have also received much attention in the literature.  However, these two extremes (very hot plasmas at atmospheric pressure or cold plasmas at low pressures) are not best suited for metallurgical work.  For example, most of the aluminum melting or steel heat treatment is carried out between 600^oC and 1200^oC.  The low pressure plasma possesses very low energy density and can not be used for aluminum melting.  The very high temperature thermal plasmas result in significant heat losses and may result in poor efficiencies. The one atmosphere patented plasma generator discussed in this article is able to address both these needs.  The device converts air into a low ionized plasma at one atmosphere.  The resulting plasma at about 1200⁰C is able to provide a nitrogen cover to the metal (eliminates cover gas) as well as able to provide focused energy and vastly improved heat transfer both leading to significant energy efficiency benefits. 

A typical device which can easily be attached to any existing furnace is shown in figures 1a and 1b.  The device as shown in Figure 1c, operates with a fan and produces the required amount of plasma directly from air.  Unlike conventional plasma guns the flow of plasma is gentle and extremely quiet.

(a)	(c)	Figure 1 (a).   A picture of a typical 10kW Plasma AirTorch™.  The overall length is about 25 inches.  The diameter is 7 inches.  Figure 1(b) shows a typical nozzle which can be attached i.e. added-on to an existing furnace. Figure 1(c) shows Plasma which exits from the nozzle.
(b)

A furnace incorporating such a plasma torch generally displays the following benefits:

• Reduced Energy Costs - Energy efficiencies of 0.2kWhr/lb for melting with no emissions.
• Improved melting because of ultra clean metal.
• Clean melting, with dross values less than 0.5%.
• No requirement for nitrogen, argon, or chemical fluxes - significantly reducing operating costs and the environmental impact.
• Extremely small equipment foot-print as shown above in Figure 1.
• Quiet operation - Zero noise -much lower than typical, conventional plasma systems.
• High energy density nearly four times compared to the standard

Although the main method of use is to add the torch to an existing furnace in a retroactive manner the Plasma AirTorch™ can also be used as the only heat source in a new furnace.  Such a Plasma Aluminum Melting (PAM) furnace (a typical design is shown in Figure 2) is a possible method to deal with the next generation melting problems, allowing energy rates as low as 0.198 kWh/lb, as opposed to induction melting energy rates of 0.345 kWh/lb. The PAM can be constructed as shown an automated furnace which allows quick charging, rapid melting, pouring, and disposal of dross. The combined effects of conduction from the hearth, forced convection from plasma, and radiation, contribute to the concentration of heat. Such a furnace may be constructed for a variety of melting needs, ranging from ingot melting, sprue melting and scrap melting for recycling. Several custom footprints are possible.   In addition, there is no noise or foul burning gas smell.  Table 1 below shows typical melting parameters observed for a 23KW system.

Figure 2.  A furnace chamber with continuous loading and unloading can be can be constructed with the Plasma AirTorch™ as the only heat source.  A cut-away from such a furnace is shown. 

Table 1:  Typical melting results from a 23kW melting furnace.  The furnace had a footprint of about 36 square inches.

For Aluminum a 23kW system yields:
Energy to melt	0.2kWhr /lb
Dross/Total Metal Loss	~0.5% or lower depending on alloy
Melt Rate	~12.7 g/s  (compare with 3g/s for conventional) ~1 Ton / day for 23kW.
Energy Concentration	269,146 BTU/ft3  (~107kJ/m3)

Energy Density

Conventional aluminum furnaces generally do not focus on energy density as this the numbers are low for wire-wound, gas fired and silicon carbide powered aluminum furnaces. However high energy densities of the furnace allow more heat to be transferred to the charge and less heat to the outside and a furnace with high energy density is desirable.  Thus energy density is important for energy conservation as well as for reducing the foot print of the furnace.  A typical electric resistance melting furnace exhibits an energy density concentration of 64,557 BTU/ft³ as opposed to the new PAM with 269,146 BTU/ft³, for equal volume of hot zones. The PAMF has four times higher energy per unit volume compared to electric resistance furnace, thus making it a unique furnace with highly concentrated power.

In addition, as the power density (i.e. energy density per unit time) is increased in conventional furnaces the melt rate tapers-off as shown in figure 3.  In contrast furnaces which have a Plasma AirTorch™ the melt rate remains high because of the enhanced heat transfer that the plasma provides

Figure 3.  Conventional furnaces are unable to create heat on the charge and their efficiency falls off. Plasma assisted furnaces automatically have high power densities and heat transfers directly to the part.

Why does this happen?  The reason lies in the fact that the heat transfer coefficient increases with the gentle plasma even where the convection velocity is small.  Figure 4 below shows the basic plasma heating and nitrogen cover principle.  Figure 5 is a plot which shows the enhancement in the surface heat transfer coefficient to the charge.

Figure 4.   The basics of plasma heating

Figure 5.  Variation of heat transfer coefficient with the degree of ionization for aluminum.  A similar result is expected for all metals.

Economic Comparisons

For economic comparisons between the several techniques used for aluminum melting, the factors to be taken into account are: (1) The cost of equipment and installation and (2) the operating costs, which depend on (a) the utility costs in the area (b) the energy efficiency of the equipment chosen (c) the quality requirements of the finished casting (d) the metal losses (dross) to be expected as a result of the melting process. In additions there is a cost associated with (i)Regulation and comfort factors, such as EPA considerations, heat, noise, and air pollution and (ii) The casting size range and the weight of metal required per day and associated storage and manpower costs.
Installation costs of electric resistance and fossil-fuel-fired furnaces are comparable. It is not practical to hypothesize a specific example, as there are too many possibilities to take into account. In general, fossil-fuel-fired furnaces require fluing, blower equipment, and in some cases heat exchangers (for preheating combustion fuels); however, on balance, power controls often result in a slightly higher investment for electric operations. Another widely used method for melting is the induction furnace. While induction furnaces cost more than resistance furnaces their production rates are generally much higher. An operating cost comparison is presented the table below, to illustrate the relative expenses for a hypothetical aluminum melting operation. Metal loss includes dross plus flue loss. The most significant operating cost consideration is not only in the relative cost of the utilities, i.e., gas, oil, electric, etc., but the relative metal losses to be expected and the reliability index. Electric resistance melting yields are high, while metal losses from fossil-fuel operations may be as high as 8 percent. When taking into account the metal loss, the current as well as the projected metal cost at the spout should be used in making investment plans. Utility costs vary widely in different localities. For example, gas prices can range from $2.50 to $4.86 per MCF, while electric costs can range from $0.032/kWhr at off-peak times to $0.08/kWhr or more. Theoretical melting for aluminum is 445BTU/lb. Efficiency is calculated as in the table below where mass is the melted mass:

------------  (1)

CONCLUSION

Based on the results obtained to date, the following energy and environmental benefits are noted from using the Plasma AirTorch™ for aluminum processing.

Measured Energy Saving
The improvement in energy efficiency over using the Convective Heat Source furnace is calculated to be approximately 73-82%.

Environmental (waste stream) savings
In addition to this, the PAM offers many other non-measurable savings, such as elimination of harmful emissions and noise (no noise), and increase of productivity. Since gas/oil burners are replaced by the ionization units in PAM, we anticipate that the harmful emissions (e.g. CO, CO2, NOx, etc.) associated with existing gas/oil-fired furnaces will be totally eliminated.

Productivity and Profitability

A small foot print furnace or a retrofit to an existing furnace is able to considerably enhance the melt rate (four times) and eliminate labor time with disposing waste.