Design Guidelines for High Performance Industrial Furnaces - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 325

learned that any faster speed would cause turbulence in flame forming
and, therefore, lower performance. Any slower speed would also produce
an inferior effect.
• Agitation of the molten bath: A thermal layer phenomenon toward the
deeper direction of the molten bath becomes stronger because some solid
aluminum remains immersed in the molten bath at the furnace bottom
and because the heat transfer rate of an aluminum molten bath is low.
Moreover, the bath surface will be covered by dross that forms through
the process of melting and drifts up, which makes it difficult for the
radiated heat from the atmosphere and the furnace wall to enter the bath.
Therefore, it is necessary to angle the jet flow from the high momentum
burners and try to remove the floating dross cover. At the same time, a
forced agitation system of the bath is necessary to break the thermal layer
and increase the heat transfer of radiated heat.
5.4.4.4 A Design Example of High Performance Aluminum-
Melting Furnace
An approach for a high performance aluminum-melting furnace is to design the
furnace based on a conventional furnace, but with modifications to integrate the
foregoing elements for high performance furnaces. Therefore, the design procedure
of a high performance aluminum-melting furnace is shown in Figure 5.78 . Figure
5.79 is an example of a high performance aluminum-melting furnace. Its major
features are reduction of CO 2 by more than 30% as energy saving, reduction of
furnace floor area by 16% as size reduction, low NO x meeting environmental regu-
lations, and reduction of metal loss as energy saving and waste reduction, as well
as a melting furnace adopting the regenerative burner combustion system. To realize
the above-mentioned high performance of the furnace, a combustion system that
exploits the combustion characteristics of high temperature air is adopted.
The furnace volume is determined by the working conditions, such as the charge
volume of the bulk cold material and the required casting volume. But thanks to the
improvement of melting rate by the increased heat transfer, the reduction of furnace
floor area is realized within the allowable limits of working conditions and the depth
of the bath.
The maximum heat transfer effect of aluminum that has a low capability of
radiation is obtainable by collision heating the center of the material heap for melting,
using directional burners of high velocity jet flow (high momentum) coupled with
a tilting movement.
Raising the entry angle of high velocity jet flow (high momentum) burners, and
by their sheer velocity, the floating dross covering the surface of the bath is removed
and the heat transfer by convection is improved. Also, by agitating the bath using a
forced agitation system, the thermal layer is broken and the radiation heat transfer
efficiency is improved. The following types of regenerative burners use oil as fuel
and are adopted for aluminum-melting furnaces. They are chosen depending on the
type of furnace.