Patent Application
20050126738
The invention generally relates to apparatus for conveying molten metal, and more specifically molten aluminum in aluminum casting processes.
In molten metal processing, troughs are generally used to convey the molten metal from the melting furnace to various process devices, such as casting moulds, analyzers, etc. Where there is ample metal flow, there is sufficient sensible heat in the molten metal to compensate for heat losses via the trough. However, in low metal flow situations, or long trough runs, some form of trough heating is required to prevent excessive metal heat loss.
U.S. Pat. No. 3,494,410 (Birchill et al) discloses a generic heated trough with provision for heating from any side, but without any detail as to how to achieve efficient thermal control.
U.S. Pat. No. 4,345,743 (Sivilotti) teaches an adiabatic tubular discharge conduit including a refractory lining for containing the molten metal and encompassed by heater elements.
U.S. Pat. No. 6,444,165 (Eckert) dated Sep. 3, 2002, describes a heated trough having heaters embedded in the side walls or bottom, in close contact with the refractory material of the trough.
These two references both use heaters which heat by conduction. Such heaters tend to form hot spots and uneven heating due to the difficulty in ensuring good contact with the surrounding refractory. They can be difficult to maintain because of a tendency get stuck by metal impregnation or expansion of the element shell. With heaters operating by thermal contact, the temperature difference between heater and surrounding materials will be low in the case of good thermal contact, and therefore a high heater temperature is needed to achieve a good watt density and energy transfer.
U.S. Pat. No. 4,531,717 (Hebrant) discloses a covered heated trough with heaters mounted in the top cover. These heaters operate principally by radiation rather than conduction and the heat flux is dependent on a fourth power of the temperature of the heater and the surface receiving the radiation. Radiative heaters are capable of high watt densities. However, heaters radiating to the surface of a molten metal are overall inefficient because of the low emissivity of the surface.
There is a common tendency for impurities, or inclusions, to form in molten metal as it travels down the trough, which is made more serious for troughs heated from above (top-heated). These inclusions grow in the presence of atmospheric oxygen at the surface of the trough, and refractory materials from which the troughs are usually made. High temperatures provided at the molten metal surface and low metal flow rates increase this effect dramatically.
It is therefore desirable to find a trough heater arrangement that will provide even and controllable heat to the traveling molten metal, while reducing formation of inclusions.
The present invention thus provides in one embodiment, a trough for carrying molten metal, comprising an outer shell defined by a bottom wall and two side walls, an insulating layer filling the outer shell and a conductive U-shaped refractory trough body for carrying molten metal, the trough body being embedded in the insulating layer. At least one heating element is positioned in the insulating layer, adjacent to but spaced apart from the trough body, to provide an air gap between the heating element and the trough body.
In a further embodiment of the invention there is provided a method for heating molten aluminum in a trough for conveying metal wherein the trough comprises an outer shell defined by a bottom wall and two side walls, an insulating layer filling the outer shell and a conductive U-shaped refractory trough body for carrying molten metal, the trough body being embedded in the insulating layer and wherein heat is provided by one or more radiant heaters embedded in the tough lining adjacent the conductive U-shaped refractory trough but spaced apart from the trough body, to provide an air gap between the heating element and the trough body.
The present invention will be described in conjunction with the following figures:
The trough body is supported within the surrounding insulation 14 on piers 19 manufactured from a refractory material, e.g. calcium silicate (wollastonite) refractory board.
A heating element 20 is positioned in the layer of insulation 14 and between the piers 19, adjacent to but spaced apart from the trough body 16, to provide an air gap 28. The air gap 28 allows for radiative heat transfer between the heating element 20 and the trough body 16. As the trough body is highly conductive, the heat balance is such that there will be a significant temperature difference between the heater and the portion of the trough body facing the heater but spaced apart from it. This allows for efficient operation of the heaters and a high heat flux even with heater temperatures lower than those used in conductive heating situation. The air gap 28 is sufficient to prevent either intermittent or accidental thermal contact between heater and trough body and therefore eliminates localized heat transfer by conduction and uneven heat and hot spots. The maximum size of the air gap 28 is not critical, and in one embodiment, a tapered gap may be used, with a larger gap width to allow for a larger heater surface area compared to the facing trough area, with resulting greater effective heat transfer. To avoid conductive heat losses to the wollastonite piers 19, the air gap is also continued between the sides of the heater elements and the piers. Although referred to as a single heating element, it is understood that the term “heating element” as used herein also includes more than one element. The heating elements are typical radiative heaters as supplied for example by Watlow.
A further closure board 21 of refractory material, such as wollastonite, is provided underneath the heater 20. This board can be removed to allow the heaters to be removed easily for servicing or replacement without dismantling the trough
The trough body material should have a high absorptivity to radiation or be coated with a conductive, high absorptivity coating to maximize the radiative heat transfer. Silicon carbide and graphite trough materials have acceptable absorptivity for these applications.
The air gap 28 between the at least one heating element and the trough body is preferably at least 0.5 cm. to avoid accidental thermal contacts in use. For practical reasons of space, a maximum air gap of 1 cm is generally used.
In a preferred embodiment shown in
A metal intrusion barrier, when manufactured from an electrically conductive metal or non-metal, may be used for detection of metal leaks by providing a means to detect a change in conductivity between the barrier and the metal in the trough. Such a detector may consist of a probe 32 that is immersed in the metal in the tough and an electrical connection 34 to the metal intrusion barrier with a conductivity detector 36 connected between the two. Normally a very low conductivity will be detected, but if metal infiltration occurs, the conductivity will rise and the conductivity detector 36 will signal a fault so that corrective action can be taken.
A further embodiment of the present invention is shown in
A suitable temperature control system, as well known in the art, can be used in conjunction with the trough 10 of the present invention. The system can comprise thermocouples placed in the one or more heating elements 20 and in sections of the trough body 16 near a top surface of the molten metal 18. A trough heater control program utilizes the output from both types of thermocouples to maintain accurate molten metal temperatures while prolonging the life of the heating element 20 by limiting heating element 20 output. One or more voltages can be used to heat and maintain the surface temperature of the molten metal in the trough body. For example, the voltages can be 220 or 110 volts.
The logic behind the heater control program uses P.I.D. closed loop control in order to maintain a tight tolerance of the molten metal temperature just prior to introduction to the casting mould. An example of a suitable temperature control system can be seen in U.S. Pat. No. 6,555,165 (Eckert), incorporated herein by reference.
