WO2008118120A1

WO2008118120A1 - Heater assembly and method for molten metal processing

Info

Publication number: WO2008118120A1
Application number: PCT/US2007/007674
Authority: WO
Inventors: Joshua C. Liu; Ronald E. Boylstein; Jake Niedling
Original assignee: Alcoa Inc.
Priority date: 2007-03-27
Filing date: 2007-03-27
Publication date: 2008-10-02

Abstract

In one embodiment, the heater assembly includes commercially available immersion heaters or heater elements nestled in a graphite support structure inside a sintered ceramic protection tube. In one embodiment, the graphite insert-based electrical heater assembly provides for a molten metal heating package that has low initial cost, low maintenance cost, improved efficiency and reliability, improved power rating, and ease of replacement of damaged heaters. In one embodiment, the entire heater assembly can be removed from the molten metal and re-installed without damaging the heater elements. Furthermore, in one embodiment, individual heater elements, as well as the graphite insert, may be easily removable and replaceable as needed.

Description

HEATER ASSEMBLY AND METHOD FOR MOLTEN METAL PROCESSING

BACKGROUND Brief Description of Related Art

[001] Degassing is a process that is generally undertaken prior to formation of semi-finished metallurgical products so as to remove dissolved gases (e.g., hydrogen) and non-metallic impurities from the raw molten metal in order to substantially prevent damage to the quality of the metal castings from the molten metal. During degassing, an inert gas (e.g., argon, which may also contain some percentage of chlorine) is injected into the molten metal (e.g., aluminum or its alloys, magnesium or its alloys, etc.) using a rotor immersed in the molten metal and receiving the inert gas via a duct connected to the rotor. In order for the degassing process to be effective and in order for the casting to be done under suitable conditions, homogeneity should be achieved in the temperature of the molten metal contained in a processing ladle or apparatus designed for degassing.

[002] The temperature of the molten metal in the ladle can be controlled or maintained with the aid of heat-producing units placed inside the ladle and immersed in the molten metal. In one embodiment, a graphite block may be used as a heater or heat-producing unit. However, such graphite blocks may fail to provide adequate heating, and may have problems with maintenance and metal quality due to the use of graphite.

[003] Other heater units may include immersion heaters including a heating part immersed in the molten metal and a riser emerging from it to provide for electrical connection to the heating part and also to provide for the handling of the heater unit. The heating part of such an immersion heater may include an electrical resistance or resistive element having an insulation between the layers of the wire windings (or coils) of the electrical resistance. The insulation may be provided using a refractory substance, such as graphite, which is heat conductive and non- electrically conductive. A plurality of such heater units may be contained in a sheath that is also highly heat conductive and capable to withstand the molten metal. The sheath may be made of sintered ceramic (e.g., sialon). Such heater units, however, suffer from, a number of problems. For example, failures occur when the insulation between layers of heater element windings is either over constrained causing increased stresses to develop in the wire windings. On the other hand, when the insulation is weak or missing between two successive windings, an electrical short or arcing may develop between the successive layers of windings. Furthermore, missing insulation between wire windings allows the wire to sag when it is at high temperature and short against itself. The reliability of such heater units is also affected by improper insulating materials and methods. In ah alternative, an overcoat of a hard ceramic is provided on two opposite sides of the wire windings.

[004] In addition to insulation-based structural defects, the heater units in current systems provide for less power output (around 6.2 kW of power in a three-element design or around 8.2 kW of power in a four-element design). Such power output levels may not be sufficient to efficiently heat the molten metal and maintain its temperature over a period of time. When damaged, the replacement of such heater units may take a long time because of their specialized design and may involve significant cost per unit. Additionally, the entire heater unit assembly is required to be replaced even when a single heater element is damaged and needs to be replaced. This results in a high cost of maintenance and upkeep for such heater units. SUMMARY

[005] In one embodiment, the present disclosure relates to a heater assembly for providing heat to a molten metal. In one embodiment, the assembly comprises a heat-conductive sheath; a heat- conductive support structure removably placed inside the sheath and in contact therewith; and a plurality of heater elements removably placed inside the sheath. In yet another embodiment, the heater elements in each pair of heater elements are physically separated by the support structure and in contact with the sheath and the support structure. In another embodiment, a geometry of the support structure allows each heater element to be individually removed from the sheath without requiring removal from the sheath of either the support structure or any other heater element in the plurality of heater elements. In a specific embodiment, the sheath may be made of sintered ceramic (e.g., sialon) and the support structure may be made of graphite. [006] In another embodiment, the present disclosure relates to a heater assembly for providing heat to a molten metal, wherein the assembly comprises a heat-conductive sheath of sintered ceramic; a heat-conductive graphite support structure configured to be removably placed inside the sheath and in contact therewith; and- a plurality of heater elements configured to be removably placed inside the sheath. In one embodiment, the heater elements in each pair of heater elements are configured to be physically separated by the graphite support structure and to be in contact with the sheath and the support structure when placed inside the sheath. In another embodiment, the support structure is configured to allow each heater element to be individually removed from the sheath without requiring removal from the sheath of either the support structure or any other heater element in the plurality of heater elements. [007] In a further embodiment, the present disclosure relates to a method of operating an apparatus for processing a molten metal, wherein the apparatus has a compartment containing the molten metal. In one embodiment, the method comprises introducing an inert gas into the compartment containing the molten metal; and controlling the temperature of the molten metal in the compartment using one or more heater assemblies immersed into the molten metal at predetermined locations throughout the compartment. In another embodiment, each heater assembly may comprise a plurality of heater elements nestled in a graphite support structure inside a sialon protection tube as discussed hereinbefore.

[008] In a still further embodiment, the present disclosure relates to an apparatus for processing a molten metal. The apparatus comprises a compartment containing the molten metal; means for introducing an inert gas into the compartment, wherein the means extends below the surface of the molten metal contained in the compartment; and a heater assembly immersed into the molten metal in the compartment so that a first portion of the heater assembly extends below the surface of the molten metal and a second portion of the heater assembly extends above the surface of the molten metal for electrical connection. In yet another embodiment, the heater assembly may comprise a plurality of heater elements nestled in a graphite support structure inside a sialon protection tube as discussed hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

[009] A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily understood by reference to the following description in connection with the accompanying drawings in which:

[010] Fig. 1 illustrates one embodiment of a molten metal processing apparatus employing one or more heater assemblies or immersion heaters constructed according to the teachings of the present disclosure; [Oil] Fig. 2 illustrates exemplary constructional details of a heater assembly according to one embodiment of the present disclosure;

[012] Fig. 3 shows the top portion or riser of the heater assembly illustrated in Fig. 2;

[013] Fig. 4 depicts a heat-conductive protection sheath or tube placed over the heater elements and graphite insert in the heater assembly shown in Figs. 2-3;

[014] Fig. 5 illustrates an exemplary cross-section depicting constituent parts of a four-element heater assembly according to one embodiment of the present disclosure;

[015] Fig. 6 illustrates an exemplary cross-section of a three-element heater assembly according to one embodiment of the present disclosure; and

[016] Figs. 7A-7D are engineering drawings illustrating various cross-sectional views and dimensions of a four-element heater assembly according to one embodiment of the present disclosure.

[017] Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various embodiments and features thereof.

DESCRIPTION

[018] The accompanying figures and the description that follows set forth the present disclosure in embodiments of the present disclosure. However, it is contemplated that persons generally familiar with extrusions and degassing of molten metals will be able to apply the teachings of the present disclosure in other contexts by modification of certain details. Accordingly, the figures and description are not to be taken as restrictive on the scope of the present disclosure, but are to be understood as broad and general teachings. In the discussion herein, when any numerical range of values is referred, such range is understood to include each and every member and/or fraction between the stated range of minimum and maximum. Finally, for purpose of the description hereinbelow, the terms "upper," "lower," "right," "left," "vertical," "horizontal," "top," "bottom," and derivatives thereof shall relate to the present disclosure as it is oriented in the drawing figures provided herein.

[019] In one embodiment, Fig. 1 illustrates a molten metal processing apparatus 10 employing one or more heater assemblies or immersion heaters 12-14 constructed according to the teachings of the present disclosure. All three heater assemblies 12-14 shown herein are substantially identical in constructional details and, hence, for the sake of brevity, a single heater assembly may be described in more detail hereinbelow. In yet another embodiment, the apparatus 10 includes an enclosure 16 that provides a processing compartment 18 separated by a partition 20 from the outlet portion 22. In one embodiment, the enclosure 16 may be provided with an external metallic envelope and an interior refractory lining (not shown). In another embodiment, during molten metal processing (e.g., during degassing of a molten metal including, for example, aluminum or its alloys, magnesium or its alloys, etc.), the enclosure 16 is supplied, through an inlet port 24, with the raw molten metal to be processed, while the processed metal emerges at an outlet port 26 to carry out the metal casting operation. In the embodiment of Fig. 1, a representative level of the molten metal is indicated by the reference numeral "28." In case of a degassing operation, the molten metal may be processed with a rotor device 30 including a duct 32 for feeding an inert gas (e.g., argon) into the molten metal 28. During such degassing operation, the temperature of the molten metal 28 may be controlled using the heater assemblies 12-14 according to the teachings of the present disclosure. Although three exemplary heater assemblies 12-14 are shown in Fig. 1, in a given application, the total number of heater assemblies may vary depending on the application at hand, the type or characteristic of the molten metal, the size of the enclosure 16, etc. The constructional details of any of the heater assemblies 12-14 are discussed later hereinbelow with reference to Figs. 2-7, however, it is noted here that a portion of a heater assembly 12-14 (e.g., the portion 34 shown with reference to the heater assembly 12) may represent the heating portion of the respective heater assembly and such heating portion may be immersed into the molten metal 28. On the other hand, a riser or top portion (e.g., the riser 36 shown with reference to the heater assembly 14) of a heater assembly may remain above the surface 28 of the molten metal and provide, among other things, ports for electrical connection 38. The heater assembly 14 in the outlet portion 22 may be similarly installed.

[020] In a further embodiment, the apparatus 10 may be an in-line degasser with a covered top (not shown) for improved performance. Additional constructional and operational details of the apparatus 10 are not shown in Fig. 1. Furthermore, although the heater assemblies 12-14 are shown in Fig. 1 to have been installed in proximity to the same wall of the enclosure 16, these heater assemblies 12-14 may be disposed at other different locations throughout the enclosure 16 as needed. Also, the apparatus 10 may include more than one processing compartment 18 to process a greater quantity of metal. Such additional compartments (not shown) may operate in series or in any other arrangement compatible with the desired operational requirements. Each such compartment may include a gas introducing device (similar to, for example, the rotor device 30) and one or more immersion heaters similar to the heaters 12-14 shown in Fig. 1 to control the temperature of the molten metal being processed. [021] Fig. 2 illustrates exemplary constructional details of a heater assembly (e.g., any of the heater assemblies 12-14 shown in Fig. 1) according to one embodiment of the present disclosure. Although the details in Fig. 2 are shown with reference to the heater assembly 12, it is noted here, as before, that the discussion equally applies to any of the other immersion heaters 13-14 because of their substantially similar construction. The heater assembly 12 is shown to include a plurality of heater elements 40 physically separated by a heat-conductive support structure 42. In the embodiment of Fig. 2, there are four heater elements 40, each of which may be an electrical resistance that generates heat when provided with suitable electrical current. In one embodiment, the heater elements 40 are commercially available immersion heaters referred to in the industry as Hot Toe heaters available from industrial heating products suppliers such as Watlow Electric Manufacturing Company. As shown in Fig. 2, each pair of heater elements 40 may be separated by the support structure 42, which is in physical contact with the heater elements 40 so as to transfer heat generated by the heater elements 40. The support structure 42 may be made of any refractory substance that is heat conductive, but electrically non-conductive. In one embodiment, the support structure 42 is made of graphite. The portion of the heater assembly 12 shown in the embodiment of Fig. 2 may be the bottom portion of the heater assembly 12 that will be immersed in the molten metal during operation of the apparatus 10. Furthermore, although the graphite insert or support structure 42 is shown in Fig. 2 to extend only to a portion of the total length of the heater elements, it is observed here that Fig. 2 is for illustration only. In practice, the graphite insert 42 may extend all the way along the length of the heater elements as shown, for example, in the cross-sectional details of Fig. 7A. On the other hand, if desired, the graphite insert 42 may extend to only such portion of the heater elements as will be immersed in the molten metal during operation of the apparatus 10. [022] Fig. 3 shows the top portion or riser 36 of the heater assembly 12 illustrated in Fig. 2. It is seen from the embodiment of Fig. 3 that the top portion 36 of each heater assembly 12-14 may include ports 36A-36D for providing electrical connections 38 to corresponding heater elements in the plurality of heater elements 40. The riser 36 may be made of machined carbon steel or stainless steel and may also be fitted to a metallic seal plate 44 of machined carbon steel or stainless steel that may be used to securely hold various components of the heater assembly 12 and also to allow removal and replacement of the heater elements 40 as discussed later hereinbelow. In one embodiment, the seal plate 44 may be in the form of a ring encircling the heater elements 40, thereby allowing easy sliding of the heater elements 40 into and out of the seal plate 44 with the help of the riser 36. Although the riser 36 in Fig. 3 is shown to have four ports 36A-36D formed integrally therein, in an alternative embodiment, the riser 36 may be constituted of four sub-parts, each sub-part being formed of the corresponding one of the four ports 36A-36D and joined (e.g., through bolts or wedge or a dove-tail joint) with other neighboring sub-parts to form the entire riser assembly 36. In such an embodiment, each port 36A-36D may be separable from the other ports, thereby allowing further flexibility in removing and replacing corresponding heater elements. In still another embodiment, the riser 36 can be eliminated in its entirety with the elements simply resting on the seal plate 44. This further enables fast and easy change-out of failed heating elements. The elimination of the riser 36, though, has some inherent risk with the heaters being unconstrained and the possibility of the graphite support structure 42 degrading to a point where the heater elements can in time contact each other causing failures to occur.

[023] Fig. 4 depicts a heat-conductive protection sheath or tube 46 placed over the heater elements 40 and graphite insert 42 in the heater assembly 12 shown in Figs. 2-3. In one embodiment, the graphite insert 42 (not shown in Fig. 4, but shown in Fig. 2), the heater elements 40, and the protection sheath 46 are all in physical contact with each other so as to provide conduction of heat from the heater elements 40 to the molten metal when the heater assembly 12 is placed in operation. In other embodiments, the insert, heater elements and the sheath are not in direct physical contact but are otherwise configured so that heat is conducted from the heather elements to the molten metal, i.e. they are in thermal communication. Other components of the heater assembly 12 in Fig. 4 (e.g., the riser 36, the ports 36A-36D, etc.) are already discussed hereinbefore with reference to Fig. 3 and, hence, additional discussion of these components is not provided herein. In some embodiments, the protection tube 46 may be made of a heat conductive material, which can also withstand the molten metal — chemically as well as temperature-wise. In one embodiment, the protection sheath 46 is made of sintered ceramic, e.g., sialon, which effectively withstands molten aluminum and its alloys. A sintered ceramic is a ceramic formed into a mass using heat and pressure. Thus, as shown in Fig. 4, the heater assembly 12 may consist of an assembly of commercially available immersion heaters, which are nestled in a graphite support structure inside a sialon protection tube in such a manner that there is physical contact among the heater surface, the graphite insert, and the sialon protection tube, thereby resulting in maximum heat transfer efficiency.

[024] Fig. 5 illustrates an exemplary cross-section depicting constituent parts of a four-element heater assembly (e.g., the heater assembly 12 shown in Figs. 2-4) according to one embodiment of the present disclosure. The structural arrangement of the heater elements 40, the graphite insert 42, and the sialon protection tube 46 is clearly visible in the cross-section. Similarly, Fig. 6 illustrates an exemplary cross-section of a three-element heater assembly according to one embodiment of the present disclosure. As noted hereinbefore, different heater assemblies according to the teachings of the present disclosure may include different number (two, three, four, etc.) of heater elements depending on the desired configuration and application. Figs. 5 and 6 provide two examples of such different configurations for the heater assembly. [025] Figs. 7A-7D are engineering drawings illustrating various cross-sectional views and dimensions of a four-element heater assembly (e.g., the heater assembly 12 in Figs. 2-5) according to one embodiment of the present disclosure. It is noted here that various dimensions shown in Figs. 7A-7D are given in inches and are exemplary in nature. Heater assemblies with dimensions different from those shown in Figs. 7A-7D may also be provided by one skilled in the art as per the teachings of the present disclosure. Fig. 7A illustrates the front view of the complete heater assembly (e.g., any of the heater assemblies 12-14 in Fig. 1) including an additional mounting bracket 50 for securely fastening the seal plate 44 and riser 36 units with the heater structure comprising the sialon tube 46 containing the heater elements 40 and the graphite insert 42. Various cross-sectional views in Figs. 7A-7D are self-explanatory and, hence, additional discussion thereof is not provided herein. It is observed, however, with reference to exemplary embodiment in Fig. 7 A that about a 19 inch portion of the sialon tube 46 may be immersed into the molten metal during operation of the apparatus 10. It is also seen from Fig. 7B that the diameter of the heater assembly 12 may be, for example, equal to about 4.75 inches or in the neighborhood of about 90 to 150 mm.

[026} It is observed from the discussion herein that in some embodiments, the geometry of the graphite insert or the overall heater assembly allows each of the heater elements 40 to be easily removed from the sialon sheath 46 and replaced without requiring simultaneous removal, or any removal, of any of the other heater elements or the graphite insert 42. In some embodiments, the graphite insert 42 itself also can be removed from the sialon sheath 46 without damaging the heater elements 40. In some embodiments, an individual heater element can be removed and replaced without damaging or having to replace any other component of the heater assembly. [027] In one embodiment, the surface area of the sialon tube 46 that is in contact with the molten metal is about 135452 mm , which is different from the contact surface area of about 93362 mm² in some prior art heater assemblies. Furthermore, a four-element design according to one embodiment of the present disclosure may be powered by about 120 volts, single-phase power, with a current draw of about 20.5 amperes per heater element (i.e., a current draw of about 82 amperes for the four-element heater assembly). Such a design may provide a maximum power output of about 10 kilowatts for the four-element heater assembly, which is higher than the power output of around 8.2 kW in some existing four-element designs. In one embodiment, a three-element heater assembly provides a maximum power output of about 7.5 kW per assembly, which is also higher than the power output of around 6.2 kW in some current three-element designs. Such higher power outputs may provide quick control of the temperature of molten metal and speedy heating of the molten metal. Additionally, as noted before, the current graphite insert-based design according to the teachings of one embodiment of the present disclosure facilitates easy replacement of failed heaters. When a heater fails, the replacement can be performed by simply disconnecting the wiring, pulling the failed heater element(s) out from the top of the heater assembly, dropping-in the replacement heater elements, and re-making the electrical connection. Such replacement can be carried out more expeditiously than with some prior art heater assembly designs. Furthermore, the cost of replacing heater elements may be less than one third of the replacement cost in previous designs. Also, once installed, each new heater rod or element can be monitored for correct performance with a hand-held ammeter or power indicating lamps so as to identify any failures within the heater assembly. When a failed heater element is identified as needing to be replaced, the cost may be lower because of the need to replace only a single heater element versus the entire heater assembly as in some prior art systems.

[028] In one embodiment, where the riser 36 is eliminated or constituted of interlocking and individually separable sub-parts, a failed heater element may be replaced without necessarily disconnecting the wiring of the other functional heater elements in the group of heater elements in the heater assembly. Such an arrangement may allow further flexibility in expeditiously replacing failed heaters at savings of time and cost.

[029] In one embodiment, a four-element heater assembly unit (including the heater elements, the graphite insert, and the sialon tube) according to the teachings of the present disclosure was baked-out (curred) by heating the entire assembly to about 1300° F at a rate not to exceed about 25° F per hour. The heater elements performed as required and resulted in a fast startup time for a new degasser unit — four days from the receiving dock to aluminum sheet production. The caster continued to run for 12 days, after which a roll change was required and the degasser successfully maintained the metal temperature at about 1350° F in the idle mode for the twelve hour time period required to complete the roll change process and restart the caster for production. In the first 45 days of production, no heater problems or failures were reported. [030] The foregoing describes some embodiments of the invention. Some of the described embodiments include a graphite insert-based electrical heater assembly that provides for a molten metal heating package that has low initial cost, low maintenance cost, improved efficiency and reliability, improved power rating (e.g., about 10 kW of output power in a four- element design), and ease of replacement of damaged heaters. In some embodiments, the heater elements can be procured from commercially available immersion heaters, without requiring reliance on special suppliers or manufacturers. In some embodiments, the entire heater assembly can be removed from the molten metal and re-installed without damaging the heater elements. Furthermore, in some embodiments, because of the geometry of the graphite insert and because the graphite insert is removably placed inside the sialon tube in the heater assembly, individual heater elements as well as the graphite insert may also be easily removable and replaceable as needed.

[031] While a number of embodiments of the present invention have been described, it is understood that these embodiments are illustrative only, and not restrictive, and that many modifications may become apparent to those of ordinary skill in the art. For example, the heater assembly may comprise six heater elements.

Claims

What is claimed is: 1. A heater assembly comprising: a heat-conductive sheath; a plurality of heater elements removably placed inside the sheath; and a heat-conductive support structure placed inside the sheath and in thermal communication therewith, wherein heater elements of the plurality of heater elements are physically separated by the support structure and in contact with the sheath and the support structure, and wherein a geometry of the support structure allows each heater element to be individually removed from the sheath without requiring removal from the sheath of either the support structure or any other heater element in the plurality of heater elements.

2. The heater assembly of claim 1 wherein the geometry of the support structure allows each heater element to be individually removed from the sheath without requiring removal of the support structure from the sheath.

3. The heater assembly of claim 1, wherein the sheath is made of sintered ceramic .

4. The heater assembly of claim 1, wherein the support structure is comprises graphite.

5. The heater assembly of claim 1, wherein the plurality of heater elements includes three heater elements.

6. The heater assembly of claim 5, wherein the heater assembly is configured to output a maximum of about 7.5 kW of power during operation.

7. The heater assembly of claim 1, wherein the plurality of heater elements includes four heater elements.

8. The heater assembly of claim 7, wherein the heater assembly is configured to output a maximum of about 10 kW of power during operation.

9. The heater assembly of claim 1, wherein each heater element is configured to be individually connected to a corresponding electrical connection, and wherein each the heater element is removable from the sheath when disconnected from the corresponding electrical connection.

10. A heater assembly comprising: a heat-conductive sheath; a plurality of heater elements configured to be removably placed inside the sheath; and a heat-conductive graphite support structure configured to be placed inside the sheath and in contact therewith; wherein heater elements of the plurality of heater elements are configured to be physically separated by the graphite support structure and to be in thermal communication with the sheath and the support structure when placed inside the sheath, and wherein the heater assmbly is configured to allow each heater element placed inside the sheath to be individually removed from the sheath without requiring removal from the sheath of any other heater element in the plurality of heater elements.

11. In a heater assembly containing a plurality of heater elements, the improvement comprising: the plurality of heater elements removably encased within a heat-conductive protection tube and separated from each other inside the protection tube by a heat-conductive support structure, wherein all heater elements are in thermal communication with the protection tube and the support structure, and wherein a geometry of the support structure allows each heater element to be individually removed from the protection tube without requiring removal from the sheath of any other heater element in the plurality of heater elements.

12. The heater assembly of claim 11 , wherein the improvement further comprises: an output power of about 2.5 kW per heater element in the heater assembly.

13. A heater assembly comprising: a heat conductive sheath; a support structure placed inside the sheath; and a plurality of heater elements placed inside the sheath and supported by the support structure; and wherein a geometry of the heater assembly allows each heater element of the plurality of heater elements to be individually replaced.