High temperature vacuum furnace heater element support assembly

Abstract

An electrical insulating and heating element support assembly for a high temperature vacuum furnace having a threaded support rod for connecting a heating element to the insulated hot-zone support ring in an electrically non-connected position includes insulator sleeves and washers surrounding the rod in contact with a series of refractory metal washers which may include graphite and/or molybdenum as shielding liners used to protect electrical insulators from having electrical short path means due to deposition of conductive materials onto the non-conducting insulators, and the use of threaded nuts and bushings to anchor the rod and shielding arrangement within the furnace hot zone. The non-conducting insulators and washers are made from materials with high thermal and electrical resistance, such as preferably alumina or mullite, and radially surround the support rod and the heating element. The electrically non-connected shielding washers and nuts, and the rod can be made from graphite or molybdenum, and are designed to be easily disassembled in order to provide relatively easier maintenance service to the vacuum furnace. This design accomplishes the dual objective of supporting both the heating element and the high temperature insulation support ring while remaining electrically non-connected from the heating element. It also allows for variations in thickness of the furnace insulation and heating elements which is common for different furnace designs. This new stand-off assembly is designed to be easily disassembled in order to provide faster maintenance turnaround time and reuse of the stand-off hardware. Another equally important advantage of this design is the absence of holes in the support rod for the placement of pin retainers, and the elimination of the pin retainers, commonly found in prior art vacuum furnace heater element support assembly designs.

Claims

1. In a high temperature vacuum furnace system having a heating chamber with insulation material and insulation retaining plate, a hot zone area for placing work piece material for treatment, at least one hot zone support ring wall, at least one heating element, at least one support rod to provide a base for securing the heating element in spaced relation to the hot zone support ring wall, and an electrical insulating and support arrangement to be used with the support rod and the heating element, the support rod and support arrangement comprising: a solid support rod with no penetrations therein, said solid support rod being threaded at least at one end thereof and electrically insulated from the heating element, a threaded nut formed to operatively engage the threaded end of said solid support rod, bushing means in operative contact with said support rod at an opposite end thereof and in proximate contact with the hot zone support ring wall for supporting the furnace insulation material and the heating element, a central insulator passing through the heating element and surrounding said solid support rod adjacent to said threaded nut, a first pair of washers operatively located between said central insulator and said threaded nut, a second pair of insulators surrounding said central insulator and formed to be in operative contact with said first pair of washers on one respective end of each one of said second pair of insulators, a second pair of washers surrounding the heating element and adjacent to said second pair of insulators on one respective surface of each one of said second pair of washers and adjacent to said first pair of washers on another respective surface of each one of said second pair of washers, a third pair of washers surrounding the heating element on one respective surface of each one of said third pair of washers and formed to be adjacent to said second pair of insulators on another respective surface of each one of said third pair of washers, an insulation retaining plate surrounding said solid support rod and in operative contact with the furnace insulation, and an annular sleeve surrounding said solid support rod and in operative contact with one of said first pair of washers on one end thereof and with said insulation retainer plate on another end thereof.

2. A support arrangement in accordance with claim 1 wherein said second pair of insulators is not fastened to any surrounding parts.

3. A support arrangement in accordance with claim 1 wherein said second pair of insulators comprises a first portion and a second portion.

4. A support arrangement in accordance with claim 3 wherein a first portion of said second pair of insulators surrounds said central insulator on a first side of the heating element, and a second portion of said second pair of insulators surrounds said central insulator on a second side of the heating element.

5. A support arrangement in accordance with claim 1 wherein said solid support rod is comprised of molybdenum.

6. A support arrangement in accordance with claim 1 wherein said solid support rod is comprised of carbon fiber composite.

7. A support arrangement in accordance with claim 1 wherein said solid support rod is comprised of graphite.

8. A support arrangement in accordance with claim 1 wherein said threaded nut is comprised of graphite.

9. A support arrangement in accordance with claim 1 wherein said central insulator is comprised of alumina.

10. A support arrangement in accordance with claim 1 wherein said central insulator is comprised of mullite.

11. A support arrangement in accordance with claim 1 wherein said first pair of washers is comprised of graphite.

12. A support arrangement in accordance with claim 1 wherein said second pair of insulators is comprised of alumina.

13. A support arrangement in accordance with claim 1 wherein said second pair of insulators is comprised of mullite.

14. A support arrangement in accordance with claim 1 wherein said second pair of washers is comprised of molybdenum.

15. A support arrangement in accordance with claim 1 wherein said third pair of washers is comprised of molybdenum.

16. A support arrangement in accordance with claim 1 wherein said insulation retaining plate is comprised of graphite.

17. A support arrangement in accordance with claim 1 wherein said annular sleeve is comprised of graphite.

18. A support arrangement in accordance with claim 1 wherein said bushing means has a continuous surface in full contact with the vacuum furnace wall.

19. A support arrangement in accordance with claim 1 wherein said bushing means is in the form of a threaded nut and is formed to threadingly engage the opposite end of said solid support rod.

20. A support arrangement in accordance with claim 1 wherein said bushing means is in the form of a nut having a twist lock design and is formed to engage the opposite end of said solid support rod.

21. A support arrangement in accordance with claim 1 wherein said bushing means has an annular surface having a first cross-sectional diameter, and a second annular surface having a second cross-sectional diameter that is less than the first cross-sectional diameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objects and features of the present invention will be better understood from the following description taken in conjunction with the drawings wherein:

(2) FIG. 1 depicts a cutaway section of a portion of a high temperature vacuum furnace illustrating a heating element support assembly that supports the heating element away from the furnace wall in accordance with the present invention.

(3) FIG. 2 depicts a dimensionally exaggerated cross-section of a heating element support assembly for supporting a heating element within a high temperature vacuum furnace including an electrically insulating assembly in accordance with a preferred embodiment of the present invention.

(4) FIG. 3 depicts a dimensionally exaggerated cutaway cross-section of another embodiment of the present invention showing a portion of a heating element support assembly.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention provides, in a preferred embodiment, an improved heating element support assembly in which the rod support design protects the ceramic components from metallization. Prior art designs can be found in U.S. Pat. Nos. 8,088,328; 7,514,035; 6,936,792; 6,111,908; 6,023,487; 6,021,555; 5,930,285; 4,559,631; 4,425,660; and 4,259,538. The disclosure of each of these patents is incorporated herein in its entirety by reference.

(6) Referring now to FIG. 1, there is shown a cutaway section of a high temperature vacuum furnace illustrating a support assembly that supports a heating element 2 away from the furnace wall 16. Heating element 2 is one of a plurality of heating elements within the hot zone chamber of the vacuum furnace that are electrically insulated from the furnace structure. The cylindrically shaped heating elements 2 are connected to the inner wall 16 of the hot zone chamber by a plurality of support rods 1 (preferably formed from relatively pure, commercially pure molybdenum, carbon fiber composite (CFC) or graphite material) which support each of the heating elements a distance away from the furnace wall 16. The support mechanism is anchored at the other end by bushing means 3. Bushing means 3 may be in the form of a full threaded nut made from preferably stainless steel, which is threaded onto the other end of support rod 1 to stabilize the support mechanism. Alternatively, bushing means 3 may be in the form of a nut with a twist lock design. In yet another embodiment of the present invention shown and more fully described in FIG. 3, for reduction of heat loss through the bushing, a modified bushing in the form of a nut with only a reduced annular portion welded to the outer wall 16 may be used. The furnace is generally formed in a substantially cylindrical shape having a substantially internal cross-section that is closed at its forward end by a releasable door (not shown).

(7) In the embodiment illustrated in FIG. 1 the heating element bank is not formed into a complete loop, but has two ends at which an electrical power source is connected. If the banks of heating elements 2 were not electrically isolated from the support rods 1, and the mounting rods were connected to ground, a short circuit would occur which could cause damage to the furnace and also possibly to the work piece parts in the furnace. It is that type of major malfunction that the present invention eliminates.

(8) As shown in FIG. 2, an electrical insulating and support assembly for both heating elements and furnace insulation shields (or other refractory insulation materials) includes a support rod 1 which is threaded at least at one end thereof. Alternatively, rod 1 can be threaded throughout its length or at both ends. Rod 1 is preferably made from a material providing reasonable strength and thermal resistance such as preferably molybdenum. Rod 1 can alternatively be made from carbon fiber composite (CFC) or graphite material in certain instances. Rod 1 is supported by a bushing 3 which is preferably machined and is designed to be welded or bolted to the furnace insulation hot zone support ring assembly 16 so as to be rigid enough to support both a heating element 2 and furnace insulation 12. The design integrity relies primarily on a threaded nut 4 made from a material such as preferably graphite to retain the assembly, which in turn retains the support structure surrounding the furnace insulation 12. Heating element 2 is electrically insulated from the furnace by a plurality of insulators made from a material with high thermal and electrical resistance, such as preferably alumina or mullite. A central insulator 13 passes through heating element 2 and surrounds rod 1 providing electrical insulation between these two elements. A pair of insulators 6 and 6a surround central insulator 13 and are spaced in such a way that they allow radial thermal expansion of heating element 2 as it increases in temperature. This preserves both the heating element and the support structure for a longer service life due to reduction of stresses caused by thermal expansion. Insulators 6 and 6a are in near proximity to a pair of washers 8 and 8a, preferably flat washers, made of some refractory metal material such as preferably molybdenum, to prevent reaction of the alumina material onto insulators 6, 6a and 13. Washers 8 and 8a are located adjacent to both surfaces of heating element 2 at both its top and bottom. Insulators 6, 6a and 13 are in proximal contact at each of their respective ends with a pair of washers 5 and 5a, which are spaced from each other and surrounding rod 1. Washers 5 and 5a are preferably made from graphite material to shield the heating element 2 and the insulators 6, 6a and 13 from deposition of conductive material as well. A pair of washers 7 and 7a are in proximal contact with both ends of insulators 6 and 13 and with each of washers 5 and 5a. Washers 7, 7a, 8 and 8a are made from a refractory metal, such as preferably molybdenum, and they are located between graphite washers 5 and 5a, and ceramic sleeves 6 and 6a, in order to prevent abutment of the graphite and ceramic materials. This physical separation is required to prevent an unwanted detrimental chemical reduction process that can occur at high temperature and high vacuum, or in a hydrogen atmosphere, between graphite and alumina. This would result in evaporative loss of both graphite material and the ceramic washer material, thus leading to short circuiting and arcing. A sleeve 10 surrounds rod 1 between lower washer 5a and an insulation retainer 11 surrounding the furnace insulation. Sleeve 10 is made from a heat resistant material that is easily machined, such as preferably graphite. Sleeve 10 provides support for the insulation retainer 11, which in turn supports the furnace insulation.

(9) The present design accomplishes the dual tasks of supporting both the heating element 2 and the furnace insulation 12 of a high temperature vacuum furnace while remaining electrically insulated from the heating element. The furnace is easily serviced by removing nut 4, which advantageously utilizes the self-lubricating properties of graphite and is not susceptible to crystallization like molybdenum, which was used in previous designs. A molybdenum nut would not be able to be removed after several heating cycles, but the graphite nut remains stable up to high temperatures above approximately 1600 F. Because the nut is easily removed for service, no parts such as molybdenum nuts or wires need to be broken in order to access the heating elements 2 for service or repair, making it very economical to periodically service the vacuum furnace. The present design also allows for variations in thickness of furnace insulation 12, which are sometimes necessary to accommodate different work pieces to be heat treated and varying cycle requirements. When adjusting the overall length of sleeve 10 to accommodate variations in insulation thickness, nut 4 can be adjusted to address changes in the thickness of insulation 12 over time or at initial installation. Another advantage of the present design is that no holes need to be drilled into rod 1 to accept a pin or wire retainer as in the previous design. This eliminates the costs of drilling and machining holes in rod 1, as well as of producing pin or wire retainers. Accordingly, replacement parts are less expensive and more readily available.

(10) Referring now to FIG. 3, another embodiment of the present invention is shown in cross-section. This embodiment is identical to the embodiment illustrated in FIG. 2 and described hereinabove, with the exception of bushing means 14, which includes a reduced annular portion 15 in contact with support ring assembly 16. Annular portion 15 is machined as one piece from bushing means 14, and portion 15 is preferably approximately 0.040 inches in thickness. However, the range of thicknesses of annular portion 15 may be between 0.020 to 0.050 inches, in order to minimize the cross-sectional area of the heat conduction path between support ring assembly 16 and support rod 1. Annular portion 15 may be typically welded to support ring assembly 16. As described above with regard to bushing means 3, bushing means 14 may be in the form of a full threaded nut made from preferably stainless steel, which is threaded onto the other end of support rod 1 to stabilize the support mechanism. Alternatively, bushing means 3 may be in the form of a nut with a twist lock design.

(11) The purpose of a reduced annular portion 15, as opposed to the full width or diameter of bushing 3 shown in FIG. 2 in contact with support ring assembly 16, is to reduce the amount of thermal heat transmission from the hot zone to bushing 14, and ultimately to support ring assembly 16. This configuration reduces energy transfer from the hot zone support ring assembly to the heat treating cold wall chamber, resulting in less power required to operate the furnace.

(12) It will be recognized by those skilled in the art that changes or modifications can be made to the above-described invention without departing from the broad inventive concepts disclosed herein. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed herein, but is intended to cover all modifications and changes that are within the scope of the invention as defined in the appended claims.