AU2010101435A4 - Tubular Heating Element and Process for Producing Same - Google Patents

Description

Our Ref: 20706554 P/00/0 Il Regulation 3:2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION INNOVATION PATENT Applicant(s): Specified Electrical Process Heating Co. Australia Pty. Ltd. 5-7 Park Road, Cheltenham, Victoria 3192, Australia Address for Service: DAVIES COLLISON CAVE Patent & Trade Mark Attorneys 255 Elizabeth Street SYDNEY NSW 2000 Invention Title: "Tubular Heating Element and Process for Producing Same" The following statement is a full description of this invention, including the best method of performing it known to me:- C:\NRPorbl\DC0WXH\3372444_ DOC-17/12/2010 TUBULAR HEATING ELEMENT AND PROCESS FOR PRODUCING SAME The present invention relates to a heating element and in particular to a heating element 5 including a seal which reduces the ingress of moisture into the heating element. The present invention also relates to a process for sealing a tubular heating element. Background of the invention 10 Tubular heating elements are generally associated with electrically powered ovens and heating devices. A typical tubular heating element consists of a core of resistance wire often composed of nichrome which is surrounded by an insulating material such as for example ceramic material of magnesium oxide or alumina powder. The insulating material is then covered by a metallic tube typically composed of stainless steel or brass wherein 15 the ends of the tube are sealed with the core of resistance wire located within the tube and connected onto a conductive rod of metal, such as steel, which passes through the seal of the tube to a terminal. Electricity applied to the terminal produces a current which passes along the metal rod through the seal to the resistance wire which due to its resistance causes heat that then radiates from through the insulating material out of the metallic tube 20 and increasing the temperature of the environment immediately surrounding the heating element. As the heat provided by a tubular heating element relies on the passage of electricity through the core of the resistance wire, any moisture that is present in the insulating layer 25 can cause the tubular heating element to short circuit and cease to function efficiently. Accordingly, seals are typically provided at the ends of the metallic tube which seek to prevent moisture from entering into the insulating material. However, seals and sealing methods to date have not been sufficiently effective and many tubular heating elements that have been left in storage situations have effectively been rendered useless due to 30 moisture passing into the insulating material via the seal.

C:\NRPortbI\DCC\WXH\3372444 1DOC-17112/2010 -2 Accordingly, there is a need for a sealing process for tubular heating elements that substantially reduces the ingress of moisture into the insulating material of a tubular heating element. 5 Summary of the Invention According to one aspect the present invention provides a tubular heating element including: a core of resistance wire with two metal rods attached to the core of 10 resistance wire at each end; a layer of insulating material surrounding the core of resistance wire; a metallic tube with two open ends, the metallic tube encasing the layer of insulating material; and, a seal located at the two open ends of the metallic tube with the two metal 15 rods at each end of the core of resistance wire passing through the seal leading to a terminal; wherein the seal substantially reduces the ingress of water into the layer of insulating material from the two open ends of the metallic tube. 20 In one form, the seal is formed by applying a first sealant to the surface of the insulating material at the open ends of the metallic tube; applying a second sealant on top of the first sealant; and then threading a bead onto the metal rod and into the open ends of the metallic tube. 25 In one form, the first sealant is applied into a cavity formed in the insulating material at the two open ends of the metallic tube and the bead is threaded into the cavity wherein an outer surface of the bead is substantially flush with the end of the metallic tube. In one form the first sealant is applied to the surface of the insulating material and is at 30 least partially absorbed therein. In one form the first sealant is allowed to at least partially absorb into the insulating material before the application of the second sealant.

C\NRortbl\DCC\WXH\3372444 .DOC-17112/2010 -3 In one form the first sealant is a high temperature silicon drop such as for example DOW CORNING SILICON 200 FLUID 50 cs. 5 In one form the second sealant is a high temperature red silicon such as for example BOSTIK RTV 936 HEAT RESISTANT INDUSTRIAL GRADE. In one form the bead is a high temperature ceramic bead. 10 According to another aspect the present invention provides a process for sealing a tubular heating element consisting essentially a core of resistance wire with two metal rods attached to the core of resistance wire at each end; a layer of insulating material surrounding the core of resistance wire; and a metallic tube with two open ends, wherein the metallic tube encases the layer of insulating material, the process including the 15 following steps: - optionally providing a cavity at the two open ends of the metallic tube by removing an amount of the insulating material; - applying a first sealant to the surface of the insulating material at the two open ends, optionally allowing the first sealant to at least partially be 20 absorbed into the insulating material, and then allowing the first sealant to cure; - applying a second sealant on top of the first sealant; - threading a bead onto the metal rods and into the second sealant before the second sealant has cured; and, 25 - allowing the second sealant to cure. Brief Description of the Accompanying Figure The present invention will become better understood from the following detailed 30 description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein: C:\NRPortbl\DCC\WXH\3372444 .DOC- 1 7/12/2010 -4 Figure 1 is a schematic cut away view of one end of a tubular heating element in accordance with an embodiment of the present invention. Detailed Description of Embodiments of the Invention and the Accompanying Figure 5 The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. 10 The present invention provides a tubular heating element that includes a core of resistance wire composed of highly resistant metal such as nichrome that is surrounded by an insulating material. Two metal rods composed of a conducting material that is resistant to corrosion, such as for example stainless steel, are attached to the two ends of the core of resistance wire. The metal rods may be attached by any suitable method, and in one form, 15 the core of resistance wire is wound around the ends of the metal rods. The insulating material can be chosen from any suitable ceramic insulating binder such as for example magnesium oxide or alumina powder. The insulating material is then encased by a metallic tube which may be composed of any suitable metallic material such as for 20 example stainless steel, copper or brass. The metallic tube has two open ends which include a seal which allows the metal rods to pass through the seal but prevents any moisture from entering through the open ends of the metallic tube and into the insulating material containing the core of resistance wire within the tubular heating element. 25 The seal is made up of three elements. The first element consists of a first sealant which is applied to the surface of the insulating material at the ends of the metallic tube. Often it is advantageous to remove some of the insulating material from the ends of the metallic tube such that a cavity is formed within the ends of the metallic tube for the seal to be located. 30 The first sealant is applied to cover the surface of the insulating material at each of the ends of the metallic tube and is then allowed to cure. The first sealant may also be allowed C:\NRorbr\DCC\WXH\3372444 1.DOC-17/12/2010 -5 to at least be partially absorbed into the insulating material when applied. The first sealant may be chosen from any suitable sealing composition that is able to withstand high temperatures often associated with oven environments. In certain embodiments, the first sealant composition is able to withstand temperatures of greater than 250*C up to 350*C 5 depending on the heat generated by the oven or heating apparatus in question. In one embodiment the first sealant is chosen from high temperature silicon drops such as for example DOW CORNING SILICON 200 FLUID 50 CS. In this embodiment, it is also preferred that the first sealant is cured by heating the sealant for a period of time. If two drops of this first sealant are used, the first sealant is heated from between 1 to 2 hours at a 10 temperature of between 100 to 150 *C to facilitate curing of the first sealant. Once the first sealant has cured, the second element in the seal comprising of a second sealant is then applied to the surface of the cured first sealant. After being applied and whilst the second sealant is still wet or has yet to cure, a ceramic bead, the third element in 15 the seal, is threaded onto the metal rods and placed into the opening of the metallic tube such that the outside surface of the ceramic bead is substantially flush with the end of the metallic tube. In this way, the ceramic bead effectively provides a plug at the end of the metallic tube. Excess second sealant is then removed from around the seal forced out when the ceramic bead was pushed into place. 20 In the event there is no excess sealant produced around the edges of the ceramic bead, more of the second sealant should be used, as the excess produced after the ceramic bead is pushed into place ensures that a solid seal results. The second sealant is then allowed to cure thereby forming the seal. 25 The second sealant may be chosen from any suitable sealing composition that is able to withstand high temperatures often associated with oven environments. In certain embodiments, the second sealant composition is able to withstand temperatures of greater than 250*C up to 350*C depending on the heat generated by the oven or heating apparatus 30 in question. In one embodiment the second sealant is chosen from high temperature red C NRPortbIWCC\WXH\337244 1.DOC-17/12/2010 -6 silicon such as BOSTIC RTV 936 HEAT RESISTANT INDUSTRIAL GRADE RED SILICON. In one embodiment the ceramic bead is chosen from a high temperature ceramic bead such 5 that it may also withstand the temperatures associated with an oven environment. In accordance with certain embodiments, the seal of the tubular heating element can be prepared by means of the following process wherein a tubular heating element is prepared where a core of resistance wire is surrounded by a layer of insulating material which is 10 then encased in a metallic tube with two open ends with the core of resistance wire attached to two metal rods at each end, where the two metal rods extend beyond the ends of the metallic tube with the core of resistance wire held within the insulating material. The two ends of the metallic tube may be subjected to sand blasting in order to remove a few millimetres of the insulating material to form a small cavity of about 7mm in depth 15 within the metallic tube at each end for the seal. In one embodiment, a ceramic bead is then threaded over the metal rods at each end of the tubular heating element and a terminal is then attached onto the end of the metal rods which provides the location where an electric current is introduced to heat the element 20 during operation. Alternatively, the ceramic bead and terminal may be threaded onto the metal rods after the application of the first or second sealant. The terminal is sandblasted to remove any excess flux. Once this is completed the tubular heating element is dried in an oven to remove moisture from the insulating material. The 25 duration and intensity of this drying step depends on how much moisture is in the initial tubular heating element as well as the actual length and diameter of the metallic tube. For example, for a short tubular heating element where there is less moisture in the air on a hot summer's day, only one hour in an oven at about 150*C is required to dry the tubular heating element. Whereas a long tubular heating element where there is a high moisture 30 content in the air during a rainy day in a cold winter's day a period of up to 8 to 12 hours in an environment of about 150*C may be required.

C:\NRPorbl\DCC\WXH\3372444.l.DOC-17/12/2010 -7 Once the tubular heating element is dried the first sealant is applied at the ends of the metallic tube to cover the surface of the insulating material within the cavity. Usually about two to three drops of sealant such as DOW CORNING SILICON 200 FLUID 50 CS 5 is all that is required in a typically sized tubular heating element. Any excess fluid can be drained out by turning the end of the metallic tube upside down. The first sealant once applied is at least partially absorbed into the insulating material The element is then placed in an oven at about 150*C for at least about one to two hours to 10 allow the first sealant to cure. The first sealant is preferably dry before the second sealant is applied and preferably the second sealant is applied once the end of the metallic tube is within the temperature range of about 40 to 100*C. The second sealant is typically chosen from high temperature red silicon such as that 15 available from BOSTIC RTV 936 HEAT RESISTANT INDUSTRIAL GRADE. Because such a red silicon sealant is quite thick and viscous it is typically syringed into the cavity at the open end of the metallic tube where the sealant is filled up to the end of the metallic tube completely filling the cavity. The ceramic bead is then pushed along the core of resistance wire into the open end of the cavity at the end of the metallic tube with excess 20 sealant exiting out the sides between the ceramic bead and the metallic tube. The ceramic bead is pushed into the end until the outside edge of the ceramic bead is flush with the end of the metallic tube with the rest of the body of the ceramic bead held within the cavity. Once the ceramic bead is in place the second sealant is allowed to cure for at least 24 hours 25 at room temperature or alternatively at 4 hours in an environment at about 100 to 150'C. Once cured, the seal provides a barrier to moisture entering into the insulated material of the tubular heating element. The seal and the process for sealing a tubular heating element as hereindescribed may be 30 used with a heating element of any sheath diameter, material and terminal.

C\NRPortbi\DCC\WXH\3372444 1 DOC-17/12/2010 -8 Referring now specifically to Figure 1 there is shown the end of a tubular heating element 10 which includes a metal rod 11 attached to a core of resistance wire (not shown) which is surrounded by a layer of insulating material 13 which in this embodiment is magnesium oxide. A metallic tube 12 surrounds the insulating material 13 effectively encasing the 5 metal rod 11 and insulating material 13 making up the body of the tubular heating element 10. As described above when producing a heating element, a cavity is formed at the end 19 of the metallic tube 12 into the insulating material 13 at a distance D into the end 19 of the 10 metallic tube 12. The distance D may be chosen from a distance of about 5 to 9 mm and depends on the diameter of the metallic tube 12. A first sealant 14 is then applied to the surface of the magnesium oxide 13 within the cavity formed at the end of the metallic tube 12. The first sealant 14 is then allowed to be absorbed into the insulating material and then allowed to cure. Once cured the first sealant is then covered by a second sealant 15 which 15 is applied to the surface of the first sealant 14 and fills up the remainder of the cavity. Before the second sealant 15 is allowed to cure a ceramic bead 16 is pushed into the end of the metallic tube 12 having been threaded onto the metal rod 11. The ceramic bead 19 is pushed into the cavity along the metal rod 11 until the outside surface of the ceramic bead 20 16 is substantially flush with the end of the metallic tube 12. Any excess second sealant 15 is then removed. A terminal 18 is also attached onto the metal rod 11 which enables the tubular heating element to be connected to an electric current. The second sealant 15 is then allowed to 25 cure providing a seal comprising three elements at the end of the tubular heating element 10. The first element being the first sealant 14, the second element being the second sealant 15 and the third element being the ceramic bead 19. In the context of this specification, the word "comprising" means "including principally 30 but not necessarily solely" or "having" or "including", and not "consisting only of'.

C:\NRPortbl\DCCWX H\3372444 DOC- 1 7/12/2010 -9 Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.

Claims (5)

1. A tubular heating element including: a core of resistance wire with two metal rods attached to the core of 5 resistance wire at each end; a layer of insulating material surrounding the core of resistance wire; a metallic tube with two open ends, the metallic tube encasing the layer of insulating material; and, a seal located at the two open ends of the metallic tube with the two metal 10 rods at each end of the core of resistance wire passing through the seal leading to a terminal; wherein the seal substantially reduces the ingress of water into the layer of insulating material from the two open ends of the metallic tube. 15

2. A tubular heating element according to claim I wherein the seal is formed by applying a first sealant to the surface of the insulating material at the open ends of the metallic tube; applying a second sealant on top of the first sealant; and then threading a bead onto the metal rods and into the open ends of the metallic tube. 20

3. A tubular heating element according to claim I or claim 2 wherein the first sealant is applied into a cavity formed in the insulating material at the two open ends of the metallic tube and the bead is threaded into the cavity wherein an outer edge of the bead is substantially flush with the end of the metallic tube. 25

4. A tubular heating element according to any one of claims 1 to 3 wherein the first sealant is a high temperature silicon drop and/or the second sealant is high temperature red silicon and/or the bead is a high temperature ceramic bead.

5. A process for sealing a tubular heating element consisting essentially of a core of 30 resistance wire with two metal rods attached to the core of resistance wire at each end; a layer of insulating material surrounding the core of resistance wire; and a C:\NRPortbl\DCC\WXK3372444 1.DOC- 17/12/2010 - 11 metallic tube with two open ends, wherein the metallic tube encases the layer of insulating material, the process including the following steps: - optionally providing a cavity at the two open ends of the metallic tube by removing an amount of the insulating material; 5 - applying a first sealant to the surface of the insulating material at the two open ends and allowing the first sealant to cure; - applying a second sealant on top of the first sealant; - threading a bead onto the metal rods and into the second sealant before the second sealant has cured; and, 10 - allowing the second sealant to cure.