Heating element furnace and method

Abstract

A heating element 2 comprises a plurality of resistive members 16 extending in a longitudinal direction and an assemblage of electrically conductive components. At least some of the resistive members are connected in series and are arranged to be connected to an electric power source via two connectors 8 for electric power. The assemblage of electrically conductive components comprises a contact element 4 and a resistor 6, wherein the resistor is connected in series with the contact element and with one of the two connectors 8 for electric power. The contact element 4 is arranged at an end portion 13 of the heating element 2 for the contact element 4 to be contacted by an overly elongated resistive member 16' of the plurality of resistive members. This configuration allows for indication of overly elongated resistive members 16', preventing short circuits and potential damage.

Description

[0001] Heating Element Furnace and Method

[0002] TECHNICAL FIELD

[0003] The invention relates to a heating element comprising a plurality of resistive members and to a furnace comprising a heating element. The invention further relates to a method of operating a furnace.

[0004] BACKGROUND

[0005] Resistive heating elements are used in furnaces for heating purposes. These heating elements typically comprise a number of resistive members having a longitudinal extension, and which are electrically heated. As the resistive members are cyclically heated, they undergo elongation and contraction due to material properties and temperature fluctuations. This cyclical heating process also leads to permanent residual elongation of the resistive members, which can cause problems in the operation of the heating element and the furnace.

[0006] A common design for resistive heating elements involves arranging the heating element in a cartridge, which is then mounted in a so-called radiant tube. The radiant tube is formed by a protective tube wherein the cartridge is arranged. The radiant tube is mounted in a furnace to be heated by the heating element via the radiant tube. The resistive members of the heating element are electrically connected to each other thus, forming a circuit. They are electrically isolated from each other and the radiant tube by isolator elements. These isolator elements have a short length along the longitudinal extension of the resistive members to avoid impeding heat transfer or radiation from the resistive members.

[0007] To accommodate the elongation of the resistive members, the radiant tube is made longer than the resistive members. This design aims to prevent short circuits between the resistive members and the radiant tube during use. However, the residual elongation of the resistive members can eventually lead to one or more of the resistive members causing a short circuit between the members and / or the radiant tube. This short circuit can cause the resistive members to melt and rupture, rendering the heating element ineffective. Furthermore, the short circuit can also damage the radiant tube, significantly reducing its life span and requiring costly unscheduled replacements.

[0008] Prior art, such as US 2010 / 193504, discloses an electrical heating element for vertical installation, comprising vertically suspended electrical resistance elements for the heating of furnaces in industrial operation. The heating element includes ceramic disks provided with holes through which the resistive members of the heating element pass. The ceramic disks are arranged between ceramic tubes, and a metallic central rod is positioned within the ceramic tubes, which supports the element. The heating element is surrounded by a radiant tube.

[0009] Another prior art, WO 2005 / 006812, discloses an electric heating element with a radiant tube comprising a radiation pipe and an electric heating element contained in the pipe. The heating element has legs, or resistive members, that run to and from in the pipe, and the heating element is connected at one end of the pipe close to a furnace wall with electric power outlets through which electric current is fed to the element.

[0010] DE 29606563 discloses an electrical heating element arranged in a radiant tube. In order to avoid a short circuit between resistive members of the heating element and the radiant tube, a spring component is arranged into the chain of components of the heating element: The spring component compensates for changes in length. Moreover, a centre rod of the heating element is selected having a coefficient of thermal expansion of the same order as that of the resistive members.

[0011] SUMMARY

[0012] There is a need for an improved heating element design that can effectively address the issue of residual elongation of resistive members and the associated risks of short circuits and damage to one or more of a radiant tube in which the heating element is arranged, a furnace in which the radiant tube is arranged, and a product being heated in the furnace.

[0013] It would be advantageous to achieve a heating element that reduces the risks of short circuits and damage to the radiant tube, the furnace, and / or the product being heated. In particular, it would be desirable to enable indicating of potentially occurring short circuits. To better address one or more of these concerns, one or more of a heating element, a furnace, and a method having the features defined in one or more of the independent claims is provided.

[0014] According to a first aspect of the disclosure, a heating element is described that comprises a plurality of resistive members extending in a longitudinal direction of the heating element, an assemblage of electrically conductive components, and, seen along the longitudinal direction, a first end portion and an opposite second end portion. The resistive members are connected in series and are arranged to be connected to an electric power source via two connectors for electric power. The connectors for electric power are arranged at the first end portion. The assemblage of electrically conductive components comprises a contact element and a resistor. The resistor is connected in series with the contact element and with one of the two connectors for electric power. The contact element is arranged at the second end portion, for the contact element to be contacted by an overly elongated resistive member of the plurality of resistive members.

[0015] This configuration allows for indication of at least one overly elongated resistive member of the plurality of resistive members during use of the heating element. Thus, short circuits and potential damage to the heating element and / or to an overall structure such as a radiant tube and / or a furnace, in which the heating element is arranged, can be prevented.

[0016] More specifically, when at least one of the resistive members becomes overly elongated and comes into contact with the contact element during use of the heating element, an electric current will flow through the assemblage of electrically conductive components. The current can be measured and / or a voltage over the resistor can be measured. Accordingly, when a current and / or a voltage is measured, a control system and / or an operator can be made aware of the presence of an overly elongated resistive member and thereby it is possible to take action before any damage occurs e.g., by shutting off the heating element. Thus, a short circuit potentially caused by the overly elongated resistive member is avoided. At a suitable occasion, the overly elongated resistive member or the heating element including the overly elongated resistive member can be replaced.

[0017] According to a second aspect of the disclosure, a furnace is described that comprises at least one radiant tube and a heating element according to any one of aspects and / or examples discussed herein arranged in each of the at least one radiant tube.

[0018] Thus, the furnace is configured for indication of at least one overly elongated resistive member of the heating element. As discussed above, thus, short circuits and potential damage to the heating element and / or to the at least one radiant tube and / or the furnace, can be prevented.

[0019] According to a third aspect of the disclosure, a method of operating a furnace is described, which furnace comprises a number of radiant tubes and a heating element according to any one of aspects and / or examples discussed herein arranged in each of the radiant tubes of the number of radiant tubes. The method comprises steps of:

[0020] - supplying electric power from an electric power source to the heating elements arranged in the radiant tubes, and - indicating when an overly elongated resistive member comes into contact with the contact element of one of the heating elements.

[0021] In analogy with the discussions of the heating element and the furnace above, the method allows for indication of a faulty heating element, preventing damage to the heating elements, the radiant tubes, the furnace, and / or the product / goods in the furnace.

[0022] The disclosure presents a heating element with a unique assemblage of electrically conductive components that allows for indication of overly elongated resistive members, preventing short circuits and potential damage to the heating element. The heating element can be used in a furnace, which can be operated using a method that allows for indication of faulty heating elements, preventing damage to the furnace. As will be discussed, various optional features provide additional advantages such as easy and convenient monitoring, efficient identification of overly elongated resistive members, targeted disconnection of faulty heating elements, increased lifespan of the heating element, prevention of damage to the radiant tube, the furnace and / or a product being heated in the furnace, as well as efficient maintenance of the furnace.

[0023] The heating element may be provided for use in a furnace. In a furnace, the heating element is arranged in a radiant tube of the furnace. The heating element may be arranged with its longitudinal direction extending either vertically or horizontally in the furnace. Depending on whether the heating element is mounted vertically or horizontally, it may require a different support structure, but the principle of an overly elongated resistive member coming into contact with the contact element is applicable in both installation cases.

[0024] The radiant tube is a tube that forms a space for the heating element, which space is closed off from a heating chamber of the furnace. Thus, the heating element is not affected by the atmosphere within the heating chamber. Accordingly, the radiant tube also forms a protective tube for the heating element. Sometimes the radiant tube is called a protective tube.

[0025] The resistive members of the heating element are heated by an electric current. The heat from the resistive members radiates via the radiant tube and heats the heating chamber of the furnace.

[0026] As such, the heating element may form part of a cartridge. The cartridge is insertable into, and removable from, the radiant tube. Thus, the furnace and the heating element is set up for easy maintenance, such as easy removal, checking and replacing of heating elements and / or cartridges and / or parts thereof.

[0027] The plurality of resistive members may form all resistive members of the heating element. Alternatively, the heating element may comprise further resistive members, separate from the above-mentioned plurality of resistive members. Such further resistive members may be connected in series and arranged to be connected to an electric power source via two connectors for electric power, e.g. when the further resistive members are to be connected to one or more different phases of electric mains power than the plurality of resistive members.

[0028] The resistive members can be of various types or forms, such as wire members, strip members, or tubular members. The resistive members extend along the longitudinal direction and are electrically isolated from each other by isolator members. During use, the resistive members are subjected to fluctuations in temperature, as a consequence of regulating the temperature of the heating element so that it fulfils the temperature requirements for the specific use. For instance, an increase in temperature is required when a furnace is opened and / or when cold goods or product is introduced in a furnace.

[0029] The temperature fluctuations lead to the resistive members being elongated and contracted. Over time, the temperature fluctuations lead to permanent residual elongation of the resistive members. This is a natural characteristic of the resistive members. The heating element is devised to operate with elongated resistive members, be it from thermal elongation and / or due to residual elongation. The term “overly elongated resistive member” refers to a resistive member that due to its permanent residual elongation, and optionally in combination with thermal elongation related to current temperature conditions, has become too long for ensuring short circuit free operation of the heating element, e.g. in a radiant tube.

[0030] Optionally, one or more of the resistive members or the entire heating element may be replaced since the residual elongation may add up and the resistive members can cause short circuits within the heating element or with the radiant tube, in which it is placed.

[0031] Herein, an overly elongated resistive member makes contact with the contact element before it can cause a damaging short circuit. Accordingly, in the context of the present configuration of the heating element including inter alia the contact element, a short circuit between an overly elongated resistive member and the contact element is desirable since it enables prevention of a damaging short circuit within the heating element or the radiant tube. Accordingly, an electric current will flow through the assemblage of electrically conductive components. The electric current is driven by the electric power connection to the resistive members of the heating element. Namely, a closed electric circuit is formed by the overly elongated resistive member and the assemblage.

[0032] Herein, the assemblage of electrically conductive components is alternatively referred to as the assemblage.

[0033] Alternating current, AC, electric power or direct current, DC, electric power may be connected to the two connectors for electric power in order to supply electric power to the resistive members.

[0034] For instance, two connectors may be utilised when one phase AC or DC is supplied to the plurality of resistive members. A further connector may be utilised when three phase AC is supplied to the plurality of resistive members and further resistive members.

[0035] Electrical connections, such as the two connectors for electric power and a connector for the contact element, are arranged at the first end portion of the heating element, which may extend outside the radiant tube when the heating element is mounted therein. Thus, the electrical connectors are not directly subjected to the heat generated by the resistive members, at least no to any large extent.

[0036] The contact element is arranged at the second end portion of the heating element opposite to the first end portion, seen along the longitudinal direction.

[0037] In a new or little used heating element, the contact element is arranged at a distance from ends of the resistive members. Thus, space is provided within the heating element for the elongations and contractions of the resistive members during normal operation of the heating element. The distance between the contact element and the ends of the resistive members is chosen such that the resistive members can be subjected to permanent residual elongation before one or more of them becomes overly elongated and makes contact with the contact element. The precise length of the distance may depend on the initial length of the resistive members.

[0038] The heating element may be arranged in any kind of furnace comprising radiant tubes configured for receiving heating elements comprising a number of resistive members having a longitudinal extension. Mentioned purely as examples, the heating element may be arranged in a furnace forming part of continuous annealing and / or galvanizing lines e.g., in sheet metal production, in immersion heaters, in thermal oxidizers, and in incinerators e.g., for destruction of hydrocarbons.

[0039] Optionally in some examples, the assemblage of electrically conductive components comprises an electrical conductor connected to the contact element and the resistor. In this manner, the contact element may be electrically connected with other components of the assemblage at the first end portion of the heating element via the electrical conductor.

[0040] For instance, the electrical conductor may extend centrally within the heating element along the longitudinal direction from the contact element to a position of the heating element, which is positioned outside the radiant tube during use of the heating element. Here, the resistor may be connected to the electrical conductor.

[0041] Optionally in some examples, the electrical conductor extends in the longitudinal direction between the contact element and the first end portion. In this manner, the heating element may be configured for being positionable in a radiant tube with the assemblage accessible for measuring a current therethrough or a voltage thereof.

[0042] Optionally in some examples, the contact element is disc-shaped and extends perpendicularly to the longitudinal direction. This design allows for any overly elongated resistive member to make contact with the contact element.

[0043] Namely, by being disc-shaped and extending perpendicularly to the longitudinal direction, the contact element is arranged such that ends of the longitudinally extending resistive members will make contact with the contact member once sufficiently elongated.

[0044] Optionally in some examples, the contact element is arranged at a distance from ends of the plurality of resistive members in an unelongated state. This arrangement allows for predetermined residual permanent elongation of the resistive members before an overly elongated resistive member makes contact with the contact element.

[0045] Optionally in some examples, the heating element further comprises isolator members arranged at intervals along the longitudinal direction to electrically isolate the resistive members from each other and from a surrounding radiant tube. In this manner, is may be ensured that short circuits between the resistive members and the radiant tube along the longitudinal direction are avoided. Moreover, the resistive members may be supported in at least some of the isolator members. Thus, internal positions of the resistive members within the heating element may be maintained.

[0046] Optionally in some examples, the assemblage of electrically conductive components may produce a measurable current and / or voltage when the contact element is contacted by the overly elongated resistive member during use of the heating element. This configuration allows for measurement of current and / or voltage, enabling indication and / or early detection of overly elongated resistive members.

[0047] Optionally in some examples, the assemblage of electrically conductive components may enable identifying the overly elongated resistive member among other resistive members of the plurality of resistive members based on a measured current through the assemblage of electrically conductive components and / or a voltage over the resistor during use of the heating element. In this manner, efficient and accurate identification of which of the resistive members has become overly elongated may be enabled.

[0048] More specifically, since the resistive members of the plurality of resistive members are connected in series between the two connectors for electric power, during use of the heating element different electrical potentials will prevail at different positions along the resistive members. Herein, the different electrical potentials at the ends of the resistive members closest to the contact element are utilised for identifying an overly elongated resistive member that makes contact with the contact element.

[0049] Namely, the electric potential of the resistive member that has become overly elongated will determine the current flowing through the arrangement of electrically conductive components and / or the voltage over the resistor. By measuring the current and / or the voltage, it becomes known which of the resistive members has become overly elongated. A control arrangement and / or an operator may note the relevant resistive member.

[0050] The heating element comprising the overly elongated resistive member, suitably, is disconnected from electric power. At a suitable point in time, the heating element may be replaced, or the overly elongated resistive member of the heating element may be replaced.

[0051] The possibility to note the resistive member which has become overly elongated is beneficial. Namely, after the heating element has cooled down, the individual resistive members have contracted from their elongated heated lengths and it is not necessarily visible in this cooled down state, which of the resistive members has become overly elongated. Therefore, the above-described possibility of identifying the relevant resistive member which has become overly elongated, ensures that the right resistive member can be replaced.

[0052] Optionally in some examples, the heating element is configured to be arranged within a radiant tube. This arrangement allows for the heating element to be safely housed within the radiant tube, protecting the heating element from an atmosphere of a heating chamber of a relevant furnace.

[0053] For instance, the heating element may have an elongated extension along the longitudinal axis for it to fit inside a radiant tube.

[0054] Further features of, and advantages with, the invention will become apparent when studying the appended claims and the following detailed description.

[0055] BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Various aspects and / or examples of the invention, including its particular features and advantages, will be readily understood from the examples discussed in the following detailed description and the accompanying drawings, in which:

[0057] Fig. 1 schematically illustrates a heating element according to examples,

[0058] Fig. 2 schematically shows a plurality of resistive members and an assemblage of electrically conductive components of a heating element,

[0059] Fig. 3 schematically shows a section through an example furnace, and Fig. 4 illustrates a method of operating a furnace.

[0060] DETAILED DESCRIPTION

[0061] Aspects and / or examples of the invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and / or clarity.

[0062] Fig. 1 schematically illustrates a heating element 12 according to examples. Fig. 1 shows a sectional view through the heating element 12 and a radiant tube 20 along a longitudinal direction 5. The heating element 12 comprises a plurality of resistive members 16 extending along the longitudinal direction 5 of the heating element 12 and an assemblage 2 of electrically conductive components.

[0063] The heating element 12 and the assemblage 2 are shown arranged within a radiant tube 20, which is part of a larger furnace installation, see below with reference to Fig. 3. The heating element 12 and the plurality of resistive members 16 extend along the longitudinal direction 5. These resistive members 16 are electrically isolated from each other and the radiant tube 20 by isolator members 18. At least some of the resistive members 16 are connected in series and are arranged to be connected to a mains electric power source via two connectors 8 for electric power.

[0064] During use, the resistive members 16 are subjected to cyclical heating, leading to their elongation and contraction. The supply of an electric current to the resistive members 16 causes them to be heated. The resistive members 16 can be of various types, such as wire members, strip members, or tubular members.

[0065] The resistive members 16 may be made from one or more of the materials in the group including iron-chromium-aluminium alloys (FeCrAI-alloys), nickel-chromium alloys (NiCr- alloys), nickel-chromium-iron alloys (NiCrFe-alloys), molybdenum silicide compositions, and molybdenum disilicide compositions.

[0066] The assemblage 2 of electrically conductive components comprises a contact element 4 and a resistor 6. The resistor 6 is connected in series with the contact element 4 and with one of the two connectors 8 for electric power.

[0067] The assemblage 2 of electrically conductive components further comprises an electrical conductor 10. The contact element 4 is arranged within the radiant tube 20 and is electrically isolated from it. During use of the heating element 2, the contact element 4 is contacted by an overly elongated resistive member 16’ of the plurality of resistive members 16 before the overly elongated resistive member 16’ makes contact with the radiant tube 20.

[0068] Seen along the longitudinal direction 5, the heating element 12 comprises a first end portion 11 and an opposite second end portion 13. The two connectors 8 for electric power are arranged at the first end portion 11 and the contact element 4 is arranged at the second end portion 13. The electrical conductor 10 extends in the longitudinal direction 5 between the contact element 4 at the second end portion 13 and the first end portion 11 and forms part of an electrical connection between the contact element 4 and the resistor 6.

[0069] In the assemblage 2, the contact element 4, the electrical conductor 10, and the resistor 6 are connected in series to one of the two connectors 8 for electric power.

[0070] The electrical conductor 10 may extend through one or more pipe sections arranged between the isolator members 18.

[0071] The electrical conductor 10 may comprise a heat resistant electrically conductive material and as such may extend as one component through the isolator members 18 without extending through any pipe sections.

[0072] In Fig. 1 , the electrical conductor 10 extends centrally within the heating element 12 along the longitudinal direction 5. Alternatively, the electrical conductor 10 may extend along the longitudinal direction 5 at any other radial position within, or along, the heating element 12.

[0073] The contact element 4 is positioned at the second end portion 13 of the heating element 2 to be contacted by an overly elongated resistive member 16’ of the plurality of resistive members 16 at the second end portion 13 of the heating element 12.

[0074] In Fig. 1 , two resistive members 16 are shown having a length along the longitudinal direction 5 that corresponds to the length of unused resistive members 16. Further, two resistive members 16 are shown after frequent use and them having become permanently residually elongated.

[0075] In Fig. 1 , a contact between the contact element 4 and the overly elongated resistive member 16’ is indicated with a flash-shaped arrow. When making contact, a current flows through the overly elongated resistive member 16’, the contact member 4, the electrical conductor 10, and the resistor 6, to the one of the two connectors 8, to which the assemblage 2 is connected.

[0076] In some examples, the contact element 4 is disc-shaped and extends perpendicularly to the longitudinal direction 5. The contact element 4 is arranged at a closed end of the radiant tube 20. A ceramic disc 26 may arranged at the second end portion 13 of the heating element 12 and may at least partially support the assemblage 2 of electrically conductive components. For instance, the ceramic disc 26 may support the electrical conductor 10 at the second end portion 13.

[0077] The ceramic disc 26 may be designed to withstand high temperatures and by supporting the electrical conductor 10, the ceramic disc 26 may prevent the electrical conductor 10 from bending and making contact with any of the resistive members 16 when they become elongated.

[0078] The contact element 4 is positioned at the second end portion 13 such that an overly elongated resistive member 16’ will make contact with the contact element 4 before it makes contact with the radiant tube 20. This feature allows for the indication of overly elongated resistive members 16’, preventing short circuits and potential damage to the radiant tube 20. The contact element 4 may be made of a sheet metal disc and is arranged at the second end portion 13 of the heating element 12. Alternatively, the contract element 4 may be made e.g. of a metal wire mesh disc. These discs may be circular, square, rectangular, or any other shape that ensures that the contact element 4 extends perpendicularly to the longitudinal direction 5 in front of all relevant resistive members 16.

[0079] The ceramic disc 26 may extend in a direction perpendicularly to the longitudinal direction 5 and in parallel with the contact element 4.

[0080] The contact element 4 is arranged in an axial position between the ceramic disc 26 and the ends of the resistive members 16.

[0081] The contract element 4 may be sized and arranged such that each of the resistive members 16 will make contact with the contact element 4 in case one of them should become overly elongated. However, as discussed below, some of the resistive members 16 may be more prone to permanent elongation than others. Accordingly, the contact element 4 may be sized and arranged such that only each of the resistive members 16 that are most prone to permanent elongation will make contact with the contact element 4 in case one of them should become overly elongated.

[0082] The two connectors 8 are connected to the serially connected resistive members 16 to supply a current from mains electric power to the heating element 12. The two connectors 8 are arranged at the first end portion 11 of the heating element 12. In one configuration, the isolator members 18 are arranged at intervals along the longitudinal direction 5 to electrically isolate the resistive members 16 from each other and from the surrounding radiant tube 20. The isolator members 18 allow for the elongation and contraction of the resistive members 16 while preventing them from making contact with each other and / or the radiant tube 20 e.g., by preventing excessive sagging of the resistive members 16. The isolator members 18 are considerably shorter in length along the longitudinal direction 5 compared to the resistive members 16 to avoid impeding heat transfer from the resistive members 16 to the radiant tube 20.

[0083] The heating element 12 may be arranged to form part of a cartridge 14 supporting and containing the plurality of resistive members 16, the assemblage 2 including the contact element 4, as well as the two connectors 8 for electric power. For instance, an end member 30 of the cartridge 14 may form a structure to which the parts of the heating element 12 are directly or indirectly connected. Suitably, the end member 30 is electrically isolated from the two connectors 8, the electrical conductor 10, the radiant tube 20 and the furnace. The end member 30 is electrically isolated from the resistive members 16. The end member 30 may comprise more than one part.

[0084] The cartridge 14 is a structure that contains and supports the heating element 12. As such, the cartridge 14 provides for the heating element 12 to be conveniently handled e.g., during being arranged within the radiant tube 20 and / or during being removed therefrom.

[0085] Also, in applications where the heating element 12 is utilised without any radiant tube, the present way of indication of overly elongated resistive members 16’ is beneficial since e.g., short circuits between individual resistive members 16 can be avoided.

[0086] Mentioned purely as examples, the heating element 12 may have a length along the longitudinal direction 5 within a range of 0.5 - 5 m or within a range of 0.5 - 10 m. The heating element may have a diameter, perpendicularly to the longitudinal direction 5, within a range of 60 - 300 mm.

[0087] Fig. 2 schematically shows a plurality of resistive members 16 and an assemblage 2 of electrically conductive components of a heating element 12.

[0088] The heating element 12 may be a heating element 12 as discussed above with reference to Fig. 1. Accordingly, in the following reference is also made to Fig. 1. Again, the resistive members 16 and the electrical conductor 10 of the assemblage 2 extend in a longitudinal direction 5 of the heating element 12. Again, in the assemblage 2, the resistor 6 is connected in series with the contact element 4 via the electrical conductor 10. Also, the resistor 6 is connected with one of the two connectors 8 for electric power.

[0089] The resistive members 16 are shown in a folded-out state in parallel with the longitudinal direction 5. In these examples, pairs of resistive members 16 are formed by U-shaped members having bent portions 32 positioned at the second end portion 13 of the heating element 12. The free ends of the resistive members 16 are connected to each other at the first end portion 11 of the heating element 12 in a manner such that the resistive members 16 are connected in series.

[0090] When electric power is connected to the two connectors 8, a current flows through the resistive members 16, causing them to be heated. Moreover, the bent portions 32 of the resistive members 16 will have different electrical potentials. With substantially identical resistive members 16, the electrical potentials at the bend portions 32 will be one or more multiples of a fraction of the total voltage, Umain, between two of the connectors 8.

[0091] In the example of Fig. 2, there are 24 resistive members 16. This means that the fraction in this example is 1 / 24 and the difference between the electrical potential U at each of the 12 bent portions 32 is incrementally 2 / 24 of the voltage, Umain, between the connectors 8. In sequence, from the top of Fig. 2, the electrical potential U at each of the bent portions 32 will be 1 / 24 * Umain, 3 / 24 * Umain, 5 / 24 * Umain, 7 / 24 * Umain, 9 / 24 * Umain, 11 / 24 * Umain, 13 / 24 * Umain, 15 / 24 * Umain, 17 / 24 * Umain, 19 / 24 * Umain, 21 / 24 * Umain, and 23 / 24 * Umain.

[0092] For instance, should one or both of the 7thand 8thresistive members 16, counted from the top of Fig. 2, become overly elongated and their connecting bent portion 32’ make contact with the contact element 4 - then the electrical potential U1 = 7 / 24 * Umain will drive an electric current through the assemblage 2.

[0093] In a different example, should one or both of the 17thand 18thresistive members 16, counted from the top of Fig. 2, become overly elongated and their connecting bent portion 32” make contact with the contact element 4 - then the electrical potential U2 = 17 / 24 * Umain will drive an electric current through the assemblage 2. The electrical currents in these two examples will accordingly differ and by measuring the relevant current, as indicated by a coil 33, it can be established that the 7thand 8thresistive members 16 and the 17thand 18thresistive members 16, respectively, need being replaced. Alternatively, or additionally, a voltage UR over the resistor 6 may be measured to establish which of the resistive members 16 have become overly elongated. Namely, the different electrical currents will produce different voltages over the resistor 6.

[0094] To summarise, by measuring the current and / or the voltage in the assemblage 2, it becomes known which of the resistive members 16 has become overly elongated and made contact with the contact element 4. A control arrangement and / or an operator may note the current / voltage and draw the relevant conclusions to identify which of the resistive members 16 has become overly elongated. Accordingly, this measurement allows for the indication of overly elongated resistive members 16, preventing short circuits and potential damage to the heating element 12 and / or the radiant tube 20 wherein it is positioned during use.

[0095] By selecting a suitable resistive value of the resistor 6, a value range of the electric current through the assemblage 2 can be selected. For instance, the highest electric potential that is expected to drive a current through the assemblage 2 may determine the value of the resistor 6. For instance, if a maximum current of 20 mA is desired and the 17th / 18thresistive members 16 are expected to have the highest electric potential making contact with contact element 4, the value of the resistor 6 can be calculated, R = U / 1, i.e., in this example R = (17 / 24 * Umain) / 20 [kOhm],

[0096] Depending on their relative positions, the resistive members 16 may be subject of larger or smaller cyclical elongations from being cyclically heated. Larger cyclical elongations will lead to larger permanent residual elongations. This is indicated in Fig. 2 where the 12 centrally shown resistive members 16 have a larger residual elongation than the respective two sets of 6 outer resistive members 16.

[0097] In such heating elements 12, where the resistive members 16 are subjected to different residual elongations, it may be sufficient for the contact element 4 to be arranged only in front of the resistive members 16 subject to the longest residual elongation, as indicated in Fig. 2.

[0098] For instance, alternating current, AC, electric power with a voltage, Umain, may be connected to the connectors 8 for electric power. The electric power may be supplied from one phase of a three phase mains electric power supply. If provided with three connectors for electric power, the heating element may be designed for connection to three phase electric power. A reference ground point may be introduced, such as in a three-phase wye connection to three phase electric power. Taking account of the phase difference between the three phases, the above discussed measurement of current / voltage may be applied in such a heating element connected to three phase electric power in order to identify an overly elongated resistive member.

[0099] Fig. 3 schematically shows a section through an example furnace 22.

[0100] The furnace 22 comprises at least one radiant tube 20 and a heating element 12 arranged in the radiant tube 20. In the section of Fig. 3, three radiant tubes 20 and three heating elements 12 are shown. Depending on the size of the furnace 22 and the heat to be produced, the furnace 22 may comprise numerous radiant tubes 20 and a heating element 12 arranged in each radiant tube 20.

[0101] At least some of the heating elements 12 may be heating elements 12 as discussed above with reference to Figs. 1 and 2. Accordingly, in the following reference is also made to Figs.

[0102] 1 and 2 and the discussions related thereto.

[0103] Accordingly, the heating elements 12 may be provided in the form of cartridges 14, which can be replaced within the respective radiant tubes 20. As discussed above, each of the heating elements comprises an assemblage 2 of electrical components, which allows for the identification of overly elongated resistive members 16’ of the heating elements 12.

[0104] The shown three heating elements 12 are connected in a delta connection to a three-phase electric power supply with each heating element 12 being connected to one of the three phases via the connectors 8 for electric power.

[0105] Alternatively, three heating elements 12 are connected in a wye connection to a three-phase electric power supply with each heating element 12 being connected to one of the three phases via the connectors 8 for electric power.

[0106] In a furnace, two or more heating elements 12 may be connected in parallel over each phase of a three-phase electric power supply. Each of such heating elements 12 is arranged in one radiant tube 20. The furnace 22 further comprises a furnace housing 24. The furnace housing supports the at least one radiant tube 20 and delimits a heating chamber 28. In this manner, a controlled environment for the heating process is provided in the heating chamber 28. Moreover, the heating element 12 within the radiant tube 20 is not subjected to the atmosphere of the heating chamber 28.

[0107] The furnace 22 is designed to provide a controlled environment for the heating process, with the heating elements 12 generating heat that is transferred via the radiant tube 20 to the heating chamber 28. In the heating chamber 28 delimited by the furnace housing 24, the controlled environment for the heating process is provided.

[0108] The furnace housing 24 is designed to withstand the high temperatures generated by the heating element 12. Thus, in a known manner, the furnace housing 24 may be made of heat- resistant materials to withstand the high temperatures generated by the heating element 12.

[0109] The furnace 22 is configured to support multiple radiant tubes 20, such as by the furnace housing 24 supporting the radiant tubes 20. Each radiant tube 20 contains a heating element 12, allowing for efficient and uniform heating of the heating chamber 28.

[0110] The radiant tubes 20 may be made from one or more of the materials in the group including FeCrAI-alloy, NiCr-alloy, and NiCrFe-alloy, Molybdenum silicide composition, Molybdenum disilicide composition, and ceramic materials, such as SiC and SiAION.

[0111] The radiant tubes 20 are longer than the resistive members 16 in order to accommodate their elongation and contraction due to cyclical heating. A radiant tube 20 can be damaged if a short circuit occurs between the resistive members 16 and the tube 20. Therefore, the radiant tube 20 is designed to be electrically isolated from the resistive members 16 and the contact element 4 by the isolator members 18 and the ceramic disc 26, respectively.

[0112] Moreover, the present indication of overly elongated resistive members 16’ enables electrically disconnecting of heating elements 12 that contain such overly elongated resistive members 16’ before they cause short circuits with the respective radiant tubes 20.

[0113] Disconnection of heating elements 12 that contain overly elongated resistive members 16’ may be performed via automatically or manually controlled switches (not shown) arranged in the connection between the electric power supply and the heating elements 12. One switch may be provided for each heating element 12. Fig. 4 illustrates a method 100 of operating a furnace 22 comprising a number of radiant tubes 20 and a heating element 12 arranged in each of the radiant tubes 20 of the number of radiant tubes.

[0114] The furnace 22 may be a furnace 22 as discussed above with reference to Fig. 3 and the heating elements 12 may be heating elements 12 as discussed above with reference to Figs. 1 and 2. Accordingly, in the following reference is also made to Figs. 1 - 3.

[0115] The method 100 comprises steps of:

[0116] - Supplying 102 electric power from an electric power source to the heating elements 12 arranged in the radiant tubes 20. The heating elements 12 are connected to the mains power source via their respective two connectors 8 for electric power. The supply of electric power enables the heating elements 12 to generate heat, which is transferred to the radiant tubes 20 and then to the heating chamber 28 of the furnace 22.

[0117] - Indicating 104 when an overly elongated resistive member 16’ comes into contact with the contact element 4 of one of the heating elements 12. This step may be performed by detecting and / or indicating whether an electric current flows through the assemblage 2 and / or whether a voltage is formed over the resistor 6 of the assemblage 2 and / or by measuring a value of the current / voltage. As discussed above, the contact element 4 is contacted by an overly elongated resistive member 16’ before the member 16’ makes contact with the radiant tube 20. This allows for the indication of overly elongated resistive members 16’.

[0118] Thus, short circuits in the furnace 22 may be avoided and damage to the heating elements 12 and the radiant tubes 20 may be avoided.

[0119] Optionally in some examples, the method 100 comprises a step of disconnecting 106 the heating element 12 comprising the overly elongated resistive member 16’ from the electric power source. In this manner, damage to the relevant radiant tube 20 and the relevant heating element 12 may be prevented when an overly elongated resistive member 16’ is detected.

[0120] Optionally in some examples, the method 100 comprises a step of continue operating 108 other heating elements 12 of the number of heating elements 12 while the heating element 12 comprising the overly elongated resistive member 16’ is disconnected. In this manner, uninterrupted operation of the furnace 22 may continue even when e.g., one heating element 12 is faulty.

[0121] The other heating elements 12 remain connected to the mains power source via their respective two connectors 8 for electric power. The supply of electric power enables the remaining heating elements 12 to generate heat, which is transferred to the radiant tube 20 and then to the heating chamber 28 of the furnace 22, wherein the heating process can continue.

[0122] Thus, the furnace 22 may be operated without unvoluntary interruption caused by short circuits in between service opportunities, such as scheduled service opportunities. This means that the furnace 22 can be operated efficiently without unplanned costly stops of the heating process in the furnace 22.

[0123] Optionally in some examples, the step of indicating 104 when the overly elongated resistive member 16’ comes into contact with the contact element 4 of one of the heating elements 12 comprises a step of measuring 110 a current in the assemblage 2 of electrically conductive components and / or a voltage over the resistor 6. In this manner, basis may be provided for accurately identifying the overly elongated resistive member 16’.

[0124] Optionally in some examples, the method 100 comprises a step of identifying 112 the overly elongated resistive member 16’ among the plurality of resistive members 16 of the heating element 12 comprising the overly elongated resistive member 16’, based on a result of the step of measuring 110 a current in the assemblage 2 of electrically conductive components and / or a voltage over the resistor 6. In this manner, targeted replacement of the faulty resistive member 16’ is enabled.

[0125] As discussed above with reference to Fig. 2, the overly elongated resistive member 16’ of a heating element 12 can be identified based one a measured current in the assemblage 2 of electrically conductive components and / or a measured voltage over the resistor 6 of the assemblage 2. This measurement allows for the identification of which of the resistive members 16 has become overly elongated, enabling the disconnection of the faulty heating element 12 from the mains power source and eventually, replacement of the overly elongated resistive member 16’.

[0126] Optionally in some examples, the method 100 comprises a step of replacing 114 the overly elongated resistive member 16’ at a next service opportunity of the furnace 22 or replacing 116 the heating element 12 comprising the overly elongated resistive member 16’ at a next service opportunity of the furnace 22. In this manner, efficient maintenance of the furnace 22, ensuring its longevity and optimal performance, is provided for.

[0127] Thus, service opportunities of the furnace 22, such as scheduled service opportunities, can be used efficiently for refurbishing the furnace 22 and unplanned stops of the heating process in the furnace 22 can be avoided.

[0128] Moreover, this allows for the timely replacement of the faulty resistive member 16’, preventing damage to the radiant tube 20 and the heating element 12. The service opportunity provides a convenient time for the replacement of the overly elongated resistive member 16’, ensuring the continued operation of the furnace 22 and the longevity of the heating element 12.

[0129] Alternatively, this allows for timely replacing the entire heating element 12 comprising the overly elongated resistive member 16’. This may be done to expedite service of the furnace 22. Namely, replacing an entire heating element 12 may be quicker than repairing the heating element 12 comprising the overly elongated resistive member 16’. Thus, the heating process in the furnace 22 is not stopped over unnecessarily long time periods.

[0130] The overly elongated resistive member 16’ of the heating element 12 comprising the overly elongated resistive member 16’ can be replaced while the furnace 22 has been started again. Thus, this heating element 12 is refurbished and can be used to replace a different faulty heating element at a next service opportunity of the furnace 22.

[0131] The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein specify the presence of stated features, integers, actions, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, and / or groups thereof.

[0132] It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0133] It is to be understood that the foregoing is illustrative of various examples and that the invention is defined only by the appended claims. A person skilled in the art will realize that the examples may be modified, and that different features of the examples may be combined to create examples other than those described herein, without departing from the scope of the invention, as defined by the appended claims.

Claims

CLAIMS1. A heating element (12) comprising a plurality of resistive members (16) extending in a longitudinal direction (5) of the heating element (12), an assemblage (2) of electrically conductive components, and, seen along the longitudinal direction (5), a first end portion (11) and an opposite second end portion (13), wherein the resistive members (16) are connected in series and are arranged to be connected to an electric power source via two connectors (8) for electric power, wherein the two connectors (8) for electric power are arranged at the first end portion(11), wherein the assemblage (2) of electrically conductive components comprises a contact element (4) and a resistor (6), wherein the resistor (6) is connected in series with the contact element (4) and with one of the two connectors (8) for electric power, and wherein the contact element (4) is arranged at the second end portion (13), for the contact element (4) to be contacted by an overly elongated resistive member (16’) of the plurality of resistive members (16).

2. The heating element (12) according to claim 1 , wherein the assemblage (2) of electrically conductive components comprises an electrical conductor (10) connected to the contact element (4) and the resistor (6).

3. The heating element (12) according to claim 2, wherein the electrical conductor (10) extends in the longitudinal direction (5) between the contact element (4) and the first end portion (11).

4. The heating element (12) according to any one of claims 1 to 3, wherein the contact element (4) is disc-shaped and extends perpendicularly to the longitudinal direction (5).

5. The heating element (12) according to any one of claims 1 to 4, wherein the contact element (4) is arranged at a distance from ends of the plurality of resistive members (16) in an unelongated state.

6. The heating element (12) according to any one of claims 1 to 5, further comprising isolator members (18) arranged at intervals along the longitudinal direction (5) to electrically isolate the resistive members (16) from each other and from a surrounding radiant tube (20).

7. The heating element (12) according to any one of claims 1 to 6, wherein the assemblage (2) of electrically conductive components produces a measurable current and / or voltage when the contact element (4) is contacted by the overly elongated resistive member (16’) during use of the heating element (12).

8. The heating element (12) according to any one of claims 1 to 7, wherein the assemblage (2) of electrically conductive components enables identifying the overly elongated resistive member (16’) among other resistive members (16) of the plurality of resistive members (16) based on a measured current through the assemblage (2) of electrically conductive components and / or a voltage over the resistor (6) during use of the heating element (12).

9. The heating element (12) according to any one of claims 1 to 8, being configured to be arranged within a radiant tube (20).

10. A furnace (22) comprising at least one radiant tube (20) and a heating element (12) according to any one of claims 1 to 9 arranged in each of the at least one radiant tube (20).

11. The furnace (22) according to claim 10, further comprising a furnace housing (24), wherein the furnace housing (24) supports the at least one radiant tube (20) and delimits a heating chamber (28).

12. A method (100) of operating a furnace (22) comprising a number of radiant tubes (20) and a heating element (12) according to any one of claims 1 to 9 arranged in each of the radiant tubes (20) of the number of radiant tubes (20), the method (100) comprising steps of:- supplying (102) electric power from an electric power source to the heating elements (12) arranged in the radiant tubes (20), and- indicating (104) when an overly elongated resistive member (16’) comes into contact with the contact element (4) of one of the heating elements (12).

13. The method (100) according to claim 12, comprising a step of:- disconnecting (106) the heating element (12) comprising the overly elongated resistive member (16’) from the electric power source.

14. The method (100) according to claim 13, comprising a step of:- continue operating (108) other heating elements (12) of the number of heating elements (12) while the heating element (12) comprising the overly elongated resistive member (16’) is disconnected.

15. The method (100) according to any one of claims 12 to 14, wherein the step of indicating (104) when the overly elongated resistive member (16’) comes into contact with the contact element (4) of one of the heating elements (12) comprises a step of: - measuring (110) a current in the assemblage (2) of electrically conductive components and / or a voltage over the resistor (6).

16. The method (100) according to claim 15, comprising a step of:- identifying (112) the overly elongated resistive member (16’) among the plurality of resistive members (16) of the heating element (12) comprising the overly elongated resistive member (16’), based on a result of the step of measuring (110) a current in the assemblage (2) of electrically conductive components and / or a voltage over the resistor (6).

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