HEATING ELEMENT UNIT

Abstract

A heating element unit for an electric resistance heater comprises: a casing; a heating element within the casing; an electrical supply pin in electrical contact with the heating element; an electrically insulating filler between the heating element and the casing; and an electrically insulating barrier provided between portions of the heating element, the electrical supply pin and/or the casing. The electrically insulating barrier has a greater dielectric strength than the electrically insulating filler, and the dielectric strength of the electrically insulating barrier is greater than about 1500 kV/m (greater than about 40 V/mil).

Claims

1. A heating element unit for an electric resistance heater, the heating element unit comprising: a casing; a heating element within the casing; an electrical supply pin in electrical contact with the heating element; an electrically insulating filler between the heating element and the casing; and an electrically insulating barrier provided between portions of the heating element, the electrical supply pin and/or the casing, the electrically insulating barrier having a greater dielectric strength than the electrically insulating filler, wherein the dielectric strength of the electrically insulating barrier is greater than about 1500 kV/m (greater than about 40 V/mil).

2. A heating element unit according to claim 1, wherein the electrically insulating barrier is provided between portions of the heating element and the casing.

3. A heating element unit according to claim 1, wherein the electrical supply pin is a first electrical supply pin provided at a first end of the heating element unit where the first electrical supply pin is in electrical contact with the heating element, the heating element unit comprises a second electrical supply pin provided at a second, opposing end of the heating element unit where the second electrical supply pin is in electrical contact with the heating element, the heating element extends between the first and second ends of the heating element unit, and the electrically insulating barrier is provided between portions of the heating element and the casing, between portions of the first electrical supply pin and the casing, and/or between portions of the second electrical supply pin and the casing.

4. A heating element unit according to claim 1, wherein the electrical supply pin is a first electrical supply pin provided at a first end of the heating element unit where the first electrical supply pin is in electrical contact with the heating element, the heating element unit comprises a second electrical supply pin also provided at said first end of the heating element unit where the second electrical supply pin is also in electrical contact with the heating element, the heating element comprises at least first and second sections spaced apart from one another, and the electrically insulating barrier is provided between portions of the at least first and second sections of the heating element, between portions of the first and second electrical supply pins, between portions of the first electrical supply pin and the casing, between portions of the second electrical supply pin and the casing, and/or between portions of the heating element and the casing.

5. A heating element unit according to claim 1, wherein the heating element is a first heating element, the heating element unit further comprises a second heating element, and the electrically insulating barrier is provided between portions of the first and second heating elements.

6. A heating element unit according to claim 1, wherein the electrically insulating barrier extends along at least a portion of a length of the heating element unit, substantially parallel to a longitudinal axis of the heating element unit.

7. A heating element unit according to claim 6, wherein the electrically insulating barrier has a substantially rectangular shape in cross-section in a plane perpendicular to the longitudinal axis.

8. A heating element unit according to claim 6, wherein the electrically insulating barrier comprises a plurality of electrically insulating barrier wall portions when viewed in cross-section in a plane perpendicular to the longitudinal axis.

9. A heating element unit according to claim 8, wherein the electrically insulating barrier wall portions are arranged radially around the longitudinal axis.

10. A heating element unit according to claim 1, wherein the electrically insulating barrier is an electrically insulating sleeve.

11. A heating element unit according to claim 1, wherein the electrically insulating barrier extends between first and second longitudinal ends, and the electrically insulating barrier tapers in a radial direction towards the first end.

12. A heating element unit according to claim 11, wherein the electrically insulating barrier is insertable into the casing in an insertion direction, wherein the first end of the electrically insulating barrier is the first part of the electrically insulating barrier to be inserted into the casing in the insertion direction.

13. A heating element unit according to claim 1, wherein the electrically insulating barrier extends only along a length of the electrical supply pin within the casing, along the length of the electrical supply pin within the casing and at least part of the length of the heating element, or along no greater than about 50%, for example, no greater than about 25%, or no greater than about 10 %, of the length of the heating element unit.

14. A heating element unit according to claim 1, wherein the electrically insulating filler is a granular material.

15. A heating element unit according to claim 1, wherein the electrically insulating filler comprises one or more of: a metal oxide such as an alkaline earth metal, for example, magnesium oxide (MgO) or beryllium oxide (BeO), a transition metal oxide, for example, titanium dioxide (TiO2), zirconium dioxide (ZrO2), or hafnium dioxide (HfO2), or a post transition metal oxide, for example, aluminium oxide (Al2O3); a nitride such as a Group 13 nitride, for example, boron nitride (BN) or aluminium nitride (AIN), or a Group 14 nitride, for example, silicon nitride (SiN); a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral such as mullite or mica; a glass such as soda-lime glass, borosilicate glass or aluminosilicate glass; a ceramic; a glass ceramic such as a machinable glass ceramic; a polymer such a fluoropolymer, for example, polytetrafluoroethylene (PTFE), or a silicone.

16. A heating element unit according to claim 1, wherein the electrically insulating barrier comprises one or more of: a metal oxide such as an alkaline earth metal oxide other than magnesium oxide (MgO), for example, beryllium oxide (BeO), a transition metal oxide, for example, titanium dioxide (TiO.sub.2), zirconium dioxide (ZrO.sub.2), or hafnium dioxide (HfO.sub.2), or a post transition metal oxide, for example, aluminium oxide (Al.sub.2O.sub.3); a nitride such as a Group 13 nitride, for example, boron nitride (BN) or aluminium nitride (AIN), or a Group 14 nitride, for example, silicon nitride (SiN); a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral such as mullite or mica; a glass such as soda-lime glass, borosilicate glass or aluminosilicate glass; a ceramic; a glass ceramic such as a machinable glass ceramic; a polymer such a fluoropolymer, for example, polytetrafluoroethylene (PTFE), or a silicone.

17. A heating element unit according to claim 1, wherein the electrically insulating barrier has a dielectric strength greater than about 1500 kV/m and no greater than about 39000 kV/m (greater than about 40 V/mil and no greater than about 1000 V/mil), for example, no less than about 7800 kV/m and no greater than about 39000 kV/m (no less than about 200 V/mil and no greater than about 1000 V/mil), or no less than about 15000 kV/m and no greater than about 39000 kV/m (no less than about 400 V/mil and no greater than about 1000 V/mil), or no less than about 23000 kV/m and no greater than about 39000 kV/m (no less than about 600 V/mil and no greater than about 1000 V/mil).

18. A heating element unit according to claim 1, wherein the electrically insulating barrier comprises one or more materials having a melting point no less than about 1000° C., for example, no less than about 2000° C., or no less than about 3000° C., for example wherein the electrically insulating barrier has a melting point no less than about 1000° C., or no less than about 2000° C., or no less than about 3000° C.

19. A method of manufacturing a heating element unit for an electric resistance heating, the method comprising: providing a heating element within a casing; providing an electrical supply pin in electrical contact with the heating element; providing an electrically insulating filler between the heating element and the casing; and providing an electrically insulating barrier within the casing between portions of the heating element, the electrical supply pin and/or the casing; the electrically insulating barrier having a greater dielectric strength than the electrically insulating filler, wherein the dielectric strength of the electrically insulating barrier is greater than about 1500 kV/m (greater than about 40 V/mil).

20. An electric resistance heater comprising a heating element unit, wherein the heating element unit comprises: a casing; a heating element within the casing; an electrical supply pin in electrical contact with the heating element; an electrically insulating filler between the heating element and the casing; and an electrically insulating barrier provided between portions of the heating element, the electrical supply pin and/or the casing, the electrically insulating barrier having a greater dielectric strength than the electrically insulating filler, wherein the dielectric strength of the electrically insulating barrier is greater than about 1500 kV/m (greater than about 40 V/mil).

Description

FIGURES

[0232] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0233] FIGS. 1a-1c are cross sectional views of a first example heating element unit;

[0234] FIGS. 2a-2c are cross sectional views of a second example heating element unit;

[0235] FIGS. 3a-3c are cross sectional views of third, fourth and fifth heating element units; and

[0236] FIG. 4 is a schematic diagram of an electrical resistance heater.

DETAILED DESCRIPTION OF THE INVENTION

[0237] FIGS. 1a-1c show a first example heating element unit 100 which comprises a heating element 102, an electrically insulating filler 104, an electrically insulating barrier 108, a casing 106, and two terminal pins 110. The casing 106 is substantially cylindrical and is in the form of a tubular sheath which surrounds the heating element 102, the electrically insulating filler 104 and the electrically insulating barrier 108.

[0238] The heating element 102 is in the form of a coil which extends within the casing 106. A first coil portion 112 extends in a substantially straight line along a longitudinal axis of the heating element unit, and curves around 180° (not shown) before passing back on itself within the casing, forming a second coil portion 114 which is substantially straight and substantially parallel to the first coil portion 112. The first and second coil portions 112, 114 are spaced apart by a distance D.

[0239] Each of the first and second coil portions 112, 114 of the heating element 102 are connected to a respective terminal, or electrical supply, pin 110. The terminal pins 110 are located mostly within the casing 106, and each comprises a respective end portion 116 which extends beyond the casing for connection to a power supply (not shown), so as to allow the application of a voltage to the end portions 116 of the terminal pins.

[0240] The electrically insulating barrier 108 is provided between adjacent portions of the pins 110 and of the first and second coil portions 112, 114. The electrically insulating barrier 108 has the form of a panel having a substantially rectangular cross section, when viewed along the longitudinal axis. In this example, the barrier 108 is located centrally between the terminal pins and centrally between the first and second coil portions 112, 114, is substantially cuboidal and extends longitudinally parallel to the first and second coil portions 112, 114. In this example, the barrier 108 extends along the entire length of the terminal pins within the casing 106, and also extends past the terminal pins 110 to extend between the first and second coil portions 112, 114. In other embodiments, the barrier 108 may extend a different length along the heating element unit 100.

[0241] The electrically insulating barrier 108 has a higher dielectric strength than the electrically insulating filler 104 (i.e. the electrically insulating barrier 108 is typically formed from higher dielectric strength materials than the electrically insulating filler 104). The electrically insulating barrier 108 therefore functions as a dielectric shield, effectively increasing the local dielectric strength of electrical insulation between components of the heating element unit 100 where the electrically insulating barrier 108 is provided. The presence of the electrically insulating barrier 108 therefore reduces the likelihood of local dielectric breakdown and thus increases the maximum volage at which the heating element unit 100 can be safely operated.

[0242] In the configuration shown in FIGS. 1a-c, in which the two terminal pins are located adjacent one another at the same end of the casing 106, it is desirable for the barrier 108 to extend at least between (e.g. the entire lengths of) the terminal pins 110 within the casing 106 as, in use of the heating element unit 100, the voltage is highest in this location and, thus, dielectric breakdown of any insulating material is most likely in this region.

[0243] In other examples, the barrier 108 may be formed of a plurality of barrier wall portions which are arranged within the casing, between portions of the heating element, the terminal pins and/or the casing. In some examples, the plurality of barrier wall portions may be discontinuous. In some examples, the plurality of barrier wall portions may for a X-shaped or a star-shaped cross section, when viewed along the longitudinal axis.

[0244] For example, cross sectional views of heating element units 600, 700, 800 having an alternative barrier arrangement are shown in FIGS. 3a-3c.

[0245] In FIG. 3a, the heating element unit 600 comprises a barrier 608 which is substantially X-shaped in cross-section, i.e. the barrier 608 has four barrier wall portions which meet in the centre of the heating element unit 600 and outwardly extend. Heating element portions 602 are disposed between each extending arm. Heating element portions 602 may be portions of the same, single heating element which extends back and forth along the length of the heating element unit 600. Alternatively, some of the heating element portions 602 may be portions of different heating elements, i.e. the heating element unit 600 may contain two or more heating elements which are not in electrical contact with one another. For example, the heating element unit 600 may contain four different heating elements which are electrically isolated from one another, in part, by the barrier 608. In another embodiment, the heating unit 600 contains two different heating elements which are electrically isolated from one another, each of the two heating elements extending in a substantially straight line parallel to the longitudinal axis of the heating element unit, curving around 180° and passing back on itself within the casing, thus forming two heating element portions which are spaced apart from one another at least partly by the barrier 608.

[0246] In FIG. 3b, the heating element unit 700 comprises four barrier wall portions 708a, 708b, 708c, 708d which are arranged to extend outwardly from the centre to form an X-shape, similarly to the arrangement of FIG. 3a. However, the barrier wall portions 708a, 708b, 708c, 708d of this heating element unit 700 do not meet at the centre of the unit 700 (i.e. there is insulating filler 704 disposed in the central region between each barrier wall portion 708a, 708b, 708c, 708d). Barrier wall portions 708a, 708b, 708c, 708d space apart heating element portions 702.

[0247] In FIG. 3c, the heating element unit 800 comprises six barrier wall portions 808a, 808b, 808c, 808d, 808e, 808f which are arranged to extend outwardly from the centre of the unit 800 in a star shape. The barrier wall portions 808a, 808b, 808c, 808d, 808e, 808f of this example do not meet in the centre, however in other arrangements, they may be arranged to be in contact with one, or integrally formed with, another.

[0248] It will be appreciated that barrier arrangements comprising any number of barrier wall portions are possible, the number and arrangement of the barrier wall portions being selected based on the number and arrangement of heating elements within the heating element unit. The barriers or barrier wall portions may also extend outwardly to the casing 106, 206, 606, 706, 806. For example, the barrier or barrier wall portions may extend the whole way from one side of the casing to the other side of the casing (e.g., across an entire diameter of the casing cross-section).

[0249] During manufacture of the heating element unit 100 (or equivalently, heating element units 600, 700 or 800), the barrier 108, or barrier wall portions may be inserted into the casing 106 before or after the filling of the casing 106 with the filler 104.

[0250] As mentioned above, the electrically insulating barrier 108 has a higher dielectric strength than the electrically insulating filler 104. In particular, the barrier 108 comprises a material having a dielectric strength greater than about 1500 kV/m (about 40 V/mil). In one example, the electrically insulating barrier 108 comprises powdered boron nitride, having an electrical resistivity of about 2.5x10.sup.6 MΩm (about 9.85x10.sup.7 MΩ.Math.in) at ambient temperature, a dielectric strength of about 37500 kV/m (about 950 V/mil), and a thermal conductivity of about 30 W/mK. It will be appreciated that, in other embodiments, other materials having a dielectric strength greater than about 1500 kV/m (about 40 V/mil) may be used as the barrier. For example, the barrier may comprise a metal oxide such as an alkaline earth metal oxide other than magnesium oxide (MgO), for example, beryllium oxide (BeO), a transition metal oxide, for example, titanium dioxide (TiO2), zirconium dioxide (ZrO.sub.2), or hafnium dioxide (HfO2), or a post transition metal oxide, for example, aluminium oxide (Al2O3); a nitride such as a Group 13 nitride, for example, boron nitride (BN) or aluminium nitride (AIN), or a Group 14 nitride, for example, silicon nitride (SiN); a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral such as mullite or mica; a glass such as soda-lime glass, borosilicate glass or aluminosilicate glass; a ceramic; a glass ceramic such as a machinable glass ceramic; a polymer such a fluoropolymer, for example, polytetrafluoroethylene (PTFE), or a silicone. The particular material used may be selected to target a desired dielectric strength, electrical resistivity, thermal conductivity, melting temperature or other physical or chemical property for a given heating element unit design.

[0251] The remainder of the space within the casing and around the heating element 102, electrical supply pins, 110 and the barrier 108 is filled with the electrically insulating filler 104, or base fill material. The electrically insulating filler 104 is thermally conducting. In this example, the filler 104 is a powder comprising about 92 wt. % magnesium oxide (the remainder of the powder being made up, for example, of other oxides such as silica, calcium oxide, alumina and iron oxide, and impurities). In this example, the filler 104 has an electrical resistivity of about 2x10.sup.6 MΩm (about 8x10.sup.7 MΩ.Math.in) at ambient temperature, a dielectric strength of about 1500 kV/m (about 40 V/mil), and a thermal conductivity of about 5.2 W/mK. In other examples, the powder comprises alternative thermally conducting, electrically insulating materials such as beryllium oxide, titanium dioxide, zirconium dioxide, hafnium dioxide, aluminium oxide, boron nitride, aluminium nitride, silicon nitride, mullite or mica. In yet further examples, a prefilled ceramic, such as a prefilled MgO ceramic, may be used.

[0252] Thus, the barrier 108 comprises a material having a dielectric strength greater than the dielectric strength of the material from which the filler 104 is formed. By using a barrier 108 which comprises a material having a dielectric strength which is (a) greater than 1500 kV/m (about 40 V/mil) where the filler 104 comprises MgO, or more generally (b) greater than the dielectric strength of the material from which the filler 104 is formed, the effective dielectric strength in the heating element unit is increased compared to a heating element unit without such a barrier 108.

[0253] The heating element 102 is formed from a material having high electrical resistivity, for example, no less than about 0.90 Ω mm.sup.2/m (540 Ω/cmf) and no greater than about 1.60 Ω mm.sup.2/m (960 Ω/cmf). In the example, the heating element 102 is formed from a nichrome alloy, such as Nikrothal® 80 available from Kanthal AB, Sweden and has an electrical resistivity of about 1.09 Ω mm.sup.2/m (654 Ω/cmf).

[0254] In use, a voltage is applied across the terminal pins 110, which causes the flow of current through the heating element 102. As current passes through the heating element, the heating element 102 heats up due to its high electrical resistance. The heat is passed to the casing (and the surroundings) via the barrier and the filler which have good thermally conducting properties. The barrier and the filler are electrically insulating and have high dielectric strengths (i.e., a high dielectric breakdown strength), which inhibits the flow of current from the heating element 102 to the casing 106. By using a barrier having an increased dielectric strength and a filler, higher voltages may be achieved without a concomitant increase in the amount of filler required or the size of the heating element, such that the process of transferring heat to the surroundings is rendered more efficient.

[0255] An alternative arrangement of the heating element unit is shown in FIGS. 2a-2c. In this arrangement, features corresponding to those previously described in relation to FIGS. 1 are indicated with like reference numerals, increased by 100. For instance, the heating element 200 of FIGS. 2a-2c comprises a casing 206 and a heating element 202.

[0256] The differences between the heating element unit 200 and the heating element unit 100 will now be described. In the heating element unit 200, the barrier is in the form of a sleeve 208 which is formed to fit around the heating element 202 and the terminal pins 210. The sleeve barrier 208 is a preformed sleeve. During manufacture of the heating element unit 200, the preformed sleeve 208 may be inserted into the casing 206 before or after the filling of the casing 206 with the filler 204.

[0257] The sleeve barrier 208 may be formed of, for example, a thermally conducting, electrically insulating polymer such as a silicone, hydrocarbon-based polymer or fluoropolymer (e.g., FEP/PTFE/PFA), a ceramic, glass, a glass ceramic, and/or a mineral such as mica, depending upon the desired application. In this example, the sleeve barrier is formed of FEP/PTFE.

[0258] In other embodiments, the sleeve barrier 208 is replaced by a coating barrier 208, which is formed on the heating element 202 by, for example, spraying or painting. In other arrangements, the sleeve barrier or coating barrier 208 may be formed to fit or to be applied on the inside of the casing 206. The coating barrier 208 may be ceramic-based (e.g. formed from ceramic-based materials such as Cerakote® coatings available from NIC Industries, Inc., USA, Duracote® coatings available from Duracote Corporation, USA, CeraGlide® coatings available from Saint Gobain Ceramics, France, or Aluma-Hyde® coatings available from Brownells, Inc., USA), and/or polymer-based (e.g. silicone based, hydrocarbon-based polymer based or fluoropolymer based) depending upon the desired application. In this example, the coating barrier 208 is formed of Cerakote.or CeraGlide®

[0259] The barrier 108, barrier wall portions and/or sleeve barrier 208 may comprise a first end and a second end. During manufacture, the first end may be inserted into the casing 106, 206 first, i.e. before the second end. The barrier 108, barrier wall portions and/or sleeve barrier 208 may be tapered in a radial direction from the second end to the first end such that the barrier 108, barrier wall portions and/or sleeve barrier 208 can be easily inserted into the casing 106, 206. This is particularly advantageous if the barrier 108, barrier wall portions and/ or sleeve barrier 208 are inserted into the casing 106, 206 after the casing 106, 206 has been filled with filler 104, 204.

[0260] It will be understood that the heating element unit may include a combination of the barriers of FIGS. 1a-c or FIGS. 3a-c and the sleeve barrier or coating barrier of FIGS. 2a-c.

[0261] In examples, in addition to use of an electrically insulating barrier, the dielectric strength of the filler 104 itself may be improved by using a combination of at least two different electrically insulating granular materials as the filler within the casing. For example, the filler may comprise a mix of powdered magnesium oxide and powdered boron nitride. In this example, the magnesium oxide powder has an electrical resistivity of about 2x10.sup.6 MΩm (about 8x10.sup.7 MΩ.Math.in) at ambient temperature, a dielectric strength of about 1500 kV/m (about 40 V/mil), and a thermal conductivity of about 5.2 W/m.Math.K. In this example, the boron nitrite powder has an electrical resistivity of about 2.5x106 MΩm (about 9.85x107 MΩ.Math.in) at ambient temperature, a dielectric strength of about 37500 kV/m (about 950 V/mil), and a thermal conductivity of about 30 W/m.Math.K. As a result, the filler (i.e. the combination of the powders, has a dielectric strength greater than about 1500 kV/m (about 40 V/mil).

[0262] In other examples, other combinations of powdered materials may be used to result in an insulator having a dielectric strength greater than about 1500 kV/m (about 40 V/mil). The proportions of the different powdered materials used may be varied to target desired levels of electrical resistivity, dielectric strength and/or thermal conductivity, as well as other physical or chemical properties. In some examples, ceramic binding materials may also be used.

[0263] It will be understood that a heating element unit may include a combination of the barriers of FIGS. 1a-c or FIGS. 3a-c and/or the sleeve barrier or coating barrier of FIGS. 2a-c and a combination of at least two different electrically insulating granular materials as a filler.

[0264] As shown in FIG. 4, there is provided an electrical resistance heater 400. The electrical resistance heater comprises any of the heating element units 100, 200, 600, 700, 800 as described above, connected to an electrical power supply 500.