HEATING ELEMENT UNIT

Abstract

A heating element unit for an electric resistance heater comprises: a casing; a heating element within the casing; and an electrical insulator between the heating element and the casing. In some embodiments, the electrical insulator comprises first and second layers, the second layer having a greater dielectric strength than the first layer, and the second layer having: a dielectric strength of greater than about 1500 kV/m (greater than about 40 V/mil). In some embodiments, the electrical insulator comprises an electrically-insulating granular material and has a dielectric strength 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; and an electrical insulator between the heating element and the casing; wherein the electrical insulator comprises first and second layers, the second layer having a greater dielectric strength than the first layer, and the second layer having: a dielectric strength of greater than about 1500 kV/m (greater than about 40 V/mil).

2. A heating element unit according to claim 1, wherein the second layer has a dielectric strength 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).

3. A heating element unit according to claim 1, wherein the second layer comprises one or more of: a metal oxide; a nitride; a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral; a glass; a ceramic; a glass ceramic; a polymer.

4. A heating element unit according to claim 1 wherein the first layer has a dielectric strength of no greater than about 9800 kV/m (250 V/mil).

5. A heating element unit according to claim 1, wherein the first layer is an inner layer and the second layer is an outer layer, or the first layer is an outer layer and the second layer is an inner layer.

6. A heating element unit according to claim 1, wherein the second layer is a sleeve.

7. A heating element unit according to claim 6, wherein the sleeve comprises one or more of: a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral; a glass; a ceramic; a glass ceramic; a polymer.

8. A heating element unit according to claim 1, wherein the second layer is a coating.

9. A heating element unit according to claim 8, wherein the coating is a ceramic-based coating or a polymer-based coating.

10. A heating element unit according to claim 1, for use in an electric resistance heater having a maximum operating temperature of 1200° C.

11. A heating element unit for an electric resistance heater, the heating element unit comprising: a casing; a heating element within the casing; and an electrical insulator between the heating element and the casing; wherein the electrical insulator comprises an electrically-insulating granular material and has a dielectric strength greater than about 1500 kV/m (greater than about 40 V/mil).

12. A heating element unit according to claim 11, wherein the electrically-insulating granular material is a first electrically-insulating granular material and the electrical insulator further comprises a second electrically-insulating granular material different from the first electrically-insulating granular material.

13. A heating element unit according to claim 12, wherein the second electrically-insulating granular material is magnesium oxide (MgO).

14. A heating element unit according to claim 11, wherein the electrically-insulating granular material has a dielectric strength greater than about 1500 kV/m (40 V/mil).

15. A heating element unit according claim 11, wherein the electrically-insulating granular material has a dielectric strength 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).

16. A heating element unit according to claim 11, wherein the electrically-insulating granular material comprises one or more of: a metal oxide; a nitride; a silicate, aluminium silicate, aluminosilicate or phyllosilicate mineral; a glass; a ceramic; a glass ceramic; a polymer.

17. A heating element unit according to claim 11, for use in an electric resistance heater having a maximum operating temperature of 1200° C.

18. An electric resistance heater comprising a heating element unit, the heating element unit comprising: a casing; a heating element within the casing; and an electrical insulator between the heating element and the casing; wherein the electrical insulator comprises: first and second layers, the second layer having a greater dielectric strength than the first layer, and the second layer having a dielectric strength of greater than about 1500 kV/m (greater than about 40 V/mil); and/or an electrically-insulating granular material, the electrical insulator having a dielectric strength greater than about 1500 kV/m (greater than about 40 V/mil).

Description

FIGURES

[0199] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0200] FIGS. 1a-1c are cross sectional views of a first example heating element unit;

[0201] FIGS. 2a-2c are cross sectional views of a second example heating element unit;

[0202] FIGS. 3a-3d are cross sectional views of four additional examples of heating element units; and

[0203] FIG. 4 is a schematic diagram of an electrical resistance heater.

DETAILED DESCRIPTION OF THE INVENTION

[0204] FIGS. 1a-1c show a first example heating element unit 200 which comprises a heating element 202, a first insulator layer 204, a second insulator layer 208, a casing 206, and two terminal pins 210. The casing 206 is substantially cylindrical and is in the form of a tubular sheath which surrounds the heating element 202, and the first and second insulator layers 204, 208.

[0205] The heating element 202 is in the form of a coil which extends within the casing 206. A first coil portion 212 extends in a substantially straight line, and curves around 180° (not shown) before passing back on itself within the casing, forming a second coil portion 214 which is substantially straight and substantially parallel to the first coil portion 212. The first and second coil portions 212, 214 are spaced apart by a distance D.

[0206] Each of the first and second coil portions 212, 214 of the heating element 202 are connected to a terminal, or electrical supply, pin 210. The terminal pins 210 are located mostly within the casing 206, and each comprises a respective end portion 216 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 216 of the terminal pins.

[0207] Around each of the first and second coil portions 212, 214, there is provided a first layer 204 of a thermally conducting electrical insulator. In this example, the first layer 204 of insulator 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 first layer 204 of insulator has an electrical resistivity of about 2×10.sup.6 MΩm (about 8×10.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.

[0208] Within the casing 206, there is also provided a second layer of insulator 208 in the form of a sleeve which is formed to fit around the heating element 202 and the terminal pins 210. The sleeve 208 is a preformed sleeve. The sleeve 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 is formed of FEP/PTFE.

[0209] In other embodiments, the sleeve 208 is replaced by a coating 208, which is formed on the heating element 202 by, for example, spraying or painting. In other arrangements, the sleeve or coating 208 may be formed to fit or to be applied on the inside of the casing 206. The coating 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 is formed with Cerakote®.

[0210] The second layer of insulator 208 comprises a material having a dielectric strength greater than about 1500 kV/m (about 40 V/mil). In one example, the second layer comprises powdered boron nitride, having an electrical resistivity of about 2.5×10.sup.6 MΩm (about 9.85×10.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 second layer. In yet further embodiments, the second layer of insulator 208 comprises a material having a dielectric strength greater than the dielectric strength of the material from which the first layer of insulator 204 is formed.

[0211] By using a second layer 208 of insulator which comprises a material having a dielectric strength which is (a) greater than 1500 kV/m (about 40 V/mil) where the first layer 204 of insulator comprises MgO, or more generally (b) greater than the dielectric strength of the material from which the first layer of insulator 204 is formed, the dielectric strength of the insulator (i.e., the first and second layers combined) is increased.

[0212] The heating element 202 is formed from a material having high electrical resistivity, for example, no less than about 0.90 Ωmm.sup.2/m (540 Ω/cmf) and no greater than about 1.60 Ωmm.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 Ωmm.sup.2/m (654 Ω/cmf).

[0213] In use, a voltage is applied across the terminal pins 210, which causes the flow of current through the heating element 202. As current passes through the heating element, the heating element 202 heats up due to its high electrical resistance. The heat is passed to the casing (and the surroundings) via the insulator which has good thermally conducting properties. The insulator is electrically insulating and has a high dielectric strength (i.e., a high dielectric breakdown strength), which inhibits the flow of current from the heating element 202 to the casing 206. By using an insulator having an increased dielectric strength, higher voltages may be achieved without a concomitant increase in the amount of insulator required or the size of the heating element, such that the process of transferring heat to the surroundings is rendered more efficient.

[0214] Alternative arrangements of the heating element unit are shown in cross section in FIGS. 2a-2c, and in FIG. 3a-3d. In these arrangements, features corresponding to those previously described in relation to FIG. 1 are indicated with like reference numerals, increased in increments of 100. For instance, the heating element 300 of FIG. 2a-2c comprises a casing 306 and a heating element 302, and the heating elements 400, 500, 600 and 700 of FIGS. 3a-3d comprise respective casings 406, 506, 606, 706 and heating elements 402, 502, 602, 702.

[0215] The differences between the heating element unit 300 of FIGS. 2a-2c and the heating element unit 200 will now be described. In FIGS. 2a-2c, the dielectric strength of the insulator is improved by the combination of at least two different electrically insulating granular materials within the casing. The improvement in dielectric properties is provided by the use of a combination of different powdered materials intimately mixed with one another. In this example, the insulator comprises a mix of powdered magnesium oxide and powdered boron nitride. In this example, the magnesium oxide powder has an electrical resistivity of about 2×10.sup.6 MΩm (about 8×10.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 2.5×106 MΩm (about 9.85×107 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 insulator (i.e. the combination of the powders, has a dielectric strength greater than about 1500 kV/m (about 40 V/mil).

[0216] 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.

[0217] FIGS. 3a-3d show various different arrangements of heating element units in cross-section. In the heating element unit 400 of FIG. 3a, there is a single heating element 402 provided, which is surrounded by a second layer of insulator 408 which is in turn surrounded by a first layer of insulator 404 and encased within a sheath 406. As described in relation to FIG. 1 above, the second layer of insulator 408 has a higher dielectric strength than that of the first layer of insulator 404. It will be appreciated that the second layer of insulator 408 may be a sleeve or coating as described in relation to FIG. 1 above.

[0218] FIG. 3b shows a heating element unit 500 comprising two heating element portions 502, each of which is surrounded by a second layer of insulator 508. A first layer of insulator 504 surrounds both of the heating element portions 502 and second layers of insulator 508, and is encased within a sheath 506. As described in relation to FIG. 1 above, the second layer of insulator 508 has a higher dielectric strength than that of the first layer of insulator 504. It will be appreciated that the second layer of insulator 508 may be a sleeve or coating as described in relation to FIG. 1 above.

[0219] FIGS. 3c and 3d show heating element units 600 and 700 respectively. The heating element unit 600 is substantially corresponding to that of FIG. 3a, however the order of the first and second layers of insulator 604, 608 have been swapped so that the second layer of insulator 608 is the outermost layer, with the first layer 604 being the inner layer. Similarly, the heating element unit 700 is substantially corresponding to that of FIG. 3b, however the order of the first and second layers of insulator 704, 708 have been swapped so that the second layer of insulator 708 is the outermost layer, with the first layer 704 being the inner layer surrounding each of the two heating elements 704.

[0220] It will be appreciated by the skilled person that the layer thicknesses of the first and second layers of insulator may be varied, to adapt the dielectric properties as desired.

[0221] It will be understood that a heating element unit may include a combination of the sleeve or coating of FIGS. 1a-c or FIGS. 3a-3d and a combination of at least two different electrically insulating granular materials within the first and/or second insulator layers, as described in relation to FIGS. 2a-2c. Moreover, in embodiments such as shown in FIGS. 3b and 3d, both sets of heating element portions 502 and 702 could be surrounded by the same, single second layer of insulator 508 or 708.

[0222] 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, 300 as described above, connected to an electrical power supply 500.