Electrical heater

Abstract

An electrical heater having at least two elongate conductors 4,6 extending along the length of the heater, and a heating element 8 electrically connecting the conductors. The element having a silicone

Description

The present invention relates generally to an electrical heater comprising a heating element, in particular a heating cable comprising a heating element. Parallel resistance self-regulating heating cables are well known. Such cables normally comprise two conductors (known as buswires) extending longitudinally along the cable. Typically, the conductors are embedded within a resistive polymeric heating element, the element being extruded continuously along the length of the conductors. The cable thus has a parallel resistance form, with power being applied via the two conductors to the heating element connected in parallel across the two conductors. The heating element usually has a positive temperature coefficient of resistance. Thus as the temperature of the heating element increases, the resistance of the material electrically connected between the conductors increases, thereby reducing power output. Such heating cables, in which the power output varies according to temperature, are said to be self-regulating or self-limiting. High-temperature heating cables are known that comprise fluoropolymers in the heating element. Fluoropolymers have a number of advantages, such as temperature resistance and chemical inertness. However, under certain conditions (such as radiation) fluoropolymers can react to produce compounds such as hydrofluoric acid, which are highly undesirable. Furthermore, there is steadily increasing regulatory pressure to restrict or ban the production and use of fluoropolymers. It is an example aim or example embodiments of the present invention to at least partially overcome or avoid one or more disadvantages of the prior art, whether identified herein or elsewhere, or to at least provide a viable alternative. According to the present invention there are provided products as set forth in the claims that follow. Other features of the invention will be apparent from the dependent claims, and the description which follows. According to a first aspect of the present invention, there is provided an electrical heater comprising a heating element, wherein the heating element comprises a silicone material. Suitably, the heater is a self-regulating electrical heater. The heater is preferably suitable for use at high temperatures. The heater may suitable for use at a temperature of at least 100°C, preferably a temperature of at least 150°C, for example a temperature of at least 200°C. The heater suitably comprises one or more conductors. The heating element is suitably connected to the one or more conductors. Preferably, the heater comprises two or more conductors. The heating element is suitably connected between the two or more conductors. Preferably the two or more conductors are electrically connected by the heating element. The two or more conductors are suitably parallel to each other. The conductors may be formed of a material having a higher conductivity (i.e. lower resistivity) than the heating element. Suitably, the conductors are metallic. The conductors suitably comprise a metal. Any suitable metal may be used. The metal preferably comprises copper or aluminium. The conductors are suitably elongate. The conductors are suitably in the form of a wire or a sheet. The wire or sheet may be solid, stranded, braided or segmented. Stranded, braided or segmented wires or sheets have the advantage that they may be more flexible and can accommodate thermal expansion of the electric heater in use. In some embodiments, the conductors are stranded wires. The conductors may be stranded copper wires (also known as buswires). Suitably, the silicone material surrounds the conductors and penetrates the spaces between the conductors (in particular when the conductors are stranded or segmented wires or sheets). This ensures a strong mechanical and electrical contact between the heating element and the conductors. The two or more conductors may have a cross sectional area in a plane normal to a length of the electrical heater of at least 0.1 mm2, preferably at least 0.5 mm2, such as at least 1 mm2. The cross sectional area of the two or more conductors may be from 0.1 to 10 mm2, preferably from 0.5 to 5 mm2, such as from 1 to 2 mm2 (in particular when the conductors are wires). These cross sectional areas refer to the area of each conductor individually. Alternatively, the two or more conductors may each have a cross sectional area in a plane normal to a length of the electrical heater of at least 10 mm2. The cross sectional area of each of the two or more conductors is preferably at least 20 mm2. The cross sectional area of each of the two or more conductors is more preferably approximately 40 mm2. The heating element is suitably arranged to generate heat when an electrical current is passed through the conductors, for example between the two or more conductors. Suitably, the heater is elongated and is configurable, in use, such that the length of the heater extends along a length of an object to be heated. In some embodiments, the electrical heater comprises a stack wherein the two or more conductors and the heating element comprise layers of the stack. The stack is preferably planar. The heating element in the stack is suitably has a sheet-like form. The two or more conductors may be formed from metal foils. The term "metal foil" is intended to mean any sheet-like form of metal. However, it will be appreciated that while a foil is usually continuous, it may also be discontinuous. For example, a foil may comprise a sheet of metal containing a plurality of apertures. A metal foil may have a thickness of, for example, around 0.15 mm. A metal foil may, for example, have a thickness of up to around 0.5 mm. Each layer of the stack may have a substantially uniform thickness. The electrical heater may extend in a first direction to a significantly lesser extent than in a second direction, the first direction being perpendicular to the second direction, and the second direction being along a length of the electrical heater. Each layer of the stack may lie substantially parallel to a plane. The electrical heater may extend in a first direction parallel to the plane to a significantly lesser extent than in a second direction parallel to the plane, the first direction being perpendicular to the second direction. The two or more conductors in the stack may have a cross sectional area in a plane normal to a length of the electrical heater of at least 0.1 mm2, preferably at least 0.5 mm2, such as at least 1 mm2. The cross sectional area of the two or more conductors may be from 0.1 to 10 mm2, preferably from 0.5 to 5 mm2, such as from 1 to 2 mm2 (in particular when the conductors are wires). Alternatively, the two or more conductors in the stack may have a cross sectional area in a plane normal to a length of the electrical heater of at least 10 mm2. The cross sectional area of the two or more conductors is preferably at least 20 mm2. The cross sectional area of the two or more conductors is more preferably approximately 40 mm2. The larger the cross sectional area of the conductors in an electrical heater, the smaller the voltage drop along the conductors when a current is passed along them in use. The use of a conductor having an increased cross sectional area therefore provides an advantage over smaller cross sectional area conductors by enabling an electrical heater to extend fora greater length. The heating element may have a first thickness in a first region and a different thickness in a second region. The thickness of a heating element may vary and may therefore deliver a different heat output to different locations. For example, an electrical heater in the form of a ribbon may have a heating element thickness which varies along the length of the ribbon. A particular region may be required to deliver a higher heat output than another region along the ribbon, and be designed to have a different thickness. For example, a thinner region of ribbon will result in a higher current flowing through that region of the ribbon and a higher heat output being generated in that region. Conversely, a thicker region will result in a lower current flowing through that region of the ribbon and a lower heat output being generated in that region. In general the choice of materials used in and the dimensions of an electrical heater will determine the power output per unit area of a particular electrical heater. For example, a thicker heating element will produce a lower heat output for the same current passed through it, due to the larger resistance. However, it will require a larger voltage supplied to it to deliver the same current. A thinner heating element will allow a lower voltage to be used to power the electrical heater than would be required by a similar electrical heater with a thicker heating element and may be appropriate where a lower heat output is required. A further advantage of using a thin heating element is that a thin heating element will be more flexible and formable than a thick heating element. Additionally, a thin heating element will require less raw materials, and therefore be less expensive to manufacture than a thick heating element. The thickness of the heating element may vary between various applications. The thickness of the heating element may for example be greater than or equal to 0.1 mm. The thickness of the heating element may for example be less than or equal to 20 mm. In some embodiments layers of the stack may be curved. For example, an electrical heater may be considered to be substantially planar, each layer of the stack having a fixed separation. Each layer of the stack may have a substantially uniform thickness. However, such an electrical heater may be applied to a curved article (e.g. a pipe) such that the layers of the stack are each arranged to follow a curved surface of the article. Such an electrical heater may still be regarded as being substantially planar, in spite of the layers not lying substantially parallel to a plane. It will be appreciated that the generally flexible nature of electrical heaters according to embodiments of the invention allows such electrical heaters to conform to a large number of shapes, as required by a desired application. The electrical heater may perform a mechanical function. The electrical heater may comprise a fluid carrying conduit. The electrical heater may be arranged to receive a fluid carrying conduit. The electrical heater of the present invention comprises a heating element. The heating element suitably has a positive temperature coefficient of resistance (PTC). The electrical heater may be self-regulating by virtue of the positive temperature coefficient of resistance (PTC) of the heating element. The heating element comprises a silicone material. The heating element may consist of the silicone material. The silicone material comprises a silicone polymer. The silicone material preferably comprises a silicone rubber, for example a crosslinked silicone rubber. Compared to thermoplastic materials, the use of silicone materials may advantageously enable the extrusion of more complex shapes because silicone polymers typically hold their form when exiting an extruder. Silicone rubbers are stable at high temperatures. Therefore, the use of a silicone rubber in the heating element instead of a conventional thermoplastic material advantageously enables the heater to be used at higher temperatures and / or improves the safety of the heater at high temperatures. The silicone material suitably comprises an electrically conductive filler. Suitably, the electrically conductive filler is distributed in the silicone polymer. Since silicone polymers are typically electrically insulating, the silicone material suitably has an electrical conductivity between those of the silicone polymer and the electrically conductive filler. In some preferred embodiments, the silicone material comprises a silicone rubber and an electrically conductive filler. The electrically conductive filler suitably comprises conductive particles. The conductive particles may be selected from carbon black, graphite, graphene, carbon fibres, carbon nanotubes, metal powders, metal strand, metal coated fibre, or positive temperature coefficient of resistance (PTC) ceramics. Embodiments of the invention may use carbon black as the conductive filler. Carbon black is straightforward to use because it is widely available and has known properties when used as a conductive filler material in electrical heaters. However, alternative conductive fillers may be used instead of carbon black. If such alternative materials are used, then an adjustment to the proportions used may be made to achieve similarly performing electrical heaters to those achieved with carbon black. It will be appreciated that conductive fillers with a higher aspect ratio than spherical carbon black, such as, for example carbon fibres and carbon nanotubes, will lead to significantly different conductive pathways within the heating element. A conductive pathway within the heating element is likely to consist of alternately a portion within a conductive particle, and a portion between conductive particles where the conductive pathway bridges between adjacent conductive particles. It is these gaps which limit the conductivity of the heating element, and also which control the self-regulating behaviour of the heating element. Therefore, any change to the proportion of a conductive pathway which is made up of conductive particles rather than gaps between particles will have a significant impact on the conductivity and self-regulating behaviour of the heating element. In general, up to around 45% by weight of carbon black may be used in the heating element. If more than around 45% were to be used then there may be a risk that the electrical heater includes too many conductive pathways without significant amounts of polymer, such that heater does not provide useful self-regulation. In general, around 5% or more by weight of carbon black may be used in the heating element. If less than 5% were to be used then there may be a risk that the electrical heater does not include enough conductive pathways, such that the heater does not conduct sufficient electricity to allow it to be used as an electrical heater. Within the range 5% to 45% the amount of carbon black which is used may be selected depending upon the specific application for which the electrical heater will be used (e.g. taking into account the voltage that will be applied to the heater in use, which may vary from 12V to kVs). If a conductive material other than carbon black is used which has a similar aspect ratio to carbon black, then the same or similar proportions of conductive filler and silicone polymer may be used. The conductivity of the conductive filler may be taken into account, and this may modify slightly the amount of conductive filler used. For example, if metal powder were to be used instead of carbon black, the higher conductivity of metal powder may be such less is needed than is the case for carbon black. For example, as little as 2 % metal powder may be needed to make a useable electrical heater. Similarly, more than 35% of metal powder could provide conductivity which is so high that the heater does not provide useful self-regulation. The use of high aspect ratio particles of a filler material will allow a conductive pathway within a single particle to cover a significant distance, with fewer gaps required for each conductive pathway than would be required if particles with a lower aspect ratio were used. Thus, less filler material may be used. For example, if carbon fibres were to be used instead of carbon black, then the inclusion of 5-10 % by weight of the carbon fibres could provide a conductivity equivalent to the inclusion of 35 % by weight of carbon black. In general, as little as 2% by weight of carbon fibre may be used. If carbon nanotubes were included, then 2-3 % by weight of nanotubes might have the same effect on conductivity as 35 % by weight of carbon black. In general, the proportion of conductive filler with high aspect ratio used may be selected based upon the aspect ratio of that conductive filler and the specific application for which the electrical heater may be used (e.g. taking into account the voltage that will be applied to the heater). In this document the term "high aspect ratio" may be interpreted as meaning an aspect ratio which is significantly larger than the aspect ratio of carbon black. In general, alterations to the composition of silicone materials can be made to take advantage of the different properties of alternative materials. A combination of different conductive fillers could be used. In particular, the silicone material may comprise a mixture of conductive fillers of different aspect ratios. In such embodiments, the proportions of the conductive fillers may be selected to provide self-regulating behaviour to the heating element. For example, a blend or mixture of carbon black particles and carbon nanotubes could be comprised in or used as a conductive filler in a silicone material for use in electrical heaters. An adjustment to the proportions of each filler may be required to take into account the difference in aspect ratio of the particular fillers used. The silicone material may comprise further components. The silicone material may comprise a crosslinking agent. The crosslinking agent enables the creation of a 3D lattice network of the silicone polymer (especially silicone rubber), thereby constraining the movement and self-agglomeration of the electrically conductive filler (especially conductive particles such as carbon black). The silicone material may comprise an adhesive agent. The presence of an adhesive agent advantageously assists the adhesion of the silicone material to the conductors. This enhances the stability and robustness of the electrical heater, thereby improving performance and extending product life. For example, delamination is prevented during thermal cycling. Further, it allows non-conventional formats of heaters to be created. For example, when the conductors are metal foils, the presence of the adhesive agent ensures good contact between the conductors and the heating element and prevents delamination of the layers in the electrical heater. The silicone material may comprise a stabilising agent, such as a thermal stabiliser. Thermal stabilisers may be added in the range of approximately 1 to 15 wt%. When there is a risk of damage to the silicone materials due to them being subjected to harsh mechanical processing conditions (e.g. shear forces, friction, temperature rises) during processing, the addition of thermal stabilisers may act to reduce or prevent any such damage. The electrical heater may further comprise an outer insulative jacket. The outer insulative jacket is suitably formed from an insulative material such as mineral tape or insulative silicone rubber. The outer insulative jacket is suitably arranged to cover the heating element and conductor(s). The electrical heater may further comprise a continuous conductive covering. This may be included for additional mechanical protection and / or use as an earth wire. The continuous conductive covering may be a metallic braid, such as a tinned copper braid. The continuous conductive covering is suitably arranged to cover the outer insulative jacket. The electrical heater may further comprise a thermoplastic overjacket. The thermoplastic overjacket is suitably formed from an insulative material such as mineral tape or insulative silicone rubber. The thermoplastic overjacket is suitably arranged to cover the continuous conductive covering. The electrical heater may be manufactured at any suitable length. Suitably, the electrical heater is cuttable. The electrical heater may therefore be cut to any particular length required for installation. In some embodiments, the electrical heater comprises: at least two elongate conductors extending along the length of the heater, and a heating element electrically connecting the conductors, the heating element comprising a silicone material comprising an electrically conductive filler, wherein the silicone material further comprises an adhesive agent that enhances bonding between the heating element and the conductors to prevent delamination during thermal cycling. In some embodiments, the electrical heater comprises: at least two elongate conductors each having a cross sectional area of at least 10 mm2, and a heating element electrically connecting the conductors, the heating element comprising a silicone material comprising an electrically conductive filler and an adhesive agent that enhances bonding between the heating element and the conductors. In some embodiments, the electrical heater is in the form of a heating cable. The heating cable is suitably a self-regulating heating cable. The heating cable suitably comprises one or more conductors extending along the length of the cable. The conductors are suitably as described herein. Preferably, the heating cable comprises two or more conductors extending along the length of the cable. The conductors are suitably in electrical connection with each other via the heating element. The conductors may be in electrical connection with each other only via the heating element. In some embodiments, the two or more conductors comprise an inner conductor and an outer conductor. The heating element surrounds the inner conductor and is surrounded by the outer conductor. Preferably, the inner conductor, the heating element, and the outer conductor are concentrically arranged. In some embodiments, the heating cable has only two conductors. In some embodiments, the heating cable has only three conductors. The three conductors may be arranged with one conductor between the other two conductors. In some embodiments, the heating cable comprises four or more conductors. The four or more conductors suitably include one central conductor that runs along the central axis of the heating cable and three or more peripheral conductors that run along the length of the heating cable and are spaced from the central axis of the heating cable. The peripheral conductors may be equally spaced around the central conductor. Each peripheral conductor is preferably connected to the central conductor via the heating element. The heating element may be present or absent from the space between the peripheral conductors. In some embodiments, each peripheral conductor is equally spaced from the central conductor. In some embodiments, at least one of the peripheral conductors has a different spacing from the central conductor compared to another of the peripheral conductors. In some embodiments, each peripheral conductor has a different spacing from the central conductor. In one embodiment, the electrical heater is a self-regulating electrical heating cable comprising: a first power supply conductor extending along the length of the cable; a second power supply conductor extending along the length of the cable; a third power supply conductor extending along the length of the cable; the first and second power supply conductors being in electrical connection with each other via a first electrically conductive heating element body having a positive temperature coefficient of resistance, and the second and third power supply conductors being in electrical connection with each other via a second electrically conductive heating element body having a positive temperature coefficient of resistance, wherein the first and second electrically conductive heating element bodies comprise a silicone material and wherein, in use, the first, second and third power supply conductors are not physically connected to one another. First ends of each power supply conductor may be, in use, connected to a power supply, for example a three phase power supply. Second, remote ends of each power supply conductor are not physically connected together. In other words, these second ends of the power supply conductors (and, forthat matter, all parts of the conductors other than the respective first ends) are in electrical connection with each other only via the electrically conductive heating element. The first, second and third power supply conductors may extend alongside one another in a substantially planar arrangement. The second power supply conductor may be located between the first and third power supply conductors. The first and third power supply conductors may be equally spaced from the second power supply conductor. The second power supply conductor may be provided with a coating of material. The coating of material may have a higher electrical resistance than the electrical resistance of the electrically conductive heating element body or bodies. Such a higher resistance may help to achieve a balanced resistance between the conductors, allowing a load to also be balanced between the conductors. The first body may form part of a substantially hollow cylinder, and the second body may form part of substantially hollow cylinder. The self-regulating electrical heating cable may further comprise a third electrically conductive heating element body having a positive temperature coefficient of resistance wherein the third electrically conductive heating element body comprises a silicone material, the third body forming part of substantially hollow cylinder and being arranged to electrically connect the third and first power supply conductors. The first, second and third power supply conductors maybe equally spaced apart around the substantially hollow cylinder. The first, second and third power supply conductors maybe equally spaced from a central longitudinal axis of the substantially hollow cylinder. One or more of the power supply conductors maybe encased in material having a negative temperature coefficient of resistance. The material having a negative temperature coefficient of resistance maybe in the form of a sheath. One or more heating element bodies may comprise two components, each component having a different positive temperature of resistance characteristic. One or more heating element bodies may comprise a material having a negative temperature coefficient of resistance. One or more heating element bodies may together form a single heating element body. One of more of the power supply conductors may be embedded in a heating element body. In one embodiment, the electrical heater is a self-regulating electrical heating cable comprising: a first power supply conductor extending along the length of the cable; a second power supply conductor extending along the length of the cable; a third power supply conductor extending along the length of the cable; one or more of the first, second and third power supply conductors being encased in silicone material having a positive temperature coefficient of resistance, the first and second power supply conductors being in electrical connection with each other via a first electrically conductive heating element body having a negative temperature coefficient of resistance, and the second and third power supply conductors being in electrical connection with each other via a second electrically conductive heating element body having a negative temperature coefficient of resistance, and wherein, in use, the first, second and third power supply conductors are not physically connected to one another. First ends of each power supply conductor may be, in use, connected to a power supply, for example a three-phase power supply. Second, remote ends of each power supply conductor are not physically connected together. In other words, these second ends of the powersupply conductors (and, forthat matter, all parts of the conductors other than the respective first ends) are in electrical connection with each other only via the electrically conductive heating element. Brief Description of the Drawings Embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings, in which: Figure 1 is a partially cut away perspective view of a parallel resistance self-regulating heating cable in accordance with an embodiment of the present invention; Figure 2 is a perspective view of an electrical heater in accordance with an embodiment of the present invention; Figure 3 is an end-on view of an electrical heater in accordance with an alternative embodiment of the present invention; Figure 4 is an end-on view of an electrical heater in accordance with an alternative embodiment of the present invention; Figure 5 is a perspective view of an alternative heating cable in accordance with an embodiment of the present invention; Figure 6 is a perspective view of an alternative heating cable in accordance with an embodiment of the present invention; and Figure 7 is a perspective view of an alternative heating cable in accordance with an embodiment of the present invention. Detailed Description of the Drawings Figure 1 illustrates a parallel resistance, semi-conductive, self-regulating heating cable 2. The cable consists of a semi-conductive silicone material 8 extruded around the two parallel conductors 4,6. The silicone material serves as the heating element. A polymeric insulator jacket 10 is then extruded over the heating element 8. Typically, a conductive outer braid 12 (e. g. a tinned copper braid) is added for additional mechanical protection and / or use as an earth wire. Such a braid may be covered by a thermoplastic overjacket 14 for additional mechanical and corrosive protection. The polymeric insulator jacket 10 and the thermoplastic overjacket 14 are suitably formed from an insulative material such as mineral tape or insulative silicone rubber (which does not contain a conductive filler). Figure 2 illustrates schematically a self-regulating electrical heater 20 in accordance with an embodiment of the present invention. The electrical heater 20 may be a heating mat. The electrical heater 20 comprises a stack of elements. A heating element 21 extends throughout the centre of the electrical heater 20. The heating element 21 is sheet-like in form, having a substantially uniform thickness. The heating element 21 may extend in a first dimension x and a second dimension y to a significantly greater extent than the thickness, which is in the third dimension z. Alternatively, the heating element 21 may extend along a length in a first dimension x to a significantly greater extent than the width in a second dimension y and the thickness in a third dimension z. The width may be at least 5 mm, for example from 5 to 5000 mm. The thickness may be at least 0.1 mm, for example from 0.1 to 10 mm. Suitably, the width may be from 10 to 15 mm and the thickness may be from 5 to 8 mm. The heating element 21 has a positive temperature coefficient, such that resistance of the element 21 increases with temperature. The heating element 21 comprises a conductive filler distributed within a matrix of an insulative material. The insulative material is a silicone polymer. The conductive filler may be conductive particles. The conductive particles may be particles of carbon black. The carbon black is suitably present in the heating element in an amount of from 2 to 45 wt% based on the total weight of the insulative material and the carbon black. The conductive particles may be formed from a number of other suitable materials, such as graphite, graphene, carbon fibre, nanotubes, metal powders, metal strand, metal coated fibre, or a combination thereof. The heating element 21 is sandwiched between a first conductor 22 and a second conductor 23. The first and second conductors 22, 23 are formed of a metal foil. The metal foil may be made from any suitable metal, such as, for example, aluminium or copper. The first and second conductors 22, 23 are fixed to opposite sides of the heating element 21. The heating element 21 may further operate as a temperature regulation element. The electrical heater 20 may have low temperature self-regulating characteristics by virtue of the positive temperature coefficient of resistance (PTC) characteristic of the heating element 21. At normal operational temperatures (i.e. below the self-regulating temperature of the electrical heater 20) the heating element 21 will have a first electrical resistance. A voltage applied between the first and second conductors 22, 23 will cause current to flow through the heating element 21. The heating element 21 will deliver heat by converting electrical energy supplied as current through the conductors 22, 23 to thermal energy, through resistive heating. However, as the temperature approaches the self-regulating temperature, the resistance of the heating element 21 will rise to a second resistance which is greaterthan the first resistance. The increased resistance between the conductors 22, 23 causes the current flowing through the electrical heater 20 to be reduced, reducing the amount of thermal energy produced by the heating element 21. A failure mode of prior art parallel resistance self-regulating heating cables is loss of, or reduction in, electrical contact between the power conductors and the extruded resistive matrix forming the heating element. For example, differential expansion of the components and thermal cycling may lead to such failure or reduction in electrical contact overtime. Such a reduction in electrical contact may lead to electrical arcing within the cable, and a consequent loss in thermal output. The operational life of the electrical heater may thus be dependent upon the bond between the conductors and the heating element. Electrical contact between the heating element comprising the silicone material and the conductors may be improved by including an adhesive agent in the silicone material. Figure 3 shows a further embodiment of an electrical heater 30 in which the electrical heater 30 is arranged in a circular form, allowing the electrical heater to extend along and around a tube 31. In such an embodiment the electrical heater 30 may be used to heat the contents of the tube 31, so as to prevent it from freezing, suitably raise the temperature or maintain an elevated temperature. The electrical heater 30 comprises a first conductor 32 which extends around the tube 31, a heating element 33 which extends around the first conductor 32, and a second conductor 34 which extends around the heating element 33. The heating element 33 comprises a silicone material. The stack of elements comprising the electrical heater 30 forms a continuous sheath around the tube 31. Suitably, the first conductor 32, the heating element 33, and the second conductor 34 are concentrically arranged around the longitudinal axis of the electrical heater 30. In a further embodiment of the invention, as shown in Figure 4, an electrical heater 40 is a heating cable having a first conductor 41, a heating element 42 and a second conductor 43. The heating element 42 comprises a silicone material. The electrical heater 40 has a circular cross section, having an axis at the centre of the circular cross section. The electrical heater 40 is elongate, extending along the axis. Thus, the electrical heater 40 may be in the form of a cable. The first conductor 41 is a metal wire having a circular cross-section. The first conductor may be solid, stranded, bunched, segmented or otherwise subdivided. The first conductor 41 forms the centre of the electrical heater 40, extending along the length of the electrical heater 40. The heating element 42 surrounds the first conductor 41, and also extends along the length of the electrical heater 40. The second conductor 43 surrounds the heating element 42 (and therefore also first conductor 41), and also extends along the length of the electrical heater 40. Suitably, the first conductor 41, the heating element 42, and the second conductor 43 are concentrically arranged around the longitudinal axis of the electrical heater 40. The operation of the electrical heater 40 is similar to that of the electrical heaters described with reference to previously described embodiments of the invention, for example the electrical heater of Figure 2. In use, a voltage is applied between the first and second conductors 41,43, causing current to flow between the conductors 41, 43 and through the heating element 42, causing electrical energy to be dissipated as heat. The geometry of the various components which form the electrical heater 40 (i.e. the first conductor 41, heating element 42, and second conductor 43) define the output power and performance characteristics of the electrical heater. For example, the output power per unit length of electrical heater 40 will be set by the resistivity of the heating element 42 (which may be a function of temperature), the thickness of the heating element 42, and the width of the heating element 42 (i.e. if the heating element 42 was to be unrolled from around the first conductor 41 , it could be considered to have a 'width'). The thickness of the heating element 42 may be constant (i.e. the separation between the first conductor 41 and the second conductor 42 in a radial direction). However, the area of the heating element 42 which is in contact with the first conductor 41 (i.e. at the circumference of the first conductor 41) will be less than the area of the heating element 42 which is in contact with the second conductor 43 (i.e. at the inner circumference of the second conductor 43). The area is the product of the 'width' as described above, and the length along the electrical heater 40. Therefore, the heating element may be considered to have a single effective width which is between the circumference of the first conductor 41 and the inner circumference of the second conductor 43. Another characteristic of the electrical heater 40 which is influenced by geometry is the resistance of the conductors 41,43. While in earlier described embodiments of the invention the use of metal foils is discussed, it will be appreciated that thicker metal layers may alternatively be used. This may be particularly appropriate in embodiments which are elongate. In such embodiments, thicker metal layers may be used to reduce the resistance of the conductors. In some applications, especially where electrical heaters are required to cover large distances (e.g. oil pipelines, railway lines), voltage drop along the conductors of an electrical heater can severely limit the length of heater which can be deployed, necessitating electrical power supply connections at regular intervals. Reducing the resistance of the conductors reduces the voltage drop along their length allowing fewer electrical connections to be made. This may provide a significant advantage where providing electrical connections is expensive or inconvenient. For example, in the electrical heater 40, the first conductor 41 may have a cross-sectional area of around 40 mm2 (which corresponds to a diameter of ~7.14 mm). The heating element 42 has a thickness of 2 mm. The inner circumference of the second conductor 43 is ~11.14 mm. The second conductor 43 has a thickness of around 1.04 mm, and therefore has a cross-sectional area of 40 mm2 (i.e. the same as that of the first conductor41). By providing large cross-section conductors, it is possible to provide an electrical heater which can be deployed in applications which require a long heater length. Large cross-section conductors can be matched (i.e. both the first and second conductors having similar large cross-sections) so as to ensure that a similar voltage drop is experienced by both conductors. For example, when compared to a conventional heating cable having bus-wires each having a cross-sectional area of around 1.25 mm2, a reduction in voltage drop along the length of an electrical heater of approximately an order of magnitude can be brought about by using conductors each having a cross sectional area of 40 mm2. An electrical heater may be designed such that the voltage drop along the length of the electrical heater is less than a predetermined amount. For example, a voltage drop of 10% of the supply voltage may be permitted along the length of a conductor within an electrical heater (i.e. a 10% voltage drop along each of the two conductors, and the remaining 80% of the voltage dropped across the heating element). For example, a conventional heating cable having copper conductors each having a cross-sectional area of 1.25 mm2, and an output power of 30 W / m when supplied with a voltage of 230 V, may extend to around 100 m in length before the voltage across the heating element at the end of the heater distant from the supply is reduced to around 80% of the supply voltage. Conversely, an electrical heater according to an embodiment of the invention having aluminium conductors each having a cross- sectional area of 40 mm2, and an output power of 30 W / m when supplied with a voltage of 230 V, may extend to approximately 500 m or more in length before the voltage across the heating element at the end of the heater distant from the supply is reduced to around 80% of the supply voltage. Increasing the cross-sectional area of the conductors may thus allow the length of an electrical heater to be extended significantly. Conductors having a cross sectional area of at least 10 mm2 may be considered large crosssection conductors for the purpose of the invention. Such large cross-section conductors may provide a useful reduction in voltage drop when compared to conventional heating cables having a cross-sectional area of, for example, around 1.25 mm2. The upper limit in useful conductor cross-sectional area may be determined by factors such as material cost, cable weight, or cable flexibility. Conductors having a cross-sectional area of upto around 100 mm2 may, for example, provide a useful reduction in voltage drop when compared to conventional heating cables having a cross-sectional area of, for example, around 1.25 mm2, while still enabling a cost-effective and useable electrical heater. In some applications conductors with larger cross-sectional areas may be used. It is appreciated that increasing the cross-sectional area of a conductor within a prior art heating cable would have the effect of reducing the resistance of that conductor, and therefore reducing any voltage drop along the length of that conductor. However, if large cross-section conductors were used in conventional prior art heating cables, this would result in the heating element having to be increased in cross-sectional area so that it would entirely surround the enlarged conductors, so as to ensure contact was maintained between the conductors and the heating element. If the heating element was not enlarged so as to entirely surround the conductors, the poor bond between the conductors and the heating element which is present in known heating cables would cause the conductors to separate from the heating element, losing electrical contact and causing poor electrical performance and reliability of the heating cable. It will therefore be appreciated that the enhanced bonding brought about by the use of an adhesive agent as a component part of the silicone material, as described above, allows the use of large cross-section conductors in electrical heaters with a wide range of geometries. The use of the arrangement of Figure 4 will also ensure that the conductors cannot separate from the heating element, each layer in the device being entirely surrounded by the next layer. Further, the use of the arrangement shown in Figure 4 allows a smaller overall cross-section to be achieved in heating cables having a given conductor cross-section when compared to conventional heating cables. In addition to the arrangement shown in Figure 4, it will be appreciated that large cross-section conductors can also be used in other embodiments of the invention described herein. The thickness of each conductor can be selected fora particular electrical heater taking into account the intended power output of that electrical heater and the desired length of that conductor, so as to mitigate the effect of voltage drop along the length of the conductor. For example, thick metal foils could be used in combination with an electrical heater to provide an electrical ribbon heater which extended for tens or hundreds of metres without suffering from a significant voltage drop. The use of an electrical heater having conductors and a heating element in a circular arrangement, as shown in Figure 4, allows the electrical heater to be bent in any direction. For example, an electrical heater as shown in Figure 4 could be wound around a fluid carrying conduit. In such an application, it would be possible to bend the electrical heater around corners in the conduit without having to arrange the electrical heater in a particular plane in which it was able to bend. This can be understood in comparison with the substantially planar electrical heater shown in Figure 2, which, while able to bend easily in the y-z and x-z planes (depending on the thickness in the z-direction), may be difficult to bend in the x-y plane, because of its planar structure. Figure 5 depicts a heating cable 50 in accordance with an embodiment of the present invention. The heating cable 50 is provided with three electrical conductors 51a, 51b, 51c (e.g. copper wires, or the like) running along the length of the cable. Each of the conductors 51a, 51b, 51c are equally spaced apart from one another, and lie in substantially the same plane. The conductors 51a, 51b, 51c are embedded in an electrically conductive body 52 of a silicone material having a positive temperature coefficient of resistance (hereinafter referred to as 'the PTC body 52'). The conductors 51a, 51b, 51c may be embedded in the PTC body 52 in any appropriate manner. For example, the PTC body 52 may be extruded over and around the conductors 51a, 51b, 51c. Alternatively, the PTC body 52 may be formed (e.g. moulded) around the conductors 51a, 51b, 51c. The conductors 51 a, 51 b, 51 c of Figure 5 can be formed from any suitable material that conducts electricity. For example, the conductors can be formed from copper, aluminium, steel, etc. The electrically conductive PTC body 52 is typically formed from carbon particles embedded in a silicone rubber. The PTC body 52 may be formed from any suitable silicone material which has a positive temperature coefficient of resistance. For example, the PTC body 52 may typically be formed from a mixture of a conductive material and a silicone rubber. The conductive material may be a metal powder, carbon black, carbon fibres, carbon nanotubes or one or more PTC ceramics. The PTC body 52 is surrounded by an insulating sheath 53. The insulating sheath 53 electrically isolates the PTC body 52 from a continuous conductive covering, such as a metallic braid 54. The metallic braid 54 gives the heating cable mechanical stability and strength. The metallic braid 54 is encased in an insulating jacket 55. The insulating jacket 55 electrically insulates the heating cable and reduces or eliminates the effects of wear and tear and the ingress of water, dirt etc. In use, each of the conductors 51a, 51b, 51c will be attached to an output of a three-phase power supply (not shown). The heating cable can be cut to length. Figure 6 depicts a cross-section of a heating cable 60 in accordance with an embodiment of the present invention. The heating cable is provided with three peripheral electrical conductors 61a, 61b, 61c arranged around a central electrical conductor 62. The peripheral conductors 61a, 61b, 61c are equally spaced apart from one another and from the central conductor 62. The conductors 61a, 61b, 61c, 62 run along the length of the cable 60 and are embedded in an electrically conductive body 63 of a silicone material having a positive temperature coefficient of resistance (hereinafter referred to as 'the PTC body 63'). In the embodiment shown, the cross-section of the PTC body 63 forms a three-point star with one of the peripheral conductors 61a, 61b, 61c at each point. Such a cross-section allows for a lighter heating cable and less silicone material to be used. Alternatively, the cross-section of the PTC body 63 may take other shapes, such a circle, which may provide a more rigid and robust heating cable. The conductors 61a, 61b, 61c, 62 may be embedded in the PTC body 63 in any appropriate manner. Suitably, the PTC body 63 may be extruded over and around the conductors 61a, 61b, 61c, 62. The use of silicone material enables the extrusion of more complex shapes because silicone polymers typically hold their form when exiting an extruder. Alternatively, the PTC body 63 may be formed (e.g. moulded) around the conductors 61a, 61b, 61c, 62. The PTC body 63 may be formed from any suitable silicone material which has a positive temperature coefficient of resistance. For example, the PTC body 63 may typically be formed from a mixture of a conductive material and a silicone polymer. The conductive material may be a metal powder, carbon black, carbon fibres, carbon nanotubes or one or more PTC ceramics. The PTC body 63 is suitably surrounded by further layers of material (not shown), such as an insulating sheath, a metallic braid, and an insulating jacket. In use, each of the conductors 61a, 61b, 61c, 62 will be attached to an output of a power supply (not shown). The central conductor 62 will be neutral, while the peripheral conductors 61a, 61b, 61c will be live, or vice versa. The heating cable can be cut to length Figure 7 depicts a cross-section of a heating cable 70 in accordance with an embodiment of the present invention. The heating cable is provided with peripheral electrical conductors 71 arranged around a central electrical conductor 72. The peripheral conductors 71 are equally spaced around the central conductor 72. However, each of the peripheral conductors 71 has a different spacing from the central conductor 72. The conductors 71, 72 run along the length of the cable 70 and are embedded in an electrically conductive body 73 of a silicone material having a positive temperature coefficient of resistance (hereinafter referred to as 'the PTC body 73'). The conductors 71,72 may be embedded in the PTC body 73 in any appropriate manner. The PTC body 73 may be extruded over and around the conductors 71, 72. Alternatively, the PTC body 73 may be formed (e.g. moulded) around the conductors 71,72. The PTC body 73 may be formed from any suitable silicone material which has a positive temperature coefficient of resistance. For example, the PTC body 73 may typically be formed from a mixture of a conductive material and a silicone polymer. The conductive material may be a metal powder, carbon black, carbon fibres, carbon nanotubes or one or more PTC ceramics. The PTC body 73 is suitably surrounded by further layers of material (not shown), such as an insulating sheath, a metallic braid or other continuous conductive covering, and an insulating jacket. In use, the central conductor 72 and one of the peripheral conductors 71 will be attached to an output of a power supply (not shown). The central conductor 72 will be live, while the peripheral conductor 71 will be neutral, or vice versa. The heating cable can be cut to length. Due to the different spacings between the peripheral conductors 71 and the central conductor 72, the power output of the cable 70 can be controlled by selecting which peripheral conductor 71 to connect to the power supply. For example, the spacings between the peripheral conductors 71 and the central conductor 72 may include spacings of 2, 2.5, 3, 3.5, and 4 mm etc., and connecting the peripheral conductor 71 which is 2 mm away from the central conductor 72 will result in a higher power output than connecting the peripheral conductor 71 which is 4 mm away from the central conductor 72. In this way, a single heating cable can be produced for a variety of possible power outputs, and the power output can be selected at the installation of the heating cable. Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. 5

Claims

1. An electrical heater comprising:at least two elongate conductors extending along the length of the heater, anda heating element electrically connecting the conductors, the heating element comprising a silicone material comprising an electrically conductive filler, wherein the silicone material further comprises an adhesive agent that enhances bonding between the heating element and the conductors to prevent delamination during thermal cycling.

2. An electrical heater comprising:at least two elongate conductors each having a cross sectional area of at least 10 mm2, anda heating element electrically connecting the conductors, the heating element comprising a silicone material comprising an electrically conductive filler and an adhesive agent that enhances bonding between the heating element and the conductors.

3. An electrical heater according to claim 1 or 2, wherein the silicone material comprises a mixture of conductive fillers of different aspect ratios, the proportions of the fillers being selected to provide self-regulating behaviour of the heating element.

4. An electrical heater according to claim 1, wherein each conductor has a cross-sectional area between 0.5 and 5 mm2.

5. An electrical heater according to any preceding claim, wherein the silicone material comprises a silicone rubber, preferably a crosslinked silicone rubber.

6. An electrical heater according to any preceding claim, wherein the electrically conductive filler comprises one or more of carbon black, graphite, graphene, carbon fibres, carbon nanotubes, metal powder, metal strand, metal-coated fibre, or a positive temperature coefficient (PTC) ceramic.

7. An electrical heater according to claim 6, wherein the electrically conductive filler comprises a mixture of carbon black and carbon nanotubes.

8. An electrical heater according to any preceding claim, wherein the silicone material further comprises a crosslinking agent.

9. An electrical heater according to any preceding claim, wherein the silicone material further comprises a stabilising agent selected from thermal stabilisers in an amount of 1-15 wt%.

10. An electrical heater according to any preceding claim, wherein the heater is suitable for continuous operation at temperatures of at least 150 °C, preferably at least 200 °C.

11. An electrical heater according to any preceding claim, wherein the heater is in the form of a cable having an inner conductor, a surrounding heating element, and an outer conductor concentrically arranged.

12. An electrical heater according to any of claims 1 to 10, wherein the heater is in the form of a planar stack comprising a sheet-like heating element between two metal foil conductors.

13. An electrical heater according to any preceding claim, wherein the heater further comprises an outer insulative jacket and optionally a metallic braid or other continuous conductive covering.

14. An electrical heater according to any preceding claim, wherein the heater is self-regulating by virtue of a positive temperature coefficient of resistance of the heating element.

15. An electrical heater according to any preceding claim, wherein the heater is cuttable to length for installation.A

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