INDUCTION HEATING METHOD AND APPARATUS

Abstract

Aspects of the present invention relate to an induction heating system for heating a component. The induction heating system includes a power module for outputting an alternating current, the power module being operable to output the alternating current at a variable supply frequency. A controller is provided to identify at least one resonant frequency of the alternating current supplied to the at least one induction element. The controller is configured to determine an operating temperature of the component in dependence on the at least one identified resonant frequency. Aspects of the present also relate to an induction heating controller; a component comprising an induction heating element; and to a method of heating a component by inductive heating.

Claims

1. An induction heating system for heating a component, the induction heating system comprising: a plurality of induction elements for positioning proximal to an exterior of the component; at least one power module for outputting an alternating current to the at least one induction element; and a controller configured to identify a plurality of resonant frequencies of the alternating current supplied to the plurality of induction elements; wherein the controller is configured to determine an operating temperature of the component in dependence on the identified resonant frequencies.

2. The induction heating system as claimed in claim 1, the controller being configured to: control the power module to change the supply frequency of the alternating current; and measure a current across the plurality of induction elements as a function of the supply frequency; wherein identifying the plurality of resonant frequencies comprises identifying the supply frequency of the alternating current corresponding to a plurality of peaks in the measured current.

3. The induction heating system as claimed in claim 1, wherein the controller is configured to: control the power module to change the supply frequency of the alternating current; and determine a phase difference between the voltage applied to the at least one induction element and the current in the induction element; wherein identifying the plurality of resonant frequencies comprises identifying the supply frequency which results in the phase difference being at least substantially zero.

4. An induction heating system for heating a component, the induction heating system comprising: a plurality of induction elements for positioning proximal to an exterior of the component; at least one power module for outputting an alternating current to the induction elements; and a controller configured to identify a plurality of resonant frequencies of the alternating current supplied to the at least one induction element.

5. The induction heating system as claimed in claim 4, wherein the controller is configured to determine an operating temperature of the component in dependence on the identified resonant frequencies.

6. The induction heating system as claimed in claim 1, wherein the controller is configured to determine how many induction elements are connected in parallel to the power module in dependence on the number of identified resonant frequencies.

7. The induction heating system as claimed in claim 1, wherein the controller is configured to control the power module to reduce a frequency offset between the supply frequency and the identified resonant frequency to increase or maintain heating of the component; and/or to increase a frequency offset between the supply frequency and the identified resonant frequency to reduce heating of the component.

8. The induction heating system as claimed in claim 1, wherein the induction elements are connected to the power module in parallel.

9. The induction heating system as claimed in claim 8, wherein each induction element has a capacitor associated therewith, each capacitor having a different capacitance.

10. An induction heating system for heating a component, the induction heating system comprising: a power module for outputting an alternating current to a first induction element and a second induction element, the first induction element and the second induction element being connected in parallel; a first capacitor being associated with the first induction element and a second capacitor being associated with the second induction element, the first and second capacitors having different capacitances.

11. The induction heating system as claimed in claim 1 comprising a plurality of the power modules, each power module being configured to supply the alternating current to one or more of a plurality of induction elements.

12. The induction heating system as claimed in claim 11, wherein the power modules are connected to each other in a daisy-chain arrangement.

13. The induction heating system as claimed in claim 11, wherein the controller is configured to control the power modules independently of each other.

14. An induction heating controller for controlling a variable frequency alternating current power module configured to supply an alternating current to a plurality of induction elements, the controller comprising at least one processor and a memory device, the at least one processor being configured to: change a supply frequency of the alternating current; and monitor a current in the plurality of induction elements as the supply frequency of the alternating current changes; identify a plurality of resonant frequencies of the alternating current supplied to the at least one induction element by the power module; and determine an operating temperature of the component in dependence on the identified resonant frequencies.

15. A component comprising at least one integrated induction element (27-n) for connection to an alternating current power module to generate an alternating magnetic field for generating electrical currents inside the component to perform heating.

16. An induction heating device comprising an induction element for connection to an alternating current power module to generate an alternating magnetic field for generating electrical currents inside an electrically conductive component to perform heating, the induction element having a longitudinal alternating configuration.

17. The induction heating device as claimed in claim 16, wherein the induction element is disposed on an electrically insulating member for positioning against the component, wherein the member is deformable to facilitate positioning of the induction element proximal to or in contact with the component.

18. A method of heating a component by inductive heating, the method comprising: outputting an alternating current to a plurality of induction elements disposed proximal to an exterior of the component; determining a plurality of resonant frequencies of the alternating current output to the plurality of induction elements; and determining an operating temperature of the component in dependence on the identified resonant frequencies.

19. The method as claimed in claim 18 comprising measuring a current across the at least one induction element as a function of the supply frequency and monitoring changes in the measured current in dependence on changes in the supply frequency of the alternating current; wherein identifying the at least one resonant frequency comprises identifying at least one peak in the measured current and identifying a corresponding resonant frequency of the alternating current output by the power module.

20. The method as claimed in claim 18 comprising determining a phase difference between the voltage applied to the at least one induction element and the current in the induction element in dependence on changes in the supply frequency of the alternating current; wherein identifying the or each resonant frequency comprises identifying when the phase difference is at least substantially zero.

21. The method as claimed in claim 18 comprising determining how many induction elements are connected in dependence on the plurality of resonant frequencies.

22. The method as claimed in claim 18 comprising reducing a frequency offset between the supply frequency and the identified resonant frequency to increase or maintain heating of the component; and/or increasing a frequency offset between the supply frequency and the identified resonant frequency to reduce heating of the component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0101] FIG. 1 shows a schematic representation of a thermal management system comprising an induction heating system in accordance with an embodiment of the present invention;

[0102] FIG. 2 shows a schematic representation of a first induction heating device for the thermal management system shown in FIG. 1;

[0103] FIG. 3 shows a first circuit representing the connection of a first induction heating element to a first power module;

[0104] FIG. 4 is a first graph representing the current measured across the first induction heating element shown in FIG. 3 with respect to supply frequency;

[0105] FIG. 5 is a schematic representation of a thermal management system comprising a single induction heating element connected to a power module;

[0106] FIG. 6 is a flow diagram illustrating operation of the thermal management system shown in FIG. 5;

[0107] FIG. 7 is a schematic representation of a thermal management system comprising a plurality of induction heating elements having power modules connected in a daisy-chain arrangement;

[0108] FIG. 8 is a schematic representation of a thermal management system comprising a plurality of induction heating elements connected in parallel to a common power module;

[0109] FIG. 9 shows a second circuit representing the parallel connection of the induction heating elements shown in FIG. 8; and

[0110] FIG. 10 is a second graph representing the current measured across each of the induction heating elements shown in FIG. 9.

DETAILED DESCRIPTION

[0111] An induction heating system 1 in accordance with an embodiment of the present invention will now be described with reference to the accompanying Figures. In the present embodiment the induction heating system 1 forms part of a thermal management system (TMS) (denoted generally by the reference numeral 2). The induction heating system 1 is operative to perform inductive heating of an electrically conductive component 3. In the present embodiment, the component is in the form of a conduit 3 comprising one or more sub-sections 3-n.

[0112] The induction heating system 1 is operative to control the temperature of an exhaust system 4 for conveying process gases to an abatement device 5. The exhaust system 4 may, for example, be provided to transport deposition gases and associated powders expelled from a chemical vapour deposition (CVD) process. The induction heating system 1 is configured to control the temperature of the exhaust system 4 to ensure that compounds remain volatile, thereby preventing or suppressing the accumulation of solids which may partially or completely block the exhaust system 2. It will be understood that the induction heating system 1 can be used in other industrial processes.

[0113] As shown in FIG. 1, the exhaust system 4 comprises a conduit 3. The conduit 3 is in the form of a tube composed of a metal, such as stainless steel. The conduit 3 may, for example, comprise a DN40 pipe having an internal diameter of 40 mm. The conduit 3 may have a wall thickness of approximately 1 mm or 2 mm, for example. It will be understood that the conduit 3 may have larger or smaller wall thicknesses. The conduit 3 may, for example, be 10 metres or more in length and may follow a convoluted path. The conduit 3 forms a substantially continuous fluid path for conveying exhaust gases to the abatement device 4. The conduit 3 could consist of a single length of pipe. However, the conduit 3 typically comprises a plurality of subsections 3-1, 3-2 joined together in a fluid-tight manner. The conduit 3 may comprise one or more bends to provide the required connection to the abatement device 4. The conduit 3 is supported along its length by a plurality of supports 6. An inlet coupling 9 is provided at an inlet 10 of the exhaust system 4; and an outlet coupling 11 is provided at an outlet 12 of the exhaust system 4. The outlet coupling 11 is provided to connect the exhaust system 4 to the abatement device 4. The inlet and outlet couplings 9, 11 each comprise an 0-ring for forming a fluid-tight seal with the associated components. A valve 13 is provided at the outlet 12 of the exhaust system 4. The valve 13 is operable to selectively open and close the outlet 12. The valve 13 may be heated to reduce the build-up of solids. A lagging 14 is provided around an exterior of the conduit 3 in order thermally to insulate the conduit 3.

[0114] The induction heating system 1 comprises a controller 20, at least one alternating current (AC) power module 21-n and at least one induction heating device 22-n. The controller 20 comprises at least one electronic processor 23 and a system memory 24. A set of computational instructions is stored in the system memory 24. When executed, the computational instructions cause the processor 23 to perform the method(s) described herein. The power module 21-n has an electrical input 25 connected to a mains electricity supply RMS or other electrical power source; and an electrical output 26 connected to the at least one induction heating device 22-n. The power module 21-n is configured to output a high-frequency alternating current. The power module 21-n may, for example, output alternating current having a frequency greater than or equal to 10 kHz. The power module 21-n may be a Radio Frequency (RF) power module for outputting alternating current having a supply frequency comprising an RF signal, for example having a supply frequency greater than or equal to 20 kHz. In the present embodiment, the power module 21-n is configured to output alternating current having a frequency greater than or equal to 100 kHz. In a variant, the power module 21-n may be configured to generate an alternating current having a frequency in the range 100 kHz to 1000 kHz. The power module 21-n in the present embodiment is a variable frequency AC power module (21-n). The controller 20 is connected to the power module 21-n (either by a wired connection or a wireless connection). The controller 20 is operative to control operation of the power module 21-n to control the supply frequency of the alternating current output to the at least one induction heating device 22-n via the output 26. As illustrated in FIG. 1, the controller 20 transmits a control signal CS1 to the power module 21-n to control the supply frequency. The controller 20 may optionally be configured to receive one or more signals from the power module 21-n. In the present embodiment, the controller 20 is configured to receive a current measurement signal from the power module 21-n. The power module 21-n may be configured to operate at a relatively low voltage (for example less than 40V) and a relatively high current (for example 10 Amps).

[0115] The at least one induction heating device 22-n is configured to be positioned against (i.e. in contact with), or in close proximity to the conduit 3. A plurality of the induction heating devices 22-n may be provided on the conduit 3. The induction heating devices 22-n may be disposed on the conduit 3 in a non-overlapping arrangement. The at least one induction heating device 22-n in the present embodiment is configured to extend at least substantially around the circumference of the conduit 3. The plurality of the induction heating devices 22-n may be connected to the power module 21-n in series or parallel. In the arrangement illustrated in FIG. 1, a plurality of the induction heating devices 22-n are provided on the conduit 3. The induction heating devices 22-n provide a plurality of temperature-controlled zones Z(n) for controlling the temperature of corresponding sections of the conduit 3. One or more induction heating devices 22-n may be provided in each of the temperature-controlled zones Z(n). The induction heating devices 22-n each have the same configuration. For the sake of brevity, the description herein is directed towards the configuration and operation of a first of the induction heating devices 22-1.

[0116] As shown in FIG. 2, the first induction heating device 22-1 comprises an induction element 27-1, a first protective member 28 and a second protective member 29. The induction element 27-1 comprises an inductive coil 30; and first and second electrical connectors 31A, 31B for connection to the output 26 of the power module 21-n. The inductive coil 30 is configured to establish a concentrated magnetic field which penetrates the conduit 3. The inductive coil 30 is disposed between the first and second protective members 28, 29. In the present embodiment, the inductive coil 30 comprises an electrically conductive elongated member. The inductive coil 30 has a low resistance at a target operating frequency, or within a target operating frequency range. The inductive coil 30 may be formed from one or more wires, for example in the form of a Litz wire; or may be machined from a continuous sheet of electrically conductive material. The inductive coil 30 has a longitudinally alternating configuration, for example comprising or consisting of a sine wave-like curved configuration or a serpentine configuration. The inductive coil 30 is formed in a single plane and this arrangement is referred to herein as a “longitudinal coil”. The inductive coil 30 is supported between the first and second protective members 28, 29. The inductive coil 30 may be bonded to at least one of the first and second protective members 28, 29. Alternatively, or in addition, the first and second protective members 28, 29 may be bonded to each other to form the first induction heating device 22-1. The inductive coil 30 may be disposed in a recessed track or channel formed in at least one of the first and second protective members 28, 29.

[0117] The first protective member 28 is suitable for positioning against an exterior of the conduit 3. The first and second protective members 28, 29 are electrically insulating. The first and second protective members 28, 29 may optionally be thermally insulating to reduce thermal losses from the conduit 3. The first induction heating device 22-1 is deformable to facilitate positioning against, and preferably around, an exterior of the conduit 3. The first and second protective members 28, 29 comprise flexible panels. The first and second protective members 28, 29 may, for example, be formed from rubber or an elastomeric compound. The first induction heating device 22-1 may comprise at least one fastener (not shown) for securing the first induction heating device 22-1 to the conduit 3. The at least one fastener may, for example, be provided on the second protective member 29. The at least one fastener may be releasable to facilitate positioning and/or removal of the first induction heating device 22-1. A suitable fastener may comprise a hook and loop fastener disposed on the second protective member 29. Other types of fastener may be employed to secure the first induction heating device 22-1.

[0118] The induction heating system 1 in the present embodiment comprises at least one temperature sensor 32 for outputting a temperature signal T(n) to the controller 20. The temperature sensor 32 may, for example, comprise a thermistor or the like. In the arrangement shown in FIG. 1, a plurality of temperature sensors 32 are provided on the conduit 3. The temperature sensors 32 are associated with separate temperature-controlled zones Z(n) of the induction heating system 1. The temperature sensors 32 are thermally coupled to the conduit 3. The temperature sensors 32 could, for example, be bonded to the conduit 3. In a variant, the temperature sensor 32 could be incorporated into the first induction heating device 22-1, for example in an outer surface of the first protective member 28 for positioning against the exterior of the conduit 3. Other techniques may be employed to determine the temperature of the conduit 3. As described herein, the electrical behaviour of the inductive coil 30 may be monitored to determine the temperature of the conduit 3.

[0119] A schematic representation of a first circuit EC1 comprising the power module 21-n and the induction element 27-n is shown in FIG. 3. As outlined above, the power module 21-n is operable to output alternating current to the at least one induction heating device 22-n. The oscillating electric field in the induction element 27-n creates an oscillating magnetic field which penetrates the conduit 3 of the exhaust system 4. The conduit 3 is composed of an electrically conductive material and the changing magnetic field generates eddy currents therein. The eddy currents are effective in heating the conduit 3 at a rate determined by the resistance of the conduit 3. Although the first circuit EC1 has an inherent ability to store electric charge (i.e. a capacitance), a separate capacitor 33 is added to control electrical behaviour. The induction element 27-n, the capacitor 33 and a resistor 34 are arranged in series in the first circuit EC1. The resistor 34 may be a discrete component provided in the first circuit EC1. Alternatively, the resistor 34 may represent the resistance of the inductive load with eddy current developing losses which are the source of heating. A resonant inductive coupling is established when the supply frequency is at a resonant frequency f.sub.0. The effective impedance of the first circuit EC1 is reduced (typically at a minimum value) and the current and power transfer is maximised. The resonant frequency f.sub.0 may be determined by the following equation:

[00001]$\begin{array}{cc}{f}_0=\frac{{\omega }_0}{2\pi }=\frac{1}{2\pi \sqrt{\mathrm{LC}}}& \left(1\right)\end{array}$ [0120] where: f.sub.0 is the resonant frequency; [0121] ω.sub.0 is the wavelength; [0122] L is the inductance; and [0123] C is the capacitance.

[0124] The inductance (L) of the first circuit EC1 is a function of the material properties of the conduit 3, as well of the configuration of the coil formed by the induction element 27-n. A first graph 50 is shown in FIG. 4 representing the current (I) measured in the first circuit EC1 at a range of wavelengths output by the power module 21-n. For the purposes of this example, the resistance (R) is one (1) Ohm (Ω); the inductance (L) is one (1) Henry (H); the capacitance (C) is one (1) Farad (F); and the voltage is one (1) volt (V). There are two control strategies available for controlling the power input into the conduit 3, namely: (a) selectively control the frequency of the alternating current supplied to the induction element 27-n; and (b) modulating the supply voltage (for example turning the supply voltage ON/OFF) while maintaining a fixed frequency of alternating current. The power module 21-n in the present embodiment is a variable frequency AC power module (21-n) and the preferred control strategy is to vary the supply frequency of the alternating current. The technique of controlling the frequency of the alternating current to control power transfer to the conduit 3 is illustrated by the arrows in the first graph 50. The controller 20 is configured to control the power module 21-n to adjust the frequency of the alternating current output to the induction element 27-n. The controller 20 can be configured to implement incremental changes (i.e. stepped changes) or substantially continuous changes in the supply frequency. By measuring the current (I) in the first circuit EC1, the controller 20 can identify the resonant frequency f.sub.0. The resonant frequency f.sub.0 is influenced by changes in the temperature of the base material, namely the conduit 3. By monitoring the resonant frequency f.sub.0, the controller 20 can estimate a temperature of the conduit 3. Thus, the temperature of the conduit 3 can be determined without the need for a temperature sensor 32.

[0125] A flow diagram 100 representing operation of the TMS 2 is shown in FIG. 6. The alternating current is output from the power module 21-n to the to at least one induction element 27-n (BLOCK 110). The supply frequency of the alternating current output to the at least one induction element is varied, for example by scanning the supply frequency within a range (BLOCK 120). A current (I) across the at least one induction element 27-n is measured as a function of the supply frequency (BLOCK 130). At least one peak in the measured current is identified and the associated supply frequency is identified as the resonant frequency (f.sub.0) (BLOCK 140). A temperature of the conduit 3 is determined (BLOCK 150). The temperature of the conduit 3 may be determined in dependence on the identified resonant frequency (f.sub.0) or in dependence on a temperature signal received from the temperature sensor 32. The determined temperature of the conduit 3 is compared to a target temperature (BLOCK 160). The controller 20 controls the supply frequency of the alternating current in dependence on the determined temperature of the conduit 3. If the temperature of the conduit 3 is less than the target temperature, the controller 20 controls the supply frequency to reduce a frequency offset between the current supply frequency and the identified resonant frequency (f.sub.0) (BLOCK 170). If the temperature of the conduit 3 is greater than the target temperature, the controller 20 controls the supply frequency to increase a frequency offset between the current supply frequency and the identified resonant frequency (f.sub.0) (BLOCK 180). The process operates continuously while the industrial process is ongoing.

[0126] The operation of the TMS 2 will now be described in more detail. The controller 20 sets a target temperature for each of the temperature-controlled zones Z(n). The target temperature may be set according to operating or process characteristics, for example depending on a composition of exhaust gases to be transported within the conduit 3. The operation of an embodiment of the TMS 2 comprising a single induction heating device 22-1 will now be described with reference to FIG. 5. The first induction heating device 22-1 is disposed against the exterior of the conduit 3 and secured in position using the fasteners. The induction element 27-1 of the first induction heating device 22-1 extends at least substantially around the circumference of the conduit 3 to promote uniform heating. The first induction heating device 22-1 is connected to the output 26 of the first power module (21-n) 21-1. The controller 20 is connected to the first power module (21-n) 21-1 and outputs control signals CS1 to control the supply frequency of the alternating current output to the first induction heating device 22-1. The changing electric field in the induction element 27-1 creates an oscillating magnetic field which induces eddy currents which cause direct heating of the conduit 3. The temperature sensor 32 measures the temperature of the conduit 3 and outputs a temperature signal T(1) to the first power module (21-n) 21-1 and/or the controller 20. The controller 20 is configured to control the supply frequency of the alternating current in dependence on the measured temperature. If the temperature signal T(n) indicates a measured temperature below the target temperature, the controller 20 is configured to change the supply frequency (either by increasing or decreasing the supply frequency) to a frequency closer to the resonant frequency f.sub.0 to increase the power transfer. A frequency offset between the current supply frequency and the identified resonant frequency f.sub.0 is thereby reduced in order to increase heating of the component. If the temperature signal T(n) indicates a measured temperature above the target temperature, the controller 20 is configured to change the supply frequency (either by increasing or decreasing the supply frequency) such that it is further away from the resonant frequency f.sub.0 to decrease the power transfer. A frequency offset between the current supply frequency and the identified resonant frequency f.sub.0 is thereby increased in order to reduce heating of the component.

[0127] As outlined above, the resonant frequency f.sub.0 varies in dependence on temperature. The relationship between the resonant frequency f.sub.0 and the measured temperature of the conduit 3 may be predefined, for example in a look-up table stored in the system memory 24. The controller 20 may thereby determine the temperature of the conduit 3 in dependence on the determined resonant frequency f.sub.0. Alternatively, or in addition, the controller 20 may be configured to control the power module 21-n to vary the supply frequency to determine the resonant frequency f.sub.0. The supply frequency may be varied within a range, for example to perform a scan or sweep. The controller 20 may, for example, vary the supply frequency between a lower frequency limit and an upper frequency limit. The controller 20 in the present embodiment is configured to vary the supply frequency substantially continuously between the lower frequency limit and the upper frequency limit In a variant, the controller 20 may be configured to implement incremental changes, for example comprising a plurality of step changes in the supply frequency. The controller 20 measures the current (i.sub.1) across the first induction heating device 22-1 as the supply frequency changes. The controller 20 is configured to identify a peak in the measured current (i.sub.1) as a function of the supply frequency. The peak may be identified by determining when the rate of change of the measured current is at least substantially equal to zero (0) as the supply frequency changes. The controller 20 identifies the resonant frequency f.sub.0 as the frequency corresponding to the peak current. The controller 20 may selectively increase or decrease the supply frequency in dependence on whether the rate of change of the measured current is positive or negative in order to determine the resonant frequency f.sub.0. Alternatively, the controller 20 may measure the current for a predefined range of supply frequencies. This process may be performed periodically, for example as a calibration operation.

[0128] A development of the TMS 2 is illustrated in FIG. 7. This implementation of the TMS 2 comprises a plurality of the induction heating devices 22-n each associated with a separate section of the conduit 3. The TMS 2 in this arrangement comprises a plurality of temperature-controlled zones Z(n) each comprising at least one induction heating device 22-n. In the illustrated arrangement, the TMS 2 comprises four (4) induction heating devices 22-1, 22-2, 22-3, 22-4 and four (4) associated power modules 21-1, 21-2, 21-3, 21-4. The induction heating devices 22-1, 22-2, 22-3, 22-4 are each connected to a respective one of the power modules 21-1, 21-2, 21-3, 21-4. The power modules 21-1, 21-2, 21-3, 21-4 are daisy-chained together and powered over a common line-voltage connection, thereby minimising the number of connections back to a central power distribution board. The power modules 21-1, 21-2, 21-3, 21-4 each receive a temperature signal T(n) from an associated temperature sensor 32. The controller 20 is configured to control the supply frequency of the alternating current output from each of the power modules 21-1, 21-2, 21-3, 21-4 in dependence on the temperature signal T(n). The supply frequency of the alternating current output to each induction heating device 22-1, 22-2, 22-3, 22-4 is controllable independently. Thus, the temperature of the separate temperature-controlled zones Z(n) can be controlled independently.

[0129] The power modules 21-n may be configured to communicate directly with the controller 20 (as represented by the dashed lines shown in FIG. 5). In a variant, the power modules 21-n are configured to communicate with one another over the daisy-chained line-voltage connection. Each power module 21-n is configured to transmit and receive signals over the line-voltage connection. The first power module (21-n) 21-1 associated with the first induction heating device 22-1 may function as a master unit which communicates directly with the controller 20. The second, third and fourth power modules 21-2, 21-3, 21-4 associated with the subsequent induction heating devices 22-2, 22-3, 22-4 function as slave units. This connection offers the advantage of providing a single point of communication connection to the controller 20. In this arrangement the line-voltage connection is operative to transmit control signals CS1 to the respective power modules 21-n to control the supply frequency output to each induction heating devices 22-2, 22-3, 22-4. The temperature signal T(n) and other operating signals may optionally be transmitted over the line-voltage connection. The current across each induction heating devices 22-2, 22-3, 22-4 could optionally be measured and transmitted over the line-voltage connection. It will be understood that each power module 21-n is operable independently of the other power modules 21-n. The power requirement at the mains electrical connection is equal to the sum of the individual power requirements of each of the AC power module (21-n) units 21.

[0130] A further embodiment of the TMS 2 is illustrated in FIG. 8. Like reference numerals are used for like components. The TMS 2 comprises a plurality of temperature-controlled zones Z(n) each comprising at least one section of the conduit 3. The TMS 2 comprises a plurality of the induction heating devices 22-n each connected in parallel to a common power module 21-n. In the illustrated embodiment, there are three (3) of the induction heating devices 22-1, 22-2, 22-3 connected in parallel to the first power module (21-n) 21-1. The first, second and third induction heating devices 22-1, 22-2, 22-3 are each associated with a separate section of the conduit 3 (corresponding to separate temperature-controlled zones Z(n)). The temperature of the separate sections of the conduit 3 can be controlled independently.

[0131] A second circuit EC2 representing this embodiment of the TMS 2 is shown in FIG. 9. The second circuit EC2 comprises a plurality of branches connected in parallel to the AC supply module 21-1. The branches each correspond to one of the temperature-controlled zones Z(n). In the illustrated arrangement, the second circuit EC2 comprises three (3) branches, but it will be understood that the second circuit EC2 may comprise two (2) branches or more than three (3) branches. The second circuit EC2 comprises first, second and third capacitors 33-1, 33-2, 33-3 having respective first, second and third capacitances C1, C2, C3. The first, second and third capacitors 33-1, 33-2, 33-3 may each have capacitances C1, C2, C3 which are different from each other to alter the resonant frequency of each branch of the second circuit EC2. The current flow in each branch of the second circuit EC2 is dependent on the supply frequency of the current output by the first power module (21-n) 21-1. It will be understood that more than one induction heating device 22-n may be provided in each temperature-controlled zone Z(n), for example two or more induction heating devices 22-n may be connected in series within each branch of the second circuit EC2.

[0132] The controller 20 is configured to control the power module 21-n to vary the supply frequency to determine the resonant frequency f.sub.0 of each branch of the second circuit EC2. The supply frequency may be varied within a range, for example between a lower frequency limit and an upper frequency limit In the present embodiment, the controller 20 controls the power module 21-n such that the supply frequency varies substantially continuously. The current (I) in each branch of the second circuit EC2 is measured as the supply frequency is varied across the range. In the present embodiment, the controller 20 measures a first current (i.sub.1), a second current (i.sub.2) and a third current (i.sub.3) across the first, second and third induction heating devices 22-1, 22-2, 22-3 respectively. The controller 20 is operative to identify peaks in each of the first current (i.sub.1), the second current (i.sub.2) and the third current (i.sub.3) as a function of the supply frequency. The peak in each of the first current (i.sub.1), the second current (i.sub.2) and the third current (i.sub.3) corresponds to a resonant frequency f.sub.0 in each of the first, second and third induction heating devices 22-1, 22-2, 22-3. The controller 20 is configured to determine the temperature of each section of the conduit 3 in dependence on the resonant frequency f.sub.0 for each branch of the second circuit EC2. The controller 20 can also control the temperature of each section of the conduit 3 by controlling the AC supply module 21-1 to adjust the supply frequency. Alternatively, or in addition, the temperature of each section of the conduit 3 may be varied by selectively adjusting the inductance and/or capacitance of each branch of the second circuit EC2.

[0133] The controller 20 is described as measuring the current (I) across each of the induction heating devices 22-1, 22-2, 22-3. The total current (I) is equal to the sum of the first current (i.sub.1), the second current (i.sub.2) and the third current (i.sub.3) (i.e. I=i.sub.1+i.sub.2+i.sub.3). The first current (i.sub.1), the second current (i.sub.2) and the third current (i.sub.3) each comprise a vector described in the complex domain with a phase and a module. The total current (I) could be measured and the presence or absence of resonant frequencies f.sub.0 determined by detecting one or more peaks in the total current (I) as a function of the supply frequency. The controller 20 may monitor the total current (I) and count the total number of peaks present in the total current (I) across the range of the supply frequency. The identification of each peak in the total current (I) is indicative of a resonant frequency f.sub.0 of a separate temperature-controlled zone connected in parallel to the power module 21-n. By determining how many peaks are present across the supply frequency range, the controller 20 can determine how many (n) induction heating devices 22-n are connected. The controller 20 may determine that there are no induction heating devices 22-n connected (i.e. n=0); or that there are one or more induction heating devices 22-n connected (n>=1). The controller 20 may thereby discover how many temperature-controlled zones Z(n) are connected to the power module 21-n. The ability to determine how many induction heating devices 22-n are connected may enable implementation of an automated or semi-automated control system.

[0134] A second graph 60 is shown in FIG. 10 representing the current (I) measured in the second circuit EC2 at a range of wavelengths output by the first power module (21-n) 21-1. The power module 21-n is a variable frequency AC power module (21-n) and the control strategy comprising controlling the supply frequency of the alternating current output to the induction elements 27-1, 27-2, 27-3. The controller 20 can be configured to implement incremental changes (i.e. stepped changes) or substantially continuous changes in the supply frequency. By measuring the first current (i.sub.1), the second current (i.sub.2) and the third current (i.sub.3), the controller 20 can identify the resonant frequency f.sub.0 of each induction element 27-1, 27-2, 27-3. The resonant frequency f.sub.0 is influenced by changes in the temperature of the base material, namely the conduit 3. By monitoring the resonant frequency f.sub.0, the controller 20 can estimate a temperature of each section of the conduit 3. The controller 20 is configured to control the first power module (21-n) 21-1 in dependence on the determined temperature of the corresponding sections of the conduit 3.

[0135] A variable capacitor and/or a variable inductor may be provided in each branch of the second circuit EC2. The controller 20 could be configured to control the capacitance and/or inductance in each branch to adjust the resonant frequency f.sub.0 of each induction element 27-1, 27-2, 27-3.

[0136] The at least one induction heating device 22-n is described herein as a separate device which is located on the conduit 3. In a variant, the induction heating device 22-n could be integrated into the component 3. In particular, the induction element 27-n could be integrated into the conduit 3. The induction element 27-n could comprise a helix extending around the circumference of the conduit 3. Alternatively, the induction element 27-n could comprise a longitudinal coil of the type described herein extending at least partially around the circumference of the conduit 3. The induction element 27-n would be electrically and optionally also thermally insulated from the conduit 3. For example, an electrical insulating sheath may be provided around an exterior of the conduit 3. Each section 3-1, 3-2 of the conduit 3 may comprise electrical connectors. The electrical connectors may be used to connect the induction elements 27-n to each other (for example in parallel or series) and/or to the power module (s) 21-n. The conduit 3 may comprise coupling means for forming a fluid-tight seal with an adjacent conduit 3. A thermal insulating layer and/or an electrical insulating layer may be provided around the outside of the induction element 27-n.

[0137] It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

[0138] The controller 20 has been described herein as determining the resonant frequency f.sub.0 by measuring the current i.sub.1 across the first induction heating device 22-1 as the supply frequency changes. The resonant frequency f.sub.0 is identified as the supply frequency corresponding to a peak in the measured current (i.sub.1). It will be understood that other techniques may be used to determine the resonant frequency f.sub.0. For example, the resonant frequency f.sub.0 may be determined by monitoring the phase of the voltage applied to the induction element 27-n and the current. At resonance, the phase is null since the circuit is purely resistive (corresponding to the maximum power transmitted). If the frequency is greater than the resonant frequency f.sub.0 (f>f.sub.0), the circuit behaves more like an inductance with a current ‘behind’ the voltage. If the frequency is less than the resonant frequency f.sub.0 (f<f.sub.0), the circuit behaves more like a capacitor with a current ‘ahead’ of the voltage. Thus, phase detection can be used to determine the resonant frequency f.sub.0. The controller 20 may be configured to control the frequency of the alternating current supplied to the induction element 27-n in dependence on the determined phase.

[0139] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0140] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.