Patent Description:
In a chromatography system, different types of devices can be used that may be rapidly heated to facilitate separation of analytes. Such devices may, e.g., include a sample introduction inlet or injector, a column oven, a heated transfer line, and a cold trap. For example, an external resistive heater or a hot air heater can be used for heating the device to a high temperature. External heaters can be attached to the device externally, for example by making physical contact between the heater and the device body to be heated. Due to a relatively high thermal mass of the heater and the device body, and the thermal resistance between the contacting surfaces, external resistive heaters may have high thermal inertia, and as a result, may have a slow heating and cooling rate.

<CIT> describes a direct heating tube aimed at ensuring a uniform temperature distribution in the whole part thereof or a temperature distribution having a desired temperature gradient, and making it possible to keep constant the temperature of a fluid which is caused to flow through the tube or to give a desired change to the temperature of the fluid. Thereto, in a desired portion of the tube to be heated, a second heated tube connected to a first heated tube is provided outside the first heated tube, and an electrode portion is connected to the second heated tube. As further background, <CIT> describes another resistance heater.

A potential drawback of having a heater integrated in the device body is that a failure in one of these components may lead to the other component becoming unusable. Considering the normal operating conditions of these systems, which may e.g. include devices undergoing multiple cycles of being heated up to <NUM> within a few seconds and cooled back to room temperature before receiving a next analyte, the effect of temperature on durability and reliability can be a limiting factor. Compared to systems in which a device is heated by an external heater, replacing or servicing of components may be more challenging in integrated solutions.

Accordingly there remains a desire to address these and other drawbacks, to further advance the field of gas chromatography, by providing a system for heating an analyte with improved reliability and durability.

Aspects of the present disclosure relate to a heating assembly. The assembly comprises an inner tube with a primary heating tube made of electrically conductive material, and outer tube with an auxiliary heating tube made of (the same or another, preferably at least similar) electrically conductive material. The heating assembly comprises connection means (e.g. referred herein as flanges) which mechanically and electrically interconnect the inner tube with the outer tube. The primary and auxiliary heating tubes axially overlap at least along a subsection length of the inner and outer tubes for transferring auxiliary heat from the auxiliary heating tube to the primary heating tube over a radial gap between the inner and outer wall surfaces. For example, an electrical path is formed through the primary and auxiliary heating tubes via the electrically conductive material.

By providing an electrically insulating section parts of the electrically conductive material can be electrically separated while maintaining a mechanical connection. By separating parts of the outer tube between the flanges and connecting electrodes on either sides of the insulating section, the electrical path can be directed to exclusively pass in series through respective parts of both the primary and auxiliary heating tubes. Advantageously, this may ensure the same current runs through both heating tubes.

By providing an extendible section in at least one of the tubes the tubes may be allowed to axially extend or shorten for compensating any difference in thermally induced contraction or expansion, respectively, between the inner tube and the outer tube. By providing the extendible section in the outer tube, the inner tube may be relatively unaffected. Accordingly, while ensuring a uniform temperature distribution along the length of the primary heating tube during heating of the heating assembly, internal thermal expansion differences, e.g. caused by cyclic thermal loading, can be compensated by the extendible section in a controlled fashion, to avoid that these thermal expansion differences lead to stresses in undesired locations within the heating assembly. As such, deformation and misalignment of components, such as sensors or couplings, can be minimalized, while structural damage due to thermal expansion can be reduced, to improve the reliability and durability of the heating assembly.

When the extendible section is part of the auxiliary heating tube, it can be arranged for generating and transferring auxiliary heat from said part of the auxiliary heating tube to the primary heating tube. As such, the extendible section can also be used to generate and transfer auxiliary heat to the primary heating tube, e.g. to heat sections of the primary heating tube that are connected to a relatively large thermal mass. As a result, the function of providing auxiliary heat as well as the function of compensating for thermal expansion can synergistically be integrated in the extendible section, to provide a heating assembly with a uniform temperature distribution, while avoiding damage due to cyclic thermal loading of the heating assembly.

By having the extendible section configured to axially extend or shorten by elastic and/or reversible deformation thereof, the number of load cycles, e.g. elastic deformation cycles, that the extendible section can endure before the extendible section is damaged by mechanical fatigue can be significantly increased, e.g. compared to plastic deformation of the extendible section.

In a particularly advantageous embodiment, the extendible section comprises a foldable structure, most preferably a bellows. A bellows is generally understood as an expandable object or device, e.g. having concertinaed sides to allow it to expand and contract. Typically, the bellows is formed by an axially corrugated tube comprising a series of at least two corrugations (undulations, convolutions), preferably at least three, five, e.g. up to twenty or more. For example, the corrugations have corrugation amplitudes arranged in radial direction of the auxiliary heating tube and corrugation pitches arranged in axial direction of the auxiliary heating tube. In this way a relative high degree of resilience of the extendible section can be provided, e.g. combined with a relatively low degree of stiffness of the extendible section, along the axial direction of the inner and outer tubes, thereby allowing (elastic and/or reversible) deformation of the extendible section to compensate any thermal expansion differences between the inner and outer tubes.

By having a uniform corrugation amplitude of the series of corrugations, the stiffness and resilience characteristics of the extendible section can be constant along the axial length of the extendible section. As a result, elastic deformation of the extendible section, to compensate thermal expansion differences between the inner and outer tubes, can be evenly distributed along the axial length of the extendible section, to minimize fatigue. Alternatively or additionally, the stiffness and resilience characteristics of the extendible section can be made constant along the axial length of the extendible section by having a uniform corrugation pitch of the series of corrugations. As such, elastic deformation of the extendible section can be evenly distributed along the axial length of extendible section, to minimize fatigue.

By having the series of corrugations form part of the electrical path through the primary and auxiliary heating tubes, the series of corrugations can be used to generate and transfer auxiliary heat to the primary heating tube. As a result, the series of corrugations provide a synergetic combination of the function of providing auxiliary heat and the function of compensating for thermal expansion, integrated in the extendible section, to provide a heating assembly with a uniform temperature distribution, that can avoid damage to the heating assembly due to cyclic thermal loading.

To match the electrical resistivity of the extendible section with the electrical resistivity of the primary heating tube, e.g. to have approximately similar heating characteristics of the extendible section and the primary heating tube, the primary heating tube can have a first wall thickness, the extendible section can have a second wall thickness, and the second wall thickness can be the same or similar as the first wall thickness, e.g. within factor two. To further match the electrical resistivity of the extendible section with the electrical resistivity of the primary heating tube, e.g. these are preferably made of the same conductive material. Advantageously, by using identical materials, the thermal expansion of the extendible section can also be better matched with the thermal expansion of the primary heating tube, to reduce the axial extension and shortening of the extendible section required for compensating a thermal expansion difference between the inner and outer tubes.

To minimize damage or deformation of the primary heating tube, e.g. due to cyclic thermal loading, the extendible section, forming part of auxiliary heating tube, can be designed having an axial stiffness lower than that of the primary heating tube, e.g. by at least a factor two. As such, axial thermal expansion differences between inner tube, comprising primary heating tube, and outer tube, comprising extendible section, are more likely to be compensated by extendible section, thereby minimizing stresses in other parts.

By having the extendible section arranged for providing a sealed environment between the inner tube and the outer tube, the sealed environment can provide a synergetic advantage in that it can additionally or alternatively be used for receiving coolant medium inside the heating assembly, e.g. along a cooling path that passes though radial gap past the inner surface of the auxiliary heating tube and past the outer surface of the primary heating tube, to cool these heating tubes. Alternatively, or additionally, a cooling circuit can be arranged to flow in the radial gap between the primary or auxiliary heating tubes. As will be appreciated this can provide an advantageous heating/cooling arrangement, also without the extendible section.

Other aspects or further aspects relate to a temperature controllable device comprising the heating assembly as described herein, e.g. with its temperature controlled based on sensor input. Preferably, the temperature sensor is arranged for measuring a temperature of the primary heating tube. For example, the temperature sensor can be used for determining the temperature of the heating assembly and/or an analyte heated by the heating assembly (inside the primary heating tube). An an electric current passing through the electrical path can be controlled based on measurements from the temperature sensor, e.g. with a closed loop temperature control of the device to follow a specific heating profile.

In some embodiments, the device may comprise a cooling circuit, arranged for cooling the inner and outer tubes. As such, cooling of the device, e.g. from a heated temperature back to room temperature or to a lower temperature, can be performed more rapidly compared to conventional heating devices. For example when device is used as an inlet, this may reduce the time between injection of samples into a gas chromatography system, thereby increasing throughput. For example when device is used as a cold trap, this may increase the rate of cooling of the analyte.

By having the cooling circuit arranged for allowing cooling medium to flow through the radial gap between the inner and outer wall surfaces of the auxiliary and primary heating tubes, respectively, cooling medium can flow past and come into contact with the inner and outer wall surfaces of the auxiliary and primary heating tubes, respectively, to simultaneously cool the auxiliary and primary heating tubes, to increase the cooling rate of the device.

When the device forms an inlet for a gas chromatography system, the inlet can have improved reliability and durability by comprising a heating assembly with an extendible section arranged for compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading of inlet, while ensuring a uniform temperature distribution during heating of a sample.

When the device forms a cold trap for a gas chromatography system, a uniform temperature distribution can be ensured during heating of a sample after cryogenic trapping of the sample by the device comprising a heating assembly as described herein, while having improved reliability and durability by compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading.

Further aspects relate to a chromatography system comprising the heating assembly and/or the device as described herein. Accordingly, while ensuring uniform temperature distribution during heating and/or reheating of samples, the chromatography system can have improved reliability and durability by comprising one or more heating assemblies and/or devices with one or more expansion mechanisms arranged for compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading.

Yet further aspects relate to specific use of the heating assembly and/or the device as described herein, e.g. as an inlet and/or a cold trap or other part of a chromatography system. This may provide similar and/or further advantages as noted herein.

It will be further understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

<FIG> illustrates an embodiment of a heating assembly <NUM>, e.g. an inlet for a gas chromatograph, while <FIG> illustrates another or further embodiment of a heating assembly <NUM>, e.g. a cold trap for a gas chromatograph. According to the invention, the heating assembly <NUM> comprises a primary heating tube <NUM>, made of an electrically conductive material and forming at least part of an inner tube <NUM>, and an auxiliary heating tube <NUM>, made of the same or another electrically conductive material and forming at least part of an outer tube <NUM>. Preferably, the conductive materials are the same or similar, e.g. at least having similar thermal expansion coefficient within. Suitable electrically conductive materials for the primary heating tube <NUM> and/or the auxiliary heating tube <NUM> may, e.g., be dependent on the specific temperature ranges at which the heating assembly <NUM> is intended to be used. For example, suitable electrically conductive materials may include materials having a melting point higher than a maximum heating temperature of the primary and/or auxiliary heating tubes <NUM>, <NUM>, e.g. at least <NUM> degrees Celsius, preferably at least <NUM> degrees Celsius. Preferred materials may, for example, include (at least partially conductive) metals or alloys, such as (possibly different) stainless steels, nickel, and/or other hard metal alloys. Alternatively, or in addition, refractory metals, including the wider definition of refractory metals, and their alloys can be suitable materials for their relatively high resistivity to heat and wear. Alternatively, or additionally, electrically conductive ceramics or electrically conductive plastics can be used in some applications of a heating assembly <NUM> as described herein.

According to the invention, the heating assembly further comprises a pair of flanges <NUM> which mechanically and electrically interconnect the inner tube <NUM> with the outer tube <NUM> (e.g. a pair of rings, washers, or other conductive pieces, radially interconnecting respective mid and/or end sections of the tubes), wherein the inner and outer tubes <NUM>, <NUM> extend between the flanges with the inner tube <NUM> arranged inside the outer tube <NUM>, wherein the primary and auxiliary heating tubes <NUM>, <NUM> axially overlap at least along a subsection length L of the inner and outer tubes <NUM>,<NUM>, respectively, between the flanges. For example, the inner and outer tubes <NUM>, <NUM> can have axial lengths ranging between <NUM> millimeter and <NUM> millimeter. Dimensions may depend on the type of application of the heating assembly <NUM>, e.g. depending on the size of a liner comprising a sample to be heated by the heating assembly <NUM>, or a column outside diameter. The length and wall thickness of the inner tube <NUM> and/or outer tube <NUM> may vary while the primary and auxiliary heating tubes <NUM>, <NUM> preferably have a relatively high resistance, to facilitate Ohmic heating. As such, the primary and auxiliary heating tubes <NUM>, <NUM> preferably have a relatively thin wall thickness, e.g. selected in a range between <NUM> - <NUM> millimeter, preferably between <NUM> millimeter and <NUM> millimeter. Alternatively, or additionally, the (overlapping) subsection length L is preferably selected in a range between <NUM> - <NUM> millimeter, or in a range between <NUM> - <NUM> millimeter. For example, values in these range may be selected as an ideal compromise between mechanical strength and electrical resistance.

The subsection length L between the auxiliary primary and auxiliary heating tubes <NUM>, <NUM> may, in principle, span the full axial length of primary heating tube <NUM>, although it is preferred to span only part thereof, e.g. between <NUM> - <NUM>% of the total length between the flanges <NUM>. The auxiliary heating tube <NUM> and the primary heating tube <NUM> can for example overlap such that axial ends of the auxiliary heating tube <NUM> and the primary heating tube <NUM> are aligned on a plane, or the auxiliary heating tube <NUM> can overlap the primary heating tube <NUM>, such that there is an offset between axial ends of the primary heating tube <NUM> and the auxiliary heating tube <NUM>.

According to the invention, along the subsection length L, an inner wall surface <NUM> of the auxiliary heating tube <NUM> faces an outer wall surface <NUM> of the primary heating tube <NUM>. Accordingly, auxiliary heat can be transferred from the auxiliary heating tube <NUM> to the primary heating tube <NUM> over a radial gap G between the inner and outer wall surfaces <NUM>, <NUM>. For example the auxiliary heat may be transferred from the auxiliary heating tube <NUM> to the primary heating tube <NUM> across the radial gap G by radiation and/or convection. At the same time, the radial gap G may electrically isolate the inner and outer wall surfaces <NUM>, <NUM> from each other. As such, the radial gap G between the inner and outer wall surfaces <NUM>, <NUM> (along the subsection length L) is preferably selected in a range between <NUM> - <NUM> millimeter, more preferably between <NUM> - <NUM> millimeter.

According to the invention, the heating assembly further comprises a pair of electrodes <NUM>, <NUM>, connected to respective parts of the electrically conductive material and arranged for forming an electrical path <NUM> through the primary and auxiliary heating tubes <NUM>, <NUM> via the electrically conductive material. Electrodes can e.g. be made of metal or other electrically conductive material, and connected via wiring to an electric power source, such as an AC or DC power source. The outer tube <NUM> can comprise an electrically insulating section <NUM> arranged for mechanically connecting, while electrically separating (isolating), parts of the electrically conductive material. As such, the electrical path <NUM> passes exclusively in series through respective parts of both the primary and auxiliary heating tubes <NUM>, <NUM>. For example, a ceramic or a high temperature plastic ring <NUM> can be placed between upper <NUM> and lower <NUM> electrodes to provide the electrical isolation between them. To create a mechanically stable construction, primary heating tube <NUM> and auxiliary heating tube <NUM> may be mounted in one assembly using ceramic or high temperature plastic washers <NUM>, and screws <NUM>. Preferably, all parts used in and around the electrically insulating section <NUM> have a sufficiently high (continuous) operating or working temperature, e.g. at least <NUM> degrees Celsius, most preferably at least <NUM> degrees Celsius. To achieve this, typically the melting temperature should be at least twice higher. For example, the melting temperature is at least <NUM> degrees Celsius, preferably at least <NUM> degrees Celsius.

According to the invention, the auxiliary heating tube <NUM> comprises an extendible section <NUM>, configured to axially extend or shorten for compensating any difference in thermally induced contraction or expansion, respectively, between the inner tube <NUM> and the outer tube <NUM>. Preferably, the extendible section <NUM> comprise a foldable structure, most preferably a closed foldable structure such as a bellows. Also other or further foldable structures can be envisages such as a leaf springs, membranes, corrugated elements, split joints, helical or coil springs, Belleville washers, or other mechanism for compensating a thermal expansion difference, e.g. an axial and/or radial expansion difference, between inner tube <NUM> and outer tube <NUM>. Depending on the application, the heating assembly <NUM> may benefit from very rapid heating and/or cooling, e.g. heating up with a rate ranging between <NUM> - <NUM> degrees (Celsius or Kelvin) per second, preferably at least <NUM> degrees per second, with corresponding electrical currents, e.g., between <NUM> - <NUM> Ampere passing through the primary and auxiliary heating tubes <NUM>, <NUM>. For such extreme conditions, thermal fatigue, e.g. causing failure with macroscopic cracks resulting from cyclic thermal stresses and strains due to temperature changes, spatial temperature gradients, and high temperatures under constrained thermal deformation, may be a significant factor determining the lifespan and reliability of the heating assembly <NUM>. In some preferred embodiments, including the extendible section <NUM> as described, thermal and/or other stresses in the heating assembly <NUM> may be alleviated, e.g. compensating expansion differences between the inner and outer tubes <NUM>,<NUM> to improve lifespan and/or reliability. Synergistically, by the extendible section <NUM> being a part of the auxiliary heating tube <NUM>, the extendible section <NUM> can provide a uniform temperature distribution along the primary heating tube <NUM> during heating, while improving the durability and reliability of the heating assembly <NUM>.

In some embodiments, e.g. as shown in <FIG> and <FIG>, the extendible section <NUM>, as part of the auxiliary heating tube <NUM>, can be arranged for generating and transferring auxiliary heat from said part of the auxiliary heating tube <NUM> to the primary heating tube <NUM>. For example, the extendible section <NUM> can be arranged for generating auxiliary heat, e.g. by ohmic heating caused by electrical path <NUM> passing through the extendible section <NUM>, and for transferring auxiliary heat to primary heating tube <NUM>, e.g. by radiation or convection along radial gap G. As such, the extendible section <NUM> can also be used to generate and transfer auxiliary heat to primary heating tube <NUM>, e.g. to heat sections of primary heating tube <NUM> that are connected to a relatively large thermal mass. As a result, the function of providing auxiliary heat as well as the function of compensating for thermal expansion can synergistically be integrated in extendible section <NUM>, to provide a heating assembly <NUM> with a uniform temperature distribution, while avoiding damage due to cyclic thermal loading of heating assembly <NUM>.

In other or further embodiments, the extendible section <NUM> can be configured to axially extend or shorten by elastic deformation thereof. For example, the extendible section <NUM> can be an elastic element, e.g. designed with specific material and/or geometric properties to provide a relatively high degree of resilience in one or more directions. In other words, in one or more directions the extendible section <NUM> can have a relatively low stiffness combined with a relatively high yield strength. For example, the extendible section <NUM> can have a relatively high degree of resilience along the axial direction of primary and auxiliary heating tubes <NUM>, <NUM>, to compensate any thermal expansion differences present in axial direction between inner and outer tubes <NUM>, <NUM> by elastic deformation of the extendible section <NUM>. For example, the extendible section <NUM> can be configured to axially extend or shorten at least between a factor <NUM> - <NUM> times its original (unloaded) length by elastic deformation of the extendible section <NUM>, preferably at least between a factor <NUM> - <NUM>. As such, the number of load cycles, e.g. elastic deformation cycles, that the extendible section <NUM> can endure before the extendible section <NUM> is damaged by mechanical fatigue, e.g. metal fatigue, can be increased compared to when the extendible section <NUM> is configured to compensate the thermal expansion difference by plastic deformation of the extendible section <NUM>. Accordingly, extendible section <NUM> can be arranged for compensating the thermal expansion difference between the inner and outer tubes <NUM>, <NUM> by being an elastically deformable structure, e.g. an elastically foldable structure, such as a foldable plate or tube structure, e.g. a bellows. A foldable structure can for example be arranged for extending and shortening along the axial direction of extendible section <NUM>, to compensate for thermal expansion differences between inner and outer tubes <NUM>, <NUM> in an axial direction. Alternatively, extendible section <NUM> can comprise a corrugated membrane, or a split joint, having sliding members. A split joint, however, may in some embodiments need a seal between sliding members, which can cause additional wear and/or damage.

In some further embodiments, the extendible section <NUM> is configured to elastically deform along the axial direction of the auxiliary heating tube <NUM> by at least <NUM> millimeter, preferably by at least <NUM> millimeter, and more preferably by at least <NUM> millimeter. For example, extendible section <NUM> can be configured to elastically deform along the axial direction of the auxiliary heating tube <NUM> between <NUM> millimeter and <NUM> millimeter, e.g. depending on the total length, operating temperatures, and electrically conductive materials of the heating assembly <NUM>. As a result, the extendible section <NUM> can compensate thermal expansion differences, in the axial direction of auxiliary heating tube <NUM>, between the inner tube <NUM> and the outer tube <NUM> of at least <NUM> millimeter, e.g. to avoid fatigue in components of heating assembly <NUM> under cyclic thermal loading. Calculations show that, for instance, an about <NUM> millimeter long primary heating tube <NUM> made out of a stainless steel may elongate approximately <NUM> when being heated from room temperature to <NUM> degrees Celsius. In case the axial ends of inner tube <NUM> and outer tube <NUM> are restricted, such elongation may cause significant internal stress that can lead to damage of heating assembly <NUM> after a number of thermal cycles. By having the extendible section <NUM> configured to elastically deform along the axial direction of the auxiliary heating tube <NUM> by at least <NUM> millimeter, e.g. between <NUM> millimeter and <NUM> millimeter, stresses due to thermal expansion of heating assembly <NUM> can be relieved by the extendible section <NUM>. By having the deformation of the extendible section <NUM> in an elastic deformation range, i.e. with stresses in the extendible section <NUM> below the yield strength of the extendible section <NUM>, the number and frequency of thermal load cycles can be increased compared to having the deformation of the extendible section <NUM> in a plastic deformation range, i.e. with stresses in the extendible section <NUM> beyond the yield strength of the extendible section <NUM>. For example, the extendible section <NUM> can be arranged for elastically deforming along the axial direction of the auxiliary heating tube <NUM> by at least <NUM> millimeter, and for at least <NUM> thermal load cycles in which the heating assembly <NUM> is heated from room temperature to approximately <NUM> degrees Celsius within about <NUM> seconds, and then cooled back to room temperature.

<FIG> illustrate cross-section views of embodiments of the extendible section <NUM>. In some embodiments, e.g. as shown, the extendible section <NUM> comprises a bellows formed by an axially corrugated tube, comprising a series of corrugations <NUM> having corrugation amplitudes 127A arranged in radial direction of the auxiliary heating tube <NUM> and corrugation pitches 127P arranged in axial direction of the auxiliary heating tube <NUM>. By having a series of corrugations, a bellows is advantageously able to compensate for expansion differences between the inner tube <NUM> and the outer tube <NUM> in an axial direction as well as in a radial direction, lateral to the axial direction. A bellows may also be able to compensate for slight misalignments or angular offsets between the inner tube <NUM> and the outer tube <NUM>, that could otherwise result in internal stresses in the heating assembly <NUM> introduced during assembly of the heating assembly <NUM> or during thermal loading of the heating assembly <NUM>. As such, using a bellows may further reduce internal stresses in the heating assembly <NUM>, thereby improving the reliability and durability of heating assembly <NUM>. For example, the extendible section <NUM> can comprise a bellows formed by a tube comprising a plurality of corrugations, e.g. two or more corrugations, between five and twenty corrugations, up to thirty corrugations, or more, depending on the total length of extendible section <NUM>. For example, the bellows can comprise five corrugations, seven corrugations, ten corrugations, fourteen corrugations, twenty corrugations, or twenty four corrugations. The bellows can for example be made of an electrically conductive material, such as stainless steel or other hard metal alloy, for the bellows to also work as an auxiliary heating tube. Bellows dimensions can be restricted by the size of the primary heating tube <NUM> to create a good auxiliary heating solution.

The corrugation amplitude 127A extends in a radial direction, e.g. between an inner diameter of extendible section <NUM> and an outer diameter of extendible section <NUM>. For example, for a bellows having an inner diameter of <NUM> millimeter and an outer diameter of <NUM> millimeter, corrugation amplitude 127A is equal to <NUM> millimeter. As another example, for a bellows having an inner diameter of <NUM> millimeter and an outer diameter of <NUM> millimeter, corrugation amplitude 127A is equal to <NUM> millimeter. The corrugation pitch 127P extends in an axial direction, e.g. forming at least part of the axial length of extendible section <NUM>. For example, for a bellows having seven corrugations spanning a corrugated axial length of <NUM> millimeter, corrugation pitch 127P is equal to <NUM> millimeter. As another example, for a bellows having fourteen corrugations spanning a corrugated axial length of <NUM> millimeter, corrugation pitch 127P is equal to <NUM> millimeter.

The series of corrugations can for example be single-sided, e.g. having corrugations <NUM> either all extending radially inward or all extending radially outward. Alternatively, the series of corrugations can be double-sided, e.g. having corrugations <NUM> extending radially inward as well as outward. While having a corrugation amplitude in the radial direction and a corrugation pitch in the axial direction, corrugations can for example comprise sharp edges in the wall of the tube, such as folds, creases, or plies. Alternatively, corrugations can comprise rounded edges in the wall of the tube, such as waves, undulations, or indents. In bellows, corrugations can sometimes also be referred to as convolutions. As such, an axially corrugated tube can provide a relative high degree of resilience of the extendible section <NUM>, e.g. combined with a relatively low degree of stiffness of the extendible section <NUM>, along the axial direction of the inner and outer tubes, thereby allowing elastic deformation of the extendible section <NUM> to compensate any thermal expansion differences between the inner tube <NUM> and the outer tube <NUM>.

Buckling, or squirming, of the extendible section <NUM> may occur under compressive loading of the extendible section <NUM>, e.g. when the extendible section <NUM> is shortened to compensate for an axial expansion difference between the inner tube <NUM> and the outer tube <NUM>. Buckling may be a result of a relatively low lateral stiffness of the extendible section <NUM>, e.g. caused by a relatively long and slender extendible section <NUM>, in combination with a lateral load on the extendible section <NUM>, e.g. due to small assembly misalignments of the extendible section <NUM> or due to the own weight of the extendible section <NUM> under specific orientations with respect to gravity. Buckling may cause a number of corrugations to shift laterally away from the axial centerline of the outer tube <NUM>, thereby potentially short circuiting and/or damaging the heating assembly <NUM>. To avoid buckling while allowing the extendible section <NUM> to compensate thermal expansion differences between the inner tube <NUM> and the outer tube <NUM>, and while allowing transfer of auxiliary heat from the auxiliary heating tube <NUM> to the primary heating tube <NUM>, a design of the extendible section <NUM> may include optimization for the number of corrugations of the extendible section <NUM>, the inner and outer diameter of the extendible section <NUM>, the wall thickness of the extendible section <NUM> and/or the total length of the extendible section <NUM>.

In some further embodiments, the series of corrugations <NUM> have a uniform corrugation amplitude 127A. For example, the series of corrugations <NUM> have a uniform corrugation amplitude 127A along the axial length of the extendible section <NUM>, such that the corrugation amplitude 127A runs between lines parallel to the centerline of the extendible section <NUM> on the inside and on the outside of the extendible section <NUM>. A uniform corrugation amplitude can e.g. be uniform along the axial length of the extendible section <NUM> within <NUM>%, preferably within <NUM>%. For example, each corrugation of the series of corrugations <NUM> can comprise a uniform corrugation amplitude 127A, which is uniform along the axial length of the extendible section <NUM>, between about <NUM> millimeter and <NUM> millimeter, or between about <NUM> millimeter and <NUM> millimeter. Accordingly, the stiffness and resilience characteristics of the extendible section <NUM> can be constant along the axial length of the extendible section <NUM>. As a result, elastic deformation of the extendible section <NUM>, to compensate thermal expansion differences between the inner tube <NUM> and the outer tube <NUM>, can be evenly distributed along the axial length of the extendible section <NUM>, to minimize fatigue.

Additionally, or alternatively, the series of corrugations <NUM> have a uniform corrugation pitch 127P. For example, each corrugation of the series of corrugations <NUM> can comprise a uniform corrugation pitch 127P, which is uniform along the axial length of the extendible section <NUM>, equal to <NUM> millimeter. Similarly, each corrugation <NUM> can comprise a uniform corrugation amplitude 127P equal to e.g. <NUM> millimeter or <NUM> millimeter. Accordingly, the stiffness and resilience characteristics of the extendible section <NUM> can be constant along the axial length of the extendible section <NUM>. As a result, elastic deformation of the extendible section <NUM>, can be evenly distributed along the axial length of the extendible section <NUM>, to further minimize fatigue.

In some embodiments, the series of corrugations <NUM> form part of the electrical path <NUM> through the primary and auxiliary heating tubes <NUM>, <NUM>. For example, the electrical path <NUM> may axially pass through the series of corrugations <NUM>, thereby following the convoluted shape of the wall of the bellows. As such, the series of corrugations <NUM> are arranged for conducting current passing along the extendible section <NUM> of the auxiliary heating tube <NUM>. As such, the series of corrugations <NUM> can be used to generate and transfer auxiliary heat to the primary heating tube <NUM>. As a result, the series of corrugations <NUM> provide a synergetic combination of the function of providing auxiliary heat and the function of compensating for thermal expansion, integrated in extendible section <NUM>, to provide a heating assembly <NUM> with a uniform temperature distribution, that can avoid damage to heating assembly <NUM> due to cyclic thermal loading.

In other or further embodiments, e.g. as shown in <FIG>, the primary heating tube <NUM> has a first wall thickness 110T, wherein the extendible section <NUM> has a second wall thickness 125T, and wherein the second wall thickness 125T is the same or similar as the first wall thickness 110T, e.g. within a ratio between <NUM> - <NUM>, preferably between <NUM> and <NUM>. For example, for the primary heating tube <NUM> shown in <FIG> the first wall thickness 110T is about <NUM> millimeter, and the second wall thickness 125T of the extendible section <NUM> is about <NUM> millimeter. As another example, for the primary heating tube <NUM> shown in <FIG>, the first wall thickness 110T is about <NUM> millimeter, and the second wall thickness 125T of the extendible section <NUM> is about <NUM> millimeter. Accordingly, the electrical resistivity of the extendible section <NUM> can be matched with the electrical resistivity of primary heating tube <NUM>, e.g. to have approximately similar heating characteristics of extendible section <NUM> and primary heating tube <NUM>.

In some embodiments, the primary heating tube <NUM> is made of a first electrically conductive material, the extendible section <NUM> is made of a second electrically conductive material, and the second electrically conductive material is identical to the first electrically conductive material. For example, the primary heating tube <NUM> can be made of a stainless steel, and the extendible section <NUM> can be made of an identical stainless steel. As another example, the primary heating tube <NUM> can be made of a refractory metal, and the extendible section <NUM> can be made of an identical refractory metal. Accordingly, the electrical resistivity of the extendible section <NUM> can be further matched with the electrical resistivity of the primary heating tube <NUM>, e.g. to have approximately similar heating characteristics of the extendible section <NUM> and the primary heating tube <NUM>. Also, by using identical materials, the thermal expansion of the extendible section <NUM> can be matched with the thermal expansion of the primary heating tube <NUM>, to reduce the axial extension and shortening of the extendible section <NUM>, e.g. used for compensating a thermal expansion difference between the inner tube <NUM> and the outer tube <NUM>.

In some embodiments, for example as shown in <FIG>, the extendible section <NUM> has an inner diameter 125B, and the inner diameter 125B defines at least part of the radial gap G between the inner and outer wall surfaces <NUM>, <NUM> of the auxiliary and primary heating tubes <NUM>, <NUM>, respectively. In other words, the extendible section <NUM> may e.g. have an inner wall surface that at least partially defines the inner wall surface <NUM> of auxiliary heating tube <NUM>. For example, as shown in <FIG>, the inner diameter 125B can be defined as the smallest distance between corrugations <NUM> on opposing sides of the center line of the extendible section <NUM>. As a result, transfer of auxiliary heat from the extendible section <NUM> to the primary heating tube <NUM>, e.g. by radiation and/or convection, can be defined by the inner diameter of the extendible section <NUM> relative to the outer diameter of the primary heating tube <NUM>.

As also shown in <FIG>, in some embodiments, the primary heating tube <NUM> can have a first outer diameter 110D, the extendible section <NUM> can have a second outer diameter 125D, and wherein the second diameter 125D is between a factor <NUM> and <NUM> larger than the first diameter 110D. For example, the second outer diameter 125D of the extendible section <NUM> can be more than a factor <NUM> larger than the first outer diameter 110D of the primary heating tube <NUM>, e.g. such that the radial gap G between the extendible section <NUM> and the primary heating tube <NUM> provides electric insulation between the extendible section <NUM> and the primary heating tube <NUM>. For example, the second outer diameter 125D of the extendible section <NUM> can be less than a factor <NUM> larger than the first outer diameter 110D of the primary heating tube <NUM>, e.g. such that the radial gap G between the extendible section <NUM> and the primary heating tube <NUM> allows transfer of auxiliary heat from the extendible section <NUM> to the primary heating tube <NUM>, e.g. by radiation and/or convection. For example, the second outer diameter 125D of the extendible section <NUM> can be between a factor <NUM> and <NUM> larger than the first outer diameter 110D of the primary heating tube <NUM>, e.g. to allow passing of cooling medium through the radial gap G between the extendible section <NUM> and the primary heating tube <NUM> to cool the extendible section <NUM>, the auxiliary heating tube <NUM> correspondingly, and/or the primary heating tube <NUM>. As such, the second outer diameter 125D of the extendible section <NUM> can for example be dependent on the first outer diameter 110D of the primary heating tube <NUM>, e.g. to limit the size of the heating assembly <NUM>. The second outer diameter 125D is defined as the largest distance between opposing corrugations, measured along the radial direction of the extendible section <NUM>, perpendicular to the axial direction of the extendible section <NUM>.

In other or further embodiments, the inner tube <NUM> has a first axial stiffness, the extendible section <NUM> has a second axial stiffness, and the second axial stiffness is less than fifty percent of the first axial stiffness, e.g. down ten percent or even lower. Because of their ohmic heating function, the primary and auxiliary heating tubes <NUM>, <NUM> may be designed as relatively thin walled compared to other components of the heating assembly <NUM>, while also forming structural elements. As such, the primary and auxiliary heating tubes <NUM>, <NUM> can be regarded as the structurally most delicate parts of the heating assembly <NUM>. To minimize damage or deformation of the primary heating tube <NUM>, e.g. due to cyclic thermal loading, the extendible section <NUM>, forming part of the auxiliary heating tube <NUM>, can be designed having a second axial stiffness lower than a first axial stiffness of the primary heating tube <NUM>. As such, axial thermal expansion differences between the inner tube <NUM>, comprising the primary heating tube <NUM>, and the outer tube <NUM>, comprising the extendible section <NUM>, are more likely to be compensated by the extendible section <NUM>, thereby minimizing stresses in the primary heating tube <NUM>.

In yet other or further embodiments, the extendible section <NUM> is arranged for providing a sealed environment between the inner tube <NUM> and the outer tube <NUM>. For example, the extendible section <NUM> can be a tube-like structure that is axially connected e.g. welded, brazed, or pressed, to another part of the outer tube <NUM> around the circumference, to provide a sealed environment between inner tube <NUM> and outer tube <NUM>. Accordingly, the sealed environment can for example be used for receiving coolant medium inside the heating assembly <NUM>, e.g. along a cooling path that passes though the radial gap G past the inner surface <NUM> of the auxiliary heating tube <NUM> and past the outer surface <NUM> of the primary heating tube <NUM>, for example to cool these heating tubes <NUM>, <NUM>.

<FIG> illustrate a device <NUM>, e.g. an inlet for a gas chromatograph, and <FIG> illustrate another device <NUM>, e.g. a cold trap for a gas chromatograph, comprising the heating assembly <NUM> described herein. For example, when the device <NUM> is an inlet for injection of an analyte into a gas chromatography system, the heating assembly <NUM> can e.g. be arranged for heating the analyte before or during injection. As another example, when the device <NUM> is a cold trap - sometimes referred to as a cryogenic trap - in which an analyte in the column of a gas chromatography system is first cooled and then rapidly heated to narrow the chromatographic band an improve the detection limit, the heating assembly <NUM> can e.g. be arranged for heating the analyte. Other types of devices <NUM> comprising the heating assembly <NUM>, e.g. in the field of chromatography, can be envisioned by the person skilled in the art. Accordingly, the reliability and durability of the device <NUM> can be improved by comprising the heating assembly <NUM> arranged for compensating thermal expansion differences, e.g. caused by heating and cooling cycles of the device <NUM>, while having a uniform temperature distribution in the whole part thereof.

In some embodiments, e.g. as shown in <FIG> and 6A-B, the device <NUM> comprises a temperature sensor <NUM> arranged for measuring a temperature of the primary heating tube <NUM>. For example, the temperature sensor <NUM> can be a thermocouple, such as an N- or K-type thermocouple, or a high temperature resistance temperature detector (RTD), or any other suitable type of temperature sensor. To measure the temperature of the primary heating tube <NUM>, the temperature sensor <NUM> can be mechanically attached to a central area of the primary heating tube <NUM>. Alternatively, the temperature sensor <NUM> can be mechanically attached to a distal area of the primary heating tube <NUM>. As such, based on the measured temperature of the primary heating tube <NUM>, the temperature sensor can e.g. be used for determining the temperature of the heating assembly <NUM> and/or e.g. an analyte heated by the heating assembly <NUM>.

In other or further embodiments, the device <NUM> can comprise a controller <NUM> arranged for controlling an electric current passing through the electrical path <NUM> based on measurements from the temperature sensor <NUM>. For example, the controller <NUM> can be part of a software workstation, a computer, or a processing unit. The controller <NUM> can for example be arranged on board of the device <NUM>, e.g. as an integrated part, or can be a separate unit, operatively connected to the device <NUM>. As such, heating of the device <NUM> can be controlled by the controller <NUM> based on actual measurements of the temperature sensor <NUM> in an accurate and automated fashion, e.g. by having closed loop temperature control of the device <NUM> to follow a specific heating profile. Accordingly, heating of an analyte by the device <NUM> can be performed with sophisticated control.

In yet other or further embodiments, e.g. as shown in detail in <FIG> and <FIG>, the device <NUM> can comprise a cooling circuit <NUM>, arranged for cooling the inner and outer tubes <NUM>, <NUM>. In some embodiments, the cooling circuit <NUM> can e.g. extend between an inlet <NUM>, such as an inlet duct or pipe, and an outlet <NUM>, such as an outlet duct or pipe, disposed on the outer tube <NUM>, such that the cooling circuit <NUM> passes through the radial gap G between the inner wall <NUM> of the auxiliary heating tube <NUM> and the outer wall <NUM> of the primary heating tube <NUM>. The outlet can also be e.g. a cutout, slot, or hole in the heating assembly <NUM>, for example provided in the outer tube <NUM> with the extendible section <NUM> arranged between the inlet <NUM> and the outlet <NUM>, such that cooling medium passing along the cooling circuit <NUM> is brought into contact with the inner wall <NUM> of the auxiliary heating tube <NUM>, and correspondingly the extendible section <NUM>. Alternatively, or additionally, the inner tube <NUM> and/or outer tube <NUM> can comprise cooling fins, e.g. extending within the radial gap G and/or extending outward beyond an external surface of the outer tube <NUM>, and the cooling circuit <NUM> can be arranged for cooling the inner and outer tubes <NUM>, <NUM> by passing cooling medium past the cooling fins. As such, cooling of the device <NUM>, e.g. from a heated temperature back to room temperature or to a lower temperature, can be performed more rapidly compared to conventional heating devices, by having both the inner tube <NUM> and outer tube <NUM> cooled simultaneously. For example when the device <NUM> is used as an inlet, this may reduce the time between injection of samples into a gas chromatography system, thereby increasing throughput. For example when the device <NUM> is used as a cold trap, this may increase the rate of cooling of the analyte.

In some further embodiments, the cooling circuit <NUM> can be arranged for allowing cooling medium to flow through the radial gap G between the inner and outer wall surfaces <NUM>, <NUM> of the auxiliary and primary heating tubes <NUM>, <NUM>, respectively. Any suitable cooling medium can be supplied via cooling inlet <NUM>, such as compressed air, CO<NUM>, liquid nitrogen, and/or cold dry gas, e.g. air or nitrogen , for example. Accordingly, cooling medium can flow past and come into contact with the inner and outer wall surfaces <NUM>, <NUM> of the auxiliary and primary heating tubes <NUM>, <NUM>, respectively, to simultaneously cool the auxiliary and primary heating tubes <NUM>, <NUM>, to increase the cooling rate of the device <NUM>.

In some embodiments, the device <NUM> may form an inlet <NUM> for a gas chromatography system. The inlet <NUM> can for example be used as a sample introduction port, e.g. for a gas chromatograph. It may comprise a few weldments, such as a main heater tube weldment, a second heater weldment, and a head weldment. The main heater weldment is built around the primary heating tube <NUM>, shown in <FIG>. The primary heating tube <NUM> can e.g. be welded or brazed to the inlet top <NUM> that may act as a base for mounting of the inlet <NUM> and for electrical <NUM>, carrier gas split line <NUM> and cooling medium <NUM> connections. A lower part of the primary heating tube <NUM> may e.g. be welded or brazed to a bottom inlet plate <NUM>. The bottom part of the main heater weldment can provide a connection to a separation column <NUM>. The separation column <NUM> can for example be connected using a ferrule <NUM> and a nut <NUM> to ensure a leak tight connection.

The second heater weldment for example comprises a bellows <NUM> with an extension <NUM>, an outer tube <NUM> and a lower electrode <NUM>. The bellows <NUM> can act as a thermal expansion compensation means but also as an auxiliary heating tube <NUM>, e.g. to ensure good temperature profile at a bottom section of the inlet <NUM>. In general, the bellows <NUM> can be placed anywhere along the second heater weldment length, for instance, just below the lower electrode <NUM>. In such case the auxiliary heating tube <NUM> at the bottom of the inlet <NUM> can e.g. be made out of thin walled material. The second heater weldment can for example be brazed or welded to a main heater weldment bottom plate <NUM>, forming a base for the inlet <NUM>. A ceramic or a high temperature plastic ring <NUM> may be placed between the upper and lower electrodes <NUM>, <NUM> of the heater weldments to provide electrical insulation between them. The ring <NUM> may not only act as an insulating part, but can e.g. also ensure leak tightness at the top of an inlet cooling chamber provided between the inner and outer walls <NUM>, <NUM> of the auxiliary and primary heating tubes <NUM>, <NUM>, respectively. The inlet <NUM> can for example be cooled using any suitable cooling medium supplied via a cooling inlet pipe <NUM>. Cooling medium can escape the cooling chamber via hole <NUM> at the bottom of the bellows extension <NUM>. To create a mechanically stable construction, the primary heating tube <NUM> and the second heater weldments can for example be mounted in one assembly using ceramic or high temperature plastic washers <NUM> and screws <NUM>. To determine the temperature of the inlet <NUM>, a temperature sensor <NUM> can be mechanically attached to a central area of the primary heating tube <NUM>.

In some embodiments, the head weldment can permit opening the inlet port to exchange a liner and to place a sample, e.g. for heating and injecting an analyte to be analyzed by a chromatography system. The liner is typically a glass or metal tube, which can be inserted into the inlet <NUM> to ensure an inert environment for the introduction of a sample into the separation column <NUM>. The head weldment may comprise a top boss <NUM>, a carrier gas supply inlet <NUM>, a high temperature rubber septum <NUM>, a septum nut <NUM>, and a needle guide <NUM>. To ensure leak tightness of the inlet <NUM>, a high temperature rubber O-ring <NUM> can be placed on the liner and in-between the top boss <NUM> and the inlet top <NUM>. Accordingly, a liquid sample can be introduced into the inlet <NUM> using a syringe via an opening in the head weldment, which is sealed by the septum <NUM>.

Accordingly, while ensuring a uniform temperature distribution during heating of a sample, the inlet <NUM> can have improved reliability and durability by comprising a heating assembly <NUM> with an extendible section <NUM> arranged for compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading of the inlet <NUM>.

In other or further embodiments, the device <NUM> may form a cold trap <NUM>, sometimes referred to as a cryogenic trap, for a gas chromatography system. The cryogenic trap <NUM> can for example be used for cooling a fraction of a separation column <NUM> inserted through a primary heating tube <NUM>, see <FIG>. A purpose behind this is to improve the separation of low boiling temperature sample components, i.e. volatiles. The trap main heater weldment may consist of a primary heating tube <NUM>, which can e.g. be welded or brazed on both ends to a bellows <NUM>, for example via small center rings. The bellows <NUM> can act as a thermal expansion compensation means but also as an auxiliary heating tube <NUM>, e.g. to ensure a uniform temperature distribution along the length of the primary heating tube <NUM> of the cold trap <NUM>. Axial ends of the bellows <NUM> can e.g. be welded or brazed to electrodes <NUM>, e.g. arranged for providing electrical connections to a current source via wires <NUM>. The electrodes <NUM> are e.g. rigidly mounted onto a cooling chamber body <NUM> via insulating washers <NUM> which may be made out of a high temperature plastic, e.g. having a melting temperature of at least <NUM> degrees Celsius. The washers <NUM> can for example act not only as insulating parts but may also ensure leak tightness of the cooling chamber. Having a leak tight cooling chamber can significantly increase the cooling performance of the heating assembly <NUM> and/or the cold trap <NUM>. The cold trap <NUM> can be cooled using any suitable cooling medium, which may be supplied via a cooling inlet <NUM>. Cooling medium can e.g. escape the cooling chamber via an exhaust pipe <NUM>. To measure the temperature of the cold trap <NUM>, a temperature sensor <NUM> can e.g. be mechanically attached to a central area of the primary heating tube <NUM>.

As such, the cold trap <NUM> can for example be formed by a device <NUM> comprising a heating assembly <NUM> with an extendible section <NUM> at each axial end of the cold trap <NUM>, to ensure a uniform temperature distribution during heating of a sample after cryogenic trapping of the sample, while having improved reliability and durability by compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading.

<FIG> illustrates a chromatography system <NUM> comprising a heating assembly <NUM> and/or a device <NUM>. The chromatography system <NUM> can for example be arranged for separating components of a mixture, and the heating assembly <NUM> and/or the device <NUM> may e.g. form an inlet <NUM> for introducing a sample to be analyzed by the chromatography system <NUM> and/or a cold trap <NUM> for improving the separation of low boiling temperature sample components. In gas chromatography, the mixture is vaporized and carried through a stationary phase, e.g. a metal or glass separation column <NUM>, with a carrier gas, typically helium, hydrogen, nitrogen or argon. Larger molecules in the mixture take longer to pass from the inlet through the column and reach the detector <NUM> at the far end. Typical inlet pressure can range from several kilopascal and up to <NUM> kilopascal. Inlet pressure values can e.g. be used for controlling gas flow through the column in the range <NUM> - <NUM> milliliter per minute, or higher. The column <NUM> typically has an internal diameter ranging between <NUM> millimeter and <NUM> millimeter, and a length ranging between <NUM> centimeter up to <NUM> meter, or more.

For example, the chromatography system <NUM> can comprise a heating assembly <NUM> as part of one or more devices <NUM> of the chromatography system <NUM>. The device <NUM> can e.g. be an inlet <NUM>, for injecting a sample into chromatography system <NUM> and/or a cold trap <NUM>, for cooling the sample downstream the inlet <NUM>. The chromatography system <NUM> can further comprise e.g. a column <NUM>, extending between the inlet <NUM> and a detector <NUM>. The detector <NUM> can for example be arranged for analyzing the sample passing through the column <NUM>.

Accordingly, while ensuring uniform temperature distribution during heating and/or reheating of samples, the chromatography system <NUM> can have improved reliability and durability by comprising one or more heating assemblies <NUM> and/or devices <NUM> with one or more extendible sections <NUM> arranged for compensating internal thermal expansion differences, e.g. caused by cyclic thermal loading.

Other aspects of the present disclosure relate to use of the heating assembly <NUM> and/or the device <NUM> described herein as an inlet and/or a cold trap in a chromatography system. For example, the heating assembly <NUM> and/or device <NUM> can be used as an inlet and/or a cold trap in a laboratory setting, or other analytical setup or configuration for a chromatography system, e.g. for analyzing a sample by separating components of a mixture.

The various elements of the embodiments as discussed and shown offer certain advantages, such as improved heating uniformity as well as improved structural reliability of a temperature controlled device. It is appreciated that this disclosure offers particular advantages to gas chromatography, and in general can be applied also in other systems and methods employing a heating and/or cooling tube - particularly those applications benefitting from rapid heating and/or cooling.

Claim 1:
A heating assembly (<NUM>), comprising
a primary heating tube (<NUM>) made of electrically conductive material and forming at least part of an inner tube (<NUM>);
an auxiliary heating tube (<NUM>) made of electrically conductive material and forming at least part of an outer tube (<NUM>);
a pair of flanges (<NUM>) which mechanically and electrically interconnect the inner tube (<NUM>) with the outer tube (<NUM>), wherein the inner and outer tubes (<NUM>,<NUM>) extend between the flanges (<NUM>) with the inner tube (<NUM>) arranged inside the outer tube (<NUM>), wherein the primary and auxiliary heating tubes (<NUM>, <NUM>) axially overlap at least along a subsection length (L) of the inner and outer tubes (<NUM>,<NUM>), respectively, between the flanges (<NUM>);
wherein, along the subsection length (L), an inner wall surface (<NUM>) of the auxiliary heating tube (<NUM>) faces an outer wall surface (<NUM>) of the primary heating tube (<NUM>) for transferring auxiliary heat from the auxiliary heating tube (<NUM>) to the primary heating tube (<NUM>) over a radial gap (G) between the inner and outer wall surfaces (<NUM>, <NUM>); and
a pair of electrodes (<NUM>, <NUM>), connected to respective parts of the electrically conductive material and arranged for forming an electrical path (<NUM>) through the primary and auxiliary heating tubes (<NUM>, <NUM>) via the electrically conductive material;
wherein the outer tube (<NUM>) comprises an electrically insulating section (<NUM>) arranged for mechanically connecting, while electrically separating, parts of the electrically conductive material, to have the electrical path (<NUM>) exclusively passing in series through respective parts of the primary and auxiliary heating tubes (<NUM>, <NUM>);
characterized in that the auxiliary heating tube (<NUM>) comprises an extendible section (<NUM>), configured to axially extend or shorten for compensating any difference in thermally induced contraction or expansion, respectively, between the inner tube (<NUM>) and the outer tube (<NUM>).