Heat exchanger comprising a fiber optical sensor for determining the tube wall thickness of a heat transfer tube

Abstract

The invention relates to a heat exchanger (1), more particularly a high-pressure heat exchanger for urea synthesis, having a plurality of heat transfer tubes (3) for transporting a first fluid (F1) in order to transfer heat between the first fluid (F1) and a second fluid (F2) via the heat transfer tubes (3). In order to improve the availability, the invention proposes that on one or more of the heat transfer tubes (3), an optical fiber sensor (2) is arranged in each case, the optical fiber sensor (2) being designed to ascertain elastic vibrations, more particularly natural vibrations, of the respective heat transfer tube (3) during operation of the heat exchanger (1) by means of interference, and the optical fiber sensor (2) being designed to ascertain the elastic vibrations, more particularly natural vibrations, of the respective heat transfer tube (3) during operation of the heat exchanger (1). In order to determine the tube wall thickness of the respective heat transfer tube (3) during operation of the heat exchanger (1). The invention further relates to a method for operating a heat exchanger (1).

Description

Technical Field

[0001] The present invention relates to a heat exchanger comprising a plurality of heat transfer tubes for conveying a first fluid so as to transfer heat between the first fluid and a second fluid via the heat transfer tubes, in particular a high-pressure heat exchanger for urea synthesis.

[0002] The invention furthermore relates to a method for operating a heat exchanger. Background Art

[0003] From the prior art it is known to use a heat exchanger for exchanging thermal energy between a first fluid flow and a second fluid flow. The heat exchanger generally comprises a plurality of heat transfer tubes for conveying a first fluid flow so as to transfer heat via the heat transfer tubes to a second fluid flow flowing around the heat transfer tubes, or to absorb heat from said second fluid flow via the heat transfer tubes. As part of the urea synthesis, heat exchangers are generally used in which the first fluid flow or the second fluid flow has a high pressure (generally greater than 30 bar) and a high temperature (generally greater than 80° C.). The conveyance of the first fluid flow through the heat transfer tubes is thus generally associated with the removal of wall material of the heat transfer tubes (in particular corrosive removal and / or erosive removal) or the formation of deposits inside the heat transfer tubes, respectively, so that the wall thickness of the heat transfer tubes changes during operation of the heat exchanger, generally decreases. A reduction in the integrity of the wall of the heat transfer tubes may be problematic for the operational safety of the heat exchanger. Therefore, based on chronological maintenance intervals, it is generally necessary to shut down the heat exchanger regularly and measure the wall thickness of the heat transfer tubes.

[0004] For this purpose, measuring probes are usually inserted into the corresponding heat transfer tubes in the non-operating state of the heat exchanger in order to determine the inner radius or the wall thickness of the heat transfer tubes. Measuring probes comprising ultrasonic sensors, optical sensors or eddy current sensors are known in order to determine the wall thickness.

[0005] Particularly in the case of heat exchangers operating at pressures of the first and / or second fluid flows greater than 30 bar and temperatures greater than 80° C., such service interruptions for determining the wall thickness of the heat transfer tubes are often time-consuming and laborious and are associated with high costs. Summary of the invention

[0006] This is achieved by the present invention.It is an object of the present invention to specify a heat exchanger of the type named at the outset with optimized availability, in particular optimized operation.

[0007] It is also an object of the invention to specify a method for operating a heat exchanger making it possible to optimize the use or operation of the heat exchanger.

[0008] According to the invention, the object is achieved in that, for a heat exchanger of the type named at the outset, optical fiber sensors are respectively arranged on one or more of the heat transfer tubes, wherein the optical fiber sensors are designed to detect elastic vibrations, in particular natural vibrations, of the corresponding heat transfer tubes by interference during operation of the heat exchanger in order to determine the tube wall thickness of the corresponding heat transfer tubes during operation of the heat exchanger.

[0009] The basis of the invention is the idea of ​​increasing the availability of a heat exchanger, in particular a heat exchanger designed for high operating pressures and / or high operating temperatures of a first fluid and / or a second fluid, in that the wall thickness of at least one, preferably a plurality of, heat transfer tubes of the heat exchanger is determined during operation of the heat exchanger. As a result, operation, in particular process management and / or maintenance of the heat exchanger can be carried out according to the wall thickness determined using the fiber optic sensor. In particular, it is not necessary to interrupt the operation of the heat exchanger for determining the wall thickness. This can be practically achieved if, in particular as described, the fiber optic sensor is arranged on one or more of the heat transfer tubes, respectively. In fact, the wall thickness of the tube wall of the respective heat transfer tube can be determined during operation of the heat exchanger using the respective fiber optic sensor, in that the elastic vibrations, in particular the frequencies of the elastic vibrations, of the respective heat transfer tube are detected interferometrically using the fiber optic sensor. The elastic vibrations are typically elastic natural vibrations of the heat transfer tube, or the frequencies are natural frequencies of the heat transfer tube. Conveniently, a plurality of elastic natural vibrations or a plurality of natural frequencies of the respective heat transfer tube can be detected using the fiber optic sensor in order to determine the wall thickness.

[0010] The operation of the heat exchanger means a state in which a first fluid is conducted through a heat transfer tube so as to exchange heat with a second fluid via the heat transfer tube. High operating pressure and high operating temperature respectively mean an operating pressure of greater than 30 bar of the first fluid and / or the second fluid and an operating temperature of greater than 80°C of the first fluid and / or the second fluid. In particular, the operating pressure is between 30 bar and 200 bar, preferably about 180 bar, and / or the operating temperature is between 80°C and 300°C, preferably about 230°C. Typically, the first fluid and / or the second fluid has this type of operating pressure and this type of operating temperature during operation of the heat exchanger, or the heat exchanger is designed for this type of operation. Therefore, it is beneficial if the corresponding optical fiber sensor is designed to be used or measured at an operating pressure and an operating temperature corresponding to the operating pressure and the operating temperature, respectively. The heat exchanger is preferably a high-pressure heat exchanger. Preferably, the heat exchanger or the optical fiber sensor is designed to determine the wall thickness of the corresponding heat transfer tube due to in-situ determination and in operation, or the wall thickness of the corresponding heat transfer tube is determined in-situ and in operation using the optical fiber sensor.

[0011] It is particularly advantageous if the heat exchanger is a stripper for performing stripping. It is advantageous if the stripper is used for urea synthesis. The stripper can be implemented to synthesize urea by stripping, usually inside heat transfer tubes.

[0012] It is advantageous if the fiber optic sensor comprises an optical measuring fiber constituting an optical measuring section and an optical reference fiber constituting an optical reference section, wherein the measuring fiber is connected to the heat transfer tube in a vibration-transmitting manner, preferably wound around the heat transfer tube, so as to detect the interference signal created by the electromagnetic waves guided along the measuring section and the electromagnetic waves guided along the reference section using the detector of the fiber optic sensor. The elastic vibrations of the heat transfer tube can thus be detected in a practical manner. The use of optical fibers to realize the measuring section and the reference section enables highly robust measurements, which are generally necessary under the conditions of use in heat exchangers, in particular high-pressure heat exchangers. In this document, in particular hereinafter, for better readability, in particular without changing the terminology, the optical measuring fiber and the optical reference fiber as well as the optical measuring section and the optical measuring section are also simply referred to as the measuring fiber and the reference fiber as well as the measuring section and the reference section.

[0013] Typically, a mass change of the corresponding heat transfer tube (in particular a change in the wall thickness) leads to an elastic vibration, in particular a natural vibration, of the heat transfer tube. By detecting or measuring the elastic vibration, in particular its frequency or natural frequency, the wall thickness of the heat transfer tube can be determined. It is generally provided that the elastic vibration of the heat transfer tube generates a change in the length of the measuring section (in particular the measuring fiber). Conveniently, the measuring fiber can be correspondingly connected to the heat transfer tube in a vibration-transmitting manner (usually in a material bonding manner) in order to achieve this. Typically, the measuring fiber is connected to the corresponding heat transfer tube so that an expansion or contraction of the heat transfer tube associated with the elastic vibration of the heat transfer tube leads to an elastic expansion and relaxation, in particular a change in length, of the measuring fiber corresponding to the vibration of the heat transfer tube. The reference fiber is generally arranged (in particular connected to the corresponding heat transfer tube) so that the length of the reference fiber is substantially unaffected by a mass change or a change in the elastic vibration of the heat exchanger tube. Preferably, the reference fiber and the measuring fiber are arranged so that the fibers are substantially subject to the same temperature influence and / or pressure influence. The measuring fiber and the reference fiber (in particular their respective interaction sections) are generally arranged on the same heat transfer tube or connected to the same heat transfer tube. Generally, the electromagnetic wave conducted on the measuring fiber or along the measuring section is called the measuring wave, and the electromagnetic wave conducted on the reference fiber or along the reference section is called the reference wave. The measuring wave and the reference wave may have a path difference, in particular a phase difference, caused by the elastic vibration of the heat transfer tube. The path difference or phase difference thus generally depends on the frequency, in particular the natural frequency, of the elastic vibration of the heat transfer tube. It is generally provided that the measuring fiber and the reference fiber are coupled to each other so that the measuring wave and the reference wave interfere with each other to create an interference signal. The interference signal (also referred to as an electromagnetic interference wave) generally depends on the path difference, in particular the phase difference, of the electromagnetic wave (in particular the measuring wave and the reference wave) so that the interference signal corresponds to the elastic vibration of the heat transfer tube, in particular the frequency or natural frequency of the elastic vibration. The phase difference thus generally depends on the mass of the heat transfer tube, wherein the mass is generally related to the natural frequency or resonant frequency of the elastic vibration of the heat transfer tube. It should be understood that the optical fiber sensor can be implemented accordingly. The measuring wave and the reference wave are generally implemented to be coherent with each other so as to create an interference signal by interference after passing through the measuring section and the reference section. It has proven to be effective if the measuring wave and the reference wave originate from a shared electromagnetic emission source, so that the measuring wave and the reference wave are formed such that they are coherent with each other.

[0014] Typically, one or more frequencies (particularly natural frequencies (also referred to as resonant frequencies)) of the elastic vibrations of the heat transfer tube are detected using an optical fiber sensor. The elastic vibrations of the heat transfer tube (particularly its frequency or natural frequency) are generally related to the wall thickness of the heat transfer tube, so that by detecting or measuring the elastic vibrations (particularly its frequency or natural frequency), the wall thickness of the heat transfer tube can be determined. Typically, the frequency (particularly the natural frequency) increases as the wall thickness of the heat transfer tube decreases. In particular, it has been shown that there is generally a substantially linear relationship between the frequency (particularly the natural frequency) of the elastic vibrations of the heat transfer tube and the wall thickness of the heat transfer tube. In this way, a robust evaluation can be implemented practically. Conveniently, the intensity of the interference signal can be detected in a time-dependent manner. The detected intensity of the interference signal can be converted into a mathematical frequency space for evaluation (generally with the aid of a Fourier transform) in order to detect the natural frequency, in particular a plurality of natural frequencies. The wall thickness can be determined from one or more natural frequencies. Conveniently, the optical fiber sensor may include one or more detectors to detect the interference signal. The evaluation of the interference signal, in particular the detection of the natural frequency or the determination of the tube wall thickness, can be performed using an electronic data acquisition unit. The electronic data acquisition unit can be part of the heat exchanger, in particular part of the fiber optic sensor. The electronic data acquisition unit can be designed for computer-aided evaluation and, in particular, can be formed such that it includes a microcontroller. The accuracy of the determination of the tube wall thickness of the heat transfer tube is typically less than 100 μm.

[0015] The measuring fiber and the reference fiber are usually embodied to conduct electromagnetic waves, in particular to conduct measuring waves and reference waves, respectively. The measuring fiber and the reference fiber are usually dielectric waveguides, which are usually formed such that they comprise or consist of quartz glass or plastic (preferably fiber optic cable). It is advantageous if, in the non-operating state of the heat exchanger, the measuring fiber or the measuring section and the reference fiber or the reference section have substantially equal lengths.

[0016] The measuring fiber is usually connected to the heat transfer tube in a vibration-transmitting manner so that the length of the measuring fiber or the length of the measuring section changes correspondingly to the elastic vibration of the heat transfer tube (in particular the frequency of the elastic vibration) during the operation of the heat exchanger. It is beneficial if for this purpose, the measuring fiber includes an interaction section connected to the heat transfer tube in a vibration-transmitting manner. The interaction section can be wound around the circumference of the heat transfer tube at least once, preferably multiple times. The interaction section of the measuring fiber can form one or more windings around the circumference of the heat transfer tube, in particular between 2 windings and 10 windings, preferably between 4 windings and 7 windings, usually about 5 windings. In this way, the elastic vibration of the heat transfer tube can act efficiently on the measuring fiber during the operation of the heat exchanger. Alternatively or cumulatively, the interaction section can have a meandering shape. In a simple embodiment, the interaction section can be shaped as a straight line. The measuring fiber, in particular its interaction section, can be connected to the heat transfer tube in a vibration-transmitting manner by material bonding (in particular by means of an adhesive). For this purpose, the adhesive can be implemented to produce a rigid connection between the measuring fiber, in particular its interaction section, and the heat transfer tube.

[0017] It is advantageous if the reference fiber is connected to the heat transfer tube in a vibration-decoupled manner, preferably wound around the heat transfer tube. This is usually achieved because during operation of the heat exchanger, the length of the reference fiber or the length of the reference section does not substantially change with the elastic vibrations of the heat transfer tube, in particular the frequency of the elastic vibrations. For this purpose, the reference fiber can include an interaction section connected to the heat transfer tube in a vibration-decoupled manner. In this way, similar boundary conditions can be achieved for the measuring fiber or measuring section and the reference fiber or reference section, so as to achieve a high accuracy in determining the tube wall thickness using the sensor. The interaction section can be wound around the circumference of the heat transfer tube at least once, preferably multiple times. The interaction section of the reference fiber can form one or more windings around the circumference of the heat transfer tube, in particular between 2 windings and 10 windings, preferably between 4 windings and 7 windings, typically about 5 windings. Alternatively or cumulatively, the interaction section can have a meandering shape. In a simple embodiment, the interaction section can be shaped as a straight line. The measuring fiber, in particular its interaction section, can be connected to the heat transfer tube in a vibration-decoupled manner by material bonding, in particular by means of an adhesive. For this purpose, the adhesive can produce an elastic connection between the measuring fiber and the heat transfer tube, in particular its interaction section. It has proven to be effective if the adhesive is formed so that it includes polydimethylsiloxane (PDMS). Preferably, the interaction section of the measuring fiber and the interaction section of the reference fiber have the same shape. It is advantageous if the interaction section of the measuring fiber and the interaction section of the reference fiber form an equal number of windings around the heat transfer tube. Typically, the interaction section of the measuring fiber and the interaction section of the reference fiber are arranged adjacent to each other on the same heat transfer tube, in particular connected to the same heat transfer tube, in particular along the longitudinal extension of the heat transfer tube. It has proven to be effective if the spacing between the interaction sections is thereby less than 30 mm, preferably less than 10 mm. Typically, the spacing is thereby between 1 mm and 30 mm, preferably about 5 mm.

[0018] The heat transfer tube is usually implemented for conducting a first fluid in order to transfer heat between the first fluid and the second fluid through the tube wall of the heat transfer tube. It is preferably provided that during operation of the heat exchanger, the second fluid is in contact with the heat transfer tube or the tube wall, in particular directly in contact with the heat transfer tube or the tube wall. The first fluid is usually a first fluid flow conducted through the heat transfer tube during operation of the heat exchanger. During operation of the heat exchanger, the second fluid may be a second fluid flow that usually flows around the heat transfer tube.

[0019] The heat exchanger generally comprises a fluid chamber for accommodating a second fluid, wherein a heat transfer tube extends inside the fluid chamber. The fluid chamber generally forms a fluid chamber cavity between a fluid chamber wall of the fluid chamber and a heat transfer tube, so as to accommodate the second fluid with the fluid chamber cavity for transferring heat between the first fluid and the second fluid. The heat transfer tube generally extends through the fluid chamber cavity. Generally, it is provided that during operation of the heat exchanger, the second fluid is conducted through the fluid chamber cavity, in particular so that the second fluid flows around the heat transfer tube. Conveniently, the fluid chamber cavity can be implemented in the form of one or more channels, so as to conduct the second fluid using the channel during operation of the heat exchanger. The fluid chamber generally comprises at least one fluid chamber inlet and at least one fluid chamber outlet, so as to conduct the second fluid into the fluid chamber (in particular the fluid chamber cavity) via the fluid chamber inlet, and generally after heat transfer has occurred between the first fluid and the second fluid, the second fluid is removed from the fluid chamber (in particular the fluid chamber cavity) again via the fluid chamber outlet. The fluid chamber is generally formed so that it comprises the following components, in particular made of the following components: metal, preferably an iron alloy, particularly preferably steel.

[0020] Typically, the heat transfer tubes are spaced apart from each other at least in sections so that during operation of the heat exchanger, a second fluid can flow between the heat transfer tubes for transferring heat therewith. This applies in particular to the interior of the fluid chamber or its fluid chamber cavity.

[0021] Typically, the first fluid and the second fluid are implemented as liquid and / or gaseous. For example, the first fluid and the second fluid can be formed so that they include the following components, in particular made of the following components: liquid water and gaseous water. It can be provided that the first fluid and the second fluid are implemented so that they include the following components, in particular made of the following components: a liquid medium and a gaseous medium, wherein the liquid medium and the gaseous medium of the corresponding fluid flow through the heat exchanger in opposite directions, typically so that they are in contact with each other. For example, the first fluid can be formed so that it includes a liquid medium and a gaseous medium, wherein in the corresponding heat transfer tubes, the media flow through the heat transfer tubes in opposite directions, so that they are in contact with each other.

[0022] The heat transfer tubes usually extend between a first tube sheet and a second tube sheet, wherein the tube sheets define a fluid chamber cavity for accommodating a second fluid, wherein the heat transfer tubes terminate in a through opening of the corresponding tube sheet, or are guided through the through opening. Usually, the fluid fed through the through opening of one of the plates is conducted to the through opening of the other tube sheet through the heat transfer tube. The heat transfer tubes are usually connected to the tube sheet in a fluid-tight manner. Usually, the corresponding tube sheet is implemented as a plate-shaped having a plurality of flow channels oriented transversely (especially orthogonally) relative to the longitudinal extension of the tube sheet, and the flow channels form corresponding through openings. The tube sheet can be implemented as part of the fluid chamber wall of the fluid chamber. The heat exchanger usually includes at least one first tube sheet of this type and at least one second tube sheet of this type. The tube sheets are usually formed so that they include the following components, in particular made of the following components: metal, preferably an iron alloy, particularly preferably steel.

[0023] In the fluid chamber, one or more fluid guiding surfaces may exist so as to define the flow path of the second fluid using the fluid guiding surface. The corresponding fluid guiding surface is generally implemented as a segmented block of the fluid flow of the second fluid between the heat transfer tubes. The fluid guiding surface may define a flow path having a plurality of deflection curves, along which the second fluid is guided from the fluid chamber inlet to the fluid chamber outlet. For example, the flow path may have a meandering shape. Typically, a plurality of heat transfer tubes pass through the corresponding guiding surface. Typically, a plurality of guiding surfaces intersecting the heat transfer tubes and spaced apart from each other are provided. The corresponding fluid guiding surface is generally oriented transversely (particularly orthogonally) relative to the longitudinal extension of the heat transfer tube. Typically, a plurality of fluid guiding surfaces spaced apart from each other in the longitudinal direction of the heat transfer tube are provided. Typically, the intermediate space between the plurality of heat transfer tubes is substantially closed by the corresponding fluid guiding surface so as to prevent the second fluid from flowing through the intermediate space. The corresponding fluid guiding surface may be implemented as a majority of the intermediate space between the heat transfer tubes closed for the flow of the second fluid in a cross section passing through the fluid chamber. The fluid guiding surface may be formed using a guiding wall arranged in the fluid chamber. The fluid guiding surfaces are usually implemented in plate shape. A fluid chamber usually comprises one or more guiding surfaces of this type.

[0024] Typically, a plurality of heat transfer tubes are interconnected by stabilizing elements in order to stabilize the heat transfer tubes during operation of the heat exchanger. The corresponding stabilizing elements can be implemented in a plate shape, wherein the longitudinal extension of the stabilizing elements is typically oriented transversely (in particular orthogonally) relative to the longitudinal extension of the heat transfer tubes connected by the stabilizing elements. Typically, the heat transfer tubes pass through the stabilizing elements. The stabilizing elements are generally referred to as baffles. Typically, a plurality of stabilizing elements that are spaced apart from each other and that connect the heat transfer tubes to each other are provided along the longitudinal extension of the heat transfer tubes. In particular, the fluid guiding surface can be formed by the stabilizing elements. The stabilizing elements can then be used both to stabilize the heat transfer tubes and to define the flow path of the second fluid.

[0025] Typically, the measuring fiber of the corresponding optical fiber sensor is connected to the corresponding heat transfer tube in the interior of the fluid chamber in a vibration-transmitting manner. If the detector of the optical fiber sensor is arranged in the fluid chamber (especially outside the fluid chamber cavity) for detecting the interference signal of the electromagnetic wave guided along the measuring section and the electromagnetic wave guided along the reference section. Typically, the electronic data acquisition unit is also arranged outside the fluid chamber (especially the fluid chamber cavity). As a result, the detector and the electronic data acquisition unit are protected from the load, especially the pressure load and / or temperature load of the first fluid and the second fluid. Typically, the measuring fiber and the reference fiber extend at least partially through the fluid chamber (especially the fluid chamber cavity) or through the second fluid during the operation of the heat exchanger. Typically, the interaction section of the measuring fiber and the interaction section of the reference fiber are connected to the corresponding heat transfer tube inside the fluid chamber. The fluid chamber may include one or more optical fiber feedthroughs, with which the measuring fiber and the reference fiber are guided through the fluid chamber wall of the fluid chamber in a fluid-tight manner, especially out of the fluid chamber.

[0026] Conveniently, the fiber optic sensor may include an electromagnetic emission source, preferably a laser, for generating and emitting electromagnetic waves (especially measuring waves and reference waves), wherein the electromagnetic emission source is coupled to the measuring fiber and the reference fiber so as to introduce the electromagnetic waves into the measuring fiber and the reference fiber. The electromagnetic waves introduced into the measuring fiber and the reference fiber are generally coherent with each other so that the electromagnetic waves can create interference or interference signals after passing through the measuring section and the reference section. It is advantageous if the electromagnetic emission source or the electromagnetic waves (especially measuring waves and reference waves) have a coherence length greater than 0.5 mm, particularly greater than 1 mm, preferably greater than 2 mm, particularly preferably greater than 5 mm. Typically, the coherence length is between 1 mm and 10 mm. Preferably, the electromagnetic emission source (especially the laser) is implemented as an electromagnetic wave, especially a light wave, emitting a wavelength between 500 nm and 2000 nm, particularly between 1000 nm and 1500 nm, preferably about 1300 nm. It is advantageous if the laser is constructed to emit electromagnetic waves with a vertical cavity surface emitting laser diode (VCSEL). The interference signal generally has a substantially periodic structure. It is beneficial if, downstream of the electromagnetic emission source in the feeding direction, an optical isolator (also referred to as an optical diode) is arranged downstream in order to minimize (in particular prevent) electromagnetic back reflections to the electromagnetic emission source. The feeding direction generally denotes the direction along which the electromagnetic waves are fed to the measuring fiber and the reference fiber using the electromagnetic emission source.

[0027] It has been shown that if the coherence length is between 10 μm and 500 μm, in particular between 20 μm and 100 μm, preferably between 25 μm and 50 μm, for example about 30 μm, the tube wall thickness can also be determined, although this is not so preferred in terms of robustness. Typically, the interference signal in this case has a non-periodic structure with maxima, in particular a plurality of maxima. The interference signal or the non-periodic structure typically comprises one or more Gaussian-like substructures. The maximum values ​​(in particular the distances between a plurality of maxima) of the interference signal (in particular the Gaussian-like substructure) typically correspond to the path difference between the measurement wave and the reference wave. It is advantageous if, in the non-operating state of the heat exchanger, the length difference between the length of the measurement fiber or measurement section and the length of the reference fiber or reference section is less than the coherence length.

[0028] It is advantageous if the measuring fiber and the reference fiber respectively comprise a reflective element or are connected to such a reflective element so as to use the reflective element to reflect the electromagnetic waves conducted along the measuring section and the reference section, in particular to reflect them back along the measuring section and the reference section, respectively. Typically, with the reflective element, the measuring wave is reflected back after passing through the measuring section, and the reference wave is reflected back after passing through the reference section, respectively along the measuring section and the reference section. The reflective element preferably comprises a reflective surface to reflect the corresponding electromagnetic waves. It is expedient if the reflective element or the reflective surface has a reflectivity greater than 90%, in particular greater than 95%, preferably greater than 98%. The reflective element can be a mirror. It has been proven to be effective if this type of reflective element is arranged at the fiber end of the measuring fiber and the reference fiber, respectively. Typically, the fiber end is one of the fiber ends of the measuring fiber and the reference fiber that is arranged downstream along the corresponding fiber in the feeding direction of the electromagnetic wave with the electromagnetic emission source. The reflective element is usually arranged downstream of the measuring section and the reference section of the corresponding fiber in the feeding direction. The reflective element can be formed so that it includes a metal layer arranged at the fiber ends of the measuring fiber and the reference fiber, in particular formed by a metal layer arranged at the fiber ends of the measuring fiber and the reference fiber. The metal layer can be formed so that it includes gold, silver and / or aluminum, in particular made of gold, silver and / or aluminum. Preferably, the metal layer is applied to the fiber ends using a sputtering method. The reflective element can be a Bragg mirror, in particular a dielectric mirror. The reflective element can be formed so that it includes one or more thin films that can be stacked on top of each other. The thin film can be applied using thin film deposition. The thin film can be formed so that it includes magnesium fluoride, silicon dioxide, tantalum oxide (V), zinc sulfide and / or titanium dioxide, in particular substantially made of magnesium fluoride, silicon dioxide, tantalum oxide (V), zinc sulfide and / or titanium dioxide.

[0029] Preferably, the optical fiber sensor can have an interferometer design according to the type of Michelson interferometer, in which electromagnetic waves travel along the measuring section via the measuring fiber and along the reference section via the reference fiber, and are reflected at the corresponding ends of the measuring section and the reference section (in particular the corresponding fiber ends) so as to superimpose the reflected electromagnetic waves to create an interference signal.

[0030] It is beneficial if the measuring fiber and the reference fiber are coupled to each other at a coupling site in order to create an interference signal using electromagnetic waves transmitted along the measuring section and electromagnetic waves transmitted along the reference section. The coupling site is usually realized by a mechanical connection of the measuring fiber and the reference fiber.

[0031] It has been proven to be effective if the fiber optic sensor includes an optical coupler having multiple input lines and multiple output lines, wherein the input lines and the output lines are connected to each other for distributed transmission of electromagnetic waves. The optical coupler can form a coupling site. Typically, the input lines and the output lines are connected so that the electromagnetic waves fed to the optical coupler via the input lines are transmitted to multiple output lines, and the electromagnetic waves fed to the optical coupler via one of the output lines are transmitted to multiple input lines. It is practical if an electromagnetic emission source is connected to one of the input lines, and the measuring fiber and the reference fiber are respectively connected to one of the output lines so that the electromagnetic waves introduced into the input line using the emission source are conducted to the measuring fiber and the reference fiber via the output line. Typically, the measuring fiber and the reference fiber are respectively connected to the output line so that the electromagnetic waves introduced into the output line via the measuring fiber and the reference fiber (especially the electromagnetic waves reflected back along the measuring fiber and the reference fiber) are transmitted to one or more of the input lines so that the electromagnetic waves are output as interference signals at the corresponding input lines. Conveniently, a detector for detecting interference signals can be arranged on one or more of the input lines. It is beneficial if a detector (which is preferably formed such that it comprises a photodiode) is connected to one or more of the input lines, respectively, so that the electromagnetic waves reflected back along the measuring fiber and the reference fiber, respectively, into the output line are detected as interference signals using the corresponding detectors at the input lines. For example, the optical coupler may comprise at least three input lines and at least two output lines, wherein the electromagnetic emission source is connected to one of the input lines and the detectors are connected to the other two input lines, respectively, and wherein the measuring fiber is connected to one of the output lines and the reference fiber is connected to the other output line.

[0032] The detector can be implemented to detect the interference signal, in particular the intensity of the interference signal, in a time-dependent manner. The detector is typically an optoelectronic detector, which is typically implemented such that it comprises a photodiode or is a photodiode. The detector is typically connected to an electronic data acquisition unit for transmitting data.

[0033] The second fluid typically has a pressure of more than 30 bar, in particular between 30 bar and 200 bar, preferably about 180 bar, and / or a temperature of more than 80° C., in particular between 80° C. and 300° C., preferably about 230° C., during operation of the heat exchanger. The first fluid can have a higher pressure and / or a higher temperature than the second fluid. Typically, it is provided that during operation of the heat exchanger the measuring fiber at least partially extends through the second fluid, or the heat exchanger is embodied such that during operation of the heat exchanger the measuring fiber at least partially extends through the second fluid. It is advantageous if the fiber-optic sensor is embodied such that the measuring fiber and the reference fiber can be used at an operating pressure of more than 30 bar and / or an operating temperature of more than 80° C. In particular, it is advantageous if the operating pressure is between 30 bar and 200 bar, preferably about 180 bar, and / or the operating temperature is between 80° C. and 300° C., preferably about 230° C.

[0034] It is advantageous if the measuring fiber and the reference fiber extend at least in sections inside the protective sheath for protection from the influence of the ambient pressure and / or the ambient temperature. Typically, the protective sheath is formed such that it comprises metal, in particular is made of metal, in particular such that it comprises copper and / or iron, preferably steel, particularly preferably austenitic steel, or is made of copper and / or iron, preferably steel, particularly preferably austenitic steel. Alternatively, the protective sheath can be formed such that it comprises plastic (in particular polyimide), in particular is essentially made of plastic (in particular polyimide). It is advantageous if a section of the measuring fiber and a section of the reference fiber, which section extends inside the fluid chamber during operation of the heat exchanger, in particular through the second fluid, extends inside a protective sheath of this type. The section can be formed such that it comprises the main part of the longitudinal extension of the measuring fiber and the reference fiber extending inside the fluid chamber (in particular substantially the entirety). The protective sheath can be realized such that it comprises or is a coating applied to the measuring fiber and the reference fiber. The measuring fiber and the reference fiber can each be embodied as part of an optical cable, wherein the protective sheath forms the outer jacket of the optical cable.

[0035] It is advantageous if a plurality of optical fiber sensors arranged on various heat transfer tubes for determining the corresponding tube wall thickness have a shared electromagnetic emission source. Conveniently, the electromagnetic emission source can be coupled to the measuring fiber and the reference fiber of the optical fiber sensor so that the electromagnetic waves generated using the electromagnetic emission source are split and conducted into the measuring fiber and the reference fiber. This can be achieved using one or more optical feed fibers that connect the electromagnetic emission source with the measuring fiber and the reference fiber for transmitting electromagnetic waves. Conveniently, the feeding fiber may include a main branch and a plurality of side branches branching out from the main branch so that the electromagnetic waves conducted into the main branch using the electromagnetic emission source are split into the side branches so as to guide the electromagnetic waves to the measuring fiber and the reference fiber of the corresponding optical fiber sensor via the side branches. The corresponding optical fiber sensor may include an optical coupler, in particular the aforementioned optical coupler, wherein the feeding fiber is connected to the input line of the corresponding optical coupler so as to feed the electromagnetic waves into the input line via the feeding fiber. Conveniently, each of the side branches may be connected to the input line of the corresponding optical fiber sensor for transmitting electromagnetic waves.

[0036] It is advantageous if a plurality of fiber optic sensors arranged on various heat transfer tubes for determining the respective tube wall thicknesses have a shared electronic data acquisition unit (in particular the aforementioned electronic data acquisition unit) or are connected to such a unit.

[0037] It is advantageous if the fiber optic sensor has multiple electromagnetic emission sources, and the multiple electromagnetic emission sources have different wavelengths and / or different coherence lengths of the electromagnetic waves that can be generated. The electromagnetic emission source can be implemented as described in this document. It is beneficial if the electromagnetic emission source is coupled to the measuring fiber and the reference fiber so that the electromagnetic waves generated using different emission sources can be transmitted via the measuring fiber and the reference fiber in a superimposed manner. It is advantageous if multiple detectors are provided, wherein the detectors are implemented and / or connected to the measuring fiber and the reference fiber (in particular via optical couplers) so that various interference signals are detected using the detectors. It is practical if the detector is coupled to the measuring fiber and the reference fiber via a wavelength-selective demultiplexer so as to output interference signals from electromagnetic waves of different wavelengths at different outputs of the demultiplexer. The detector is usually connected to different outputs of the demultiplexer in this case for detecting interference signals. The input of the demultiplexer can be connected to the measuring fiber and the reference fiber (usually via optical couplers) for transmitting electromagnetic waves. Different emission sources are electromagnetic waves that usually have different wavelengths that can be generated.

[0038] In order to determine the wall thickness with high accuracy, it is advantageous if a correlation function between the wall thickness and one or more natural frequencies of the elastic vibrations of the heat transfer tube is established. The correlation function can be established such that it depends on the material and the dimensions (in particular the diameter and / or the length) of the heat transfer tube. Alternatively or cumulatively, the correlation function can be established by calibration, wherein typically the natural frequencies of the elastic vibrations of the heat transfer tube are measured at different known wall thicknesses of the heat transfer tube.

[0039] Typically, the optical fiber sensor, in particular its measuring fiber or reference fiber, is arranged on a plurality of heat transfer tubes of the heat exchanger, respectively. It is also possible to arrange a plurality of optical fiber sensors on a plurality of heat transfer tubes, respectively. The heat transfer tube is typically formed such that it comprises, in particular is made of, a metal, in particular an iron alloy, preferably a steel alloy.

[0040] Typically, the fiber optic sensor, in particular its measuring fiber or reference fiber, is arranged in an arrangement area on the respective heat transfer tube. The arrangement area is preferably an area in which corrosion preferably occurs during operation of the heat exchanger. The arrangement area generally depends on the production capacity. It is advantageous if the respective fiber optic sensor, in particular its measuring fiber or reference fiber, is arranged in an arrangement area on the respective heat transfer tube, wherein the arrangement area (in particular in the flow direction of the first fluid through the heat transfer tube, starting from the inlet of the heat transfer tube into the fluid chamber (in particular the fluid chamber cavity), along the longitudinal extension of the heat transfer tube) is defined by two thirds of the longitudinal extension of the heat transfer tube inside the fluid chamber or the fluid chamber cavity. Specifically, the arrangement area of ​​the respective heat transfer tube may extend, in particular in the flow direction of the first fluid through the heat transfer tube, starting from the inlet of the heat transfer tube into the fluid chamber (in particular the fluid chamber cavity), along the longitudinal extension of the heat transfer tube, inside the fluid chamber or the fluid chamber cavity, by a length of 30%, in particular 20%, preferably 10% of the longitudinal extension of the heat exchanger tube. This preferably applies if the heat exchanger, in particular the stripper, is part of a urea plant for producing urea, in particular for urea synthesis, wherein the urea plant has a production capacity of less than 2700 MTPD (metric tons per day). Alternatively, the arrangement region of the respective heat transfer tubes can be defined by a section of two thirds of the longitudinal extension of the heat transfer tubes inside the fluid chamber or the fluid chamber cavity, in particular in the flow direction of the first fluid through the heat transfer tubes, typically starting from the inlet of the heat transfer tubes along the longitudinal extension of the heat transfer tubes into the fluid chamber (in particular the fluid chamber cavity). This preferably applies if the heat exchanger, in particular the stripper, is part of a urea plant for producing urea, in particular for urea synthesis, wherein the urea plant has a production capacity of greater than or equal to 2700 MTPD (metric tons per day). It has been shown that material removal or wear of the heat transfer tubes is usually particularly large in this arrangement region of the respective heat transfer tubes, which is why it is beneficial to position the optical fiber sensor in said region. Typically, the arrangement area (particularly in the flow direction of the first fluid flowing through the heat transfer tube) is arranged in one third and / or two thirds of the length of the fluid chamber or the length of the fluid chamber cavity. The area in the center of the length of the fluid chamber or the length of the fluid chamber cavity is preferably in two thirds.

[0041] Another object is achieved with a method for operating a heat exchanger of the type named at the outset, if on one or more heat transfer tubes (with which a first fluid is transported in order to transfer heat between the first fluid and a second fluid via the heat transfer tube), optical fiber sensors are respectively arranged, wherein elastic vibrations (in particular natural vibrations) of the corresponding heat transfer tubes are detected by interference using the optical fiber sensors during operation of the heat exchanger in order to determine the wall thickness of the corresponding heat transfer tube during operation of the heat exchanger. The method can be implemented in particular using the aforementioned heat exchanger. Typically, the wall thickness of the heat transfer tube represents the (in particular radial) distance between the inner surface and the outer surface of the wall of the heat transfer tube in a cross section of the heat transfer tube. Typically, the second fluid is located outside the heat transfer tube so that heat is transferred between the first fluid and the second fluid via the wall of the heat transfer tube.

[0042] In particular, as described above, a robust determination of the wall thickness of the tube wall of the respective heat transfer tube in the heat exchanger (particularly in a high pressure and / or high temperature environment) is thus enabled during operation of the heat exchanger. As a result, operation of the heat exchanger (particularly process management and / or maintenance) can be performed based on the tube wall thickness determined using the fiber optic sensor. In this way, an optimized availability of the heat exchanger and an optimized operation of the heat exchanger are possible.

[0043] It should be understood that the method for operating a heat exchanger can be implemented according to the features and effects described in this document (especially above) in the context of a heat exchanger. As far as the method is concerned, the same also applies to the heat exchanger.

[0044] It is advantageous if the optical fiber sensor comprises an optical measuring fiber constituting a measuring section and an optical reference fiber constituting a reference section, wherein the measuring fiber is connected to the heat transfer tube in a vibration-transmitting manner, wherein the elastic vibration of the heat transfer tube (particularly the frequency of the elastic vibration) is detected by detecting (particularly measuring) interference signals from electromagnetic waves guided along the measuring section and electromagnetic waves guided along the reference section. Typically, interference is generated between the electromagnetic waves guided along the measuring section and the electromagnetic waves guided along the reference section, so that the electromagnetic waves create an interference signal. The interference signal is typically detected using a detector so that the frequency (particularly the natural frequency) of the elastic vibration of the heat transfer tube is detected using the detected interference signal. Multiple frequencies (particularly the natural frequencies) of the elastic vibration can be detected. The tube wall thickness can be determined by one frequency or multiple frequencies (particularly the natural frequencies).

[0045] Preferably, the fiber optic sensor is implemented to determine the wall thickness of the heat transfer tube with an accuracy of less than 100 μm.

[0046] It is particularly beneficial if the method for operating a heat exchanger is used for urea synthesis. The heat exchanger can be implemented as a stripper for stripping, in particular for urea synthesis, in which it is generally provided that the liquid phase and the gas phase with opposite flow directions are brought into contact with each other, usually inside the heat transfer tube. It is beneficial if the first medium flows through the corresponding heat transfer tube in the flow direction and the second medium flows through the heat transfer tube in the direction opposite to the flow direction so as to react with each other, wherein one of the media is generally liquid and the other medium is gaseous. This usually occurs inside the fluid chamber or fluid chamber cavity. The first fluid can be formed so that it includes the first medium and the second medium, or is made of the first medium and the second medium. It is beneficial if the orientation of the heat transfer tube and the flow direction of the first fluid flowing through the heat transfer tube is substantially perpendicular, in particular if the heat exchanger is a stripper. The heat exchanger or stripper generally comprises a plurality of, in particular more than 10, preferably more than 50, particularly preferably more than 100, particularly preferably more than 1000 heat transfer tubes.

[0047] Typically, a heat exchanger, in particular if the heat exchanger is a stripper, comprises a first inlet via which a first medium is fed into a heat transfer tube and a second inlet via which a second medium is fed into the heat transfer tube, so that inside a fluid chamber or fluid chamber cavity, the media flow through the heat transfer tube in opposite flow directions in order to react with each other. With respect to the fluid chamber cavity, the first inlet and the second inlet are typically connected to the heat transfer tube in a fluid-conducting manner at different ends of the heat transfer tube. The heat exchanger typically comprises at least one outlet for removing products formed by the reaction between the first medium and the second medium from the heat transfer tube. Practically, the heat exchanger may comprise a first outlet via which a first product can be removed from the heat transfer tube and a second outlet via which a second product can be removed from the heat transfer tube, wherein with respect to the fluid chamber cavity, the outlets are connected to the heat transfer tube in a fluid-conducting manner at different ends of the heat transfer tube. The first product and the second product are typically formed by means of a reaction between the first medium and the second medium or as a result of a reaction between the first medium and the second medium. This is particularly applicable if the heat exchanger is implemented as a stripper.

[0048] For urea synthesis, the first medium is generally formed such that it comprises the following components, in particular is made of the following components: urea, ammonium carbamate and ammonia, and the second medium is formed such that it comprises gaseous carbon dioxide (CO2), in particular is made of gaseous CO2. In this way, urea (in particular high-purity urea) can be separated as a product (in particular a first product), the urea being led out of the heat transfer tube, typically at one of the ends of the heat transfer tube or via a first outlet. Conveniently, the formed process gas, typically gaseous ammonia (NH3) and / or gaseous carbon dioxide (CO2), can be led out of the heat transfer tube, typically at the other end of the heat transfer tube or via a second outlet. The second fluid can be formed such that it comprises liquid water and / or gaseous water, in particular is made of liquid water and / or gaseous water. The stripper can be implemented and operated as described in this document, in particular with respect to the heat exchanger.

[0049] It is expedient if the electromagnetic waves guided using the measuring fiber or the reference fiber have a coherence length greater than 2 mm, in particular greater than 5 mm. As a result, a robust interference signal can be achieved by generating interference between the electromagnetic waves guided along the measuring section and the electromagnetic waves guided along the reference section. In particular, the coherence length can be as described above.

[0050] The second fluid typically has a pressure greater than 30 bar, in particular between 30 and 200 bar, preferably about 180 bar, and / or a temperature greater than 80°C, in particular between 80 and 300°C, preferably about 230°C.

[0051] It should be understood that the optical fiber sensor and its implementation and / or arrangement on the heat transfer tube generally refers to the heat transfer tube on which the optical fiber sensor is arranged, or the heat transfer tube to which the optical fiber sensor is connected for measuring the wall thickness of the heat transfer tube. Conveniently, multiple optical fiber sensors can be arranged on various heat transfer tubes or connected to various heat transfer tubes, in particular arranged or connected by the corresponding implementations described in this document. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Further features, advantages and effects of the invention are derived from the following description of exemplary embodiments. In the accompanying drawings to which reference is made:

[0053] Figure 1 A schematic diagram showing a heat exchanger with a fiber optic sensor;

[0054] Figure 2 A schematic diagram showing an optical fiber sensor arranged on a heat transfer tube;

[0055] Figure 3 showing a graph illustrating resonant frequency over tube wall thickness;

[0056] Figure 4 a schematic diagram showing a further heat exchanger with a fiber optic sensor;

[0057] Figure 5 A schematic diagram showing a fiber optic sensor arranged on a heat transfer tube having a shared electromagnetic emission source;

[0058] Figure 6 A schematic diagram showing a further optical fiber sensor arranged on the heat transfer tube;

[0059] Figure 7 A schematic diagram of a heat exchanger implemented as a stripper with a fiber-optic sensor is shown. DETAILED DESCRIPTION

[0060] exist Figure 1 In the embodiment of the present invention, a heat exchanger 1 is schematically illustrated, wherein the heat exchanger 1 comprises a plurality of heat transfer tubes 3 and a fluid chamber 4, wherein the heat transfer tubes 3 pass through the fluid chamber 4 so as to conduct a first fluid F1 through the heat transfer tubes 3 and conduct a second fluid F2 through the fluid chamber 4 so that the second fluid F2 surrounds the heat transfer tubes during operation of the heat exchanger 1, so that heat is transferred between the first fluid F1 and the second fluid F2 through the tube wall of the heat transfer tubes 3. The fluid chamber 4 forms a fluid chamber cavity 5 between the fluid chamber wall and the heat transfer tubes 3 so as to accommodate the second fluid F2, and the second fluid F2 is conducted through the cavity. The fluid chamber 4 comprises a fluid chamber inlet 6 for feeding the second fluid F2 into the fluid chamber 4 (particularly the fluid chamber cavity 5) and a fluid chamber outlet 7 for removing the fluid from the fluid chamber 4 (particularly the fluid chamber cavity 5). Typically, the heat transfer tubes 3 are guided through the fluid chamber 4 so that they are spaced apart from each other so that the second fluid F2 can flow between the heat transfer tubes 3. The heat exchanger 1 can be implemented as a stripper. The heat exchanger 1 , in particular a stripper, is usually embodied or oriented such that the longitudinal direction of the heat transfer tubes 3 is oriented substantially vertically.

[0061] The optical fiber sensors 2 are arranged on a plurality of heat transfer tubes 3 so as to determine the tube wall thickness of the corresponding heat transfer tubes 3 using the corresponding heat exchanger 1 during operation of the heat exchanger 1. The corresponding optical fiber sensors 2 are implemented to detect the frequency (particularly the natural frequency) of the elastic vibration of the heat transfer tubes 3 by interference during operation of the heat exchanger 1. The corresponding optical fiber sensors 2 include an optical measurement fiber M and an optical reference fiber R so as to guide an electromagnetic measurement wave along a measurement section using the measurement fiber M and guide an electromagnetic reference wave along a reference section using the reference fiber R, which is Figure 2. The measuring fiber M and the reference fiber R are connected to the same heat transfer tube 3, wherein the interaction section 21 of the measuring fiber M and the interaction section 21 of the reference fiber R on the heat transfer tube 3 are adjacently wound around the same heat transfer tube 3 multiple times. The corresponding interaction section 21 is formed using the end region of the measuring fiber M or the reference fiber R. The measuring fiber M is connected to the heat transfer tube 3 in a vibration-transmitting manner so that the elastic vibration of the heat transfer tube 3 changes the optical length of the measuring section. The reference fiber R is connected to the heat transfer tube 3 in a vibration-decoupled manner so that the optical length of the reference section is not significantly affected by the elastic vibration of the heat transfer tube 3. The optical sensor 2 includes a laser as an electromagnetic emission source L, so that an electromagnetic wave is introduced into the measuring fiber M as an electromagnetic measuring wave using the laser and an electromagnetic wave is introduced into the reference wave as an electromagnetic reference wave. The change in the length of the measuring section can generate a path difference between the measuring wave and the reference wave, so that the elastic vibration (especially the natural frequency of the elastic vibration) can be detected or measured using the detector PD of the optical fiber sensor 2 by generating interference of the measuring wave and the reference wave to create an interference signal. The measuring fiber M and the reference fiber R respectively include a reflective element at the respective fiber ends so that the measuring wave and the reflected wave are reflected back along the measuring fiber M and the reference fiber R again. The measuring fiber M and the reference fiber R are coupled to each other at the coupling site so as to create an interference signal using the measuring wave and the reference wave. The interaction section 21 of the measuring fiber M and the interaction section 21 of the reference fiber R are connected to the corresponding heat transfer tube 3 inside the fluid chamber 4 (especially the fluid chamber cavity 5), and the measuring fiber M and the reference fiber R are guided to the outside through the fluid chamber wall of the fluid chamber 4 so as to measure the interference signal outside the fluid chamber 4 using the detector PD. The detector PD and the electromagnetic emission source L are located outside the fluid chamber 4 or the fluid chamber cavity 5 (usually inside the sensor housing 9). The measuring fiber M and the reference fiber R are usually guided through the fluid chamber in a fluid-tight manner using one or more optical fiber feedthroughs 8. The optical fiber sensor 2 can be connected to a (especially shared) electronic data collection unit 18 for transmitting data (usually via an electronic data line 10). The electronic data collection unit 18 can be, for example, an electronic data processing system. In order to withstand the high temperatures and / or pressures in the heat exchanger 1, it is advantageous if, inside the fluid chamber 4, the measuring fiber M and the reference fiber R, respectively, extend inside a protective sheath, which can be implemented as a coating applied to the measuring fiber M and the reference fiber R. In order to keep the length of the measuring fiber M and the reference fiber R short, it is advantageous if a plurality of individual fiber-optic sensors 2, which in particular each comprise an individual sensor housing 9 and an individual electromagnetic emission source L, are present.

[0062] Typically, the heat transfer tubes 3 extend between a first tube sheet 11 and a second tube sheet 12, respectively, wherein the tube sheets are implemented as part of a fluid chamber wall of the fluid chamber 4 or delimit a fluid chamber cavity 5. The respective heat transfer tubes 3 are guided through the first tube sheet 11 and the second tube sheet 12. The fluid chamber 4 comprises a plurality of stabilizing elements 13, which are typically indicated as baffles, which connect the plurality of heat transfer tubes 3 to each other in order to stabilize the heat transfer tubes 3 using the stabilizing elements 13 during operation of the heat exchanger 1. It is advantageous if the interaction sections 21 of the measuring fibers M and the reference fibers R of the respective optical fiber sensors 2 are arranged in an arrangement region on the respective heat transfer tubes 3, which is located in one third and / or two thirds of the longitudinal extension of the heat transfer tubes 3 inside the fluid chamber 4 or the fluid chamber cavity 5 in the flow direction of the first fluid F1 flowing through the heat transfer tubes 3. Preferably, the interaction sections 21 of the measuring fiber M and the reference fiber R are connected to the heat transfer tube 3 between the first tube sheet 11 and a first of the stabilizing elements 13 in the flow direction of the first fluid F1 through the heat transfer tube 3 .

[0063] Figure 2 Show Figure 1Schematic diagram of the design of the optical fiber sensor 2 in the figure, the optical fiber sensor 2 is arranged on the corresponding heat transfer tube 3. The optical fiber sensor 2 includes an optical coupler 19, which forms a coupling site so that the measuring fiber M and the reference fiber R are coupled to each other to create an interference signal. The optical coupler 19 includes multiple (for example, three) inputs and multiple (for example, two) outputs. The electromagnetic emission source L is connected to one of the inputs, and the measuring fiber M and the reference fiber R are respectively connected to one of the outputs, so that the electromagnetic wave generated using the electromagnetic emission source L (especially in the feeding direction S) is transmitted as a measuring wave into the measuring fiber M and is transmitted as a reference wave into the reference fiber R. The detector PD is respectively connected to multiple other inputs so that the measuring wave reflected back via the measuring fiber M and the reference wave reflected back via the reference fiber R can be detected as an interference signal at the other inputs using the corresponding detector PD. Generally, the input and output of the optical coupler 19 are connected to each other so that the electromagnetic wave conducted via one of the inputs is transmitted so that it is distributed to the output, and the electromagnetic wave conducted via one of the outputs is transmitted so that it is distributed to the input. In this way, the interference signal corresponding to the elastic vibration (especially the natural vibration) of the heat transfer tube 3 can be measured using the detector PD. The detector PD is usually implemented as a photodiode. The detector PD is usually connected to an electronic data acquisition unit 17 for transmitting data, usually connected via an electronic data line 10. The electronic data acquisition unit 17 can be connected to an electronic data collection unit 18 for transmitting data. Between the electromagnetic emission source L and the optical coupler 19, an optical isolator 14 can be arranged to minimize the back reflection of the electromagnetic wave fed to the optical coupler 19 using the electromagnetic emission source L. The electromagnetic emission source L is usually electrically connected to an electrical control unit 15 for controlling the emission source L. Between the corresponding detector PD and the electronic data acquisition unit 17, an electrical amplifier 16 (especially a transimpedance amplifier) ​​can be arranged to amplify the interference signal detected by the detector PD. At various heat transfer tubes 3, the optical fiber sensor 2 implemented in this way can be arranged to determine the wall thickness of the corresponding heat transfer tube 3.

[0064] Figure 3 A graph showing an exemplary relationship between the measured resonant frequency or natural frequency of the elastic vibration of the heat transfer tube 3 and the wall thickness of the tube wall of the heat transfer tube 3 is shown. The linear relationship between the resonant frequency and the tube wall thickness is illustrated by a curve fitted by a straight line. Multiple resonant frequencies or natural frequencies can be detected to determine the tube wall thickness.

[0065] Figure 4 A schematic diagram of a further heat exchanger 1 with a plurality of optical fiber sensors 2 is shown. The heat exchanger 1 can be configured according to Figure 1 The heat exchanger 1 is implemented as described in the embodiment of the present invention. Figure 1In contrast, the optical fiber sensor 2 of the heat exchanger 1 in Figure 4 The fiber optic sensor 2 has a shared electromagnetic emission source L in the form of a laser. Figure 5 is illustrated in the figure. Figure 5 A schematic diagram of optical fiber sensors 2 arranged on different heat transfer tubes 3 having a shared electromagnetic emission source L is shown. Figure 5 Each individual optical fiber sensor 2 in Figure 2 Design of optical fiber sensor 2. Figure 2 on the contrary, Figure 5 The optical coupler 19 of the corresponding optical fiber sensor 2 is connected to the shared electromagnetic emission source L via an optical feeding fiber, so that the electromagnetic wave generated using the electromagnetic emission source L is fed to the optical coupler 19 so that the wave is split into the optical coupler 19. The feeding fiber includes a main branch and a plurality of side branches branching from the main branch, so as to guide the electromagnetic wave conducted into the main branch using the electromagnetic emission source L to the input of the corresponding optical coupler 19 so that the wave is split into the side branches. In addition, each individual optical fiber sensor 2 can have a shared electronic data acquisition unit 17, and the detector PD of the optical fiber sensor 2 is connected to the electronic data acquisition unit 17 for transmitting data.

[0066] Figure 6 A schematic diagram showing a further optical fiber sensor 2 arranged on the heat transfer tube 3. The optical fiber sensor 2 may be Figure 1 The optical fiber sensor 2 of the heat exchanger 1 in the embodiment of the present invention can be used according to Figure 2 The characteristics of the optical fiber sensor 2 are implemented. Figure 2 In contrast to the fiber optic sensor 2, Figure 6 The optical fiber sensor 2 in has two electromagnetic emission sources L whose electromagnetic wave wavelengths are different and whose coherence lengths are different. For example, one of the electromagnetic emission sources L may be a laser having a laser wavelength of 1300 nm, and the other electromagnetic emission source L may be a laser having a laser wavelength of 1550 nm. One of the lasers may have a coherence length between 0.5 mm and 10 mm, for example, about 5 nm, and the other laser may have a coherence length between 10 μm and 500 μm, for example, about 30 μm. The two electromagnetic emission sources L1, L2 are coupled to an optical coupling unit 20, usually respectively connected to the input line of the optical coupling unit 20, so that the electromagnetic waves generated using the electromagnetic emission sources L are output on the shared optical output line of the optical coupling unit 20 in a superimposed manner. The output line of the optical coupling unit 20 is connected to the input of an optical coupler 19 for transmitting electromagnetic waves, so as to feed the electromagnetic waves to the measuring fiber M and the reference fiber R via the optical coupler 19. Between the optical coupling unit 20 and the optical coupler 19 , an optical isolator 14 may be arranged to minimize back reflection.

[0067] In this way, superimposed measurement waves of different wavelengths and different coherence lengths can be used on the measurement fiber M or along the measurement section, and superimposed reference waves of different wavelengths and different coherence waves can be used on the reference fiber R or along the reference section. Therefore, at the other input of the optical coupler 19, to which the detector PD is connected for detecting the interference signal, as a result of the measurement wave reflected back along the measurement fiber M and the reference wave reflected back along the reference fiber R, two superimposed interference signals are generated for detection using the detector PD. Between the detector PD and the corresponding input of the optical coupler 19, a wavelength selective demultiplexer DM is each arranged so as to output interference signals from electromagnetic waves of different wavelengths at different outputs of the demultiplexer DM. A detector PD is each connected to the output of the corresponding demultiplexer DM to measure the interference signal. In this way, two different interference signals can be detected simultaneously. Due to different coherence lengths, interference signals of different shapes are generated. This enables particularly accurate determination of the natural frequency or the pipe wall thickness. The detector PD can be connected to a shared electronic data acquisition unit 17 for transmitting data.

[0068] Figure 7 A schematic diagram of a further heat exchanger 1 implemented as a stripper for stripping is shown, wherein optical fiber sensors 2 are arranged on a plurality of heat transfer tubes 3 of the heat exchanger 1 to determine the tube wall thickness of the heat transfer tubes 3. Typically, this type of heat exchanger 1 is used for urea synthesis. The heat exchanger 1 can be configured according to the invention. Figures 1 to 6 , or may include a corresponding optical fiber sensor 2. The heat exchanger 1 is generally oriented so that the longitudinal extension of the heat transfer tube 3 is oriented substantially vertically. For urea synthesis, it is provided that the first fluid F1 is formed so that it includes a first medium M1 and a second medium M2 or is made of a first medium M1 and a second medium M2, wherein inside the fluid chamber 4 or the fluid chamber cavity 5, the first medium M1 and the second medium M2 flow through the corresponding heat transfer tube 3 in opposite flow directions. Typically, the first medium M1 is formed so that it includes the following components, in particular is made of the following components: urea, ammonium carbamate and ammonia, and the second medium M2 is formed so that it includes gaseous carbon dioxide (CO2), in particular is made of gaseous CO2. The heat exchanger 1 or stripper generally includes a plurality of, in particular more than 10, preferably more than 50, particularly preferably more than 100, particularly preferably more than 1000 heat transfer tubes 3. The heat exchanger 1 is usually oriented so that the first tube sheet 11 is located vertically above the second tube sheet 12. Preferably, the respective fiber optic sensor 2 is located between the first tube sheet 11 and the first of the stabilizing elements 13.

[0069] The heat exchanger 1 comprises a first inlet 22 through which a first medium M1 can be fed into the heat transfer tube 3 and a second inlet 24 through which a second medium M2 can be fed into the heat transfer tube 3, so that inside the fluid chamber 4 or the fluid chamber cavity 5, the media M1, M2 flow through the heat transfer tube 3 in opposite flow directions so as to react with each other. With respect to the fluid chamber cavity 5, the first inlet 22 and the second inlet 24 are connected to the heat transfer tube 3 in a fluid-conducting manner at different ends of the heat transfer tube 3. For this purpose, the first inlet 22 and the second inlet 24 can be connected to the fluid distribution chamber in a fluid-conducting manner, respectively, wherein the ends of the heat transfer tube 3 are connected to the fluid distribution chamber in a fluid-conducting manner, respectively, so that the first medium M1 and the second medium M2 fed into the corresponding fluid distribution chamber via the first inlet 22 and the second inlet 24 are respectively conducted into the heat transfer tube 3 so that they are distributed to the heat transfer tube 3. The heat exchanger 1 comprises a first outlet 23 and a second outlet 25, through which the first product Z1 can be removed from the heat transfer tube 3, and the second product Z2 can be removed from the heat transfer tube 3 via the first outlet 23, wherein the first outlet 23 and the second outlet 25 are connected to the heat transfer tube 3 in a fluid-conducting manner at different ends of the heat transfer tube 3 with respect to the fluid chamber cavity 5, preferably the first outlet 23 and the second outlet 25 are respectively connected to one of the fluid distribution chambers in a fluid-conducting manner, so that the first product Z1 and the second product Z2 leaving the heat transfer tube 3 can be removed via the corresponding outlets 23 and 25. The first product Z1 is usually urea, in particular high-purity urea. The second product Z2 is usually gaseous ammonia (NH3) and / or gaseous carbon dioxide (CO2). The second fluid F2 is usually formed so that it includes liquid water and / or gaseous water, in particular made of liquid water and / or gaseous water.

[0070] If, on one or more of the heat transfer tubes 3 of the heat exchanger 1, optical fiber sensors 2 implemented to detect the natural frequency or resonance frequency of the elastic vibration of the corresponding heat transfer tube 3 by interference during the operation of the heat exchanger 1 are arranged respectively, the tube thickness of the corresponding heat transfer tube 3 can be practically determined during the operation of the heat exchanger 1. Preferably, the optical fiber sensor 2 is designed so that at an operating pressure greater than 30 bar, in particular between 30 bar and 200 bar, and / or an operating temperature greater than 80° C., in particular between 80° C. and 300° C., the measuring fiber M and the reference fiber R can be used or can be arranged on the corresponding heat transfer tube 3. This makes it possible to optimize the availability of the heat exchanger 1.

Claims

1. A heat exchanger (1), in particular a high-pressure heat exchanger for urea synthesis, said heat exchanger (1) comprising a plurality of heat transfer tubes (3) for conveying a first fluid (F1) so as to transfer heat between said first fluid (F1) and a second fluid (F2) via said heat transfer tubes (3), characterized in that The optical fiber sensors (2) are respectively arranged on one or more of the heat transfer tubes (3), wherein the optical fiber sensors (2) are designed to detect the elastic vibration, in particular the natural vibration, of the corresponding heat transfer tube (3) by interference during the operation of the heat exchanger (1), so as to determine the tube wall thickness of the corresponding heat transfer tube (3) during the operation of the heat exchanger (1).

2. The heat exchanger (1) according to claim 1, characterized in that The optical fiber sensor (2) comprises an optical measuring fiber (M) and an optical reference fiber (R), wherein the optical measuring fiber (M) constitutes a measuring section and the optical reference fiber (R) constitutes a reference section, wherein the measuring fiber (M) is connected to the heat transfer tube (3) in a vibration-transmitting manner, preferably wound around the heat transfer tube (3), so as to use a detector (PD) of the optical fiber sensor (2) to detect an interference signal created by electromagnetic waves guided along the measuring section and electromagnetic waves guided along the reference section.

3. The heat exchanger (1) according to claim 2, characterized in that The reference fiber (R) is connected to the heat transfer tube (3) in a vibration-decoupled manner, preferably wound around the heat transfer tube (3).

4. The heat exchanger (1) according to claim 2 or 3, characterized in that The heat exchanger (1) comprises a fluid chamber (4) for accommodating the second fluid (F2), wherein the heat transfer tube (3) extends inside the fluid chamber (4), and the measuring fiber (M) is connected to the heat transfer tube (3) inside the fluid chamber (4) in a vibration-transmitting manner, wherein the detector (PD) of the optical fiber sensor (2) is arranged outside the fluid chamber (4) for detecting the interference signal.

5. Heat exchanger (1) according to one of claims 2 to 4, characterized in that The optical fiber sensor (2) comprises an electromagnetic emission source (L), preferably a laser, for generating electromagnetic waves, wherein the emission source (L) is coupled to the measuring fiber (M) and the reference fiber (R) so as to introduce electromagnetic waves into the measuring fiber (M) and the reference fiber (R).

6. Heat exchanger (1) according to one of claims 2 to 5, characterized in that The measuring fiber (M) and the reference fiber (R) respectively comprise a reflecting element or are connected to such a reflecting element in order to reflect electromagnetic waves conducted along the measuring section and the reference section using the reflecting elements.

7. Heat exchanger (1) according to one of claims 2 to 6, characterized in that The measuring fiber (M) and the reference fiber (R) are coupled to each other at a coupling site so as to create an interference signal using an electromagnetic wave transmitted along the measuring section and an electromagnetic wave transmitted along the reference section.

8. Heat exchanger (1) according to one of claims 2 to 7, characterized in that The optical fiber sensor (2) includes an optical coupler (19) having a plurality of input lines and a plurality of output lines, wherein the input lines and the output lines are connected to each other for distributed transmission of electromagnetic waves, wherein the electromagnetic emission source (L) is connected to one of the input lines, and the measuring fiber (M) and the reference fiber (R) are respectively connected to one of the output lines, so that the electromagnetic waves introduced into the input lines using the emission source (L) are conducted to the measuring fiber (M) and the reference fiber (R) via the output lines.

9. The heat exchanger (1) according to claim 8, characterized in that It is preferably formed so that a detector (PD) comprising a photodiode is respectively connected to one or more of the input lines so that the electromagnetic waves reflected back into the output line along the measuring fiber (M) and the reference fiber (R) are detected as interference signals using the corresponding detector (PD) at the input line.

10. Heat exchanger (1) according to one of claims 2 to 9, characterized in that The measuring fiber (M) extends at least partially through the second fluid (F2) during operation of the heat exchanger (1), wherein the optical fiber sensor (2) is designed so that the measuring fiber (M) and the reference fiber (R) can be used at an operating pressure greater than 30 bar and / or an operating temperature greater than 80°C.

11. Heat exchanger (1) according to one of claims 2 to 10, characterized in that The measuring fiber (M) and the reference fiber (R) extend at least in sections inside a protective sheath, which is preferably formed such that it comprises metal or polyimide for protection against ambient pressure and / or ambient temperature.

12. A method for operating a heat exchanger (1), in particular a heat exchanger (1) according to one of claims 1 to 11, characterized in that The optical fiber sensors (2) are respectively arranged on one or more heat transfer tubes (3), and a first fluid (F1) is transmitted by means of one or more heat transfer tubes (3) so as to transfer heat between the first fluid (F1) and a second fluid (F2) via the heat transfer tubes (3), wherein the elastic vibration, in particular the natural vibration, of the corresponding heat transfer tube (3) is detected by interference using the optical fiber sensors (2) during the operation of the heat exchanger (1) so as to determine the wall thickness of the corresponding heat transfer tube (3) during the operation of the heat exchanger (1).

13. The method according to claim 12, characterized in that The optical fiber sensor (2) comprises an optical measuring fiber (M) and an optical reference fiber (R), wherein the optical measuring fiber (M) constitutes a measuring section and the optical reference fiber (R) constitutes a reference section, wherein the measuring fiber (M) is connected to the heat transfer tube (3) in a vibration-transmitting manner, wherein the elastic vibration of the heat transfer tube (3) is detected by detecting interference signals from electromagnetic waves guided along the measuring section and electromagnetic waves guided along the reference section.

14. The method according to claim 13, characterized in that The electromagnetic waves guided using the measuring fiber (M) or the reference fiber (R) have a coherence length of greater than 2 mm, in particular greater than 5 mm.

15. The method according to claim 12, characterized in that The second fluid (F2) typically has a pressure greater than 30 bar, in particular between 30 and 300 bar, preferably about 180 bar and / or a temperature greater than 80°C, in particular between 80 and 300°C, preferably about 230°C.

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