A heat exchanger comprising an optical fiber sensor for determining the wall thickness of a heat transfer tube of the heat exchanger, and a method for operating such a heat exchanger

Abstract

The present invention relates to a heat exchanger (1) comprising a plurality of heat transfer tubes (3) for transporting a first fluid (F1) for transferring heat between a first fluid (F1) and a second fluid (F2) through the heat transfer tubes (3), in particular a high-pressure heat exchanger for urea synthesis. In order to improve the usefulness, an optical fiber sensor (2) is respectively arranged for one or more of the heat transfer tubes (3), wherein the optical fiber sensor (2) is configured to, during operation of the heat exchanger (1), determine the wall thickness of each of the heat transfer tubes (3) by using interference spectroscopy to confirm elastic vibrations, in particular natural vibrations, of each of the heat transfer tubes (3) during operation of the heat exchanger (1). Furthermore, the present invention relates to a method for operating the heat exchanger (1).

Description

Technical Field

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

[0002] The present invention further relates to a method for operating the heat exchanger.

[0003] Exchanging thermal energy between a first fluid stream and a second fluid stream using a heat exchanger is known from the prior art. Heat exchangers often comprise a plurality of heat transfer tubes, which transport a first fluid stream in order to transfer heat to or absorb heat from a second fluid stream flowing around the heat transfer tubes via the heat transfer tubes. As part of urea synthesis, typically a heat exchanger is used, in which the first or second fluid stream usually has a high pressure above 30 bar (3 MPa) and a high temperature above 80°C. Thus, the transport of the first fluid stream through the heat transfer tubes is often associated with the removal of the tube wall material of the heat transfer tubes, in particular due to corrosion and / or erosion, or the formation of respective deposits inside the heat transfer tubes. As a result, the wall thickness of the heat transfer tubes changes, usually decreases, during operation of the heat exchanger. The integrity of the tube walls of the heat transfer tubes being impaired can be a problem in the safety of the operation of the heat exchanger. Therefore, it is usually necessary to stop the heat exchanger regularly and measure the wall thickness of the heat transfer tubes based on maintenance intervals over time.

[0004] For this purpose, it is common to insert a measurement probe into each heat transfer tube in a non-operating state of the heat exchanger in order to determine the inner radius or the wall thickness of the heat transfer tube. Measurement probes equipped with ultrasonic sensors, optical sensors, or eddy current sensors are known for determining the wall thickness.

[0005] In the case of a heat exchanger operating at a pressure above 30 bar (3 MPa) and a high temperature above 80 °C of the first and / or second fluid stream, in particular, an interruption of this type of operation for determining the wall thickness of the heat transfer tubes is usually laborious and associated with high costs.

[0006] This is addressed by the present invention. The object of the present invention is to define a heat exchanger of the type mentioned at the beginning having optimized utility, in particular optimized operation.

[0007] It is also an object of the present invention to define a method for operating a heat exchanger that enables optimized use or operation of the heat exchanger.

[0008] According to the present invention, an optical fiber sensor is respectively arranged for one or more of the heat transfer tubes using a heat exchanger of the type mentioned at the beginning, wherein the optical fiber sensor is designed to confirm the elastic vibration, in particular the natural vibration, of the respective heat transfer tubes during operation of the heat exchanger by using interference spectroscopy in order to determine the wall thickness of the respective heat transfer tubes during operation of the heat exchanger. In this way, the object is achieved.

[0009] The basis of the present invention is the idea of improving the usefulness of a heat exchanger, in particular a heat exchanger designed for high operating pressures and / or high operating temperatures of the first and / or 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, the operation of the heat exchanger, in particular the process control and / or maintenance, can be carried out depending on the wall thickness determined using an optical fiber sensor. Specifically, it is not essential to interrupt the operation of the heat exchanger to determine the wall thickness. This can be practically achieved, especially when the optical fiber sensors are respectively arranged for one or more of the heat transfer tubes, as will be particularly explained. Practically, using an optical fiber sensor, the wall thickness of the tube wall of each heat transfer tube can be determined during operation of the heat exchanger in that the elastic vibration, in particular the frequency of the elastic vibration of each heat transfer tube, is confirmed using interferometry. The elastic vibration is typically the elastic natural vibration of the heat transfer tube or the frequency is the natural frequency of the heat transfer tube. Advantageously, a plurality of elastic natural vibrations or natural frequencies of each heat transfer tube can be confirmed using an optical fiber sensor to determine the wall thickness.

[0010] The operation of the heat exchanger shows a state in which the first fluid is induced through the heat transfer tubes to exchange heat with the second fluid via the heat transfer tubes. High operating pressure and high operating temperature indicate an operating pressure of the first fluid and / or the second fluid exceeding 30 bar (3 MPa) respectively, and an operating temperature of the first fluid and / or the second fluid exceeding 80 °C. In particular, the operating pressure is between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or the operating temperature is between 80 °C and 300 °C, preferably approximately 230 °C. Usually, the first and / or the second fluid has this type of operating pressure and this type of operating temperature during the operation of the heat exchanger, or the heat exchanger is designed for this type of operation. Therefore, it is beneficial if each optical fiber sensor is designed for use or measurement 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 for in-situ determination and in-operando determination of the wall thickness of each heat transfer tube, or the wall thickness of each heat transfer tube is determined in-situ and in-operando using the optical fiber sensor.

[0011] It is particularly beneficial if the heat exchanger is a stripper for performing stripping. It is beneficial if the stripper is used for urea synthesis. The stripper can usually be embodied inside the heat transfer tubes for synthesizing urea by stripping.

[0012] The optical fiber sensor has an optical measurement fiber constituting an optical measurement section and an optical reference fiber constituting an optical reference section. The measurement fiber is connected in a manner that transmits vibration to the heat transfer tube so as to detect, using a detector of the optical fiber sensor, an interference signal created using an electromagnetic wave guided along the measurement section and an electromagnetic wave guided along the reference section, and is preferably wound around the heat transfer tube, which is advantageous. Thereby, the elastic vibration of the heat transfer tube can be practically confirmed. The implementation of the measurement section and the reference section using optical fibers enables a high robustness of measurement, which is typically required under the usage conditions of a heat exchanger, particularly a high-pressure heat exchanger. Hereinafter, in particular, the optical measurement fiber and the optical reference fiber, as well as the optical measurement section and the optical reference section, are simply also referred to as the measurement fiber and the reference fiber, and the measurement section and the reference section, without specifically changing the meaning of the terms for readability.

[0013] Typically, a change in the mass of each heat transfer tube, particularly a change in the wall thickness of the tube, results in a change in the elastic vibration of the heat transfer tube, particularly the natural vibration. By checking or measuring the elastic vibration, particularly its frequency or natural frequency, the wall thickness of the heat transfer tube can be determined. Typically, it is brought about that the elastic vibration of the heat transfer tube generates a change in the measurement section, particularly in the length of the measurement fiber. Conveniently, the measurement fiber can be correspondingly connected to the heat transfer tube in a manner that transmits vibration, typically physically coupled, to achieve this. Usually, the measurement fibers are connected to each heat transfer tube, and as a result, the expansion or contraction of the heat transfer tube associated with the elastic vibration of the heat transfer tube causes elastic expansion and relaxation of the measurement fiber corresponding to the vibration of the heat transfer tube, particularly a change in length. The reference fiber is usually arranged, particularly connected, relative to each heat transfer tube, and as a result, the length of the reference fiber is not essentially affected by a change in the mass of the heat exchanger tube or a change in the elastic vibration. Preferably, the reference fiber and the measurement fiber are arranged such that the fibers are exposed to essentially the same temperature influence and / or pressure influence. The measurement fiber and the reference fiber, particularly their respective interaction segments, are typically arranged relative to or connected to the same heat transfer tube. Typically, the electromagnetic wave conducted across the measurement fiber or along the measurement section is called the measurement wave, and the electromagnetic wave conducted across the reference fiber or along the reference section is called the reference wave. The measurement wave and the reference wave can have an optical path difference, particularly a phase difference, caused by the elastic vibration of the heat transfer tube. Therefore, the optical path difference or phase difference usually depends on the frequency of the elastic vibration of the heat transfer tube, particularly the natural frequency. Typically, it is provided that the measurement fiber and the reference fiber are coupled to each other to interfere the measurement wave and the reference wave with each other to create an interference signal. The interference signal, also called the electromagnetic interference wave, typically depends on the optical path difference, particularly the phase difference, of the electromagnetic waves, particularly the measurement wave and the reference wave, and as a result, the interference signal corresponds to the elastic vibration of the heat transfer tube, particularly the frequency or natural frequency of the elastic vibration.Thus, the phase difference usually depends on the mass of the heat transfer tube, where typically this mass correlates with the natural frequency or resonance frequency of the elastic vibration of the heat transfer tube. It will be understood that an optical fiber sensor can be embodied accordingly. The measurement wave and the reference wave are typically embodied to be coherent with each other in order to create an interference signal by interference after passing through the measurement section and the reference section. It has been found to be effective when the measurement wave and the reference wave are derived from a shared electromagnetic radiation source in order to form them such that they are coherent with each other.

[0014] Typically, one or more frequencies of the elastic vibration of the heat transfer tube, in particular the natural frequency also referred to as the resonance frequency, are confirmed using an optical fiber sensor. The elastic vibration of the heat transfer tube, in particular its frequency or natural frequency, usually correlates with the wall thickness of the heat transfer tube, so that as a result, the wall thickness of the heat transfer tube can be determined by confirming or measuring the elastic vibration, in particular its frequency or natural frequency. Usually, the frequency, in particular the natural frequency, increases as the wall thickness of the heat transfer tube decreases. In particular, it has been shown that there is essentially a linear relationship between the frequency of the elastic vibration of the heat transfer tube, in particular the natural frequency, and the wall thickness of the heat transfer tube in many cases. In this way, a robust evaluation can be practically realized. Advantageously, the intensity of the interference signal can be detected in a time-dependent manner. The detected intensity of the interference signal can typically be transformed into the mathematical frequency space for evaluation using a Fourier transform in order to confirm the natural frequency, in particular multiple natural frequencies. The wall thickness can be determined from the natural frequency or frequency. Advantageously, the optical fiber sensor can comprise one or more detectors for detecting the interference signal. The evaluation of the interference signal, in particular the confirmation of the natural frequency or the determination of the 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 optical fiber sensor. The electronic data acquisition unit can be designed for computer-aided evaluation, and in particular can be formed to comprise a microcontroller. The accuracy of the determination of the wall thickness of the heat transfer tube is typically less than 100 μm.

[0015] The measurement fiber and the reference fiber are typically embodied to conduct electromagnetic waves, particularly the measurement wave and the reference wave, respectively. The measurement fiber and the reference fiber are usually dielectric waveguides, preferably optical fiber cables, that are generally formed such that they comprise or are made from silica glass or plastic. In a non-operating state of the heat exchanger, it is beneficial if the measurement fiber or measurement section and the reference fiber or reference section have essentially equal lengths.

[0016] The measurement fiber is typically connected to the heat transfer tube in a manner that transmits vibrations thereto, such that the length of the measurement fiber or measurement section changes in correspondence with the elastic vibrations of the heat transfer tube during operation of the heat exchanger, particularly the frequency of the elastic vibrations. For this purpose, it is beneficial if the measurement fiber comprises an interaction segment that is connected to the heat transfer tube in a manner that transmits vibrations thereto. The interaction segment can be wound around the circumference of the heat transfer tube at least once, preferably a plurality of times. The interaction segment of the measurement fiber can form one or more windings around the circumference of the heat transfer tube, particularly between two windings and ten windings, preferably between four windings and seven windings, generally approximately five windings. In this way, the elastic vibrations of the heat transfer tube can act efficiently on the measurement fiber during operation of the heat exchanger. Alternatively or cumulatively, the interaction segment can have a meandering shape. In a simple implementation, the interaction segment can be in a straight shape. The measurement fiber, particularly its interaction segment, can be connected to the heat transfer tube in a manner that transmits vibrations thereto by physical bonding, particularly using an adhesive. For this purpose, an adhesive can be embodied to create a strong connection between the measurement fiber, particularly its interaction segment, and the heat transfer tube.

[0017] It is beneficial if the reference fiber is connected to the heat transfer tube in a way that decouples vibrations, preferably if it is wound around the heat transfer tube. Usually, this is achieved in that during operation of the heat exchanger, the length of the reference fiber or reference section essentially does not change with the elastic vibrations of the heat transfer tube, in particular with the frequency of the elastic vibrations. For this purpose, an interaction segment can be provided in which the reference fiber is connected to the heat transfer tube in a way that decouples vibrations. In this way, similar boundary conditions can be achieved with respect to the measurement fiber or measurement section and the reference fiber or reference section in order to achieve a high accuracy in the determination of the tube wall thickness using the sensor. The interaction segment can be wound around the circumference of the heat transfer tube at least once, preferably multiple times. The interaction segment of the reference fiber can form one or more windings around the circumference of the heat transfer tube, in particular between two windings and ten windings, preferably between four windings and seven windings, mostly approximately five windings. Alternatively or cumulatively, the interaction segment can have a meandering shape. In a simple implementation, the interaction segment can be in a straight shape. The reference fiber, in particular its interaction segment, can be connected to the heat transfer tube in a way that decouples vibrations using a physical connection, in particular an adhesive. For this purpose, the adhesive can create an elastic connection between the reference fiber and the heat transfer tube, in particular its interaction segment. It has been found to be effective if the adhesive is formed such that it comprises polydimethylsiloxane (PDMS). Preferably, the interaction segment of the measurement fiber and the interaction segment of the reference fiber have the same shape. It is advantageous if the interaction segment of the measurement fiber and the interaction segment of the reference fiber form an equal number of windings around the heat transfer tube. Usually, the interaction segment of the measurement fiber and the interaction segment of the reference fiber are arranged adjacent to each other with respect to the same heat transfer tube, specifically, in particular along the longitudinal extension of the heat transfer tube, and are connected to the same heat transfer tube. Thus, it has been found to be effective if the distance between the interaction segments is less than 30 mm, preferably less than 10 mm.Typically, and thus the above interval is between 1 mm and 30 mm, preferably approximately 5 mm.

[0018] The heat transfer tube is typically embodied to direct the first fluid in order to transfer heat between the first fluid and the second fluid through the tube wall of the heat transfer tube. During operation of the heat exchanger, it is preferably provided that the second fluid is in contact with the heat transfer tube or the tube wall, particularly directly. The first fluid is typically a first fluid stream directed through the heat transfer tube during operation of the heat exchanger. During operation of the heat exchanger, the second fluid can typically be a second fluid stream flowing around the heat transfer tube.

[0019] The heat exchanger typically comprises a fluid chamber for containing the second fluid, where the heat transfer tube extends inside the fluid chamber. The fluid chamber typically forms the fluid chamber cavity between the fluid chamber wall of the fluid chamber and the heat transfer tube for containing the second fluid using the fluid chamber cavity to transfer heat between the first fluid and the second fluid. The heat transfer tube typically extends through the fluid chamber cavity. Typically, during operation of the heat exchanger, it is provided that the second fluid is directed through the fluid chamber cavity, particularly such that the second fluid flows around the heat transfer tube. Conveniently, the fluid chamber cavity can be embodied in the form of one or more channels for directing the second fluid using channels during operation of the heat exchanger. The fluid chamber typically comprises at least one fluid chamber inlet and at least one fluid chamber outlet for directing the second fluid into the fluid chamber, particularly into the fluid chamber cavity, through the fluid chamber inlet, and for removing the second fluid from the fluid chamber, particularly from the fluid chamber cavity, again through the fluid chamber outlet after heat transfer has occurred between the first fluid and the second fluid. The fluid chamber is typically formed such that it comprises metal, preferably an iron alloy, particularly preferably steel, and is particularly made therefrom.

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

[0021] Typically, the first fluid and the second fluid are embodied to be a liquid and / or a gas. For example, the first fluid and the second fluid can be formed such that they include liquid and gaseous water, in particular such that they are made from them. The first fluid and the second fluid are embodied such that they comprise a liquid medium and a gaseous medium, in particular such that they are made from them, and it can be provided that the liquid medium and the gaseous medium of each fluid flow through the heat exchanger in opposite directions to each other, typically such that they contact each other. For example, the first fluid can be formed such that it comprises a liquid medium and a gaseous medium, where in each heat transfer tube, the media flow through the heat transfer tube in opposite directions to each other such that they contact each other.

[0022] The heat transfer tubes typically extend between a first tube plate and a second tube plate, the tube plates defining the boundaries of fluid chamber cavities for containing a second fluid, and the heat transfer tubes terminate at or are guided through passage openings in the respective tube plates. Typically, fluid fed through a passage opening in one of the plates is induced through the heat transfer tubes to the passage opening in the other tube plate. The heat transfer tubes are typically connected to the tube plates in a liquid-tight manner. Typically, each tube plate is embodied in the form of a plate provided with a plurality of flow channels, which are oriented transversely, in particular orthogonally, to the longitudinal extension of the tube plate, and the flow channels form the respective passage openings. The tube plate can be embodied as part of the fluid chamber wall of the fluid chamber. The heat exchanger typically comprises at least one first tube plate and at least one second tube plate of this type. The tube plates are typically formed such that they are made of metal, preferably an iron alloy, particularly preferably steel, in particular from it.

[0023] The fluid chamber may have one or more fluid guiding surfaces for defining a flow path of a second fluid using the fluid guiding surface. Each fluid guiding surface is typically embodied, at least in part, to impede the flow of the second fluid between the heat transfer tubes. The fluid guiding surface can 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 extend through each guiding surface. Usually, a plurality of guiding surfaces that intersect the heat transfer tubes and are spaced apart from each other are provided. Each fluid guiding surface is typically oriented in a direction transverse to, in particular perpendicular to, the longitudinal extension of the heat transfer tube. Usually, 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 each fluid guiding surface to impede the flow of the second fluid through the intermediate space. Each fluid guiding surface can be embodied to close most of the intermediate space between the heat transfer tubes against the flow of the second fluid in a cross-section taken through the fluid chamber. The fluid guiding surface may be formed using a guiding wall disposed in the fluid chamber. The fluid guiding surface is typically embodied to be plate-like. The fluid chamber usually comprises one or more guiding surfaces of this type.

[0024] Typically, a plurality of heat transfer tubes are connected to each other by stabilizing elements in order to stabilize the heat transfer tubes during operation of the heat exchanger. Each stabilizing element may be embodied to be plate-shaped, and the longitudinal extension of the stabilizing element is usually oriented in a transverse direction, particularly orthogonally, with respect to the longitudinal extension of the heat transfer tubes connected by the stabilizing element. Typically, the heat transfer tubes extend through the stabilizing elements. The stabilizing elements are customarily referred to as baffles. Usually, a plurality of stabilizing elements that are spaced apart from each other and 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 may be formed by the stabilizing elements. In this case, the stabilizing elements may function to stabilize the heat transfer tubes and to define a flow path for a second fluid.

[0025] Typically, the measuring fiber of each optical fiber sensor is connected to each heat transfer tube in such a way that vibrations are transmitted inside the fluid chamber. It is beneficial if the detector of the optical fiber sensor is arranged outside the fluid chamber, particularly outside the fluid chamber cavity, for the detection of 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 likewise arranged outside the fluid chamber, particularly outside the fluid chamber cavity. As a result, the detector and the electronic data acquisition unit are protected, in particular, against the loads of the first and second fluids, particularly the pressure load and / or the temperature load. Usually, the measuring fiber and the reference fiber extend at least partially through the fluid chamber, particularly through the fluid chamber cavity, or through the second fluid during operation of the heat exchanger. Typically, the interaction segment of the measuring fiber and the interaction segment of the reference fiber are connected to the respective heat transfer tubes inside the fluid chamber. The fluid chamber may comprise one or more fiber feed-throughs, by means of which the measuring fiber and the reference fiber are guided through the fluid chamber wall of the fluid chamber and guided out of the fluid chamber, particularly in a liquid-tight manner.

[0026] Advantageously, the optical fiber sensor may comprise an electromagnetic radiation source, preferably a laser, for generating and emitting electromagnetic waves, in particular a measurement wave and a reference wave, where the electromagnetic radiation source is coupled to the measurement fiber and the reference fiber to bring the electromagnetic waves to the measurement fiber and the reference fiber. The electromagnetic waves brought to the measurement fiber and the reference fiber are usually coherent with each other, so that the electromagnetic waves can create an interference or interference signal after passing through the measurement section and the reference section. It is advantageous if the electromagnetic radiation source or the electromagnetic waves, in particular the measurement wave and the reference wave, have a coherence length exceeding 0.5 mm, in particular exceeding 1 mm, preferably exceeding 2 mm, particularly preferably exceeding 5 mm. Typically, the coherence length is between 1 mm and 10 mm. Preferably, the electromagnetic radiation source, in particular the laser, is embodied to emit electromagnetic waves, in particular light waves, having a wavelength between 500 nm and 2000 nm, in particular between 1000 nm and 1500 nm, preferably approximately 1300 nm. If the laser is constructed for the emission of electromagnetic waves using a vertical cavity surface emitting laser diode (VCSEL), it is advantageous for a robust measurement or determination of the tube wall thickness. The interference signal typically has an essentially periodic structure. It is beneficial if an optical isolator, also referred to as an optical diode, is arranged downstream in the feed direction from the electromagnetic radiation source in order to minimize, in particular prevent, electromagnetic back-reflection to the electromagnetic radiation source. The feed direction typically indicates the direction in which the electromagnetic waves are fed to the measurement fiber and the reference fiber using the electromagnetic radiation source.

[0027] Although less preferable from the viewpoint of robustness, it has been shown that it is similarly possible to determine the wall thickness when the coherence length is between 10 μm and 500 μm, particularly between 20 μm and 100 μm, preferably between 25 μm and 50 μm, for example, approximately 30 μm. Usually, the interference signal in this case has a non-periodic structure with a maximum value, particularly a plurality of maximum values. The interference signal, or the non-periodic structure, often comprises one or more sub-structures in the shape of a Gaussian function. The distance between the maximum values of the interference signal, particularly the sub-structures in the shape of a Gaussian function, particularly the plurality of maximum values, usually corresponds to the optical path difference between the measurement wave and the reference wave. In the non-operating state of the heat exchanger, it is beneficial if the difference in length 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 measurement fiber and the reference fiber each comprise a reflection element or are connected to such a reflection element in order to reflect electromagnetic waves conducted along the measurement section and the reference section, respectively, in particular back along the measurement section and the reference section, using the reflection element. Typically, the reflection element is used to reflect the measurement wave back along the measurement section and the reference wave back along the reference section after they have passed through the respective sections. The reflection element preferably comprises a reflective surface for reflecting the respective electromagnetic waves. It is advantageous if the reflection element or the reflective surface has a reflectivity of more than 90%, in particular more than 95%, preferably more than 98%. The reflection element can be a mirror. This type of reflection element has been found to be effective when arranged at the fiber ends of the measurement fiber and the reference fiber, respectively. Typically, the fiber end is one of the fiber ends of the measurement fiber and the reference fiber arranged downstream along the respective fiber in the direction of propagation of the electromagnetic wave using an electromagnetic radiation source. The reflection element is usually arranged downstream from the measurement section and the reference section along the respective fiber in the direction of propagation. The reflection element can be formed, in particular, such that it comprises a metal layer arranged at the fiber ends of the measurement fiber and the reference fiber. The metal layer can be formed such that it comprises gold, silver, and / or aluminum, in particular made therefrom. Preferably, the metal layer is applied to the fiber end using sputtering. The reflection element can be a Bragg mirror, in particular a dielectric mirror. The reflection element can be formed such that it comprises one or more thin films that can be applied overlapping each other. The thin films can be applied using thin film deposition. The thin films can be formed such that they comprise magnesium fluoride, silicon dioxide, tantalum(V) oxide, zinc sulfide, and / or titanium dioxide, in particular essentially made therefrom.

[0029] Preferably, the fiber optic sensor may have an interferometer design according to the type of Michelson interferometer, where electromagnetic waves traveling along the measurement section via the measurement fiber and along the reference section via the reference fiber are reflected at the respective ends of the measurement and reference sections, in particular at the respective fiber ends, in order to superimpose the reflected electromagnetic waves to create an interference signal.

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

[0031] It has been found to be effective when an optical fiber sensor includes an optical coupler having a plurality of input lines and a plurality of output lines, where the input lines and output lines are connected to each other for the distributed transmission of electromagnetic waves. The optical coupler can form a coupling site. Typically, the input lines and output lines are connected such that an electromagnetic wave fed to the optical coupler via an input line is transmitted to a plurality of output lines, and an electromagnetic wave fed to the optical coupler via one of the output lines is transmitted to a plurality of input lines. An electromagnetic radiation source is connected to one of the input lines, and a measurement fiber and a reference fiber are respectively connected to one of the output lines. As a result, it is practical when the electromagnetic wave brought to the input line using the radiation source is conducted to the measurement fiber and the reference fiber via the output line. Typically, the measurement fiber and the reference fiber are respectively connected to the output line. As a result, an electromagnetic wave brought to the output line via the measurement fiber and the reference fiber, particularly an electromagnetic wave reflected to return along the measurement fiber and the reference fiber, is transmitted to one or more of the input lines. As a result, the electromagnetic wave is output as an interference signal at each input line. Conveniently, a detector for detecting the interference signal can be arranged at one or more of the input lines. Preferably, a detector formed to include a photodiode is respectively connected to one or more of the input lines to detect, using each detector, an electromagnetic wave reflected to return to the output line along the measurement fiber and the reference fiber as an interference signal at the input line, which is beneficial. For example, the optical coupler can include at least three input lines and at least two output lines. The electromagnetic radiation source is connected to one of the input lines, the detector is respectively connected to two other input lines, the measurement fiber is connected to one of the output lines, and the reference fiber is connected to another output line.

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

[0033] The second fluid typically has a pressure exceeding 30 bar (3 MPa), in particular between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or a temperature exceeding 80 °C, in particular between 80 °C and 300 °C, preferably approximately 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 or embodied such that the measurement fiber extends at least partially through the second fluid during operation of the heat exchanger. It is beneficial if an optical fiber sensor is embodied such that the measurement fiber and the reference fiber can be used at an operating pressure exceeding 30 bar (3 MPa) and / or an operating temperature exceeding 80 °C. In particular, it is correspondingly beneficial if the operating pressure is between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or the operating temperature is between 80 °C and 300 °C, preferably approximately 230 °C.

[0034] It is advantageous if the measurement fiber and the reference fiber at least partially extend inside the protective sheath for protection against ambient pressure and / or ambient temperature. Typically, the protective sheath is formed such that it comprises metal, in particular is made therefrom, in particular if it comprises or is made from 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 therefrom. During operation of the heat exchanger, it is beneficial if segments of the measurement fiber and segments of the reference fiber that extend inside the fluid chamber, in particular through the second fluid, extend inside a protective sheath of this type. The segments can be formed such that they comprise the dominant part, in particular essentially the whole, of the longitudinal extensions of the measurement fiber and the reference fiber that extend inside the fluid chamber. The protective sheath can be realized as comprising a coating applied to the measurement fiber and the reference fiber or as a coating applied to the measurement fiber and the reference fiber. The measurement fiber and the reference fiber can each be embodied as part of an optical cable, where the protective sheath forms the outer sleeve of the optical cable.

[0035] It is advantageous if a plurality of optical fiber sensors arranged for respective heat transfer tubes to determine respective wall thicknesses have a shared electromagnetic radiation source. Conveniently, the electromagnetic radiation source can be coupled to the measurement fiber and the reference fiber of the optical fiber sensor, such that electromagnetic waves generated using the electromagnetic radiation source are split and conducted to the measurement fiber and the reference fiber. This can be achieved using one or more optical feed fibers connecting the electromagnetic radiation source and the measurement fiber and the reference fiber for transmission of the electromagnetic waves. Conveniently, the feed fiber can have a main branch and a plurality of tributary branches branching from the main branch, such that electromagnetic waves conducted to the main branch using the electromagnetic radiation source are split into the tributary branches to guide the electromagnetic waves to the measurement fiber and the reference fiber of respective optical fiber sensors. Each optical fiber sensor can comprise an optical coupler, particularly as previously described, and the feed fiber is connected to the input line of each optical coupler for feeding the electromagnetic waves to the input line via the feed fiber. Conveniently, one tributary branch can be connected to the input line of each respective optical fiber sensor for transmission of the electromagnetic waves.

[0036] It is beneficial if a plurality of optical fiber sensors arranged for respective heat transfer tubes to determine respective wall thicknesses have a shared, particularly as previously described, electronic data acquisition unit or are connected to such a unit.

[0037] An optical fiber sensor is advantageous when it has a plurality of electromagnetic radiation sources capable of producing electromagnetic waves of different wavelengths and / or different coherence lengths. The electromagnetic radiation sources can be embodied as described herein. It is beneficial when the electromagnetic radiation sources are coupled to a measurement fiber and a reference fiber, such that electromagnetic waves generated using the various radiation sources can be transmitted through the measurement fiber and the reference fiber in an overlapping manner. It is advantageous when a plurality of detectors are provided, and the detectors are embodied and / or connected to the measurement fiber and the reference fiber, particularly via an optical coupler, such that various interference signals are detected using the detectors. It is practical when the detectors are coupled to the measurement fiber and the reference fiber via a wavelength-selective demultiplexer to output interference signals from electromagnetic waves of different wavelengths at different outputs of the demultiplexer. The detectors are in this case typically connected to different outputs of the demultiplexer for the detection of the interference signals. The inputs of the demultiplexer can be connected to the measurement fiber and the reference fiber, typically via an optical coupler, for the transmission of the electromagnetic waves. In that case, the different radiation sources typically have different wavelengths of the electromagnetic waves that can thereby be generated.

[0038] For high accuracy in the determination of the tube wall thickness, it is advantageous when a correlation function is established between the tube wall thickness and one or more natural frequencies of the elastic vibrations of the heat transfer tube. The correlation function can be established such that it is determined by the material and size of the heat transfer tube, particularly the diameter and / or length. Alternatively or cumulatively, the correlation function can be established using calibration, typically by measuring the natural frequencies of the elastic vibrations of the heat transfer tube at various known tube wall thicknesses of the heat transfer tube.

[0039] Typically, an optical fiber sensor, particularly its measurement fiber or reference fiber, is respectively arranged for a plurality of heat transfer tubes of a heat exchanger. It is also possible for a plurality of optical fiber sensors to be respectively arranged for a plurality of heat transfer tubes. The heat transfer tubes are usually formed such that they contain, particularly are made from, metal, particularly an iron alloy, preferably a steel alloy.

[0040] Typically, the optical fiber sensors, particularly their measurement fibers or reference fibers, are arranged within the arrangement areas on the respective heat transfer tubes. Preferably, the arrangement area is an area where corrosion preferably occurs during the operation of the heat exchanger. The arrangement area often depends on the production capacity. Each optical fiber sensor, particularly its measurement fiber or reference fiber, is arranged in the arrangement area on the respective heat transfer tube, and the said arrangement area, particularly in the flow direction of the first fluid through the heat transfer tube, starts from the inlet to the fluid chamber of the heat transfer tube, particularly the fluid chamber cavity, and along the longitudinal extension of the heat transfer tube, it is beneficial if it is defined by two-thirds of the longitudinal extension of the heat transfer tube inside the fluid chamber or the fluid chamber cavity. In particular, the arrangement area of each heat transfer tube, particularly in the flow direction of the first fluid through the heat transfer tube, typically starts from the inlet of the heat transfer tube to the fluid chamber, particularly the fluid chamber cavity, and extends along the longitudinal extension of the heat transfer tube, and may have a length of 30%, particularly 20%, preferably 10% of the longitudinal extension of the heat transfer tube inside the fluid chamber or the fluid chamber cavity. Preferably, this applies when the heat exchanger, particularly the stripper, is part of a urea plant for urea production, particularly for urea synthesis, and the urea plant has a production capacity of less than 2700 MTPD (metric tons per day). Alternatively, the arrangement area of each heat transfer tube, particularly in the flow direction of the first fluid through the heat transfer tube, typically starts from the inlet of the heat transfer tube to the fluid chamber, particularly the fluid chamber cavity, and may be defined along the longitudinal extension of the heat transfer tube by a segment of the second third of the longitudinal extension of the heat transfer tube inside the fluid chamber or the fluid chamber cavity. Preferably, this applies when the heat exchanger, particularly the stripper, is part of a urea plant for urea production, particularly for urea synthesis, and the urea plant has a production capacity equal to or greater than 2700 MTPD (metric tons per day). It has been shown that material removal or consumption of the heat transfer tube is usually particularly large in this arrangement area of each heat transfer tube, which is why it is beneficial to position the optical fiber sensor within this area.Typically, the placement area is located within and / or within the second third of the first third of the length of the fluid chamber or fluid chamber cavity, particularly in the flow direction of the first fluid through the heat transfer tube. The central region of the length of the fluid chamber or fluid chamber cavity is preferably thus in the second third.

[0041] Another object is one or more heat transfer tubes, and optical fiber sensors are respectively arranged for one or more heat transfer tubes used when the first fluid is transported through the heat transfer tube to transfer heat between the first fluid and the second fluid. Here, when the elastic vibration, particularly the natural vibration, of each heat transfer tube is confirmed by using an optical fiber sensor during the operation of the heat exchanger to determine the wall thickness of the tube wall of each heat transfer tube by using interferometric spectroscopy during the operation of the heat exchanger, the method of the type mentioned at the beginning for operating the heat exchanger is achieved. The method can be implemented particularly by using the aforementioned heat exchanger. Typically, the wall thickness of the heat transfer tube indicates the distance, particularly the radial distance, between the inner surface and the outer surface of the tube wall of the heat transfer tube in the cross-section of the heat transfer tube. Typically, the second fluid is positioned outside the heat transfer tube, and as a result, heat is transferred between the first fluid and the second fluid through the tube wall of the heat transfer tube.

[0042] Particularly as described above, a robust determination of the wall thickness of the tube wall of each heat transfer tube in the heat exchanger, particularly in a high-pressure and / or high-temperature environment, is thereby made possible during the operation of the heat exchanger. As a result, the operation of the heat exchanger, particularly process management, and / or maintenance can be carried out according to the wall thickness determined by using the optical fiber sensor. In this way, an optimized usefulness of the heat exchanger or an optimized operation of the heat exchanger becomes possible.

[0043] It should be understood that the method for operating the heat exchanger can be embodied within the scope of the heat exchanger in accordance with the features and effects particularly described above herein. The same also applies to the heat exchanger regarding the method.

[0044] The optical fiber sensor comprises an optical measurement fiber constituting a measurement section and an optical reference fiber constituting a reference section. The measurement fiber is connected in a manner that transmits vibration to the heat transfer tube. It is advantageous to detect an interference signal from the elastic vibration of the heat transfer tube, particularly when the frequency of the elastic vibration is guided along the measurement section and the electromagnetic wave guided along the reference section, especially when confirmed by measurement. Typically, interference is generated between the electromagnetic wave guided along the measurement section and the electromagnetic wave guided along the reference section, and as a result, the electromagnetic waves create an interference signal. The interference signal is typically detected using a detector to confirm the frequency of the elastic vibration of the heat transfer tube, particularly the natural frequency, using the detected interference signal. A plurality of frequencies of the elastic vibration, particularly the natural frequency, can be confirmed. It is possible to determine the wall thickness of the tube from one or more frequencies, particularly the natural frequency.

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

[0046] A method for operating a heat exchanger is particularly beneficial when used in urea synthesis. The heat exchanger may be embodied as a stripper for stripping, particularly for urea synthesis, and typically it is provided that a liquid phase and a gas phase having opposite flow directions are brought into contact with each other, usually inside the heat transfer tubes. For reacting with each other, a first medium flows through each heat transfer tube in the flow direction, and a second medium flows through the heat transfer tubes in a direction opposite to the flow direction, and it is beneficial when one of the media is usually a liquid and the other medium is a gas. This typically takes place inside a fluid chamber or a fluid chamber cavity. The first fluid may be formed such that it contains or is made from the first medium and the second medium. Particularly when the heat exchanger is a stripper, it is beneficial when the heat transfer tubes and the flow direction of the first fluid through the heat transfer tubes are essentially vertically directed. The heat exchanger or stripper typically comprises a plurality of heat transfer tubes, particularly more than 10, preferably more than 50, particularly preferably more than 100, and especially preferably more than 1000.

[0047] Typically, a heat exchanger, particularly when the heat exchanger is a stripper, includes a first inlet through which a first medium can be fed to a heat transfer tube and a second inlet through which a second medium can be fed to the heat transfer tube. As a result, inside a fluid chamber or a fluid chamber cavity, the media flow through the heat transfer tube in opposite flow directions 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 manner that induces fluid at different ends of the heat transfer tube. The heat exchanger typically includes at least one outlet for removing a product formed by the reaction between the first medium and the second medium from the heat transfer tube. Practically, the heat exchanger may include a first outlet through which a first product can be removed from the heat transfer tube and a second outlet through which a second product can be removed from the heat transfer tube. With respect to the fluid chamber cavity, the outlets are connected to the heat transfer tube in a manner that induces fluid at different ends of the heat transfer tube. The first product and the second product are typically formed using or as a result of the reaction between the first medium and the second medium. This is particularly true when the heat exchanger is embodied as a stripper.

[0048] For urea synthesis, the first medium is typically formed such that it contains urea, ammonium carbamate, and ammonia, especially as made therefrom, and the second medium is formed such that it contains gaseous carbon dioxide (CO₂), especially as made therefrom. In this way, the product, especially the first product, especially high-purity urea, can be separated, and the urea is typically induced out of the heat transfer tube at one of the ends of the heat transfer tube or via the first outlet. Conveniently, the formed process gas, usually gaseous ammonia (NH₃) and / or gaseous carbon dioxide (CO₂), can generally be induced out of the heat transfer tube at the other end of the heat transfer tube or via the second outlet. The second fluid can be formed such that it contains liquid and / or gaseous water, especially as made therefrom. The stripper can be embodied and operated as described herein, especially with respect to the heat exchanger.

[0049] It is advantageous if the electromagnetic wave guided using the measurement fiber or the reference fiber has a coherence length exceeding 2 mm, especially exceeding 5 mm. As a result, a robust interference signal can be realized by generating interference between the electromagnetic wave guided along the measurement section and the electromagnetic wave guided along the reference section. In particular, the coherence length can be as described above.

[0050] The second fluid typically has a pressure exceeding 30 bar (3 MPa), especially between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or a temperature exceeding 80 °C, especially between 80 °C and 300 °C, preferably approximately 230 °C.

[0051] Embodiments and / or arrangements of the optical fiber sensor and its heat transfer tube typically refer to the heat transfer tube where the optical fiber sensor is disposed or to which the optical fiber sensor is connected to measure the wall thickness of the heat exchanger tube. Advantageously, a plurality of optical fiber sensors may be disposed or connected to various heat transfer tubes, particularly using the respective implementations described herein.

Brief Description of the Drawings

[0052] Additional features, advantages, and effects of the present invention will be understood from the following description of exemplary embodiments. In the drawings referred to thereby:

[0053]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Modes for Carrying Out the Invention

[0054] In FIG. 1, a heat exchanger 1 is schematically illustrated, where the heat exchanger 1 comprises a plurality of heat transfer tubes 3 and a fluid chamber 4. Here, the heat transfer tubes 3 are for guiding a first fluid F1 through the heat transfer tubes 3 during operation of the heat exchanger 1, and for guiding the second fluid F2 through the fluid chamber 4 such that the second fluid F2 surrounds the heat transfer tubes. As a result, heat is transferred between the first fluid F1 and the second fluid F2 through the tube walls 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 for accommodating the second fluid F2, and the second fluid F2 is guided through the cavity. The fluid chamber 4 comprises a fluid chamber inlet 6 for feeding the second fluid F2 to the fluid chamber 4, in particular to the fluid chamber cavity 5, and a fluid chamber outlet 7 for removing fluid from the fluid chamber 4, in particular from the fluid chamber cavity 5. Typically, the heat transfer tubes 3 are guided through the fluid chamber 4 such that they are spaced apart from each other, and as a result, the second fluid F2 can flow between the heat transfer tubes 3. The heat exchanger 1 may be embodied as a stripper. The heat exchanger 1, in particular the stripper, is often embodied or oriented such that the longitudinal direction of the heat transfer tubes 3 is essentially vertically oriented.

[0055] The optical fiber sensor 2 is arranged for a plurality of heat transfer tubes 3 to determine the wall thickness of each heat transfer tube 3 using each heat exchanger 1 during the operation of the heat exchanger 1. Each optical fiber sensor 2 is embodied to confirm the frequency, particularly the natural frequency of the elastic vibration of the heat transfer tube 3 during the operation of the heat exchanger 1, using the interference spectroscopy method. Each optical fiber sensor 2 includes an optical measurement fiber M and an optical reference fiber R to guide the electromagnetic measurement wave along the measurement section using the measurement fiber M and to guide the electromagnetic reference wave along the reference section using the reference fiber R, which can also be understood from FIG. 2. The measurement fiber M and the reference fiber R are connected to the same heat transfer tube 3, where the interaction segments 21 of the measurement fiber M and the reference fiber R on the heat transfer tube 3 are wound around the same heat transfer tube 3 adjacent to each other a plurality of times. Each interaction segment 21 is formed using the end region of the measurement fiber M or the reference fiber R. The measurement fiber M is connected to the heat transfer tube 3 in a manner that transmits vibration, and as a result, the elastic vibration of the heat transfer tube 3 changes the optical length of the measurement section. The reference fiber R is connected to the heat transfer tube 3 in a manner that decouples vibration, and as a result, 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 the laser as an electromagnetic radiation source L to bring the electromagnetic wave as an electromagnetic measurement wave to the measurement fiber M and the electromagnetic wave as an electromagnetic reference wave to the reference fiber R using the laser. Due to the change in the length of the measurement section, an optical path difference between the measurement wave and the reference wave can be generated, and as a result, the elastic vibration, particularly the natural frequency of the elastic vibration, can be detected or measured using the detector PD of the optical fiber sensor 2 by generating the interference of the measurement wave and the interference of the reference wave to create an interference signal. The measurement fiber M and the reference fiber R each include a reflection element at the end of the fiber to reflect the measurement wave and the reference wave back along the measurement fiber M and the reference fiber R. The measurement fiber M and the reference fiber R are coupled to each other at the coupling site to create an interference signal using the measurement wave and the reference wave.The interaction segment 21 of the measurement fiber M and the interaction segment 21 of the reference fiber R are each connected to a respective heat transfer tube 3 inside the fluid chamber 4, particularly inside the fluid chamber cavity 5. The measurement fiber M and the reference fiber R are guided outwards through the fluid chamber wall of the fluid chamber 4 in order to measure the interference signal outside the fluid chamber 4 using the detector PD. The detector PD and the electromagnetic radiation source L are located outside the fluid chamber 4 or the fluid chamber cavity 5, typically inside the sensor housing 9. The measurement fiber M and the reference fiber R are typically guided through the fluid chamber wall in a liquid-tight manner using one or more fiber feed-throughs 8. The optical fiber sensor 2 may typically be connected via an electronic data line 10 to an electronic data collection unit 18 for data transmission, particularly a shared one. The electronic data collection unit 18 may be, for example, an electronic data processing system. In order to withstand high temperatures and / or high pressures inside the heat exchanger 1, it is beneficial if, inside the fluid chamber 4, the measurement fiber M and the reference fiber R each extend inside a protective sheath that can be embodied as a coating applied to the measurement fiber M and the reference fiber R. It is beneficial if there are a plurality of separate optical fiber sensors 2, particularly if these each comprise an individual sensor housing 9 and an individual electromagnetic radiation source L, in order to keep the lengths of the measurement fiber M and the reference fiber R short.

[0056] Typically, the heat transfer tubes 3 extend between the first tube plate 11 and the second tube plate 12, and the tube plates are embodied such that they are part of the fluid chamber wall of the fluid chamber 4 or define the boundary of the fluid chamber cavity 5. Each heat transfer tube 3 is guided through the first tube plate 11 and the second tube plate 12. The fluid chamber 4 typically comprises a plurality of stabilizing elements 13, shown 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. The interaction segments 21 of the measurement fiber M and the reference fiber R of each optical fiber sensor 2 are arranged in the arrangement area on each heat transfer tube 3, and it is advantageous if this arrangement area is at the position of the first third and / or the second third of the longitudinal extension of the heat transfer tube 3 inside the fluid chamber 4 or the fluid chamber cavity 5 in the flow direction of the first fluid F1 through the heat transfer tube 3. Preferably, the interaction segments 21 of the measurement fiber M and the reference fiber R are connected to the heat transfer tube 3 between the first tube plate 11 and the first stabilizing element 13 in the flow direction of the first fluid F1 through the heat transfer tube 3 among the stabilizing elements 13.

[0057] Figure 2 shows a schematic diagram of the design of the optical fiber sensor 2 from Figure 1, and this optical fiber sensor 2 is arranged with respect to each heat transfer tube 3. The optical fiber sensor 2 includes an optical coupler 19 that forms a coupling site to couple the measurement fiber M and the reference fiber R to each other to create an interference signal. The optical coupler 19 includes a plurality of, for example, three inputs and a plurality of, for example, two outputs. The electromagnetic radiation source L is connected to one of the inputs, and the measurement fiber M and the reference fiber R are each connected to one of the outputs. As a result, the electromagnetic wave generated using the electromagnetic radiation source L is transmitted as a measurement wave to the measurement fiber M and as a reference wave to the reference fiber R, particularly in the transmission direction S. Each detector PD is connected to a plurality of other inputs. As a result, the measurement wave reflected back through the measurement fiber M and the reference wave reflected back through the reference fiber R can be detected as an interference signal at the other inputs using each detector PD. Usually, the inputs and outputs of the optical coupler 19 are connected to each other. As a result, the electromagnetic wave conducted through one of the inputs is transmitted so that it is distributed to the output, and the electromagnetic wave conducted through 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 of the heat transfer tube 3, particularly the natural frequency, can be measured using the detector PD. The detector PD is typically embodied as a photodiode. The detector PD is typically connected to an electronic data acquisition unit 17 for the transmission of data, usually via an electronic data line 10. The electronic data acquisition unit 17 can be connected to an electronic data collection unit 18 for the transmission of data. An optical isolator 14 can be arranged between the electromagnetic radiation source L and the optical coupler 19 to minimize the back reflection of the electromagnetic wave fed to the optical coupler 19 using the electromagnetic radiation source L. The electromagnetic radiation L is typically electrically connected to an electrical control unit 15 for controlling the radiation source L. An electrical amplifier 16, particularly a transimpedance amplifier, can be arranged between each detector PD and the electronic data acquisition unit 17 to amplify the interference signal detected using the detector PD.In various heat transfer tubes 3, the optical fiber sensors 2 embodied in such a manner can each be arranged to determine the wall thickness of each heat transfer tube 3.

[0058] FIG. 3 shows, as an example, a graph showing the relationship between the measured resonance frequency or natural frequency of the elastic vibration of the heat exchanger tube 3 and the wall thickness of the tube wall of the heat exchanger tube 3. The linear relationship between the resonance frequency and the wall thickness is shown by a linear fitting curve. A plurality of resonance frequencies or natural frequencies can be confirmed to determine the wall thickness.

[0059] FIG. 4 shows a schematic view of a further heat exchanger 1 comprising a plurality of optical fiber sensors 2. The heat exchanger 1 can be embodied according to the description related to the heat exchanger 1 from FIG. 1. In contrast to the optical fiber sensors 2 of the heat exchanger 1 from FIG. 1, the optical fiber sensors 2 according to FIG. 4 have a shared electromagnetic radiation source L in the form of a laser. This is shown in FIG. 5. FIG. 5 shows a schematic view of the optical fiber sensors 2 arranged for various heat transfer tubes 3 with a shared electromagnetic radiation source L. The individual optical fiber sensors 2 from FIG. 5 can be designed corresponding to the optical fiber sensors 2 from FIG. 2. In contrast to FIG. 2, the optical coupler 19 of each optical fiber sensor 2 from FIG. 5 is coupled to the shared electromagnetic radiation source L via an optical feed fiber in order to feed the electromagnetic wave generated using the electromagnetic radiation source L to the optical coupler 19 such that the wave is split into the optical coupler 19. The feed fiber comprises a main branch and a plurality of branch branches branching from the main branch in order to guide the electromagnetic wave conducted to the main branch using the electromagnetic radiation source L to the input of each optical coupler 19, such that the wave is split into the branch branches. Further, the individual optical fiber sensors 2 can have a shared electronic data acquisition unit 17 to which the detector PD of the optical fiber sensor 2 is connected for data transmission.

[0060] FIG. 6 shows a schematic view of a further optical fiber sensor 2 arranged with respect to the heat transfer tube 3. The optical fiber sensor 2 can be the optical fiber sensor 2 of the heat exchanger 1 from FIG. 1 or can be embodied according to the characteristics of the optical fiber sensor 2 from FIG. 2. In contrast to the optical fiber sensor 2 from FIG. 2, the optical fiber sensor 2 from FIG. 6 has two electromagnetic radiation sources L of electromagnetic waves of different wavelengths and different coherence lengths. For example, one of the electromagnetic radiation sources L can be a laser having a laser light wavelength of 1300 nm, and the other electromagnetic radiation source L can be a laser having a laser light wavelength of 1550 nm. One of the lasers can have a coherence length between 0.5 mm and 10 mm, for example approximately 5 mm, and the other laser can have a coherence length between 10 μm and 500 μm, for example approximately 30 μm. The two electromagnetic radiation sources L1, L2 are coupled to the optical coupling unit 20 and are typically each connected to the input line of the optical coupling unit 20 so that the electromagnetic waves generated using the electromagnetic radiation source L are output to 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 the optical coupler 19 for the transmission of electromagnetic waves in order to feed the electromagnetic waves to the measurement fiber M and the reference fiber R via the optical coupler 19. An optical isolator 14 can be arranged between the optical coupling unit 20 and the optical coupler 19 to minimize back reflection.

[0061] In this way, the superimposed measurement waves of different wavelengths and different coherence lengths can be used across the measurement fiber M or along the measurement section, and the superimposed reference waves of different wavelengths and different coherence lengths can be used across the reference fiber R or along the reference section. Accordingly, at the other input of the optical coupler 19 to which the detector PD is connected for detecting the interference signal, two superimposed interference signals are generated for detection using the detector PD as a result of the measurement wave reflected to return along the measurement fiber M and the reference wave reflected to return along the reference fiber R. Between each input of the detector PD and the optical coupler 19, one wavelength-selective demultiplexer DM is respectively arranged to output interference signals from electromagnetic waves of different wavelengths at different outputs of the demultiplexer DM. For measuring the interference signal, one detector PD is respectively connected to the output of each demultiplexer DM. 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 a particularly accurate determination of the natural frequency or the wall thickness of the pipe. The detector PD can be connected to the shared electronic data acquisition unit 17 for data transmission.

[0062] FIG. 7 shows a schematic view of a further heat exchanger 1 embodied as a stripper for stripping, where the optical fiber sensor 2 is arranged with respect to a plurality of heat transfer tubes 3 of the heat exchanger 1 in order to determine the 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 embodied according to the explanations related to the heat exchanger 1 and the optical fiber sensor 2 of FIGS. 1 to 6, or can be provided with a corresponding optical fiber sensor 2. The heat exchanger 1 is typically oriented such that the longitudinal extension of the heat transfer tubes 3 is essentially vertically oriented. For urea synthesis, a first fluid F1 is formed such that it comprises or is made from a first medium M1 and a second medium M2, and inside the fluid chamber 4 or the fluid chamber cavity 5, the first medium M1 and the second medium M2 are provided to flow through their respective heat transfer tubes 3 in opposite flow directions. Usually, the first medium M1 is formed such that it comprises, in particular is made from, urea, ammonium carbamate, and ammonia, and the second medium M2 is formed such that it comprises, in particular is made from, gaseous carbon dioxide (CO2). The heat exchanger 1 or stripper typically comprises a plurality of, in particular more than 10, preferably more than 50, particularly preferably more than 100, especially preferably more than 1000 heat transfer tubes 3. The heat exchanger 1 is typically oriented such that a first tube plate 11 is positioned above a second tube plate 12 in the vertical direction. Preferably, each optical fiber sensor 2 is positioned between the first tube plate 11 and the first of the stabilizing elements 13 of the stabilizing elements 13.

[0063] The heat exchanger 1 comprises a first inlet 22 through which a first medium M1 can be fed to the heat transfer tubes 3, and a second inlet 24 through which a second medium M2 can be fed to the heat transfer tubes 3. As a result, inside the fluid chamber 4 or the fluid chamber cavity 5, the media M1, M2 flow through the heat transfer tubes 3 in opposite flow directions in order 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 tubes 3 in a way that guides the fluid at different ends of the heat transfer tubes 3. For this purpose, the first inlet 22 and the second inlet 24 can be connected to a fluid distribution chamber in a way that guides the fluid respectively, and the ends of the heat transfer tubes 3 are connected to the fluid distribution chamber in a way that guides the fluid respectively. As a result, the first medium M1 and the second medium M2 fed to the respective fluid distribution chambers through the first inlet 22 and the second inlet 24 are respectively guided to the heat transfer tubes 3, and as a result, they are distributed to the heat transfer tubes 3. The heat exchanger 1 comprises a first outlet 23 through which a first product Z1 can be taken out from the heat transfer tubes 3, and a second outlet 25 through which a second product Z2 can be taken out from the heat transfer tubes 3. With respect to the fluid chamber cavity 5, the first outlet 23 and the second outlet 25 are connected to the heat transfer tubes 3 in a way that guides the fluid at different ends of the heat transfer tubes 3. Preferably, the first outlet 23 and the second outlet 25 are respectively connected to one of the fluid distribution chambers in a way that guides the fluid. As a result, the first product Z1 and the second product Z2 exiting from the heat transfer tubes 3 can be taken out through the respective outlets 23, 25. The first product Z1 is usually urea of particularly high purity. The second product Z2 is usually gaseous ammonia (NH3) and / or gaseous carbon dioxide (CO2). Usually, the second fluid F2 is formed such that it contains liquid and / or gaseous water, particularly such that it is made from them.

[0064] When optical fiber sensors 2 are respectively arranged for one or more of the heat transfer tubes 3 of the heat exchanger 1 to confirm the natural frequency or resonance frequency of the elastic vibration of each heat transfer tube 3 during the operation of the heat exchanger 1 by using optical interference spectroscopy, the tube thickness of each heat transfer tube 3 can be practically determined during the operation of the heat exchanger 1. Preferably, the optical fiber sensor 2 can be used at an operating pressure exceeding 30 bar (3 MPa), particularly between 30 bar (3 MPa) and 200 bar (20 MPa), and / or at an operating temperature exceeding 80°C, particularly between 80°C and 300°C, or is designed to be arranged for each heat transfer tube 3. Thereby, it becomes possible to optimize the usefulness of the heat exchanger 1. (Other possible items) (Item 1) A heat exchanger (1) comprising a plurality of heat transfer tubes (3) for transporting the first fluid (F1) for transferring heat between the first fluid (F1) and the second fluid (F2) through the heat transfer tubes (3), particularly a high-pressure heat exchanger for urea synthesis, wherein optical fiber sensors (2) are respectively arranged for one or more of the heat transfer tubes (3), and wherein the optical fiber sensors (2) are designed to confirm the elastic vibration, particularly the natural vibration, of each heat transfer tube (3) during the operation of the heat exchanger (1) by using optical interference spectroscopy in order to determine the wall thickness of each heat transfer tube (3) during the operation of the heat exchanger (1). (Item 2) The optical fiber sensor (2) has an optical measurement fiber (M) constituting a measurement section and an optical reference fiber (R) constituting a reference section. The measurement fiber (M) is connected in a manner of transmitting vibration to the heat transfer tube (3) for detecting, by using a detector (PD) of the optical fiber sensor (2), an interference signal created by using an electromagnetic wave guided along the measurement section and an electromagnetic wave guided along the reference section, and is preferably wound around the heat transfer tube (3). The heat exchanger (1) according to Item 1. (Item 3) The reference fiber (R) is connected to the heat transfer tube (3) in a manner that decouples vibration, and is preferably wound around the heat transfer tube (3). The heat exchanger (1) according to item 2. (Item 4) The heat exchanger (1) includes a fluid chamber (4) for accommodating the second fluid (F2), the heat transfer tube (3) extends inside the fluid chamber (4), and the measurement fiber (M) is connected to the heat transfer tube (3) inside the fluid chamber (4) in a manner that transmits vibration. The detector (PD) of the optical fiber sensor (2) is disposed outside the fluid chamber (4) for detecting the interference signal. The heat exchanger (1) according to item 2 or 3. (Item 5) The optical fiber sensor (2) includes an electromagnetic radiation source (L) for generating electromagnetic waves, preferably a laser. The radiation source (L) is coupled to the measurement fiber (M) and the reference fiber (R) to bring electromagnetic waves to both the measurement fiber (M) and the reference fiber (R). The heat exchanger (1) according to any one of items 2 to 4. (Item 6) The measurement fiber (M) and the reference fiber (R) each include or are connected to such a reflection element for reflecting electromagnetic waves conducted along the measurement section and the reference section using the reflection element. The heat exchanger (1) according to any one of items 2 to 5. (Item 7) The measurement fiber (M) and the reference fiber (R) are coupled to each other at a coupling site to create an interference signal using the electromagnetic waves transmitted along the measurement section and the electromagnetic waves transmitted along the reference section. The heat exchanger (1) according to any one of items 2 to 6. (Item 8) The optical fiber sensor (2) has an optical coupler (19) including a plurality of input lines and a plurality of output lines, the input lines and the output lines being connected to each other for the distributed transmission of electromagnetic waves, the electromagnetic radiation source (L) being connected to one of the input lines, and the measurement fiber (M) and the reference fiber (R) being respectively connected to one of the output lines, such that the electromagnetic wave introduced into the input line using the radiation source (L) is conducted via the output line to the measurement fiber (M) and the reference fiber (R), the heat exchanger (1) according to any one of items 2 to 7. (Item 9) The detector (PD), preferably formed to include a photodiode, is respectively connected to one or more of the input lines to detect, using the respective detectors (PD), the electromagnetic waves reflected back to the output line along the measurement fiber (M) and the reference fiber (R) as an interference signal in the input line, the heat exchanger (1) according to item 8. (Item 10) The measurement fiber (M) extends at least partially through the second fluid (F2) during operation of the heat exchanger (1), and the optical fiber sensor (2) is designed such that the measurement fiber (M) and the reference fiber (R) can be used at an operating pressure exceeding 30 bar (3 MPa) and / or an operating temperature exceeding 80°C, the heat exchanger (1) according to any one of items 2 to 9. (Item 11) The measurement fiber (M) and the reference fiber (R) extend at least partially, preferably inside a protective sheath formed to include metal or polyimide for protection against ambient pressure and / or ambient temperature, the heat exchanger (1) according to any one of items 2 to 10. (Item 12) A method for operating a heat exchanger (1), in particular the heat exchanger (1) according to any one of items 1 to 11, comprising one or more heat transfer tubes (3) through which a first fluid (F1) is transported for transferring heat between the first fluid (F1) and a second fluid (F2), and an optical fiber sensor (2) is respectively arranged with respect to each of the one or more heat transfer tubes (3), wherein the elastic vibration, in particular the natural vibration, of each of the heat transfer tubes (3) is confirmed by using the optical fiber sensor (2) during operation of the heat exchanger (1) to determine the wall thickness of each of the heat transfer tubes (3) during operation of the heat exchanger (1) by using interferometry. (Item 13) The optical fiber sensor (2) has an optical measurement fiber (M) constituting a measurement section and an optical reference fiber (R) constituting a reference section, the measurement fiber (M) is connected in a manner of transmitting vibration to the heat transfer tube (3), and the elastic vibration of the heat transfer tube (3) is confirmed by detecting an interference signal from an electromagnetic wave guided along the measurement section and an electromagnetic wave guided along the reference section. The method according to item 12. (Item 14) The electromagnetic wave guided by using the measurement fiber (M) or the reference fiber (R) has a coherence length exceeding 2 mm, in particular exceeding 5 mm. The method according to item 13. (Item 15) The second fluid (F2) typically has a pressure exceeding 30 bar (3 MPa), in particular between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or a temperature exceeding 80 °C, in particular between 80 °C and 300 °C, preferably approximately 230 °C. The method according to any one of items 12 to 14.

Claims

**Claim 1** A heat exchanger comprising a plurality of heat transfer tubes for transporting a first fluid for transferring heat between a first fluid and a second fluid through the plurality of heat transfer tubes, in particular a high-pressure heat exchanger for urea synthesis, wherein an optical fiber sensor is respectively arranged for one or more of the plurality of heat transfer tubes, wherein the optical fiber sensor is designed to confirm the elastic vibration, in particular the natural vibration, of each of the heat transfer tubes during operation of the heat exchanger by using interference spectroscopy in order to determine the wall thickness of each of the heat transfer tubes during operation of the heat exchanger. Heat exchanger. **Claim 2** The optical fiber sensor has an optical measurement fiber constituting a measurement section and an optical reference fiber constituting a reference section, and the optical measurement fiber is arranged along the measurement section. The electromagnetic wave and the interference signal generated by using the electromagnetic wave guided along the reference section are detected by using the detector of the optical fiber sensor, and are connected to the heat transfer tube in a manner of transmitting vibration, preferably wound around the heat transfer tube. The heat exchanger according to claim 1. **Claim 3** The optical reference fiber is connected to the heat transfer tube in a manner of decoupling vibration, preferably wound around the heat transfer tube. The heat exchanger according to claim 2. **Claim 4** The heat exchanger includes a fluid chamber for accommodating the second fluid, the heat transfer tube extends inside the fluid chamber, and the optical measurement fiber is arranged inside the fluid chamber. The heat exchanger is connected to the heat transfer tube in a manner of transmitting vibration, and a detector of the optical fiber sensor is arranged outside the fluid chamber for detecting the interference signal. The heat exchanger according to claim 2. **Claim 5** The optical fiber sensor includes an electromagnetic radiation source, preferably a laser, for generating an electromagnetic wave, and the electromagnetic radiation source is coupled to the optical measurement fiber and the optical reference fiber for bringing the electromagnetic wave to the optical measurement fiber and the optical reference fiber. The heat exchanger according to claim 2. **Claim 6** The heat exchanger according to claim 2, wherein the optical measurement fiber and the optical reference fiber each include or are connected to such a reflection element for reflecting electromagnetic waves conducted along the measurement section and the reference section using a reflection element.

7. The heat exchanger according to claim 2, wherein the optical measurement fiber and the optical reference fiber are coupled to each other at a coupling site to create an interference signal using the electromagnetic waves transmitted along the measurement section and the electromagnetic waves transmitted along the reference section.

8. The optical fiber sensor has an optical coupler including a plurality of input lines and a plurality of output lines, the plurality of input lines and the plurality of output lines are connected to each other for distributed transmission of electromagnetic waves, the electromagnetic radiation source is connected to one of the plurality of input lines, and the optical measurement fiber and the optical reference fiber are respectively connected to one of the plurality of output lines. As a result, the electromagnetic waves provided to the input line using the electromagnetic radiation source are conducted to the optical measurement fiber and the optical reference fiber via the plurality of output lines. The heat exchanger according to claim 5.

9. The detector, preferably formed to include a photodiode, is respectively connected to one or more of the plurality of input lines to detect, as an interference signal, the electromagnetic waves reflected back to the output lines along the optical measurement fiber and the optical reference fiber using each of the detectors. The heat exchanger according to claim 8.

10. The optical measurement fiber extends at least partially into the second fluid during operation of the heat exchanger, and the optical fiber sensor is designed such that the optical measurement fiber and the optical reference fiber can be used at an operating pressure exceeding 30 bar (3 MPa) and / or an operating temperature exceeding 80°C. The heat exchanger according to claim 2.

11. The optical measurement fiber and the optical reference fiber extend at least partially inside a protective sheath preferably formed to include metal or polyimide for protection against ambient pressure and / or ambient temperature. The heat exchanger according to claim 2.

12. A method for operating a heat exchanger, in particular a heat exchanger according to any one of claims 1 to 11, comprising one or more heat transfer tubes through which a first fluid is transported for transferring heat between the first fluid and a second fluid, wherein an optical fiber sensor is respectively arranged for each of the one or more heat transfer tubes, and wherein the elastic vibration, in particular the natural vibration, of each of the heat transfer tubes is confirmed by using the optical fiber sensor during operation of the heat exchanger to determine the wall thickness of each of the heat transfer tubes during operation of the heat exchanger by using interference spectroscopy.

13. The optical fiber sensor has an optical measurement fiber constituting a measurement section and an optical reference fiber constituting a reference section, the optical measurement fiber is connected to the heat transfer tube in a manner of transmitting vibration thereto, and the elastic vibration of the heat transfer tube is confirmed by detecting an interference signal from an electromagnetic wave guided along the measurement section and an electromagnetic wave guided along the reference section. The method according to claim 12.

14. The electromagnetic wave guided by using the optical measurement fiber or the optical reference fiber has a coherence length exceeding 2 mm, in particular exceeding 5 mm. The method according to claim 13.

15. The second fluid typically has a pressure exceeding 30 bar (3 MPa), in particular between 30 bar (3 MPa) and 200 bar (20 MPa), preferably approximately 180 bar (18 MPa), and / or a temperature exceeding 80 °C, in particular between 80 °C and 300 °C, preferably approximately 230 °C. The method according to claim 12.

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