WO2023088930A1

WO2023088930A1 - Method of removing deposits from a surface of a heat exchanger

Info

Publication number: WO2023088930A1
Application number: PCT/EP2022/082062
Authority: WO
Inventors: Benjamin DUGONE
Original assignee: Hitachi Zosen Inova Ag
Priority date: 2021-11-17
Filing date: 2022-11-16
Publication date: 2023-05-25

Abstract

Method of removing deposits from a heat exchanger surface (50) arranged in a boiler system, comprising the steps of: - applying a cleaning excitation force to the heat exchanger surface (50) to induce cleaning vibrations to remove deposits; characterized by a step of detecting deposits on the heat exchanger surface (50) comprising the steps of: - applying a detection excitation force to the heat exchanger surface (50) to induce detection vibrations of the heat exchanger surface; - measuring amplitudes and frequencies of the detection vibrations; - estimating their deviation from reference amplitudes and frequencies of the heat exchanger surface (50); - assessing the presence of deposits (40) by means of the deviations; - determining a deposit region (100b) on which deposits must be removed; and by a step of choosing the cleaning excitation force that induce a deflection maximum of the cleaning vibrations at least in the deposit region (50).

Description

Method of removing deposits from a surface of a heat exchanger

The present invention relates to a method of removing deposits from a surface of a heat exchanger arranged in a boiler system according to the preamble of claim 1 and a device for generating and applying an excitation force to the surface of the heat exchanger according to the preamble of claim 18 .

WO2007136698A2 discloses a device coupled to a heat exchanger for mitigating fouling by applying a mechanical force to a fixed heat exchanger to generate vibrations in the heat exchange surface and produce shear waves in the fluid adj acent the heat exchange surface while the heat exchanger is in operation . An electromagnetic driven impulse device in the form of an impactor striking an impact surface connected to the heat exchange surface induces vibration onto heat exchanger tubes and/or an acoustic wave through the fluid to reduce fouling . The device can be mounted directly onto the outer part or piping and produces acoustical/vibrational waves onto the tube or near the surface of tubes .

WO2007136697A2 relates to fouling of heat exchange surfaces , wherein fouling is mitigated by a process in which a mechanical force is applied to a fixed heat exchanger to generate vibrations in the heat exchange surface and produce shear waves in the fluid adj acent the heat exchange surface . The mechanical force is applied by a dynamic actuator coupled to a controller to produce vibration at a controlled frequency and amplitude output that minimi zes adverse ef fects to the heat exchange structure . The dynamic actuator may be coupled to the heat exchanger in place and operated while the heat exchanger is in operation . US2016/ 025485 discloses a boiler system that includes a boiler having at least one heat exchanger having a surface on which a deposit may form . The boiler system further includes at least one retractable soot blower having a lance tube for carrying a high pressure fluid into the boiler . The lance tube is configured such that the high pressure fluid impacts the heat exchanger surface to ef fect a vibration thereof . The boiler system also includes at least one vibration measuring device coupled to the boiler system . The vibration measuring device is configured to measure the vibration of the heat exchanger surface , and the measured vibration indicates presence or absence of the deposit on the heat exchanger surface . The vibration measuring device may optionally detect a vibration caused by the release of the deposit from the surface of the heat exchanger or the impact of the released deposit with a surface in the boiler system .

US 8156819B2 discloses a method that is provided for checking a knocking device which is intended for cleaning a surface of a heat exchanger element arranged on an inside of a boiler housing of a garbage incineration plant . The knocking device has a knocking ram guided through a wall of the boiler housing and movable in a direction toward the inside of the boiler . The heat exchanger element is deflected out of its position of rest into a deflection position by the knocking ram, and the force required for deflecting the heat exchanger element and/or the attenuation behavior of the heat exchanger element released after deflection are/ is determined .

However, there is a need to develop methods for removing deposits from a surface of a heat exchanger to allow cleaning of speci fic heat exchanger regions in which deposits are present . There is a need to remove deposits while the heat exchanger is operating . Further, there is a need to provide for methods in which the energy used for cleaning is applied speci fically in the heat exchanger regions in which deposits is present . There is also a need to extend the li fetime of the heat exchangers and reduce the need for expensive high-strength materials by preventing that the trans fer of energy is performed by sudden impacts that involve high stresses in the structure .

The obj ect of the present invention is to provide for a method of removing deposits from a surface of a heat exchanger arranged in a boiler system, wherein cleaning is performed in speci fic heat exchanger regions in which deposits are present . A further obj ect of the present invention is to provide for a device for generating and applying vibrational excitation to the surface of the heat exchanger preventing that the trans fer of vibrational excitation is performed by impacts on the heat exchanger .

The problem is solved by a method according to claim 1 and the device according to claim 18 . Preferred embodiments are disclosed in the dependent claims .

The method of removing deposits from a heat exchanger surface of a heat exchanger arranged in a boiler system, comprises the steps of providing a heat exchanger having a heat exchanger surface , providing a device for generating and applying a cleaning excitation force to the heat exchanger surface , wherein the cleaning excitation force is a force applied to generate vibrational excitation intended to be used for cleaning purpose , and applying the cleaning excitation force to the heat exchanger surface to induce cleaning vibrations of the heat exchanger surface . The boiler system comprises a heat exchanger and walls forming a duct designed to conduct a fluid . The heat exchanger is arranged within the walls , i . e . in the duct . While flowing in a direction of flow in the duct through the heat exchanger, the fluid exchanges heat with the heat exchanger, the heat exchanger being designed to collect heat by way of a heat exchanger medium circulating inside the heat exchanger . It is noted that the direction of flow can be parallel to the heat exchanger surface , for example due to convection, or perpendicular to the heat exchanger surface , for example i f the fluid is sucked by an arrangement of fans . The heat is collected for reuse or for convers ion into other forms of energies . At the same time , particles contained in the fluid can form deposits on the surface of the heat exchanger forming a deposit layer acting as an insulation layer between the fluid and the heat exchanger resulting in degradation of the heat collection by the heat exchanger .

Deposition of particles can occur for example in regions of the boiler system in which the fluid stagnates or in which particles accumulate under the ef fect of gravity . Temperature can also play a role because of chemical reactions happening in certain temperature ranges at the heat exchanger surface . Since the temperature of the heat exchanger surface can also vary, deposits can be formed in di f ferent quantities in di f ferent regions of the boiler system . Therefore , the deposit layer can be formed as a layer having a varying thickness and extending over a region having an irregular contour of the heat exchanger surface or over the whole heat exchanger surface . The deposit layer can also be formed as a layer having a plurality of separated regions having an irregular contour . In a preferred embodiment , the boiler system is arranged in an incinerator plant and fluid for example in the form of flue gases resulting from waste combustion in a combustion chamber are directed to the boiler system . The flue gases flow in the duct formed by the walls of the boiler system and heat is extracted by the heat exchanger and collected by a heat exchanger medium for example in the form of water or steam for trans formation in further modules of the incineration plant . Typically, flue gases , i . e . the fluid flowing through the heat exchanger, can have a temperature of 100 ° C to 800 ° C and the heat exchanger medium circulating inside the heat exchanger to collect heat from the fluid has a temperature of 20 ° C to 500 ° C . Typically, deposits formed as a result of the flue gas flow are a mixture of ashes and various salts .

The heat exchanger can comprise a plurality of heat exchanger tubes having an outer surface and being supported by a supporting structure , the outer surface of the plurality of heat exchanger tubes forming the surface of the heat exchanger . It is noted that the heat exchanger surface can comprise a plurality of separate surfaces spaced apart from each other and/or partially connected to each other . It is noted that the heat exchanger tubes can be connected by elements , for example transversal elements designed to enable the flow of medium in the heat exchanger and/or to sti f fen the plurality of heat exchanger tubes , that can also be seen as belonging to the heat exchanger surface .

In a preferred embodiment , the heat exchanger tubes are arranged in a plurality of groups , so-called "harps" , of heat exchanger tubes . Each group of heat exchanger tubes extends in a plane , wherein the respective planes of the groups extend preferably parallel to each other . The planes can be at a constant distance from each other, or the distance can vary along the boiler . In each group of the plurality of groups , the heat exchanger tubes extend longitudinally preferably parallel to each other at a preferably constant distance from each other . Each group of the plurality of groups is supported by the supporting structure inside the duct . In a known manner, the plurality of groups of heat exchanger tubes can be fluidly connected to each other and in each group the heat exchanger tubes can be fluidly connected with each other to allow the circulation of the heat exchanger medium inside the heat exchanger tubes .

In a preferred embodiment , the heat exchanger tubes of each group are attached together in a first end portion of the heat exchanger tubes by a first transversal element and in a second end portion of the heat exchanger tubes opposed to the first end portion by a second transversal element , wherein the first transversal element is supported by the supporting structure and the second transversal element is free or is guided to prevent exces sive displacements of the second end portion . In a preferred embodiment , the first end portion is the upper end portion, and the second end portion is the lower end portion, as seen in the direction o f gravity .

The heat exchanger surface , in particular the heat exchanger tubes , and the walls can be made of metal , preferably steel alloy . Typically, the supporting structure is made of metal , preferably an iron alloy .

Vibrational excitation can be provided by any well-known device for generating and applying vibrational excitation to the heat exchanger . The device can be in the form of a vibrator device , for example an electromagnetic shaker or a pneumatic vibrator, comprising a vibrational excitation source capable of creating vibrational energy .

In a preferred embodiment , the device for generating and applying vibrational excitation is designed to induce vibrations of a heat exchanger surface of a heat exchanger arranged in a boiler system preferably of an incineration plant . The device comprising the vibrational excitation source , a vibrator arm having a first end mechanically connected to the heat exchanger surface and a second end opposed to the first end that is acted upon by the vibrational excitation source .

According to the invention, the first end is in continuous contact with the heat exchanger surface and the second end is in continuous contact with the vibrational excitation source at anytime during application of the cleaning excitation force and of a detection excitation force . . This embodiment has the advantage that the vibrator arm is at anytime in continuous contact with the heat exchanger surface . Consequently, the vibrator arm and the heat exchanger surface are less subj ect to wear for example compared to solutions including an impactor striking an impact surface connected to the heat exchange surface . Consequently, there is also no need to provide for higher grade material in the region of the heat exchanger surface to which the vibrational excitation is applied and a more cost-ef fective manufacturing o f heat exchangers can be achieved . Further, the arrangement allows the application of the detection excitation force and/or the cleaning excitation force in a continuous manner with a constant amplitude to the heat exchanger surface . In a preferred embodiment , the device is characteri zed by an intermediate element arranged between the vibrator arm and the heat exchanger surface and removably fixed at his side facing the heat exchanger to the heat exchanger surface as well as removably fixed to the first end of vibrator arm . I f necessary, only the intermediate element can be replaced in case of wear so that this embodiment allows a simple maintenance of the device .

The intermediate element can also be removably fixed at his side facing the heat exchanger to the supporting structure , to the first transversal element , to the second transversal element of the heat exchanger, to a group of heat exchanger tubes or a single heat exchanger tube with the same advantage .

In a preferred embodiment , the device is attached to the walls by way of a fixing element to provide for a simple interface means between the heat exchanger and the device .

In a preferred embodiment , a plurality of vibrator devices is used, wherein each vibrator device is connected to a group of heat exchangers tubes of the plurality of heat exchanger tubes for applying vibrational excitation to the group of heat exchangers tubes . However, it is also conceivable to connect each time a vibrator device to a heat exchanger tube .

In a preferred embodiment , the vibrator device can be mounted for transmitting vibrational excitation axially to the heat exchanger surface , in particular to the heat exchanger tubes , i . e . to induce longitudinal modes of vibration of the heat exchanger surface , in particular of the heat exchanger tubes .

In a preferred embodiment , the vibrator device can be mounted for transmitting vibrational excitation orthogonal to the heat exchanger surface , in particular to the heat exchanger tubes , i . e . to induce transverse modes of vibration of the heat exchanger surface , in particular of the heat exchanger tubes .

In a preferred embodiment , a plurality of vibrator devices can be mounted for transmitting vibrational excitation axially and perpendicularly to the heat exchanger surface , in particular to the heat exchanger tubes .

The process includes controlling by a controller the frequency or frequencies as well as amplitude of the vibrational excitation, i . e . of the vibrations generated by the vibrator device and induced in the heat exchanger, in particular of the vibrations of the heat exchanger surface . The process can also include controlling the phase of the vibrations generated by the vibrator device and induced in the heat exchanger, in particular of the vibrations of the heat exchanger surface .

Frequency, amplitude and phase can be independently controlled from each other . For example , one signal can be given as an input to the device for generating the cleaning excitation force , but the signal can be generated by independently adj usting the frequencies , amplitudes and phases of components of the signal , e . g . y ( t ) = Al . sin ( 2n . f 1 . t+cpl ) + A2 . sin ( 2n . f 2 . t+cp2 ) + ... + Ai . sin ( 2n . f i . t+cpi ) .

Frequency, amplitude and phase measurements can be measured by way of a plurality of sensors arranged on the heat exchanger surface and connected to the controller, wherein the controller can act on frequency, amplitude and phase of the vibrational excitation depending on the measurements of the plurality of sensors . For example , sensors like strain gauges , piezoelectric accelerometer or proximity sensors can be used for this purpose .

In the context of the present invention, a natural frequency is defined as a vibration frequency of the heat exchanger, in particular of the heat exchanger surface , occurring in the absence of any driving or damping force . Natural frequencies are speci fic to the construction, the geometry and the securing of the heat exchanger . A modal analysis can be performed either with a numerical model or on a real structure of the heat exchanger to find out the natural frequencies and mode shapes of the heat exchanger . In addition, this modal analysis is also used to define best locations to position sensors .

Further, a forced vibration frequency is defined as a vibration frequency that happen at the frequency of an applied force . A resonance frequency is defined as a forced vibration frequency equal to a natural frequency and resulting in an amplitude increase of the vibration .

According to the invention, the method comprises a step of detecting deposits on the heat exchanger surface .

The step of detecting deposits comprises the step of applying the detection excitation force to the heat exchanger surface to induce detection vibrations of the heat exchanger surface , and the step of measuring amplitudes and frequencies of the detection vibrations of the heat exchanger surface . A detection excitation force must be understood as a force generating vibrational excitation intended to be used for detection purpose .

Further, the step of detecting deposits comprises the step of estimating deviations of measured amplitudes and frequencies of the detection vibrations from reference amplitudes and reference frequencies of reference detection vibrations of the heat exchanger surface . As already mentioned, the deposit layer acts as an insulation layer at the heat exchanger surface resulting in degradation of the heat collection by the heat exchanger . Another ef fect of the deposit layer is an increase of mass , sti f fness and damping of the heat exchanger locally so that amplitudes , frequencies , and phases of the vibrations propagating in the heat exchanger di f fer from amplitudes , frequencies , and phases measured at a past point of time at which the heat exchanger mass , sti f fness and damping had locally a di f ferent value because the deposit layer was thinner or not yet present . The definition of reference amplitudes , reference frequencies and, i f applicable , reference phases for reference detection vibrations of the heat exchanger surface is advantageous to monitor the evolution of the deposit layer to decide i f cleaning of the heat exchanger surface is required .

Further, the step of detecting deposits comprises the step of assessing the presence of deposits on the heat exchanger surface by means of the deviations of measured amplitudes and frequencies to determine i f deposits are present . Depending on the condition defined, for example i f no deviation is observed or i f the deviations are within acceptable limits for operation of the heat exchanger or i f the deviations are within the precision limits of the measurement , it can be concluded that deposits are not present . A region of the heat exchanger for which deviations of measured amplitudes , frequencies and, i f applicable , phases are observed can be identi fied as a deposit region . A deviation of measured frequencies can be the result of a shi ft of natural frequency values because the deposit layer has increased locally the heat exchanger mass and/or sti f fness . A deviation of measured amplitudes can be a change of the measured amplitude resulting from di f ferent inertia, sti f fness or damping ef fect caused by the deposit layer .

Further, the step of detecting deposits comprises the step of determining a deposit region of the heat exchanger surface on which deposits must be removed . A region of the heat exchanger for which deviations of measured amplitudes , frequencies and, i f applicable , phases are observed can be identi fied as a deposit region . For proper operation of the heat exchanger an action, for example the removal of the deposit layer, can be required in the deposit region .

Further, according to the invention, the method comprises a step of choosing the cleaning excitation force that induce a deflection maximum of the cleaning vibrations at least in the deposit region . Therefore , the step of applying the cleaning excitation force to the heat exchanger surface can be optimi zed to speci fically target the deposit region determined previously . The term deflection must be construed as a physical displacement or deformation of the heat exchanger surface in at least one direction in comparison to a state of the heat exchanger surface at rest , i . e . not submitted to excitation .

A first advantage is that the region of the heat exchanger where high strain is caused by cleaning vibrations is reduced to a minimum to speci fically address deposit regions while the rest of the heat exchanger remains at least approximately free of induced vibrations , i . e . of strains . The term " at least approximately free" must be construed in view of the vibrational nature of the cleaning vibrations . By construction, vibrations applied in a first region of the heat exchanger can generate vibrations of a second region of the heat exchanger region in mechanical contact with the first region . Therefore , the term characteri zes vibrations having an amplitude at least an order of magnitude less than the amplitude of the induced vibrations , here of the induced cleaning vibrations , and being neglected or having a neglectable ef fect in the method .

A further advantage is that the energy needed to remove the deposit layer on the heat exchanger surface can be reduced because the energy is applied only to deposit regions instead to the whole heat exchanger region . It leads to lower requirements in terms of equipment si ze and material strength compared to prior art solution using for example impactors .

The local deflection maximums created by the cleaning vibrations at least in the deposit region cause cracks in the deposit layer because of high strains induced in the deposit layer by the vibration of the heat exchanger surface . Further, the high acceleration caused by the cleaning vibrations in the deposit layer generate a pull-out force removing the deposit layer .

It is noted that at least one local deflection maximum of the cleaning vibrations is induced at least in the deposit region . Due to the nature of vibrations , a plurality of deflection maximums of the cleaning vibrations can be induced by the cleaning excitation force at least in the deposit region . Further, also due to the nature of vibrations , a further deflection maximum of the cleaning vibrations can be induced simultaneously in a region di f ferent to the deposit layer . However, should it be the case , this latter deflection maximum is not detrimental to the present invention, because the cleaning excitation force achieves its primary effect, i.e. removing deposits in the deposit region.

Further, the duration of the application of the cleaning excitation force can be used as a parameter to improve cleaning of the heat exchanger surface as explained below.

In a preferred embodiment, the cleaning excitation force is composed by one or a plurality of frequencies corresponding to 0.85 and 1.15 times a natural frequency of the heat exchanger surface. This frequency range has been found to maximize the ratio between deflection amplitude/strains at the deflection maximums and to maximize the amplitude of induced cleaning vibrations because of the amplification phenomenon that takes place close to a resonance. Therefore, smaller excitation systems can be used to achieve the strain levels required for cleaning. In a more preferred embodiment, the cleaning excitation force is composed by one or a plurality of frequencies corresponding to 0.95 and 1.05 times a natural frequency of the heat exchanger surface. In this frequency range, the amplification of vibrations is further increased so that higher strains are generated at the deflection maximums. In a most preferred embodiment, the cleaning excitation force is at least approximately at the natural frequency. At this frequency, the amplification of the vibrations is maximum. The wording "at least approximately at the natural frequency" must be understood as a value corresponding to or close to the natural frequency within the material limitation used to generate and induce cleaning vibrations.

In a preferred embodiment, the cleaning excitation force and the detection excitation force have a frequency in the range of 5 Hz to 2000 Hz. The induced cleaning and detection vibration excitation are forced and have consequently a frequency that matches or at least approximately matches the frequency of the corresponding excitation . This frequency range was found to cover the frequencies and mode shapes identi fied to provide an ef ficient removal of deposits in the plurality of regions of the heat exchanger described above . In a more preferred embodiment , the cleaning excitation force and the detection excitation force have a frequency in the range of 10 Hz to 1500 Hz . This frequency range has the additional advantage that more cost-ef fective equipment can be selected to generate the excitation vibrations . In a most preferred embodiment , the cleaning excitation force and the detection excitation force have a frequency in the range of 10 Hz to 1000 Hz . This frequency range was found to provide an optimi zed solution in terms of removing the deposit layer and minimi zing the costs of the necessary equipment .

In a preferred embodiment , the cleaning excitation force is periodic and can be applied to the heat exchanger surface in a continuous manner for more than one period . In other words , in di f ference to US2016/ 025485 or US 8156819B2 , the cleaning excitation force is not applied in a discrete manner and repeated after a time interval after which the cleaning vibrations have damped, but continuously applied so that the amplitude of the excitation force and of the resulting cleaning vibrations are kept constant for the complete duration of the cleaning . The advantage of this embodiment is that cleaning is more ef ficient because deflection maximum are generated for a period of time .

In a preferred embodiment , the detection excitation force is periodic and can be applied to the heat exchanger surface in a continuous manner for more than one period . The advantage of this embodiment in which the amplitude of the detection vibrations is kept at least approximately constant is that the detection is more ef ficient . Indeed, measured detection vibrations and their amplitude remains high for a period time so that a more accurate signal processing can be performed in this period of time .

In a more preferred embodiment , both the cleaning excitation force and the detection excitation force are periodic and applied to the heat exchanger surface in a continuous manner for more than one period . This embodiment combines the advantages j ust mentioned above .

In a preferred embodiment , the cleaning excitation force is applied for at least 10 periods . Simulation have shown that a minimum number of periods , i . e . of oscillations , is necessary to cause cleaning of the heat exchanger surface . Preferably, the cleaning excitation force is applied for 15 to 100 periods . In combination with the frequency ranges of the cleaning excitation force disclosed above , the periods can last for at least 5 milliseconds to 2 seconds . Ranges of 10 milliseconds to 4 seconds and of 25 milliseconds to 10 seconds can also be obtained . These durations ensure that the cleaning excitation force is applied to obtain an ef ficient cleaning .

In a preferred embodiment , the detection excitation force is applied for at least 10 periods . Preferably 15 to 100 periods . The advantage of this embodiment is an accurate signal processing because of the duration of the excitation force applied .

In a preferred embodiment , the amplitude of the cleaning excitation force is at least 5 times larger than the amplitude of the detection excitation force . Preferably 5 to 20 times larger . Advantageously, the detection excitation force is kept low to save energy and reduce mechanical stress applied to the heat surface of the heat exchanger .

The cleaning excitation force and the detection excitation force can be sinusoidal . The cleaning excitation force and the detection excitation force can also be of the general form F ( t ) = Bl . sin ( 2n . Fl . t+<I>l ) + B2 . sin ( 2n . F2 . t+ t>2 ) + ... + Bi . sin ( 2n . Fi . t+ t>i ) , wherein the number " i" of terms can be chosen depending on the region in which the deflection maximum of the cleaning vibrations must be generated . In a known manner, the term Bi represent the amplitude , Fi represent the frequency, and t>i represent the phase of the oscillation i .

In a preferred embodiment , the step of applying detection excitation force to the heat exchanger surface to induce detection vibration of the heat exchanger surface is performed as a sequence of steps in which each time a di f ferent frequency is applied . This embodiment has the advantage that the analysis of detection vibrations induced involves simpler methods and more accurate results , thereby improving detection and subsequently cleaning . The amplitude of the detection excitation force can be the same for each frequency to further simpli fy the analysis .

In a preferred embodiment , the step of applying cleaning excitation force to the heat exchanger surface to induce cleaning vibration of the heat exchanger surface is performed as a sequence of steps in which each time a di f ferent frequency is applied . Advantageously, a simpler excitation source can be used to apply cleaning vibrations , thereby reducing the costs . The complete energy can then be concentrated on cleaning one region of the heat exchanger .

In a preferred embodiment , the step of detecting deposits further comprises measuring phases of the detection vibrations of the heat exchanger surface , estimating deviations of measured phases of the detection vibrations from reference phases of reference detection vibrations of the heat exchanger surface , and assessing the presence of deposits on the heat exchanger surface by means of the deviations of measured phases to determine i f deposits are present . A deviation of measured phases can be the result of a modi fied mode shape of the system due to regions of the heat exchanger having increased thicknesses of deposit layer . The measured phases and their deviations are an additional information that can be used to refine the choice of cleaning vibrations .

In a preferred embodiment , the same vibrator device is used to induce cleaning vibrations and detection vibrations of the heat exchanger surface . This embodiment has the advantage to be economical .

In a preferred embodiment , the re ference amplitudes , reference frequencies , and reference phases of the reference detection vibrations are measured for the heat exchanger surface free of deposits , for example when the heat exchanger is commissioned, or simulated numerically for the heat exchanger surface free of deposits , which has the advantage of a simple implementation .

In a preferred embodiment , the reference amplitudes , reference frequencies , and reference phases of the reference detection vibrations are measured for the heat exchanger surface with a known quantity and spatial distribution of deposits , for example after inspection when the heat exchanger has been operated for a certain time which has the advantage of taking into account real distribution of the deposit layer, or simulated numerically for the heat exchanger surface with a known quantity and spatial distribution of depos its , which has the advantage of a simple implementation . Referring to detection vibrations measured for the heat exchanger surface with a known quantity and spatial distribution of deposits can be the case for example when the method is applied to a heat exchanger surface after a retrofit of the cleaning process .

In a preferred embodiment , the step of assessing the presence of deposits comprises the step of calculating an estimated quantity and spatial distribution of deposits by means of measured amplitudes , frequencies and phases of the detection vibrations . Deviations of measured amplitudes , frequencies and phases can be used to detect the presence of deposits by way of vibration amplitude and phase changes , shi ft of natural frequencies and/or di f ferent damping characteristics . The deviations to measured or simulated reference amplitudes , reference frequencies , and reference phases of the reference detection vibrations for a known quantity and spatial distribution of deposits can be used to determine the actual quantity and spatial distribution of deposits . This has the advantage that the step of choosing the cleaning excitation force can be further refined to induce the deflection maximum of the cleaning vibrations in a more limited deposit region . As a result , cleaning ef ficiency is further improved .

In a preferred embodiment , the step of determining the deposit region is based on the comparison of the deviations of measured amplitudes , frequencies and phases of the detection vibrations to a threshold value defined for the deviations of measured amplitudes , frequencies and phases of the detection vibrations , respectively . In this way it is possible to discriminate between heat exchanger regions having thin deposit layer for which the deviations are below the threshold value and no action is required, and heat exchanger region having thick deposit region for which the deviations are above the threshold value and cleaning is preferable to ensure proper operation of the heat exchanger . The threshold value can be defined in relation to the thickness of the deposit region . The use of a threshold value is beneficial for simple discrimination of deposit layer thicknesses and as result of heat exchanger regions for which an action is required . I f necessary, the threshold value can be adapted to adapt the determination of the heat exchanger regions considered as having a thin deposit layer or a thick deposit layer . In this manner, the thickness of deposit layer considered as requiring an action can be adapted .

In a preferred embodiment , the method comprises a step of checking i f deposits have been removed that is performed after the step of applying the cleaning excitation force to the heat exchanger surface . For this purpose , the step of detecting deposits is performed .

In a preferred embodiment , the heat exchanger surface is split in a plurality of sections and a plurality of cleaning excitation forces are determined to generate each time a local deflection maximum of the cleaning vibrations in a corresponding section of the plurality of sections of the heat exchanger surface . Further, cleaning excitation forces of the plurality of cleaning excitation forces are chosen that induce a local deflection maximum of the cleaning vibrations in sections that overlap at least partially the deposit region . For each section of the heat exchanger surface , cleaning excitation forces are determined, based on calculation or on measurements , to generate deflection maximums of the cleaning vibrations in the section . Therefore , it is possible to clean one or a plurality of sections of the heat exchanger surface instead of cleaning the whole exchanger surface . Further, cleaning can be performed in a sequence of steps , i . e . corresponding each time to the selection of an excitation frequency, amplitude and duration, each corresponding to the cleaning of a section, wherein the steps can be performed simultaneously . Alternatively, the steps can be performed one after the other . The provision of sections is particularly beneficial to simpli fy cleaning in complex and/or large heat exchanger surface .

In a preferred embodiment , the plurality of sections forms subdivisions of the heat exchanger surface in the form of arrays having, referring to a longitudinal direction of the heat exchanger surface , n sections counted longitudinally with respect to the longitudinal direction and m sections counted transversally with respect to the longitudinal direction, wherein m and n are integer in the range 1 to 10 . The provision of sections forming an array is particularly beneficial to simpli fy cleaning in complex and/or large heat exchanger surface . Preferably m and n are integer in the range 1 to 4 to find a compromise between the number of sections that must be included in the cleaning process and precision of cleaning .

The list of sections of the heat exchanger surface is not exhaustive and a further division in two or more of the sections listed can also be considered to focus cleaning of deposits on an even more limited surface of the heat exchanger surface with a correspondingly optimi zed energy use .

In a preferred embodiment , the plurality of sections are quadrilateral and the array of n times m sections covers the heat exchanger surface preferably completely . Preferably, rectangular sections are used to simpli fy the simulation and the application of cleaning vibrational energy .

In a preferred embodiment , the plurality of sections are rounded, preferably elliptical sections . The provision of rounded section allows more flexibility to match the form of deposit region that by nature have an irregular shape .

In a preferred embodiment , the cleaning excitation force is chosen in the group consisting of cleaning excitation forces determined to generate the deflection maxima of the cleaning vibrations in sections of the heat exchanger surface corresponding to , referring to a longitudinal direction of the heat exchanger surface ,

- an array of 2 times 1 , i . e . a first transversal hal f of the heat exchanger surface extending transversally with respect to the longitudinal direction and a second transversal hal f of the heat exchanger surface extending transversally with respect to the longitudinal direction and adj acent to the first transversal hal f ,

- an array of 4 times 1 , that can be defined in a similar manner as the array of 2 times 1 by splitting transversally in two the first transversal hal f and the second transversal hal f ,

- an array of 1 times 2 , i . e . a first longitudinal hal f of the heat exchanger surface extending longitudinal ly with respect to the longitudinal direction, and a second longitudinal hal f of the heat exchanger surface extending longitudinally with respect to the longitudinal direction and adj acent to the first longitudinal hal f ,

- an array of 1 times 4 , that can be defined in a similar manner as the array of 1 times 2 by splitting longitudinally in two the first longitudinal hal f and the second longitudinal hal f ,

- an array of 3 times 1 , i . e . a first transversal third of the heat exchanger surface extending transversally with respect to the longitudinal direction, a second transversal third of the heat exchanger surface extending transversally with respect to the longitudinal direction and adj acent to the first transversal third and a third transversal third of the heat exchanger surface extending transversally with respect to the longitudinal direction and adj acent to the second transversal third,

- an array of 1 times 3 , i . e . a first longitudinal third of the heat exchanger surface extending longitudinal ly with respect to the longitudinal direction, a second longitudinal third of the heat exchanger surface extending longitudinally with respect to the longitudinal direction and adj acent to the first longitudinal third, and a third longitudinal third of the heat exchanger surface extending longitudinally with respect to the longitudinal direction and adj acent to the second longitudinal third,

- or a combination of thereof .

The reference to a fraction of the heat exchanger surface like a hal f o f or a third of the heat exchanger surface is measured relatively to the dimension of the heat exchanger surface extending in the longitudinal direction. Therefore, the fraction considered can be defined independently of the shape of the heat exchanger surface.

In a preferred embodiment, the heat exchanger surface is arranged such that the longitudinal direction of the heat exchanger surface extends in a vertical direction and the cleaning excitation force is chosen in the group consisting of cleaning excitation forces determined to generate a deflection maximum of the cleaning vibrations in sections of the heat exchanger surface corresponding to, in reference to the vertical direction,

- an array of 2 times 1, i.e. an upper transversal half of the heat exchanger surface and a lower transversal half of the heat exchanger surface,

- an array of 4 times 1, that can be defined in a similar manner as the array of 2 times 1 by splitting transversally in two the upper transversal half and the lower transversal half,

- an array of 1 times 2, i.e. a right longitudinal half of the heat exchanger surface and a left longitudinal half of the heat exchanger surface,

- an array of 1 times 4, that can be defined in a similar manner as the array of 1 times 2 by splitting longitudinally in two the right longitudinal half and the left longitudinal half,

- an array of 3 times 1, i.e. an upper transversal third of the heat exchanger surface, a lower transversal third of the heat exchanger surface and a central transversal third of the heat exchanger surface ,

- an array of 1 times 3 , i . e . a right longitudinal third of the heat exchanger surface , a central longitudinal third of the heat exchanger surface and a left longitudinal third of the heat exchanger surface ,

- or a combination of thereof .

In a preferred embodiment , the method comprises the step of approximating the shape of the deposit region by a section overlapping at least partially the deposit region, and the cleaning excitation force determined to generate a deflection maximum of the cleaning vibrations in the section is applied to the heat exchanger surface . Due to the generally irregular contour of deposit layer, it is necessary to approximate the deposit region by a known shape , here a section of the heat exchanger surface , for which the cleaning excitation force has already been determined to generate a deflection maximum of the cleaning vibrations in the section . By applying the cleaning excitation force to the section, deposits are removed at least in the section . The approximation of the shape of the deposit region by a section can be such that the section overlaps at least partially the whole deposit region to obtain at least a partial cleaning . Preferably, the method comprises the step of approximating the shape of the deposit region by a section overlapping the whole deposit region to obtain an at least approximately complete cleaning of the deposit region .

Presently, the term " at least approximately complete cleaning" is to be understood as corresponding possibly to a residual quantity that has a neglectable ef fect on the operation condition of the heat exchanger, for example in terms of insulation, or a residual quantity that results in deviations of amplitude , frequency and phases that are below the threshold value used to determine the presence of a deposit region .

The approximation by a section overlapping at least partially the deposit region or the whole deposit region has the advantage of simplicity and ease o f implementation . Therefore , it can be used for a routine cleaning of the heat exchanger surface .

However, this solution might require applying cleaning vibrational energy to a large section to cover a deposit region having an irregular shape .

For example , in the case of a heat exchanger surface arranged such that the longitudinal direction of the heat exchanger surface extends in the vertical direction, a deposit region in the form of a "L" extending in the left longitudinal third of the heat exchanger surface and in the upper third of the heat exchanger surface can be approximated in a first step by a section of the heat exchanger surface corresponding to the upper third, i . e . chosen in an array of 3 times 1 sections , or - for more overlapping - the upper hal f of the heat exchanger surface , i . e . chosen in an array of 2 times 1 sections . Applying the corresponding cleaning vibrational energy to the upper third or to the upper hal f of the heat exchanger surface results in cleaning of consequent portion of the deposit region that might be enough in a first step to operate the heat exchanger . The method can be further optimi zed as shown below .

In a preferred embodiment , the method comprises the step of approximating the shape of the deposit region by a group of sections chosen in the plurality of sections and overlapping at least partially the deposit region on which deposits must be removed, wherein the cleaning excitation force determined to generate a deflection maximum of the cleaning vibrations in each section of the group of sections is applied to the heat exchanger surface simultaneously or one after the other . This arrangement allows to clean more precisely the heat exchanger surface . Referring to the example above of the deposit region having a "L" -shape , the deposit region can be approximated by a group of sections chosen in the plurality of sections of the heat exchanger surface , wherein the group of sections comprises an upper third of the heat exchanger surface and a left longitudinal third of the heat exchanger surface . The cleaning excitation force determined to generate a deflection maximum of the cleaning vibrations in the upper third of the heat exchanger surface and then in the upper third of the heat exchanger surface can be applied to clean the heat exchanger surface . Alternatively, the cleaning excitation force can be applied simultaneously to both sections in this example .

The cleaning excitation force and the detection excitation force induce vibrations of the heat exchanger surface of the general form y ( t ) = Al . sin ( 2n . f 1 . t+cpl ) + A2 . sin ( 2n . f 2 . t+cp2 ) + ... + Ai . sin ( 2n . f i . t+cpi ) , wherein the number " i" of terms can be chosen depending on the region in which the deflection maximum of the cleaning vibrations must be reached . In a known manner, the term Ai represents the amplitudes , fi represents the frequency, and epi represents the phase of the oscillation i .

In a preferred embodiment , the cleaning vibrations induced are composed by one or a plurality of frequencies corresponding to 0 . 85 and 1 . 15 times a natural frequency of the heat exchanger surface . This frequency range has been found to maximi ze the ratio between deflection amplitude/ strains at the deflection maximums and amplitude of excitation vibrations because of the ampli fication phenomenon that takes place close to a resonance . Therefore , smaller excitation systems can be used to achieve the strain levels required for cleaning . In a more preferred embodiment , the cleaning vibrations induced are at a frequency between 0 . 95 and 1 . 05 times a natural frequency of the heat exchanger surface . In this frequency range , the ampli fication of vibrations is further increased so that higher strains are generated at the deflection maximums . In a most preferred embodiment , the cleaning vibrations induced are at least approximately at the natural frequency . At this frequency, the ampli fication of the vibrations is maximum . The wording " at least approximately at the natural frequency" must be understood as a value corresponding to or close to the natural frequency within the material limitation used to generate and induce cleaning vibrations .

In a preferred embodiment , the cleaning vibrations and the detection vibrations have a frequency in the range of 5 Hz to 2000 Hz . This frequency range was found to cover the frequencies and mode shapes identi fied to provide an ef ficient removal of deposits in the plurality of regions of the heat exchanger described above . In a more preferred embodiment , the cleaning vibrations and the detection vibrations have a frequency in the range of 10 Hz to 1500 Hz . This frequency range has the additional advantage that more cost-ef fective equipment can be selected to generate the excitation vibrations . In a most preferred embodiment , the cleaning vibrations and the detection vibrations have a frequency in the range of 10 Hz to 1000 Hz . This frequency range was found to provide an optimi zed solution in terms of removing the deposit layer and minimi zing the costs of the necessary equipment . For measuring the deposit , a sweep through a frequency range of the detection vibration can be used .

Description of figures

The present invention will now be described, by way of an example , with reference to the accompanying drawings in which :

Fig . 1 shows a front view of a heat exchanger with a deposit layer ;

Fig . 2 shows a flowchart illustrating the steps of the method according to the invention, as it could be implemented to remove deposits from the surface of the heat exchanger according to Fig . 1 ;

Fig . 3 shows a perspective view of a heat exchanger harp having induced cleaning vibrations according to a first vibrational excitation;

Fig . 4 shows a perspective view of a heat exchanger having induced cleaning vibrations according to a second vibrational excitation;

Fig . 5 shows a perspective view of a heat exchanger having induced cleaning vibrations according to a third vibrational excitation;

Fig . 6 shows a schematic illustration of an embodiment of a vibrator device mounted for transmitting vibrational excitation to the heat exchanger ; Fig . 7 shows schematically the application of the cleaning excitation force to the heat exchanger surface according to prior art and the resulting cleaning vibration; and

Fig . 8 shows schematically the application of the cleaning excitation force to the heat exchanger surface according to an embodiment of the invention and the resulting cleaning vibration .

Referring to Fig . 1 , it is disclosed a heat exchanger 10 of a boiler system, wherein walls 20 forming a duct 30 designed to conduct a fluid are represented schematically . Fig . 1 is used later in relation to Fig . 2 to illustrate an embodiment of the method according to the invention .

The heat exchanger 10 is arranged within the walls 20 . While flowing in a direction of flow F, presently perpendicularly to the plane of a heat exchanger surface , here in the form of a heat exchanger harp, in the duct 30 through the heat exchanger 10 , a f luid exchanges heat with the heat exchanger 10 and, in the present embodiment , the walls 20 . The heat exchanger 10 is designed to collect heat by way of a heat exchanger medium circulating inside the heat exchanger 10 and the walls 20 .

At the same time , particles contained in the fluid can form deposits on the surface of the heat exchanger 10 forming a deposit layer 40 acting as an insulation layer between the fluid and the heat exchanger . This deposit layer 40 is detrimental to the heat collection by the heat exchanger 10 and partially obstructs the path of the fluid, creating higher losses .

The heat exchanger 10 comprises a heat exchanger surface 50 in the form of a plurality of heat exchanger tubes 50 having an outer surface and being supported by a supporting structure arranged outside the duct 30 , the outer surface of the plurality of heat exchanger tubes 50 forming the surface of the heat exchanger .

In the present embodiment , the plurality of heat exchanger tubes 50 are arranged in a single group, a so-called "harp" of heat exchanger tubes . It is noted that in an industrial boiler system, a plurality of harps can be arranged in the duct , spaced apart from each other and connected to each other to increase the heat exchanger surface .

The heat exchanger tubes are attached together in a first end portion 68 of the heat exchanger tubes by a first transversal element 70 in the form of an upper collector and in a second end portion 80 of the heat exchanger tubes opposed to the first end portion 68 by a second transversal element 90 in the form of a lower collector, wherein the first transversal element 70 is supported by the supporting structure and the second transversal element 90 is free .

The group of heat exchanger tubes 50 extends in a plane , wherein the heat exchanger tubes extend longitudinally parallel to each other at a distance from each other . The group of heat exchanger tubes 50 is supported via the upper collector 70 by the supporting structure arranged outside the duct 30 . Each heat exchanger tube is fluidly connected at his end to the adj acent heat exchanger tube to allow the circulation of the heat exchanger medium inside the heat exchanger tubes 50 .

In the embodiment illustrated, deposition of particles occurred in such a way that the deposit layer 40 is formed as a layer having a varying thickness and extending over a region having an irregular contour of the heat exchanger surface . The deposit layer 40 is divided in a first region 100b, and a second region 100a represented as a hatched region, wherein the deposit layer 40 has a thickness bigger than a threshold thickness value in the first region 100b and a thickness smaller than a threshold thickness value in the second region 100a .

Fig . 2 discloses an exemplary embodiment of the method implemented to remove deposits from the surface of the heat exchanger according to Fig . 1 .

The method comprises a step of detecting deposits 200 on the heat exchanger surface .

The step of detecting deposits 200 comprises the step of applying a detection excitation force 210 to the heat exchanger surface , i . e the outer surface of the plurality of heat exchanger tubes and transversal elements , to induce detection vibrations of the heat exchanger surface , and the step of measuring amplitudes , frequencies and phases 220 of the detection vibrations of the heat exchanger surface . The result of this step is a set of measured amplitudes A_m, measured frequencies F_m and measured phases P_m schematically represented by measured output 220a .

Further, the step of detecting deposits 200 comprises the step of estimating deviations of measured amplitudes , measured frequencies , and measured phases 230 of the detection vibrations from reference amplitudes A_o, reference frequencies F_o and reference phases P_o of reference detection vibrations of the heat exchanger surface . The result of this step is a set of amplitude deviations D_A = A_m - A_o, frequency deviations D_F = F_m - F_o and phase deviations D_P = P_m - P_o schematically represented by deviations output 230a . Indeed, an ef fect of the deposit layer is an increase of mass , sti f fness and damping of the heat exchanger locally so that amplitudes , frequencies , and phases of the vibrations propagating in the heat exchanger di f fer from amplitudes , frequencies , and phases measured at a past point of time at which the heat exchanger mass , sti f fness and damping had locally a di f ferent value because the deposit layer was thinner or not yet present .

In the present embodiment , we assume that a step of measuring the reference amplitudes , reference frequencies , and reference phases 225 of the reference detection vibrations is the result of a dedicated measurement for the heat exchanger surface free of deposits , wherein this step has been performed for example at the time of commissioning the heat exchanger . Alternatively, the step of measuring the reference amplitudes , reference frequencies , and reference phases 225 of the reference detection vibrations can be the result of numerical simulation for the heat exchanger surface free of deposits . The result of this step is a set of reference amplitudes A_o, reference frequencies F_o and phases P_o schematically represented by reference output 225a . Further, it is assumed that in the same step reference amplitudes , reference frequencies , and reference phases of the reference detection vibrations are simulated for the heat exchanger surface with each time a known quantity and spatial distribution of deposits , for example corresponding the state of the heat exchanger surface after having been operated for a certain time .

Further, the step of detecting deposits 200 comprises the step of assessing the presence of deposits 240 on the heat exchanger surface by means of the deviations of measured amplitudes , frequencies and phases to determine i f deposits are present . The result of this step is the comparison of the estimated amplitude deviations D_A, frequency deviations D_F and phase deviations D_P to reference values and schematically represented by test output 240a .

A deviation of measured frequencies can be the result of a shi ft of natural frequency values because the deposit layer has increased locally the heat exchanger mass and/or sti f fness . A deviation of measured amplitudes can be a change of the measured amplitude resulting from di f ferent inertia, sti f fness or damping ef fect caused by the deposit layer . A deviation of measured phases can be the result of a modi fied mode shape of the system due to a di f ference in propagation of the detection vibrations between regions of the heat exchanger having di f ferent increased thicknesses of deposit layer . I f no deviation is observed or i f the deviations are within the precision limits of the measurement , it can be concluded that deposits are not present . A region of the heat exchanger for which deviations of measured amplitudes , frequencies and phases are observed can be identi fied as a deposit region .

In the present embodiment , the step of assessing the presence of deposits 240 comprises the step of calculating an estimated quantity and spatial distribution of deposits by means of measured amplitudes , frequencies and phases of the detection vibrations .

For this purpose , the deviations of measured amplitudes , frequencies and phases to the simulated or measured reference amplitudes , frequencies and phases for a known quantity and spatial distribution of deposits can be used to determine the actual quantity and spatial distribution of deposits . Further, the step of detecting deposits 200 comprises the step of determining a deposit region 250 of the heat exchanger surface on which deposits must be removed . A region of the heat exchanger for which deviations of measured amplitudes , frequencies and phases are observed can be identi fied as the deposit region .

For this purpose , the deviations of measured amplitudes , frequencies and phases are compared to the respective threshold values defined for amplitudes , frequencies and phases that can be defined in relation to the thickness of the deposit layer . The result of this step is the comparison of the estimated amplitude deviations D_A, frequency deviations D_F and phase deviations D_P to the threshold value and schematically represented by threshold test output 250a .

A second heat exchanger region 100a having thin deposit layer for which the deviations are below the threshold value and no action is required is represented in hatched lines in Fig . 1 . A first heat exchanger region 100b having thick deposit region for which the deviations are above the threshold value and cleaning is preferable to ensure proper operation of the heat exchanger is also represented in Fig . 1 .

Further, according to the invention, the method comprises a step of choosing a cleaning excitation force 260 that induce a local deflection maximum of the cleaning vibrations at least in the deposit region . For this purpose , a set of cleaning vibrations inducing a local deflection maximum in sections of the heat exchanger surface has been determined in a separate step 255 and is available . Advantageously, the further step of applying the cleaning excitation force 270 to the heat exchanger surface can be limited to the deposit region determined previously .

A first advantage is that the region of the heat exchanger where high strain is caused by cleaning vibrations is reduced to a minimum to speci fically address deposit regions while the rest of the heat exchanger remains at least approximately free of induced vibrations , i . e . of strains

The further step of applying the cleaning excitation force 270 to the heat exchanger surface includes a step of controlling by a controller 692 ( further described in relation to Fig . 6 ) amplitudes , frequencies , and phases of induced vibrations in the heat exchanger to adj ust , i f necessary, the operation of a vibrator device 600 ( further described in relation to Fig . 6 ) designed to create vibrational excitation .

A step of checking i f deposits have been removed 280 is performed at the end of the application of the cleaning vibration by running the step of detecting deposits on the heat exchanger surface .

In Fig . 3 , Fig . 4 and Fig . 5 induced vibrations , technical known as modal deflection shapes , of the heat exchanger surface corresponding each time to di f ferent cleaning vibration frequencies are illustrated . The heat exchanger 10 represented is the same as in Fig. 1 and the reference signs refer to the same features as in Fig. 1.

The heat exchanger is arranged such that the longitudinal direction of the heat exchanger surface extends in a vertical direction. The heat exchanger surface can be segmented in the form of arrays having, in reference to the vertical direction,

- an array of 3 times 1, i.e. an upper transversal third of the heat exchanger surface, a lower transversal third of the heat exchanger surface and a central transversal third of the heat exchanger surface,

- an array of 1 times 3, i.e. a right longitudinal third of the heat exchanger surface, a central longitudinal third of the heat exchanger surface and a left longitudinal third of the heat exchanger surface.

In general, the reference to the right and to the left of the heat exchanger surface can be assigned initially, for example in relation to a direction facing the heat exchanger surface that is kept during operation of the heat exchanger. The same applies to the references to the upper, central and lower sections of the heat exchanger.

Different cleaning excitation forces can be defined for the sections of the heat exchanger surface. In the present case, three exemplary cleaning vibrations induced are illustrated in Fig. 3, Fig. 4 and Fig. 5, wherein regions having high deflections are represented in dark shades and regions having smaller deflections are represented in light shades.

In Fig. 5, the heat exchanger surface is divided in an array of n = 1 times m = 3, i.e. a right longitudinal third of the heat exchanger surface, a central longitudinal third of the heat exchanger surface and a left longitudinal third of the heat exchanger surface. Cleaning vibrations are induced in a region S5 corresponding to the left longitudinal third of the heat exchanger surface, i.e. n = 1 and m = 1.

It is noted that other methods of splitting the heat exchanger surface are possible, for example with more precise mesh. For example, in Fig. 3, the heat exchanger surface is divided in an array of n = 6 times m = 1 and cleaning vibrations are induced in a region S3 corresponding to a lower transversal half of the upper transversal third of the heat exchanger surface, i.e. n = 2 and m = 1.

It is noted that splitting the heat exchanger surface is not limited to sections having a quadrilateral form like a rectangle. For example, in Fig. 4, cleaning vibrations are induced in a region S4 corresponding to a central elliptical region of the upper transversal half of the heat exchanger surface .

In the concrete example of Fig. 1, the method disclosed in relation to Fig. 2 would advantageously include in the step of choosing a combination of a first cleaning excitation force that induces cleaning vibration in a region S2 corresponding to a left longitudinal third of the heat exchanger surface, as illustrated in Fig . 5 , and a second cleaning excitation force that induces cleaning vibration in a region S I corresponding to an upper transversal third of the heat exchanger surface .

A device 600 for generating and applying an excitation force to the heat exchanger in the form of a vibrator device 600 is represented schematically in Fig . 6 . The vibrator device 600 is embodied as an electromagnetic shaker 600 comprising a vibrational excitation source 600a arranged to apply vibrational excitation to a heat exchanger 610 having the same configuration as the heat exchanger disclosed in Fig . 1 . The heat exchanger 610 comprises a plurality of heat exchanger tubes 650 which outer surface forms the surface of the heat exchanger and is arranged within walls 620 of the boiler system, said walls 620 forming a duct 630 . Only an end portion of the heat exchanger 610 to which vibrational excitation is transmitted is represented in Fig . 6 .

The heat exchanger tubes 650 are attached together in a first end portion of the heat exchanger tubes by a first transversal element (not represented) and in a second end portion 680 of the heat exchanger tubes opposed to the first end portion by a second transversal element 690 , wherein the first transversal element is supported by a supporting structure and the second transversal element 690 is guided to limit the lateral displacements of the second transversal element 690 at least in the direction perpendicular to the plane of the heat exchanger .

The electromagnetic shaker 600 comprising a vibrator arm 621 is removably attached to the walls 620 by way of a fixing element 600b and is mechanically connected by way of an intermediate element 621a and the vibrator arm 621 to the heat exchanger surface . The vibrator arm 621 is removably attached at a first end 622 to the intermediate element 621a that is removably attached to the heat exchanger surface 650 , here speci fically to the second transversal element 690 , for example by way of a mechanical fastening system . This embodiment has the advantage that the vibrator arm 621 is at anytime in continuous contact with the heat exchanger surface . Consequently, the intermediate element 621a and the heat exchanger surface 650 are less subj ect to wear compared to the prior art using impactors . Further, i f necessary, only the intermediate element 621a can be replaced in case of wear .

A second end 624 of the vibrator arm 621 opposed to the first end 622 is acted upon by the vibrational excitation source ( 600a ) .

In the embodiment of Fig . 6 , the electromagnetic shaker 600 is mounted for transmitting vibrational excitation perpendicularly to the heat exchanger tubes to induce transverse modes of vibration the heat exchanger tubes .

The controller 692 is provided for the controlling of the frequency, amplitude and phase of the vibrational excitation, i . e . of the vibrations generated by the electromagnetic shaker 600 and induced in the heat exchanger tubes 650 . Frequency, amplitude and phase of the vibrations generated are measured by a plurality of sensors 694 arranged on the exchanger tubes 650 as well as on the second transversal element 690 . The sensors 694 are connected to the controller that can act on frequency, amplitude and phase of the vibrational excitation depending on the measurements made by the plurality of sensors 624 . In Fig . 7 and Fig . 8 the application of the cleaning excitation force to the heat exchanger surface according to prior art is compared to the application of the cleaning excitation force to the heat exchanger surface according to an embodiment of the invention . The resulting cleaning vibrations are also illustrated in the lower part of the figures . The same comparison can also be made for the detection excitation force that can be applied in the same manner according to an embodiment of the present invention .

According to prior art like US 8156819B2 mentioned in the introduction or similar others , the method provided for cleaning a surface of a heat exchanger element implies that a knocking device , for example a knocking ram, that is moved in a direction toward the surface of a heat exchanger and knocks the surface of a heat exchanger . The amplitude A of the corresponding cleaning excitation force is represented in function of time t and frequency F in the first and second plots , as seen from the top to the bottom of Fig . 7 , respectively . The cleaning excitation force has an amplitude di f ferent to zero for the time of the contact between the knocking device and the surface of the heat exchanger as shown in the first plot . The amplitude is the same over the frequency spectrum of the cleaning excitation force . The amplitude V of the induced cleaning vibrations is represented in function of time t and frequency F in the third and fourth plots of Fig . 7 , respectively . The induced cleaning vibrations have an amplitude decaying over time as shown schematically in the third plot and are composed of several frequencies defined by the structural characteristics of the heat exchanger, including frequencies that form noise in the response . In difference to the prior art and referring now to Fig. 8, the amplitude A of the cleaning excitation force is represented in function of time t in a case in which it is applied for an excitation time of 5 periods in the first plot and at a frequency F as shown in the second plot of Fig. 8, as seen from the top to the bottom of Fig. 8. The amplitude V of the induced cleaning vibrations are represented in function of time t and frequency F in the third and fourth plots of Fig. 8, respectively. The induced cleaning vibrations have an amplitude that remains at least approximately constant over the excitation time as shown schematically in the third plot and occurs at the frequency defined by the cleaning excitation force, thereby reducing frequencies that form noise in the response. Signal processing is consequently more accurate and easier to perform.

List of reference signs heat exchanger 10, 610 walls 20, 620 duct 30, 630 deposit layer 40 heat exchanger surface, heat exchanger tubes 50, 650 first, second end portion of heat exchanger tubes 68, 80, 680 first, second transversal element 70, 90, 690 first, second region of the deposit region 100b, 100a step of detecting deposits 200 step of applying a detection excitation force 210 step of measuring amplitudes , frequencies and phases 220 set of measured amplitudes A_m, measured frequencies F_m and measured phases P_m = output 220a step of estimating deviations of measured amplitudes , measured frequencies , and measured phases 230 step of measuring the reference amplitudes , reference frequencies , and reference phases 225 set of reference amplitudes A_o, reference frequencies F_o and phases P_o = output 225a set of amplitude deviations D_A = A_m - A_o, frequency deviations D_F = F_m - F_o and phase deviations D_P = P_m - P_o = output 230a step of assessing the presence of deposits 240 set of comparisons of the estimated amplitude deviations D_A, frequency deviations D_F and phase deviations D_P to reference values = output 240a step of determining a depos it region 250 set of comparison of the estimated amplitude deviations D_A, frequency deviations D_F and phase deviations D_P to the threshold value = output 250a step of determining a set of cleaning vibrations 255 step of choosing the cleaning excitation force 260 step of applying cleaning excitation force to the heat exchanger surface 270 step of checking i f deposits have been removed 280 fixing element 600b vibrator arm 621 intermediate element 621a controller 692 first , second end of the vibrator arm 622 , 624

Claims

- 45 -

Claims

1. Method of removing deposits from a heat exchanger surface

(50) of a heat exchanger (10) arranged in a boiler system, the method comprising the steps of:

- providing a heat exchanger (10) having a heat exchanger surface ( 50 ) ;

- providing a device (600) for generating and applying a cleaning excitation force to the heat exchanger surface;

- applying the cleaning excitation force to the heat exchanger surface (50) to induce cleaning vibrations of the heat exchanger surface to remove deposits on the heat exchanger surface; characterized by a step of detecting deposits on the heat exchanger surface (50) comprising the steps of: applying a detection excitation force to the heat exchanger surface (50) to induce detection vibrations of the heat exchanger surface;

- measuring amplitudes and frequencies of the detection vibrations of the heat exchanger surface (50) ; estimating deviations of measured amplitudes and frequencies of the detection vibrations from reference amplitudes and reference frequencies of reference detection vibrations of the heat exchanger surface (50) ;

- assessing the presence of deposits (40) on the heat exchanger surface (50) by means of the deviations of - 46 - measured amplitudes and frequencies to determine i f deposits are present ; and determining a deposit region ( 100b ) of the heat exchanger surface on which deposits must be removed; and by a step of choosing the cleaning excitation force that induce a deflection maximum of the cleaning vibrations at least in the deposit region ( 50 ) . Method according to claim 1 , characteri zed in that the step of detecting deposits further comprises measuring phases of the detection vibrations of the heat exchanger surface , estimating deviations of measured phases of the detection vibrations from reference phases of reference detection vibrations of the heat exchanger surface ( 50 ) , and assessing the presence of deposits ( 40 ) on the heat exchanger surface ( 50 ) further by means of the deviations of measured phases to determine i f deposits are present . Method according to claim 2 , characteri zed in that the reference amplitudes , reference frequencies , and reference phases of the reference detection vibrations are measured for the heat exchanger surface ( 50 ) free of deposits or simulated numerically for the heat exchanger surface free of deposits . Method according to claim 2 , characteri zed in that the reference amplitudes , reference frequencies , and reference phases of the reference detection vibrations are measured for the heat exchanger surface ( 50 ) with a known quantity and spatial distribution of deposits or simulated numerically for the heat exchanger surface with a known quantity and spatial distribution of deposits . Method according to any one of claims 1 to 4 , characteri zed in that the step of assessing the presence of deposits comprises the step of calculating an estimated quantity and spatial distribution of deposits by means of measured amplitudes , frequencies and phases of the detection vibrations . Method according to claim 5 , characteri zed in that the deposit region is determined as the heat exchanger surface ( 50 ) on which the estimated quantity and spatial distribution of deposits exceeds a threshold quantity and spatial distribution of deposits . Method according to any one of claims 1 to 6 , characteri zed in that the step of determining the deposit region is based on the comparison of the deviations of measured amplitudes , frequencies and phases of the detection vibrations to a threshold value defined for the deviations of measured amplitudes , frequencies and phases of the detection vibrations . Method according to any one of claims 1 to 7 , characteri zed in that the step of detecting deposits on the heat exchanger surface ( 50 ) is performed in addition after the step of applying the cleaning excitation force , to check i f deposits have been removed . Method according to any one of claims 1 to 8 , characteri zed in that the heat exchanger surface ( 50 ) is split in a plurality of sections and a plurality of cleaning excitation forces are determined to generate each time a deflection maximum of the cleaning vibrations in a corresponding section of the plurality of sections , and in that cleaning excitation forces of the plurality of cleaning excitation forces are chosen that induce a deflection maximum of the cleaning vibrations in sections that overlap at least partially a deposit region ( 100b ) on which deposits must be removed . Method according to claim 9 , characteri zed in that the plurality of sections forms subdivisions of the heat exchanger surface in the form of arrays covering the heat exchanger surface and having, referring to a longitudinal direction o f the heat exchanger surface ( 50 ) , n sections counted longitudinally with respect to the longitudinal direction and m sections counted transversally with respect to the longitudinal direction, wherein m and n are integer in the range 1 to 10 , preferably 1 to 4 . Method according to any one of claims 9 to 10 , characteri zed by the step of approximating the shape of the deposit region by a group of sections chosen in the plurality of sections and overlapping at least partially the deposit region ( 100b ) on which deposits must be removed, wherein the cleaning excitation force determined to generate a deflection maximum of the cleaning vibrations in each section of the group is applied to the heat exchanger surface simultaneously or one after the other . Method according to any one of claims 1 to 11 , characteri zed in that the cleaning excitation force is composed by one or a plurality of frequencies corresponding to 0 . 85 and 1 . 15 times a natural frequency of the heat exchanger surface , preferably at a frequency between 0 . 95 and 1 . 05 times a natural frequency of the heat exchanger surface (50) , most preferably at least approximately at the natural frequency.

13. Method according to any one of claims 1 to 12, characterized in that the cleaning excitation force and the detection excitation force are periodic.

14. Method according to claim 13, characterized in that the cleaning excitation force and the detection excitation force are applied to the heat exchanger surface in a continuous manner for more than one period.

15. Method according to claim 14, characterized in that the cleaning excitation force is applied for at least 10 periods, preferably 15 to 100 periods.

16. Method according to any one of claims 1 to 15, characterized in that the cleaning excitation force and the detection excitation force have a frequency in the range of 5 Hz to 2000 Hz, preferably 10 HZ to 1500 Hz, most preferably 10 Hz to 1000 Hz.

17. Method according to any one of claims 1 to 16, characterized in that the amplitude of the cleaning excitation force is at least 5 times larger than the amplitude of the detection excitation force, preferably 5 to 20 times larger.

18. Device (600) for generating and applying an excitation force to a heat exchanger surface (50) of a heat exchanger (10) arranged in a boiler system, the device (600) comprising a vibrational excitation source (600a) , a vibrator arm (621) having a first end (622) mechanically connected to the heat exchanger surface (650) and a second end (624) opposed to the first end (622) that is acted upon by the vibrational excitation source (600a) , characterized in that the first end (622) is in continuous contact with the heat exchanger surface (50) and the second end (624) is in continuous contact with the vibrational excitation source (600a) at anytime during application of cleaning excitation force and of detection excitation force.