The present invention relates systems and methods for cleaning devices holding fluid such as heat exchanges, in particular to systems and methods wherein the cleaning is performed by using a transducer assemblies that can operate at their natural resonance frequencies.

BACKGROUND

Fouling within industry has an impact on both capital and operation costs. An increase in internal fouling results in poor thermal efficiency. This is coupled with poor heat and mass transfer to the metal surface of designed heat exchangers, pipes and other equipments. The cleaning of fouled heat exchanges presents a significant challenge to the maintenance and operation of e.g. chemical, petro leum and food processes. Despite efforts in the design of processes and hard ware to minimize fouling, eventually the intricate interior surface of the ex- changer require cleaning to restore the unit to the required efficiency.

Heat exchangers are typically cleaned onsite by removing the exchanger and by placing the unit on a wash pad for spraying with high pressure water to re move foulants. Cleaning heat exchangers in an ultrasonic bath requires specially designed vessels that allow coupling sound into them and that are capable of holding sufficient fluid to affect the cleaning, and that feature specific design to allow easy removal of the foulant material from the immersed device.

US 2012055521 discloses a segmental ultrasonic cleaning apparatus config ured to remove scales and/or sludge deposited on a tube sheet. The segmental ultrasonic cleaning apparatus includes a plurality of segment groups arranged in a ring shape on a top surface of a tube sheet along an inner wall of the steam generator, in which each segment groups includes an ultrasonic element seg ment and a guide rail support segment loosely connected to each other by metal wires located at a lower portion of the steam generator, such that ultrasound radiated from transducer in each of the ultrasonic element segments travels along the surface of the tube sheet, with the segment groups tightly connected in the ring shape by tightening the metal wires via wire pulleys of flange units.

US 2007267176 discloses a method wherein fouling of heat exchange surfaces is mitigated by a process in which an ultrasound is applied to a fixed heat ex changer. According to the document, the ultrasound excites a vibration in the heat exchange surface and produce waves in the fluid adjacent to the heat ex change surface. The ultrasound is applied by a dynamic actuator coupled to a controller to produce vibration at a controlled frequency and amplitude that min imizes adverse effects to the heat exchange structure. The dynamic actuator may be coupled to the heat exchanger in place and operated while the heat exchanger is online.

US2008073063 discloses a method for reducing the formation of deposits on the inner walls of a tubular heat exchanger through which a petroleum-based liquid flows. The method comprises applying one of fluid pressure pulsations to the liquid flowing through the tubes of the exchanger and vibration to the heat exchanger to affect a reduction of the viscous boundary layer adjacent to the inner walls of the tubular heat exchange surfaces. Fouling and corrosion were further reduced using a coating on the inner wall surfaces of the exchanger tubes.

Figure 1 shows a typical system 100 for cleaning a device for holding fluid. The system comprises mechanical wave generating means 101 , such as an ultra sound transducer and a waveguide 102. The waveguide comprises a first end 102a adapted to be in contact with a device to be cleaned and a second end 102b which is in contact with the mechanical wave generating means, such as an ultrasound transducer. The fundamental resonance frequency of the system 100 is 20 kFIz, and there are antinodes at both ends of the system. To mimic the impact of mechanical loading by the device to be cleaned, a rigid boundary con dition is introduced at the first end 102a of the wave guide. As a result of this loading, a node is created at the first end and the new resonance frequency of the transducer assembly is 25 kFIz. The waveform of the loaded transducer as sembly is also presented in the figure. In figure 2 the system 100 is attached on an outer surface 103a of a wall 103 of a device to be cleaned. The wall is made of metal and its thickness h is 10 mm. The contact to the metal wall alters the tuning frequency of the transducer from 25 kHz to 27 kHz. Accordingly, the wall interface changes the fundamental res- onance of the transducer, and the coupled resonance at 27 kHz is damped as shown in figure 3.

As shown in figures 1 -3, the use of system like 100 for cleaning purposes has its challenges. Accordingly, there is a need for further systems and methods for cleaning of devices. SUMMARY

The present invention is based on the observation that at least some of problems related to cleaning of a device for holding fluid, such as a heat exchanger, can be avoided or at least alleviated when the cleaning is performed by using a sys tem, such as a transducer assembly which is able to operate at its fundamental resonance frequency even when in contact with the device to be cleaned.

Accordingly, it is an object of the present invention to provide a system for clean ing a device, the system comprising

o mechanical wave generating means,

o a waveguide comprising

· a first end adapted to be in contact with outer surface of the device

• a second end wherein the second end is in contact with the me chanical wave generating means,

• a cavity comprising a base portion separated from the second end by a distance I.

The mechanical wave generating means is adapted to emit mechanical waves through the waveguide to outer surface of the device. Waveform of the mechan ical waves is adapted to be such that there is an antinode positioned in the waveguide at the distance I from the second end. The maximum diameter D _max of the waveguide is less than ½ of wavelength of the mechanical waves, and ratio of diameter d of the base portion and the diameter D of the waveguide at distance I from the second end is 0.9 or less, preferably from 0.2 to 0.9, more preferably from 0.4 to 0.8.

It is another object of the present invention to provide a new method for cleaning a device with the system according to claim 1.

It is still another object of the present invention to provide a new use of the of the system according to claim 1 for cleaning a device holding fluid.

Further objects of the present invention are described in the accompanying de pendent claims.

Exemplifying and non-limiting embodiments of the invention, both as to con- structions and to methods of operation, together with additional objects and ad vantages thereof, are best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.

The verbs“to comprise” and“to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in the accompanied depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be under stood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality. The terms acoustic, elastodynamic and ultrasonic are used in this document as synonyms.

BRIEF DESCRIPTION OF DRAWINGS

The exemplifying and non-limiting embodiments of the invention and their ad vantages are explained in greater detail below with reference to the accompa- nying drawings, in which:

Figure 1 shows a transducer assembly according to prior art comprising a wave guide comprising a first end adapted to be contacted with a device to be cleaned, figure 2 shows a situation where the transducer assembly of figure 1 is con nected to an outer surface of a device to be cleaned, figure 3 shows electrical impedance curves of a situation wherein the transducer assembly of figure 1 is connected with at a 10 mm thick metal wall, figure 4A and figure 4B show the principle of present invention for cleaning a device holding fluid by using an exemplary non-limiting transducer assembly, figure 5 shows electrical impedance curves at a 10 mm thick metal wall accord ing to the system of figure 4A, figures 6A-E show exemplary designs of the system of the present invention, figure 7 shows radiated acoustic power obtainable from an exemplary system according to the present invention as a function of distance h, and figure 8 shows exemplary non-limiting designs of the first end of a system of the present invention.

DESCRIPTION Figures 1 -3 have been discussed in Background section of this document.

As defined herein, a point-like pressure source is a pressure source which has at least one of its dimension smaller, e.g. at least two times smaller than the wavelength generated by the pressure source in a fluid within the device to be cleaned and/or in a wall of the device to be cleaned. For example, for a point source contacting a metal surface utilizing longitudinal 20 kFIz ultrasound, a point-like pressure source is a source with a contact diameter significantly smaller than 25 mm, e.g. 12.5 mm, and for 100 kFIz ultrasound, significantly smaller than 5 mm, e.g. 2.5 mm. For different wave modes, these diameters are adjusted according to the speed of sound of the mode. In the following text, the system and the method of the present invention is ex emplified by different transducer assemblies. The principle of the method of the present invention for cleaning a device holding fluid, such as liquid, is presented using an exemplary non-limiting system shown in figure 4A and 4B. Accordingly, a system 200 suitable for the method com prises a mechanical wave generating means 201 and a waveguide 202. The waveguide comprises a first end 202a adapted to be positioned on outer surface 203a of a device to be cleaned and a second end 202b which is in contact with the mechanical wave generating means. The waveguide comprises a cavity 204 comprising a base portion 204a which is substantially in xz-plane of the coordi nate system 299. The cavity is preferably positioned around acoustic axis 205 of the system. Distance of the base portion from the second end of the wave guide in y-direction of the coordinate system is marked in the figure with a letter I. The distance I is selected such that when the mechanical wave generating means emits succession of mechanical waves through the wave guide to the outer surface, there is an antinode 206 at distance I from the second end, i.e. in proximity of the base portion.

Diameter of the waveguide, and diameter of the cavity at the distance I from the second end in x-direction of the coordinate system 299 is marked in figure 4A with symbols D and d, respectively. When the ratio d/D is 0.9 or less, the por tions of the waveguide marked with reference numbers 207a and 207b in figure 4B act as point like pressure sources and are adapted to emit mechanical waves, marked with arrows 208a and 208b in figure 4B, towards the outer sur face. The correct position of the antinode can be adjusted by proper design of the system as discussed later in detail. Distance of the portions 207a and 207b from the acoustic axis 205 of the system is preferably same. The maximum di ameter D _max of the waveguide is less than ½ of wavelength of the mechanical waves. In titanium l would be ca 0.4 m, whereas in copper it would be ca 0.2 m, and in steel it would be ca 0.3 m.

The portions 207a and 207b act as point pressure sources and interfere in the waveguide resulting a propagating wave marked with an arrow 209. The wave guide delivers the wave through the wall 203 to the inner surface 203b. The interfering mechanical waves 210 make the inner surface vibrate. As the vibrat ing inner surface moves, the motion produces pressure pulse 21 1 in the fluid 212 in the device. The pressure pulse cleans the device, for instance removes fouling from the device.

Figure 5 show the magnitude (Magn) and phase (Arg) of the impedance curves of a situation wherein the transducer assembly 200 is in contact with an outer surface of a device to be cleaned. The thickness of the wall is 10 mm. The res onance frequency is 20.4 kHz i.e. consistent with the fundamental resonance of the transducer, the impedance magnitude is relatively low (150 W) and the phase curve shifts from negative to positive at the resonance. The curves are very close to those of an unloaded transducer. In comparison, figure 3 shows curves for a fully mass loaded transducer. The resonance frequency is shifted to 26 kHz, the impedance magnitude is relatively high (550 W) and the phase curve does not shift from negative to positive at the resonance. Accordingly, as the mass loading of the transducer assembly to the device to be cleaned is reduced compared e.g. to the system 100, operation of the transducer close to its natural resonance frequency is permitted.

The system of the present invention must have a waveguide comprising a cavity. It is essential that ratio of the diameter of the cavity and the waveguide at dis tance I from the second end is 0.9 or less, preferably 0.2 to 0.9, more preferably from 0.4 to 0.8. This is to ensure that the system can operate at its fundamental frequency even when in contact with a device to be cleaned. Other dimensions and shapes of the cavity are not critical.

Figures 6a-c represent exemplary non-limiting cavity configurations of a cylin drical waveguide. Accordingly, the cavity can be an opening, i.e. a through hole (fig. 6 b), or a hollow portion in the wave guide (fig. 6 a,c). According to a par ticular embodiment the area of the first end of a waveguide is larger than area of the second end. This allows the acoustic radiation efficiency to be increased, by increasing the acoustic radiation impedance versus ultrasound impedance of the system. Side view of an exemplary waveguide of this type is shown in figure 6d. According to another particular embodiment the first end is shaped for interfac ing with geometry of the outer surface of the device to be cleaned. Side view of an exemplary waveguide of this type is shown in figure 6e. The first end, such as the one shown in figure 6e may also comprise clamping means for fastening and tightening the system to the outer surface of the device to be cleaned.

As shown in figure 4A, the base portion 204a of the cavity is separated from the first end by a distance marked with letter h. Figure 7 presents radiated acoustic power of a system as a function of the distance h when the actuation power was fixed. The figure is based on numerically simulated 20 kHz driving of a 38 mm cylindrical waveguide fixed at a 10 mm thick steel wall having water on the other side. As seen from the figure, the distance h has plurality optimums, one of which is about 85 mm and another one at about 210 mm.

According to still another particular embodiment, the first end of the waveguide is designed to further enhance the ability of the system to operate at its funda mental resonance frequency. Exemplary design alternatives are presented in figure 8. According to one embodiment, the first end comprises an opening 813a, wherein the opening is adapted to be towards the outer surface. According to this embodiment side walls 814 of the opening act as point like pressure sources when the system is in operation. The opening can be also such that it is not through the walls of the first end but like the one shown in figure 8 d.

According to another embodiment the first end comprises at least one pair of protrusions 813b or one or more circular protrusions 813c adapted to be posi tioned on the outer surface of the device to be cleaned. The distance d’ between the two protrusion in the x-direction of the coordinate system 899 is preferably smaller than half of the acoustic wavelength in the fluid and/or wall of the device, for example, at 20 kHz d’ <38 mm. If the wall thickness of the device to be cleaned is thin e.g. <10 mm, the protrusions should be close to each other. An exemplary distance d’ is 5-25 mm, 20 kHz. This is to ensure that an interference point is formed on the inner surface of the wall. According to an exemplary em bodiment the height of the protrusion in the y-direction of the coordinate system 899 is 1 -100 mm. An exemplary protrusion length is 10 mm. The protrusions are adapted to act as point-like pressure sources. The contact area of the first end i.e. the contact area of the protrusions is less than 100%. According to a preferable embodiment, the contact area of the at least one pair of protrusions is 1 -30%, more preferably 1 -20%, most preferably about 10% of the total area of the first end. An exemplary contact area of a protrusion or a circular protrusion acting as a point-line pressure source is 1 10-330 mm ² .

In the structure depicted in Figure 8 (804, 813a) there are two cavities, one in the wave generating means similarly as described in Figure 4, and another one in the coupling structure 812. The first cavity decouples the transducer from the load, whereas the second cavity 813a generates two point-like sources from the edges of the cavity 814 which generate waves which constructively interfere and generate a leaky Lamb wave which efficiently generates sound in the fluid. This dual cavity structure allows more freedom when selecting the size of the sec ondary cavity 813a.

According to one embodiment the mechanical wave generating means is a Langevin transducer. A Langevin transducer comprises a front mass (head), a back mass (tail) and piezoelectric ceramics. A Langevin transducer is a resonant transducer for high-power ultrasonic actuation. The transducer is composed by a stack of piezoelectric disks 201 a, e.g. 2, 4, 6 or 8 disks, clamped between two metallic bars, typically aluminum, titanium or stainless-steel, that feature a front mass and a back mass of the transducer, respectively. The length of the front mass and back mass of the transducer are tuned so that the transducer behaves as a half-wavelength resonator, i.e. a fundamental standing wave is born along the long axis of the transducer, featuring an antinode at both ends of the trans ducer. This results in an antinode at the first end 300a and at the second end 300b of the transducer assembly, and a nodal point at the middle of the wave guide. Such a transducer is narrowband featuring sharp resonance and anti resonance, separated typically by a narrow, e.g. 1 kHz, frequency interval. Op timal and natural resonance behavior occurs when the transducer is driven in free space (no mechanical load). Any loading damps the resonance, increases the bandwidth and affects the resonance frequency. Heavy loading kills the fun damental resonance. Although the transducer assembly still is able to operate at higher resonance frequencies even when heavily loaded its efficiency is re duced. The higher resonance frequencies are in this case those of the coupled system, i.e. loading-modified higher resonance frequencies of the transducer assembly.

According to another embodiment the present invention concerns a method for cleaning a device holding fluid. The method comprising the following steps a) providing a system 200 comprising

o mechanical wave generating means 201 and

o a waveguide 202 comprising

• a first end 202a adapted to be in contact with outer surface of the device 203

• a second end 202b wherein the second end is in contact with the mechanical wave generating means,

o a waveguide comprising a cavity 204 comprising a base portion 204a in xz-plane of the coordinate system 299, the base portion separated from the second end by a distance I in y-direction of the coordinate system, o the mechanical wave generating means is adapted to emit mechanical waves through the waveguide to the outer surface,

o waveform of the mechanical waves is adapted to be such that there is an antinode 205 positioned in the waveguide at the distance I from the second end,

o maximum diameter D _max of the waveguide in x-direction of the coordinate system 299 is less than ½ of wavelength of the mechanical waves and in that

o ratio of diameter d of the base portion and the diameter D of the wave guide in x-direction of the coordinate system 299 at distance I is 0.9 or less, preferably from 0.2 to 0.9, most preferably from 0.2 to 0.8.

b) contacting the first end with outer surface of the device,

c) the mechanical wave generating means emitting, via waveguide succes sion of mechanical waves inner surface of the device, d) the mechanical waves interfering at the inner surface and producing a vibrating inner surface, and

e) the vibrating inner surface producing and emitting a pressure pulse into the fluid.

The thickness of the vessel wall of the device to be cleaned is typically 2-30 mm. The point like pressure sources such as the protrusions of the waveguides of a transducer are preferably made of material that is softer than the material of surface of the device. According to an exemplary embodiment, the surface of the device is made of stainless steel and the protrusions are made of aluminum.

Experimental

Design of the transducer assembly

The transducer assembly was composed of a piezoelectric ultrasonic stack transducer (Langevin transducer, sandwich transducer) and an optional wave guide. The transducer was either a commercially available model, or a custom made one. The transducer was a narrowband (featuring typically e.g. a 1 kHz bandwidth) resonant transducer, composed by a stack of piezoelectric disks (e.g. 2, 4, 6 or 8 disks), clamped between two metallic bars (typically aluminium, titanium or stainless steel) that feature front mass and back mass of the trans ducer.

The transducer design was based on a chosen resonant frequency (e.g. 20 kHz) which determines the choice (material and dimensions) of the piezoelectric disks. The stack of piezoelectric disks features a narrowband resonator. The lengths of the front mass and back mass were tuned such that the coupled res onator (i.e. transducer) behaves as a half-wavelength (lambda/2) resonator at the chosen frequency. This is the fundamental resonance of the transducer. The bandwidth remained narrow (e.g. 1 kHz). Transducer design was based on the oretical and/or numerical modelling (finite-element simulations).

An optional waveguide was fitted as an extension on the first end of the trans ducer. The length of the waveguide was chosen/tuned so as to maintain the fundamental resonance behavior of the transducer. To this end, the waveguide length must be a multiple of lambda/2. A waveguide may be useful e.g. to in crease the q-value of the transducer assembly, to provide thermal insulation be tween the transducer and a system to be cleaned, or to provide flexibility in transducer placement in situations when the transducer cannot directly fit against the device to be cleaned. Waveguide design is based on theoretical and/or numerical modelling (e.g. finite-element simulations).

Point-like contacts (e.g. contact protrusions, openings) were machined as ex tensions on the first end of a transducer assembly. Cavities were machined in the waveguide. The shapes of the contact structures were evaluated and opti- mized by theoretical and/or numerical modelling (finite-element simulations).

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims.