A METHOD IN SONIC CLEANING

Abstract: A method in sonic cleaning of a space (13) wherein there is a tendency of a coating being formed on the surfaces thereof. By controlled relative phase shift of pressure waves emitted from two mutually spaced locations (A, B) an amplified pressure wave is produced, which scans the space said amplified pressure wave providing an effective loosening of the coating by acting on the surfaces of the space (13).

Description: A METHOD IN SONIC CLEANING Internal spaces of furnaces, heat exchangers and similar apparatuses with gas flowing therethrough such as hot flue gases or waste gases from chemical processes, tending to form a dust coating on the surfaces of the space generally should be kept free from coatings formed by ashes or soot because such coatings are heat insulat¬ ing, can change the flow pattern in the space and also can cause increased corrosion of existing metal surfaces. In order to prevent the unfavourable operating condi¬ tions thus associated with an increasing coating in the actual ' space, from arising it has become more and more common to use so-called sonic cleaning, i.e. to use gas-carried pressure waves with tone or infrasonic frequency in order to physically affect the dust of the coating because this is a cheap cleaning method as compared with e.g. sweeping by means of steam. In sonic cleani ng, vibration energy is supplied usually as pressure waves from a high-power sound emitter which is mounted in a wall with some external air cooling, and is distributed to different parts of the space, e.g. the narrow spaces between a plurality of boiler tubes in a boiler installation. A high sound pressure level is required in order to obtain a satis¬ factory cleaning. However, problems may be involved in distributing the vibration energy to all parts of the space which is often of complicated form, considering the occurence of sound shadow although there is obtained in most cases a surprisingly uniform distribution of the pressure waves due to reflection of the sound in the space to be cleaned. As far as large boiler installations are concerned where the space to be cleaned has walls with a length of 10 to 15 m the pressure wave may be attenu- ated too much to reach effectively all nooks of the space and to provide an effective cleaning of portions of the space which are remote from the sound emitter. It is not possible to locate sound emitters in this space proper due to the fact that vital parts of the sound emitter cannot stand existing high temperature and also due to the fact that the flow pattern in the space can be disturbed by objects mounted therein such as a sound emitter mounted between the tubes of a nest of boiler tubes. The range can be extended by increasing the acoustic power generated by the sound emitter in order to increase the sound pressure level but in this respect there are technical limitations because the construction of the sound emitter will be e ry compli¬ cated and expensive if a substantial increase of the power is required. Moreover, the production of acoustic power generally is relatively expensive. A better alternative of increasing the acoustic range could be to concentrate the pressure wave emitted by the sound emitter to different portions of the space to be cleaned but no practicable method has been avail¬ able so far in order to achieve by simple means such a concentration of the sound from a sound transmitter of the type commonly used in sonic cleaning. The object of the invention is to provide by simple means a concentration of the pressure wave to different portions of the space to be cleaned also when these por¬ tions are located at a great distance from the. pressure wave generator, and more particularly by using conven¬ tional pressure wave generators operating in the frequency and power ranges now commonly applied in sonic cl eani ng . To achieve this object as well as additional objects and advantages of the invention, which in part will be set forth in the following description and in part will be obvious from the description, the invention provides a method in sonic cleaning of a space where¬ in there is a tendency of a coating being formed on the surfaces thereof, characterized in that pressure waves are emitted from at least two mutually spaced locations in the space, and in that an amplified pressure wave is produced by controlled phase shift of the pressure wave emitted from one location in relation to the pressure wave emitted from the other location, said amplified pressure wave scanning the space. By applying this method it is possible to control by rather uncomplicated means the main direction of the emitted pressure wave, the power or intensity thereof in said direction being substantially increased. The invention will be described in more detail below reference being made to the accompanying drawing in which FIG. 1 diagrammatical ly illustrates how the 'wave from two synchronous sound emitters can be induced to change the direction thereof by a relative phase shift of the pressure waves emitted from the two sound emitters; and FIG. 2 discloses an arrangement of two pneumatically operated sound emitters for providing said phase shi ft. Referring to FIG. 1, there are shown therein two sound emitters A and B spaced a distance a ^ from each other. The sound emitters in this case are parallel to each other but this is not necessary; the directions of the sound emitters also can converge or diverge. It is also possible to arrange the sound emitters opposite to each other. If it is assumed that the two sound emitters generate sound waves comprising a single frequency component only, having the wave length λ, and if it is also assumed that these frequency components are fully synchronized and without phase shift, the wave front will propagate in the direction A/B - Al/Bl. If a phase shift φ is introduced between the pressure wave from the sound emitter B in relation to the pressure wave from the sound emitter A the direction of the wave front will be changed and the wave front will have the direction A/B - A2/B2. If the angle between the directions A/B - Al/Bl and A/B - A2/B2 is designated θ (generally measured from the perpendicular of the line A/B connecting the sound emitters) there is obtained the relationship a s i n 0 = — • λ = kλ s i n θ 2π where k is a constant which can be expressed as k = φ " 2τr sin θ The angle Θ is obtained from the expression sin Θ = Ψ 2τr k By cyclically changing the phase shift angle φ an instantaneous in-phase addition of the two pressure waves (at φ = 0) is obtained and also a continuous change of the angle of the direction of the combined wave at the maximum power or intensity. Thus, there is obtained a scanning pressure wave, i.e. a pressure wave moving forwards and backwards, which can be utilized in order to cover the several portions of a space of a furnace, heat exchanger or similar apparatus at maximum power or intensity also when the space has f OMPI large dimensions and/or complicated form. The two sound emitters can be located in a wall of this space or in adjacent or opposite walls at a place where they can be mounted easily. At in-phase addition of the pressure waves the total sound pressure amplitude will be twice as large, which means that the power, or intensity of the pressure wave will be increased by about 6 db. It is assumed that the amplitudes of the two sound emitters are equal . In an addition other than the in-phase addition, e.g. of two frequencies which are not adjacent to each other, there is obtained an increase of the power or intensity of about 3 db only. FIG. 2 discloses a practical embodiment of an apparatus for working the method according to the invention for sonic cleaning of a space as described with reference to FIG. 1. In this case the two sound emitters are of the pneumatic type and comprise a membrane housing 1 OA and 10B, respectively, and an acoustic horn 11A and 11B, respectively. The emitters are mounted in a wall 12 of the space 13 to be cleaned, and pressurized air at the pressure Pα is supplied through a conduit 14. Synchronism of the two pneumati¬ cally operated sound transmitters A and B can be obtained by spacing these sound emitters such a distance <_ that they interact by so-called external coupling via the acoustic horns 11A and 11B. If the distance a_ is related to the wave length λ so that a = k • λ where k is the constant referred to above, it has been found convenient in practice to choose a value of k which is less than 1 e.g. 0.7 but the distance is not critical; k can range between e.g. 0.5 and 1. The oscillating air column of the horns and the membrane mechanism the oscillation of which is controlled by the resonance conditions of the horns thus can be influenced by the pressure oscillations externally of the horns. Though synchronism can be obtained in this manner the phase position is not determined thereby. The relative phase shift can be mastered only if the chambers behind the membranes in the membrane housings 10A and 1OB are interconnected by means of an acoustic wave guide, and in FIG. 2 this connection is provided by a conduit 15 so that a stable phase position will appear at a defi¬ nite length of this conduit. This method of combining an external acoustic coupling and a coupling which is bound to a conduit provides the possibility to control the phase shift from e.g. T. radianes (synchronization in opposition) to a value sliding cyclically from π to 0 (in-phase synchronization). Sliding of this type can take place if the length of the conduit 15 is somewhat shorter or longer than the optimum length for a stable phase position. The sliding can be controlled in a desired manner if there is maintained behind the membranes during operation of the sound emitters a pressure P. which is lower than the pressure P existing at the supply side of the membranes. For this purpose, pressurized air can be supplied to the conduit 15 and the pressure in this conduit and the chambers behind the membranes can be adjusted to a desired value P b by means of a regulator 16. However, a desired pressure in the conduit 15 and the chambers behind the membranes in the membrane housings 10A and 10B can also be obtained by arranging a connection for pressurized air in one or both of the sound emitters between the supply side of the membrane < and the chamber at the rear side of the membrane, e.g. by providing an opening in the membrane, so that pressurized air is supplied from the supply side of the sound emitters, the pressure in the conduit I5 and the chambers behind the membranes being adjusted by con- trolled discharge of air through the pressure regulator 16. If a = λ i.e. k = 1, sine Θ = ■ - at the phase shift ■ϊ for the fundamental frequency (first harmonic) of a sound wave comprising several harmonics. In that case sine Θ for the fundamental frequency will be 0.25 corre¬ sponding to an angle Θ = 14.5°. For the second harmonic the phase shift would be π corresponding to sine 0 = 0.5 and an angle Θ = 30°. Thus, in dependence of the harmonics the pressure wave propagates as several separate lobes. Scanning comprising a continuously repeated move¬ ment from one side to the other of the amplified pressure wave can be obtained also in another manner, viz. if the frequencies of the sound emitters differ from each other to a minor extent, e.g. by 1 Hz, so that there is obtained a beat frequency of the pressure waves. Then, the phase shift will continuously vary between negative ' and positive values and the waves will be in-phase, i.e. there will be no phase shift, for a short moment only of each beat cycle. It should be noted that the significant increase of the power and intensity of the pressure wave by in-phase addition will have time to affect the dust coating in the space 13 also at these relatively short but repeated instances when the amplified scanning pressure wave passes a location in the space. The sound emitters can also consist of sound emitters of other types such as electric sound emitters. In that case the phase shift or phase change in order to obtain the periodical scanning of the amplified pressure wave can be obtained electronically by applying methods known in the art. In this connection it should be noted that as pressure wave generators it is possible to use sound emitters for audible sound in the approximate frequency BUREAU OMPI f . - l range 20 - 20,000 Hz as well as devices e.g. pulsators for non-audible sound, infrasound, in the approximate frequency range 2 - 20 Hz. Excellent practical results have been obtained at frequencies in the range extend¬ ing from 60 to 800 Hz. In the embodiment disclosed herein, only two sound emitters are provided but the method according to the invention can be applied by using more than two sound emitters. In sonic cleaning by applying the method according to the invention the cleaning may be performed either while the gas with dust entrained therein passes through the space to be cleaned or during a period when the gas flow is interrupted. This latter procedure provides the advantage that the loosened dust cannot be withdrawn with the gas. The method of sonic cleaning according to the invention can be applied not only in furnaces, heat exchangers or similar apparatuses through which gas is flowing at least intermittently, but also in cold stores wherein ice deposits on cooling coils and cooling-coil batteries and also on the walls cause problems by reduc¬ ing the efficiency of the cooling installation. A further field wherein the method according to the invention can be applied is in spray dryers wherein the powder produced by condensing a liquid can have a tendency of adhering to the walls and the devices in the spray drying compartment. When the pressure wave generators are of the pneumatic type an antistate agent can be supplied to the space 13 together with the drive fluid (pressurized air) in order to prevent the loosened dust from being attracted to the cleaned surfaces again due to electro¬ static forces.

Claims: CLAIMS 1. A method in sonic cleaning of a space (13) wherein there is a tendency of a coating being formed on the surfaces thereof, c h a r a c t e r i z e d in that pressure waves are emitted from at least two mutually spaced locations (A, B) in the space (13), and in that an amplified pressure wave is produced by controlled phase shift of the pressure wave emitted from one location (A) in relation to the pressure wave emitted from the other location (B), said amplified pressure wave scanning the space. 2. A method according to claim 1 wherein pneumatic pressure wave generators (A, B) are " used for generating pressure waves in dependence of the displacement of a movable valve member, c h a a c ¬ t e r i z e d in that a pressure acting on the valve member is controlled. 3. A method according to claim 2 wherein pressure wave generators of the membrane type are used, c h a r a c t e i z e d in that a connection is maintained by an acoustic wave guide (15) between the pressure wave generators (A, B) at the rear side of the respective membranes. 4. A method according to claim 3, c h a r a c - t e r i z e d in that an adjusted pressure is main¬ tained in the wave guide (15) and at the rear side of the membranes by the supply of pressurized fluid to the wave guide (15). 5. A method according to claim 3, c h a r a c - t e r i z e d in that an adjusted pressure is main¬ tained in the wave guide (15) and at the rear side of the membranes by the supply of pressurized fluid from the supply side of the membranes through a passage between the two sides of the membranes and controlled discharge of pressurized fluid from the wave guide (15) 6. A method according to claim 1, c h a r a c ¬ t e i z e d in that the pressure wave generators are operated at different frequencies to provide a beat frequency. 7. A method according to any of claims 1 to 6, c h a r a c t e z e d in that the frequency of the pressure waves is within the frequency range of audible sound. 8. A method according to any of claims 1 to 6, c h a r a c t e r i z e d in that the frequency of the pressure waves is within the frequency range of infrasound. 'BUREATJ OVPI

Inventors: HOLM B
Applicants: HOLM B, KOCKUMS AUTOMATION, KOCKUMS AUTOMATION AB
Application year: 1979
Main classification: B08B7
IPC codes: B06B1, B08B7, B08B17, F23J3, F28G7