IE913839A1

IE913839A1 - An acoustic chamber for the aerosol treatment of exhaust¹gases

Info

Publication number: IE913839A1
Application number: IE383991A
Authority: IE
Original assignee: Euratom
Priority date: 1990-11-27
Filing date: 1991-11-01
Publication date: 1992-06-03
Also published as: PT99614A; LU87850A1; EP0488097A1; EP0488097B1; DE59102932D1; WO1992009354A1; JPH06509406A; CA2097070A1

Abstract

The invention relates to an acoustic chamber (3, 6, 7) for the aerosol treatment of exhaust gases which are intended to flow through the chamber and are exposed therein to an acoustic field. According to the invention, the chamber has a square or hexagonal cross-section, and the sound sources (4, 5) emit obliquely into the chamber, so that the sound is reflected several times on the side walls of the chamber before it strikes the sound reflector (8, 9) likewise obliquely arranged at the other end of the chamber. As a result of the intensive sonic treatment, fine dust particles are caused to vibrate and coagulate to give larger grains which can then be filtered off.

Description

The invention relates to an acoustic chamber for the aerosol treatment of exhaust gases, which are to flow through the chamber and which are exposed therein to an acoustic field.

Exhaust gases often contain very fine solid particles, the rate of which is to be reduced as much as possible before the exhaust gases are freed into the environment. Up to now, such dust filters use either electrostatic fields or mechanical processes (cyclone or venturi separators) which are very ex15 pensive and the filter effectiveness of which remains limited.

From Spanish patent 459 523 a proposal is known to use an ultrasound chamber for the cleaning of fumes. In this patent, a sound field is produced in a tube axially to its axis and the gases to be cleaned are helically led through the tube, the particles being put into vibration and interaction by the sound field, so that they agglomerate. Then, the larger particles can be evacuated from the exhaust gases by simple mechanical filters.

The acoustic chamber according to this document is only suited for small fume flow rates, since with greater flow rates, the helical laminar path of the fume flow changes into a turbulent flow. Further, the pressure losses at passing through the chamber are important and may require an additional fan to finally expulse the exhaust gases into the chimney.

The object of the invention is to adapt such a chamber to the use in the industrial field, i.e. with greater flow rates, and at the same time to reduce the pressure losses in the chamber.

This object is achieved according to the invention by the acoustic chamber which comprises the features of the characteristic part of claim 1. Contrary to the known aerosol chamber, the exhauts gases flow through the chamber along its axis in a straight line, while the sound field fills the chamber in a multiply broken way. Simultaneously, by using several sound sources, an optimal sonorisation of the chamber is achieved without the different sound waves influencing each other. For constructional reasons it is useful to give the chamber a square or hexagonal constant cross-section.

By the measure according to claim 3, it becomes possible to direct the sound sources precisely to the associated reflectors, which leads to an optimal energy efficiency.

The invention will now be described by means of a preferred embodiment and with reference to the drawings.

Figure 1 shows in perspective an outside view of a chamber 20 according to the invention.

Figure 2 shows an axial cross-section through this chamber.

Figure 3 shows means that can be used for varying the length 25 of the chamber between the sound sources and the reflectors.

Figure 1 shows an acoustic chamber according to the invention for the aerosol treatment of exhaust gases. It is integrated into an installation for cleaning fumes and has, in this envi30 ronment, the task to coagulate the fine particles to larger dust particles, which can then be separated by a mechanical filter (not shown). The exhaust gases are supplied to the chamber via an inlet duct 1 in the direction of arrow 2, the chamber consisting of three duct portions disposed in align35 ment, i.e. a first duct portion 3, on which are mounted sound transmitters 4 and 5, a duct portion 6, in which the interaction between the sound field and the fume particles is intended to take essentially place, and a duct portion 7, to which are mounted sound reflectors 8 and 9. The ducts have a square cross-section and are traversed linearly by the fumes without any encumbrance. The sound sources 4 and 5 are associated to and respectively of two adjacent sides of the duct portion 3 and are disposed in such a way that their axis hits the respective opposite wall under an angle of for example 60°. In the same way, the reflectors 8 and 9 are mounted on the duct portion 7. In the cross-section view according to figure 2, the sound source 4 and the associated reflector 9 as well as the sound field resulting therefrom in the duct portion 6 are represented. If the length of the central duct portion 6 is chosen appropriately, and if the sound source 4 and the reflector are precisely aligned to each other, this results in a stationary sound field as indicated.

Figure 2 might also be regarded as an orthogonal cross-sec20 tion, which runs through the sound source 5 and the reflector 8. The sound source operates for example at a frequency of 20 kHz and the length of the chamber between the sound sources and the reflectors lies between 1 and 3 meters. If lower frequencies are used, for example 10 kHz, a stationary sound field could be produced in a substantially longer chamber of up to 6 meters, which means that the fumes stay longer in the chamber and thus the coagulation effect is increased. The total acoustic power invested is for example 300 Watt, a higher efficiency being possible also in this case with lower frequencies.

In order to be able to precisely stabilize the acoustic length between the sound sources and the reflectors, it is suggested to render the length of the central duct portion 6 slightly variable and to allow this length to be precisely varied. To this end, for example, a double flange, as it is shown in detail in figure 3, is used at one end of the central duct portion. At this point, this duct portion is not directly screwed to the reflectors or sound sources, but via an inter5 mediary member 10, the length 11 of which can be varied in axial direction of the duct by screw bolts 12, the wall continuity being ensured by two wall portions 13 and 14 sliding one on the other. The plurality of bolts 12 distributed over the duct periphery can be coupled to a common drive motor (not shown) by a chain 15, so that the length variation can be obtained by the operator at any time and in a precise manner.

In order to keep the reflection at the duct walls as lossfree as possible, it is advisable to keep the surface roughness of the inner side of the ducts as low as possible, such that, if possible, there are no protuberances exceeding 1 mm.

The invention is not restricted to the embodiment described by means of the drawings. Thus, ducts with hexagonal or even octogonal cross-section can be used and then three or four sound sources can be associated to the three or four facing wall pairs. It is also possible to invert the direction of sound propagation for all or only for single ones of the acoustic systems consisting of sound transmitter and reflector and to dispose the sound transmitters on the side of the duct portion 7 for gas evacuation.

Due to the linear and practically undisturbed flow cross-section of the chamber for the fume to be filtered, the pressure losses can be kept small, so that there is no need for an additional fan. The chamber can be mounted, as desired, with horizontal, inclined or vertical axis.

Claims

1. An acoustic chamber for the aerosol treatment of exhaust gases, which are to flow through the chamber and which are 5 exposed therein to an acoustic field, wherein the chamber has a regular polygonal cross-section with 2k sides and wherein k sound sources are provided, the axes of which, projected on a cross-section plane of the chamber, include an angle of 180/k degree and who are each associated to; a respective side wall 10 of the chamber, and that k reflectors are likewise disposed, the sound sources transmitting sound waves into the chamber under such an angle with the chamber axis that the sound is repeatedly reflected from the walls of the chamber before it hits on the other end of the chamber a sound reflector, which 15 is likewise disposed in an inclined position, so that a stationary wave is formed and that the exhaust gases flow through the chamber linearly and along the chamber axis.

2. An acoustic chamber according to claim 1, wherein k is two 20 or three.

3. An acoustic chamber according to one of claims 1 to 2, wherein means are provided allowing to change the length of the chamber between the sound sources and the reflectors.

4. An acoustic chamber according to one of claims 1 to 3, wherein the sound sources are capable to excite a sound field at a frequency of below 25 kHz. 30

5. An acoustic chamber as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.