US3668835A

US3668835A - Electrostatic dust separator

Info

Publication number: US3668835A
Application number: US8295A
Authority: US
Inventors: Pierre Vicard
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-02-13
Filing date: 1970-02-03
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: JPS4821997B1; DE2006298A1; GB1239712A; CH521170A; DE2006298C3; CA939617A; BE745288A; DE2006298B2; SE362019B; ES376456A1

Abstract

In an electrostatic dust separator having a ionizing electrode axially disposed within a Venturi, an annular nozzle produces a spray of an auxiliary substance in the gas stream issuing from the Venturi, around a polarizing electrode. The coarse particles of the spray are electrified by influence and they attract and retain the finer ionized dust particles carried by the gas. The spraying nozzle may be disposed within an auxiliary Venturi to increase the velocity of the gas and to improve impact effects between the dust particles and the sprayed particles. The ionizing electrode is preferably formed with fins to enhance the ionizing corona discharge, while the polarizing electrode is on the contrary smooth to avoid any noticeable discharge effect. The gas to be treated may be fed into the inlet of the main Venturi by an injector nozzle to produce a negative pressure whereby a portion of the gas issuing from the main Venturi may be recycled into same through an annular passage provided around the said main Venturi.

Description

United States Patent Vicard 14 1 June 13, 1972 [54] ELECTROSTATIC DUST SEPARATOR 2,852,239 9/1958 Vicard ..261/1 16 X cm 1:22:21 :1223 z1z:;:;;;;;;; [22] Filed: Feb. 3 1970 FOREIGN PATENTS OR APPLICATIONS 159,412 8/1940 Austria ..261/118 [211 App! 8295 4,824 1886 Great Britain. ...261/l16 421,811 12/1934 Great Britain ..55/107 Foreign Application Priority Data 940,930 11/1963 Great Britain ..261/DIG. 54

Feb. 13, 1969 France ..6903675 March 24, 1969 France ..6907777 jgj' jg igfg'g Pggjff [52] US. Cl ..55/l07, 55/117, 55/122, [57] ABSTRACT 55/124, 55/127, 55/129, 55/130, 55/134, 55/138, 55/146, 55/152 55/154, 55/226, 55/238, 55/257, In an electrostatic dust separator having a ionizing electrode 55/259, 55/319, 55/338, 55/345, 55/418, 55/456, axially disposed within a Venturi, an annular nozzle produces 5 5 3 261/1 16, 2 1 17 2 1 1 2 a spray of an auxiliary substance in the gas stream issuing from 54 317/3 the Venturi, around a polarizing electrode. The coarse parti- 51 1m. 01. ..B03c 3/36, B03c 3 41 cles of the p y are electrified y influence and y attract [58] Field of Search... ..55 5, 10, 11, 7, 107, 117, and retain the finer ionized dust Particles carried y the 5 g 119, 20 2 124, 2 23 30 4 The spraying nozzle may be disposed within an auxiliary Ven- 135 136, 137, 133, 139, 146 150, 151 152 154, turi to increase the velocity of the gas and to improve impact DIG 38 129, 226 238 257' 259 319 338 345, effects between the dust particles and the sprayed particles. 418,456; 261/1316 54 64, 116, 117, 8; The ionizing electrode is preferably formed with fins to 317/3 enhance the ionizing corona discharge, while the polarizing electrode is on the contrary smooth to avoid any noticeable [56] References Cited discharge effect. The gas to be treated may be fed into the inlet of the main Venturi by an injector nozzle to produce a UNITED STATES PATENTS negative pressure whereby a portion of the gas issuing from the main Venturi may be recycled into same through an annul,333,790 3/1920 Bradley "SS/I 52 X passage provided around the i main Ventmi 1,579,462 4/1926 Wintermute ..55/ X 2,357,355 9/1944 Penney ..55/l07 7 Claims, 7 Drawing Figures PATENTEnJuu 1 3 1912 3, 668 835 sum 1 or 2 I) INYENT R.

:wuf 91, VIC.

BY g

ELECTROSTATIC DUST SEPARATOR The present invention relates to electrostatic dust separators of the kind comprising an axial high voltage electrode disposed in a Venturi unit so as to ionize the dust particles in suspension in the gas which flows through the said unit, the said particles being thereafter electrostatically separated from the gas itself.

In accordance with the present invention an electrostatic dust separator of the kind above referred to, comprises means disposed downstream of the Venturi unit for producing in the gas stream which issues from the said unit a spray of particles of an auxiliary substance, these particles being appropriately electrified so as to attract and to retain the ionized dust particles in suspension in the gas stream. The auxiliary substance, which may be a divided solid or preferably a liquid such as water, may be sprayed around a polarizing electrode disposed in the gas stream.

The axial ionizing electrode is conveniently formed of a rod provided with thin radially extending longitudinal fins between which the gas stream may circulate so as to dislodge any dust particle which could have settled on the said electrode, while the polarizing electrode is preferably made of a mere smooth rod.

A portion of the gas issuing from the outlet of the Venturi unit is preferably recycled into the inlet thereof in order that the ionized dust particles entrained by the recycled gas may form nuclei adapted to attract and to retain the dust particles in suspension in the untreated gas.

In the annexed drawings FIG. 1 is a general vertical section of an electrostatic dust separator according to the invention.

FIG. 2 reproduces to an enlarged scale the upper portion of FIG. 1, but showing the butterfly valve at its transverse position.

FIGS. 3 and 4 are cross-sections taken along lines III-III and IV-IV ofFIG. 1.

FIG. 5 is a view similar to FIG. 1, but illustrating a modified embodiment.

FIG. 6 is a transverse section taken along line VI-VI of FIG. 5.

FIG. 7 is a vertical section showing the lower portion of another embodiment of the invention.

The electrostatic dust separator illustrated in FIGS. 1 to 4 comprises a vertical cylindrical casing 1 having at its upper end a converging injector nozzle 2. This injector nozzle opens in front of the converging inlet 3 of a conduit comprising a neck portion 4 followed by a diverging portion 5, the said conduit thus forming a Venturi unit which is axially maintained within the outer casing 1 by radial arms such as 6.

An insulating cross-member 7 is provided in the vicinity of the lower end or outlet of Venturi unit 3-4-5. This crossmember carries two vertical

axial electrodes

8 and 9. The first one extends upwardly through the Venturi unit and it terminates in the vicinity of the neck of the said unit, while the other one 9 extends downwardly well below the lower end thereof.

As shown in FIG. 3 the first electrode 8, or ionizing electrode, is formed of a rod provided with a relatively large number of longitudinal radial fins 10 of quite reduced thickness which therefore have a relatively sharp outer edge. These fins may for instance be mounted in longitudinal slots milled in the periphery of the rod and in which they are retained by soldering or otherwise. As to the second electrode 9, or polarizing electrode, it is formed of a smooth rod of relatively large diameter, as illustrated in FIG. 4.

Below the lower end of the lower electrode 9 there is provided within the casing 1 an annular nozzle 11 carried by a tube 12 which extends through the conical bottom 13 of the casing, its outer end 14 being adapted to be connected with an appropriate supply. The nozzle 11 is so arranged as to produce a slightly diverging annular spray, concentric to the lower electrode 9 without any intermediate screen and at a relatively small distance thereof. This spray strikes the casing wall, as indicated at 15. The spraying nozzle 11 is supplied with an appropriate liquid under pressure, as for instance water, it being however noted that it would also be possible to spray solid particles in suspension in a gas such as air. The nozzle, the liquid or the pulverized solid arc electrically conducting and the nozzle is grounded.

The bottom 13 of casing 1 has a lower outlet 16 closed by an appropriate air-lock device, while a lateral exhaust 17 is provided somewhat above the said bottom.

In the embodiment illustrated in FIGS. 1 and 2,the upper nozzle 2 is equipped with a valve 18 of the butterfly type, pivoted about a transverse horizontal axis, this valve, of circular contour, being of such diameter that at the horizontal position (see FIG. 2), it does not fully close the nozzle but leaves an annular gas passage 19 between its periphery and the inner side of this nozzle.

Both

electrodes

8 and 9 are connected by means (not illustrated) with an appropriate high voltage source, also not shown.

In operation the dust-laden gas enters the apparatus through nozzle 2, valve 18 being fully open as illustrated in FIG. 1. This gas flows downwardly in the form of a jet through the converging portion 3, the neck 4 and the diverging portion 5. There is thus obtained an injector effect whereby gas is sucked from the annular space which surrounds the Venturi unit 3-4-5. The gaseous mass thus realized flows downwardly at a high velocity along the ionizing electrode 8, the water vapor contained by the gas condensing on the dust particles in the neck portion 4, in a per se known manner. Owing to the presence of the sharp outer edges of the fins 10, the high voltage applied to electrode 8 results in a sparkless electric discharge of the type generally called corona effect, between this electrode and the surrounding walls of the Venturi, whereby the suspended particles are strongly ionized.

A substantial portion of the gas issuing from the lower end of the Venturi unit 3-4-5 is recycled upwardly between the said unit and easing 1, together with the suspended particles, due to the injector effect of the jet from nozzle 2. The gaseous mass which flows through the Venturi unit is thus, so to speak, inseminated with already ionized particles which play the role of nuclei on which the other particles tend to agglomerate. This progressive growdi of the suspended particles facilitates their separation.

The remainder of the gas issuing from the Venturi unit 3-4-5 flows downwardly towards outlet 17 around the polarizing electrode 9 and through the spray 15 of relatively coarse particles issuing from the spraying nozzle 11. Owing to the vicinity of this high voltage polarizing electrode 9, which due to its smooth outer surface produces no corona effect, these coarse particles become electrified by influence as they issue from nozzle 11, in such a manner that the gas flows through a zone which contains coarse particles carrying an electric charge of opposed polarity with respect to the ionization charge. These coarse particles therefore attract and retain electrostatically the finer ionized dust particles in suspension in the gas, which are therefore entrained towards the bottom 13 of easing 1, wherefrom they may be periodically evacuated through the air-lock or like valve device 16. Of course some of these coarse particles may be entrained by the gas through the outlet 17, but owing to their dimensions they are easily separated by means of a conventional sleeve filter, centrifugal separator or similar apparatus, together with the finer particles which they have retained.

Owing to the insemination of the incoming gas with already ionized particles entrained by the recycled gas, the ionization effects within the Venturi unit 3-4-5 are extremely rapid. The velocity of the gas flow through the latter may therefore be quite high. The production of filtered gas is thus important and moreover the dust particles cannot settle on the inner walls of the Venturi, which eliminates the frequent cleaning operations generally required in conventional electrostatic separators. Owing to the particular construction of the ionizing electrode 8, the gas stream may freely circulate between the successive radial fins where dust particles cannot therefore be retained. When the flow of the incoming dust-laden gas is so small that the velocity of the gas jet issuing from nozzle 2 becomes insufficient, it is possible to reduce the effective cross-sectional area of this nozzle by closing valve 18 (or more accurately by bringing it to the horizontal position), which of course increases the velocity of the jet.

It may furthermore be noted that the recycled gas stream is submitted to a sudden change of direction at the outlet of the Venturi unit 3-4-5, as shown by the arrows in FIG. 1. This entails a quite noticeable centrifugal acceleration which tends to separate the biggest particles from this stream. In other words the zone immediately below the Venturi unit acts as a centrifugal separator for the particles in the recycled gas stream, those which have grown by collecting and retaining smaller particles within the Venturi, being selectively projected downwardly towards the spraying nozzle 1 1.

It is obvious that the butterfly valve 18 is of no use when the flow (quantity per unit of time) of dust-laden gas to be filtered is substantially constant. The inlet portion or the neck of the Venturi unit could be provided with helicoidal vanes imparting to the gas stream a rotational motion of high angular velocity in order to separate the biggest particles. In such a case the fins should also be helicoidal so as not to hinder circulation of the gas between them also care would have to be taken that the vanes do not short-circuit the axial electrode (the latter could be of reduced length or the vanes could be made of an insulating material). The apparatus could comprise a lower settling chamber of large volume for a better separation of the coarse particles from the gas before the latter reaches the outlet 17. The spray of electrified particles below the Venturi unit could be realized in any appropriate manner. The inner side of the walls of the Venturi unit 34-5 could be provided with fins or blades in order to enhance ionization of the incoming dust particles. In some cases the recycling could be dispensed with.

In the modification illustrated in FIG. 5 the Venturi unit 3-4-5 is surrounded by an intermediate cylindrical casing 20 of substantially larger diameter (about twice the diameter of the neck portion 4 of the Venturi), this intermediate casing being spaced from the main outer casing 1 so as to leave an annular passage for the recycled gas stream. The upper and lower edges of this intermediate casing 20 are joined with the upper and, respectively, the lower end of the Venturi so as to form therewith a single member enclosing an inner closed annular space, this member being supported by the radial arms 6. Radial partitions such as 21 are disposed within the said closed space between the Venturi unit 3-4-5 and the intermediate casing 20. These partitions, as well as the intermediate casin g, the Venturi unit itself and the supporting arms 6 are made of a conducting material such as a metal. The closed space may further be filled with a conducting substance such as water.

With such an arrangement the Venturi unit 345 is only connected electrically with the intermediate casing 20 which is itself grounded through arms 6.

Under such conditions when a high-voltage is applied to the upper electrode 8, the Venturi unit 3-4-5 becomes electrified by influence, which means that electric charges of a polarity opposed to that of electrode 8 appear on its inner surface, while charges of the other polarity flow freely from the Venturi unit towards the intermediate casing 20 through the radial partitions 21 (and also through the conducting substance which fills the hollow space between the Venturi and the intermediate casing when such a filling is provided). Owing to the distance between the Venturi unit 3-4-5 and the intennediate casing, the electrostatic field which appears between the venturi and the upper electrode 8 is much stronger. Furthermore the ionization current which circulates between the electrode.

and the Venturi may freely flow towards the intermediate casing 20 without determining on the inner surface of the Venturi localized electric charges which could limit the ionization effects.

It is obvious that similar considerations would apply to the lower spray 15, namely that in order to enhance electrostatic influence effects the spraying nozzle 11 should be grounded through a connection of substantial length. But in this case the said connection is comprised of the oblique tube 12 which may be provided relatively long without-requiring any particular arrangement for this purpose.

In the embodiment of FIG. 7 an additional Venturi unit 22-23 is disposed below the outlet of the diverging portion 5 of the main Venturi unit with which it is connected by a perforated intermediate zone 24. This zone may be formed of a perforated plate or of a grid, as shown. The spraying nozzle 11 is axially disposed within the additional Venturi unit 22-23 and it is carefully stream-lined, as illustrated. Its annular spray is so directed as to flare upwardly and to pass through the perforated zone or grid 24. The lower end or outlet of the additional Venturi unit is flanged outwardly as shown as 25, so as to close the annular space between the said Venturi unit and the outer casing, apart from some holes 26 of reduced crosssection.

In operation the gas stream issuing from the diverging portion 5 of the main Venturi unit is considerably accelerated in the converging inlet 22 of the additional Venturi unit. Under such conditions the impact effects between the dust particles or agglomerates of dust particles and the coarse liquid particles from the spraying nozzle 11 are greatly enhanced. The coarse particles collect between the additional Venturi and the outer casing together with the dust particles which they have entrained, and the liquid flows through the holes 26 towards the bottom of casing 1, wherefrom it is evacuated through the air-lock device 16.

The angle of the spray with respect to the axis of the apparatus in the zone 24 is preferably below 45.

It is further to be noted that the recycled gas stream passes through the grid 24 as indicated by the arrows 27.

I claim:

1. An electrostatic dust separator comprising:

a Venturi unit having an inlet and an outlet for a dust-laden a high-voltage ionizing electrode disposed axially of said Venturi and terminating substantially at the outlet thereof, said ionizing electrode being in the form of a rod having thin longitudinally extending radial fins to ionize dust particles in the gas within said unit, and said ionizing electrode having a predetermined polarity;

a high-voltage polarizing electrode forming an extension of said ionizing electrode beyond the outlet of said Venturi unit, said electrode being in the form of a smooth rod, said polarizing electrode having the same polarity as said ionizing electrode;

spraying means to produce around said polarizing electrode a spray of particles of an auxiliary substance;

and means to collect said sprayed particles together with the dust particles which they have attracted and retained.

2. In a dust separator as claimed in claim 1, means to recycle into the inlet of said Venturi unit a portion of the gas issuing from the outlet thereof.

3. In a dust separator as claimed in claim 1:

an outer casing axially enclosing said Venturi unit while leaving an annular gas-recycling passage around same;

an axial injector nozzle in said outer casing in front of the inlet of said Venturi unit to receive the gas to be treated and to inject same into said inlet together with a portion of the gas issuing from the outlet of said Venturi unit upstream of the spraying means, said portion thus being recycled through said passage.

4. In a dust separator as claimed in claim 3, a butterfly valve disposed within said injector nozzle, the diameter of said valve being such that at its position transverse to said injector nozzle it leaves an annular gas passage through said injector nozzle.

5. In a dust separator as claimed in claim 1, an intermediate casing axially enclosing said Venturi unit while being spaced therefrom, said intermediate casing having a first and a second end respectively joined to the inlet and to the outlet of said Venturi unit so as to determine therewith a closed annular space, and said Venturi unit and said intermediate casing being made of an electrical conducting material;

means within said closed annular space to form a radially directed electrical connection between said Venturi unit and said intermediate casing;

an electrically conducting outer casing enclosing said intermediate casing while leaving an annular gas passage between itself and said intermediate casing;

means to form a radially directed connection between said intermediate casing and said outer casing;

means to recycle into the inlet of said Venturi unit through said annular gas passage a portion of the gas issuing from the outlet of said Venturi unit;

and means to electrically connect said intermediate cas-ing with said outer casing.

6. An electrostatic dust separator comprising:

a Venturi unit having an inlet at one end for the dust-laden gas to be treated, and an outlet at the other end through which said gas issues;

a high-voltage ionizing electrode axially disposed in said Venturi unit to ionize dust particles in the gas flowing therethrough;

spraying means adjacent the outlet to produce in the gas issuing from said Venturi unit a spray of particles of an auxiliary substance;

electrifying means adjacent the outlet to electrify said sprayed particles with such a polarity that they attract and retain ionized dust particles in the gas issuing from said Venturi unit;

and means adjacent said outlet to collect said sprayed particles together with the dust particles which they have retained;

an auxiliary Venturi unit disposed downstream of the first named unit to receive the gas issuing from the outlet of said first-named Venturi unit, said auxiliary Venturi unit having an inlet portion disposed adjacent the outlet of the first named Venturi unit, and having a neck portion and an outlet portion with said spraying means and said electrifying means being so disposed within said auxiliary Venturi unit that the spray of particles of said auxiliary substance is produced in the vicinity of the neck portion of said auxiliary Venturi unit in order that said particles of said auxiliary substance may act on the dust particles in a zone wherein the gas which contains said dust particles flows with a high velocity.

7. An electrostatic dust separator comprising:

an elongated outer casing having a first end and a second end;

an injector nozzle disposed in axial alignment with the axis of said outer casing and at the first end of said outer casing to receive the dust-laden gas to be treated;

a main Venturi unit axially disposed within said outer casing while leaving an annular gas-recycling passage between said main Venturi unit and said outer casing, said main Venturi unit having an inlet portion, a neck portion and an outlet portion, with said inlet portion disposed in spaced relationship to said injector nozzle, said inlet portion being disposed between said injector nozzle and said second end, said inlet portion being disposed downstream of said injector nozzle to receive the gas issuing therefrom, and at a distance therefrom so as to form therewith a gas-recycling injector device;

a high-voltage electrode axially disposed within said main Venturi unit to ionize dust particles in the gas flowing therethrough;

insulating means to support said ionizing electrode;

an auxiliary Venturi unit axially disposed within said outer casing downstream of said main Venturi unit to receive the gas issuing therefrom and leaving an annular space between said outer casing and said auxiliary Venturi unit,

said auxiliary Venturi unit havingan inlet portion, a neck portion and an outlet portion, with said inlet portion of said auxiliary Venturi unit being spaced from the outlet portion of said main Venturi unit so as to leave an intermediate annular zone;

means to substantially prevent gas flow through said annular space;

a high-voltage polarizing electrode extending axially of and adjacent to the inlet portion of said auziliary Venturi unit;

insulating means to support said polarizing electrode;

an annular spraying nozzle axially disposed in the outlet portion of said auxiliary Venturi unit to produce an annular diverging spray of electrically conducting liquid particles which surrounds said polarizing electrode and passes through said intermediate annular zone so as to collect within said annular space together with the ionized dust particles which they have retained;

means to supply an electrically-conducting liquid to said annular nozzle;

means to remove collected liquid and dust particles from said annular space;

and means for exhausting gas from the second end of said outer casing downstream of the outlet portion of said auxiliary Venturi unit.

Claims

1. An electrostatic dust separator comprising: a Venturi unit having an inlet and an outlet for a dust-laden gas; a high-voltage ionizing electrode disposed axially of said Venturi and terminating substantially at the outlet thereof, said ionizing electrode being in the form of a rod having thin longitudinally extending radial fins to ionize dust particles in the gas within said unit, and said ionizing electrode having a predetermined polarity; a high-voltage polarizing electrode forming an extension of said ionizing electrode beyond the outlet of said Venturi unit, said electrode being in the form of a smooth rod, said polarizing electrode having the same polarity as said ionizing electrode; spraying means to produce around said polarizing electrode a spray of particles of an auxiliary substance; and means to collect said sprayed particles together with the dust particles which they have attracted and retained.

3. In a dust separator as claimed in claim 1: an outer casing axially enclosing said Venturi unit while leaving an annular gas-recycling passage around same; an axial injector nozzle in said outer casing in front of the inlet of said Venturi unit to receive the gas to be treated and to inject same into said inlet together with a portion of the gas issuing from the outlet of said Venturi unit upstream of the spraying means, said portion thus being recycled through said passage.

5. In a dust separator as claimed in claim 1, an intermediate casing axially enclosing said Venturi unit while being spaced therefrom, said intermediate casing having a first and a second end respectively joined to the inlet and to the outlet of said Venturi unit so as to determine therewith a closed annular space, and said Venturi unit and said intermediate casing being made of an electrical conducting material; means within said closed annular space to form a radially directed electrical connection between said Venturi unit and said intermediate casing; an electrically conducting outer casing enclosing said intermediate casing while leaving an annular gas passage between itself and said intermediate casing; means to form a radially directed connection between said intermediate casing and said outer casing; means to recycle into the inlet of said Venturi unit through said annular gas passage a portion of the gas issuing from the outlet of said Venturi unit; and means to electrically connect said intermediate cas-ing with said outer casing.

6. An electrostatic dust separator comprising: a Venturi unit having an inlet at one end for the dust-laden gas to be treated, and an outlet at the other end through which said gas issues; a high-voltage ionizing electrode axially dIsposed in said Venturi unit to ionize dust particles in the gas flowing therethrough; spraying means adjacent the outlet to produce in the gas issuing from said Venturi unit a spray of particles of an auxiliary substance; electrifying means adjacent the outlet to electrify said sprayed particles with such a polarity that they attract and retain ionized dust particles in the gas issuing from said Venturi unit; and means adjacent said outlet to collect said sprayed particles together with the dust particles which they have retained; an auxiliary Venturi unit disposed downstream of the first named unit to receive the gas issuing from the outlet of said first-named Venturi unit, said auxiliary Venturi unit having an inlet portion disposed adjacent the outlet of the first named Venturi unit, and having a neck portion and an outlet portion with said spraying means and said electrifying means being so disposed within said auxiliary Venturi unit that the spray of particles of said auxiliary substance is produced in the vicinity of the neck portion of said auxiliary Venturi unit in order that said particles of said auxiliary substance may act on the dust particles in a zone wherein the gas which contains said dust particles flows with a high velocity.

7. An electrostatic dust separator comprising: an elongated outer casing having a first end and a second end; an injector nozzle disposed in axial alignment with the axis of said outer casing and at the first end of said outer casing to receive the dust-laden gas to be treated; a main Venturi unit axially disposed within said outer casing while leaving an annular gas-recycling passage between said main Venturi unit and said outer casing, said main Venturi unit having an inlet portion, a neck portion and an outlet portion, with said inlet portion disposed in spaced relationship to said injector nozzle, said inlet portion being disposed between said injector nozzle and said second end, said inlet portion being disposed downstream of said injector nozzle to receive the gas issuing therefrom, and at a distance therefrom so as to form therewith a gas-recycling injector device; a high-voltage electrode axially disposed within said main Venturi unit to ionize dust particles in the gas flowing therethrough; insulating means to support said ionizing electrode; an auxiliary Venturi unit axially disposed within said outer casing downstream of said main Venturi unit to receive the gas issuing therefrom and leaving an annular space between said outer casing and said auxiliary Venturi unit, said auxiliary Venturi unit having an inlet portion, a neck portion and an outlet portion, with said inlet portion of said auxiliary Venturi unit being spaced from the outlet portion of said main Venturi unit so as to leave an intermediate annular zone; means to substantially prevent gas flow through said annular space; a high-voltage polarizing electrode extending axially of and adjacent to the inlet portion of said auziliary Venturi unit; insulating means to support said polarizing electrode; an annular spraying nozzle axially disposed in the outlet portion of said auxiliary Venturi unit to produce an annular diverging spray of electrically conducting liquid particles which surrounds said polarizing electrode and passes through said intermediate annular zone so as to collect within said annular space together with the ionized dust particles which they have retained; means to supply an electrically-conducting liquid to said annular nozzle; means to remove collected liquid and dust particles from said annular space; and means for exhausting gas from the second end of said outer casing downstream of the outlet portion of said auxiliary Venturi unit.