Electrogasdynamic Precipitator Utilizing Retarding Fields

Abstract

Electrostatic precipitating apparatus in which particles entrained in a gas stream are charged in an ionization zone established between one or more ionizer electrodes and a movable attractor electrode and collected, by reaction with a transverse precipitation field, in an adjacent downstream zone including a movable dielectric collection surface. The dielectric collection surface may be carried by the same conductive member constituting the attractor electrode. Electric fields that are non-attractive to the charged particles, and that have components directed against gas flow, are established in the vicinity of the ionization zone-collection zone interface, thus increasing collection efficiency by retarding downstream movement of the particles, including turbulence in the gas stream, and creating non-linear precipitation field conditions. A series of such collection zones or of additional ionization zones and associated collection zones may be spaced along the gas stream to further enhance precipitation efficiency and to reduce re-entrainment of particles in the gas stream. The ionizer and attractor electrodes and the dielectric collection surfaces are movable to a cleaning zone remote from the gas stream where collected particles are neutralized and removed.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the electro-static precipitation of particles entrained in a gas stream and, in particular, to apparatus for achieving high precipitating efficiency, while affording significant reductions in the size of precipitator units, through the establishment of electric fields tending to retard movement of the entrained particles through the precipitator.

Conventional practice in the electrostatic precipitator technology has been to precipitate particles carried by a gas stream by passing the particles through a transverse electric field in which the particles both acquire electrical charges and are caused to be brought into contact with a collection surface. This is the familiar single-stage or Cottrell precipitator. Alternatively, the particles may be charged and collected in a two-stage operation, including an upstream charging field and a separate precipitation field located in a downstream region of the stream. A precipitator of this type is, for example, disclosed in U.S. Pat. No. 2,318,093, issued to Penney on May 4, 1943.

Both foregoing types of precipitators, however, utilize charging and precipitation fields in which the chief component is directed transverse to the gas stream, with little or no component oriented parallel to the gas flow. Moreover, such parallel components as do occur are generally balanced in both the upstream and downstream directions, so that no net electric repelling force exists to retard downstream movement of the charged particles. Accordingly, the particles move through the precipitators at virtually the speed of the gas stream. The residence time of any given particle within the precipitation field is thus determined by the gas stream velocity. Hence to increase residence time in such precipitators, with the object of increasing efficiency of collection by prolonging particle exposure in the precipitation field, it is necessary to decrease the velocity of the gas stream or enlarge, usually by lengthening, the collection zone. Neither of these steps is desirable, however, as the former requires a reduction in throughput capacity and the latter an increase in the size of the precipitating unit.

Typically, the removal of particles deposited on the collection surfaces of the foregoing prior art precipitation systems is accomplished by vibrating or rapping the surfaces, which may be in the form of plates positioned along the gas stream, to dislodge the particles. However, this causes a substantial amount of re-entrainment of the particles in the gas stream. These re-entrained particles must be recharged and again collected for efficient cleaning of the gas stream. The prior art solution has been to lengthen the collection zone to compensate for such re-entrainment of particles during the cleaning operation. Attempts to produce high efficiency, on the order of 99%, for example, units by this approach have resulted in significant increases in the overall size and complexity of the precipitation system. Manufacturing costs, of course, are correspondingly increased.

A further difficulty associated with known precipitation techniques is that the operative elements of the particle charging and precipitation zones, e.g., the ionizer and attractor electrodes and the particle collection surfaces, in time become saturated with precipitated particles, so that the charging or precipitation fields, or both, are quenched to the point that further particle collection is impeded. Such quenching of the charging and precipitating fields results in reduced collection efficiencies since, on the one hand, the electric charge imparted to each particle will drop in value as the charging field becomes weaker and, on the other, the electrostatic forces tending to move the particles to the collection surface will also be of a lower magnitude by virtue of the combined effect of the lower particle charge and the decreased field strength of the precipitation field. This situation is aggravated, and collection efficiency still further reduced, when particles of low conductivity are being precipitated, a common circumstance in many industrial and environmental applications.

Efforts directed toward overcoming the foregoing difficulties associated with cleaning of the collection surfaces and quenching of the ionization and/or precipitation fields, have included the use of moving electrodes that are cleaned at quiescent locations so as to minimize re-entrainment and to avoid field deterioration. Some measure of improvement in performance has been achieved in this way. But current technology has failed to recognize that important advantages are obtainable through the appropriate utilization of such cleaning and field maintenance procedures in combination with and complementary to the establishment of particle retarding electric fields in the gas stream.

These and other disadvantages of prior art precipitation systems are overcome by the present invention.

SUMMARY OF THE INVENTION

In accordance with the invention, a movable surface having one or a series of associated conductive regions and dielectric regions, spaced alternately in the direction of gas flow, is located along a boundary of the gas stream. Particles entrained in the gas stream are charged electrically in an ionization zone or zones established with one or more of the conductive regions of the movable surface and are thereafter precipitated in a collection zone or zones established with the downstream dielectric regions of the surface. Movement of the charged particles through the precipitator is retarded, and the residence time of the particles within the precipitator increased, by electric fields set up in the gas stream in the vicinity of each ionization zone-collection zone interface. For this purpose, the geometries of adjacent ionization zones and collection zones are such that electric fields that are non-attractive to the charged particles and that have components in opposition to the gas flow are established across the zone interfaces.

Significant advantages contributing to increased collection efficiency are realized by the use of particle retarding fields. Notably, increased residence time in the ionization and precipitation fields yields more effective movement of the particles to the collectors; eddy currents induced in the gas stream by rearwardly directed force lines lead to an increased incidence of collision between particles and thus to agglomeration of the particles and a consequent increase in electrostatic mobility; and the average precipitation field strength is elevated due to the creation of a non-linear field at the area of transition between ionization and collection zones.

In preferred embodiments, the movable surface is constituted by a single conductive member, with each dielectric region of the surface being formed by a layer of dielectric material on the surface of the member facing the gas stream. The region of the conductive member immediately upstream of the respective dielectric layers then forms the associated conductive region of the surface. Other constructions of the movable surface may be used, including the use of a dielectric member having one or more strips of conductive material provided on the facing surface.

Each ionization zone preferably is formed by a plurality of ionizer electrodes positioned in opposed spaced relation to a conductive region of the surface, which functions as an attractor electrode for corona discharges emanating from the ionizer electrodes. A transverse precipitation field extending from a high-potential passive electrode, desirably in the plane of the ionizer electrodes, is established in each collection zone normal to the corresponding dielectric region of the surface.

During operation, charged particles and molecular ions are collected rapidly at the upstream areas of each dielectric region, where they set up electric fields terminating at the downstream area of the preceding conductive region. These fields, being non-attractive to the charged particles, have a repelling effect on the charged particles and therefore slow downstream movement of the particles. As a result, the particles remain within the influence of the ionization and precipitation fields for a longer time, acquire higher electric charges, and thus are moved in a shorter trajectory to the dielectric collection surface. Particle movement is most influenced by the retarding fields when laminar flow conditions prevail in the gas stream, but improved collection efficiency is obtained with turbulent flow as well since residence time is thereby prolonged.

In one embodiment, additional particle retarding fields are maintained in the gas stream by appropriate positioning of the upstream end of the passive electrode in each collecting zone. Preferably, the passive electrode is located so that the upstream end is at least no farther upstream than the leading edge of the associated dielectric region of the surface. By this configuration, the change in the magnitudes and directions of the electric field lines is created between the upstream end of the passive electrode and the downstream area of the adjacent upstream conductive region. As the polarity of each passive electrode is the same as that of the charged particles, an electric force in opposition to particle flow is thus set up in the gas stream, with the result that the rate of particle flow relative to the gas stream is decreased.

In another embodiment, each passive electrode is spaced farther downstream and a pre-charging ionizer electrode is located within the collection zone in spaced relation to the upstream edge of the associated dielectric region. A corona discharge occurs between the pre-charging electrode and the adjacent conductive region, thereby setting up an electrostatic force directed in opposition to particle flow. A further advantage of this configuration is that it enhances the retarding field set up by the particles and ions deposited on the upstream area of the dielectric region. This results by virtue of a field being established between the precharging ionizer and the upstream area of the dielectric region, so that a significant portion of the charges generated by the ionizer electrode is attracted to and retained on the dielectric surface. Such "pre-charging" of the leading area of the collector surface assures a high charge level irrespective of the rate at which charges are carried to the surface by the collected particles. The strength of the retarding fields is therefore preserved at all times.

To prevent reverse corona conditions from arising at the ionization zone-collection zone interface when a precharging ionizer is used, particularly at high operating potentials, or when high resistivity particulate materials are collected, an air gap is interposed between the downstream edge of the attractor region and the leading edge of the dielectric region. The air gap is sized to reduce electrical stresses across the interface to below corona initiation conditions.

According to still another aspect of the invention, particles collected on the conductive regions and dielectric regions of the movable surface are removed at a cleaning zone located remote from the gas stream to avoid re-entrainment of the collected particles. For greater cleaning efficiency, neutralization of the particle charges accompanies the cleaning operation. If desired, the ionizer electrodes, including the pre-charging ionizer, may also be movable to a remote location for cleaning.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made to the following description of exemplary embodiments, taken in conjunction with the figures of the accompanying drawings, in which:

FIG. 1 is a sectional view of one embodiment of an electrostatic precipitator constructed in accordance with the invention, showing the distribution of the electric fields established within the ionization and collection zones;

FIG. 2 is a sectional view of a second embodiment of the invention, also showing electric field patterns within the precipitator;

FIG. 3 is a sectional view of a cascaded-zone precipitator unit having the basic configuration of FIG. 1, but including an additional collection zone;

FIG. 4 portrays still a further variation of the embodiment of FIG. 1, in which a series of associated ionization zones and collection zones are spaced along the gas stream;

FIG. 5 is a sectional view of another plural-zone precipitator according to the invention, showing the use of multi-point ionizer electrodes, but with the field lines within the ionization and collection zones omitted for clarity;

FIG. 6 is a sectional view of a further embodiment of the invention, depicting the configuration of FIG. 3 incorporated into a cascaded-zone precipitator;

FIG. 7 illustrates yet another embodiment of the invention, in which an extended ionizer module is used in the collection zone;

FIG. 8 is a pictorial illustration of a precipitator embodying certain novel features of the invention, and in which the movable surface constitutes an endless belt structure;

FIG. 9 is a graph depicting collection efficiency percentage (.eta..sub.c) versus .lambda. for Cottrell precipitation systems; and

FIG. 10 is a bar graph portraying collection area (in.sup.2) versus collection efficiency percentage (.eta..sub.c).

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

As illustrated in FIG. 1, a representative embodiment of the invention includes a pair of spaced members 10 and 12 that define the boundaries of a gas stream (indicated by the arrows) from which dust particles, such as the fly ash entrained in the exhaust gases of incinerator stacks or the like, are to be removed. Preferably, the members 10 and 12 are electrically conductive and, as illustrated, connected to electrical ground. They may, however, be maintained at any appropriate reference potential.

In accordance with the invention, an upstream region 14 and 16 of the members 10 and 12, respectively, is exposed to the gas stream. Downstream from these conductive regions, the members 10 and 12 are blocked from exposure to the gas flow by bodies 18 and 20 of dielectric material, thus defining dielectric regions 22 and 24 along the downstream extent of the members 10 and 12. Preferably, the dielectric bodies 18 and 20 constitute layers formed on the surfaces of the members 10 and 12, but an independent construction may be used and the members 10 and 12 may take the form of conductive substrates located behind the dielectric bodies. Similarly, the conductive and dielectric regions, for example, the regions 14 and 22, respectively, of the member 10, need not be carried by the same member, although for simplicity of construction it is preferable that they are.

If desired, the members 10 and 12 may be constructed of dielectric material and the conductive region of each member might then be constituted by a conductive strip carried by the facing surface of the member. With this arrangement, a conductive substrate would be located adjacent to the dielectric region of each member and external to the gas stream to provide a reference electrode to maintain a strong electric field tending to hold the collected particles on the dielectric surface.

Regardless of the specific construction used, however, the conductive and dielectric regions together define a surface located in proximity to the gas stream. As explained more fully hereinafter, this surface is supported in a manner to be movable to a quiescent location for cleaning.

One or more ionizer electrodes 26 are located centrally of the members 10 and 12 opposite the conductive regions 14 and 16, and a passive electrode 28 is disposed in spaced relation to the dielectric regions 22 and 24. The ionizer electrodes 26 and the passive electrode 28 desirably are located in the same plane relative to the members 10 and 12.

As schematically illustrated in FIG. 1, the ionizer electrodes 26, depicted as elongate wire-like elements, are connected to a power source 30 to set up a corona discharge in the gas stream between the wires 26 and the conductive regions 14 and 16 of the members 10 and 12. For clarity, only half of the field distribution lines are shown in FIG. 1, but it will be apparent that the corona discharge field surrounding each electrode 26 is symmetrical.

In a like manner, the passive electrode 28 is connected to a power source 32 of the same polarity as the source 30 to establish a precipitation field normal to the dielectric regions 22 and 24. Again, the field lines in this respect are shown in FIG. 1 over only one-half of the unit. Although a separate source 32 is illustrated for driving the passive electrode 28, the electrode 28 may, if desired, be connected to the same source 30 used for the ionizer electrodes 26. Also, while four ionizer electrodes are depicted in FIG. 1, this is for illustrative purposes only and any suitable number may be used.

Dust particles entering the precipitator are impressed with unipolarity electrical charges upon traversing the ionization zone defined by the ionizer electrodes 26 and the conductive regions 14 and 16, which function as attractor electrodes for the corona discharge currents emanating from the ionizer electrodes. Thereafter, the charged particles enter the collection zone, which functionally may be considered as beginning at the upstream edges of the dielectric regions 22 and 24, although some particle collection does, of course, occur on the conductive regions 14 and 16 within the ionization zone. Most particles, however, pass through the ionization zone and are deposited on the dielectric surfaces of the collection zone.

It is a feature of this invention that, complementary to the transverse ionization and precipitation fields, additional electric fields are established in the vicinity of the interface, indicated generally in FIG. 1 at 34, of the ionization and collection zones. These fields are non-attractive to the charged particles and have components directed in opposition to particle flow. The existence of such non-attractive or repelling fields in the gas stream results in electric repulsion forces being exerted against the downstream movement of the charged particles, causing them to slow down relative to the velocity of the gas stream. Accordingly, the individual particles remain in the transverse ionization and precipitation fields for a longer time, acquire higher electrical charges, and hence are caused to be moved more effectively to the collection surfaces. Thus, fewer particles escape through the outlet end of the precipitator.

Other benefits are also derived from the presence of the retarding fields. For instance, a certain degree of turbulence, i.e., eddy currents, is generated in the gas stream by the upstream field components. Such turbulence induces chaotic motion of the particles within the stream, with the result that the incidence of collision among the particles is increased. Since particles tend to agglomerate upon colliding and because large particles acquire charges more rapidly then do small particles, greater electro-static mobility is therefore imparted to the particles. The particles, as a consequence, drift toward the collection surfaces at an increased rate. Collection efficiency is thereby further enhanced.

Precipitation is still further aided by the retarding fields inasmuch as a non-linear field condition is created in the vicinity of transition from a conductive region to a dielectric region. The effect is to increase the average precipitation field strength in the transitional area. An obvious contribution to improved collection performance results.

Referring to FIG. 1, one such electric retarding field is seen as being established across the ionization zone-collection zone interface between the leading area of the dielectric region 22 and the adjacent downstream area of the conductive region 14. This occurs because the charged particles and molecular ions collected on the dielectric region 22 are prevented from leaking their charges to the grounded member 10. Consequently, a high-strength electrical disruption exists across the interface 34. Charged particles entering the area of this disruption encounter a repulsive force in the upstream direction and are accordingly slowed in their downstream movement. This effect is especially pronounced when gas flow is laminar.

A further electric field having a component in opposition to particle flow is established in one embodiment of the invention by appropriate positioning of the passive electrode 28. More specifically, the passive electrode is located such that its upstream end 36 extends no farther upstream than the upstream end of the dielectric region 22. With the passive electrode thus positioned, a high-strength electrical disruption is established across the gas stream between the end 36 of the passive electrode and the downstream area of the conductive region 14. A similar disruption (not shown) is of course established with the conductive region 16. Inasmuch as the passive electrode 28 extends across the full width of the gas stream, that is to say, in a direction perpendicular to the plane of FIG. 1, all charged particles in the stream will be subjected to the retarding forces of this field.

The beneficial effects of these retarding fields on the collection process may be further appreciated by consideration of the following simplified analysis. The precipitating or collection efficiency (.eta.c), assuming uniform charging in the ionization zone and negligible re-entrainment of collected particles, may be expressed as:

(1) .eta.c = 1-e.sup.-

and the coefficient .lambda. as:

(2) .lambda. = [v.sub.dy /v.sub.p ] [1/d] = k.sub.p E.sub.p l/v.sub.p d

where:

v.sub.dy is the drift velocity of a particle perpendicular to the collector surface (meters per second);

v.sub.dx is the particle drift velocity in the upstream direction imparted by the retarding electric fields (meters/seconds);

v.sub.g is the gas velocity (meters/seconds);

v.sub.p = v.sub.g - v.sub.dx, i.e., the actual particle velocity downstream (meters/seconds);

1 is the length of the precipitation zone (meters);

E.sub.p is the average precipitation field strength (volts per meter);

d is a multiple of the distance between the plane of the passive electrode and the plane of the surface (meters); and

k.sub.p is particle mobility in an electrostatic field (meters.sup.2 /volt-second).

Typical values of collection efficiency (.eta..sub.c) for specific values of .mu. are tabulated in Table I. --------------------------------------------------------------------------- TABLE I

.lambda. .eta. c(%) __________________________________________________________________________ 0 0 2 86.5 3 95.0 4 98.0 7 99.0 __________________________________________________________________________

Analyzing equations (1) and (2), it is apparent that for a precipitator having given values of 1 and d the magnitude of .lambda., and hence the collection efficiency (.eta..sub.c), may be increased by increasing k.sub.p and/or E.sub.p, or by decreasing the value of v.sub.p. With this in mind, it is clear that significant advantages in precipitator performance are attributable to the presence of retarding fields within the gas stream, since all three of these desired parameter changes are thereby realized.

As previously described, k.sub.p is increased by the agglomeration mechanism produced by the retarding fields, and E.sub.p is increased by the creation of a non-linear precipitation field in the area of transition between the ionization zone and the collection zone, as is clearly depicted in FIG. 1. Also, as pointed out above, the net particle velocity (v.sub.p) in the downstream direction is lower than the gas velocity, due to the slowing influence of the electric repulsion forces set up by the retarding fields.

Conversely, for a given collection efficiency (.eta.c) reductions in the size of the precipitator unit required to obtain that particular efficiency may be achieved in accordance with the present invention. Turning again to equations (1) and (2), it may be seen that the length of the precipitation zone 1 may be decreased if k.sub.p and/or E.sub.p is increased or if v.sub.p is decreased, assuming a constant value of d. It has already been shown that the values of all three parameters are improved through the use of the retarding electric fields of the present invention. Accordingly, it is obvious that the utilization of such fields allows the reduction of precipitator size, through reduction in the length of the precipitation zone required, without sacrifice of collection efficiency.

A further benefit of the invention is that it allows optimization of the parameters of equation (2) in order to obtain a high .lambda. value, and accordingly a high collection efficiency (.eta.c), while keeping 1 (collector size) as small as possible. Experiments have established, for example, certain preferred values of h (see FIG. 1) which give good ionizer performance without unduly lengthening the ionization zone. An appropriate value of h, which is measured from the emission surface of the electrode, in this respect has been found to be 11/4 the distance between the plane of the electrodes 26 and the plane of the conductive region 14. The term "plane of electrodes" refers to the plane containing the emission surfaces of the electrodes nearest to the facing conductive region. The ionizer electrodes 28 may be positioned farther apart than the foregoing value of h at little or no loss in charging efficiency, but with some sacrifice in compactness of the ionization zone. On the other hand, some loss in ionizer performance results if the electrodes are spaced closer together than 11/4 the distance to the plane of the conductive regions 14 or 16.

To further optimize ionizer performance, the foremost ionizer electrode should be located at least as far as 3/4 h (see FIG. 1) downstream from the upstream edge of the conductive region 14 or 16. This assures that there will be no downstream distortion of the ionization field due to unduly high current flow to the leading ionizer electrode.

Similarly, for best results, the distance f in FIG. 1, i.e., the distance between the upstream edge of the dielectric region 22 and the leading end 36 of the passive electrode 28, should not exceed fifteen-sixteenths the distance between the plane of the electrodes 26 and the plane of the conductive region 14. If the end 36 is more distantly located from the upstream edge of the region 22, a retarding field might still be created with the downstream areas of the conductive regions 14 and 16, but the length of the precipitation zone would be unnecessarily enlarged.

Other preferred dimensions are indicated in FIG. 1 and the other figures of the drawings. It will be understood, of course, that these are merely illustrative of preferred values and that other configurations and locations of the various components may be used without departing from the scope of the invention.

The above-described beneficial effects of the electric retarding fields, and particularly those due to the buildup of charges on the upstream areas of the dielectric regions, are further enhanced in the embodiment of the invention represented in FIG. 2. It incorporates generally the same overall organization as that of FIG. 1, except for the location of the trailing ionizer electrode 38, the position of the passive electrode 40, and the formation of an air gap 42 at the interface of the ionization and collection zones.

As shown in FIG. 2, the trailing electrode 38 is positioned in overlying relation to the upstream area of the dielectric regions 44 and 46 of the collection zone. So positioned, a corona discharge is established in the upstream direction between the electrode 38 and the downstream area of the conductive regions 48 and 50, and a strong electrical disruption terminating at the upstream area of the dielectric regions 44 and 46 is also set up. Therefore, not only is a retarding field maintained between the ionizer electrode 38 and the conductive regions 48 and 50, but the charge level on the upstream areas of the dielectric regions leading to the establishment of electric retarding fields between the dielectric and conductive regions is maintained at a high order of magnitude irrespective of the rate at which charges are carried to the dielectric regions by collected particles. By thus "precharging" the adjacent areas of regions 44 and 46, the electrode 38 assures that substantial electrostatic forces will be maintained in opposition to the gas stream at all stages of operation.

Although the ionizer electrode 38 is physically located within the collection zone, some particle charging occurs by virtue of the corona current between the electrode 38 and the conductive regions 48 and 50. In effect, therefore, the zone of ionization extends to the upstream ends of the dielectric regions 44 and 46, notwithstanding that an air gap 42 is formed between the conductive and dielectric regions.

For optimum performance, the electrode 38 should be positioned downstream from the upstream edges of the dielectric regions 44 and 46 at least as far as five-sixteenths the distance between the plane of the ionizer electrodes, including, of course, the precharging electrode 38, and the plane of the conductive region 48 or 50. In the same vein, superior operating conditions are achieved when the upstream end of the passive electrode 40 is spaced from the precharging electrode 38 by a distance at least as great as 11/4 the distance between the electrodes and the conductive regions.

At high operating potentials, high charge levels are impressed on the upstream areas of the dielectric regions 44 and 46 by the precharging electrode 38. This gives rise to very high electrical stresses across the ionization zone-collection zone interface, tending to initiate reverse corona conditions in the vicinity of particles collected on the grounded conductive regions 48 and 50. Reverse corona conditions are especially likely to occur when particulate materials having a resistivity greater than about 10.sup.9 ohm-centimeters are being collected. It is to avoid the creation of such corona conditions that an air gap 42 is interposed between the conductive and dielectric regions.

Desirably the size w (see FIG. 2) of the air gap is at least as great as 0.063 inches per kilovolt per inch of the average field strength between the precharge ionizer 38 and the planes of the conductive regions 48 and 50. However, to assure that reverse corona discharges will not occur even at low potentials, w should not be less than 0.125 inches. An air gap sized in this manner reduces the electrical stress between the conductive and dielectric regions to below corona initiation levels.

Still further advantages of increased collection efficiency and size reduction are obtainable if a plurality of collection zones or a series of ionization zones followed by associated collection zones are spaced along the gas stream. With such plural-zone, or cascaded-zone, configurations, the extent of the individual ionization zones, and particularly of the individual collection zones, may be shortened considerably from those required in a unit utilizing only a single ionization zone and a single collection zone. In addition, particles which may become re-entrained at upstream collection zones are again precipitated, since they retain their charges by virtue of the dielectric collection surfaces controlling charge leakage, at a downstream zone. In embodiments utilizing a series of ionization zones as well as a plurality of collection zones, still better performance is achieved inasmuch as any particles not collected in an upstream collection zone are further charged in the succeeding ionization zone or zones and thus react strongly with the electric retarding fields at the zone interfaces and the precipitation fields within the downstream collection zones.

Since each collection zone is preceded by a conductive region, constituting either a termination zone or an ionization zone depending on whether additional ionizer electrodes are used, a series or cascade of particle retarding fields is established along the gas stream. Those particles not collected in the upstream portions of the stream are therefore subjected to additional retarding forces tending to further slow their downstream movement. Of course, the agglomerating and precipitation field strengthening effects of the retarding fields are similarly multiplied. Consequently, precipitation efficiency is improved since even fewer particles will escape collection.

Turning now to FIG. 3, one plural-zone arrangement includes at the upstream end an ionization zone 52 and a collection zone 54 configured as in FIG. 1, followed by a second collection zone 56 of similar configuration spaced downstream from the zone 54. As illustrated in FIG. 3, the dimensional relationships between the various components of the zones 52, 54 and 56 are the same as described in connection with FIG. 1.

In order to establish an electric retarding field between the upstream end of the passive electrode 58 of the second collection zone 56, conductive regions 60 and 62 are provided between the dielectric regions of the collection zones 54 and 56. The regions 60 and 62, therefore, function as termination zones for force lines extending upstream from the passive electrode 58 and the leading areas of the dielectric regions of the zone 56. For purposes of clarity, the latter field lines are omitted from FIG. 3 as are the lines for the equivalent fields extending across the interface of the ionization zone 52 and the collection zone 54.

Electric fields will, of course, be set up between the downstream end of the foremost passive electrode 64 and the adjacent conductive regions 60 and 62. However, the component of these fields in the downstream direction, and hence the propelling effect of the fields on the particles, is minimized by positioning the trailing end of the electrode 64 at least as far downstream as the upstream edges of the regions 60 and 62, or, in other words, at least as far as the downstream edges of the foremost dielectric regions. In a preferred arrangement, the electrode 64 extends beyond the downstream edge of the dielectric regions by a distance at least as great as five-sixteenths the distance between the plane of the electrodes and the plane of the conductive region 60 or 62. Such spacing of the passive electrode 64 creates field distributions between it and the adjacent conductive regions that are substantially transverse to the gas stream, with negligible downstream components. Accordingly, the electric field system within the precipitator is maintained unbalanced in the upstream direction to yield a net retarding influence on particle flow.

Optimally, the passive electrode 58 is spaced downstream from the electrode 64 by a distance at least as great as 11/4 the distance between the plane of the electrodes and the plane of the conductive region 60 or 62.

As with the embodiment of FIG. 1 and as schematically represented in FIG. 3, separate power sources 66 and 68 may be used to drive the ionizer electrodes and the passive electrodes 58 and 64, although a single source may be used. Alternatively, individual power supplies may be used for each of the passive electrodes 58 and 64 as well as for the ionizer electrodes.

FIG. 4 depicts a further embodiment of a cascaded-zone precipitator modeled after the basic configuration of FIG. 1, and including at least one additional ionization zone 70 and associated collection zone 72. Although only one ionizer electrode 74 is shown in the ionization zone 70, any suitable number may be used. It will be understood, also, that the spacing among the components of the ionization zones and collection zones and between the respective zones are as described above in connection with FIGS. 1 and 3.

In this respect, it need only be mentioned that the leading ionizer electrode of each succeeding ionization zone, for example, the electrode 74 in zone 70 of FIG. 4, preferably is spaced from the trailing end of the preceding passive electrode by a distance at least as great as the previously defined distance h. Also, any additional ionizer electrodes provided in the successive ionization zones should be spaced downstream from the foremost electrode by a distance equally as great. For maximum compactness commensurate with high performance, the minimum spacing would of course be adopted.

Where only a single ionizer electrode is used in succeeding zones, it is desirable that the upstream edges of the dielectric regions of the collection zone associated therewith be located no closer to the downstream end of the preceding passive electrode than twenty five-sixteenths the distance between the plane of the electrodes and the plane of the conductive regions at the gas stream boundary.

Yet another version of a plural-zone precipitator constructed in accordance with the invention is portrayed in FIG. 5. The field distribution lines are omitted from FIG. 5 for clarity.

While wire-like ionizer electrodes have been illustrated and described in connection with the embodiments of FIGS. 1 to 4, the invention is not limited to the use of that particular type of ionizer electrode. Indeed, any suitable configuration capable of establishing a strong ionization current across the gas stream may be used. One such configuration of ionizer electrode is depicted in FIG. 5, where a plurality of electrodes 76, each having a multiplicity of pointed discharge members 78, are shown. The electrodes 76 may, for example, constitute pointed saw-tooth blades in order to promote very high electrical stress on the discrete corona points so as to burn off and prevent the accumulation of particulate matter on the ionizer. This improves charging efficiency since such accumulations reduce the local electrical field intensity and thus seriously degrade ionization current. As indicated in FIG. 5, the spacing between the individual ionizer electrodes 76 preferably conforms to the dimensions earlier described in connection with the wire-like electrodes.

Also illustrated with the embodiment of FIG. 5 is that the polarity of the ionizer electrodes and the passive electrodes may be either positive or negative so long as they are the same, though negative is advantageous as it usually gives a higher charging efficiency. Thus, the ionizer electrodes and passive electrodes of FIG. 5 are shown connected to the positive terminals of the power supplies.

A cascaded-zone precipitator having a representative arrangement of the ionization and collection zone configurations of FIG. 2 is shown in FIG. 6. In sequence, the precipitator includes an ionization zone 80, a collection zone 82, a second ionization zone 84, and a second collection zone 86. Additional ionization zones and collection zones may be added if desired.

With reference to the second ionization zone 84, as mentioned previously although the precharging ionizer electrode 88 is located within the collection zone 86, an ionization current is nevertheless established between the electrode 88 and the conductive wall regions of the zone 84. Accordingly, the zone 84 is in fact an ionization zone in which particles carried by the gas stream will receive electrical charges.

The embodiment of FIG. 7 represents still a further configuration of ionization zones and collection zones in accordance with the invention. In particular, an integral module 90 is shown as incorporating both a passive electrode 92 and ionizer electrodes 94 of the multi-point type. This modular construction of the ionizer and passive electrodes facilitates assembly and reduces costs by minimizing the number of electrode supports and electrical connectors required.

FIG. 7, in addition, illustrates the use of a multi-point precharging ionizer electrode 96 within the collection zone. This type ionizer electrode, or any other suitable type ionizer, may be used as well as the precharging electrode in the embodiments of FIGS. 2 and 6 in place of the wire-like electrodes there shown. Since an air gap is not included in the FIG. 7 embodiment, despite the use of a precharging electrode, it is especially adapted for use in applications where conductive dust particles are to be collected or where high operating potentials are not required. Regardless of the type of precharging ionizer electrode used, however, if highly resistive dust particles are being precipitated or if operating potentials are elevated, an air gap is preferred to avoid the creation of reverse corona conditions.

It is another feature of the invention that the establishment of particle retarding electric fields within the gas stream, which yield substantial benefits in improved precipitator performance as described above, is complemented by providing for movement of the member or members carrying the conductive and dielectric regions to a location remote from the gas stream where particles collected on the regions are removed without danger of re-entrainment. After cleaning, the surface carrying the conductive and dielectric regions is then available in a cleaned condition to be moved back into operating position within the ionization and collection zones. In this way, the buildup of saturation level charges on the collection zones and the buildup of deposited particles on the conductive, attractor electrode regions of the surface are avoided. The high efficiency performance characteristics attributable to the retarding fields of the present invention are thus preserved.

A pictorial representation of a movable wall, or "surface," precipitator is contained in FIG. 8. In FIG. 8, an endless belt 98 constructed of conductive material is carried by spaced rollers 100 and 102 for movement into and out of proximity to the gas stream. A dielectric layer 104 is formed on the belt surface, leaving a bare region 106 at the upstream portion of the belt exposed to the gas stream. Corona discharges are established between the conductive region 106 and one or more ionizer electrodes 108 extending transverse to the direction of particle flow.

As schematically shown in FIG. 8, it is a further feature of the invention that the ionizer electrodes 108 may also be mounted for movement to a quiescent location for cleaning. A pair of spools 110 and 112 associated with each ionizer electrode 108 move the electrode, either intermittently or continuously, through the gas stream. A cleaning element 114, such as a brush, wiper, or the like, is provided at a location outside of the gas stream to remove particles adhering to the electrode.

Charged particles leaving the ionization zone are caused to be precipitated on the dielectric surface 104 by precipitation fields established between the grounded belt 98 and a high-potential passive electrode 116. These particles are retained on the dielectric surface 104 against forces tending to re-entrain them by the electrostatic attraction of the charged particles for the metal belt 98. Some particles collect on the conductive attractor 106 as well.

However, as mentioned, it is desirable that the collected particles be removed from the belt to prevent quenching of the charging and precipitation fields. This is accomplished by contacting the belt with a brush 118, or some other suitable cleaning device, at a location remote from the gas stream. As the belt 98 is moved past the brush 118, the particles adhering to the conductive region 106 and the dielectric region 104 of the belt are removed. Since those particles collected on the dielectric region 104 retain electrical charges, it is preferable that the brush 118 be constructed of an electrically conductive material and be connected to ground so that the particle charges may be neutralized to facilitate their removal. Other techniques of discharging the charges carried by the particles in advance of or simultaneously with the cleaning operation may of course be used.

Where the dust particles are electrically conductive, either brushes constructed of dielectric material or electrically isolated brushes are preferred.

Movable or moving surfaces other than endless belts may be used in accordance with the invention. Examples of these are discs or drums which are rotated past the gas stream. Also, it is not necessary that a surface susceptible of continuous movement be used. Instead, the surface may be reciprocated between a position of proximity to the gas stream, where particle charging and collection are carried out, and a cleaning position outside of the stream. Similarly, the ionizer electrodes, or the precharging ionizer electrode if one is used, may be reciprocated into and out of the gas stream for cleaning at a remote location.

Moreover, both surfaces of the movable member may be exposed to the gas stream, notwithstanding that the description heretofore has made reference to only one side of a movable member. For example, in the figurative embodiment portrayed in FIG. 8, an additional passive electrode 120 may be located in juxtaposition to the inner surface of the belt 98. In this instance, the inner surface would also be provided with a dielectric layer to define a collection zone and an upstream ionization zone. Appropriate ionizer electrodes would of course be located in particle charging relation to the upstream conductive region on the inner surface of the belt. A third passive electrode 122, and additional ionizer electrodes, could be positioned to allow for collection on the outer surface of the belt 98 on its return run after being cleaned by the brush 118. If collection proceeds on both surfaces of a member, so should cleaning. To that end, it is contemplated that a brush or the like would be provided to engage each surface.

The substantial advantages flowing from the present invention will become additionally apparent from a consideration of the following test data, and upon analysis of the graphs of FIGS. 9 and 10.

TESTS OF RETARDING-FIELD UNIT

A cascaded-zone precipitator in the FIG. 4 configuration, with a series of three ionization zones and three collection zones, was tested. A movable surface was located on either side of the gas stream. The surface was constituted by an endless Mylar belt having strips of brass shim stock secured to its facing surface to define the conductive regions of the surface. The belt speed was 1.33 inches per second, the air velocity 19 feet per second, and the volumetric flow rate of the air stream 443 cubic feet per minute. An absolute filter was connected across the exhaust end of the precipitator to trap all particles not collected within the unit.

Five ionizer electrodes were used in each ionization zone. Except for the trailing electrode in each zone, the ionizers were maintained at a voltage of -28 kv., and a current of approximately 3.5 to 4 milliamps. A separate power supply was used for the first four electrodes in each ionization zone. Each trailing ionizer electrode was energized by an independent power supply at a voltage of -20 kv. and a current of one milliamp. The passive electrodes were raised to a voltage of -19 kv., with negligible current. The effective volume of the precipitating unit was 1.23 cubic feet.

The results obtained are listed below: --------------------------------------------------------------------------- TABLE II

D.sub.L .eta. c T __________________________________________________________________________ (a) 12.8 99.2 57.2 (b) 13.1 98.6 56.7

where:

D.sub.L is the dust loading in grains per cubic foot of dry fly ash;

.eta.c is the collection efficiency determined by weighing collections at the absolute filter, i.e., .eta.c = (weight of fly ash at entrance -- weight of fly ash at absolute filter) divided by weight of fly ash at entrance; and

T is the time of the run in minutes.

TESTS OF COTTRELL UNITS

For purposes of comparison, a Cottrell-type precipitator was tested, both with collection surfaces constituted by stationary walls, as in the conventional Cottrell unit, and with a moving dielectric wall but without the retarding fields of the present invention.

A total of 35 ionizer wires spaced 1 inch apart were used. The collection surfaces, one on each side of the gas chamber, were constituted by brass shim stock and were grounded. Seven power supplies ranging from voltages of -25 kv. to -19.5 kv., at a current of approximately 4.6 milliamps to 1 milliamp, where used to drive the ionizer electrodes. Belt speed (where applicable), gas velocity, and volumetric flow rate were the same as for the retarding-field unit.

The following results were obtained: --------------------------------------------------------------------------- TABLE III

D.sub.L .eta. c T (c) 10 93.85 67.33 (d) 12.3 95.46 61.7 (e) 11.1 74.2 67.2

(belt held stationary, collected particles removed by rapping the walls)

The data in Table II verifies that a small (1.23 cubic feet effective volume) precipitator with the moving dielectric surface and retarding field features of the present invention is capable of handling high dust mass loads (10 to 15 grains per cubic foot) at high gas velocities (19.5 to 20 feet per second) while providing collection efficiencies up to 99.2 percent, using the cascaded triple ionization-collection zone geometry.

However, initially it might appear that the Cottrell units perform nearly as well as the unit of the present invention. Comparing lines (a) of Table II with lines (d) and (e) of Table III, there is only approximately a 25 percent difference in efficiency between the retarding-field unit (line (a)) and the stationary wall Cottrell unit (line (e)) and only a 4 percent difference between the retarding-field unit and the moving wall Cottrell unit (line (d)). But this is illusory, as the following analysis of FIGS. 9 and 10 reveals.

Again considering equation (1), for sake of simplicity, the collection efficiency (.eta.c) percentage becomes:

(1 a) .eta.c = 100 (1-e.sup.- )

where:

.lambda. may be expressed in an alternative version of equation (2) as:

(2 a) .lambda. = k.sub.p E.sub.p .SIGMA.A/V.sub.g

where:

.SIGMA.A is the total collection surface area; and V.sub.g is the volumetric rate of gas flow.

FIG. 9 depicts a graph of collection efficiency (.eta.c) percentage versus .lambda.; i.e., equation (1a). Studying FIG. 9, it is evident that the "cost" of improvement in collection efficiency depends upon the efficiency range involved. For instance, an improvement of 4 percent in collection efficiency from 70 percent to 74 percent requires only a small change (.DELTA..lambda..sub.1 of FIG. 9) in .lambda., while approximately the same improvement from 95 percent to 99.2 percent necessitates a very large change (.DELTA..lambda..sub.3) in .lambda.. It is assumed here that k.sub.p, .SIGMA.A, and V.sub.g are held constant.

Thus for the collection efficiency of the Cottrell units to be raised to that of the retarding field unit, without sacrificing throughput capacity by lowering the volumetric rate of flow (V.sub.g), the area available for particle collection (.SIGMA.A) must be increased. Turning again to the test data for the Cottrell units and looking once more at FIG. 9, to improve the moving wall Cottrell system from 94.6 percent (line (d) of Table III) to the 99.2 percent value (line (a) of Table I) of the retarding-field precipitator, the size of the collector surface, and hence of the precipitator, must be increased by a factor of at least 1.6. Similarly, an improvement from 74.2 percent (line (e) of Table III) to 99.2 percent requires that the collection surface area be increased by a factor of approximately 3.5.

As the cost of manufacturing a precipitator is proportional to the size of the collection surface, FIG. 9 indeed reflects that a high "cost" must be paid for seemingly small improvements in collection efficiency at the efficiency levels here involved.

FIG. 10 illustrates the relationship between collection efficiency and the size of the collection area even more clearly. (FIG. 10 is based on a dust loading of 15 grains per cubic foot, a gas velocity of 20 feet per second, and a volumetric flow rate of 500 cubic feet per minute.) It is obvious from FIG. 10 that substantial economies in the size of the collection area, and, consequently, of the precipitator unit as a whole, are obtainable at any given collection efficiency by the techniques of the present invention. Moreover, the Cottrell units were tested under conditions that must be considered as ideal for that type of system. For example, the buildup of a heavy dust layer on the collection surfaces was eliminated by moving or frequently rapping the collectors, which in turn eliminated the re-entrainment and field quenching problems inherent in Cottrell systems. Therefore, in actual practice the collection efficiency of a Cottrell unit might be significantly less than those observed in the reported tests. If so, the factor by which .lambda., i.e., the collection area, must be increased to achieve collection efficiencies on the order of those of the present invention would be raised substantially, and may go up as high as 10 to 25. This would be especially true where highly resistive dusts are collected since the buildup of charges on the collection surfaces would greatly degrade the performance level of the Cottrell type units.

Additional costs savings over the Cottrell systems are realized with the present invention inasmuch as the Cottrell units require nearly three times the number of corona wires as does the retarding-field unit. The power requirements are therefore also nearly tripled. Finally, an automatic voltage regulator is needed for the stationary wall Cottrell system to compensate for charge buildup on the collection surfaces. At the usual voltage of 60 kv. to 100 kv., typically used in Cottrell units, such regulators are very expensive.

It will be readily apparent to those skilled in the art that the above-described embodiments are intended to be merely exemplary, in that they are susceptible of modification and variation without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

We claim:

1. Apparatus for electrostatically precipitating particles entrained in a gas stream comprising:

means defining at least one surface adjacent to and movable along the gas stream boundary;

means, including at least one ionizer electrode, defining an ionization zone at an upstream region of the movable surface for imparting unipolarity electric charges to the entrained particles;

means defining a collection zone at an adjacent downstream region of the movable surface; and

means for establishing electric fields in the gas stream in the vicinity of the ionization zone-collection zone interface that are non-attractive to the charged particles and that have components in opposition to the gas flow thereby to retard particle flow downstream, the field establishing means including the region of the movable surface within the collection zone being constructed of dielectric material and the region within the ionization zone being constructed of electrically conductive material, and further including high-potential electrode means, of like polarity as the ionizer electrode, located within the collection zone in spaced relation to the downstream edge of the electrically conductive region of the surface to establish with the electrically conductive region an electric field extending in the upstream direction.

2. Apparatus according to claim 1 further comprising:

means for moving the surface to a cleaning zone remote from the gas stream; and

means at the cleaning zone for removing particles from the conductive and dielectric regions of the surface.

3. Apparatus according to claim 2 in which the movable surface comprises an endless belt structure, and in which the means for moving the surface includes drive means for successively moving the conductive and dielectric regions of the surface through the ionization and collection zones, respectively, and the cleaning zone.

4. Apparatus according to claim 2 in which the cleaning means includes means for neutralizing the charges remaining on the collected particles to facilitate the removal thereof from the surface.

5. Apparatus according to claim 4 in which the cleaning means comprises at least one electrically conductive brush and the means for neutralizing the charges comprises a conductor connecting the brush to electrical ground.

6. Apparatus according to claim 1 further comprising means for retaining charged particles collected on the dielectric region of the surface against forces tending to re-entrain them in the gas stream.

7. Apparatus according to claim 6 in which the particle retaining means comprises an electrically conductive member located closely adjacent the dielectric region and exterior to the gas stream.

8. Apparatus according to claim 7 in which the upstream portion of the electrically conductive member forms the conductive region of the movable surface.

9. Apparatus according to claim 8 in which the dielectric region of the surface comprises a dielectric layer on the electrically conductive member.

10. Apparatus according to claim 1 in which the means for defining the ionization zone includes a plurality of ionizer electrodes spaced along the conductive region of the surface in the direction of gas flow and arranged in the same plane relative to the conductive region.

11. Apparatus according to claim 10 in which the spacing between adjacent electrodes is at least as great as 1 1/4 the distance from the plane of the ionizer electrodes to the plane of the conductive region of the surface.

12. Apparatus according to claim 1 further comprising a plurality of collection zones spaced along the movable surface downstream from the ionization zone.

13. Apparatus according to claim 1 in which the means defining the collection zone includes a high-potential passive electrode, of like polarity as the ionizer electrode, located in spaced, parallel relation to the dielectric region of the movable surface, and in which the high-potential electrode means comprises the upstream end of the passive electrode.

14. Apparatus according to claim 13 in which the upstream end of the passive electrode is spaced at least no farther upstream than the upstream edge of the dielectric region of the surface.

15. Apparatus according to claim 14 in which the upstream end of the passive electrode is located downstream of the upstream end of the dielectric region a distance no greater than fifteen-sixteenths the distance between the plane of the electrode and the plane of the conductive region of the surface.

16. Apparatus according to claim 13 in which the means defining the ionization zone includes a plurality of electrodes spaced along the conductive region of the surface in the direction of gas flow, and in which the ionizer electrode nearest to the passive electrode is spaced from the upstream end of the passive electrode a distance at least as great as 1 1/4 the distance between the plane of the ionizer electrodes and the plane of the conductive region of the surface.

17. Apparatus according to claim 16 in which the ionizer electrodes and passive electrode are aligned in the direction of gas flow.

18. Apparatus according to claim 1 in which the high-potential electrode means comprises a precharging ionizer electrode located within the collection zone in overlying relation to the upstream portion of the dielectric region of the surface, thereby to maintain the charge level on the upstream portion of the dielectric region independent of the charges carried to the dielectric surface by the collected particles.

19. Apparatus according to claim 18 in which an air gap is formed in the movable surface between the upstream edge of the dielectric region and the downstream edge of the conductive region.

20. Apparatus according to claim 19 in which the width of the air gap is at least 0.063 inches per kilovolt per inch of the average field strength between the plane of the precharging electrode and the plane of the conductive region of the surface, but not less than 0.125 inches.

21. Apparatus according to claim 19 in which the precharging electrode is spaced downstream from the upstream edge of the dielectric region by a distance no greater than five-sixteenths the distance between the plane of the precharging electrode and the plane of the conductive region of the surface.

22. Apparatus according to claim 18 in which the collection zone defining means includes a high-potential passive electrode, of like polarity as the precharging electrode, located in spaced, parallel relation to the dielectric region of the surface.

23. Apparatus according to claim 22 in which the upstream end of the passive electrode is spaced from the precharging electrode a distance at least as great as 1 1/4 the distance between the plane of the precharging electrode and the plane of the conductive region of the surface.

24. Apparatus according to claim 23 in which the passive electrode and the precharging electrode are aligned in the direction of gas flow.

25. Apparatus for electrostatically precipitating particles entrained in a gas stream comprising:

means defining at least one surface adjacent to and movable along the gas stream boundary;

means, including at least one ionizer electrode, defining an ionization zone at an upstream region of the movable surface for imparting unipolarity electric charges to the entrained particles;

means defining a plurality of collection zones at spaced downstream regions of the movable surface;

means defining a termination zone between adjacent collection zones; and

means for establishing electric fields in the gas stream in the vicinity of the ionization zone-collection zone interface, and at the interface of each termination zone and collector zone, that are non-attractive to the charged particles and that have components in opposition to the gas flow thereby to retard particle flow downstream, the field establishing means including the region of the movable surface within each collection zone being constructed of dielectric material and the regions within the ionization zone and each termination zone being constructed of electrically conductive material, and further including a high-potential passive electrode, of like polarity as the ionizer electrode, located within each collection zone in spaced relation to the downstream edge of the preceding electrically conductive region of the surface to establish between the upstream end of the passive electrode and the electrically conductive region an electric field extending in the upstream direction.

26. Apparatus according to claim 25 in which the downstream end of the passive electrode of each collection zone in advance of the trailing collection zone is located downstream of the downstream edge of the dielectric region of the zone.

27. Apparatus according to claim 26 in which the downstream end of each passive electrode in advance of the trailing zone is located downstream of the downstream edge of the associated dielectric region a distance at least as great as five-sixteenths the distance between the plane of the electrode and the plane of the associated conductive region of the surface.

28. Apparatus according to claim 26 in which the upstream end of each succeeding passive electrode is spaced from the downstream end of the next preceding passive electrode a distance at least as great as 1 1/4 the distance between the plane of the electrode and the plane of the associated conductive region of the surface.

29. Apparatus for electrostatically precipitating particles entrained in a gas stream comprising:

means defining at least one surface adjacent to and movable along the gas stream boundary;

means defining with the movable surface a plurality of ionization zones spaced along the surface in the direction of gas flow for imparting unipolarity electrical charges to the entrained particles, at least the foremost ionization zone including at least one ionizer electrode;

means defining with the movable surface a collection zone downstream of and adjacent to each ionization zone; and

means for establishing electric fields in the gas stream in the vicinity of each ionization zone-collection zone interface that are non-attractive to the charged particles and that have components in opposition to the gas flow thereby to retard particle flow downstream, the field establishing means including the region of the movable surface within each collection zone being constructed of dielectric material and the region within each ionization zone being constructed of electrically conductive material, and further including high-potential electrode means, of like polarity as each ionizer electrode, located within each collection zone in spaced relation to the downstream edge of the electrically conductive region of the preceding ionization zone to establish with said electrically conductive region an electric field extending in the upstream direction.

30. Apparatus according to claim 29 further comprising:

means for moving the surface to a cleaning zone remote from the gas stream; and

means at the cleaning zone for removing particles from the conductive and dielectric regions of the surface.

31. Apparatus according to claim 29 further comprising means for retaining charged particles collected on the dielectric regions of the surface against forces tending to re-entrain them in the gas stream.

32. Apparatus according to claim 31 in which the particle retaining means includes an electrically conductive member located closely adjacent each dielectric region and exterior to the gas stream.

33. Apparatus according to claim 32 in which the conductive member is coextensive with the movable surface and forms the conductive regions of the surface.

34. Apparatus according to claim 33 in which the dielectric regions of the surface comprise spaced dielectric layers on the electrically conductive member.

35. Apparatus according to claim 29 in which the means defining the collection zones includes in each collection zone a high-potential passive electrode, of like polarity as the ionizer electrode, located in spaced, parallel relation to the dielectric region of the zone, and in which the high-potential electrode means in each zone comprises the upstream end of the passive electrode.

36. Apparatus according to claim 35 in which the upstream end of each passive electrode is spaced at least no farther upstream than the upstream edge of the dielectric region of the zone.

37. Apparatus according to claim 36 in which the upstream end of each passive electrode is located downstream of the upstream edge of the dielectric region a distance no greater than fifteen-sixteenths the distance between the plane of the electrode and the plane of the associated conductive region of the surface.

38. Apparatus according to claim 37 in which each ionization zone includes at least one ionizer electrode.

39. Apparatus according to claim 38 in which the foremost ionizer electrode of each successive ionization zone is spaced from the downstream end of the next preceding passive electrode a distance at least as great as 1 1/4 the distance between the plane of the ionizer electrodes and the plane of the associated conductive region of the movable surface.

40. Apparatus according to claim 39 in which the upstream edge of the dielectric region of each succeeding collection zone is spaced downstream from the downstream end of the next preceding passive electrode by a distance at least as great as twenty-five sixteenths the distance between the plane of the electrode and the plane of the associated conductive region of the surface.

41. Apparatus according to claim 29 in which the high-potential electrode means in each collection zone comprises a precharging ionizer electrode located within the collection zone in overlying relation to the upstream portion of the dielectric region of the zone, thereby to maintain the charge level on the upstream portion of the dielectric region independent of the charges carried to the dielectric surface by the collected particles.

42. Apparatus according to claim 41 in which an air gap is formed in the movable surface between the upstream edge of the dielectric region of each collection zone and the downstream edge of the conductive region of the adjacent ionization zone.

43. Apparatus according to claim 42 in which the width of each air gap is at least 0.063 inches per kilovolt per inch of the average field strength between the plane of the precharging electrode and the plane of the associated conductive region of the surface, but not less than 0.125 inches.

44. Apparatus according to claim 42 in which each precharging electrode is spaced downstream from the upstream edge of the dielectric region of the zone by a distance no greater than five-sixteenths the distance between the plane of the precharging electrode and the plane of the associated conductive region of the surface.

45. Apparatus according to claim 41 further comprising a high-potential passive electrode, of like polarity as the precharging electrode, in each collection zone located in spaced, parallel relation to the dielectric region of the zone.

46. Apparatus according to claim 45 in which each precharging electrode is spaced downstream from the downstream end of the next preceding passive electrode by a distance at least as great as 1 1/4 the distance between the plane of the precharging electrode and the plane of the associated conductive region of the surface.

47. Apparatus according to claim 1 further comprising:

means for moving each ionizer electrode to a cleaning zone remote from the gas stream; and

means at the cleaning zone for removing particles collected on each ionizer electrode.

48. Apparatus according to claim 29 further comprising:

means for moving each ionizer electrode to a cleaning zone remote from the gas stream; and

means at the cleaning zone for removing particles collected on each ionizer electrode.