US3258897A

US3258897A - Electrical precipitator

Info

Publication number: US3258897A
Application number: US149812A
Authority: US
Inventors: Mayer Friedrich; Messen-Jaschin Gregori
Original assignee: Messen-Jaschin GA
Current assignee: Messen-Jaschin GA
Priority date: 1960-10-28
Filing date: 1961-10-25
Publication date: 1966-07-05
Anticipated expiration: 1983-07-05
Also published as: CH395936A; SE301135B; GB999155A; DE1284947B

Description

y 5, 1966 F. MAYER ETAL 7 ELECTRICAL PRECIPITATOR Filed 001;. 25, 1961 2 Sheets-Sheet 1 aa I :i ll T 0 I; I

SOURCE July 5, 1966 F. MAYER ETAL ELECTRICAL PRECIPITATOR 2 Sheets-Sheet 2 Filed Oct. 25. 1961 0C V047A6 SOURCE United States Patent 3,258,897 ELECTRICAL PRECIPITATOR Friedrich Mayer and Gregori Messen-Jaschin, both of Sarnen, Switzerland, assignors to G. A. Messen-Jaschm, Sarnen, Switzerland, a corporation of Switzerland Filed Oct. 25, 1961, Ser. No. 149,812 Claims priority, application Switzerland, Oct. 28, 1960;

8 Claims. (Cl. 5513.7)

The present invention relates to an electrical precipitator.

Electrical precipitator technique is based on four main requirements regarding the ionization of the gas and the precipitation of the particles therein. The first requirement is to keep the ionization voltage to a minimum, the second is that the number of free ions produced should be a maximum, the third is that the rate of ionization should be a maximum and the fourth requirement is that the electrical field of the precipitator should be just below the breakdown field strength.

From theoretical field considerations, the geometry of precipitator apparatus is significant. The ionization and charging process is dependent on the structure of the electric field in which these processes take place. It is found that all of the forms of field, the rotation-symmetrical field (i.e. one which is symmetrical about an axis in any plane to which the axis is normal) has the greatest advantage, since the distribution of potential and field lines is symmetrical, decreasing rapidly with distance from the source of the field, but is maintained to a considerable extent if an ionic cloud is formed round the ionization source. The field distortion due to the occurrence of an ionic cloud at the source of ionization, which very considerably affects the ionization and precipitation performance in other geometrical lay-outs, is least significant in a rotation-symmetrical field. Therefore a rotation-symmetrical tubular element, i.e. one having a circular cross-section is the most favourable form for the ionization and charging space and also for the precipitation space.

According to the present invention there is provided an electrical precipitator comprising a first electrode in the form of a duct which, considered in any plane to which the axis of the duct is normal, is of circular cross-section, and second and third electrodes constituted by ioniser electrodes situated within the first electrode co-axial therewith. The second and third electrodes have pointed tips directed towards one another, are conical with a small or acute angle of conicity, and have radii of curvature substantially less than the radius of curvature of any other part of the ionizer electrodes. The second and third electrodes are spaced apart by a distance substantially equal to the internal diameter which the first electrode has midway between the second and third electrodes. The electrical precipitator of the invention further comprises electric potential-providing means which provide high unidirectional potential, and conductors connecting the second and third electrodes to said potential-providing means.

The ionizer construction indicated above is thought to meet rather well the first three requirements set forth above. The screening or counter-field thus produced provides in the charging space, for minimum ionization voltage, a maximum of free ions for maximum rate of ionization. The voltage can be kept so low that corona discharge at the electrodes is avoided with certainty. Thus, in no case is corona ionization employed, the ionizer being strong enough, due to the screening field which is produced, to produce a maximum ionic avalanche by providing a large number of free ions. Since it is possible to produce with the ionizer electrodes an extremely high field, which is not diminished by corona Ice elfects, there is very high probability of the ions produced becoming attached to the liquid or solid particles to be precipitated.

For the process of attachment of ions to particles, in fact, the so-called electric saturation field is decisive, this field increasing with increase in the ions attached to a particle. The attachment of ions is therefore retarded by the saturation field. The higher the velocity of the ions, however, the greater is the possibility of their penetrating the saturation field and increasing the number of attached ions.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made to the accompanying drawings in which:

FIGURE 1 shows diagrammatically an axial section section through an electrical precipitator for ionization of gas and deposition of particles;

FIGURE 2 shows the form of the field lines in the gas ionization chamber according to FIGURE 1;

FIGURE 3 is an axial sectional view of one form of electrical precipitator according to the invention;

FIGURE 4 is a view similar to FIGURE 3 but showing a modified construction;

FIGURE 5 is a transverse section on the line VV of FIGURE 4;

FIGURE 6 is an axial sectional view of another modified construction;

FIGURE 7 is an axial sectional view of a further modified construction;

FIGURE 8 is a transverse section on the line VIII-VIII of FIGURE 7; and

FIGURE 9 is an axial sectional view of a still further modification.

FIGURE 1 of the drawings illustrates the basic construction of an electrical precipitator according to the invention. In the figure, 1 denotes a cylindrical boundary Wall of a rotation-symmertical charging space, the Wall also serving as a ground counter-electrode within which there are two pointed ionizer electrodes 2 directed towards each other, co-axial with the wall 1 and spaced apart by a distance 21. p (rho) is the internal radius of the cylindrical wall 1 and ,8 (beta) is the point angle of the two electrodes 2. A high unidirectional voltage source is applied to the electrodes 2 by

conductors

101, 102, 103.

For the ionization and charging process of an electrical precipitator, the decisive factor according to the aforementioned requirements is that the field strength drop between ionizer electrodes and the wall 1 bounding the ionization space should be as small as possible. It can now be shown at the boundary wall 1 in the radial plane passing through the point O(FIGURE 2), the maximum field strength occurs when the distance [between each electrode point and the point 0 is equal to the radius p.

It is known that at points, very thin wires and sharp corners, a concentration of field strength is set up and that, therefore, the effectiveness of a field for ionization purposes is greater the lower the radii of curvature of the ionizer electrodes.

Electrodes with absolute points cannot be made. In practice, every such point will really approximate to a hyperboloid of revolution. On the surface of this hyperboloid of revolution at the distance I from the point 0 there prevails a field which is stronger by the factor A as compared with the field prevailing in a vertical plane. This factor A depends considerably on the point angle 3 of the electrodes. In practice, points can be made with a conicity of for example 0.25 mm. on a length of 70 mm., which corresponds to a small angle of conicity 3 of 012 and which would give for example a factor A of about 10 In the case of the field arrangement shown, this leads to an exceptionally high increase in field intensity. FIGURE 2 shows on the one hand the equi-potentials, and on the other hand the electric field lines superimposed orthogonally on the former. The field liner issuing from the ends of the electrodes 2 meet in the meridian plane passing through the point 0; they thus form a screen-shaped or fan-shaped field distribution which, regarded rotation-symmetrically, will be a blocking field. Similar considerations are of course applicable to very thin wire electrodes.

As already mentioned, the ionization electrodes should not produce a corona discharge. As soon as the pointed electrode or the wire begins to give a corona discharge, the field in the vicinity of the electrode drops considerably, due to the ion cloud occurring. For example, if air ionization is concerned, the nitrogen atoms are excited to luminosity and form a very powerful ion cloud, that is to say a conducting plasma, in which the field, dependent on the plasma density, collapses.

If the voltage applied to the pointed electrode or the wire is kept below the limit voltage for the release of free electrons from the electrode or point surface, the electric field in the vicinity of the electrode is then only capable of accelerating the free ions in the atmos phere. If these charge carriers are accelerated in the high field, they produce by so-called secondary collision processes an ionic avalanche, which however, is not sufiicient to produce maximum ionization. One cannot, therefore, do without the electrons released from the electrode surface, since it is only thereby that the second requirement, that the number of ions produced should be a maximum, can be satisfied, the released electrons being accelerated by the high electric field at the electrode to velocities of up to 9 10 cm./sec., and hence receiving sufiicient energy for ionizing the nitrogen and oxygen atoms. Practical experiments have shown that the release of electrons in the arrangement selected occurs already at field strengths at which no corona discharge yet occurs. It has been possible to produce field strengths of 10' to 10 v./cm. at th electrodes without corona and hence to satisfy the requirements for maximum number of ions.

In the constructional example shown in FIGURE 3, a cylindrical tube 3 serves as a passage for the gas current to be cleaned; the tube 3 is grounded and serves as a counter electrode for the ionizer and also as the precipitation electrode. Two pointed electrodes 4 connected to the same unidirectional voltage source 300 through conductors Ui project co-axially into the tube, from the two ends of the tube, and so far into the tube that the distance 21 between their points is approximately equal to the

internal diameter

2,0 of the tube. An electrical precipitator of this character is suitable for those cases where the speed of migration of the particles to be charged and precipitated does not exceed 1.5 m./sec. in the tube. In this case, the space containing the points may be so designed that ion production, ion attachment to particles and particle precipitation occur within the length marked L of the tube. This length L depends of the radius p of the tube, the velocity of flow of the aerosol and the electrical velocity, the latter being a function of the magnitude and charge number of the particle, the dynamic viscosity of the aerosol and the field strength. Here also it is found that for obtaining maximum precipitation, the selection of electrode spacing is of decisive importance, since the maximum field strength on the tube Wall, necessary for maximum precipitation, can be obtained only in the case of approximate equality of, on the one hand, the distance between the electrode points and, on the other hand, the tube diameter.

FIGURES 4 and 5 show an electrical precipitator in which the particle separator is separate from the ionizer. In a grounded cylindrical ionizer tube 5, there are again provided two pointed electrodes 6 projecting into the tube from opposite ends of the latter and both connected to the same direct-current voltage source 400 through conductors Ui. Here again, there is produced the hereinbefore described screening field, which for minimum voltage gives a maximum number of ions with maximum charge. stream electrode 6 projects into a grounded precipitation tube 7 and is supported by struts 8 projecting radially from the outside through openings in the wall of the tube 7. There also projects into the downstream end of the precipitation tube 7 a voltage-carrying counter-electrode 9 connected to a conductor Ua to which is applied a DC. voltage, from a source 491, different from that applied to the pointed electrodes. The precipitation field strength produced lies just below the breakdown field strength.

It is also possible to make the voltage applied to the electrode 9 equal to the voltage applied to the elec trode 6.

In FIGURE 6, there is provided an ionization tube 11 and a separate grounded precipitation tube 10 co-axial therewith, into which project the pointed electrodes 12 which are connected to a voltage source 600 so that they both have the same unidirectional potential. An axial, cylindrical extension 12a of the downstream point electrode 12 of the ionizer forms the voltage-carrying counter-electrode of the precipitation section. The radius r of the counter-electrode 12a is selected relatively to the internal radius R of the precipitation tube 10 so that the precipitation field strength in this case also lies just below the breakdown field strength and a minim-um precipitation length L is obtained.

The advantage of this two-chamber precipitator with the ionizer separate from the precipitation tube is that it is practically unnecessary to take into consideration any limitation of the volume rate of flow of the aerosol. The limit for this rate of flow is generally set by the adhesion of the particles to the precipitation electrode 10. The rate of flow will be higher in the case of aerosols with liquid particles than for aerosols with solid} particles. The high electric field still operative at the precipitation electrode 10 exerts an electrical pressure on the wall of the precipitation electrode, and thus contributes in increasing still further the adhesion of the slowly discharging particles and hence the rate of flow of the aerosol and the throughput volume can be increased.

In the precipitator shown in FIGURES 7 and 8, ionization of gas and deposition of particles of dirt which become charged by the generated ions are effected within a single tubular duct 21. This duct is of circular crosssection and of uniform internal diameter and it serves as a particle-collecting electrode, that is to say the charged particles are attracted to it and become deposited on the inner surface of it. Within the duct 21 and coaxial with it are elongated and pointed ionizer electrodes. These include first and second electrodes 22 and 23, which are coaxial with the duct 21 and are mechanically and electrically connected together and point 'way from one another, in opposite directions along the axis of the duct 21. They are supported within the duct 21 by three electrically conductive stays 24 which extend into the duct from the outside, through apertures 25 in the duct and have their inner ends secured to the electrodes 22 and 23. The outer ends of the struts are rigidly connected to a housing (not shown) of the precipitator. In a similar manner third and fourth ionizer electrodes 26 and 27 are positioned and supported within the duct 21, and so are fifth and sixth ionizer electrodes 28 and 29. The second and third ionizer electrodes 23 and 26 point towards one another and they have their points spaced apart by a distance 2! which is substantially equal to the internal diameter 2 of the duct 21 and the same applies to the fourth and fifth ionizer electrodes 27 and 28. The stays 24 are connected by conductors, shown diagram- A cylindrical extension 6a of the down- I matically at 30, to the high-potential terminal of a source 31 of high unidirectional potential, the other terminal of which is grounded. The duct 21 is grounded also.

When the dirt-laden gas flows downwardly in the duct 21, there is ionization of gas around the points of the electrodes 22 and 23, charging of particles of dirt by means of the generated ions and attraction of charged particles to the inner surface of the duct 21, upon which the particles collect. Somewhat cleaner gas then proceeds to the electrodes 26 and 27 and there the process is repeated, so that a progressive or multi-stage removal of dirt is effected.

In the example shown in FIGURE 9, the rotationsymmetrical charging space is formed by a grounded tube 16 which widens conically in the direction of flow. At 17 are also again shown the pointed electrodes acting counter to each other and producing the screening or counter-field, which electrodes carry diiferent unidirectional voltages from two

separate voltage sources

900 and 901. The distance between the points of the electrodes 17 is approximately equal to the diameter 2 which the tube has midway between the electrodes 17. Since the value of the permissible voltage depends on the radial distance of the points from the tube wall, the same voltage could be applied to both electrodes 17, or else, as shown in FIGURE 9, a higher voltage could be applied to the downstream electrode 17 than to the upstream electrode 17. To avoid excessive field distortions relatively to the normal case of a cylindrical charging space, the angle at which the tube tapers should not be too large; the practical limits would be at about 20.

We claim:

1. An electrical precipitator comprising a first electrode in the form of a duct which, considered in any plane to which the axis of said duct is normal, is of circular crosssection; second and third electrodes of circular cross section throughout constituting ionizer electrodes situated within the first electrode coaxial therewith, having conical tip portions directed towards one another and of a small angle of conicity and having radii of curvature at their tips substantially less than the radius of curvature of any other part of the ionizer electrodes, said tips of said sec 0nd and third electrodes being spaced apart by a distance substantially equal to the internal diameter of said first electrode midway between the tips of said second and third electrodes; and means applying high unidirectional potential to said second and third electrodes.

2. An electrical precipitator according to claim 1 in which said first electrode is a tube of unvarying circular internal diameter.

3. An electrical precipitator according to claim 1 in which said first electrode is a tube tapering in the direction of its longitudinal axis.

4. An electrical precipitator according to claim 1 in 'which said means applying high unidirectional potential to said second and third electrodes comprises a single potential source; and means so connecting said source to said second and third electrodes that the same voltage is applied to each of said second and third electrodes.

5. An electrical precipitator according to claim 1 in which said means applying high unidirectional potential to said second and third electrodes comprises two different potential sources; and means connecting said two sources respectively to said second and third electrodes.

6. An electrical precipitator according to claim 1 further including a second duct located downstream of the first-mentioned duct and which is grounded; and means in said second duct for effecting deposition of particles charged in said first-mentioned duct.

7. An electrical precipitator according to claim 6 in which said third electrode is downstream with respect to said second electrode and projects into said second duct and acts therein as a counter electrode for the deposition.

8. An electrical precipitator according to claim 1 further including fourth and fifth electrodes within said duct, spaced along the duct axis, and being formed and dimensioned and spaced from one another in the same way as said second and third electrodes.

References Cited by the Examiner UNITED STATES PATENTS 2,295,152 9/1942 Bennett 138 2,567,709 9/1951 Hedberg 55-154 2,934,648 4/1960 Leupi et al 55102 FOREIGN PATENTS 873,565 7/1961 Great Britain.

ROBERT F. BURNETT, Primary Examiner. HARRY B. THORNTON, Examiner.

Claims

1. AN ELECTRICAL PRECIPITATOR COMPRISING A FIRST ELECTRODE IN THE FORM OF A DUCT WHICH, CONSIDERED IN ANY PLANE TO WHICH THE AXIS OF SAID DUCT IS NORMAL, IS OF CIRCULAR CROSSSECTION; SECOND AND THIRD ELECTRODES OF CIRCULAR CROSS SECTION THROUGHOUT CONSTITUTING IONIZER ELECTRODES SITUATED WITHIN THE FIRST ELECTRODE COAXIAL THEREWITH, HAVING CONICAL TIP PORTIONS DIRECTED TOWARDS ONE ANOTHER AND OF A SMALL ANGLE OF CONICITY AND HAVING RADII OF CURVATURE AT THEIR TIPS SUBSTANTIALLY LESS THAN THE RADIUS OF CURVATURE OF ANY OTHER PART OF THE IONIZER ELECTRODES, SAID TIPS OF SAID SECOND AND THIRD ELECTRODES BEING SPACED APARD BY A DISTANCE SUBSTANTIALLY EQUAL TO THE INTERNAL DIAMETER OF SAID FIRST ELETRODE MIDWAY BETWEEN THE TIPS OF SAID SECOND AND THIRD ELECTRODES; AND MEANS APPLYING HIGH UNIDIRECTIONAL POENTIAL TO SAID SECOND AND THIRD ELECTRODES.