GB2240495A

GB2240495A - Electrostatic precharger

Info

Publication number: GB2240495A
Application number: GB9101281A
Authority: GB
Inventors: Keith Morris
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1990-02-01
Filing date: 1991-01-21
Publication date: 1991-08-07
Anticipated expiration: 2011-01-21
Also published as: GB2240495B; GB9101281D0

Abstract

An electrostatic precharger for a gas stream in a duct comprises a corona discharge electrode transverse to the stream and a surface of low electrostatic potential parallel to and upstream of the electrode. In an embodiment a venturi aperture 16 is provided between earthed cylinders 14 upstream of the corona discharge electrode 20 of stainless steel at - 20 Kv. The cylinders are held in place by plates 18. In another embodiment (Fig 2) an array of earthed cylinders 34 is provided upstream of discharge electrodes 42 at high negative voltage. The cylinders may be replaced by an array of earthed parallel wires. Dust particles are charged negatively but are inhibited from settling on the earthed cylinders by the high gas velocity thus preventing fouling of the precharger. <IMAGE>

Description

Electrostatic Precharger This invention relates to a precharger for providing electrostatic charge to particles suspended in a gas stream.

It is known to remove particles (e.g. dust particles) from a gas stream by passing the gas stream through an electrostatic precipitator, in which the particles are charged and collected. It is also known to pass a dust-laden gas stream through a filter, for example a fabric filter, on which the particles collect to form a cake, and it has been proposed that the performance of such a filter might be improved by first charging the particles.

Prechargers proposed for such a purpose have hitherto been large, and therefore expensive, and have tended to act as precipitators, collecting the particles and so becoming fouled up. It has proved very difficult in practice to design a precharger which provided adequate charging of dust particles without at the same time becoming fouled by settling out dust particles; this problem has been found to occur even where there is a rapid air flow through the precharger.

According to the present invention there is provided an electrostatic precharger for a gas stream within a duct, the precharger comprisinq a high-emission corona discharge electrode extending transverse to the gas flow direction, and means forming a surface of low electrostatic potential within the duct, the surface extending parallel to, and being upstream of, the corona discharge electrode, and being such that the electric field adjacent to the surface is not sufficient for sparking to occur, such that in operation particles in the gas stream become charged in the vicinity of the corona discharge electrode but gas flow through the duct inhibits particle deposition on the surface-forming means.

The surface-forming means might be gas-permeable, for example an array such as a grid, grille or mesh of wires or rods extending generally transverse to the gas flow direction, for example in a plane across the duct. The gas would flow through the plurality of slots or holes defined by the grid, grille or mesh. For example the means might comprise an array of parallel wires each of diameter about 1 mm and at a spacing of about 6 mm; the spacing should be no more than a quarter of the distance the array is upstream of the discharge electrode. The array is preferably earthed.

Alternatively the surface-forming means might comprise two low-voltage electrodes each presenting a continuous curved surface to the gas, this surface having a large enough radius of curvature to prevent sparking, the lowvoltage electrodes being spaced apart so as to define a generally rectangular aperture between them for the gas flow, and the corona discharge electrode being downstream of the aperture and extending parallel to a side of the aperture. The low voltage electrodes would usually be earthed, and their radius of curvature is typically at least 5 mm.

The discharge electrode itself must provide sufficient corona current to charge all particles in the gas flow, and might be at a positive or negative potential of between 10 kV and 100 kV, preferably between 20 kV and 60 kV, for example 50 kV, and emit a current of about 1.5 mA per metre of its length. It must have a much smaller radius of curvature than the low potential surface, and for example may comprise a metal wire of diameter less than imam, for example between 0.2 and 0.7mm, for example 0.5 mm.

Alternatively it may define a large number of points or edges with small radii of curvature, to enhance the emission. It might be in the form of a saw-like wire with a plurality of sharp outwardly pointing teeth, or comprise a plurality of metal fibres each of diameter 0.1 mm or less twisted to make a thread or string of diameter a few millimetres.

In a further embodiment the precharger comprises a plurality of low voltage electrodes defining at least two such apertures, the gas flows through which are in parallel. The corona discharge electrodes downstream of adjacent apertures may be of the same polarity, or might be of opposite polarity so that particles passing through the adjacent apertures are given charges of opposite sign.

This enhances the tendency for the particles subsequently to agglomerate.

The precharger of the invention may be used in conjunction with a filter, such as a fabric bag filter, or with an electrostatic precipitator, or with other gas cleaning equipment such as a scrubber.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a sectional view in a horizontal plane through a precharger of the invention; and Figure 2 shows a sectional view through an alternative precharger of the invention.

Referring to figure 1, a precharger 10 is shown installed in a duct 12 of rectangular cross-section and extending horizontally. The precharger 10 comprises two upright steel cylinders 14 each of diameter 20 mm extending between the top and the bottom walls of the duct 12 and arranged symmetrically on either side of the mid-plane P of the duct 12 so as to leave an aperture or gap 16 between them of width 25 mm. The gap between each cylinder 14 and the respective side wall of the duct 12 is blocked by a respective fillet plate 18. Gas flowing along the duct 12 as indicated by the arrows A is consequently constrained to flow through the gap 16, and flows through the gap 16 faster than along the duct 12.On the mid plane P of the duct 12, and 25 mm downstream of the plane containing the longitudinal axes of the two cylinders 14, is a corona discharge electrode 20 in the form of a stainless-steel wire of diameter 0.5 mm, this wire extending between the top and the bottom walls of the duct 12 and being supported at each end by an electrically insulating block (not shown). The cylinders 14 are both earthed electrically; while the electrode 20 is connected to a high voltage d.c.

supply, usually at a negative potential, for example - 20 kV when energised.

In operation of the precharger 10, when a dust-laden gas stream flows along the duct 12 it is constrained to flow through the gap 16 and then to pass the electrode 20.

Gas molecules in the vicinity of the electrode 20 are ionized by the strong electric field, which is progressively stronger the closer to the wire electrode 20, and the negative ions are repelled by the electrode 20; a corona discharge takes place between the wire electrode 20 and the two earthed cylinders 14. Dust particles pick up the negative ions, but are inhibited from settling out on the outside of the earthed cylinders 14 by the comparatively high gas velocity against which they would have to diffuse to reach the cylinders 14. Hence a stream of gas laden with negatively charged dust particles emerges from the precharger 10.

Referring now to Figure 2 there is shown a precharger 30 with some similarities to the precharger 10 of Figure 1: the precharger 30 is located in a horizontally extending duct 32 of rectangular cross-section, and includes an array of five upright steel cylinders 34 each of diameter 20 mm spaced apart across the duct 32 so as to define four rectangular apertures 36 each of width 25 mm. The gaps between the side walls of the duct 32 and the outermost cylinders 34 are blocked by fillet plates 38. The arrow A indicates the direction of flow of the dust-laden gas along the duct 12. An insulating plate 40 is attached to the downstream side of the middle cylinder 34, extending along its whole length and extending 50 mm downstream.

At a distance of 25 mm downstream of the plane containing the longitudinal axes of the cylinders 34 are four corona discharge electrodes 42 each in the form of a steel wire of diameter 0.5 mitt; each is aligned with the centre of one of the apertures 36, and each extends between the top and the bottom walls of the duct 32 being supported at each end by an electrically insulating block (not shown).

All the cylinders 34 are earthed. In operation the two discharge electrodes 42a at one side of the duct 32 are connected to a negative high voltage d.c. supply, and the other two electrodes 42b to a positive high voltage d.c.

supply. Corona discharges occur as described earlier, so the dust particles in the gas stream passing the discharge electrodes 42a become negatively charged, while the dust particles in the gas stream passing the electrodes 42b become positively charged. Downstream of the plate 40 these gas streams tend to mix, so the dust particles tend to agglomerate before reaching the next stage in the dust-removal plant.

It will be appreciated that a precharger of the invention may differ in many respects from the embodiments described above. In particular the size, shape, and orientation of the duct does not matter; in a wider duct more apertures and discharge wires could be provided, and in a non-rectangular duct the lengths of the apertures might not be the same across the whole width of the duct.

The low voltage (or earthed) electrodes need not be cylindrical, but the surface nearest the discharge electrode should be of much larger radius of curvature than that of the discharge electrode, to ensure that corona discharge rather than sparking occurs. The width of the apertures is not critical, although it is generally preferably to minimize pressure drop across the precharger by providing apertures at least 10 mm wide and providing a greater area than that obstructed by the intervening low voltage electrodes.

In a further modification (not shown) to the prechargers described above the cylinders 14, 34 are replaced by an array of parallel wires each of diameter 1 mm and spaced 6 mm apart, extending across the entire width of the duct 12, 32. These wires are all earthed, and lie in a plane 25 mm upstream of the discharge electrodes 20 or 42. This plane is entirely at earth potential, but obviously imposes very little restriction to the gas flow.

Claims

1. An electrostatic precharger for a gas stream within a duct, the precharger comprising a high-emission corona discharge electrode extending transverse to the gas flow direction, and means forming a surface of low electrostatic potential within the duct, the surface extending parallel to, and being upstream of, the corona discharge electrode, and being such that the electric field adjacent to the surface is not sufficient for sparking to occur, such that in operation particles in the gas stream become charged in the vicinity of the corona discharge electrode but gas flow through the duct inhibits particle deposition on the surface-forming means.

2. A precharger as claimed in Claim 1 wherein the corona discharge electrode provides a radius of curvature less than lmm.

3. A precharger as claimed in Claim 2 wherein the corona discharge electrode comprises a wire of radius of curvature of between 0.2 and 0.7mm.

4. A precharger as claimed in Claim 2 wherein the corona discharge electrode comprises a plurality of metal fibres each of diameter no more than 0.lmm.

5. A precharger as claimed in any one of the preceding Claims wherein the surface-forming means comprises two low-voltage electrodes each presenting a continuous curved surface to the gas, this surface having a large enough radius of curvature to prevent sparking, the low-voltage electrodes being spaced apart so as to define a generally rectangular aperture between them for the gas flow, the corona discharge electrode being downstream of the aperture and extending parallel to a side of the aperture.

6. A precharger as claimed in Claim 5 comprising a plurality of said low voltage electrodes defining at least two such apertures through which the gas flows are in parallel.

7. A precharger as claimed in Claim 6 wherein the corona discharge electrodes downstream of adjacent apertures are of the same polarity.

8. A precharger as claimed in Claim 6 wherein the corona discharge electrodes downstream of adjacent apertures are of opposite polarity.

9. A precharger as claimed in any one of Claims 5 to 8 wherein the radius of curvature of the low-voltage electrodes is at least 5 mm.

10. A precharger as claimed in any one of Claims 1 to 4 wherein the surface-forming means comprises a gas-permeable plate arranged for gas flow there through.

11. A precharger as claimed in Claim 10 wherein the gas-permeable plate comprises an array of parallel spaced-apart wires and is arranged at a distance upstream from the corona discharge electrode at least four times the spacing between -the wires.

12. A precharger as claimed in Claim 11 wherein the wires are of width about 1 mm and are spaced about 6 mm apart.

13. An electrostatic precharger for a gas stream substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings. t