WO2016120416A1

WO2016120416A1 - Electrostatic precipitator

Info

Publication number: WO2016120416A1
Application number: PCT/EP2016/051855
Authority: WO
Inventors: Neil VERNER
Original assignee: Sirius Products Limited
Priority date: 2015-01-28
Filing date: 2016-01-28
Publication date: 2016-08-04
Also published as: GB201501427D0; EP3250324A1

Abstract

An electrostatic precipitator for removing pollution from exhaust air. The precipitator comprises at least one pair of electrodes (10, 12) with at least one gap between the electrodes through which air may flow. The electrodes (10, 12) are connected to a generating circuit (20) which generates a potential difference between corresponding electrodes (10, 12), and a measuring means (36) to measure the potential difference between corresponding electrodes. The measuring means (36) is connected to the generating circuit (20), so that the generating circuit (20) can vary a current supplied to the electrodes (10, 12), so that the potential difference between corresponding electrodes (10, 12) is maintained within a predetermined range.

Description

[Corrected under Rule 26, 15.03.2016] ELECTROSTATIC PRECIPITATOR

The present invention relates to electrostatic precipitators.

Electrostatic precipitators are used in a range of technical fields, for example to remove particles from air by providing electrically charged pairs of electrodes with a potential difference – voltage - generated between them.

The potential difference creates an electric field between electrodes. An electric field exerts a force on charged particles, so as to direct particles out of, for example, a stream of air flowing through a channel.

If the potential difference between electrodes is sufficiently high, a “corona” of surplus electrons may be formed in the vicinity of an electrode.

A corona is a region in which the air surrounding an electrode becomes ionised, forming an electrically conductive region. In this situation electrons may escape from the negatively charged electrode – cathode - and flow towards the positively charged electrode - anode.

A corona may assist in removing particles from the air by providing electrons, and therefore a negative charge, to some of the particles to be removed from the air. These, now charged, particles will be attracted to the anode and therefore may be removed from the air flow.

In the catering industry, for example, electrostatic precipitators may be used to remove pollution from exhaust air generated by cooking. Such pollution may comprise particles of smoke, grease, cooking oil or cooking by-products such as volatile organic compounds which may be odorous, or other waste matter.

Known precipitators are generally capable of two operating states, namely on and off. The current supplied to such known precipitators is generally constant irrespective of the condition of the exhaust air being passed through them. The result of this is that the potential difference between corresponding electrodes, may during certain periods, be unsuitable for the prevailing conditions.

For example, dry air with relatively few particles has a high electrical resistance compared with moist air, which has a lower electrical resistance. Cool air is denser and therefore has a lower electrical resistance than hot air.

At constant current, if the resistance of the air drops then the potential difference between corresponding electrode drops. If the potential difference between electrodes falls, the corona may not be sufficient to remove many particles from the exhaust air, and thus dirty air may be discharged to the atmosphere, or further steps may be required to remove pollution from the exhaust air before atmospheric discharge. If the resistance of the exhaust air rises, the potential difference between electrodes rises. At very high potential differences arcing or sparking may occur between electrodes and even between parts of the electrical circuitry driving the electrodes. Arcing between electrodes or parts of the circuitry may be dangerous in environments where grease, oil or cooking fuel are present. As well as safety concerns, arcing is likely to damage components of the precipitator.

Furthermore, an excessive voltage between electrodes may cause grease removed from the exhaust air to bake onto electrodes, thus shortening the useful lifetime of the electrodes.

As well as directing particles as described above, the potential difference between the electrodes may cause oxygen O2 molecules in the ambient air to dissociate, creating oxygen-containing radicals – for example O- oxide anions – and ozone O3 molecules.

Furthermore, the potential difference between the electrodes may cause at least some volatile organic compounds to dissociate, to form charged particles.

Oxygen-containing radicals and ozone molecules may subsequently react with some of the constituents of the pollution to be removed from the exhaust air, especially volatile organic compounds, whether in an uncharged state or in a dissociated state as described above.

Thus, at a suitable potential difference, an electrostatic precipitator may operate synergistically to provide three pollution removal functions namely the removal of already charged particles, the dissociation and subsequent removal of volatile organic compounds, and the generation of ozone which subsequently reacts with for example volatile organic compounds to render them odourless.

To maximise the removal of pollution as described above, it is desirable to maximise the density of the electrons in the corona. The density of free electrons appears to be maximised at a potential difference slightly below that at which arcing occurs. For example, the density of free electrons may increase by up to ten times by increasing the potential difference by a relatively small percentage, when the potential difference is slightly below the level required for arcing. For example, the density of electrons may increase from 1,000,000 per cm3 to 10,000,000 per cm3 by changing the potential difference by a relatively small percentage.

In a high-density state, the electrons may cause a visible glow.

In a state of heightened electron density, the beneficial effects of dissociation of oxygen molecules in the ambient air, and of volatile organic compounds may improve the removal of pollution from exhaust air as described above, in addition to the attraction to electrodes of already charged particles. Heightened electron density also aids the removal of non-dissociated matter, as described above.

Therefore it is an aim of the present invention to provide an electrostatic precipitator which provides efficient removal of particles from exhaust air while minimising the risk of arcing and baking.

Accordingly, the present invention is directed to an electrostatic precipitator for removing pollution from exhaust air, the precipitator comprising at least one pair of electrodes with at least one gap between the electrodes through which air may flow, the electrodes being connected to a generating circuit which generates a potential difference between corresponding electrodes, and a measuring means to measure the potential difference between corresponding electrodes, the measuring means being connected to the generating circuit, so that the generating circuit can vary a current supplied to the electrodes, so that the potential difference between corresponding electrodes is maintained within a predetermined range.

Preferably, the potential difference is maintained at a pre-determined level.

Preferably, the potential difference is maintained at approximately 15.5 kV DC.

Preferably, the anode is maintained at approximately 0 V DC and the cathode is maintained at approximately -15.5 kV DC.

A highly negative cathode voltage, relative to ground, is preferred because it aids the generation of a suitable electron density.

An electrostatic precipitator as described thus removes a higher proportion of pollution from exhaust air, and the precipitator itself has a longer service.

The generating circuit or measuring means may comprise an ammeter. The generating circuit or measuring means may comprise a voltmeter. The generating circuit or measuring means may comprise a resistance meter.

The measuring means may further comprise one or more signal inputs and a signal processor.

The measuring means may further comprise digital sampling means.

The measuring means may further comprise a digital signal processor.

The measuring means may further comprise a pre-programmed or programmable logic controller.

The measuring means may provide one or more signals to the signal input or inputs of the generating circuit.

The measuring means uses information about the state of the electrostatic precipitator to control the generating circuit so that the generating circuit can react dynamically to changes in the resistance of the exhaust air due to changes in the composition of the exhaust air, so that the generating circuit can maintain the potential difference between electrodes within a predetermined range or level.

The measuring means may detect a rapid acceleration in current and corresponding reduction in resistance between the electrodes, indicating the likelihood of arcing. The measuring means may as a consequence generate a signal to the generating circuit to reduce the potential difference between the electrodes so as to prevent or minimise arcing.

Ideally, the feedback mechanism maintains a potential difference in a range or level high enough to generate a high density of free electrons and low enough that arcing does not occur or is minimised.

A further advantage of the arrangement as set out above is that a single unit may remove pollution by electrostatic precipitation and by reaction with ozone and oxygen radicals, whereas previously a separate ozone generator would have been needed, for example an ultraviolet light ozone generator.

In an example of the present invention, the resistance of air flowing through a precipitator was measured to be between 2.5 MΩ when the air was hot, humid and viscous, and 6.25 MΩ when the air was cold and dry. The resistance of air may be as high as 80 MΩ under certain conditions.

More especially, the resistance provided by air flowing through a precipitator was measured to be between about 7.75 MΩ and 77.75 MΩ.

To generate a suitable corona, the potential difference between electrodes in the example setup was approximately 15.5 kV, and to maintain an approximately constant potential difference between electrodes, the current supplied was varied between about 0.002 A and 0.0002 A.

If a fixed current had been used, as with previously-known precipitators, then when the air passing through is of higher resistance (such as when cold and dry) then the potential difference would rise, risking arcing; if the air passing through is of lower resistance (such as when hot, humid and viscous) then the potential difference would fall, reducing the potency of the corona and also reducing the ability of the precipitator to remove particles (such as grease) from the air.

By varying the current in response to changes in the resistance of the air passing through the precipitator, the potential difference can be maintained within a range so that pollution can be removed effectively from exhaust air without generating arcing.

An embodiment of the present invention will now be described, with reference to the drawings, in which:

Figure 1 is a diagrammatic cross-sectional representation of a pair of electrodes suitable for the present embodiment;

Figure 2 is a circuit block diagram of a suitable arrangement for the present embodiment; and

Figure 3 is a perspective view of a unit comprising an electrostatic precipitator of the present embodiment.

Figure 1 shows a diagrammatic representation of a cross-section of a pair of electrodes suitable for use in the present invention. It will be clear to the skilled reader that some elements of the electrodes are not shown in the drawing, for reasons of clarity to aid understanding. Dashed lines are intended to indicate that the elements which are shown are part of a larger structure, full details of which are not shown. A first electrode 10 is an electrically conductive rod, which is located axially within an electrically conductive tube which forms a second electrode 12. The distance d between the outer surface of electrode 10 and the inner surface of electrode 12 is uniform, in other words rod electrode 10 is centred within tube electrode 12, which is approximately cylindrical and has approximately uniform diameter. The skilled reader will understand that other configurations may be suitable, for example the

electrodes

10 and 12 may be formed by a spaced plates.

Electrode 10 extends beyond at least one end of the tube of electrode 12 so that a portion 14 of electrode 10 is not located within electrode 12. Portion 14 of electrode 10 is in electrical contact with a plate 16 which is in turn connected to a generating circuit (not shown). The plate 16 may in addition be in electrical contact with further electrodes (not shown). The end of electrode 10 away from portion 14, i.e. within electrode 12, need not be co-terminal with electrode 12.

Figure 2 shows a cell 18 which is an array of pairs of

electrodes

10 and 12. In Figure 2 the cell 18 is shown as a simplified cross-section for clarity, but the skilled reader will understand that the cell 18 may comprise a series of tubes as described with respect to Figure 1, or any other suitable arrangement.

Each of electrodes 10 is connected to a common plate 16 which in turn is connected to the generating circuit 20. Each of electrodes 12 is connected to a common connection 22 which leads to earth.

The generating circuit 20 comprises a main switch 24 with a fuse and isolator (sometimes known as an incomer), which enables connection to or isolation from a mains electricity supply, for example 110 – 240 V AC. Beyond the main switch 24 there is a step-down transformer and rectifier 26 which supplies an approximately +12 V DC supply to a polarity inverter 28. The polarity inverter supplies an approximately -12 V DC to a step-up transformer 30. The step-up transformer 30 connects the generating circuit 20 to a resistance meter 32, and a logic controller 34 is connected to the resistance meter 32 and the generating circuit 20 via the step-up transformer 30. The step-up transformer is connected to the common plate 16 which is in turn connected to the electrodes 10. The step-up transformer 30 is further connected to earth via the polarity converter 28 and the step-down transformer and rectifier 26. Together, the resistance meter 32 and logic controller 34 form a measuring means 36.

In use, the generating circuit 20 produces a potential difference between electrodes 10 and electrodes 12. The potential difference between the electrodes is monitored by the resistance meter 32, and the measured potential difference is used by the logic controller 34 to set the output of the generating circuit 20. Thus, as air of varying quality (not shown) passes between the

electrodes

10 and 12, the measuring means 36 continuously monitors the potential difference between the electrodes and controls the output of the generating circuit 20 so as to maintain the potential difference within a pre-determined range, and preferably at a pre-determined level, ideally -15.5 kV DC.

Figure 3 shows an example unit 38 with cells 18 having a multitude of electrodes 10 located inside electrodes 12. The generating circuit and measuring means are not shown in Figure 3. The unit 38 may be located within a channel or duct of a kitchen extraction arrangement, between a kitchen hood, for example, and a chimney, neither of which are shown.

In use, exhaust air from a kitchen hood would pass through the unit 38, by way of the cells 18 of

electrodes

10 and 12. Thus, pollution would be removed from the exhaust air by the unit 38 before the exhaust air is subsequently passed via a chimney to the environment.

Various modifications may occur to the skilled person, such as a variation in size or number of electrodes, or a substitution of electrical or electronic components, which nonetheless fall within the scope of the present invention.

Claims

An electrostatic precipitator for removing pollution from exhaust air, the precipitator comprising at least one pair of electrodes with at least one gap between the electrodes through which air may flow, the electrodes being connected to a generating circuit which generates a potential difference between corresponding electrodes, and a measuring means to measure the potential difference between corresponding electrodes, the measuring means being connected to the generating circuit, so that the generating circuit can vary a current supplied to the electrodes, so that the potential difference between corresponding electrodes is maintained within a predetermined range.
An electrostatic precipitator according to claim 1, in which the potential difference is maintained at a pre-determined level.
An electrostatic precipitator according to claim 2, in which the potential difference is maintained at approximately 15.5 kV DC.
An electrostatic precipitator according to any preceding claims, in which the anode is maintained at approximately 0 V DC and the cathode is maintained at approximately -15.5 kV DC.
An electrostatic precipitator according to any one of claim 1 to 3, in which a highly negative cathode voltage, relative to ground, is used.
An electrostatic precipitator according to any preceding claims, in which the generating circuit or measuring means comprise an ammeter.
An electrostatic precipitator according to any one of claims 1 to 5, in which the generating circuit or measuring means comprise a voltmeter.
An electrostatic precipitator according to any one of claims 1 to 5, in which the generating circuit or measuring means comprise a resistance meter.
An electrostatic precipitator according to any preceding claims, in which the measuring means further comprise one or more signal inputs and a signal processor.
An electrostatic precipitator according to any preceding claims, in which the measuring means further comprise digital sampling means.
An electrostatic precipitator according to any preceding claims, in which the measuring means further comprise a digital signal processor.
An electrostatic precipitator according to any preceding claims, in which the measuring means further comprise a pre-programmed or programmable logic controller.
An electrostatic precipitator according to any preceding claims, in which the measuring means may provide one or more signals to the signal input or inputs of the generating circuit.