The present invention relates to an electrofilter which consists in principle of two respectively positive and negative electrodes which are connected to a high voltage source. The type of voltage is normally a pulsating direct voltage generated by rectifying single-phased alternating voltage. The voltage level is 20 to 100 kV, depending on the size and design of the filter.

The positive electrodes of the electrofilter consists normally of steel plates, so- called collection plates, suspended at a mutual distance of 100 to 400 mm, whereby parallel ducts are created between the plates through which the smoke to be purified passes in a horizontal flow. For safety reasons the collection plates are always earthed, see fig. 2. The negative electrodes, so-called discharge wires, can be either thin wires, springs or rods suspended midway between the collection plates.

When the voltage between the discharge wires and the collection plates is sufficiently high, the discharge wires will begin emitting electrons which will immediately move towards the positive collection plates. On the way the electrons will ionize the air molecules which will then be attracted by the neutral dust particles whereby these become negatively charged and attracted by the collection plates, see fig. 2. The dust particles from the smoke will little by little settle as a layer on the collection plates. The layer thickness will be largest at the inlet and will diminish exponentially in the longitudinal direction of the filter. Usually the dust layer is subsequently removed from the collection plates by knocking on these with hammers or vibrators. This frees the dust which falls down into dust hoppers underneath the collection plates. As the electric current runs from the discharge wires towards the collection plates, the collected dust must have some conducting capacity if the process is to continue unhindered with dust-coated collection plates.

The conducting capacity is most often expressed by the resistivity which can be determined experimentally. If the resistivity in the collected dust is above a certain value, a relatively high voltage drop will occur above the dust layer on the collection plates.

If this voltage drop exceeds a certain level, the air molecules between the particles in the dust layer will be ionized, and the dust layer will begin emitting positive ions which will move towards the discharge wires and neutralize the negative ions. This will cause the corona current to increase drastically, and the filtering effect will drop catastrophically. This phenomenon is called reverse flow. In large coal-fired stations the problem can be reduced by continuously adding water or chemical compounds to the flue gas, which will improve the conducting capacity of the dust. In minor straw boilers this is not realistic. It is the purpose of the present invention to describe an electrofilter that can be used to purify the smoke from especially minor straw boilers.

This is achieved by designing the electrofilter as described in the characterizing part of claim 1. Thereby reverse flow is prevented, and the filtering effect can be maintained without continuously adding water or chemicals to the flue gas.

Claim 2 describes the design of the cutouts in the collection plates in an

electrofilter according to the invention. Claim 3 describes an alternative design of the cutouts in the collection plates in an electrofilter according to the invention. With the design described in claim 4 a cutting effect will be obtained at

temperature fluctuations in a collection plate that can break up any lumps of dust that may have collected in the cutouts. Claim 5 describes a third design of the cutouts in the collection plates of an eleetrofilter according to the invention.

With the design described in claim 6 the shape stability of the collection plates is further improved.

The invention is explained in detail below with reference to the drawing, in which

Fig. 1 is a known eleetrofilter, Fig. 2 shows the purification principle of the eleetrofilter shown in fig. 1 ,

Fig. 3 is a perspective and partial section view of an eleetrofilter with rotating collection plates, Fig. 4 is a side view of an eleetrofilter with rotating collection plates

Fig. 5 shows a drawing corresponding to the drawing shown in fig. 4, viewed from one end and with the discharge wires in drawn out position, Fig. 6 shows a rotating collection plate in which each quadrant is designed with cutouts shaped as parts of circular rings which are mutually offset at the transition from one quadrant to the next.

Fig. 7 shows a sectional view following the line l-l in fig. 6, Fig. 8 shows a rotating collection plate with rectilinear, approximately radial cutouts, and

Fig. 9 shows a rotating collection plate in which the cutouts are shaped as through bores.

When the voltage between the discharge wires and the collection plates is sufficiently high, the discharge wires will start emitting electrons e, which will immediately move towards the positive collection plates 1 which are earthed by a wire 1'. This is called corona charging. On the way the electrons e will ionize the air molecules m, which will then be attracted by the neutral dust particles s whereby these become negatively charged and attracted by the collection plates, see fig. 2. As shown in fig.s 3 and 4 an electrofilter 6 according to the invention is designed with a row of, by way of example eleven, rotating circular collection plates 1 mounted at a uniform mutual distance on a rotating shaft 8. Between each collection plate 1 and a neighbouring plate discharge wires are placed which can be mounted on a frame 4, on which a vibrator 5 is mounted that can clean the discharge wires 3 of dust. Moreover, the electrofilter 6 is fitted with stationary scrapers 10 which bear against each of the rotating collection plates 1 and clean them of dust, which drops into hoppers 11 and 12 below. The gas to be purified is led into the electrofilter through an entry hole 13 and after purification out through an exit hole 14.

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As shown in fig.s 6 and 8 the rotating collection plates 1 are designed with cutouts 15, which can be circular as shown in fig. 6 or rectilinear and approximately radial as shown in fig. 8. As shown in fig. 7 one side 16 of a cutout can be slightly inclined in relation to the other side 17 which is perpendicular to the surface of the collection plate 1. This way a cutting effect can be obtained at temperature fluctuations in a collection plate 1, which can break up any lumps of dust that may have collected in the cutouts.

As shown in fig. 9 the collection plates 1 may also be designed with rows of through bores 18 instead of cutouts.

The cutouts may also be shaped otherwise than shown in fig.s 6 and 8.

Designing the collection plates with cutouts or through bores has the effect that the collection plates do not shrink and/or buckle at temperature fluctuations. Thus the desired and necessary cleaning effect can be obtained if stationary scrapers are used. To make it even more certain that the collection plates 1 will not buckle, guide rollers not shown in the drawing are mounted at each collection plate.