It is well known that electrical discharge through gases, whereby corona discharges are formed, may affect the components of which the gas is made up or which it carries.

A corona discharge is a discharge that occurs in a non- homogenous electric field if the field strength is sufficiently high. As an example, the non-homogenous field may arise if one or both electrodes have a large curvature.

In order to increase the discharge, use is normally made of very short electrical pulses, so that the voltage periodically exceeds the breakdown voltage achieved for direct-current voltage.

The energy of the discharge normally goes into producing electrons. Upon breaking up as a result of collisions, these

electrons produce radicals and ions that may contribute to the dissolution of e. g. contaminants. It is possible by matching the strength and frequency of the discharges to the gas mixture in question, to produce a field in which e. g. contaminants may unite into larger particles or form new chemical compounds that are more easily separated from the gas.

There is known a considerable number of different solutions designed to produce electrical discharges through gases.

Several of these use cables as conductors for the electric pulses. Pulses of the type in question must contain a relatively large amount of energy and be very brief. It is obvious that a cable with its resistive, inductive and capacitive resistance will affect the rise time, form, duration and energy content of the pulse to a significant degree. Often, use-is also made of complex spark gaps that require pulse capacitors to be connected. Due to the great strain to which they are subjected, both spark gaps and pulse capacitors of this type have a relatively short lifetime.

Conventional pulse generators for known equipment typically have an efficiency of around 10 percent. Moreover, it is necessary to use transformers with an advanced constructional design in order to achieve a sufficiently stable power supply.

The object of the invention is to remedy the disadvantages of prior art.

The object is achieved in accordance with the invention by the characteristics specified in the description below and in the appended claims.

It has proven expedient to let the gases that are to be cleaned, and which are mixed with air, flow through tube-like ducts during the cleaning process. The ducts are relatively simple to adapt in terms of dimensions, and the number of parallel ducts may be adjusted according to the volume of gas in question. In a preferred embodiment, an electrically conductive wire/tube/plate connected to a high voltage power supply runs concentrically through each cleaning duct.

By making the pipe wall from a material having a relatively high electrical resistance where the inside of the pipe wall is equipped with a large number of small disk electrodes, the pipe wall that is enclosed by an electrically conductive jacket, the jacket being connected to the other terminal of the power supply, constitutes an electrical resistance R1, while the jacket forms a capacitor C1 together with each disk electrode.

Between the disk electrodes and the wire electrode extending centrally in the duct, a capacitor/capacitance C2 forms through the gas/air mixture. In this electrical circuit, other resistive, capacitive and inductive components also have an effect, but their values are relatively low, thus having only a minor influence on the functioning of the device. It has proven advantageous to cover most of the disk electrodes with insulating material in order to prevent flashovers between the disk electrodes.

Other embodiments, in which the terminals/poles may be made up of mainly parallel plate-like elements, may in principle be regarded as a folded-out tube.

The operation of the invention is explained in the specific part of the description with reference to the appended drawings.

The following describes a non-limiting example of a preferred embodiment illustrated in the accompanying drawings, in which: Figure 1 shows a cleaning duct according to the invention; Figure 2 shows a simplified electrical system diagram; and Figure 3 shows a detail where the disk electrodes are partially covered by an electrically insulating material.

In the drawings, reference number 1 denotes a cleaning duct comprising an electrically conductive jacket 2, a pipe wall 4 made from a high-resistivity material such as semiconductive ceramic material, disk electrodes 6 rigidly mounted to the high-resistivity material 4, and a wire electrode 8.

The jacket 2 and the wire electrode 8 are electrically connected to a high voltage power source 10 via lines 12 and 14. A DC power source, preferably without smoothing, is preferred. The gas that is to be cleaned flows through the central through opening 16 of the duct 1, thus surrounding the wire electrode 8. The disk electrodes 6, which may be made of e. g. acid proof metal or conductive ceramic material, protrude somewhat from the wall 4, are preferably slightly pyramidal and positioned so that the tip of the pyramid faces the wire electrode 8. Advantageously, the disk electrode 6 is partially covered by an electrically insulating material 18,

see figure 3, in order to prevent flashover between the disk electrodes 6.

In figure 2, U1 denotes the voltage/potential from the power source 10, U2 the voltage/potential at the plate electrodes 6, and U3 the voltage from the other terminal of the power source 10, e. g. earth.

R1 and C1 denote the resistance and capacitance, respectively, through the pipe wall 4 between the jacket 2 and a disk electrode 6, while C2 and G2 denote the capacitance and spark gap, respectively, between a disk electrode 6 and the wire electrode 8.

The voltages U1 and U2 from the power source 10 are connected to the jacket 2 and the wire electrode 8. At the moment of switch-on, the capacitor C1 conducts the voltage U1 to the disk electrode 6 and the spark gap G2. Thus the relative voltage between U2 and U3 will be equal to the delivery voltage of the supply point 10, causing a flashover to occur between the disk electrode 6 and the wire electrode 8.

Following the flashover, C1 is charged, so that U2 is approximately equal to U3. Before the next discharge can take place, C1 must be completely or partially discharged. The discharge takes place via R1.

At the same time as the voltage across C1 falls, the voltage between the disk electrodes 6 and the wire electrode 8 increases. As soon as this voltage exceeds the flashover voltage in the gas in question, a flashover occurs whereby the capacitor C1 is charged again.

This cycle continues as long as the circuit is tuned as an oscillatory circuit. The oscillation time is affected mainly by the values of Ri, C1 and impressed voltage, and may be calculated by means of known oscillation theory.

The many disk electrodes 6 cause the gas to experience a multitude of discharges while flowing through the duct 1.

As mentioned in the introductory part of the description, it is necessary in prior art to use a pulse generator connected to a spark gap via conductors. By the device according to the invention, the pulse generation takes place in the duct 1, and the connector lines 12 and 14 will only affect the properties of the pulses, such as rise time, form, duration and strength, to a minor degree.

The voltage source/supply point 10 may comprise a transformer of a simple and reasonable construction, and experiments have shown that in practice, efficiencies in the order of 50% may be achieved. The construction of the duct 1 leads to a significantly more rapid pulse frequency and a shorter rise and fall time for the individual pulse, than when using prior art. An increased pulse frequency leads to an improvement in cleaning effect relative to the physical size of the installation. The rapid and intense electrical variations in the local field strength caused by short rise and fall times for each individual pulse have proven to give significantly higher efficiencies than when using prior art.

The components 2,4,6 and 8 of the duct 1 all have a low sensitivity to mechanical and electrical stresses, whereby a long service life may be expected without costly maintenance.

The power of each flashover may be adjusted to certain reactions and types of gas. The system may operate continuously over the flashover limit, whereby a high corona discharge occurs, or immediately below the flashover limit, so as to make the system self-quenching in the case of a flashover.