NO168977B - DEVICE FOR AIR TRANSPORT - Google Patents

Description

The present invention relates to a device for transporting air using so-called ion wind or corona wind, the device being of the type that appears in the preamble to claim 1.

The device has been developed mainly for use in connection with air cleaning devices, such as electrostatic dust counters, and air treatment systems such as, for example, ventilation systems and air conditioning systems, although the invention can also be advantageously used in many other connections where air is to be transported, such as in cooling of electrical appliances or electrical equipment, and in connection with heating devices such as electric hot air guns.

Today, air is transported in the aforementioned devices and systems almost exclusively by means of mechanical fans of various designs. Such mechanical fans and associated drive motors are relatively expensive, in addition they are also heavy and require considerable space. They also have a relatively large energy requirement and are consequently expensive to operate. During operation, the fans also produce a significant amount of noise, which is very annoying in many places where such fans or blowers are used, for example in living rooms and in certain workplaces.

It is known that transport of air can in principle be achieved with the help of so-called ion wind or corona wind. An ion wind is created when a corona electrode and a counter electrode are arranged at a distance from each other and each is connected to a respective clamp on a direct voltage source, the construction of the corona electrode and the voltage on the direct voltage source being such that a corona discharge is produced at the corona electrode. This corona discharge results in ionization of the air in such a way that the ions get the same polarity as the polarity of the corona element, and possibly also electrically charged so-called aerosols, i.e. solid particles or liquid particles that are present in the air and become electrically charged on collision with the electrically charged air ions. The air ions move rapidly under the influence of the electric field from the corona electrode to the counter electrode, where they release their electric charge and are converted into electrically neutral air molecules. During their passage between the electrodes, the air ions are constantly in collision with the electrically neutral air molecules, whereby the electrostatic forces are also transferred to these latter air molecules, which are thus pulled with the air ions in the direction from the corona electrode to the measuring electrode, thereby producing air transport in the form of so-called ion wind or corona wind .

Devices for transporting air using ion wind are known, and examples of such devices are described and illustrated in e.g. DE-OS 2 854 716, DE-OS 2 538 959, GB-A 2 112 582, EP-A1-29 421 and US 4 380 720. However, these previously known air transport devices using ion wind or corona wind have been shown to be extremely ineffective and have gained no practical significance. It seems that the reason for this is a lack of understanding of the physical mechanisms that determine the total transport of air through a device of this kind. Consequently, it is not possible with the previously proposed embodiments of air transport devices powered by means of ion wind, to achieve in practice the transport of significant amounts of air without having to raise the corona current to levels that are significantly above the levels that can be considered acceptable when using such device in populated environments. It is well known from e.g. the electrostatic precipitators that

an electric corona discharge generates chemical compounds, mainly ozone and nitrogen oxides, which have an irritating effect on humans and which can be harmful to health when present in air in very high concentrations. In the case of a corona discharge, these chemical compounds are generated at a rate that depends on the magnitude and polarity of the electrical corona current. Current electrostatic air filters for use in human or populated environments therefore operate with a positive corona discharge and a corona current with an amperage substantially proportional to the amount of air passing through the filter per second. time unit under normal operating conditions. In this connection, the corona current is often in the order of 40-80 microamperes at an air flow rate of 100 m<3>/h, the strength of the current being adapted to the requirement for an acceptable level of production of ozone and Nox. It will be understood that the corona current used in air transport devices that work with ion wind and are used in

presence of people, must also be limited to the aforementioned size. This is not possible to achieve with the previously known air transport devices which make use of ion wind

due to the devices' poor efficiency. With the device proposed in EP-A1-29 421 and US 4 380 720 it is, according to reports, for example possible to achieve an air flow rate of 1 liter per second using a corona effect of 1 Watt at a preferred corona voltage of 15 kV. Transformed to an air flow of 100 m<3>/h, this device will therefore consume around 1900 microamps, which is roughly thirty times higher than the corona current value that is acceptable in environments where people are located.

An object of the present invention is consequently to provide an improved and more efficient air transport device of the kind mentioned in the introduction, and one which is so efficient that it also enables practical use in environments where there are people.

The device according to the invention is based on a deeper and better understanding than previously had of the mechanisms that determine the total transport of air through a device of this type, and it is characterized by the characterizing features that appear in the attached requirements.

The invention will now be described in more detail with reference to the attached drawings, where

Fig. 1 is a schematic illustration of ion migration between a corona electrode and a counter electrode; Fig. 2 - 7 and 9 - 13 schematically illustrate a number of different embodiments of a device according to the invention; and Fig. 8 is a diagram showing corona current as a function of voltage.

A summary will first be given of the fundamental conditions that determine the air transport that can be achieved by means of an ion wind or corona wind produced between a corona electrode and a counter electrode arranged axially downstream of the corona electrode in the desired direction of flow. Figure 1 schematically illustrates a corona electrode K in the form of a thin wire which extends over the air flow path, for example across an air flow channel, and a counter electrode M which also extends over the air flow path and which is shown schematically and as an example, in the form of a net or a grid structure that is permeable to the air flow. The counter electrode M is positioned downstream of the corona electrode K in the desired air flow direction, shown by an arrow w at an axial distance H from the corona electrode K.

As mentioned, the corona discharge created at the corona electrode gives rise to electrically charged air ions, which migrate in the direction towards the counter electrode under the influence of the electric field present between the corona electrode and the counter electrode.

The mobility of the ions varies within a wide spectrum, although for present purposes it can be assumed that light ions have the mobility

is predominant, and that any electrically charged aerosols present, which are far less mobile than the air ions, only make up a negligible proportion of the total charge in the system. It can also be assumed that the air ions make up a very small fraction of the total air mass inside the system, and that the flow speed of the air is at least one order of magnitude lower than the speed of movement of the air ions. In relation to the traveling speed of the air ions, the surrounding air ions can therefore be considered to be stationary. The migration speed v of electrically charged air ions relative to the surrounding air is proportional to the product of their mobility c and the strength E of the electric field, and thus It is also assumed that stable conditions prevail so that the charge density in a given partial volume of the system is constant , i.e. that electric charge per unit of time delivered to the system is equal to that removed from the system. Consequently, the current density in the air can be expressed as the product of the charges' travel speed v and the charge density

where i is the current density.

The specific volume force in the air is the product of the charge density p and the electric field strength E and thus

where f" is the driving force per unit volume of air.

When the above-mentioned equations (1), (2) and (3) are used, is thus obtained

i.e. that the specific volume force can be expressed as the ratio between the current density and the ion mobility.

As illustrated in figure 1, we can now consider a "current channel", which conducts an infinitesimal part dl of the total ion current I between the two electrodes K and M. The center line of this current channel is always parallel to the current density vector i, and its cross-sectional area ds has a surface normal parallel to the current density vector.

We now consider a volume element

of this current channel, where dV is an infinitesimal volume and dl is an infinitesimal length in the direction of the current channel. The force acting in the direction of the surface normal on each such volume element in the flow channel becomes

This volume force dF has a component in the direction w of the air transport and a component at right angles to this direction. It is believed that when these transverse forces in the device are summed over the entire cross-sectional area of the air flow path or channel, they will cancel each other out and that they can therefore be overlooked. The total transport force in a current channel is therefore

where H is the distance between the corona electrode H and the counter electrode M in the direction of the air flow. The total transport force FT in the air flow channel can thus be expressed as where S is the total cross-sectional area of the air flow channel and I is the total ion or corona current. The average pressure build-up can thus be written as

The transport force is thus proportional to the product of the total ion or corona current I and its path H, i.e. proportional to the so-called "current-distance" H.I.

It can be shown that the total airflow as a result of this pressure build-up can be written as

where Q is the air flow, k is a dimensionless aerodynamic drag coefficient and "y^ is the density of the air.

It can be seen from equation (10) that the size of the air transport is directly proportional to the square root of the product between the total ion or corona current I and its traveling distance H.

To achieve a high air flow in the desired direction, i.e. in a direction away from the corona electrode and towards the counter electrode, one should therefore seek to achieve a high product between ion current and its travel distance in a direction downstream from the corona electrode, i.e. from the corona electrode towards the counter electrode. An increase in the transport power and thus in the total air flow can be achieved either by increasing the strength of the total ion flow, or by increasing the distance between the corona electrode and the counter electrode. When used in human environments, however, as mentioned, it is not permitted to increase the strength of the ion or corona current to a level that exceeds a given maximum, due to the resulting production of harmful ozone and nitrogen oxides (Nox), as this the output is mainly proportional to the corona current. The only remaining parameter that can be influenced in this respect is thus the distance traveled by the ion stream, i.e. the axial distance or distance between the corona electrode and the counter electrode. In connection with the present invention, it is therefore proposed that the distance between the corona electrode and the part of the counter electrode which receives the majority of the ion current is at least 50 mm, and preferably measures at least 80 mm.

It will also be seen that when an air transport device of the type described is used, a stream of air ions is also capable of migrating from the corona electrode in an upstream direction, i.e. in a direction opposite to the desired air transport direction, if there is an electrically conductive object or subject upstream of the corona electrode that has an electrical potential in relation to the corona electrode that makes such migration of air ions possible. It will be understood that this greatly reduces the total desired transport of air through the device. To the extent that this possibility for an ion current which. passing from the corona electrode in an upstream direction from it, have been taken into account during the construction of known air transport devices of the kind discussed here, it seems to have been considered sufficient to ensure that electrically conductive objects upstream of the corona electrode are arranged in a considerable distance from this, and that the flow of the ion current in the upstream direction is small. However, since the transport force created by the ion flow is proportional to the product of the strength of the flow and the distance covered, which is clear from equation (9) above, it will be seen that even a very small flow of ions from the corona electrode in an upstream direction from this, can give rise to a significant transport force in a direction opposite to the desired direction of the air transport, when this upstream directed flow of ions has a long path to cover.

It must be noted that in the present connection, the expression "electrically conductive" must be interpreted in relation to the extremely small currents that prevail in a device of the present type, these currents normally being of the order of 1 mA/m<2>.

In the case of an air transport device of the kind to which the invention relates, objects which can be considered to be electrically conductive or which have a surface which can be considered to be electrically conductive will, in practice, always be found upstream of the corona electrode. These objects can, for example, comprise grids or net structures or other parts of the device itself arranged at the inlet to the air flow channel in the device. Even in the absence of such device components, such objects as wall surfaces, pieces of equipment or furniture and even people present in the area where the device is placed and located near the inlet of the air flow channel in the device can serve as electrically conductive surfaces for which a stream of ions can migrate from the corona electrode upstream in the channel.

This sought improvement in efficiency, i.e. a high air flow by means of a corona current limited to an acceptable value is achieved in the air transport device according to the invention partly by locating the counter electrode at such a distance from the corona electrode that the distance from the corona electrode to the part of the counter electrode that receives the largest part of the ion current, i.e. the traveling distance of the ion current downstream from the corona electrode, at the shortest is 50 mm and preferably not shorter than 80 mm, and partly by ensuring that the product of the ion current strength and the distance covered by the current in the upstream direction away from the corona electrode is practically zero, or in all cases much less than the corresponding product of ion current strength and distance traveled by the current in the downstream direction, away from the corona electrode. The latter is achieved according to the invention by effectively shielding the corona electrode in the upstream direction, so that no ion current is able to flow from the corona electrode in the upstream direction, or at least so that any ion current that is capable of flowing in the upstream direction is only very small and only covers a very short distance. According to an embodiment of the invention, the aforementioned necessary shielding of the corona electrode in the upstream direction can be achieved by connecting the terminal of the direct current source connected to the corona electrode to a potential which mainly coincides with the potential of the device's immediate surroundings, i.e. which in practice is grounded in the same way as the housing that houses the device and as the rest of the non-active electrical components. To the extent that it has previously been proposed in connection with air transport devices of this kind, to place the corona electrode at ground potential instead of a high potential, these two alternatives have previously been considered equivalent to each other with respect to the air transport mechanism, and coupling of the corona electrode to the ground potential has not been carried out as an endeavor to

to shield the corona electrode in the upstream direction.

In many cases, however, it is not desirable to connect the corona electrode to earth potential, since for many practical reasons it may be desirable to connect the counter electrode to earth potential or to connect the corona electrode and counter electrode to opposite polarities in relation to earth, thereby reducing the need for high-voltage insulation . In such cases, the desired shielding of the corona electrode in the upstream direction can be achieved in accordance with another embodiment of the invention, by means of a method known from other areas of the electro-technical field, by arranging an electrically conductive shield element upstream of the corona electrode and providing the element a potential which substantially coincides with the potential of the corona electrode, so that upstream of the corona electrode they form an equipotential barrier which is substantially impermeable to ions flowing in the upstream direction. To the extent that the provision of a screen electrode upstream of the corona electrode and connected to the same potential as the electrode has previously been proposed in connection with air transport devices of the type in question, such proposals have been made in connection with an air transport device of cascade construction, comprising a array of corona electrode arrays and counter electrode arrays arranged in an axial sequential relationship in an air flow channel. It has not previously been understood or perceived that effective shielding of the corona electrode against an ion current in the upstream direction under all conditions is essential for the efficiency of the air transport device.

A third and extremely surprising possibility for effecting the necessary shielding of the corona electrode against an unwanted ion flow in the upstream direction lies in extending an air flow channel which encloses the electrodes in the device a considerable distance upstream of the corona electrode, i.e. at the inlet end of the air flow channel, the walls of such a channel suitably consisting of a dielectric material, for example a suitable plastic material, in a known manner. Tests have shown that during operation of air transport devices of the type in question, an excess of electric surface charges appears on the dielectric walls of the air flow channel which remains throughout the time the material is exposed to the prevailing electric field. By "excess charges" is meant here electrical charges on the surface of the dielectric material in addition to the surface charges that are taken into account in the classical understanding of dielectric material with weak electrical conductivity. It has not been clearly established why these excess charges appear on the dielectric walls of the air flow channel, although the phenomenon itself has been established experimentally. The phenomenon seems to be related to the phenomena used in the production of the electric electrets. In this latter case, a special dielectric material is exposed to a combination of a high electric field and ion currents. Electrical excess charges are thereby permanently bound in the material's structure and are not led away despite the fact that the material is electrically conductive to a certain extent. In connection with the aforementioned phenomenon which is encountered in air transport devices of the type in question, it is consequently an obvious assumption from a person skilled in the art that the electrical excess charges on the dielectric walls of the air flow channel are also bound to the structure of the dielectric material, but only provided that the material is exposed to the action of an electric field. This phenomenon can be advantageously used to achieve the necessary shielding of the corona electrode in the upstream direction by extending the air flow channel and its dielectric walls upstream, away from the corona electrode, i.e. at the inlet end of the channel, over a distance such that the excess charges that appear on the channel walls under the influence of an ion current from the corona electrode immediately after switching on the device, effectively shield the ion cloud that is present around the corona electrode, against the possible appearance of an electric field upstream of the corona electrode, thereby achieving an effective shield against an upstream directed ion current from the corona electrode. It will be seen that the longer the air flow channel is extended upstream of the corona electrode, the greater the efficiency of the screen provided. Tests have shown that a satisfactory shielding effect can be achieved when the distance the airflow channel is extended upstream of the corona electrode is at least 1.5 times the distance between the corona electrode and the counter electrode. You will also see that the shielding effect becomes more effective with decreasing widths of the air flow channel, i.e. the smaller the distance between mutually opposite dielectric walls, the greater the effectiveness of the produced shielding effect. In the case of an air flow channel with a relatively large cross-sectional area, the shielding effect can be significantly increased by dividing the channel into a number of mutually parallel sub-channels upstream of the corona electrode, by means of elongated partitions extending parallel to the walls of the channel, for example partition walls in the form of strips or the like of dielectric material. Such an arrangement will enable effective shielding of the corona electrode against an ion flow in the upstream direction, even if the remote air flow channel is extended by upstream of the corona electrode, only approximately equal to the distance between the corona electrode and the measuring electrode.

Another serious problem encountered with air transport devices of this kind for use in human environments is that they must be safe to touch despite the high voltages used. Touch protection can of course be provided by means of mechanical devices, by providing the air flow channel that surrounds the electrodes in the device with completely impermeable walls and equipping the channel with a protective grid at both the inlet and outlet ends, so that it is impossible to touch the live electrodes in the device, both intentionally and through carelessness. However, such protections provide a significant flow resistance and thereby reduce the air transport through the device to a significant extent, and thereby its efficiency. In a device according to the invention, however, it has been found possible to provide completely satisfactory safety measures against contact with the device in a much simpler and more advantageous way. As described in the foregoing, a device constructed in accordance with the invention works with an extremely low corona current, in the order of 20-50 microamps per 100 m<3>/h transported air. This extremely low specific value of the corona current is made possible due to the large axial distance between the corona electrode and the counter electrode, and the effective shielding of the corona electrode in the upstream direction. As a result of this low current consumption, the voltage-carrying electrodes of the device, whether corona electrode or counter electrode, can be connected to their associated terminal on the voltage source through an extremely high resistance, without having to increase the voltage of the voltage source to an unacceptable level . It has been found that this series resistance can easily be given, without any difficulty, a resistance value of such a high magnitude that in the event of a direct short-circuit of the voltage-carrying electrode, the short-circuit current is so low as to be completely harmless. A limit value of 2 mA is usually considered a harmless short-circuit current in the case of body contact with such electrical devices. If the short-circuit current is made as small as about 100-300 microamperes, one will not experience any discomfort at all when touching the live electrode. This can easily be achieved with a device according to the invention.

If, for example, it is assumed that the live electrode

in a device must have an operating voltage of 20 kV and the corona current is 50 microamps, the live electrode can

is connected to the corresponding terminal on the voltage source through a resistance of, for example, 150 Mfi, whereby the voltage source itself must have a terminal voltage of 27.5 kV. When the live electrode is directly short-circuited, the short-circuit current will thereby be only about 185 microamps, which is so small that it would not cause any discomfort if the short-circuit were caused by direct contact with the electrode.

However, this limitation of the short-circuit current to a value which does not cause discomfort when coming into direct personal contact with the live electrode has been completely unattainable in practice with the large corona currents, of the order of 2000 microamperes, which must necessarily be used in previously known air transport -devices that work with electric ion wind. Another important factor in contact protection besides the low level of the short-circuit current is the capacitive discharge current that can occur when an electrode of a given capacitance is touched.

However, in the case of electrodes of such construction as to have significant capacitance, the capacitive discharge current can be reduced to completely acceptable levels by making these electrodes of a high resistivity material, in accordance with the invention. This creates no other disadvantages since the electrodes do not need to be highly conductive, in view of the low currents that can be used according to the invention, and still provide an efficient air transport device.

Figure 2 in the attached drawings illustrates schematically and as an example the principle construction of a first embodiment of an air transport device according to the invention. This device comprises an air flow channel 1 which is made of an electrically insulating material and through which an air flow is to be produced in the direction shown by an arrow 2. Arranged in the air flow channel is a corona electrode K which is permeable to the air flow, where axially downstream of the corona electrode a counter electrode M is arranged, which is also permeable to the air flow. The corona electrode K comprises an electrically conductive material which is preferably ozone- and ultraviolet-resistant, and which can be constructed in a number of different known ways to produce an electric corona discharge under the influence of an electric field. As an example, the corona electrode K in figure 2 is shown to comprise a thin wire or a filament which extends over the air flow channel 1. However, the corona electrode can have many other different forms. For example, it may comprise a number of thin threads or filaments arranged either parallel to each other or in the form of an open grid or net. Instead of using straight, thin wires or filaments, the wires may be spirally wound, or thin strips exhibiting straight, serrated, or wavy edge surfaces may be arranged in a similar manner. The corona electrode may also comprise one or more needle-like electrode elements which are directed mainly axially 1 the air flow channel 1. The counter electrode M comprises an electrically conductive or semi-conductive material, or a material coated with an electrically conductive or semi-conductive surface, and provided with surfaces that will not give give rise to a strong concentration of electric fields. The counter electrode can also be constructed in a variety of known ways, depending in part on the construction of the corona electrode. In the embodiment in Figure 2, the counter electrode M is shown as an example to comprise two mutually parallel plates arranged in the direction of the air flow channel. In the case of a needle-shaped corona electrode, the counter electrode preferably has the shape of a cylinder arranged coaxially with the air flow channel. An electrically conductive surface coating on the inside of the air flow channel 1 can also serve as a counter electrode. The counter electrode can also comprise a number of planar or cylindrical electrode elements arranged side by side, with their side surfaces mainly parallel to the longitudinal axis of the air flow channel 1. The counter electrode can also comprise straight or spirally wound wires, or straight rods which can be arranged parallel to each other or so that they cross each other and form a lattice structure, or it can have the form of a perforated disc. However, a particular advantage is achieved when the counter electrode takes the form of an electrically conductive or semi-conductive surface which surrounds the airflow channel in the form of a frame, and which has an extension parallel to the airflow

the direction corresponding to at least one fifth of the distance between the corona electrode and the counter electrode.

The aforementioned examples of embodiments of the corona electrode and the counter electrode can in principle be used in all the embodiments or devices according to the invention described below.

In the arrangement described in Figure 2, the corona electrode K and the counter electrode M are each connected in a conventional manner to a respective pole or terminal of a DC voltage source 3.

In the illustrated example, the corona electrode K is connected to the positive terminal of the voltage source 3, to achieve a positive corona discharge. In principle, however, the polarity of the voltage source 3 can also be opposite, in order to achieve a negative corona discharge. A positive corona discharge is usually preferred since less ozone, which is a toxic gas, is produced with a positive corona discharge than with a negative discharge.

In the arrangement illustrated in Figure 2, the clamp on the voltage source 3 which is connected to the corona electrode K is grounded according to the invention, so that the potential of the corona electrode K mainly coincides with the potential of all other electrically inactive parts of the arrangement in question which likewise is grounded, and also with the potential of the device's immediate surroundings. The potential of the corona electrode K will in this way be the same as the potential of the ambient conditions located upstream of the corona electrode K, with any electrically conductive objects or surfaces placed in said surroundings, and thus no unwanted ion current will be obtained from the corona electrode K in a direction upstream from this.

As mentioned above, the axial distance between the corona electrode K and the part of the counter electrode M that receives the largest part of the ion current is at least 50 mm and preferably at least 80 mm, whereby air can be transported through the air flow channel with a passage of, for example 100 m<3>/h with the help of a low corona current of the order of 20 - 50 microamperes, which is an acceptable value with regard to the production of ozone and nitrogen oxides. As previously mentioned, an advantage is further obtained when the counter electrode M is connected to the direct voltage source 3 through a large limiting resistor 8, which, in the event of a short circuit caused by contact with the counter electrode M, limits the short circuit current to a value of about 300 microamps at the most. Since the counter electrode M, as a result of its construction, does not have a negligible capacitance, it may conveniently be made of a high resistivity material. A suitable material in this connection which has a high resistivity and which at the same time has the necessary ability to conduct electricity, is a plastic material which includes a finely divided electrically conductive material such as, for example, carbon black. Known materials of this kind from which the counter electrode can be made have a surface resistivity of the order of 110 kf? and more.

It will be understood from the foregoing that a device constructed in accordance with the invention, for example in the manner illustrated in Figure 2, is quite safe to touch, and thus it is not necessary to take any other safety precautions or to provide any form of safety device to prevent intentional or accidental contact with either the corona electrode K or the counter electrode M. Moreover, since the corona electrode K is grounded, there is no risk of ion current flowing through places other than the counter electrode. Taken as a whole, this surprisingly actually enables the construction of an air transport device according to the invention without any form of air flow channel 1 at all, at least when the primary purpose of the device is to move air in that space or area where the device is installed. A device constructed according to the invention can, for example, have the extremely simple form shown in Figure 3. This embodiment of the device according to the invention comprises a corona electrode K in the form of a wire which is stretched between holding devices (only shown schematically) carried by suitable frame devices ( not shown in detail) and a counter electrode M which is separated from the corona electrode K and which is also carried by the aforementioned frame device. The counter electrode M can comprise two mutually parallel, electrically conductive surfaces, which also lie parallel to the corona electrode K. Alternatively, the counter electrode M can comprise a rectangular or circular frame-like electrode surface whose axial extent coincides with the desired air flow direction 2, as illustrated in the figure, as this embodiment of the counter electrode is the preferred one. It will be seen that in this embodiment there is no air flow channel at all surrounding the two electrodes K and M. As with the embodiment of Figure 2, the corona electrode K is connected to ground and to a terminal on the DC voltage source 3, while the counter electrode M is connected to the other terminal of the source 3 through a large ohmic resistance effective to limit a short circuit current to an acceptable value, in the event of a short circuit produced by contact with the counter electrode M. The counter electrode M is also made of a high resistivity material, for to limit the capacitive discharge current in contact with the counter electrode. Tests carried out with a device constructed in the manner illustrated in Figure 3 showed that the device is capable of transporting air very efficiently in the direction indicated by arrow 2, within the area surrounded by the counter electrode M. The tested device comprised a rectangular, frame-like counter electrode M with a cross-sectional area of 600 x 60 mm and an axial length of 25 mm. The distance between the counter electrode and the corona electrode K

was 100 mm. A voltage of 25 kV was applied to the counter electrode M, and the corona current was 30 microamps. The direct voltage source 3 had a clamping voltage of 29 kV, and the series resistor 8 had a resistance of 132 Mfl. This extremely simple arrangement resulted in an air flow of 60 m<3>/h through the area enclosed by the counter electrode M. When the counter electrode in this arrangement was short-circuited, the short-circuit current was only about 220 microamps, i.e. a current that can hardly be felt if a person were to come into contact with the counter electrode M. The device is thus completely safe to touch, provided that the relevant voltage source 3 itself is electrically safe to touch.

As mentioned, there can be many cases where it is not desirable for the corona electrode to be connected to ground potential. In such cases, the necessary shielding of the corona electrode according to the invention can be achieved with an arrangement of the kind illustrated schematically and as an example in figure 4. In this arrangement, the negative terminal of the direct voltage source 3 and thus also the counter electrode M is connected to ground, while the corona electrode K is connected to the positive terminal through a large resistance effective to limit the short-circuit current to an acceptable value in the event of a short circuit due to contact with the corona electrode K. To prevent ions from migrating upstream from the corona electrode K, it is provided a screen electrode S upstream of the corona electrode and connected to this, so that the screen electrode S and the corona electrode K both have the same potential. The screen electrode S may have one of a number of different shapes, depending on the construction or shape of the corona electrode used. When the corona electrode K comprises a thin, straight wire, the shield electrode can, for example, take the form of a rod or a spiral wire. The screen electrode can also comprise a number of rods or wires arranged parallel to each other or in a grid pattern. The screen electrode S can also be in the form of a net or a lattice-like structure. Alternatively, the screen electrode may comprise electrically conductive surfaces placed near the wall in the air flow channel 1, or on the inner surfaces of the wall. In principle, the screen electrode S is given a geometric shape and position in relation to the corona electrode K, so that the screen electrode S forms an equipotential barrier or surface that is impermeable to ions coming from the corona electrode.

The screen electrode S does not necessarily have to be electrically connected directly to the corona electrode K, but can also be connected to one terminal of a further DC voltage source 4, as illustrated schematically in figure 5, in such a way that the screen electrode S has the same polarity as the corona electrode K in relative to the counter electrode M, and preferably a potential which mainly coincides with the potential of the corona electrode K. The shield electrode S is thereby connected to the voltage source 4 through a large resistance 9 which is effective in limiting the short-circuit current in the event of contact with the shield electrode S.

It will be seen that in the case of an arrangement according to Figure 5, when the shield electrode S has a higher positive potential with respect to the counter electrode M than the corona electrode K, the flow of ions in an upstream direction from the corona electrode K is also effectively prevented. Although the screen electrode S may have a slightly lower positive potential than the corona electrode K, so that a small ion current is able to flow from the corona electrode to the screen electrode S upstream of this, this can be accepted provided that there is only a short distance between the corona electrode K and the screen electrode S, so that the distance the ion current travels over in the upstream direction is very short, and thereby also the so-called current distance.

It will be understood that when the screen electrode S in the embodiment of Figure 4 or 5 has a shape or construction that exhibits a significant capacitance, the electrode is preferably made of a material with a high resistivity in order to limit the capacitive discharge current to an acceptable level in the range of contact with the electrode. This generally applies to all voltage-carrying electrodes including a device constructed in accordance with the invention, when these electrodes have a not insignificant capacitance. However, the corona electrode is usually always designed to have a very small capacitance, so that it is unable to give rise to significant capacitive discharge currents. Another general feature is that all electrodes in a device according to the invention, which are connected to a non-grounded terminal of a direct voltage source, are preferably connected to the source through a resistance of such a size that in the event of a short circuit created by contact with the electrode, the short-circuit current is limited to no more than 300 microamps.

As mentioned above, the necessary shielding of the corona electrode against an unwanted flow of ions in the upstream direction can also be achieved electrostatically, for example in the manner illustrated in figure 6. In this embodiment, the air flow channel 1, the walls of which consist of a dielectric material such as a plastic material extended over a certain considerable distance from the corona electrode K in the upstream direction. When the device is in its operating mode, it is generated on

the walls of the channel 1 an excess of surface charges which generate an effective screen against the ion cloud in the vicinity of the corona electrode K, provided that the channel 1 extends over a sufficient distance from the corona electrode in the upstream direction. This effectively prevents the migration of an ion stream in an upstream direction from the corona electrode K. The efficiency of the screen can be further improved by dividing the upstream air flow channel

for the corona electrode K, in a series of sub-channels by means of elongated partitions, plates or strips 7 made of a dielectric material, as illustrated schematically in Figure 6. To provide an effective screen, the length of the channel located upstream of the corona electrode K should be at least equal to the distance between the corona electrode and the counter electrode M, and preferably at least 1.5 times this distance. The length of the channel which is necessary to provide an effective screen depends on the geometry of the airflow channel 1, and then primarily on its cross-sectional shape, and on whether the electrical partitions 7 are arranged in the channel 1 or not, upstream of the corona electrode 7. Considered in general, it will also be understood that the requirements set for this shielding of the corona electrode will depend on the potential difference between the corona electrode and the grounded surroundings, as a small potential difference will reduce the requirements that must be set for the shield.

When the corona electrode in an air transport device according to the present invention is effectively shielded in one of the aforementioned ways, so that essentially no ions will flow in the upstream direction from the corona electrode, the effective transport of air through the arrangement is mainly determined by the transport force generated by the ion current flowing from the corona electrode K to the counter electrode M, and is proportional to the product of the ion current and the distance between the corona electrode and the counter electrode.

An increase in the distance between the corona electrode K and the counter electrode M, while at the same time maintaining an unchanged ion current between the electrodes, can be achieved by increasing the voltage connected between the two electrodes, from the voltage source 3. According to the invention, it is therefore advantageously applied between the corona electrode and the counter electrode a potential difference of a higher magnitude than has hitherto been common, for example in electrostatic filters or dust precipitators of the kind used in homes. It will be understood that when the potential on the corona electrode is increased in relation to the surroundings, there is an even greater need to shield the corona electrode in the aforementioned manner. However, an increase in voltage is also fraught with an increase in costs, i.a. because of the high-voltage insulation in both the voltage source and the ion wind device, and because of this there is of course an upper limit to how much the voltage can be increased in practice. An advantageous method to reduce these difficulties is to connect the corona and counter electrodes to potentials of opposite polarities in relation to earth.

According to a further development of the invention, however, it has proved possible to significantly increase the distance between the corona electrode K and the counter electrode M, and thus the traveling distance of the ion current, without any decisive reduction in the strength of the ion current between these two electrodes and without it being necessary to increase the voltage level, by arranging a so-called excitation electrode E in the vicinity of the corona electrode K, as illustrated by means of an example in Figure 7. In the embodiment example in Figure 7, this excitation electrode E has the form of a rotationally symmetrical ring E comprising an electrically conductive material, or in the smallest has a partially electrically conductive inner surface, which is arranged coaxially around the corona electrode K, which in this embodiment has the form of a needle electrode. Considering the special shape of the corona electrode K in the illustrated embodiment, the counter electrode M has the shape of a cylinder arranged coaxially in the channel, while the shield electrode S has the shape of a ring arranged coaxially in relation to the corona electrode K and upstream of it. Thus, the excitation electrode E is placed at a shorter axial distance from the corona electrode K than the counter electrode M, and in the illustrated embodiment is connected to the same clamp on the direct voltage source 3 as the counter electrode M, through a high-ohmic resistance 6. The excitation electrode E thus assumes a potential which has the same polarity as the potential of the counter electrode M in relation to the corona electrode K. The potential difference between the excitation electrode E and the corona electrode K, however, becomes smaller than the potential difference between the counter electrode M and the corona electrode K. The excitation electrode E helps to generate a corona discharge and maintain it by the corona electrode K, even when the distance between the corona electrode K and the counter electrode M is increased without increasing the voltage on the voltage source 3 at the same time. Only a minor part of the ion current originating from the corona electrode K will pass to the excitation electrode E, while the main part of this corona flow or current will still pass to the counter electrode M and help to transport air through the arrangement.

The effect produced by the excitation electrode E can be illustrated using the diagram in figure 8, where curve A illustrates the corona current I as a function of the voltage U between the corona electrode and the counter electrode in the absence of an excitation electrode. As will be seen, no corona discharge and thus no corona ion current will take place at all until a given threshold voltage UT is exceeded. When, on the other hand, an excitation electrode is arranged next to the corona electrode, the conditions illustrated by curve B still prevail, namely that a corona ion current is initiated at a much lower voltage with the axial distance between the corona electrode and the counter electrode unchanged. Only part of this corona ion current will flow to the excitation electrode, while the rest passes to the counter electrode.

The excitation electrode together with the counter electrode can also be considered as a two-part counter electrode, one part of which is placed close to the corona electrode seen in the axial direction, and serves as an excitation electrode, while the other part is placed at a considerable axial distance from the corona electrode and serves as a counter electrode for that part of the corona ion current which provides the driving force for the airflow.

Accordingly, an "excitation electrode" can be obtained, for example, in the manner illustrated in figure 9, by extending a part of the counter electrode M axially towards the corona electrode K, up to the vicinity of the electrode or even past it; in that the counter electrode M in this embodiment comprises a number of mutually parallel plates which extend axially in the channel 1. In this case, the parts of the counter electrode M which are located axially closest to the corona electrode K function as an excitation electrode, even though the main part of the corona ion current will flow to the part of the counter electrode which is farthest away from the corona electrode in the axial direction, to generate the desired ion wind. When the excitation electrode E is combined with the counter electrode M in this way, by extending the counter electrode M axially to a location near the corona electrode, the counter electrode may advantageously comprise a high resistive material or a high resistive surface coating applied to the inner surface of a tube of insulating material , the distal end of the counter electrode M in relation to the corona electrode K being connected to a clamp on the DC voltage source 3. The part of the counter electrode which is closest to the corona electrode K in the axial direction will thereby serve as an excitation electrode E, which receives only a smaller part of the corona ion current. Alternatively, a combined target and excitation electrode can be obtained by providing the counter electrode M with parts which extend axially towards the corona electrode K and up to its vicinity and which exhibit a much smaller electrically conductive area than the main part of the counter electrode M if located further away from the corona electrode K and is connected to a terminal on the DC voltage source. The parts of the counter electrode with small conductive areas located axially in the vicinity of the corona electrode K will thus serve as an excitation electrode, to which only a smaller part of the total corona ion current originating from the corona electrode K will pass.

The excitation electrode can be shaped and arranged in many different ways. Any electrode form placed in the axial proximity of the corona electrode K which does not itself produce a corona discharge and which is connected to one terminal of a DC voltage source, the other terminal being connected to the corona electrode, is capable of serving as an excitation electrode if only a minor part of the total corona ion current flows to this excitation electrode while the largest part of the corona ion current flows to the counter electrode. A screen electrode which is placed upstream of the corona electrode and arranged to receive a given, small ion current, such as for example in accordance with the embodiment of Figure 5, is thus able to function as an excitation electrode.

The geometric shape of the excitation electrode E can also vary depending on the construction of the corona electrode K. When, for example, the corona electrode comprises a number of geometrically separated but electrically connected electrode elements, for example straight, thin wires arranged side by side, the excitation electrode can advantageously also comprise a series of geometrically separated but electrically connected electrode elements, which are then arranged between the electrode elements of the corona electrodes so that they are shielded from each other, which in connection with such a corona electrode is advantageous for generating the corona ion current.

Figure 10 illustrates schematically and as an example a device according to the invention which includes a corona electrode K, a counter electrode M, a screen electrode S and an excitation electrode E. In this embodiment, each electrode comprises a number of geometrically separated but electrically connected electrode elements, as in in the case of the corona electrode K comprises the straight, thin wire made of tungsten for example, while the other electrodes comprise spirally formed wires of stainless steel for example.

Since a device according to the invention can be easily constructed, which is clear from the foregoing, so that all the electrodes are safe to touch, it will be understood that the illustrated embodiments, for example in figures 4, 5, 7, 9 and 10, where the counter electrode M is grounded and the corona electrode K and the shield electrode and also preferably the excitation electrode E are connected to a higher potential, may also be constructed to exclude an air flow channel surrounding the electrodes, provided that the shield electrode is constructed in such a way as to ensure that it effectively will prevent the ion current originating from the corona electrode from flowing in any direction other than towards the counter electrode.

Although a device according to the invention is able to function quite satisfactorily in the absence of any form of air flow channel around the electrodes in the device, the provision of such a channel may be desirable in certain cases, for example for psychological reasons or because such a channel will lead the air through the device in a more orderly manner. The provision of such a channel may also be unavoidable in certain cases, for example when the device is to be placed inside a ventilation channel in a ventilation system, or in other cases where the air flow generated by the device is to be directed from and to special places. However, the presence of such an air flow channel which encloses the electrodes in the device and whose walls quite naturally consist of an electrically insulating material gives rise to troublesome problems. As discussed above in connection with Figure 6, an excess of electric surface charges occurs on the inner surfaces of the wall in such a channel. A similar excess of surface charges will naturally also occur on the part of the channel wall located between the corona electrode and the counter electrode, and will affect the desired ion current flowing from the corona electrode downstream towards the counter electrode, in a way that tends to limit the ion current to the central area of the cross-sectional area of the air duct, which results in an uneven distribution of the air flow across the width of the duct, thereby adversely affecting air transport through the duct. This problem is greatly aggravated by variations in the voltage applied to the corona electrode and the counter electrode through the aforementioned voltage source. A temporary increase in the voltage will result in an increase in the mentioned surface charges, as these charges persist even when the voltage is lowered significantly, thereby causing a strong reduction in the corona current and thereby in the air transport through the device. The disadvantages created by this phenomenon can be overcome, or at least greatly reduced, by stabilizing the voltage supplied by the voltage source, as this solution is not of any particular interest from other points of view in an arrangement of the type in question, or by briefly turning off the voltage to the electrodes at evenly spaced time intervals. The excess charges that are present on the inner surfaces of the channel wall disappear rather quickly when the voltage supply is disrupted and the electric field thereby removed. However, the presence of excess electrical charges on the inner surfaces of the electrically insulating channel wall gives rise to a further, highly surprising and serious problem. It has been found that when the inner surface of the insulating channel wall is touched, even briefly, the corona current will cease completely, and is not automatically stored, even after the end of a very long period of time from the moment the surface was touched. A solution to this problem must obviously be found.

A possible solution to this problem is to apply an electrically conductive layer to the outer surface of the insulating wall of the channel and ground this layer. However, this would give a high capacitance to a counter electrode arranged close to the channel wall, or placed directly on the inner surface of the wall, which, as mentioned above, is undesirable with regard to the contact safety of the counter electrode. However, it has been found that it is possible to avoid this by increasing the cross-sectional dimensions of the air flow channel to a size significantly larger than the corresponding dimensions of the area enclosed by the counter electrode, so that the counter electrode is placed at a considerable distance from the inner surface of the air flow channel. Such an embodiment is illustrated schematically in Figure 11. In this embodiment, the outer surface of the insulating wall in the channel 1 is provided with an electrically conductive layer 10, which is grounded. The channel 1 in this embodiment is also considerably further than the counter electrode M, so that the channel walls are further away from the counter electrode, which thereby achieves a much lower capacitance. In this way, the channel walls have also been placed further away from the corona electrode K, and thus the excess charges that appear on the inner wall of the insulating channel wall have a much less disruptive effect on the corona current that flows from the corona electrode K to the counter electrode M. This increase in the cross-sectional dimensions of the air flow channel 1 in relation to the cross-sectional dimensions of the counter electrode M, have been shown to have no adverse effect on the air transport through the device, but that this transport is actually increased by an unchanged corona current. In the embodiment illustrated in figure 11, the center point of the direct voltage source 3 is grounded, so that the counter electrode M and the corona electrode K have opposite polarities in relation to ground, which limits the total high voltage level required and thereby the necessity of isolating the device against high voltages, and which also reduces the requirements for shielding the corona electrode K as mentioned above. Since in this case a high voltage is applied to the screen electrode, the corona electrode and the counter electrode, all these electrodes are connected to the DC voltage source through a large resistance 8 which is effective in limiting the short circuit current in case of contact with the electrodes. Moreover, both the counter electrode M and the shield electrode 7 are conveniently made of a high resistivity material to limit the capacitive discharge current in the event of contact.

In an embodiment of this kind, an advantage is obtained when the cross-sectional dimensions of the air flow channel 1 are adapted so that the distance between the channel wall and the corona electrode K is approximately equal to half the distance between the corona electrode and the counter electrode, and so that the distance between the channel wall and the surface of the counter electrode is approximately 50% of the cross-sectional dimension of the counter electrode opening.

The aforementioned adverse effects caused by the presence of excess charges on the inner surface of the channel wall can also be reduced by means of an excitation electrode which has the function described above, this excitation electrode comprising an electrically conductive layer applied to the inner surface of the channel wall. As will be appreciated, no excess charges are able to appear on the inner surface of the channel wall in the presence of such an excitation electrode. If the cross-sectional dimensions of the air flow channel in this regard are increased to such an extent that the counter electrode is placed at a considerable distance from the channel wall, as illustrated in Figure 11 and described above, the excitation electrode mounted on the inner surface of the channel wall can very surprisingly be extended in the downstream direction to a location past the counter electrode. In this particular case, in fact, an electrically conductive layer can be provided on the inner surface of the channel wall over the entire length of the channel, i.e. also in the upstream direction to a location outside the corona electrode. Such an embodiment is illustrated schematically in figure 12. The embodiment illustrated in figure 12 thus comprises an air flow channel 1 whose wall is assumed to consist of an electrically insulating material and whose inner surface is provided with an electrically conductive coating L which is grounded and which functions as an excitation electrode in the vicinity of the corona electrode K. The cross-sectional dimensions of the channel 1 are such that a counter electrode M with a frame-like shape and which extends parallel to the walls of the channel 1 is located at a considerable distance from the inner surface of the channel wall, and thus is well insulated from the electrically conductive coating on the inner surface of the channel wall. Placed upstream of the corona electrode K, is a number of screen electrodes S, for example in the form of coarse rods. The DC voltage source is grounded at its midpoint, so that the corona electrode K and the counter electrode M have opposite polarities with respect to ground, which provides the advantages described above. The electrodes are also connected to the direct voltage source through large resistors 8 to limit the short-circuit current. It will be seen that no excess charges at all can appear on the inner surface of the channel wall in an embodiment such as this, and thus the device is not affected by the problems that arise in the presence of such excess charges. This embodiment of a device according to the invention has also been found to transport air in an extremely satisfactory manner. The conditions mentioned above with reference to figure 11 also apply with regard to the dimensioning of the air flow channel 1 in the embodiment of figure 12.

It will be understood that since it is possible with an arrangement such as that in figure 12 to provide the inner surface of the channel wall with an electrically conductive, earthed coating along the entire length of the channel, there is nothing to prevent the channel wall from consisting entirely of a electrically conductive material, which would of course greatly facilitate manufacture, and also provide other valuable advantages. It is thus possible that the inner surface of the duct can be lined, at least along a given part of its length, with a chemically adsorbing or absorbing material, for example a carbon filter, which is effective in removing gaseous pollutants from the air, such as odors and nitrogen oxides generated by the corona discharge, by absorption or adsorption. For the same purpose, it is also possible to send a thin liquid film, for example water or a chemically active liquid, along the inner surface of the air flow channel. The wall of the air flow channel may also be cooled or heated by suitable means, such as circulating water, to cool or heat the transported air. All this is made possible by the fact that the wall of the air flow channel is electrically conductive and grounded. • In the embodiments of the device according to the invention where the electrodes are enclosed in an air flow channel, it has been found advantageous to use a single corona electrode K arranged centrally in the channel, since the greatest possible distance between the channel wall and the corona electrode is achieved in this way, and thereby the least possible disturbance in the function of the corona electrode as a result of the channel wall. Alternatively, however, two corona electrodes placed symmetrically on a respective side of the plane of symmetry of the channel may be used. In this arrangement, each electrode will be affected by only one wall or side of the channel, and both electrodes will work under mutually equal conditions. However, this does not apply when more than two electrodes are installed in the duct. In those embodiments where two corona electrodes are placed symmetrically in the air flow duct, it may be advantageous to also install two counter electrodes side by side in a similar symmetrical relationship, as the counterelectron in this respect they preferably have a common electrically conductive wall.

In the case of an embodiment as illustrated in Figure 12, it will be understood that the electrically conductive and grounded coating or liner L on the inside of the insulating airflow channel 1 does not need to be extended upstream of the corona electrode K,

in which case excess charges which consequently appear on the inner surface of the electrically conductive channel wall upstream of the corona electrode K, will cooperate to establish the necessary shielding of the corona electrode K.

A further problem affecting the overall transport of air through a device of this type occurs when the corona electrode is in the form of a wire extending across the air flow path and is attached at both ends to electrically insulated fasteners. The same problem can also occur with other electrode types that extend over the air flow path. When it comes to this, it has been found that the corona electrode provides much more corona current per unit of length within the central region of the airflow path than at the end portions of the electrode. This appears to be due to a shielding effect created through the electrode attachment devices and through the wall of the channel at both ends of the electrode, when an air flow channel is included in the arrangement. In the case of a low "corona current" a significant part of both ends of the corona electrode may even be "turned off" or cut out. This results in uneven distribution of the ion current and thus uneven distribution of the air flow over the cross-sectional area of the path taken by the air flow. When the arrangement includes an air flow channel surrounding the electrodes, it has been found that when viewed in cross section, the portions of the air flow channel located opposite respective ends of the corona electrode exhibit an air flow moving in a direction opposite to that desired. This phenomenon can greatly impair and even totally eliminate effective air transport through the arrangement. However, this problem can be overcome in accordance with a further development of the invention, by giving the counter electrode and/or the excitation electrode a special shape. An embodiment of a target electrode suitably shaped for this purpose is illustrated schematically and as an example in Figure 13, which shows e n device according to the invention comprising an air flow channel 1, shown in broken lines, with a narrow, elongated, rectangular cross-section. A wire-like corona electrode K extends over the channel 1 between the two short walls. The target electrode M has the form of a conductive layer or coating on the inner surfaces of the channel wall, and in this embodiment is shaped so that, viewed in the axial direction of the channel, it lies closer to the end parts of corona electrode K than the central area of the corona electrode in the transverse direction of the channel. The axial distance between the counter electrode M and the corona electrode K at its central area can be, for example, 60 mm, while the corresponding axial distance from the counter electrode to the oppositely located end parts of the corona electrode is only 40 mm. A counter electrode M of this shape will eliminate the problem discussed above, so that a substantially uniform distribution of the corona current along the entire length of the corona electrode is achieved.

The same result can be achieved when an excitation electrode arranged between the corona electrode K and the counter electrode M is shaped in the manner described above with reference to Figure 13 in relation to the counter electrode. In this case, the counter electrode can either be shaped in the way illustrated in figure 13, or in the normal way, i.e. so that its axial distance from the corona electrode is the same at all points. A similar result can also be achieved by means of excitation electrodes which are only placed in the vicinity of both end parts of the corona electrode. A very important feature, however, is that the counter electrode and/or the excitation electrodes are shaped so that the corona electrode K, which extends over the air flow path, produces essentially the same amount of corona current per length unit over its entire length, i.e. also at the end parts of the corona electrode.

A counter electrode and excitation electrode with the shape described with reference to Figure 12 can advantageously also be used in a device where the electrodes are not enclosed in an air flow channel, since a counter electrode and excitation electrodes with this shape will provide a corona current that is more evenly distributed over the entire length of the electrode.

A device according to the invention and constructed in accordance with the embodiment illustrated in Figure 10 was used in practice for experimental purposes. In this experimental arrangement, the distance between the plane of the screen electrode S and the plane of the corona electrode K was 12 mm, while the distance between the plane of the corona electrode K and the counter electrode M was 85 mm. The mutual distance between the wire-like electrode elements in the corona electrode K was 50 mm, and the electrode element of the excitation electrode E was arranged in the same plane as the electrode elements of the corona electrode K centrally between them. The various electrodes were connected to the voltages indicated in the drawings. The air flow channel 1 measured 35 x 32 cm in cross-section, and a grounded protective grid G was arranged at the inlet of the channel. When this apparatus was placed freely on a table, an air flow velocity in excess of 0.5 m/s was achieved. The total corona current from the corona electrode K was about 50 microamps, of which about 40 microamps passed to the counter electrode M. An air flow velocity of about 0.5 m/s was achieved with a power consumption of 5-6 W/m<2> of the flow channel's area. The power necessary to achieve a suitable air flow rate in a similar device lacking the screen electrode S and the excitation electrode E, but with the same voltage on the corona electrode, was about 100 W/m<2>. In this case, the distance between the corona electrode K and the counter electrode M was about 50 mm, and the distance between the corona electrode K and the protective grid G at the channel inlet was 100 mm. In this embodiment of the apparatus according to the invention, the distance between the protective grid G and the corona electrode K had no noticeable effect on the efficiency of the apparatus.

The transport of air through a device or an apparatus constructed in accordance with the invention can be further increased by arranging a number of electrode groups, where each group comprises a corona electrode, a counter electrode, a shield electrode, and preferably an excitation electrode sequentially in one and the same air flow channel. The arrangement of a screen electrode upstream of each corona electrode in the manner described will effectively prevent the unwanted and harmful ion flow in the upstream direction, such flow being unavoidable in such a cascade arrangement in the absence of a screen electrode.

The device provides an extremely efficient air transport arrangement of relatively simple construction. A device constructed in accordance with the invention is also relatively cheap, and has small dimensions and low weight. Such a device also has a low energy consumption and is absolutely silent during operation.

When an air transport device according to the invention is used in connection with an electrostatic filter device, the counter electrode M in the air transport device can be arranged to simultaneously form parts of the dust deposition surfaces included in the electrostatic filter arrangement to receive the impurities charged by collision with air ions, for example in a condenser separator of a type known per se. When the counter electrode M acts as a deposition surface for impurities carried by the air transported through the device, the counter electrode is preferably constructed in a way that allows it to be easily disassembled for replacement or cleaning, when the electrode becomes overly coated with precipitated contaminants . It will be seen that this can easily be achieved when the device does not include an air flow channel surrounding the electrodes. In such connections, the counter electrode can advantageously take the form of strip material fed from a storage roll or fed through a cleaning device when the part of the strip material used as counter electrode has been soiled by precipitated contaminants.

Claims (26)

1. • Device for transporting air using an electric ion wind, comprising at least one corona electrode (K) and at least one counter electrode (M) which can pass through the air flowing through the device and which is arranged at a distance from and downstream of the corona electrode ( K), placed in the desired flow direction for the air, as well as a direct voltage source (3) whose one pole is connected to the corona electrode (K) and whose other pole is connected to the counter electrode (M), whereby the design of the corona electrode and the voltage between the poles of the voltage source are so that a corona discharge generating air ions occurs at the corona electrode (K), characterized in that it includes shielding means for shielding the corona electrode (K) in the direction upstream from the corona electrode or an electrical connection between the corona electrode (K) and the ground potential, so that the product of any ion current current strength and path of travel in the direction upstream from the corona electrode (K) , is approximately equal to zero or at least significantly less than the product of the current strength of the ion current and the path of travel in the direction downstream from the corona electrode (K), and that the distance between the corona electrode (K) and the part of the counter electrode (M) that receives it downstream -directed ion current, is at least 50 mm and preferably at least 80 mm. 2. Device according to claim 1, characterized in that the shielding means include an electrically conductive shield electrode (S) which is arranged upstream of the corona electrode (K) and with a potential of the same polarity in relation to the counter electrode (M) as the corona electrode's (K) potential. 3. Device according to claim 2, characterized in that the screen electrode (S) is electrically connected to the corona electrode (K). 4. Device according to claim 1, characterized in that the shielding means include an air flow channel (1) which surrounds at least the corona electrode (K), the walls of the air flow channel consisting of a dielectric material and extending upstream from the corona electrode (K) a distance that is at least 1.5 times as long as the distance between the corona electrode (K) and the counter electrode (M). 5. Device according to claim 4, characterized in that the air flow channel (1) upstream of the corona electrode (K) is equipped with intermediate walls (7) of dielectric material, which intermediate walls (7) extend mainly parallel to the longitudinal direction of the channel (1). 6. Device according to claim 1, characterized by an excitation electrode (E) arranged near the corona electrode (K) at a smaller axial distance from this than the counter electrode (M), the excitation electrode (E) being connected to a potential of the same polarity in relation to the corona electrode (K) as the counter electrode's ( M) potential, to contribute to the corona discharge at the corona electrode (K) without giving rise to a corona discharge by itself, and so that a significantly smaller part of the total ion current from the corona electrode (K) goes to the excitation electrode (E) than to the counter electrode (M). 7. Device according to claim 6, characterized in that the potential difference between the excitation electrode (E) and the corona electrode (K) is smaller than between the counter electrode (M) and the corona electrode (K). 8. Device according to claim 7, characterized in that the excitation electrode (E) is connected through a large resistance (6) to the pole of the direct voltage source (3) which is connected to the counter electrode (M). 9. Device according to claim 1, characterized in that the counter electrode (M) extends towards the corona electrode (K) and to the axial proximity of the corona electrode (K), that the electrically conductive material of the counter electrode has a high resistivity, that the aforementioned second pole of the direct voltage source (3) is connected to the part of the counter electrode (M) which lies farthest from the corona electrode (K), whereby said part of the counter electrode receives the predominant part of the downstream-directed ion current from the corona electrode, and that the part of the counter electrode which lies in axial proximity to the corona electrode, serves as an excitation electrode, which contributes to the generation of corona discharge at the corona electrode, but only receives a smaller part of the ion current. 10. Device according to claim 1, characterized in that the counter electrode (M) is designed with electrically conductive parts that extend axially towards the corona electrode (K) and to the axial vicinity of the corona electrode (K), and has a substantially smaller electrical conduction area than the main part of the counter electrode, which main part is located at a substantially axial distance from the corona electrode, whereby said main part is connected to said second pole of the direct voltage source (3) to receive the main part of the downstream-directed ion current from the corona electrode, and said parts which are located in the axial proximity of the corona electrode, serves as the excitation electrode, which contributes to the generation of corona discharge at the corona electrode, but only receives a smaller part of the ion current. 11. Device according to claim 1, characterized in that the counter electrode (M) includes electrically conductive surfaces, which are parallel to the direction of the air flow and enclose the air flow path. 12. Device according to claim 1, characterized in that the electrodes are arranged inside an air flow channel (1), and that the counter electrode (M) includes electrically conductive surfaces on the wall of the air flow channel (1). 13. Device according to claim 1, characterized in that the electrodes are arranged inside an air flow channel (1), and that the counter electrode M includes electrically conductive surfaces that extend parallel to the wall of the air flow channel, and are located at a distance within this, whereby the wall of the air flow channel (1) consists of electrically insulating material and outside it has an electrically conductive surface (10) which is grounded. 14. Device according to claim 1, characterized in that the electrodes are arranged inside an air flow channel (1) with a wall with at least one electrically conductive inside (L), which is grounded, that the counter electrode (M) includes electrically conductive surfaces that are parallel to the wall of the air flow channel (1) and located themselves at a significant distance within this, and that the counter electrode (M) and the corona electrode (K) are connected with potentials of opposite polarities in relation to earth. 15. Device according to claim 14, characterized in that the wall of the air flow channel is electrically conductive in its entirety. 16. Device according to claim 14, characterized in that the air flow channel (1) has a wall of electrically insulating material, which is equipped on the inside with an electrically conductive layer, which extends axially approximately from the corona electrode (K) downstream past the mottle electrode (M). 17. Device according to claim 13, characterized in that the distance between the wall of the air flow channel (1) and the nearest surface of the counter electrode (M) roughly corresponds to 50% of the cross-sectional dimension of the surface surrounded by the counter electrode (M). 18. Device according to claim 14, characterized in that the inside of the air flow channel is at least partially equipped with a layer of chemical absorbent material. 19. Device according to claim 14, characterized in that at least part of the inside of the air flow channel is sprinkled with water or a chemically active liquid. 20. Device according to claim 14, characterized in that it comprises an organ for regulating the temperature of the wall of the air flow channel. 21. Device according to claim 1, characterized in that electrodes that are at a high potential in relation to ground are connected to the direct voltage source (3) through a resistor (8, 9), with such a high resistance value that the short-circuit current when grounding some of these electrodes will be around 300 microamps. 22. Device according to claim 1, characterized in that electrodes that are at a potential that deviates from earth, and that have a significant capacitance, consist of a material with a high resistivity, so that the capacitive discharge current when one of these electrodes is touched is limited to an acceptable value . 23. Device according to claim 1, characterized in that the corona electrode (K) and the counter electrode (M) are connected with potentials of opposite polarities in relation to earth. 24. Device according to claim 1, characterized in that the corona electrode (K) extends across the air flow path (1), that the counter electrode (M) includes an electrically conductive surface which is parallel to the air flow path or encloses it, and that the axial distance between the corona electrode and the nearest edge of the counter electrode ( M) conductive surface, is shorter directly opposite the end parts of the corona electrode (K) than directly opposite its middle part. 25. Device according to claim 6, characterized in that the corona electrode (K) extends across the air flow path (1), that the excitation electrode (E) includes an electrically conductive surface which is parallel to the air flow path and encloses this, and that the axial distance between the corona electrode (1) and the nearest edge of the conducting surface of the excitation electrode (E) is shorter directly opposite the end parts of the corona electrode (K) than directly opposite its middle part. 26. Device according to claim 6, characterized in that the corona electrode (K) extends across the air flow path (1), that the excitation electrode (E) includes electrically conductive surfaces that are parallel to the air flow path, and that these electrically conductive surfaces that make up the excitation electrode are mainly located just axially directly opposite the corona electrode ( K) end parts.