TWO-STEP AIR FILTER HAVING EFFECTIVE IONISATION

Abstract: A device for air cleaning, preferably from suspended particles, so-called aerosols, including a so-called ion chamber (10; 10'), i.e. a space where charging of aerosols takes place, and a so-called precipitator (12; 12') for separation of charged aerosols from the air by-passing the device. The air transport through the device can be effected with the aid of a fan (26; 26'), by natural draught or in another way. The air first passes through the ion chamber (10; 10') where an ionizing source (14; 14') is located, and then passes between the electrode elements of the precipitator (12; 12'), said elements being provided at a distance from each other and applied to a voltage in such a way that a strong electrostatic field is present between adjacent electrode elements. At least the repelling electrode elements (R) of the precipitator (12; 12') are manufactured of a high-resistive material that preferably is coated with an insulating layer. The ionizing source (14; 14') could be in the shape of a so-called corona electrode that in a suitable way is connected to one terminal of a high voltage source (18; 18') whereby ions of suitable polarity are generated around said corona electrode or generated in another way, e.g. via UV-light. The dimensions and design of the ion chamber (10; 10') as well as the location of the ionizing source (14; 14') inside the ion chamber (10; 10') are such that the migration and propagation of the ions in the ion chamber (10; 10') are mostly controlled partly by the repelling forces in the ion cloud and partly by the movement of the air through the chamber (10; 10').

Description: Two step air filter having effective ionisation The present invention relates to an air cleaning apparatus, especially an apparatus for cleaning of air in habitable rooms, offices or working spaces, but also for cleaning of air in other applications, e.g. the discharge air from a vacuum cleaner, the intake air to passenger compartments of cars as well as duct filters in ventilating systems etc. More specifically the invention relates to an air cleaning apparatus that includes a ionizing device for generating iones, said device being provided in a chamber, and an electrostatic dust separator, i.e. a precipitator, and possibly also complementary electrical equipment, e.g. a high voltage unit, an air transporting device etc. Known air cleaning devices could be in the shape of two step electro filters having the ionizing device in the shape of a corona discharge electrode that is located in a ionization chamber upstream of the precipitator. The walls of the ionization chamber define a well-defined space and inside said space a charge of the particles in the air takes place before the air enters the precipitator part. The efficiency of such air cleaners is among other things dependent on the efficiency in the charge of the particles that are carried by the air stream through the ionization chamber, said particles being separated from the air in the precipitator. Therefore, most elctro filters are fed with a relatively high corona current in relation to the volume of air that per time unit is transported through the precipitator. Normal values for the corona current in such air cleaners designed around a precipitator of sheet metal, often out of aluminum plate, are within the interval of 50 - 150 μA/100 m 3 cleaned air per hour. The recent development, described in SE-B 9200515-6, discloses that if the defined space of the ionization chamber has a large extension in the direction of the air flow, the air borne particles passing through said space on their way to the precipitator, then the dwell time of the particles in said space will be sufficient long to achieve sufficient charge of said particles with an essentially lower corona current, calculated per m 3 transported air through the device per time unit. Practicable the demand of corona current has decreased with a factor 10-20 for the device according to SE-B 9200515-6 compared to corresponding prior art air cleaners. In SE-A 9400110-4 a device for air cleaning in a defined larger air volume is described, e.g. a room. Such a device includes an efficient electrostatic precipitator, i.e. a separator for charged aerosol particles, and an ionizer arranged in such a way that the charge of aerosols takes place in the space, e.g. a room, where the device is located to clean the air. According to this application, through an ionizer sufficient charge of the aerosol particles is achieved in order to make the aerosol particles to adhere on the precipitant surfaces of the precipitator when the air passes through said precipitator. An interesting feature about the device is that the low demand of corona current corresponds to a split of one μA or in the range of one μA and hence there is practically no generation of either ozone or other gaseous pollutions. Another interesting feature is the very simple design. It is known that the generation of the dangerous ozone gas increases almost proportional to the increase of the corona current. There are also concerns that a strong plasma layer around corona electrodes can influence organic substances in the air, said substances can in an uncontrollable way convert into other substances that might affect human beings. It is partly of these reasons important to reduce the corona current in electrostatic air filters. The difference between charging of aerosols in a traditional two step electro filter and charging of aerosols in a well defined larger space, e.g. a room with the aid of an ionizer, seems to be that charging with the aid of an ionizer charges most of the particles in the space with relatively low charge in opposite charging of aerosols in a traditional ionization chamber where certain particles get a high charge while others have not had time to be charged at all. The disadvantages of air cleaners according to SE-A-9200110-4, where the charging takes place in the room, is partly that an albeit small amount of the particles adhere to the walls, the ceiling and other surfaces of the room and partly that the ionizer creates an electrostatic field in the room that is stronger than the electrostacic field that is present in the nature. It is uncertain whether a strong electrostatic field and charged aerosols in the inhalation air have any medical consequences for human beings. The present invention has the aim to create an air cleaning device where the charging process corresponds to the charge of aerosols with the aid of an ionizer. However, the aerosols in the air in the space where the device is located are not charged and the electrostatic field in the space is not made stronger. The air cleaning device according to the characterizing features of the claims includes a ionizing source arranged in such a way that the generated ions to an essential degree terminate their migration within the device. The ionizing source is located in a space, below called ion chamber, that is arranged in such a way that ions generated at the ionizing source almost freely can diffuse away from the source and fill up the entire ion chamber. A surprising finding for the device is that there is no need for an ordinary ionization chamber, i.e. a chamber where the charging of aerosols takes place with the aid of a relatively high corona current that is generated between the corona electrode and one or several target electrodes, or a large ionization chamber, i.e. a defined space, e.g. a room where a very low corona current in combination with a long dwell time give the aerosols a certain charge. In accordance with the present invention it is sufficient to provide the ionizing source in a space in connection to and upstream of the precipitator of the device in such a way that the generated ions are allowed to almost freely diffuse over the entire ionization chamber. The path of migration of the ions is not only controlled by the electrostatic field but also, as is the case when using an ionizer, by the repelling forces between the ions and also by the movement of the air through the chamber. Surprisingly it has been found in laboratry tests that if the ionization section of the device is designed in accordance with the characterizing features of the invention, a corona current of a split of a μA is sufficient to effectively charge the particles in a device for about 400 m 3 air per hour. Such a low demand of corona current is close to 1000 times lower than in corresponding traditional two step electro filters and several tens times lower than in the device disclosed in SE-A-9200515- 6. In a ionization chamber having high corona current between the corona electrode and the target electrode the ion migration between the electrodes is controlled mostly by the field strength between the electrodes, thus the migration path of the ions is close to equal with the distance between the electrodes. Under influence of high electrostatic forces the dwelling time of the iones in the ionization chamber is very short since the ions under such conditions move very rapidly between the electrodes. The likelihood for collision between ions and aerosols in the air and the occured charging of said particles is relatively small. This situation is compensated in traditional electrofilters by increase of the corona current in the ionization chamber, thus generating a larger amount of iones. In a ion chamber provided in accordance with the characterizing features of the invention the electrostatic field in the ion chamber is very weak and in certain embodiments fully negligible compared to the repelling forces in the ion cloud generated by the ionizing source, and the influence of the field strength upon the migration of the ions could also be small compared to the influence generated by the displacement of the air volume through the device. This means that in ionization chambers acccording to the invention the iones move irregulary and in a way that closest remind of the natural movement of the aerosols in the air, i.e. the so called Brownian motion. Thus the migration path of the ions in a device according to the invention is considerably longer than the distance between the ionizing source and the closest precipitant electrode(s) or exitation electrode(s) . Further, their migration velocity through the chamber become essentially lower than in devices with strong electrostatic fields in the ionization chamber. Both these conditions, the longer migration path and the lower velocity, results in a substantially longer dwelling time for the ions in the ionization chamber. Thereby, the likelihood increases for the wanted collisions with the aerosols of the air, this being a circumstance that explains the excellent properties of the device. Most ionization chambers in two step electro filters use positive corona discharge and therefore most filters are designed around extended wire-shaped corona electrodes in the ionization chamber. Positive corona discharge is namely necessary due to the fact that negative corona discharge gives rise to several times higher generation of ozone at a corresponding corona current compared to positive corona discharge. The use of an extended corona electrode is necessary to charge most of the aerosols in the air volume flowing through the device. Due to the low demand of corona current both positive and negative ionizing source can be used in a device according to the invention. The ionizing source can be designed as a point, short wire, loop or in other previously known ways. There are also possibilities to use other known ways to generate ions, e.g. via use of UV-light in the ionization chamber, thus also achieving an deodorizing and bacteria-destroying effect. This way of generating ions often also generates ozone in a proportional amount. Therefore it is important to use the ions generated in the ion chamberas efficient as possible. The device according to the invention will be described below with reference to Figs.1 and 2. As shown schematically in Fig.l the ionization chamber 10 is arranged upstream of the precipitator 12, seen in the direction of air through the device. The ion chamber 10 defines a well- defined space and should preferably be designed with almost symmetrical cross-section, e.g. round or square. A ionizing source 14 is in the disclosed embodiment located closest to the entrance of the ion chamber 10 and is constituted of a point electrode that is suspended by a holder 16, the electrode having its free end pointing in the direction of the air flow, see the arrow 17. Via a very high-ohmic electric circuit (passive resistance elements) the ionizing source 14 is connected to one terminal of a high-voltage source 18. Around and at a distance from the examplified ionizing source 14 a ring-shaped exitation electrode or target electrode 20 is provided, said electrode being electrically connected to the other terminal of the high-voltage source 18 and preferably electrically earthed. The exitation electrode 20 is only one way to provide necessary field concentration around the ionizing source. Of course the exitation electrode 20 can be designed in several different ways. Common for all these designs is that the exitation electrode 20 should be located in the same plane as or upstream of the ionizing source 14, seen in the air flow direction. Another way to exitate the ionizing source 14 is to design the walls 22' of the ion chamber of conductive, semi-conductive or antistatic material 23' and preferably connect these to earth, see Fig.2. Also the entrance portion of the precipitator can contribute to the necessary exitation of the ionizing source. As descibed above there are different ways to design the ion chamber 10;10' with accompanying ionizing source 14;14' in a device according to the invention. Common for these are that strong electrostatic fields in the ion chamber 10;10' should be avoided and if possible guarantee free diffusion of the ion cloud in said chamber 10;10' . It is not necessary that the ionizing source 14;14' is arranged at the entrance of the ion chamber 10;10' but it is possible to locate said ionizing source 14;14' closest to the precipitator 12;12', i.e. at the outlet from the ion chamber 10;10'. In this latter location the ionizing source 14;14' can also be electrically and mechanically connected to the precipitator 12;12', e.g. be designed as a point arranged upon someone or some of the voltage-applied electrodes in the precipitator 12;12' , the free end of the point being directed towards the entrance of the ion chamber 10;10'. In most useful embodiments it is recommendable to use only one ionizing source and this source should preferably be located on the axis of symmetry of the ion chamber 10;10'. In order to secure sufficient charging of aerosols the length of the ion chamber 10;10', i.e. the distance between the entrance and the precipitator 12;12', should not be less than the distance between the ionizing source 14;14' and the closest wall 22;22' in the ion chamber 10;10' but preferably 1-3 times said distance or more. Since the charging of the aerosols in the passing air volume normally is low it is preferable and almost necessary that the precipitator 12;12' is of a design that allows efficient separation. Therefore, it is especially recommendable to use precipitators of the type described in SE- A-9303894-1 and especially SE-A-9403116-8. Such precipitators 12 are characterized by a stabilized voltage between adjacent electrode elements since there is a continous discharge between the electrodes of the precipitator. However, this discharge is limited to splits of a μA per electrode element since the electrodes of the precipitator are of high-resistive materials that often are unexpensive and environment friendly, e.g. based upon cellulose. A further development of precipitators is presented in SE-A-9403116-8 where the repelling electrode elements are coated with an insulating layer in order to further increase the voltage between the electrode elements and also make them moisture-proof and enable the introduction of stabilizing distance elements between the electrode elements. As is shown in Fig.l the entrance of the device is provided in the shape of a grate 24 of electrically insulating material, said grate 24 allowing the air flow to bypass. If the ionizing source is located closest to the precipitator of the device it can be a benefit if the entrance grate 24 is made of an ion leaking material, e.g. earthed sheet metal where the migration of the ions are terminated. If used the exitation electrode can also be provided upstream of the grate 24, seen from the ionizing source 14, said exitation electrode preferably being made out of perforated sheet metal. The entrance of the ion chamber 10' could also be designed in such a way that a plate 25' or disc, with the ionizing source 14' located in its middle, is arranged transverse to the direction of the air flow at a distance from the ends of the walls 22' of the ion chamber 10'. The air enters the ion chamber 10' via the opening 11' between the top plate 25' and the ends of the walls 22' of the ion chamber (Fig.2). Due to the fact that devices according to the invention in its entirety easily can be designed to present no danger when touched there is no requirement for an entrance grate. It is more a question of practical or visual reasons. In SE-A-9400110-4 reference is made in the characterizing portions to the fact that the device also can be used in ventilating ducts, whereby the duct is defining the air volume, a space where the device is located. In such applications also very strong ionizers can be used and also several in a row in the duct. Some dust will adhere to the walls of the duct, this being a disadvantage. This problem is solved by the present invention due to the knowledge that in connection with the normal air flow velocities of up to 3 m/s it is sufficient with a ion chamber having a length almost equal to the distance between the centrally located ionizing device and the wall of the duct, i.e. in ventilating systems with the standard dimension 600 mm x 600 mm it is sufficient to have a 300 mm long ion chamber to charge most of the aerosols in the by¬ passing air volume with a negligible ion current. For other dimensions of the duct, e.g. 300 mm x 600 mm, it is preferable to separate the ventilating duct into two ion chambers, each being 300 mm x 300 mm and each having a separate ionizing source. When using ventilating ducts it is preferable to combine the invention with a precipitator according to SE-A-9403116-8 where the repelling electrode elements of the precipitator, in accordance with the patent claims of said patent application, are coated with an electrically insulating layer. In these applications it is very important that the filter manage to separate also the absolutely smallest aerosols (so called micro particles) and that the separation degree is very high since the air passes the filter only once opposite to an air cleaning apparatus located in a room where the air treatment is repeated when the air circulates through the apparatus. In ventilating systems the air filter is rather often subjected to a high moisture content. Due to the insulating layer on the surface of the electrode element short-circuits or discharges between the electrode elements are prevented notwithstanding the high voltage between the electrode elements. As a consequence of the high degree of separation resulting from the device according to SE-A-9403116-8, said device can be designed with a small extension in the air flow direction and thus it becomes less voluminous. This facilitates the handling in connection with insallation and exchange. Space is also given for a conventional coarse filter in front of the precipitator. A precipitator according to SE-A-9403116-8 makes it possible to design a duct filter as a disposable electrostatic precipitator. Traditional electrostatic precipitators where the electrode elements are made out of aluminum plates have been found to be impractical to handle in ventilating ducts, e.g. in connection with periodical washing of the precipitator that is necessary to remove collected dust or the like. Traditional duct filters of barrier filter type, where the separation of aerosols takes place mechanically in a mesh, give rise to a high pressure drop in the ventilating duct. The pressure drop gives rise to a high energy consumption of the fans that pushes the air through the filter. A device according to the invention where the precipitator is designed in accordance with SE-A- 9403116-8 results i that a high degre of separation also for micro particles can be achieved with substantially lower pressure drop and lower energy consumption. In combination with ionization according to the present invention, the necessary charging of aerosols can take place in a ion chamber with relatively small extension in_ direction of the duct, an efficient two step air filter can be created that accomodates in spaces adapted for traditional mechanical filter cassettes in ventilating systems. As already described above the separation part can preferably be designed out of simple, environment friendly materials. It is of course also preferable to manufacture the ion chamber of the same type of material as the separation part of the device, e.g. of cellulose based materials. The walls of the ion chamber can also be a part of the casing of the separator. Thereby, after contamination the whole interior of the device can be exchanged. Since the ionizing source, partly due to the low demand of corona current, has a very simple design low-quality isolation materials, e.g. cellulose plastic, can be used for the securing of the ionizing source. Since it also is simple to electrically connect the ionizing source to the high-voltage source, the entire device (i.e. precipitator, ion chamber and ionizing device) can be designed as a disposable product, i.e after contamination the cleaning part of the device (except the high-voltage source) is replaced by a new one and the consumed part is burnt or recycled.

Claims: Claims 1. Air cleaning apparatus, especially for cleaning of aerosols from the air, said apparatus including at least one ion chamber (10;10') , preferably with one in said chamber provided ionizing device (14;14'), an electrostatic dust separator (12;12') having electrodes that alternately and with different polarity are applied to a voltage in a suitable way, both the ion chamber (10;10') and the separator part allow by-pass of the air, said by-pass can be effected through a fan (26;26'), through natural draught or via a so called ion wind, characterized in that the ionizing device (14;14') is arranged in such a way that the ions generated at ionizing device (14;14') can substantially freely diffuse away from the ionizing device (14;14'), preferably inside the ion chamber (10;10') . 2. Air cleaning apparatus according to claim 1, characterized in that the ionizing device (14;14') is of electrostatic type and includes preferably a so called corona electrode connected to either positive or negative terminal of a high-voltage source (18;18'), the extension of the electrode in a direction perpendicular to the air flow through the ion chamber (10;10') is negligible compared to the extension of the ion chamber (10;10') in the same direction. 3. Device according to claim 2, characterized in that the ionizing electrode (14;14') is designed as a point, short wire, loop or the like. 4. Device according to claim 2 or 3, characterized in that the ionizing electrode (14;14') is located substantially on the axis of symmetry of the ion chamber (10;10'), said axis coinciding with the air flow direction through the device. 5. Device according to any of claims 1-4, characterized in that the ionizing device (14;14') is located closest to the entrance of the ion chamber (10;10'). 6. Device according to claim 5, characterized in that a so called exitation electrode (20) is provided. 7. Device according to any of claims 1-6, characterized in that the walls of the ion chamber (10;10') can leak a low electrical charge. 8. Device according to claim 1, characterized in that the ionizing device (14;14') generates ions with the aid of UV-light. 9. Device according to any of the claims, characterized in that the cross-section of the ion chamber (10;10'), perpendicular to the air flow direction through the ion chamber, is preferably symmetrical, round or square. 10. Device according to any of the claims, characterized in that the length of the ion chamber (10;10') is larger than the distance between the ionizing device (14;14') and the closest wall (22;22') in the ion chamber (lθ;10'), preferably larger than 1 - 3 times said distance. 11. Device according to any of claims 1-10, characterized in that the device according to the invention includes more than one ion chamber with accompanying ionizing device. 12. Device according to any of the previous claims, characterized in that at least the repelling electrode elements of the precipitator (12;12') are manufactured of or coated with high-resistive material. 13. Device according to claim 12, characterized in that said material is a high-resistive fibrous material, e.g. board, paper or carton. 14. Device according to claim 12 or 13, characterized in that the repelling electrode elements entirely or to a great extent are coated with an electrically insulating layer. 15. Device according to claim 14, characterized in that on the repelling electrode elements and/or the attracting electrode elements bulgings are provided that constitute distance elements between the electrode elements. 16. Device according to claim 14, characterized in that in the space between adjacent electrode elements a distance structure or distance element of electrically insulating material is provided. 17. Device according to any of the previous claims, characterized in that the walls (20;20') of the ion chamber (10;10') are manufactured out of paper, board or similar material and preferably electrically connected to an earthed terminal of the high-voltage source (18'). 18. Device according to claim 17, characterized in that the walls (22;22') of the ion chamber (10;10') constitute a prolongation of the outer casing of the precipitator (12;12') . 19. Device according to claim 18, characterized in that the holder (16;16') of the ionizing source (14;14') is designed in a simple, environment friendly, isolating material of cellulose plastic type.

Inventors: ANDRZEJ LORETH, ANDRZEJ, LORETH
Applicants: ANDRZEJ LORETH, STRAINER LPB AKTIEBOLAG, ANDRZEJ, LORETH
Application year: 1995
Main classification: B03C3
IPC codes: B03C3