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Title:
DUST CLEANING AND COLLECTING DEVICE BASED ON ELECTROSTATIC PRINCIPLES
Document Type and Number:
WIPO Patent Application WO/2010/109261
Kind Code:
A1
Abstract:
An arrangement (1) for removing undesired particles (3) from a surfaces (2) comprising an airflow generator (4) that generates an airflow that affects the particles (3) with aerodynamic drag forces, the arrangement (1) comprises an charger (5), ionizing the airflow (B) from the airflow generator (4) with one and the same electrical polarity, positive or negative, the ionized airflow charging the particles (3) to the same polarity as the ions and an attractor (6) that comprises an electrode with the opposite electrical polarity to that of the ions generated by the airflow generator (4) and the charger (5), the attractor affecting the electrical charged particles (3) with an electrostatic attraction force that lifts the particles (3) from the surface (2), the lifted particles (3) been floating above the surface (2) and transported essentially parallel to the surface (2) under the influence of aerodynamic drag forces from the airflow (B).

Inventors:
LAOPEAMTHONG SUKHANOS (DE)
LARSEN KRISTIAN PONTOPPIDAN (DK)
Application Number:
PCT/IB2009/005801
Publication Date:
September 30, 2010
Filing Date:
May 30, 2009
Export Citation:
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Assignee:
NITO AS (DK)
LAOPEAMTHONG SUKHANOS (DE)
LARSEN KRISTIAN PONTOPPIDAN (DK)
International Classes:
A47L13/40; B03C3/00; B08B6/00
Foreign References:
US20060187609A12006-08-24
US6932857B12005-08-23
US4976749A1990-12-11
Download PDF:
Claims:
Claims

1 An arrangement (1) for removing undesired particles (3) from a surfaces (2) comprising an airflow generator (4) that generates an airflow that affects the particles (3) with aerodynamic drag forces, characterised in that the arrangement (1) comprises:

an charger (5) that ionizing the airflow (B) from the airflow generator (4) with one and the same electrical polarity, positive or negative, the ionized airflow charging the particles (3) to the same polarity as the ions and

an attractor (6) that comprises an electrode with the opposite electrical polarity to that of the ions generated by the airflow generator (4) and the charger (5), the attractor affecting the electrical charged particles (3) with an electrostatic attraction force that lifts the particles (3) from the surface (2), the lifted particles (3) been floating above the surface (2) and transported essentially parallel to the surface (2) under the influence of aerodynamic drag forces from the airflow (B).

2. An arrangement as claimed in claim 1 , characterised in that the arrangement comprises a high voltage DC generator (85), with one pole, positive or negative, connected to the charger (5) and the opposite pole connected to the attractor (6).

3. An arrangement as claimed in claim 2, characterised in that the high voltage DC generator (85) provides the charger (5) with the voltage in the span between 3.5 - 8.0 kV and the attractor (6) with the voltage in the span between 10 - 60 kV.

4. An arrangement as claimed in any of the claimsi - 3, characterised in that the airflow generator (4) producing an air jet B with the velocity between 30 -60 m/s. 5 An arrangement as claimed in any of the claimsi - 4, characterised in that the airflow generator (4) comprises a slit 42 with an opening width between 0.5 - 1 mm.

6. An arrangement as claimed in any of the claims 1 - 5, characterised in that the charger (5) preferably comprising ionising pins or other means of ionizing an airflow.

7. An arrangement as claimed in any of the claims 1 - 6, characterised in that the arrangement (1 ) comprises a dust collector (7), the dust collector (7) comprises a first movable film (707), which is supplied from a film container 709 and spooled up on a roll (710), the film (707) being be made of a conducting or non-conducting thin material, the film (707) being electrical charged to the same polarity as that of the attractor (6), the film (707) attracting the charged particles (3), the charged particles (3) being trapped between the film layer spooled up on the roll (710).

8. A dust collector (7) as claimed in the claims 7, characterised in that the dust collector (7) comprises means of ionizing the air (706), preferably one or more corona wires, at a distance from the film (707), the means of ionizing the air producing ions for charging the particles (3) of the air flow and establishing a equilibrium state charge on the films(707)

9. A dust collector (7) as claimed in the claims 7 or 8, characterised in that the dust collector (7) comprises a second movable film (707) at a distance from the , first movable film (707), the two films (707) forming an airflow channel 702, so arranged that the means (706) of ionizing the air is placed between the first film (707) and the second film (707).

10. A method for collecting airborne particles (3), the method comprises one or more movable films (707) being be made of a conducting or non-conducting thin material, the method further comprises means of ionizing the air (706) placed at a distance from the film, characterised in that

the film (707) being electrical charged to one and the same electrical polarity, positive or negative,

the means of ionizing the air (706) producing air ions with the opposite electrical charge to that of the film (707),

the air with the particles (3) is flowing over the film (707) and being charged by the air ions,

the particles (3) being electrical attracted to the film (707),

the film (707) being supplied from a film container (709) and spooled up on a roll (710),

the particles (3) attracted to the film (707) being trapped between the film layer spooled up on the roll (710).

Description:
Dust cleaning and collecting device based on electrostatic principles

Technical Field The invention relates to arrangements for removing and collecting undesired particles from air and from surfaces with the use of electrostatic principles.

State of the Art

Indoor air pollution caused by small breathable particles is an emerging problem and is becoming a subject of increasing importance worldwide. Most people spend the main part of their life indoors, in production industries, offices, public institution and private homes. Hence poor indoor air quality can have a significant impact on people's life. Airborne dust particles often contain allergens and microorganism like bacteria, fungi and viruses as well as their toxic products. Inhalation of these biological contaminants may result in lung problems and other illness, like asthmatic and allergies. Even particles that are free from biological contaminants can cause serious health problems. When the particles are smaller than 10 μm in diameter they will enter the respiratory system. Fine particles, from 0.1 to 3 μm, get into the lungs, where they deposit. Certain non-degradable mineral particles, such as quartz, can get stuck in the lungs and accumulate. Eventually they can cause a chronic state of inflammation, the often deadly disease, silicosis, to which there is no cure. Ultrafine particles, below 0.1 μm, get deep into the alveoli, and through the natural protection system of the lungs. Here the particles can damage the fine lung structure or even enter the bloodstream. These particles are suspected to cause large effect inside the body, where they are considered toxic and possible have an effect on the whole body. Generally the toxicity of particles is considered to directly depend of the size. The smaller the particles are, the more toxic they are. An add on effect is that fine and ultrafine particles will not settle by gravitation, but will stay suspended in air for long time duration (from hours to days).

There are certain well-known technologies to extract dust particles from air. HEPA filter systems can be adapted, where the most efficient ones collects up to 99.97%- 99.995% of the particles larger than 0.3 microns. Though effective, they possess problems due to cost, clogging and constant need of change and/or cleaning.

Electrostatic precipitators are another highly efficient system to extract dust particles from air. It works in the way that the particulate laden gases are drawn into one side of the box often using a perforated plate and diffusers to evenly distribute the gas. Inside, high voltage electrodes impart a negative charge to the particles entrained in the gas. These negatively charged particles are then attracted to a collecting metal surface, which is positively charged or grounded. The gas then leaves the box, up to 99.9% cleaner than when it entered. Inside the precipitator, the particles from the continuing flow of dust build up on the collection plates or tubes. At periodic intervals, to keep the efficiency and remove collected particles, the plates are mechanically cleaned by rapping or flushing, causing the particles to fall into the collection hopper, from where it is disposed. The drawbacks with the precipitators are the same as mentioned for HEPA filters together with their high cost.

Dust filters and collectors are only able to clean suspended particles in air - not surface dust. The main problems with mechanical systems relate to clogging and expensive replacements or mechanical cleaning, which creates problems with resuspended dust. Also they pose a risk as an ignition source, which can create dust explosions in high dust concentrated areas.

Another area of dust problems is surface dust. Besides the obvious need of removing indoor dust from floors and furniture, there are many sectors in the production industry where surface dust degrades the quality of the product. This is a well-known problem within electronic, semi-conductor and optical disc manufacturing industries.

In the €166bn semiconductor industry, the problem with dust adhesion exists throughout the production process, and is countered with use of Electrostatic

Discharge (ESD) control equipment and cleanroom technology. Still, estimations say that up to 8% of products are discarded due to dust. A major problem is that as technology leads to smaller feature sizes in semiconductor devices, the size of the killer particle also decreases. Smaller particles are more difficult to remove because relatively stronger binding forces due to Van der Vaals forces and Coloumb forces of static charge on surfaces.

Dust adhesion in general is a problem for many companies, in various industries, like the SME dominated sectors of printing industry, fine mechanical industry, optical lenses, packaging, polymer, glass making and medical device manufactures. Many of these sectors experience a need for higher cleanliness levels among other things because of rapid advancements in various current technologies and the constant trend in miniaturizing of components. This is described normally as precision or critical cleaning. The trend towards higher cleanliness level is reinforced for the SME supply chain, since larger manufacturers are now more inclined to pass cleanliness requirements on parts and assemblies to their SME sub-tier suppliers ( "Precision Cleaning and Verification: A Practical

Guide",www.pfonline.com/articles/0903qf1.html)

An example that can be mentioned is the sub-tier suppliers for the car industry, where e.g. plastic components for some parts are required delivered in perfect condition, but since these components are of low unit price they will not be produced in a cleanroom, so they need extra cleaning before packaging. This is an extra cost, which affect the SME sub-tier supplier's profit and/or competitiveness.

Cleaning of hazardous dust types from surfaces is another challenge which requires both that the cleaning process only produces a minimum of re-suspended particles and that the collected particles are stored and handled in a safe manner. The suspended particles are in many cases more dangerous than particles situated on a surface. When hazardous particles are collected and stored in a confined space, their concentration rises dramatically and hereby also the potential health hazard.

In order to remove surface dust certain technologies are know. Conventional cleaning/removing of dust from surfaces is by the use of a broom, cloth or sponge and maybe with the use of water and detergents. More technical/modern cleaning methods for surface cleaning are described below.

Microfiber cloths - are made from microfibres that become statically charged during use, and thereby attract the dust. It works basically on the same principle as a normal dust cloth; you need to touch the surface you are cleaning, but you can pick up more dust, and clean without the use of water and detergents. Like normal cloth a microfiber cloths must be washed after usage, because it is filled up with dust. So if not used correctly this creates the possibility of decreasing its effectiveness, moving the dust around, and resuspending dust if using it too vigorously. Also manual cleaning does not ensure that all spots are covered. Another problem is that the microfiber cloth will scratch the surface, though the best microfiber cloths today made of even finer fibres have reduced this drastically, it will still be a problem on delicate surfaces. Furthermore, a microfiber cloth is not able to clean (effectively enough) in confined spaces, e.g. notches, cracks, with many small component, etc. Finally, microfiber cloth is not optional when cleaning hazardous dust or continuously cleaning of products in production line.

Vacuum cleaning typically work with the use of an air pump that creates a vacuum to suck up the dust and dirt, which is collected by a filter system for later disposal. The most common types for house cleaning may re-suspend dust that will increase dust concentration to more than 50%, mainly because the finer dust particles are not collected by the filtering system and hence re-suspended through the outlet air. However, the amount of re-suspended dust can be considerable decreased with the use of HEPA filter systems. Cyclone is another dust collecting system for vacuum cleaning, which helps the problem of clogging. Here spinning air creates a centrifugal force, which when the dirt and debris are subjected to this it is thrown out of the air and deposited at the bottom of the cyclone. However, this system is mainly useful for heavy particles, unless it is combined with a higher efficiency filter system. But regardless of the elimination of re-suspension of dust through the outlet air, vacuum cleaning still only suspends dust simply through operation when the intake port head moves over the surface ("Indoor particulate and floor cleaning", CMHC (2003)., "The impact of vacuuming. What helps, what doesn't", Fugler, D. (2004), IAQ Applications, 5(3), 1-3.). Furthermore, vacuum cleaning generally does not remove all dust particles from the surface, and even with thorough cleaning up to 10% are left behind.

The principles behind vacuum suction to remove dust in the production industry are similar to vacuum cleaning, though vacuum cleaners use fibres/brooms (touch surface) to help eliminate the bond between dust and surface, whereas this is often not the case in industrial application. Here a stronger suction power will be used, which however can be difficult to direct (unless very close to object's surface), and is more energy costly and noisy. Vacuum suction is more often used in the industry to remove particles that are already in the air, since it takes extremely strong suction power to remove very fine dust particles from the surface because of the electrostatic attraction forces. Strong suction power forbids the usage for some application area, where the products are light, small or thin (e.g. paper and some part of the plastic industry).

The major problem of dust adhesion is from electrostatic attraction. The Electrostatic Discharge (ESD) Control sector produces many types of applications for removing electrostatic attraction from a product. One often used in the above-mentioned industries is ionizing, which to some extent can eliminate the electrostatic attraction (depending on attraction force, amount of ions provided, surface form, distance to object etc.). But this does not remove the particle from the surface, so a combination with other technologies such as vacuum suction, compressed air blowing, shaking or other is needed. These combinational methods are however not 100% effective, among other things because they create new electrostatic attraction because of friction from moving the dust.

Coulombs law states that the force that acts in two electrically charged bodies is proportional to the product of the module of their charges divided by the square of the distance d between them

0l fl2

-F oe

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Since the force (F) is divided by the squared distance (d 2 ) the forces needed to attract it from a distance would be immense. Since the distance of a molecule a surface would be given by its diameter, even stronger forces would be needed to attract or loosen smaller particles.

There are a number of devices for Electrostatic Discharge (ESD) control, which are focused on the dust problem elimination either by eliminating the electrostatic charge and hence the dust attraction or combined with dust removal methods, typically with the use of strong airflow (blowing or suction) or by touching the surface. A combination is necessary since anti-static solutions only eliminates the extra adhesion of dust from the electrostatic attraction to a surface, and hence not solve problem of dust already attracted or dust attracted due to gravity force.

US 6,076,216 describes an arrangement for cleaning dust from light-collecting surfaces of solar power generators. The idea is to use an alternating electrical field with very broad frequency range of 10 to 1 ,000 Hz and amplitude between 1 ,000 to 30,000 Volts. Dust is loosened and then has to 'fall off the object being cleaned - with the risk of dust re-suspension. This process will not remove the smaller dust particles below 5μm.

SU 698494 describes a method of cleaning the surface of laser mirrors, to remove dust particles, by moving above the surface a film, particularly a fluoropolymer film, carrying a static electrical charge. However, fine dust cannot be cleaned by this manner, and only relatively large particles are removed by attraction to the cleaning film, and the cleaning film must be periodically cleaned of dust.

US 5,634,230 describes an apparatus and for removing microscopic particle contaminants from an object such as a photomask, where the particle contaminants are identified by an inspection device. After positioning of a particle contaminant, a polarised probe is dispatched to the position to remove the particle contaminant using an electrostatic method, and hereafter moves to a cleaning compartment where the particle contaminant is removed from the probe. This is a very time consuming process, for very small amount of dust.

Ionizers ("Preventing Electrostatic Problems in Semiconductor Manufacturing", A. Steinman, Compliance Engineering, 2004) are ion generators that act by generating positive and/or negative air ions, e.g. with the use of a very high electric field (corona ionization). This increases the conductivity of the air and when the ionized air comes in contact with a charged surface, the surface attracts ions of the opposite polarity, which neutralizes the static electricity. There are various types of ionizers, which combine the ESD element with a dust removing mechanism. One example is ion blowers (or Ionized Air Knife Blowers), where the dust is removed with compressed airflow (e.g. 45m/s at 50mm target distance). The problem is that though destroying the electrostatic attraction, dust is not collected, and hence re-suspended into the air. A combination can be made with a vacuum suction device, but despite directing the compressed airflow it is hard to control the dust. Moreover, ion blowers can not deliver enough microamperes of ion current to neutralize fast-moving objects like a film web if the distance is too far, in which case the compressed airflow can not remove the dust. Furthermore, compressed air can create friction and hence increase the electrostatic attraction between dust and surface, neutralizing the ionizing effect. So the mechanical force that the air flow will apply on the particle must be bigger than the electrostatic attractive force ("Static solutions for today's need", Reinhold Rutks, Converter, December 1998, Volume 35, Issue 12). US4,751 ,759 describes an appliance to remove dust from solid objects prior to painting with the use of ionization and with incorporated air nozzle that produce an air jet to sweep over the surface that is cleaned at high velocity. Air suction nozzle and a flat brush is arranged to collect the dust. This is a high-energy consumption device with a limited application range.

Web Cleaners are typically used in the converter industry for paper or film. They can be single or double sided and like for the previous mentioned techniques dust removal is done either by surface touch or with air. The normal principle behind is that a roll is moving across the web (paper, film, foil, etc.) and removes the particles on the web. Different methods are applied, depending on the product needs. Bristles in the form of a brush rotating at a speed of 600 to 800 rpm, have been employed to lightly brush the surface of the web, which has also been combined with vacuum blowers. Another technique is using a roll coated with some kind of tacky surface (polymer), where the dust from the web will stick to the roll, when it moves over the web. Web cleaners with the sole use of airflow are also known, however only for slower moving applications, since breaking through the laminar airflow layer else is problematic. Typical, the used airflow is ionized with positive and negative charges to neutralize surface charges. Problems with web cleaners are that they only work on smooth or fairly smooth surfaces like paper or foil.

In clean-room the humidity, temperature, particle matter, and contamination are precisely controlled within specified parameters. The way to control dust in these environments are to lower the dust concentration level. This can be done by a combination of things through the use of air filters (HEPA and ULPA) and continuous air circulation, air lock and possible air shower, protective clothing, the construction material of the room, control of operating procedures, frequently cleaning, etc. The cleanliness levels of a cleanroom generally begin at 100,000 particles/ft 3 (>0.5μm in size) and extend down to 0.01 particles/ft 3 or less. This can be compared to the 400,000 particles/ft 3 that is the general outdoor atmospheric dust level. The cleanliness level needed depends on the products produced, where low-level cleanrooms are often not sterile (i.e., free of uncontrolled microbes) and more attention is given to airborne dust. But even in the very best cleanrooms you may find particles (e.g. micro fibres from protection garment) that can ruin your products, a problem known from the semiconductor industry. Furthermore, cleanrooms are extremely expensive, so with mass-production of cheap products/components you tend to compromise the need for dust free environments by using some of the earlier mentioned methods. The above mentioned state-of-the-art-technologies mainly focus on removing undesired particles from a surface and often leave the problem to get the particles disposed unsolved. If the particles are just removed, it is likely that they get resuspended to the surface. Anyhow, if the particles are not disposed in a safe way they will be suspended in the ambient air and can infiltrate the lung via respiratory passages and may cause disases. In order to solve this problem an apparatus is described in US 4,976,749. The apparatus takes use of adhesive moving film close to a lower electrode connected to ground. At a distance there is an upper electrode connected to 2OkHz oscillating 3000-4000V. Using a turbulent airflow, the idea is that uncharged particles within the air will move toward the lower plate and is caught by the adhesive film. The efficiency of this arrangement is however questionably as the turbulent airflow may have a dominating effect over the electrostatic effect.

To sum up, the existing technologies have certain limitations and drawbacks.

Microfibre cloths are for manual usage, do not ensure covering of all surface, not for hazardous dust usage without protective clothing, can damage delicate surfaces.

Vacuum cleaners are generally not efficient enough, re-suspend dust, get damaged by silicate dust, though improved with HEPA filters they clogs and need changing.

Air filter dust collecting systems are mainly for cleaning from the outlet, no cleaning/collecting dust from surfaces, expensive, may clog and pose the risk as ignition source for dust explosions.

ESD Control from ionizers do not necessarily eliminate the electrostatic attraction 100%, at least not at cost-effective price, must be combined with airflow/suction to collect dust. For web cleaners the problem is their limited usage, only for smooth surfaces. Cleanroom are too expensive for low price unit products, still fine particles like fabric fibres are possible.

Description of the invention

The main object of the present invention is to achieve an arrangement according to the ingress that makes it possible to remove undesired particles with sizes under 20 μm from a surface without the need of physical contact with the surface.

An other object is to collect the removed particles in a safe way without the risk of re-suspension.

One more object is to catch air carried particles under the size of 20 μm and collect them in a safe way without the risk of re-suspension.

Still another object is to develop a disposable unit for safe deposit of dust particles, allowing emptying the collector without the risk of re-suspension.

These objects are achieved by an arrangement according to the ingress having the characterising clauses mentioned in the claims

The arrangement according to the invention is primarily intended for the removal and collecting fine, - potentially hazardous - particles below 20um on surfaces and suspended in air. Critical side aspects for this technology were to eliminate the re- suspension of particles during the cleaning process, and safely store and handle the collected particles. Thus, the developed technology differs radically from traditional suction and filter solutions.

In a traditional dust cleaning devices, the dust particles are both released from the surface and carried by the same fast airflow. The approach according to the invention is to separate the releasing and the transporting mechanisms. This is achieved by making use of electrostatic effects to release the particles from the surface and use the airflow to a larger extends for the transport. Hereby, with this well controlled airflow the arrangement according to the invention will have a minimum of re-suspended particles. Furthermore, a circulating airflow, reusing the air will reduce suspended particles and reduce the need for expensive filtering solutions for the exhaust air.

The invention has potential applications within a wide range of industries where particles are a problem, technically and health wise. These will include paper manufacturing and processing, printing industries, parts manufacturing, optical parts manufacturing, medical/pharmaceutical industries. Furthermore, there are potential long term applications within traditional office/household cleaning emphasized with the increased focus on indoor air quality and knowledge on fine particle pollution.

Many applications are considerable, like areas within paper industry, parts cleaning (car parts/optical parts), printing industry, cleaning of foils, food industry, cleaning in hazardous environments (Nuclear, nanoparticle, quartz dust) and general air cleaning.

The main invention intended for dust removal from surfaces compromises the following logical units: an airflow generator comprising a charger that produces ions, an attractor comprising an electrode, a dust collector and preferably at least one neutralizer. An air blower preferably accomplishes the airflow. In contrast to conventional well-known surface dry cleaning methods, the technology according to the invention involves charging the dust particles with ions instead of neutralizing them. The electrostatic attraction is overcome thanks to evolving Coulombic force between the charged electrode and the opposite charged dust particles. As a result, the dust particles are lifted from the surface and transported to the dust collector via a controlled airflow.

Tests have shown that an arrangement according to the invention can remove dust particles smaller than 20 μm: on average, 85% of dust particles smaller than 15 μm (10um -15um particles) and over 95% of dust particles bigger than 15 μm. The invention is guided by certain technical principles:

• Well-controlled airflow with low leakage. • Circulating airflow, closed circuit, which further reduce the emitted number of dust particles.

• Release of particles from a surface by use of electrostatic, non-contact effects.

• Collector module, collecting the released particles from of airflow with low airflow resistance.

• A cassette system is presented, for providing safe sealing, handling and disposal options of the collected particles at a low cost.

The invention gives certain advantages:

• Collection of fine particles from surfaces with reduced re-suspension of particles during cleaning.

• Safe collection and handling of toxic particles.

• Reducing problems with re-suspension when handling fine particles.

• Cleaning efficiency for particles below 20um. • Non-contact cleaning.

• Circulating airflow, -no exhaust air, reduced re-suspension of particles.

• Integrated particle filter, self cleaning.

• Suitable for industrial setups, like stationary cleaning devices, but potentially also for handheld systems.

The invention will be described in more detail with reference to the accompanying drawings, the latter being intended to explain and not limit the invention.

Figure 1 is a schematic figure showing some logical parts in the invention.

Figure 2 is an illustration showing the velocity profile of a laminar airflow close to a surface. Figure 3 is an illustration showing the forces acting on a charged particle in an E-field and in airflow.

Figure 4 is a diagram showing the voltage waveform of an AC ioniser.

Figure 5 is a schematic figure showing the essential parts of a handheld arrangement according to the invention, partly in cross-section.

Figure 6 is a schematic figure showing the dust collector.

Figure 7 illustrates essential parts of the dust collectors cassette system seen in perspective.

Figure 8 illustrates in more detail essential parts of a handheld arrangement according to the invention, partly in cross-section.

In figure 1 a surface 2 is contaminated with dust particles 3. These dust particles are, according to DMT-Theory, bond to the surface by two dominating adhesion forces i.e. the Van der Waals (VdW) force and capillary forces. Two main electrical forces acting on a particle are the Coulomb force and the dielectrophoretic force. While the Coulomb force is directly linked to the charge of the particle, the dielectrophoretic force acts on uncharged particles due to polarization. Here the field gradient and the dielectric properties of the particles are determining factors. Particles are attracted to regions of stronger electric field when their permittivity exceeds that of the suspension medium. The main conclusion is as follows: Alternating current electrical field is usually used for transporting micro particles in a system where the distance between the electrode and particles is in a magnitude of few micrometers. Moreover, high magnitude of electric field strength in the order of MV/cm becomes necessary for attracting particles in submicron level. As a result, the use of an electric field alone to release particles from a surface will be expensive in term of unit and energy costs. Therefore, it must be used in a combination with another force to overcome the adhesion forces between dust particles and a surface in an energy- and cost- efficient way. The invention solves this problem by expose the contaminated surface 2 to an ionized airflow at a certain angle to the surface. Under the influence of an air blower air A is pressed into a plenum chamber 41 of an airflow generator 4, here an air knife. The flow is directed to a precise, slotted orifice, here called slit 42. As the primary air jet B exits the thin slit 42, it directs airflow in a perfectly straight line. This creates a uniform sheet of air across the entire length of the air knife. Velocity loss is minimised and force is maximised as the surrounding air is entrained into the primary air stream at a high ratio (up to 40:1). The result is a well-defined sheet of laminar airflow with hard-hitting force and minimal wind shear. A main concern, for the airflow transporting particles is the fact, that the flow speed distribution always approach zero at a non-slip surface according to figure 2. This means that, particles 3 ' released only a few micrometers above the surface, will only experience very slow airflow speeds. This is a main limitation for using low speed laminar flow for carrying the particles. By this reason, low-velocity laminar airflow is not appropriate for transporting dust particles 3 from a surface 2 to the dust collector 7. This is a consequence of the distance between the device and the contaminated surface. Airflow B leaving the air knife 4 has smaller influence on dusts with the increasing height (z) due the lower velocity profile. Blowing angle has also a significant effect on the transportation of dust particles. The smaller the angle between the airflow and the normal to the contaminated surface 2 is, the greater the velocity field is exerted on the contaminated surface. However, the small blowing angles initiate the generation of vortices leading to the suspension of dusts in the air, which must be avoided. Therefore, a jet of higher-velocity airflow is engaged in combination with an appropriate blowing angle for carrying particles 3 from the surface the collector module 7.

The arrangement according to figure 1 further comprises a charger 5, the charger preferably comprising ionising pins connected to a high voltage DC generator, not shown in the figure, ionizes the laminar airflow B. Instead of pins, corona wires could be used or some other means of ionizing an airflow. So the laminar airflow B is used to not only remove particles from the surface and transport them to the dust collector 7, but also to exert charges onto the adhered particles 3. The housing material of the air knife is preferably polytetrafluoroethylene (PTFE) or Teflon® and the ionising pins are preferably made of special alloy selected from the group consisting of stainless steel, platinum, and carbon.

The arrangement according to figure 1 further comprises an attractor 6 comprising an electrode with opposite polarity to that of the particles. The attractor is connected to a high voltage DC generator and so an electrical field is induced. In this electrical field, the charged dust particles 3 are floating above the surface 2. In figure 3 dust particle 3 is charged to a positive polarity. The attractor 6 is connected to the negative pole of a high voltage DC generator, preferred through a capacitively high voltage coupling to eliminate the risk of electrical shocks. The attractor 6 is situated at a certain distance z above the surface 2, here supposed to be horizontal. The released particle 3 experiences a gravity force mg, aerodynamic drag forces F x and an electric force qE y , opposite to mg. When qE y exceeds mg the particle will move towards the attractor 6. As the particle is small, it will have a rather small velocity towards the attractor, relative to the airflow direction. The airflow B from the air knife 4 provides the horizontal force F x to the particle. Hence the particle 3 will be transported towards the dust collector 7 according to figure 1. If the distance to the dust collector 7 is not far the particle 3 will go into the dust collector with the airflow C before it is trapped by the attractor 6.

Simulations and experiments have shown that the slit 42 in the air knife 4 preferably should have an opening width between 0.5 - 1 mm. With suitable pressure to the inlet air A to the air knife a laminar airflow B with the velocity 30 - 60 m/s so can be achieved. It is furthermore preferred that the charger 5, here the ionising pins, is given a positive or negative electrical potential 3.5 - 8.0 kV, preferably by connection to a high voltage DC generator. The electrical potential applied to the attractor 6, the particle-attracting electrode, must be opposite to polarity to that of the charger 5 and high enough in order to lift up particles from the surface for 0.5 to 1.0 mm. Thus, the voltage up to 60 kV is required in accordance with a 2-cm space between the electrode and the surface. This corresponds to a non-linear electric field strength from 3.5 to 10.0 kV/cm depending on the charges of the attaching particles. Preferably, the electrical potential applied to the attractor is in the span between 10- 6O kV.

By this above described arrangement with airflow generator 4, charger 5, attractor 6 and a dust collector 7, a surface 2 can be dry cleaned in an efficient way without any mechanical contact. The arrangement 1 is moved, preferably in the direction E, approximately parallel over the surface, at a distance Z between 1 -2 cm, where the distance is dependent on the system parameters. The relative speed between the arrangement 1 and the surface can be 1 cm/s. In order to protect the adhesion of particles from the ambient air on the cleaned surface, the remaining charges ought to be eliminated. The arrangement according to figure 1 further comprises preferably at least one neutralizer 8, preferably comprising alternate current ionising bars, preferably connected to 7.0 kV AC.

Positive and negative ions are produced by applying a high voltage AC waveform according to figure 4 to the neutralizer 8. Only one emitter is required to produce ions; both positive and negative ions are produced at each emitter.

The main reason to involve a neutralizer 8 is, according to the description above, to avoid adhesion of other particles to the surface been cleaned. But the neutralizer on the left side in figure 1 also helps to neutralising the charged particles, so that they can be effectively charged with the desired ions.

The arrangement 1 may further comprises walls 9 in order to control the air flow so that the air B that goes out from the air knife 4 can be the same air C that later goes into the dust collector 7. Preferably the same air is circulated back to the air knife.

In figure 5 the system for circulating the air is shown schematically. An electrical air blower 10 is pressing the air via the tube 12 into the plenum 41 in the air knife 4. Through the slit 42 a laminar airflow B is directed towards the surface (not shown in this figure) to be cleaned. As described above the airflow B is ionized by the charger 5. Dust particles on the surface are by this arrangement charged and can be attracted by the attractor 6 as described before. The dust particles are floating above the surface and transported to the inlet 701 of the dust collector 7. The dust collector comprises a channel 702. The channel is closed by sidewalls 13 so that the airflow is guided in the direction F. In this channel, 702 the most part of the dust particles are trapped. The details in the function of the dust collector 7 will be described further on. The outlet 703 of the channel 702 is preferably connected to a HEPA filter 704 where the remaining dust particles will be trapped. After the HEPA filter the airflow is guided back to the air blower through the tube 11.

An essential aspect of the invention is the dust collector 7 that is schematically described in figure 6. The dust collector comprises a flat airflow channel 702, having a symmetric set of plate electrodes 705 at the right and left sides in the figure. In the middle a number of corona wires 706, preferably thin metal wires, transverses the airflow perpendicular, which airflow is directed according to the arrow F. The right and left electrodes are covered by a movable film 707, which is supplied from the film container 709, here upper rolls and spooled up on the lower rolls 710. The electrodes 705 are partly surrounding the rolls 709 and 710 in order to keep the film charged at a longer distance. In connection to the upper and lower ends of the electrodes sealing walls 708, made of non-conducting materiel, can be arranged. The rolls are preferably rotated by the means of an electrical motor around axes 709 ' and 710 ' , respectively. It is preferred that there are two separate films 707, one left and one right. The films 707, being charged by the electrodes 705, can be made of a conducting or non-conducting thin material such as polyester, Mylar, polyethylene etc. or other materials. It is possible that the film is supplied from an other type of film container 709 then a roll, e.g. the film could be stored folded. The essential aspect is that the film is spooled up on roll as the particles 3 by this means will be trapped between the film layer spooled up on the roll (710) and hence be sealed in a safe way.

During operation, air is sent from the inlet 701 through the channel 702, and airborne particles 3 of the airflow will be collected on the charged films 707 in front of the electrodes. The collector functions basically as an electrostatic precipitator. Applying a high voltage between the electrodes 705 and the corona wires 706, generates a very strong electric field around the wires 706, eventually creating free flowing ions or electrons in the surrounding air. Thus the air is ionised, a flow of current is passing towards the collector electrodes. Instead of corona wires other known means of ionizing air could be employed. Particles 3 in the air will hit those emitted electrons or ions and accordingly be charged. Hereby, the particles 3 of the airflow will be attracted to the collecting electrodes 705 of opposite polarity. The electrodes 705 preferably have the same polarity as that of the attractor 6. By this, those particles 3 that have kept their charge from the charger 5 will be attracted to the films 707 charged by the collecting electrodes 705, without the need of be recharged by the ion current from the corona wires.

If an electrical conducting film 707 is utilized, the film could be charged by contact to electrodes in the form of edges or rolls with a small contact area. In this case the films do not need to be supported by a large contact area and can be stretched between the upper and lower rolls 709 and 710. An advantage with such an arrangement is that it is easy to build up a strong e-field between the corona wires and the films. Also the friction between the films and the electrodes will be lower. A disadvantage is that electrical conducting films are much more expensive than non- conducting film.

If the film is non-conducting and dielectric, an equilibrium state will be reached whereby a charge is built up on the film 707 surface, resulting from the charges emitted from the corona wires. The level of charge depends on the charge diffusion and charge dissipation processes at the film surface. Hereby the e-field strength and gradient inside the collector channel is reduced, but nevertheless the complete channel 702 will overall be either positive or negatively charged. This effect helps to attract and collect particles 3 to the film 707.

By spooling the film from the film container 709 in the direction H, the collected particles 3 are encapsulated and sealed. The film is moved in the direction G, opposite to the direction F of the airflow. In order to quickly and securely encapsulate and seal the deposited particles 3, the lower roll of film with collected matter is counter rotated, in the direction I, so that the film surface that is carrying the particles 3 is turned towards the rotation axes 710 ' , according to the figure 6. The speed of which the film 707 is moved depends of the amount of particles 3 collected. The film can either be moved continuously or stepwise. Typical speeds are within 1mm to 5cm per minute. The film can be made of non-conducting or conducting material, e.g. polyester with a thickness of (7-15um). The electrodes 705 forming the airflow channel are slightly narrowing giving a trapezoid cross section. The typical distance between the electrodes is 2.0-0.3 cm, and the length about 30 cm. In this way, larger particles, which are easier to collect, will be trapped earlier than smaller particles. When the distance between the corona wires 706 and the electrodes 707 are decreased the strength in the e-field will increase and hence the smaller particles will easier be trapped. For an electrode distance of 1.5cm, the typical applied voltage between the corona wires 706 and the collector electrodes 705 is 3-15kV. Both positive and negative polarity can be used, but it is preferred that the electrodes have the same polarity as that of the attractor 6. The two collector electrodes 705 are however at the same potential, i.e. both positive or both negative. In order to ensure a low friction between the film 707 and the electrodes 705, several methods can be applied, such as roughening the electrode surface, employing rollers at the curved parts of the electrodes or using a powder between the film and the electrode. It can also be possible to reduce the friction by pressing air through a set of small holes in the electrodes 705 and so achieve air lubrication.

It is preferred that the film is moved in the opposite direction of the airflow. Surface treatment of the metal electrode surface can be made to avoid the film to clamp to the electrode. Use of conductive polymer materials for the film is an option, if the highest efficiency of the precipitator is needed. It also is thinkable that the electrodes 705 are made by conductive polymer material.

It is preferred that the collector 7 comprises a cassette system according to figure 7 in order to achieve the following functionality requirements: safe sealing of the particles, disposable, interchangeable, easy operation, sufficient long operation time, end-of film and movement detection and also low operation costs.

A cassette system according to figure 7 is preferably arranged as a set comprising two carrier, one carrier 712 with the unused films and the other 711 with the used film containing the collected particles. In the figure 7, the cassette is not yet inserted into the collector 7. The principal is to have two unused films 707 initially spooled on rolls 709. The rolls 709 are turnably journalled in the carrier 712 via axis 709 ' . The carrier 711 comprises rolls 710 turnably journalled in the carrier 711 via axis 710 ' . The carriers preferably are inserted in a pair and after use, both are replaced. A DC motor preferably drives the rolls 710. Both rolls 710 preferably run with the same speed, ensured by linking gear. The collector 7 may comprises means to stretch the films 707 between the carriers 712 and 711. The films 707 with collected particles on one surface, will in this way be spooled into the rolls 710, which provides an efficient sealing of the particles as long as the roll is firm and the outer shelf defined by the carrier 711 prevents external mechanical impact. By using disposal cassette carriers for the film, safe particle collection and handling of hazardous particles is possible. Finally, the film is completely spooled into carrier 711. This ensures safe encapsulation of the collected particles. It is preferred that the thin plastic film 707 is made of polyethylene or polyester. Length of film is typical 30m. The carriers 711 and 712 can be made of extruded or injection-moulded High density PE, PVC or ABS., similar to existing disposable print toner. It is preferred that carrier 712 comprises an optical film movement sensor, using encoded lines on a gear. It is also preferred that carrier 712 comprises an optical film end-point sensor, sensing encoded lines in the film, when this film has reached a certain end-point (~0.5 m before end of film). After this point, the film will get spooled off, which encapsulates the film safely.

By this arrangement a high percentage of hazardous particles can be safely sealed before entering the HEPA filter 704 where the remaining dust particles will be trapped. This is a great advantage compared with the state of the art where a sole HEPA filter rapidly becomes clogged and need to be changed or cleansed.

The above stated design parameters have been found to be suitable in order to establish a high performance for a collector at that size. In general, higher voltages, smaller wire-to-plate distance, slower air speeds, larger distance, and surface area, can increase the efficiency of the collector.

Different types of films, paper, polymer, metal, conducting polymer etc. could be used.

The capturing of particles may be enhanced by using a sticky surface on the film.

The direction of the movement of the film can be parallel or orthogonal to the airflow. One can imagine many different ways of applying a movable film to the collector electrodes of an electrostatic precipitator. Instead of the current setup with two separate film strips, it can be made using only one film strip going all the way around. The design of the collector electrodes can be made in many different ways. It can be a parallel (uni-distance) or v-shaped (narrowing down) or other varying distance configurations. Using an orthogonal film movement relative to the airflow, will provide a method to perform particle separation. Instead of rolling up the film, an automatic cleaning system for the film is imaginable where the film is cleaned using Mylar blade, like the drum cleaning system in a photocopier.

It is preferred with an arrangement with two electrodes at the same polarity and between them a set of corona wires at the opposite polarity. By this arrangement particles will be charged and more strongly attracted to the collecting electrodes 705 of opposite polarity. It is however also possible with an embodiment comprising two electrodes with opposite charge creating an electric field forcing the charged particles to one of them and only this electrode covered with foil. This imply that the particles all are charged and with the same polarity. Having a large electrode in the airflow at a high voltage will unavoidable collect a lot of dust particles, which is a problem dealing with potentially hazardous particles. However, if there are no corona wires, there will be no charge emitter, just a strong e-field. This will lead to a Coulomb attraction of already charged particles to one of the electrodes, depending on the polarity. Particles not charged will also experience a smaller force in the strong e-field, which is depending of the dielectric properties of the particle. However, the disadvantage with such an embodiment is that the efficiency depends on the particles ability to remain their charge. Those particles that have loosed some or whole of their charge will not be recharged and are likely to not be trapped.

The collector idea is to have a method for collecting particles from an airflow on a movable film by use of electrostatic effects. It is a method to provide automatic cleaning of the electrodes in a electrostatic precipitator. With this method the collected particles are sealed in a safe way in the film roll. This part of the invention has above been described in combination with an arrangement to remove undesired particles from a surface. The collector idea can however also be used alone in order to clean air from air carried particles under the size of 20 μm and collect them in a safe way without a low risk of resuspension, for instance cleaning the indoor air from dangerous particles.

For this application, the system preferably is up scaled with wide rolls of film in order to increase the surface area and lower the air speed. E.g. 1 meter wide, 1.2 meter long and 2cm distance with 0.75m/s giving a flow of 15L/s might be durable. Electrostatic precipitators according to the present state of the art (without the film) are employed for such tasks. With the collector idea according to the invention the efficiency to trap ultrafine particles may not be better than existing systems. Nevertheless, the handling of the collected hazardous particles will be much safer, and there will be an automatic way of keeping the collection surface fresh.

In figure 8 a preferred embodiment of a handheld system 80 is presented in more details. The system comprises a cover 81 , shown in the figure as outer contours. The system further comprises a handle 82, an on/off switch 83, a battery 87, a control unit 86, a high voltage generator 85 and a film motor 84. As described for figure 5, the inlet air A to the airflow generator 4 is pressed by an electrical air blower 10 via the tube 12. The outlet air B from the airflow generator is ionized by a charger 5 placed in the vicinity to the out let of the airflow generator. The air flow B is directed towards the surface (not shown in this figure) to be cleaned. Dust particles on the surface are hence charged and attracted by the attractor 6 for further transport to the inlet 701 of the dust collector 7. In the dust collector the dust particles will be disposed according to the function described above. The outlet of the collector 7 is connected to a HEPA filter 704 where the remaining dust particles will be trapped. After the HEPA filter the airflow is guided back to the air blower through the tube 11. The upper carrier 712 comprises a film movement sensor 713 and a film endpoint sensor 714, both preferably optical. It is preferred that the system 80 also comprises a neutralizer 8, although such is not shown in the figure. The system 80 is controlled by the electronic control unit 86, which can be made based on microcontrollers. The control unit 86 controls the film movement motor 84, the air blower 10 and the high voltage generator 85 for the corona wires 706, the air charger 5, the attractor 6 and the electrodes 705. The control unit receives input from the on/off switch 83, the film movement sensor 713 and the film endpoint sensor 714.

With the above-described units: a charger 5, an attractor 6, a collector 7 and preferably a neutralizer 8, also a stationary cleaning device for industrial application can be achieved. For instance, it is possible to separate the cleaning process into four zones: charging, floating and transporting, collecting and neutralizing. Such a stationary unit could be used in areas like paper industry, parts cleaning (car parts/optical parts), printing industry, cleaning of foils, food industry, cleaning in hazardous environments (nuclear, nanoparticle, quartz dust). An obvious application is for web-cleaning where the method according to the invention differs from the state of the art by charging the particles and using the charge to float the particles in a controlled way above the surface and transport them to the collector.

The invention can differ in many other ways obvious for those skilled in the art, according to the following patent claims.