Technical Field

The present invention relates to an a method and an apparatus for electrically enhanced air cleaning, especially on diffusion-charging-based particle charging and the use of massive amount of free ions produced in the method and by the apparatus.

Background Art

Using electrical particle charging for improving air cleaning unit efficiency is a well-known technology. US7 513,933 B2, StrionAir, 17.8.2006, describes a method of operating an air filter assembly including: providing motive force to produce an airflow along a flow pathway, corona precharging particles in the air flow to produce precharged particles, polarizing a fibrous filter to act in symbiosis for enhancement of filtration efficiency by action upon the precharged particles and directing air flow across the fibrous filter including the precharged particles for enhanced removal thereof, wherein the step of polarizing the fibrous filter includes creating a potential difference between an insulated electrode at an upstream position with respect to a downstream conductive electrode, the downstream conductive electrode being in physical contact with the fibrous filter media in plurality of locations evenly distributed over the entire surface of the filter media. The publication fails to give information in structural details, throwaway distance, CADR or anything similar.

US 6,364,935B1A, Blueair AB, 6.5.1997, describes a device for cleaning of a gaseous fluid from particles present in said fluid, comprising: a permanently positive or permanently negative high voltage source; a filter spaced apart from said source, said filter comprising a fine filter medium composed of fibers having a fiber diameter of approximately 1 micron or greater, and an average spacing between said fibers of approximately ten times said fiber diameter, said fibers being easily polarized when subjected to an electric charge; means for passing a flow of said fluid past said high voltage source and through said filter; said filter further characterized by an absence of external charging means other than said particles passing through said filter. The invention fails to talk about treatment of free ions downstream the filter and it also fails to talk about the number of free ions compared to the number of particles. US 4,624,685, Butrns & McDonnell Engineering Co, 25.11.1986, describes a process for optimizing the power consumption of electrostatic precipitators communicating with a boiler or the like includes a load indexed signal fed forward to a field power controller to approximate the required power levels. An optical transducer is provided in the boiler stack for monitoring the emissions therefrom and feeds back a signal to the controller proportional to the emission from the stack to trim the power level. The controller incrementally adjusts the field power by comparing the opacity generated signal to a continuously optimized limit in order to thereby optimize the power consumption by lowering and raising the field power in response to changes in the opacity. The invention relates to conventional electrostatic precipitators using field charging to charge particles and does not tell about the use of excess free ions. One should note that in a conventional electrostatic precipitator with collection plates, free ions are effectively collected by such collection plates and free ions hardly exist in the apparatus downwards the particle charging unit.

DE3048979 (Al), Thyssen Industrie, 28.10.1982, describes an electrostatic filter which consumes relatively little power and maintains a minimum purity of outgoing waste gas and smoke from smelting processes. The instantaneous concentration of dust in the clean outgoing gas is measured and the filter's high voltage adjusted so as to keep the concentration roughly constant. The voltage is increased if the dust concentration increases and reduced if the concentration reduces. The filter consists of several filtering stages. The invention describes the use of conventional electrostatic precipitation and a key motive for the invention is to avoid breakdown between the filter electrodes. The publication neither talks about free ions nor the number of free ions compared to the number of particles.

Prior-art has technical problems in either not using diffusion charging or understanding the need for massive ion production and treatment of free ions.

Disclosure of Invention

The object of the present invention is to provide a method and apparatus so as to overcome or at least alleviate at least some of the prior art disadvantages. The objects of the present invention are achieved with an apparatus according to the characterizing portion of claim 1 and a method according to the characteristic portion of claim 6.

The art of collecting charged particles is well known. Conventional electrostatic purifiers apply field charging which is charging by unipolar ions in the presence of a strong electric field. Such electric field is generated in a corona charger by the corona electrode and an opposite electrode which normally is an electrode in another potential than the corona electrode, being situated a few centimetres from it, the electric field being generated essentially perpendicular to the stream of particle-laden air to be purified. The 'another potential' is most commonly a zero electrical potential.

With the current method no collection electrodes are used. Ions are produced to a narrow space between the first electrode and the second electrode, the first electrode being typically a high-voltage corona electrode, like corona needle, multiple corona needles or corona wire, and the second electrode being at zero potential or at some other electrical potential different from the potential of the first electrode. The first and second electrodes are placed essentially perpendicular to the air stream of the particle-laden air to be purified. The speed of the air between the electrodes is such that essentially all particles and least a fraction of the ions are captured by the air stream and transferred to a diffusion chamber placed downstream the gap formed by the charging electrodes. In the diffusion chamber the particles and ions are effectively mixed and particles form an electric charge. The amount of charge per particle depends on the particle size and in diffusion charging the number of elementary units of charging acquired is shown in Table I.

Table I. Number of elementary units of charged acquired for different particle diameters

When an ion collides with a particle, it sticks, and the particle acquires its charge. Particles mixed with unipolar ions become charged by random collisions between the ions and the particles. The process is called diffusion charging. Diffusion charging is more efficient for particle charging than field charging when ultrafine particles are considered.

We have found that for the diffusion charging to be efficient, the number of ions produced by the charging unit, N [1/s] must be at least 10 000 times, more preferably at least 100000 times and most preferably at least 1 000 000 times the number of particles entering the diffusion chamber [1/s], Producing ions at such rate for electrically enhanced particle collection based on diffusion charging of particles, is an essential feature of the present invention.

Free ions which have not been acquired by particles, further flow into a mechanical filter placed downstream the diffusion chamber. It has surprisingly been found that such filter loading by unipolar ions tends to improve the capture of charged particles by the filter. Without sticking to any theory, we assume that when the filter is essentially electrically non-conducting, ions are attached on the filter surface as well as inside the filter. Unipolar ions tend to repeal each other and thus on and in the filter, small, local electrical fields are formed which improve capturing charged particles by the filter.

Some ions may propagate through the filter and enter air outside the space where particles are removed from the particle-laden air. The effect of ions in air to human health is not well understood. Some studies claim that negative ions in the air have positive effects and positive ions in the air have negative effects to the human health. Easier is to understand that free ions in the air attach to the particulate matter in the air and charge it, and such charged particles are then collected to surfaces, like room walls and ceiling, etc. Whilst such particle removal may have a positive effect on the human health, they effectively increase surface soiling by particulate matter, which is a largely unwanted effect. Thus, the method of electrical enhancement presented in the current invention, comprises a method for removing the free ions from the air flow by using an ion trap. Such ion trap is placed downstream the filter in the flow of the particle-laden air. The method may also comprise purification of the air by an activated carbon filter and such activated carbon filter may be placed either downstream or upstream the ion trap. Activated carbon filter effectively removes Ozone (O3) generated in the ion-producing charger.

The ion trap may be based on collecting free ions with the help of an electrical field. In such method an electrical field is produced, either by one or several pairs of opposite electrodes which are placed essentially perpendicular to the air flow. Electrical field forces the free ions to be collected on the electrodes and further zeroed. In the methos such ion trap may be switched to be either on or off, depending on the method user's wish.

The present invention thus describes a method for electrically enhancing air purification. The method uses a ion generator to produce essentially unipolar ions to a space through which particle-laden air stream flows. Unipolar ions are attached at least to the particles passing through the space, and to the fibres of an electrically essentially non-conducting filter placed downstream of the space. Some unipolar ions pass through the filter. Number of ions generated in the ion generator, N, is at least 10000 times the number of particles M passing through the space at time t.

Characteristic for the present invention is that unipolar ions are produced by a corona charger having corona voltage of 10 kV or smaller, preferably maximum 8,5kV, which is considered to be a safe high voltage level at some countries, especially concerning production of ozone (O3) by a corona charger.

When designing an apparatus based on the method of the present invention, the ion current generated by the charger may essentially be calculated by following equations: I _c = M * e * C (3 ) where

Mis particle flow through the space [1/s]; p is mass flow of particles passing through the space [g/s]; p is particle density [g/cm ³ ];

V _p is volume of a single particle [cm ³ ]; d _p is the average diameter of the particles passing through the space [cm];

Ic is the ion current (corona current) generated by the charger (111) [A]; and C is the multiplication coefficient with minimum value of ten thousand (10000).

At the design, the calculated ion current may be compared with the current carried by the charged particles, which may be calculated from Equation (4).

I _c = M * e * N _e (4) where N _e is the average number of elementary charges on particle (see Table I).

For accurate results, one should know the particle size distribution in the particle-laden air. In reality this is seldom the case. A reasonable accuracy can be achieved by assuming all particles to be born in combustion engine with average diameter of lOOnm. Such particles may carry four elementary charges per particle.

Mass flow rate, p, of particles may be calculated from the flow of particle-laden air, V[m ³ /h] with particle concentration CPM [pg/m ³ ] with Equation (5). PM*V /J.g

3600 s (5 )

Example of ion current design

We assume the average particle concentration to be 100 pg/m ³ , which may be a typical case in highly polluted areas in Asia and Europe and assume the average particle diameter to be lOOnm, which gives us the following ion currents for different flows of particle-laden air in the present invention. Ion currents are compared with electrical currents carried by particles in the particle-laden air.

Table II Ion current for apparatus design for different air streams of particle-laden air with average particle concentration of 100 pg/m ³ and 100 nm average particle size and particle density of 2 g/cm ³ , and multiplication coefficient of 100000. Required power is calculated for 8,5 kV high-voltage module.

Brief Description of Drawings

In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached drawings, in which Figure 1 shows a schematic view of mechanics of the apparatus in one embodiment of the present invention;

Figure 2 shows a schematic view of electronics of the apparatus in one embodiment of the present invention;

Figure 3 shows a typical relation between the high voltage applied to a corona charger and the ion current generated by it;

Figure 4shows a schematic view of a charger used in one embodiment of the present invention; and

Figure 5shows a schematic view of an ion trap which is based on removing free ions with the help of an electrical field, used in another embodiment of the present invention.

Modes for Carrying out the Invention

Figure 1 shows a schematic view of mechanics of the apparatus in one embodiment of the present invention. It shows apparatus 101 for electrically enhanced air purification, the apparatus 101 comprising a frame 103 with an intake 105 for particle-laden air with the particle mass flow rate which is related to the particle concentration CPM in the space where the intake 105 of apparatus 101 is, and an outlet 107 for purified in the apparatus 101. A fan 109 passes air from intake 105 to outlet 107. In Figure 1, a charger 111 for generating unipolar ions to the air flow passing through the apparatus 101 is placed downstream, (concerning the air flow generated by the fan 109) the fan 109, but it may also be placed upstream the fan 109. Charger 111 generates unipolar ions for diffusion charging of particles in the particle-laden air entering the diffusion chamber 113, placed downstream the charger 111. In diffusion chamber 113 ions produced by charger 111 and particles in the air, having mass flow rate n, are effectively mixed together. Downstream of the diffusion chamber 113, an electrically essentially non-conducting filter 115 and optionally an activated carbon filter 117, are placed. Apparatus 101 also comprises functional means 125 for controlling ion current escaping from the charger 111 and functional means 123 for controlling rotation speed of the fan 109. The apparatus 101 also comprises functional means 129 for main control of the apparatus 101. Downstream the essentially non-conducting filter 115, the apparatus 101 comprises an ion trap 119 for removing free ions passing through the filter 115. The optional activated carbon filter 117 may be placed upstream or downstream the ion trap 119. At the outlet 107 of apparatus 101 the mass flow rate of particles, //, is essentially zero, or at least 90% smaller than mass flow rate //at the intake 105 of apparatus 101.

In one embodiment of the current invention, the ion trap 119 placed downstream of the filter 115, comprises means 127 with or connected to an electrical current sensor 131 for measuring the current carried by free ions entering the ion trap 119.

It is obvious that the functional means 123, 125 and 129 may also situate in one physical device. In Figure 1 they are drawn separately to clarify the different functions required the apparatus 101 according to the present invention.

In another embodiment of the present invention the means 125 for controlling ion current escaping from the charger 111 comprises means for setting output voltage of a high voltage module 133.

The ion current escaping from a corona charger 111, driven by the high voltage module 133, is highly nonlinear as can be seen from Figure 3. The figure can be used to develop a general equation (6) for designing the required high voltage of the corona charger 111 when the ion current is known. where both ion current I _c and VHV, the high voltage of the high voltage module, is given in arbitrary units [a.u.] and where CI-HV is the correlation coefficient between the 10-base logarithm of the ion current l _c and the high voltage VHV corrected by the offset voltage Voffset. Ion current is essentially zero when voltage is lower than the offset voltage.

In another embodiment of the present invention, the maximum volumetric flow through apparatus 101 is 1500 m ^3/ h and the maximum corona voltage is 8.5 kV. The functional means 123 for controlling the rotation speed of the fan 109 set the fan speed (i.e. volumetric flow of air generated by the fan 109) to 1500, 1200, 900, 600 or 300 m ³ /h. Particle concentration CPM at the intake 105 of apparatus 101 is 100 mg/m3. Using table I one gets the required ion currents Ic and from that one can calculate the set values of the high voltage, VHV-, taking into account that the maximum high voltage is 8.5kV. The results are shown in Table III

Table III Set values for the high voltage Vnvof the high voltage module 133 in one embodiment of the present invention

The control voltage for the fan for 109 different volumetric flows is usually linear, so converting the control voltage (with maximum of e.g. 15 VDC) to the control voltage of the high voltage module 133 requires typically an inverting amplifier comprising offset with another inverting amplifier with gain -1. For simplicity reason both functions are integrated to a single functional amplifier 201 in Figure 2.

Figure 2 shows a schematic view of electronics of the apparatus 101 in one embodiment of the present invention. The apparatus 101 comprises a functional amplifier 201. The input of the functional amplifier 201 is connected to the control signal of the fan 109 provided from the terminal 223 of the functional means 123 for controlling rotation speed of the fan 109. The output of the functional amplifier 201 is connected to the control input of the high voltage module 133 and the output of the high voltage module 133 is connected to the charger 111.

Apparatus 101 may also work in an automatic mode, where the concentration of particles of the space to be purified determines the speed of fan 109 and thus also the control voltage of the high voltage module 133. Apparatus 101 comprises a functional hysteresis amplifier 203 with input 1 connected to one pin in terminal 229 of functional means 129 for controlling the apparatus 101 and input 2 is connected to a particle sensor 205. Apparatus 101 also comprises another functional hysteresis amplifier 207 with input 2 connected to another pin in terminal 229 of functional means 129 for controlling the apparatus 101 and input 1 connected to a particle sensor 205, and outputs of the functional hysteresis amplifier 203 and functional hysteresis amplifier 207 are connected to different pins in terminal 229 of the functional means 129 of controlling the apparatus 101.

The functional amplifiers 203 and 207 described above tell the functional means for controlling the fan speed 123, if the fan speed setting needs to be increased, keep as it is or decreased.

The functional amplifiers described in the description may be realized by using analogue or digital techniques or such functions may be realized by software, The functional means for control, 123, 125, 129 and 127 may be realized separately or they may be separate functions in a single controller. Controllers for both purposes comprise embedded controllers, card computers, computers, programmable logics, and similar devices.

In one embodiment of the present invention, the charger 111 is a corona charger. The electrical schematic of the charger 111 is shown in Figure 4. Charger 111 comprises a frame 405 through which air flow, generated by fan 109 is blown. Two opposite ends of the frame 405 have an electrical insulation part 403, to which part 403 a high-voltage rail 407 is attached. Corona needles 409 are attached to the rail 407, so that they are in electrical contact with it. The high voltage rail 407 and the frame 405 are not in electrical contact with each other.

In the embodiment of Figure 4, air is blown through the charger 111 from two slots, both having an area of A/2 and thus the total area for air flow is A. Air flows through it with speed v ₀ /r.

Ions escaping from the tips of the corona needles 409 move towards the counter electrode, which in the case of the embodiment of Figure 4, is the wall of frame 405, with speed v _/on . Speed vion depends on the distance from the tips of needles 409 to the wall of frame 405 (the high voltage rail 407 and frame 405 are connected to different electrical potentials of high voltage source 133), and on the electrical potential which the high voltage source 133 produces to the high voltage rail 407 and further to corona needles 409. where

Z is the electrical mobility of the ion [m ² /Vs] l/wi/is the value of high voltage [V] d is the distance from the corona tip 409 to the nearest wall of the frame 405.

The required area size A can be designed when we know the width w of the wall of frame 405 which works as the counter electrode for the corona needles 409. Ions escaping from corona tips 409 must fly with the air flow having speed v ₀ /r so that the electrical mobility Z does not drive them to hit the wall of frame 405. Taking the boundary conditions into account, one gets the value for the air flow area A.

And considering the length x of the corona needle 409, one can calculate the maximum length l _max of the rectangular gap having area A/2.

The equations show that the air flow area A and thus the maximum gap length l _max need to be calculate to the maximum volumetric flow through the apparatus 101. For the boundary conditions given in Table III we can thus calculate the maximum gap length to be around 300 mm when the distance d is 25 mm and the length of the corona needle is 5 mm. Mobility of a negative ion is 1.6 x IO ^-4 m ² /Vs.

In another embodiment of the present invention the ion trap 119 is based on collection of ions with the help of an electrical field. A schematic view of such embodiment is shown in Figure 5. The ion trap 119 comprises a frame 503, two opposite ends of the rectangular frame being electrical insulators 505. The structure comprises multiple plates, 505, 509 which are connected to different electrical potentials of a direct-current (DC) voltage source 501. Switch 511, operated by means 513, can be switched to provide a difference in the electrical potential to plates 507 and 509, Alternatively the plates 507, 509 can be connected to the same electrical potential. Thus, ions entering the ion trap 119 are either captured or let pass.

The required voltage V501 to be generated by voltage source 501 can be calculated when we know the maximum volumetric flow through the ion trap 119, length /, _t of the ion trap 119 in the direction of the air flow through the ion trap 119 and distance d _p-p between the plates 507 and 509. with the values of Table III, the minimum voltage between the plates 507 and 509 in ion trap

119 is greater than 35 VDC when /, _t is 10 mm, distance d _p-p is 50 mm and air escape area t of the ion trap 119 is 200 cm ² .

It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.