Gas separation: apparatus – Electric field separation apparatus – Including gas flow distribution means
Reexamination Certificate
1999-03-23
2001-06-26
Chiesa, Richard L. (Department: 1724)
Gas separation: apparatus
Electric field separation apparatus
Including gas flow distribution means
C096S079000, C096S096000
Reexamination Certificate
active
06251171
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to air cleaners, and particularly to electrostatic air cleaners.
Various electrostatic air cleaner designs have been proposed. One significant advantage of electrostatic designs is the possibility to reduce the pressure drop across the air cleaner, when compared to conventional mechanical filter air cleaners. A high pressure drop gives rise to the need for a powerful fan in order to provide the desired air flow rate, causing noisy operation of the air cleaner.
Conventional electrostatic air cleaners comprise a charging section for charging particles in the air stream through the filter, and a dust precipitation section. The pressure drop across the air cleaner can be arranged to be near zero. The charging section typically comprises a high voltage ioniser and may be arranged as a series of corona discharge electrodes, in the form of fine wires, sandwiched between ground plates. The conditions required for corona discharge will be known to those skilled in the art. Essentially, a sufficient electric field strength is required to ionise air molecules in the vicinity of the corona discharge electrodes. The corona electrodes rapidly discharge ions of one polarity while ions of the opposite polarity drift along the electric field lines towards the ground plates. Particles entrained in the air stream become charged through collisions with these drifting ions.
An electrostatic air cleaner employing a corona discharge charging section of this type is described in U.S. Pat. No. 5,330,559.
A problem with electrostatic air cleaners of this kind is the cost and complexity of the components, including the voltage source, as a very high voltage can be required to sustain the corona discharge, for example 6 kV to 20 kV, as described in U.S. Pat. No. 5,330,559.
SUMMARY OF THE INVENTION
According to the present invention there is provided an air cleaner for removing particles contained in an air stream directed through the air cleaner, comprising a charging section for charging particles in the air stream and a precipitation section for capture of charged particles, wherein the charging section comprises a first and a second array of substantially parallel wires, each array being disposed in a respective plane substantially perpendicular to the direction of air flow, the wires of the first and second arrays being held at a first potential, and a third array of substantially parallel wires sandwiched between the first and second arrays, the wires of the third array being held at a second potential.
The design of charging section according to the invention requires three wired frames which provides a simple mechanical construction. Preferably the wires of the first and second arrays are earthed, and the wires of the third array are held at a corona discharge voltage. The design of the charging section is independent of the precipitation section design, so that both sections of the air cleaner may be optimised independently. Furthermore, the spacing between earthed wires and the spacing between corona wires can be selected independently to obtain the most favourable corona discharge conditions.
During operation of the filter, the arrays of earth wires in particular become gradually fouled with dust particles. Since the first and second arrays of earth wires are arranged at the periphery of the charging section, surrounding the corona discharge wires, they can easily be manually cleaned. Furthermore, the dust particles travelling through the filter will be charged before they reach the central array of corona discharge wires, and will therefore be repelled from the corona discharge wires. The corona wires are therefore less susceptible to fouling. The earth wires surrounding the corona wires also act as a partial Faraday cage, to minimise any influence of stray environmental electric fields on the corona discharge conditions.
Preferably, the wires of the three arrays are all parallel to each other. The wires of the first and second arrays may be equal in number and aligned with respect to the direction of air flow, and the wires of the third array may be offset from the wires in the first and second arrays with respect of the direction of air flow.
The offset of the wires in the third array (the corona discharge wires) ensures that electric field lines between the corona discharge wires and the ground wires intersect the air stream through the air cleaner. This ensures effective charging of the particles in the air stream. In addition, the charging section of the invention enables the spacing between the corona discharge wires to be selected independently of the spacing between the earth wires.
In a preferred embodiment there are fewer corona discharge wires than earth wires in the first or second arrays. It has been found that by increasing the spacing between the corona discharge wires (with respect to the spacing between earth wires) it is possible to reduce substantially the voltage at which corona discharge takes place. The increased spacing between the corona discharge wires gives rise to increased symmetry of the electric field around the corona wires, with less mutual influence of adjacent corona wires on the electric field pattern. This electric filed symmetry promotes a low corona onset voltage.
The earth wires of the first and second array preferably have diameter greater than 0.2 mm, and the corona wires of the third array preferably have diameter of 0.05 to 0.08 mm. A large thickness of the earth wires ensures mechanical robustness and enables each array of wires to be formed from a solid sheet of metal, by for example an etching process or a mechanical cutting or punching process.
The precipitation section of the air cleaner may comprise a series of alternate earth and high voltage parallel plates, each extending in a plane substantially parallel to the direction of air flow. This arrangement reduces to a minimum the pressure drop across the filter, so that a low power ventilator may be employed for providing air flow through the cleaner.
Alternatively, the precipitation section may comprise mechanical filter material sandwiched between two porous electrically-conducting gauzes, an electric potential difference being applied between the two gauzes to generate an electric field across the filter material. Although this introduces a slightly greater pressure drop, the improved dust filtering efficiency of this type of electrically-enhanced filter material may give rise to significant improvements in the overall performance of the air cleaner.
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Boogaard Arjen
Marra Johannes
Bartlett Ernestine C.
Chiesa Richard L.
U.S. Philips Corporation
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