Functional principles of electrostatic filters

Overview:
Delbag electrostatic filters separate particle sizes throughout a broad spectrum under 40 µm. The function of these filters is based on the physical principle of the deflection of electrically charged particles in electric fields. Electrostatic filtration provides unipolar charging in an ionization zone of solid and/or liquid particles contained in the medium gas (air) that is removed by suction for filtration. Separation takes place in the electrostatic field of the downstream separation zone, at the plates in this zone.

Ionizers:
The ionizer of the electrostatic-filter cell contains parallel emission electrodes that are installed between earthed plates, and that are connected to a positive high-voltage source (12 V). Part of the electrically free charge carriers (produced by corona discharge) deposit out on dust, vapour, and aerosol particles, and in this manner electrically charge these particles.

Collector:
The collector part is constructed in a manner similar to a plate capacitor. The collector contains parallel-arranged aluminium plates, which are, alternatively, earthed and connected to a high-voltage source (6 kV).

Separation:
The particles charged in the ionizer enter the collector, where they are deflected from the air flow by the coulomb forces produced by the electrical field. They then separate out on the plates. The ionization and the separation zones represent mechanically separated components that are combined to form a unit.


The design of the emission electrodes, the number of the ionization stages, as well as the levels of the ionization voltage and the emission current, are all based on knowledge gained in practical experience and on theoretical planning – in accordance with the respective application cases. In this context, modular design of the system offers the great advantage of allowing later modification to changed operational conditions.

Filter passage velocity: The velocity of passage through an electrostatic filter is directly related to the efficiency of the electrostatic filter. The detention period of the pollutants in the charging zone, the available path of separation, and the surplus of ions are important criteria in the degree of separation .

The higher the velocity, the poorer the degree of separation.


Output of an electrostatic filter:
Test-bench measurements with natural aerosols – with application of laser particle counters that detect sizes beginning at 0.1 µm – have confirmed degrees of separation over 98% for an air impingement velocity of 1.5 m/s.

Under conditions of especially difficult application, tests should first be conducted on the technical measures to be taken and their effectiveness. If required, measurement tests will also be necessary for the output-air conditions, in order to properly design the filtration system.

In order to maintain as much of the incoming pollutants in filterable condition as possible, the temperature of the air flow at the point of operation of the filter should not exceed +30°C. The electrostatic filters themselves can be used in ambient temperatures of up to approx. +60°C.

For more information on electrostatic filters, please consult VDI Directive 3678, Sheet 2. Olaf Bernstorff (Dipl.-Ing.), of GEA Delbag Lufttechnik, played a key role in the preparation of this Directive.