Fig. 1: CD 2000
The bipolar corona discharge CD 2000 is used in order to neutralize the charge of a dust or liquid aerosol or even to electrically charge it, as required. For this purpose, a current is set on the positive and negative high voltage source so high that it is still fully conducted through the corona discharge generated at the tip in the corona chamber, but no (or few) sparks are generated. This current is programmed via the current limitation of the HV power supplies. The voltage limitation is set slightly higher than is necessary for the current determined, such that this current flows under all circumstances (temperature, condition of the electrode). The positive and the negative high voltage supplies and the CD 2000 are independent of one another and are independently adjustable.
Fig. 2: Functional principle of corona discharge
A dust or liquid aerosol is supplied at the aerosol inlet. The particles in this aerosol have to be discharged. The mixed air inlet is fed with dry pressurized air. The mixed flow air is passed through two ionization chambers. In these chambers the air is enriched with ions from the corona discharge with positive and negative high voltage. The aerosol air stream and the mixed air enriched with ions are mixed in a small mixing chamber. The charges of the particles in the aerosol are neutralized by the ions in the mixed air. This takes a certain time. That means that the complete discharge only takes place in the volume after CD 2000 outlet (e.g in the tubing system or in the upper part of a filter holder). An approximate value for the dwell time is 0.5 seconds. A satisfactory discharging effect was found in configurations that ensure a dwell time of at least 0.2 seconds.
CD 2000 measurement data
Fig. 3: Fractional separation efficiency with and without CD 2000
With a filter medium or filter (exception: Electret filter medium or Electret filter) the fractional separation efficiency is normally measured higher without a discharge unit than with one.
Many standards, e.g. EN 779 for room filters, require the use of a discharge unit.