Corona discharge in a cylindrical triode charger for unipolar diffusion aerosol charging

Abstract

Abstract A cylindrical triode charger for unipolar diffusion charging of aerosol particles was designed, constructed, and evaluated. The corona discharge characteristics were studied in this cylindrical triode charger. For the process the current–voltage characteristics were determined, as were the ion number concentration, the nit product, and the mean charge per particle as a function of particle diameter. The discharge and charging currents, and ion number concentration in the charging zone of the charger increased monotonically with corona voltage. The negative corona had a higher current than the positive corona. At the same corona voltage, the ion number concentration in the discharge zone was larger than the charging current for positive and negative coronas, with values of about 197 and 32 times and 645 and 99 times for the ion-driving voltages of 0 and 310 V, respectively. The average ion penetration for positive and negative coronas was 0.64 and 0.19% and 3.62 and 1.93% for the ion-driving voltages of 0 V and 310 V, respectively. The higher flow rate, shorter residence time, gave a lower Nit product. By calculation 14% of charged particles of 10 nm in diameter were lost to the outer cylinder because of the electrostatic field effect. The charger does not use a sheath of air flow along the walls or the perforated screen opening, it has low diffusion and space charge losses due to the short column charging zone, and is a low complexity and inexpensive system. It worked as well as more sophisticated and expensive commercially available chargers.