Bipolar Structure of a Back-Discharge Field in an Electrostatic Precipitator

The current densities of positive and negative ions i₊ and i-₋ in the back-discharge field of an electrostatic precipitator (ESP) are measured with a bipolar current probe. The ratio i₊/i- rises with increasing total current density i (= i₊ + i₋) in the streamer-mode back discharge, but it remains almost constant (10-20 percent) in the glow-mode back discharge. The probe-measured values of the back-discharge-starting current density are slightly higher in pulse charging than in dc charging. In addition, the pulse charging does not cause back discharge at a resistivity below 10¹³ Ω · cm. However, beyond 10¹⁴ Ω · cm the back discharge can hardly be avoided, even by pulse charging. The probe diagnosis of i₊/i₋ in an ESP enables an approximate estimation of the charge-to-mass ratio of dust and the collection performance when i₊/i₋, averaged in space, is used.