Reflections on similarity laws concerning particle transport in electrical precipitators

Abstract

Measurements in three geometrically similar wire-plate precipitators show that in the cases of constant current density at the collecting plates or constant electrical field strength, larger geometries give better grade efficiencies. If a dimensionless voltage (found by dimensional analysis) is held constant, the grade efficiencies are no longer dependent on the absolute precipitator size. Introducing an ‘electrical drift parameter’, a generalized representation of measured and (according to Deutsch) calculated grade efficiencies can be found which turns out to be a scale-up invariant in the case of constant dimensionless voltage. Overall, the results show that with decreasing particle size the measured efficiencies become increasing better than the calculated ones. For particles smaller than 1 μm, there is little difference between the Deutschian and Laminar models. When common models are applied with space averaged values for the electrical field strength (with consideration of the ionic space charge) instead of the simple field strength value, then a qualitatively correct scale-up behaviour emerges with respect to the grade efficiency trends. Two parameters are decisive: the electrical drift parameter, which includes the operational state of the precipitator, and the relative length of the collecting zone.