Electrical similarity concerning particle transport in electrostatic precipitators

Abstract

The particle transport within electrostatic precipitators is chiefly determined by two influencing variables: the average gas velocity in the duct and a parameter which characterizes the electrical state. For this, one usually either specifies the “average” electric field strength E (= U/ s) or the electric current density j. Furthermore, it is known from experiments, that the flow field can also be influenced by the gas ions which comprise the electric current. Comparing precipitators of different absolute size the question raises how the electrical operation settings are to adjust in order to keep the particles transport independent of precipitators' size. The paper presents theoretical aspects of appropriate electrical operational settings together with measured particle size-specific collection efficiencies (grade efficiencies) from geometric-similar electrostatic precipitators. With the aid of a dimensional analysis a dimensionless current density j′ and a dimensionless voltage U′ may be derived. These parameters allow all geometric-similar precipitators to be described by a unified current-voltage characteristic. In addition to the common approach of investigating the particle collection at E = const. or j = conts., measurements were also conducted for U′ = const.. To measure grade efficiencies a particle light-scattering size analyser was used. The results confirm that the dimensionless voltage U′ is obviously the most appropriate parameter to keep particle transport independent of absolute precipitator size. Upon holding E and j constant, the larger precipitator geometries yield more favorable efficiency values. Considering the ionic space charge it can be demonstrated that the physical cause of this phenomena is the spatial electric field distribution. When E and j are held constant, locally higher relative electrical field strengths exist in the wider ducts (corresponding to a higher power consumption). The relative field strength only then remains identical when U ′ = constant, i.e. electric similar operation conditions then exist.