Glor M., 2016, Determination of Material Resistivity of Fully Assembled Spiral Coiled Tubes by Measurements and Model Calculations, Chemical Engineering Transactions, 48, 343-348.
In spiral coiled tubes electrostatic discharges – so called propagating brush discharges – have been observed during the pneumatic transfer of powders. Such discharges are rather energetic and may ignite most combustible powders. If the material in which the earthed metallic spiral coil is embedded is highly insulating, the charges resulting from friction between the powder particles and the inner tube wall cannot be quickly released to earth. As a consequence a high potential may be built-up on the inner wall giving rise to propagating brush discharges. Based on recent measurements of the charging current density and model calculations a safe upper limit of the resistivity of the material, in which the earthed coiled spiral is embedded, has been determined, below which no longer any propagating brush discharges will occur. The resistivity of a material can easily be determined from a sample in form of a cuboid or cylinder. This is however nearly impossible if the resistivity should be determined from a fully assembled spiral coiled tube without dismantling and destroying the whole tube. This paper gives guidance how this can be done by simple measurements and by interpreting the results with computer model calculations.
