Measurement of the mass flow rate of airborne powdered material in a duct

Abstract

A description is given of two instruments developed to provide, in combination, absolute on-stream measurement of the mass flow of powdered materials. The range of application is typically to duct diameters from 01 to 06 m, with powders of top size not exceeding 500 μm and mass flows from about 1 to 20 kg s−1. The instruments described are an ultrasonic velocity meter and a density meter relying on β-ray attenuation. The velocity meter measures the carrier air velocity, but it has been shown theoretically that for a wide range of conditions the mean slip between air and particles is small and calculable, so that measurement of the air velocity yields the solid velocity. The β-ray absorption gauge achieves a mean value of the density across a diameter by incorporation of a collimation system that obviates errors that would otherwise arise owing to non-uniform flow distribution across the pipe. The combined instrument is capable of measuring mass flow to 5% or better and has the advantage of not requiring the insertion of impedances into the flow.