A New Simple And Sensitive On-Line Particle Size Analyser For Fine Powders Suspended In Gases

SUMMARY This is a report of a new development in on-line particle size measurement of fine powders in dust-laden gases. A continuous sample of the solids-air suspension is drawn from the process stream via a sampling probe by means of an air operated ejector into a cyclonic separator whereby the sample is subjected to a particle size dependent separation process and split into two streams, "overflow" and "underflow". The overflow stream carries most of the fines at a relatively low concentration and the underflow stream carries the coarse particles at a relatively higher concentration. The concentration of solids in the two streams is detected by electrostatic noise level technique which utilises the electrostatic charge accumulated on the solid particles due to their high velocity in the turbulent air stream, for the mass concentration measurement. The ratio of the solids concentration of the fines stream to that of the coarse stream is then correlated against the average particle size in the main process stream by a calibration curve. The in-plant tests clearly indicate that this relatively simple instrument offers good sensitivity in the size range from 2 to 6 µm, and is capable of giving a continuous indication of average particle size for process control purposes. Although the instrument was developed for a particular application, it may be used in other situations where fine powders are conveyed in gas streams. British and foreign patents are applied for.