Patel, Ketan Shantilal; (2001) Vibro-spring particle size distribution analyser. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the design and development of a fully automated pre-production particle size distribution analyser for dry powders in the 30 - 2000 μm size range. In its most basic form, the instrument comprises a horizontally held closed coil helical spring partly filled with the test powder that is mounted at one end of a double beam cantilever system pivoted along its length. Particle size distribution data are obtained by stretching the spring to known lengths and measuring the mass of the powder discharged from the spring's coils following transverse vibration at resonance. The size of the particles on the other hand is determined from the spring 'intercoir distance. The main thrust of the thesis describes the developments of the appropriate technology and methodology for in-situ and continuos measurement of the mass of the powder within the spring and system automation. Two distinct methods were developed in order to measure sample mass. The first deceptively simple approach involved mass determination from the vibration characteristics of the spring and the supporting structure during resonance vibration. This approach was unsuccessful as it was found that apart from the sample mass, the vibration characteristics were also a strong and complicated function of spring tension. The second and far more successful technique involved a two-stage dynamic-static operation in which particulate discharge is followed by wanton termination of vibration. The sample mass is then measured by monitoring the deflection of the cantilever system about its main axis. In this way, for a typical charge sample of 10 g, discharge mass may be measured with a combined resolution and reproducibility of ca. 0.1 g. The corresponding size resolution is ca. 5 μm. Particle size distribution data relating to the performance evaluation of the system are reported in conjunction with a variety of powders differing in size and shape including ballotini, milk powder, grit stone, sawdust, clay and sand. System automation has involved work on both the mechanical and computer control aspects. The former required the developments of a specially designed non-contact gear mechanism for accurate extension of the spring to predetermined lengths as well as an automated particulate pneumatic feed system. The computerised features of the instrument include fully automated operation such as data acquisition, manipulation and display.

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