Salihu, A.; (2012) Design and optimisation of a spring particle sizer. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes the results of a series of investigations examining the operational performance followed by the fundamental re-development of two analytical instruments, namely a Pneumatic Spring Particle Sizer (PSPS) and handheld Spring Particle Sizer (handheld SPS) for size distribution analysis of dry powders in the 100 – 2000 μm size range. Each instrument shares the same basic principle of operation involving the use of a closed coil helical extension spring, which is partly filled with the test powder. Particle size distribution data is obtained by stretching the spring to known lengths and measuring the mass of the discharged particles from the spring’s coils. In the case of the Handheld SPS, aimed at on the spot quality control applications, the test particles are discharged from the spring using manual shaking. In the case of the PSPS on the other hand, the particles are discharged using pulsating pressurised air. The design, development and evaluation of two methods for the in-situ measurement of the sample mass within the PSPS are discussed. These include a full-bridge strain gauge assembly and the investigation of the correlation between the minimum fluidisation velocity and the mass of the test sample within the spring. The strain gauge proved to be a successful method producing a mass resolution of ± 1 % for a total sample mass of 280 g. The second method was unsuccessful as it was found that the minimum fluidisation velocity, in most cases did not follow a clear trend with mass. Detailed investigations are conducted aimed at understanding the processes governing the mass discharge rate from the PSPS and hence the sample analysis time by studying the particle migration behaviour. A pulsating fluidised bed of similar dimensions to the spring is used to mimic the behaviour in the spring. Tests involved pulsating fluidisation followed by particle size distribution analysis at equal distances along the length of the bed containing poly-dispersed and mono-dispersed particles. It was observed that any operating or design parameter that promoted the degree of mixing, for example, increasing the fluidising air pulse frequency would reduce the test analysis time. The analysis time also increased with the sample poly-dispersity. In an attempt to reduce the sampling time, the handheld SPS was rotated using a variable speed tumbler as an alternative to manual shaking. Despite the marked reduction in the sampling time, this method resulted in the discharge of particles larger than spring coil openings thereby producing erroneous results. Calibration experiments for the same types of powders revealed a linear relationship between the discharge sample volume and its mass, independent of the particle size in the range 212 – 1000 μm. This allows in-situ measurement of the discharge sample mass in the handheld unit by reading the sample volume collected in the integrated graduated collection cylinder and reference to a previously generated calibration line.

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