Estimating coating quality parameters on the basis of pressure drop measurements in a Wurster draft tube

The paper presents experimental work in a Wurster type coater. The pressure drop in the draft tube during the coating process was measured. The purpose of this study was to investigate the potential use of pressure drop fluctuation analysis to estimate quality parameters such as coating thickness uniformity, coating yield and degree of agglomeration. Combining different Wurster chamber geometries, intake airflow rates and draft tube heights, 20 coating experiments were performed. Based on Fast Fourier Transform analysis of raw pressure drop signals, four flow regimes were identified. The characteristic frequency bands were: 2–5 Hz (regime A), 13–16 Hz (regime B), periodicity of low frequencies (regime C) and exhibiting of two characteristic bands, 2–5 Hz and 13–16 Hz (regime D). In a particular coating experiment, the relative standard deviation (RSD) of the pressure drop signal and the identified flow regime provided sufficient information for a reliable estimate of coating quality parameters. Pressure drop fluctuations did not change significantly during the coating process, which provides an acceptable assessment of the aforementioned parameters even at the beginning of coating.