Parameter regimes and rates of fibre collection on screens of various design

Abstract

Fibre collection on screens is here used as a collective name for any preferred or non-preferred deposition of fibres, appearing in e.g. paper manufacturing, recycling of elongated particles or equipment clogging. The fibre collection on screens is typically discussed on basis of the application and may distinguish between fibre collection being a friend or foe of the process of interest. We report an extensive experimental investigation of fibre collection and provide a systematic discussion based on two parameters describing the screen geometry: the fibre length to the screen opening size (LFibre/DOpen), and fibre length to the distance between the openings (LFibre/SOpen). The first parameter, LFibre/DOpen, discriminates between the two fibre collection modes: (i) fibre retention (large LFibre/DOpen, for example paper forming) and (ii) fibre stapling (small LFibre/DOpen, for example deposition on pins and edges). The second parameter LFibre/SOpen controls the fibre collection rate for both modes (with higher collection rates for higher values), but through different physics. In fibre retention, the successful collection is probabilistic and large for small screen openings and a fibre-orientation parallel to the screen. Since a decrease of LFibre/SOpen results in a smaller open area and hence to higher acceleration of the suspension upstream of the screen, the fibre orientation is skewed towards a screen normal orientation and fibres tend to pass through the holes. In the case of fibre stapling, the successful collection comes from an immobilization of the fibre when fibre-solid friction force exceeds the hydrodynamic drag force for fibres deposited close to the edge of the holes. For LFibre/SOpen above 1 a fibre bends over the solid hole spacing and is fixated on two support points. For LFibre/SOpen below 1 restrains a successful fibre fixation on two support points why fibre collection on the solid screen is hindered and prevented. The impact of the approach velocity and fibre concentration on the fibre collection was tested and found to be negligible for fibre retention but impacting fibre stapling. This is in agreement with reports on equipment clogging in cellulose fibre processing. For fibre retention, the collection rate is high and close to total retention. However, the collection rate of stapling is much lower. At high velocities, we suggested that fibre bending can cause additional leakage through the screen.