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UBC Theses and Dissertations

Power consumption in pressure screens Li, Yubing

Abstract

Pressure screens are an important unit operation in the manufacture of high quality pulp and paper. During screening, a suspension of water, fibre and contaminants are separated based on their physical dimensions. Most of the research conducted previously focused on the fibre separation or contaminant separation performance, with few studies available in the field of power consumption. The objective of this dissertation is to investigate the factors that influence the power consumption in a pressure screen. This was accomplished by experimentally measuring the torque exerted on the rotor shaft in a laboratory Cross Sectional Screen for a wide range of design and operating conditions. The experimental results are presented in non-dimensional form and empirical correlations are derived. Two conditions were considered in this thesis: a rotor without elements on its periphery (a smooth rotor) and a rotor with elements. It was found that for smooth rotors, the spacing between the rotor and outer housing had a relatively small influence on the required torque (power). The fluid type and viscosity affected the flow behavior in different ways. "Drag reduction" was observed for pulp suspensions. The non-dimensional torque, scaled using viscosity, could be expressed as a power-law function of Reynolds number. The value of the exponent in our experiments was in the range of 1.32 to 1.68. For a rotor with elements, the additional torque generated by the elements was significantly influenced by the element height. This resulted from the high pressure drag on the leading edge of the elements and the vortices downstream. The torque coefficient based on the extra toque was independent of the Reynolds number, but was a strong function of the ratio of element height with rotor spacing.

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