| Summary: | 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. === Applied Science, Faculty of === Mechanical Engineering, Department of === Graduate
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