Measurements of pressure pulses from a solid core screen rotor.

Application: The factors that affect pulse characteristics determine how well the pressure screen performs.

Pulp screens would plug quickly if it were not for rotor elements that pass over the apertures and instantaneously backflush the pulp accumulations. The form of the pressure pulse, or its shape and magnitude, determines the minimum aperture size, and the aperture size determines the capacity of the screen and the extent to which the screen can improve pulp quality.

The researchers examined how the rotor speed, the gap between the rotor and screen cylinder, and the pulp consistency affect the form of the pressure pulse. They measured pressure pulses against a smooth, solid wall, rather than in a slotted, contoured screen basket, to obtain more reliable measurements for comparing pulse shapes.

One of the problems in the overall research study was to develop a fast way to produce rotor elements with specific shapes. To solve this problem, the researchers built rotor elements of aluminium using computer-controlled (CNC) machining. With this approach, prototype elements can be rapidly designed, built, and tested.

They found that the pressure velocity increases with tip speed, but the pressure coefficient remains approximately constant. The shape of the pressure pulse does not change significantly with rotor speed. As the gap decreases, the magnitude of the pressure pulse increases, but the pulse shape remains constant. As the pulp consistency increases, the pressure pulse decreases, even at consistencies below 2%.

Pinon and Olson are with The Pulp and Paper Centre and the Department of Mechanical Engineering, University of British Columbia, Vancouver, British Columbia, Canada. Gooding is with Advanced Fiber Technologies (AFT), Montreal, Quebec, Canada. Email Olson at [e-mail].