Filtration mechanics of sheet forming. Part I: Apparatus for determinationof constant-pressure filtration resistance

The drainage rate during sheet forming is a critical issue in the operation and design of the wet end of a paper machine. Current laboratory pulp drai nage tests involve suspensions drained to form a high basis weight mat at l ow velocities and without the presence of fine material recycled with the w hitewater. In this study, a laboratory drainage tester has been developed t o characterize the drainage of pulp slurries near industrial conditions. Dr ainage rate was monitored by measuring the change in height of the free sur face of a suspension using a high-speed triangulation sensor. Instantaneous drainage velocity is determined and used to determine the magnitude of the viscous and inertial resistance terms. Drainage velocities were measured f or several grades and former arrangements during pilot paper machine trials . The results indicate that viscous resistance is not constant, but increas es as drainage occurs, even for thin mats. Inertial terms may be present, b ut the increase in the viscous resistance coefficient is the important fact or to influence drainage velocity.