Recessed Screen Bars for Continuous Pulp Digester Withdrawal

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Solution Overview

Problem

Existing digester screens face challenges with clogging and reduced withdrawal capacity, especially at high packing conditions in continuous digesters, where the column of pulp experiences increased pressure and softening, leading to difficulties in efficiently withdrawing cooking fluids.

Innovation Solution

The design incorporates recessed screen bars with a T-formed cross-section, where every second screen bar is recessed relative to neighboring bars, creating a larger virtual suction slot and reducing vertical loads, while maintaining self-clearing effects and high bending resistance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If vertical withdrawal slots are used to prevent clogging through abrasive action, then reliability is improved, but productivity is limited due to restricted withdrawal capacity at high packing conditions

Engineering Contradiction:

Improveclogging resistanceVSAvoidwithdrawal capacity

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The screen surface is segmented into alternating recessed and protruding screen bars, creating multiple withdrawal slots. This segmentation allows the pulp column to contact different bar configurations at different positions, maintaining self-clearing action while providing sufficient total withdrawal capacity through the distributed slot arrangement.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the screen surface have different geometries - recessed bars create larger local suction gaps for high withdrawal capacity, while protruding bars provide abrasive self-clearing action. This local quality variation allows simultaneous optimization of both reliability and productivity across different screen regions.

Inventive Principle:

Principle #3Local quality

2Productivity

If screen bars are arranged to create large suction gaps for high withdrawal capacity, then productivity is improved, but structural strength deteriorates due to increased bulging and deformation

Engineering Contradiction:

Improvewithdrawal capacityVSAvoidbulging resistance

Core Design Contradiction:

ProductivityVSStrength

Solution Approach 1:

The screen surface is divided into alternating recessed and protruding bars, distributing the structural load. The protruding bars act as stiffening elements that resist bulging while the recessed bars provide the necessary suction gap for withdrawal capacity, achieving both strength and productivity requirements.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The asymmetric arrangement of recessed and protruding bars creates a pattern where structural support and fluid withdrawal functions are spatially separated. The protruding bars provide structural rigidity while recessed bars create the suction gaps, resolving the contradiction between strength and withdrawal capacity.

Inventive Principle:

Principle #4Asymmetry

3Productivity

If closely arranged withdrawal slots are used to increase withdrawal capacity, then productivity is improved, but device complexity increases due to manufacturing difficulty

Engineering Contradiction:

Improvewithdrawal capacityVSAvoidscreen manufacturing complexity

Core Design Contradiction:

ProductivityVSDevice complexity

Solution Approach 1:

Instead of creating numerous closely spaced slots in a flat screen, the invention segments the screen into alternating recessed and protruding bars. This segmentation achieves high withdrawal capacity through the distributed slot pattern while using standard bar profiles that are simpler to manufacture than closely spaced slot patterns.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention moves from a two-dimensional flat screen with closely spaced slots to a three-dimensional pattern of alternating recessed and protruding bars. This dimensional change allows withdrawal slots to be distributed over a larger surface area, increasing total withdrawal capacity without requiring closely arranged slots in any single plane, thereby reducing manufacturing complexity.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This design enhances withdrawal capacity by up to 100-250% at packed regions, reduces clogging risks, and maintains durability with a controlled bulge that enlarges the suction gap, effectively managing fiber particle passage and bulging resistance.

Implementation Method 1

creating a larger virtual suction slot and reducing vertical loads

Methodology Applied

Scientific EffectSuction: Suction

Implementation Method 2

providing an abrasive action and thus keep these slots free

Methodology Applied

Scientific EffectAbrasion: Abrasion

Implementation Method 3

maintaining durability with a controlled bulge that enlarges the suction gap

Methodology Applied

Scientific EffectElasticity: Elasticity

Data Source

PatentUS8088253B2Digester screen for a continuous cellulose pulp digester

Publication Date: 2012.01.03 METABO PAPER SWEDEN AB

AI summary

The digester screen is for a continuous digester in which cooked cellulose pulp is produced. The digester screen has a number of screen bars arranged vertically in the continuous digester that have withdrawal slots (S) between the fixed screen bars through which withdrawal slots cooking fluid can be withdrawn from the column of pulp (P) of the digester. By having every second screen bar fixedly arranged and recessed a distance (D) relative to the neighboring screen bar an increased withdrawal capacity for the screen is obtained.