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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

There are many variations on the basic perforated plate screen. As the understanding of the importance of chip thickness has grown, so as the adoption of disc screens. In this type screen, several parallel shafts each with a series of closely spaced steel discs mounted on them are arranged to create a roughly horizontal surface. The shafts are spaced such that the discs slightly overlap. This creates screening slots between the discs. All of the shafts rotate in the same direction. Profiles on the perimeter of the discs jostle the chips. If the chips are thin enough, they fall between the discs. If they are too thick, they are pushed over the rows of discs and off the end of the screen. Some systems employ a disc screen as a primary thickness screen after the scalping screen to remove over thick chips.

Chip Screening

Training Time: 26 minutes

In order to process wood chips into pulp they must be separated based on size, or "screened." Chips are screened in order to remove foreign debris and remove oversized and undersized wood pieces from the acceptable chips. This module will describe the purpose of chip screening and different types of equipment used for chip screening.

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Because screening  relies on  gravity separation, screen rooms tend to be multi-story structures, with the initial stages of separation near the top of the building.

Because screening relies on gravity separation, screen rooms tend to be multi-story structures, with the initial stages of separation near the top of the building.

In a bar screen alternate sets of bars move up and down. This action tips chips on their edges and allow them to pass through if they are thin enough.

In a bar screen alternate sets of bars move up and down. This action tips chips on their edges and allow them to pass through if they are thin enough.

Centrifugal force causes the chips to move out and then lie flat against the outer knife ring. As the inner anvil rotor advances, the chips are split.

Centrifugal force causes the chips to move out and then lie flat against the outer knife ring. As the inner anvil rotor advances, the chips are split.

Learning Objectives

  • Describe the purpose of chip screening
  • Identify the various safety hazards and the safety precautions required to safely work around chip screening operations
  • Identify the most common size classifications of screened chips
  • Identify the importance of uniformly sized chips
  • Identify and describe equipment used for chip screening
  • Identify and describe the equipment used to reclaim oversized chips
  • Identify and describe primary process flows
  • Describe some things to consider in typical chip screening operation design
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Chip Screening FAQs

What are some of the reasons that chips are screened?
Chips are screened to remove contaminants and both over- and under-sized chips.

In terms of pulping, what is the most important chip dimension?
Uniform pulp quality is best achieved by having chips of a uniform chip thickness.

What role chip slicers and chip conditioners play?
Chip slicers and conditioners process oversize chips, reduce their size and return them to the process.

What are some alternatives to simple open-hole screens?
Alternative screens include disc, roll and bar screens.

What will happen if a screen is overloaded?
An overloaded screen will not operate efficiently and proper size separation will not occur.

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