Screening Flashcards
(6 cards)
- Draw basic sketches of a disk screen and a basket screen and explain the working principles of both units! What are the advantages and disadvantages of each system?
Disc screen/plate – used after pulper to remove coarse contaminants. Has drilled screen plates and often has a turbine to provide deflaking. +Robust. +Handles high concentration of debris. +Deflaking effect. – Limited separation efficiency for small particles.
Basket screen/cylindrical – used for coarse screening usually with holes and for fine screening usually with slots. Operated under pressure. They have a higher separation efficiency than disk screens. +High capacity. +Highest separation efficiency possible. +gentle processing. – sensitive to larger abrasive particles, can damage basket.
- What are the principles of screening and fractionation?
Screening is used to separate contamination from the fibres using screens with holes or slots, the pulp should pass through the screens and contaminants should be retained on the other side. The screens decrease in size as the process progresses.
Fractionation also uses screen plates with specific conditions in order to separate longer, coarse fibres from short ones. Screening is based on shape and size.
Draw the operating principles of a centrifugal screen with the rotor at the inlet side. How is the pressure pulse of a foil at the screen cylinder opening!
Suction created behind the rotor, this pulls from the accepts side back the reject side. These pressure pulses help avoid plugging of the screen.
Which rotors would you use for LC/ fine screening and MC/coarse screening and which feature do they have/need?
For LC screening mainly the foil type rotor is used (open design), allows larger pulp flow.
For coarse screening closed design used, meaning without foils. Smaller pulp flow thickness.
What is a forward screening system?
Sequence of cleaners where the rejects of screen #1 go to screen #2, the accepts of this are then fed forward with accepts of #1. Rejects of #2 are sent to #3, accepts of #3 sent forward with the others.
What a cascade screening system?
Rejects of screen #1 fed to #2. Accepts of #2 sent to feed #1, rejects to feed #3. Accepts of #3 feed #2, rejects sent out.
