Pressure screens are used primarily to separate unwanted debris or contaminants from the desirable fibers in papermaking pulp. This is accomplished by flowing a pulp slurry over a surface that has many small holes or slots in it. Most of the small flexible fibers pass through the openings, and are accepted, and the larger particles which cannot pass through the openings are held back, or rejected.
A typical pressure screen consists of a vertical cylindrical housing that contains a perforated cylindrical screen basket. Within the basket is a spinning rotor which supports foils or protrusions that run close to the surface of screen. Screen performance is quantified by looking at such factors as the reject rate, debris removal efficiency, and long fiber reject rate.
Learning Objectives
At the end of this module, you will be able to:
- Explain the purpose of pressure screening
- Identify the major components of a cylindrical pressure screen
- Describe how screens are kept from plugging
- Describe why screening systems use multiple stages
- List three measures of screening performance
- Identify controls which affect screening performance
- List possible causes for pressure screen process and mechanical malfunctions