PERMEABILITY / OPENNESS

All paper makers will have had to deal with 1 or more of the following problems:

  • insufficient nip or Uhle box dewatering, too long startup or break-in times
  • sheet crushing by too dense felts, due to contamination, plugging or compaction
  • sheet rewetting as felts are too open or worn
  • felt base marking or suction roll shadow marking in the sheet
  • fiber bleeding, contaminating the stretcher roll

The press felt permeability or openness is usually a delicate compromise. When the felt enters the press nip, water must flow easily through the felt – the felt must be open. Milliseconds later, exiting the press, the water inside the felt may not rewet the sheet – the felt must be as closed as possible.

 

 

Feltest products to check the openness of press felts:

 
RealVac – measure the vacuum where it matters: in the Uhle box (and not at the pump).

 

Related whitepaper:

Correct felt permeability