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
Feltest AirSpeed/2 – the Uhle (or vacuum) box sucks air through the press felt. The AirSpeed/2 measures that airflow accurately and reliably.
Feltest RealVac – measure the vacuum where it matters: in the Uhle box (and not at the pump).