Press Felt Permeability

Entering the press nip, water from the sheet must flow through the felt easily. A millisecond later, exiting the press nip, the water inside the felt must be held back so that it will not rewet the sheet.

If the felt is too dense (or closed) dewatering will be difficult. It can lead to water before the nip and crushing of the sheet (see picture).

If the felt is too open, the water inside the felt will rewet the sheet when exiting the nip. This undooes all the hard work that just had been done in the nip.

The permeability of a press felt changes during its life in the machine. Initially, the felt is new and open. After time, the felt will start compacting and it might get contaminated, both making the felt more closed.

The combination of an AirSpeed/2 and Caliper Profiler gives a lot of information:

• felt is thick and open: the felt is new, danger for rewetting;
• felt is thick and dense: the felt is contaminated and needs to be washed; 
• felt is thin and open: the felt worn and needs to be replaced;
• felt is thin and dense: the felt is compacted, when not too dense for most machines a normal condition.

Feltest products to check the openness of press felts

Feltest offers the following products to measure the permeability of press felts:

Feltest AirSpeed/2

The Uhle (or vacuum) box sucks air through the press felt. The AirSpeed/2 measures that airflow and therefore gives a good and affordable indication of the felt's permeability.

Feltest RealVac

Ofcourse the above mentioned airflow not only depends on the felt permeability, but also the applied vacuum on the Uhle box. With the RealVac you can reliably measure the true vacuum in the Uhle box.