Producing paper is a capital-intensive industry. Financial experts dictate how business is run to a large degree. Reducing costs is a key responsibility for a Production Manager. Rightfully so, we won’t argue with the need for cost reduction. What we do need to address is the TYPE of cost reduction that will turn out costly for our future.
In order to remove water from the sheet in the press section, water must flow easily into the press felt. On the other hand, at the exit of the press nip, water must be kept from flowing back into the sheet. These conflicting demands ensure that felt permeability is always a compromise between good dewatering and re-wetting.
This article gives some background information on felt permeability and describes hands-on tools to determine and optimize the permeability of felts which are running in the machine.
If you’ve read our previous blogs, this statement will come as no surprise: there isn’t a standard, universal metric for ‘correct’ felt permeability. This is because achieving optimal permeability is unique to each machine. And it’s determined by two sets of variables.
So how do you find the optimal felt permeability? Well, as is often the case with improving your machine’s performance, it’s a matter of ongoing investigation. Here’s how…
After you install a new felt, it's normal for its dynamic permeability (the combination of airflow and vacuum) to fall. This means the felt is compacting. However, contamination will also make it denser.
To distinguish between these two possible causes for a denser felt, you need to take the felt caliper into account. And the only way to get an accurate point of reference—knowinwhether the felt is ‘relatively thick’ or ‘relatively thin’ today compared to previous days—is to take daily measurements.
The top layer of the batt, in direct contact with the sheet (should) act like a one-way valve. It allows water to pass through easily down towards the base but prevents it from flowing back.
You don’t want the top batt layer to be too dense, because then you’ll get dewatering problems at the nip entry.
Read this article to learn why a felt may become too dense and how to avoid over-density.
The most important job of the press felt on a paper machine is to remove the water from the sheet, without harming the sheet’s strength or surface quality.
But it’s not all plain sailing. Compared to other machine components, like press roll covers or the machine frame, press felts change behavior over their lifetime, especially in their level of permeability.
Almost every paper mill using forming fabrics has one: a mechanical tension gauge to measures the tension of Paper Machine Clothing, mostly forming fabrics. When these instruments are not in a perfect condition they can cause serious problems on the paper machine like torn fabrics, too narrow and/or too long fabrics and bearing failure.
