Why Does Press Felt Fail to Improve Paper Dryness?

Have you ever encountered the following problem during paper press production: The paper machine speed stays the same, but the drying section increases the press felt pressure by one level to maintain the paper sheet’s moisture content. However, after sampling and inspection, the paper sheet emerging from the press is still wetter than before.

You may be wondering why I can’t get it as dry as I used to?

Insufficient pressure can indeed affect dewatering, but high moisture content in the press-out may also be due to an increased amount of incoming water, the press felt being unable to absorb the water, or the pressed-out water re-entering the paper sheet.

So, you should first clarify: was the water never pushed out in the first place, or was it pushed out only to flow back into the press felt?

Press Dewatering Is Not Simply a Matter of Applying More Pressure

Once the paper sheet enters the press section, water must flow out from inside the sheet, into the press felt, and be discharged through the corresponding drainage channels.

When leaving the pressure zone, care must also be taken to prevent water from the press felt from re-entering the paper sheet.

Therefore, the dryness of the paper sheet as it exits the press depends on several factors working in concert: whether the paper sheet dehydrates easily, whether the press section provides the appropriate action, whether the press felt has the capacity to absorb and drain water, and whether the post-press separation is appropriate.

Press Felt Dewatering Is About More Than Pressure

How Press Felt Condition Affects Dewatering

If the press felt becomes contaminated and clogged, the resistance to water flow through the felt will increase.

If the press felt is compacted and its internal structure changes, the effective water-holding capacity within the compacted zone may also decrease.

In addition, if the press felt has too high a moisture content before entering the pressing zone, its ability to continue absorbing and retaining the pressed water will also be affected.

In these situations, the causes should be investigated based on the specific symptoms:

When the cleanliness or permeability of a press felt decreases, check the cleaning effectiveness and the source of contamination.

When changes occur in the structure or compaction level of the press felt, an assessment should be made based on its stage of use and operational records.

If the press felt is not sufficiently dewatered during the return cycle, check the vacuum level, seal integrity, and drainage status of the suction tank.

How to Interpret Vacuum Readings When Assessing Press Felt Dewatering

Therefore, assessing the operational status of a press felt should not be based solely on its duration of use, nor should conclusions be drawn based solely on a single vacuum level reading.

For example, an abnormally high vacuum level in the water-absorption tank may be related to a decrease in the press felt’s permeability.

If the vacuum level is too low, it may be due to a system leak or insufficient pumping capacity.

It is important to note that the vacuum gauge reflects only one aspect of the system’s operating status. It does not directly indicate the actual drainage volume, nor should this value alone be used to determine whether the press felt has been sufficiently dewatered.

How to Troubleshoot High Moisture After Pressing

During on-site troubleshooting, assess the situation systematically in this order: “inlet load—press felt dewatering—pressing zone effect—post-press separation,” to avoid immediately increasing the pressure when the pressed moisture content appears too high.

First, confirm whether the moisture level has actually increased. An increase in steam consumption in the dryer section can only serve as a clue for troubleshooting. It cannot, on its own, prove an increase in moisture content after pressing.

Next, compare the operating conditions before and after the problem occurred. Check for changes in machine speed, basis weight, the sheet entering the press section, and furnish. Also review dewatering and sheet transfer in the forming section to determine whether more water is entering the press section.

Next, check the press felt’s water intake, return-line spin, and drainage. Based on the cleaning records, press felt inspection results, the condition of the water-absorption tank, vacuum levels, and drainage performance, determine whether the press felt can effectively absorb the squeezed-out water and drain it promptly before entering the next pressing zone.

Next, verify the actual loading conditions and lateral variations in the compression zone. Based on the paper machine’s operating path, the separation point, and the moisture content of the press felt, determine whether moisture returns to the paper sheet after it leaves the press zone.

Finally, check the separation status after compression and verify the effectiveness of the adjustment.