Why Does Press Felt Have Poor Early Dewatering

Poor early dewatering of the press felt in the press section is a very troublesome problem when using a paper machine.

It may seem to happen on the press felt, but in reality, it has far-reaching consequences, directly leading to a decrease in the dryness of the paper, an increase in steam consumption, a higher risk of paper breakage, and even serious paper defects such as embossing.

The Imbalance Between Water Absorption and Water Removal 

The core function of the press felt is to receive the moisture quickly squeezed out of the paper in the press zone, then transfer and discharge it laterally through the press felt structure to prevent the paper from getting wet again.

“Press felt poor early dehydration” usually manifests as

The press felt has too high a moisture content or uneven moisture distribution before entering the pressing area.

The dryness of the paper sheets at the exit of the press section was lower than expected.

The surface of the press felt has a noticeable water film, water droplets, or even dripping water.

How to Solve Poor Early Dewatering of Press Felt

Physical Channels Blocked

Improper Selection of Press Felt

If the press felt’s air permeability is too high, moisture in the press section will evaporate easily, but its initial water absorption capacity will be poor. If the air permeability is too low, water cannot penetrate the fleece quickly and will form a water film on the press felt surface.

Therefore, the choice of press felt must be precisely matched to the paper type, machine speed, and press configuration.

The base fabric structure and fiber layer composition of the press felt will also affect its vertical drainage and lateral diffusion capabilities. If its structure is not compatible with the paper machine’s pressing method (such as shoe pressing), it may cause water to “pool” within the press felt.

Poor Coordination Between Press Rolls and Press Felt

If the vacuum roll becomes clogged during the pressing process, the suction box or vacuum roll will be unable to effectively remove water from the press felt, causing the felt to carry water into the next cycle.

Wear on the vacuum box panel, aging of the sealing strips, or leaks in the vacuum lines can result in insufficient or uneven vacuum pressure, preventing the press felt from promptly expelling the moisture it has absorbed and thereby reducing its subsequent water-absorption and dewatering capabilities.

Uneven pressure distribution in the press section or mismatched roll heights can cause the press felt to be subjected to uneven pressure throughout the press section, resulting in poor drainage in certain areas.

Press Felt Tension and Runnability

Insufficient tension in the fabric will cause the press felt to arch before the press section, preventing it from lying flat against the vacuum suction box and resulting in reduced dewatering efficiency.

If the felt runs off-center, it will cause uneven lateral tension, preventing certain areas of the press felt from maintaining good contact with the vacuum box, which in turn affects the dewatering efficiency.

Press Felt Contamination and Clogging

Accumulation of Pollutants

Substances such as fillers, fine fibers, adhesive residues, and sizing agents gradually accumulate on the surface of the press felt and within its pores, forming a “clogging layer.”

These blockages reduce the effective open area of the press felt and impede water flow. At the same time, they may form a water-repellent film that prevents the felt from effectively absorbing water.

Aging and Hardening of the Press Felt

Long-term chemical erosion (such as prolonged exposure to acidic or alkaline environments) can cause the press felt fibers to become brittle and harden, losing their elasticity and making it impossible to form proper water channels in the compression zone.

Incorrect Parameter Settings

High-Pressure Spray Water Failure

Insufficient water pressure at the spray nozzle, inadequate water flow, or a clog can prevent the initial contaminants on the press felt surface from being effectively removed.

If the spray nozzles are not positioned correctly, the press felt will not be thoroughly and effectively cleaned before entering the press section.

Improper Press Nip Pressure Profile 

Excessive pressure loading can prevent water from being expelled from the press felt in time, causing the press section to become flooded with water and disrupting the subsequent dewatering cycle.

Lack of Press Felt Routine Maintenance

Improper Installation of the Press Felt

Failure to sufficiently pre-wet a new press felt before installing it on the machine, or increasing the speed and pressure too rapidly during the break-in period, can damage the press felt’s structure.

Lack of Regular Testing

No regular testing system has been established for press felt key indicators such as the air permeability, thickness, and moisture content of the fleece, making it impossible to quantify changes in its condition.