Did Your Press Felt Occur Rewetting ?

Wet paper sheet rewetting refers to the phenomenon where some of the moisture in the press felt returns to the wet paper sheet after the sheet exits the press nip along with the press felt. We do not want to see rewetting of the paper sheet, and the degree of rewetting depends on the structure of the press felt and the exit conditions.

Theoretically, even at high machine speeds and brief dwell times in the press zone, there is a phenomenon of moisture re-absorption in the wet paper sheet. During this re-absorption process, water moves from the felt into the wet paper sheet and temporary transfer space, that is, it passes through the channels of the press rolls (such as the grooved press roll, blind hole press roll, and the vacuum suction press roll) and enters the felt, subsequently reaching the wet paper sheet. The latter situation primarily occurs when the vehicle speed is low, the press zone is wide, and the felt is soft. Coarse and hard felt, as well as fully synthetic fiber felt, can mitigate this latter type of re-absorption.

Reasons for Wet Paper Sheet Rewetting

Firstly, the wet paper sheet and the press felt are in the state of exiting the press. After exiting the press, the wet paper sheet cannot be separated from the press felt in a timely manner, and it stays on the surface of the press felt for too long, which is the key to causing the paper sheet to regain moisture.

Next comes the layer design and structural proportion of the press felt. Generally speaking, the surface layer of the press felt uses fine denier fibers, the middle layer uses medium-coarse denier fibers, and the bottom layer uses the coarsest denier fibers. The finer the surface fibers, the greater the layer thickness, and the denser it is, the greater the resistance for water in the felt to return to the paper sheet, which results in a lower probability of moisture regain. This is what we commonly refer to as the "inverted funnel-shaped drainage channel" of papermaking felt. Experiments have shown that, under the same other conditions, when a paper machine uses a felt with a uniform surface, the moisture regain is only about 5g/m2. If a rough-surfaced felt is used, the moisture regain after the paper exits the press zone can be as high as 15-20 g/m2. When the paper has a basis weight of 75 g/m2, every 10 g/m2 of moisture regain will reduce the dryness of the outgoing paper by about 1.8%, which means that the paper needs to evaporate more water in the dryer section. The smaller the structural proportion of the press felt (the ratio of the fiber layer to the base mesh layer), the more likely it is for the wet paper sheet to regain moisture. Therefore, it is necessary to ensure the front layer thickness of the press felt, and not to blindly improve the drainage performance of the felt while neglecting other aspects of performance. Table 1 illustrates the impact of different basis weights of paper sheets and the layering of coarse and fine fibers in the press felt on the dryness of the wet paper sheet exiting the press.

Next, the dryness of the wet paper sheet entering the press zone and the moisture content of the felt are also important factors that determine the dryness of the wet paper sheet exiting the press zone. Research experiments have shown that the greater the dryness of the wet paper sheet entering the press zone and the lower the moisture content of the felt, the greater the dryness of the wet paper sheet exiting the press zone. When increasing the speed of the paper machine, in order to maintain the same amount of water removed from the wet paper sheet, it is necessary to increase the linear pressure of the press. For a felt with high moisture content entering the press zone, the increase in fluid pressure in the press zone reduces the ability of the felt to absorb water discharged from the press zone, and there is also a tendency for increased embossing of the wet paper sheet.

From the above analysis of reasons, we know that the structural state of the press felt is crucial for reducing and eliminating moisture regain. Therefore, when designing and producing press felt, it is necessary to endow it with the characteristic of "inverted funnel-shaped drainage channels" in terms of process formulation and structural design, thereby enhancing the moisture regain resistance of the press felt.

SUN HONG PAPER MACHINE FELT CAN BRING YOU EXCELLENT USING EXPERIENCE