Do You Know The Function of Press Felt During Paper Machine Dehydrating Process ？

Pressing and dehydration are divided into four stages: the pressing pressure is the sum of water pressure and mechanical pressure, and the wet pressing zone is divided into four stages based on the interaction between water pressure and mechanical pressure.

In the first stage, the paper web and felt are purged of air until the paper web reaches saturation with moisture and there is no residual air. At this stage, the water pressure in the paper is not high and there is no change in dryness.

In the second stage, the paper web is saturated with moisture, and the water pressure in the paper web increases, causing water to transfer from the paper web to the press felt. When the press felt also reaches saturation, the water is discharged from the felt. In the second stage, it continues until the middle of the pressure zone, at which point the total pressure reaches its maximum.

In the third stage, the pressure zone slot begins to expand, and further dehydration is carried out until the water pressure in the paper web drops to zero, at which point the dryness of the paper web is at its maximum.

In the fourth stage, the paper web and felt begin to expand, and the paper web becomes unsaturated. At this time, the paper web returns to the paper through capillary action, internal vacuum, or void wetting mechanism, and some water returns to the paper web. The paper web out of press section should be separated from the felt as soon as possible to reduce the moisture regain effect.

Factors affecting pressing and dehydration:

① The pressure applied per unit area;

② The duration of applying pressure;

③ The number of press devices;

④ The structural type of the press roll;

⑤ The characteristics and state of press felt;

⑥ The viscosity (or temperature) of water;

⑦ Paper web characteristics, including fine fiber content, dosage of additives, and breathability of wet paper.

The main factors promoting dehydration are the water pressure difference on the paper, the residence time in the pressure zone, and the uniformity of the pressure applied. The product of the pressure in the pressure zone and the residence time is called the squeezing impulse; The main resistance is the flow resistance of the paper itself. When the water temperature increases by 1 degree, the flowability improves by about 2.5%. During the pressing process, if the temperature of the paper sheet is increased from 20 degrees to 30 degrees, the dryness of the pressed paper sheet increases by 2-3%.

Pressing and dehydrating can be divided into two categories: pressure controlled and flow controlled. Pressure controlled type is mainly used on low basis weight and high free degree paper sheets, because the paper is very thin and the paper structure does not significantly restrict the flow of water. Dehydration is mainly determined by the mechanical compression of the fiber mesh layer under the pressure of the press zone and the possibility of moisture regain from the felt to the paper sheet after the midpoint of the press zone. The flow control type is mainly used on high volume paper sheets, which are quite thick and have a structure that restricts water from flowing out of the sheet. The paper density, temperature, and felt porosity have a significant impact on water resistance, and the residence time in the press zone becomes a limiting factor. The pressure controlled dehydration and flow controlled dehydration indicate the direction of efforts to improve pressing efficiency.

The paper break is the direct result of water pressure caused by the lateral movement of water flow in the paper. The use of slow pressure can avoid the occurrence of crushing. Improving the structure of the press roll and the selection of the felt can accelerate the dehydration speed and thus avoid the occurrence of crushing. There is also a critical point issue with squeezing and dehydrating. Overpressing and dehydrating beyond the critical point is uneconomical or may excessively compress the paper, and the moisture left in the paper must be removed through evaporation. Depending on the type of pulp, pressing method, and operating conditions, the critical point is between 50% and 57%.