Paper Machine Wet Section Dehydration Principle

The constant function of the paper machine forming section, press section, and dryer section is dehydration. In the wet section, mechanical methods are often used for dehydration, while in the dryer section, evaporation methods are used for dehydration.

The dehydration of paper is the process of solidification, forming paper web in the wire section. Only vacuum action is used to achieve a maximum dryness of 18-23% for the paper roll. Within this moisture range, most of the free water has been removed. Further dehydration is achieved by compressing the paper structure and increasing the internal water pressure through pressing to achieve outward dehydration. After squeezing out the moisture from the paper web during the pressing process, the void volume of the paper sheet decreases, and a certain degree of fiber compression occurs at the contact between fibers. The applied pressure helps to distribute the moisture evenly throughout the entire paper sheet.

Pressing and dehydration are divided into four stages:

In the first stage, the paper and press felt are purged of air until the paper 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 is saturated with moisture, and the water pressure in the paper increases, causing water to transfer from the paper 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 seam begins to expand, and further dehydration is carried out until the water pressure in the paper drops to zero, at which point the dryness of the paper is at its maximum.

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

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 network layer under the pressure of the compression zone and the possibility of moisture regain from the felt to the paper sheet after the midpoint of the compression 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 feltt porosity have a significant impact on water resistance, and the residence time in the compression zone becomes a limiting factor. The pressure controlled dehydration and flow controlled dehydration indicate the direction of efforts to improve pressing efficiency.

Collapse 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 roller 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%.