There are many factors that affect the press felt dehydration, and the main ones are as follows:
Press Pressure
According to the press mechanism, the press pressure first causes the water in the paper sheet to enter the press felt, and then horizontally or vertically dehydrates through the press felt. From this, it can be seen that the first factor affecting press dewatering is the pressure applied to the wet paper and felt in the press zone. The efficiency of squeezing and dehydration is directly proportional to the squeezing ratio and increases with the line pressure index.
The compression time of the press is directly proportional to the deformation width of the press roll (the width of the shoe press belt in the shoe press), and inversely proportional to the paper machine speed. In the flat roll press, according to its dewatering mechanism, only half of the press zone width truly functions as dewatering, so the compression dewatering time is only half of the compression time.
The compression time and dehydration time of paper sheets in vacuum press, groove press, shoe press, etc. are consistent. In this type of roll press, the compression width of the press zone is also known as the deformation width of the press zone, and its deformation width c is related to the line pressure of the press zone, the straight diameter of the top and bottom rolls, the elastic coefficient, and the Poisson’s coefficient.
The roll coated rubber is beneficial for dehydration, mainly by forming a relatively wide contact surface and extending the pressing and dehydration time. But the thickness and hardness of the adhesive layer should be chosen reasonably, otherwise the grooves on the roll surface will deform, causing difficulties in drainage and affecting dehydration.
Moisture Content of Press Felt in The Press Zone
According to research experiments, there is a close correlation between the dryness of wet paper in the press zone and the moisture content of the felt in the press zone. The lower the moisture content of the felt in the press zone, the greater the dryness of the paper in the press zone.
Paper Dryness in The Press Zone
There is a close relationship between the moisture content of the paper before entering the pressing zone and the dryness of the press. Therefore, there have been multiple instances of dehydration in the pressing zone when the dryness cannot be achieved. The test results of coniferous wood sulfate pulp show that the ratio of moisture change rate in the wet paper inlet pressure zone to the outlet pressure zone is approximately 2-3.
It is obvious that the dryness of the wet paper in the compression zone is also an important factor in determining the dryness of the compression zone. Each press felt is equipped with a felt vacuum box to ensure that the moisture content of the felt after washing and pressing can be reduced.
Wet Paper Temperature
By increasing the temperature of the wet paper in the press section of the paper machine, the viscosity and surface tension of the water decrease proportionally. When the temperature of water increases from 200℃ to 500℃, its dynamic viscosity decreases by half, from the original 1.0mPa. s to 0.5mPa. s. Under the same pressing pressure, wet paper with high temperature can be dehydrated faster, thereby improving the production capacity of the paper press section of the paper machine.
The temperature of wet paper during pressing has a significant impact on the dewatering process and is one of the main influencing factors. Therefore, the press section of high-speed paper machines is often equipped with a steam box.
Pulp Properties
The effect of beating on the dryness of the paper in the press zone is also demonstrated when the pressing time of a regular paper machine is 2-10ms, which also explains the flow of rehydration water and the correlation between rehydration and time.
Under long-term pressing conditions in low-speed paper machines, wet paper made from pulp without beating is more evenly wetted than wet paper made from pulp with beating. The rewetting effect is completely generated during the expansion stage of the pressure zone. Wet paper made from pulp with high beating degree is not easy to press and dehydrate in the compression zone because the wet paper itself has significant fluid resistance, and it is also difficult for water to return to the wet paper during the expansion stage in the press zone.
The fluid resistance of paper and the moisture content in swollen fibers are determined by the type of raw material, pulping method, degree of pulping, and other comprehensive factors. Without changing the squeezing conditions of the paper machine, even if two pulp materials have the same degree of beating, the dryness of the wet paper output zone may differ by 10% or 15%. After the pulp is beaten, there is a significant correlation between its water retention value (WRV) and the dryness of the paper output pressure zone. But changing the content of fine fibers and other conditions will affect the results of the experiment.
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