Selection of the form and number of passes in the press section of paper machine

The press type and number of passes in the press section of a paper machine are selected according to the dewatering characteristics determined by the ratio of the raw materials of the paper and certain quality requirements of the paper, with reference to the configuration of the press section of the existing paper machine or the results of intermediate tests.

(1) Generally, the press section of the fine paper machine adopts three-stage dewatering press. Sometimes in order to strengthen the dewatering or improve the double-sidedness of the paper, a fourth press is added. If two shoe presses are used, it is enough. For the slurry that is easier to dewater, a high-efficiency and high-line pressure squeeze form and a small number of squeezing passes are used. For the viscous slurry, a method of dewatering with more squeezing under a lower line pressure is adopted. The former uses one or two vacuum presses on older paper machines, while in the new press section, a compound press consisting of a vacuum press roll and a groove press roll is used. The latter used three to five vacuum presses, ordinary positive presses and reverse presses to form the press section. Three to four processes such as vacuum press, groove press, separate press or hot press can also be used for dehydration.

(2) When the paper is required to have a good bulk (low tightness), it is necessary to use a lower linear pressure, a larger roller diameter, and a smaller nip. For example, most toilet paper machines have only one roller press, sometimes one more press is used, and sometimes two roller presses are required. With the speed from low to high, the form of the idler roller can be selected in turn from flat rubber roller, grooved roller, vacuum press roller, shoe press roller or hydraulic cushion press.

(3) The first press has the largest amount of dehydration, and the blanket is also the easiest to get dirty. Embossing is prone to appear on the first press, and it is often the key part that limits the speed increase. Therefore, priority should be given to the selection of a high-efficiency dewatering press, such as a vacuum press, and an efficient blanket washing device.

(4) When both sides of the paper are required to have good smoothness, in order to reduce the two-sided difference of the paper web caused by the squeeze dehydration of the press or to eliminate and reduce the two-sided difference of the paper web formed by the former, it is often in the press section Configure all kinds of back press. For some paper grades that require high smoothness on both sides, a smooth press is also required.

(5) For thick paper or cardboard with large basis weight, double-felt press or double-felt vertical flow high-efficiency dehydration press is often used as the first press. For tissues with low basis weight, the re-wetting phenomenon on both sides is relatively prominent in the double-felt press, so the double-felt press is generally not used.

(6) Multiple presses are often used to produce paper machines with lower wet strength and higher speed. For fine paper machines and corrugated paper machines, the compound press is already a standard configuration for medium and high speed paper machines. Because the double press moves the first open draw of the paper web after the third or fourth nip, the paper web has sufficient wet strength to withstand the tension generated by the open draw at high speed.

(7) For large-scale high-speed paper machines or paperboard machines, a closed straight-through press section should be configured as far as possible. The press form can be large roll diameter press or shoe press.

3. The selection of the paper threading method and the paper threading device of the paper machine

The form of the transfer device that transfers the paper web from the forming wire to the press section is related to the basis weight of the paper web and the speed of the vehicle. The low-speed Fourdrinier paper machine that produces paper with higher wet strength can use ordinary open draw paper to simplify the structure and reduce operating and maintenance costs.

For the cylinder paper machine and board machine, the traditional licking mode is generally adopted. When the vehicle speed is slightly higher, a vacuum return roller is required to ensure that the paper web does not fall off when the felt is turned. Tissue paper with low basis weight is often licked on low-speed paper machines.

At present, the newly designed Fourdrinier paper machine is generally equipped with a vacuum suction and threading device when the speed exceeds 200m/min. For paper machines that produce low basis weight paper, such as Fourdrinier toilet paper machines, a simple and compact suction box can be used.

The paper machine often needs to be equipped with a paper rope device in the back press and smooth press.