The complete set of equipment that links the pulp into paper webs is called a paper machine, which includes main equipment such as headbox, wire section, press section, dryer section, calender, winding machine and drive section, as well as auxiliary systems such as steam, water, vacuum, lubrication and heat recovery.
Main features
Papermaking machinery is a machine that produces paper from a pulp suspension that meets papermaking requirements through processes such as dewatering and forming via a filter, mechanical extrusion dewatering, and drying. Papermaking machinery includes three main parts: forming, pressing, and drying, which complete the above processes. It is also equipped with necessary finishing, winding, and transmission devices, as well as auxiliary systems such as pulp supply, pulp and white water circulation, vacuum, ventilation and exhaust, waste paper handling, lubrication, and automatic control. The specifications of a papermaking machine are often expressed by the width of the paper it produces (referred to as "paper width"), the width of the wire mesh, and the preferred operating speed. Modern large-scale papermaking machines can have a paper width of up to 11 meters, an operating speed of over 1000 m/min, a daily paper production of hundreds of tons, and a total weight of over a thousand tons, with a length exceeding one hundred meters. In most developing countries and in the production of some specialty papers, smaller papermaking machines are more commonly used, with a paper width of 1-3 meters and an operating speed of tens to 200 meters per minute.
Basic Classification
Paper machines are conventionally classified according to the type of sheet forming equipment used, such as long wire, cylinder wire, double wire, and a combination of long and cylinder wire. They are also classified according to their main product types, such as cultural paper machines (including newsprint), board machines (including packaging paper), toilet paper machines, and specialty paper machines. Alternatively, they can be classified according to the thickness of the paper produced, such as thin paper machines, paperboard machines, and conventional paper machines.
Depending on the needs of the papermaking process, paper machines come in various structural forms and are generally composed of components such as flow feeding, forming, pressing, drying, finishing, winding and transmission, as well as auxiliary systems and other supporting equipment.
Forming part
The headbox consists of a headbox, breast roll, forming wire, suction box, and couch roll. The headbox distributor evenly sprays the pulp from the feed system onto the forming wire, and the pulp speed and wire speed should be matched. Controlling the pulp speed and wire speed is the main factor in selecting the headbox type. Fully enclosed hydraulic headboxes adjust the pulp speed by regulating the feed pressure of the pulp pump, suitable for high-speed paper machines; air-cushion headboxes adjust the speed by regulating the air cushion pressure; open high-level headboxes adjust the speed by adjusting the pulp level inside the box, suitable for medium-speed paper machines; and open compartment headboxes are suitable for low-speed paper machines. The forming wire is an endless annular wire that forms a horizontal or slightly inclined flat wire surface between the breast roll and couch roll, serving as the working section for pulp dewatering. It is commonly called the wire plan, hence the name long wire. If the endless forming wire is placed on a circular wire cage, it becomes an arc-shaped dewatering and forming section, called a circular wire. If a perforated steel wire drum is used to draw a vacuum from the shaft end to accelerate the dewatering of the pulp on the forming wire surface, it is called a vacuum cylinder wire or vacuum forming wire. If the pulp is held between two long wires and both wires are dewatered and formed simultaneously, it is called a clamped wire forming device (Figure 2). These forming devices use different types of dewatering elements to accelerate the dewatering of the pulp on the wires. The elements are divided into three categories: stationary, rotating, and a combination of both. Stationary elements include dewatering plates and curved vacuum boxes, while rotating elements include table rolls and vacuum forming rolls. In addition to using single-type forming devices to form long-wire paper machines, cylinder paper machines, and clamped wire paper machines, there are also forming sections that combine multiple long wires, multiple cylinder wires, multiple clamped wires, and mixed-type long-cylindrical wire forming devices.
Pressing section
The formed wet paper sheet is further dewatered by applying pressure. It consists of a press unit composed of two rollers, one hard and one soft, such as a stone roller and a rubber roller. The press section is composed of press units of the same or different types, and conventionally includes a vacuum suction device. During pressing, the paper sheet is supported by appropriately sized felt, which can evenly distribute the pressure and remove some of the squeezed water, while also increasing the pressure and improving the dewatering capacity. Low-speed paper machines typically use a linear pressure of 20–60 kN/m, while high-speed paper machines use up to 210 kN/m. When using high-impulse pressing, such as wet paper machine pressing, the linear pressure can reach up to 350 kN/m. The rubber-coated rollers are made into vacuum press rollers, grooved press rollers, wire-lined press rollers, and blind-hole press rollers to more effectively remove press water from the press area. These are new technologies adopted since the 1950s to improve pressing dewatering efficiency. Later, a double-press structure was adopted to form a closed paper feed. It consists of two or three presses combined. A vacuum suction roller peels the wet paper from the forming wire and feeds it into the compound press, supported by a felt cloth. The paper sheet is then transferred between the press rollers, eliminating the breakage caused by the paper sheet's own weight and other factors during the press process. Since the 1980s, wide-zone pressing technology has emerged. Strong rubber belts or rollers deform under strong pressure, resulting in a linear pressure of up to 750 kN/m and a wider contact surface. This significantly extends the dewatering time and greatly improves the pressing and dewatering capacity, with even more pronounced effects on high-speed paper machines. The dryness of the paper sheet exiting the press can now be increased from 30-37% to 43-45%, and in some cases even up to 50%. The number of times the paper sheet contacts the smooth, hard roller surface or the rough rubber surface, as well as the amount of dewatering on each side, all affect the surface quality of the paper sheet. Proper handling can significantly reduce the difference in smoothness between the two sides of the paper sheet; conversely, improper handling can increase this difference.
Drying section
A drying section consists of several drying cylinders driven by gears or felt, forming a group. The linear speed of each group of drying cylinders can be adjusted individually to maintain a slight speed difference between them, compensating for paper shrinkage during the drying process. The paper exiting the drying section typically has a dryness of around 92-94%. At the end of the drying section, a cooling cylinder is usually installed to allow moisture to condense on its surface. The condensate moistens the paper surface, facilitating a smoother finish during finishing. A drying cylinder diameter of 1.5m yields the best results. For single-sided glossy paper, a higher single-sided gloss can be achieved by using large-diameter drying cylinders with high hardness and high gloss surfaces; large-diameter drying cylinders typically range from 3 to 7.5m in diameter.
Surface finishing department
Multiple processing steps are used to achieve good surface quality. Most paper undergoes mechanical calendering after the paper machine to make the paper surface smooth and uniform. Mechanical calenders are made of several cold cast iron rolls, which flatten the paper surface by their own weight or additional pressure. If a higher surface smoothness is required, a supercalender is used. It generally consists of alternating paper rolls and cold cast iron rolls. Depending on the application requirements, some varieties require surface sizing to improve surface quality; others require surface coating to improve appearance or print quality. Surface sizing and coating can be performed outside the paper machine using sizing and coating machines, or the sizing and coating equipment can be combined inside the paper machine for in-machine processing.
Transmission system
The paper machine's various parts are connected and operate synchronously, integrating mechanical and electrical systems. It consists of a mechanical reduction gear, a speed regulating and stabilizing device, and various transmission devices, all powered by electricity.
White Water System
It is a system for the recovery, treatment, and reuse of dewatered water from the forming section of a paper machine. It includes equipment for collecting, conveying, filtering or settling, and flotation of white water to separate and recover fibers and fillers for reuse, as well as the entire system for recycling clean white water in the papermaking process.
auxiliary systems
It consists of an electrical system, vacuum system, lubrication system, workshop steam supply and exhaust system, waste paper handling system, compressed air system, steam supply and water supply and drainage system, etc., which are matched with the paper machine.
