Press section, paper machine

Continuous paper machines have undergone extensive mechanical developments since the 1950s, although the principles employed have changed Utde. Cylinder machines stUl are operated and involve multiples of five to seven cylinders they are used to produce heavy multi-ply boards. Fourdriniers are common in the industry and can be used to produce virtually any grade of paper or paperboard. They vary from 1 to 10 meters in width and, including the press and dryer sections, may be more than 200 m long. 

As the machine speed has continuously has increased the dynamic forces to the web have increased and it has forced to support the web with felts to avoid web breaks. Before 30 s typically the web had open draw between press and wire section which was soon replaced with pick-up suction roll arrangement. In this arrangement web is picked up with a suction roll from wire and supported by press felt to the first nip. Most recent fast paper machines are using the same principle of suction roll and supporting fabric not only through press but also through dryer section. 

Microbiological deposits may be formed in many different places in the papermaking process. However, the closer to the wet-end the more detrimental the deposits will be for the papermaking process. Mio-obiological deposits may form on submerged surfaces as well as surfaces above the water line of the paper machine. In the latter case it is a requirement for slime formation that a surface is kept moist by splashes or aerosol formation. Examples of such surfaces are the frame of the wire section and the frame of the press section. 

Even though oxidants can be very effective and are relatively inexpensive biocides, they can also cause problems for the papermaker. Oxidants can corrode the metals used in the paper machine and destroy felts used in the press section. Oxidants can also react with and destroy other chemicals used in papermaking including... 

The great advantage in favor of ASA is the high rate of cure in comparison with AKD size. The reaction with cellulose hydroxy groups takes place rapidly in the dryer section of the paper machine at less than 5 % web humidity. More than 90% of the attainable sizing potential is, in most cases, achieved before the size press unit. The reaction rate is influenced by the same parameters as the hydrolysis, that is primarily by pH, previous residence time, and stock temperature. 

There are two reasons why there is always an excess of water present in the paper machine loop. First, the paper machine loop is continuously fed with fresh water, used for spraying and chemical dilution. Secondly, the incoming water content of the stock is higher (consistency 12-30%) than the water content in the sheet after the press section (consistency up to 50% and more). This excess water is sent, in the form of clear filtrate, from the save-all unit backwards, as make-up water, to the stock preparation, thus following the counter current principle. This make-up water is often additionally treated in a DAF with coagulation and flocculation chemicals in order to remove detrimental substances before feeding it backwards. 

The objective of the process on the paper machine is to produce a continuous paper web of the required quality, uniform in machine (MD) and cross machine (CD) directions. The process equipment consists of the approach flow system, the headbox and the wire section, the press section, the dryer section, often a size press, sometimes a coating station, a calender and a pope reeler. After the web has been reeled up this full width mother reel is cut into smaller rolls with widths and diameters according to the customers requirements. The rolls are then wrapped and made ready for shipment... 

All press nips in modern paper machines are single or double felted. Shoe presses are mainly double felted. The pick-up felt is in the first press felt position it has to transfer the wet paper web from the forming wire to the press section. Whilst the loads of the press nips increase from press position to press position of the press section, the diameter of the felt fibers on the paper side surface decreases. Changing from a coarser press felt surface to a finer one enables the transfer of the paper web, due to the increased adhesion by capillary forces and larger contact area. 

Shoe presses have been standard in press sections for board and packaging grades since the 1980s. Later, they have also become state-of-the-art presses for graphic paper machines. Today modern press sections of high-speed paper machines consist of only two nips. They have for instance two double-felted nips or one double-felted nip followed by a second nip with a felt and a transfer belt... 

A reliable web guidance system is important to prevent web breaks, especially at high machine speeds, low basis weights, and at low dry contents of the web, as found after the first press nip. In modern paper machines, especially in the press section, the web is not conveyed freely, but is nearly always supported by a felt, belt or wire, or by the surface of a roll (closed draw). 

The state-of-the-art dryer section for a high-speed paper machine has a singletier configuration with steam heated cylinders in the top row and suction rolls in the bottom row. The paper web is always supported by the dryer fabric and runs jointly around the drying cyHnders and suction roUs (Fig. 6.55). In the top row the paper web is pressed against the drying cyHnders by the dryer fabric. In the bottom... 

Figure 6.81 shows a schematic of a modem paper machine for LWC paper production started up in 2004. It includes a gap former wire section, a press section with a double-felted first press nip and a second press nip with felt and transfer... 

The press sections in specialty paper machines need to be highly flexible. For paper grades where only one smooth side is required, multi-roll press concepts with two or three press nips and a closed sheet run can be used. For increased dryness and improved runnability these press concepts can be equipped with a... 

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