Press felts

Pressed Felts and Cotton Batting These materials are used to filter gelatinous particles from paints, spinning solutions, and other viscous liquids. Filtration occurs by deposition of the particles in and on the fibers throughout the mat. 

The shortage of petroleum after World War I induced an intense search for a gasoline substitute in the form of alcohol. The trade press felt alcohol would definitely replace gasoline as a fuel at some point. The advantages ot alcohol cited in the technical press included greater power and elimination of knocking. 

At the end of the forming system, the paper web is transferred from the wire to a press felt, a fine-textured, usually synthetic fabric. At this point, the web contains about 4 or 5 parts water to 1 part solids. The wet paper web and one or more press felts pass through two or more press-roll nips, where water is squeezed out. Pressing also compacts the paper mat. This increases the potential interfiber contact areas where bonds will be formed. 

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. 

Poor retention of fillers and fines caused by the poor retention and/or wet-end imbalance can also result in deposits of filler/fines/size at the presses and press felts and pre-dryer doctor blades. 

To obtain the moisture profile in the machine direction, Nissan et al. [90] have made several assumptions about the drying process, principally that the drying was a first-order process (linear falling rate period) and that the pressing felt in phase 2 of the drying cycle reduced the evaporation rate to one tenth that in the sheet s free traverse between cylinders under similar temperature driving forces. Over each of the periods when the sheet touches the cylinder, it is assumed that the sheet temperature is constant (or linearly varying about an arithmetic mean), both in the plane of the sheet and normal to it. These concepts lead to a relatively simple relationship for the sheet temperature after a time interval t ... 

Typical materials include woven fibres, metal screens or fabrics, pressed felt or cotton batting, sheets of synthetic polymers, paper, sand, coal, silica porcelain and many more. Filtration equipment is equally diverse for both batch and continuous processing [23]. 

A paper machine comprises a framework at the tending side and one at the drive side of the paper machine supporting different kinds of rolls and stationary elements like dewatering elements or beams for sensors. Almost throughout the whole paper machine the paper web is in contact with forming wires, press felts... 

In the press section this formed web is mechanically dewatered under pressure which can be more than 100 bar. Here low flow resistance of the felt and minimized rewetting are economic factors. Again the surface of the press felt and its local elastic behavior wiU show up on the web surface. 

The resulting requirements press felts have to meet are as follows ... 

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. 

During its lifetime the press felt runs several million times through the press nip. Reduction in felt thickness, abrasion and contamination are the main reasons why a press felt has to be replaced. Fillers such as calcium carbonate, clay/kaolin, other papermaking additives, and adhesives e. g. from recovered paper deposit in the press felt structure, impeding or even locally preventing water flow. In some cases not even alkaline and acids can re-open the press felt structure. 

Figure 6.21 illustrates the principle of a typical press fdt structure. The base weave is found in the middle layer of the structure. The base weave is circularly woven this means that the weft yarn during weaving is the yarn in the length direction of the finished press felt. The warp yam is the yarn in the cross direction. The weaving loom produces a seamless hose, which can be up to 12 m in width and 70 m in length. Weaving looms can be up to 33 m in width. These endless felts are also called seamless felts and can be used in all applications. 

Seamed felts, in contrast, are used mainly in the production of board and packaging and are not endless. They are closed in the paper machine with a seam, produced in a special variation of the weaving process. They are very common in North America (about 60% of all press felts in NA are seamed felts) and are easier... 

Fig. 6. 21 Principle Structure of a doublelayer press felt A roll side fiber layer ...

The nonwoven layers on the top and bottom of the base weave consist of staple fibers vdth different yarn count (3.3 to 100 dtex which means a diameter of 20 to 100 gm). The nonwoven layer on the bottom is in contact with the roll covers and protects the press felt against abrasion. The nonwoven layer on top is in contact with the paper surface and ensures a low and uniform water flow resistance during dewatering of the paper web. A coarser nonwoven layer is used in the first press nips with a higher amount of water removed from the paper web, finer nonwoven layers have use in the further press nips with a lower amount of water extraction. 

Fig. 6.23 SEM-lmage of a press felt cross section A Top fiber layer, B Base weave, C Punched plastic film, D Bottom fiber layer (source Voith).

Some press felts have additional special layers, like membranes or punched plastic films as one of the middle layers. These have been introduced to control the water flow and to prevent rewetting or to act as an elastic damper against vibrations (Fig. 6.23). 

Transfer belts are a specialty used in the second bottom position of a double shoe press replacing a press felt These transfer belts guide the paper web from the press section to the dryer section. The requirements transfer belts have to meet are as follows ... 

The manufacture of a transfer belt is based on an endless press felt which is covered with a polyurethane or rubber layer on the paper or both sides. Figure 6.24 shows the SEM image of the cross section of a transfer belt with a smooth paper side. 

The correlation results are calculated using time synchronous averaging. A process condition monitoring system can e. g. detect whether an applicator roll is responsible for periodic coat weight deviations, or whether a moisture variation is correlated with the revolution time of a press felt. 

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