The purpose of the press section is to increase the dry content of the paper web as much as possible by compression. This kind of mechanical dewatering reduces steam consumption in the dryer section and increases the strength of the web in order to avoid web breaks during production.
The wet paper web is picked up from the wire section and is transported by felts through the press section which contains one or more press nips to be passed. The so-called press nip is formed between two opposing rolls pressed together. The paper web today is usually transported either on one felt in the top or bottom position or between two felts as a sandwich. In some cases, when the paper web is strong enough compared with the applied stress in operation, the web is trans¬ported towards the next press nip or to the dryer section without any felt support. This is called an open draw. The water removed from the web is stored and carried away in the felt and, in the case of high water extraction, also in grooves or perfora¬tions in the surfaces of the rolls behind the felt. With suction press rolls, storage and transportation of water are improved and rewetting of the paper web is re¬duced.
The felt has to be designed to meet high demands as regards large storage capacity under pressure, good water retention behavior on leaving the nip which reduces rewetting of the web, and the smoothest possible felt surface for uniform pressure transfer to the web and to avoid felt marking. For the latter, e. g., the staple fiber diameter at the web contacting felt surface goes down to 20 mm which is even less than the paper fiber thickness (about 30 to 40 mm). Different types of felts are selected for the top or bottom positions in the individual nip as well as for the different press nips, taking into account the press water flow and the target of sheet transfer. The dewatering of the felts is performed with suction pipes. Con¬stant or intermittent treatment with a fine, high-pressure water jet and/or wetting facilities (felt conditioning) keeps the felt clean and maximizes its working life.
A conventional press section usually consists of three to four successive press nips (Fig. 6.45). The rolls are pressed against each other with linear forces of 20–150 N mm–1 and up to more than 300 N mm–1 in some special designs. For conventional press rolls the magnitudes of the resulting pressure and nip length mainly depend on the diameter of the roll, the elastic characteristics of the material and the geometry of the shell and of the coating (steel, or bronze, rubber or plastic; plain, grooved or perforated), as well as on the felts; for high basis weights the visco-elastic properties of the paper web are also important.
Generally the applied nip pressure must first overcome the fiber structure re¬sistance of the web before it can generate the hydraulic pressure required for web dewatering (see Fig. 6.7 in Section 6.2.4). Fiber structure resistance at the begin¬ning is low and increases with higher dry content which is shown in Fig. 6.46 for static conditions. This means that the dewatering pressure must be elevated with increasing dry content to overcome the higher structure resistance. However, when the hydraulic pressure is too high the paper web may be damaged (“crush¬ing”), especially at higher moisture content. The higher the hydraulic pressure and the longer the time of action (both factors resulting in the “press impulse”), the better the dewatering performance.
The flow resistance of the water in the web is determined by the type of stock, the fiber treatment, and the amount of fines and fillers. Increasing the web temperature by steam or infrared heating facilitates dewatering by lowering the viscosity of the water and the structural strength of the web. The actual nip pressure is increased from nip to nip, corresponding to the decreasing water content. For symmetry in both densification and fines and ash content of the top and bottom web surface symmetrical dewatering has to be aimed for.
The time during which the pressure acts on the web can be prolonged up to about tenfold compared to conventional presses by using shoe presses. In this press type, a concave shoe presses a flexible revolving plastic sleeve or belt against the counteracting press roll (Fig. 6.47). The shoe works on the hydrostatic/hydro-dynamic lubrication principle. The increase in pressure along the press nip is by far not as steep as in conventional presses, the maximum nip pressure is lower (up to about 90 bar). Furthermore the shape of the pressure curve along the press nip can be adjusted in a certain range. Figure 6.48 shows the total pressure distribu¬tion in the machine direction in a conventional two-roll press nip and in a shoe press nip.
Nowadays, the dry content after a shoe press section reaches about 50 to 55 %, depending on the product and the raw material used. Because of these high dry contents, less thermal energy is required in the dryer section, and the resulting increased web strength results in fewer breaks.
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 ma¬chines 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 press system with two single-felted nips is mainly applied for graphic paper production. These types of press operate at line loads of up to 1250 N mm–1.
A steam box ahead of the second press nip for enhanced dewatering effect and CD moisture control is optional. The sandwich felt-paper-felt in the press nip assists symmetrical dewatering providing a good z-direction symmetry of the sheet. Dur¬ing press dewatering the web layers towards the felt are more densified. So one-sided dewatering means for instance nonsymmetrical surface characteristics such as printability of top and bottom side.
The newest development is a single nip press section where the web is dewa¬tered in just one nip (Fig. 6.50). With a shoe of more than 300 mm length a dry¬ness of about 52 % is reached in woodfree paper production at more than 1300 m min–1. The requirements placed on the felt quality and uniformity are correspondingly high. Due to the nonexisting second nip investment costs, energy requirement (vacuum and drive), press sleeve and felt costs as well as machine shutdown time for their changing are lower.
Shoe presses are also used in tissue production. Here the advantages are higher bulk and sheet dryness after the press. In this application the shoe press replaces the suction press roll of a conventional machine acting against the Yankee dryer (Fig. 6.51).
A uniform moisture profile in the cross machine direction on the reel is very important for reasons of quality and economy. This requires a uniform nip pres¬sure as well as a uniform dewatering effect of the felts in CD. Preconditions for that are uniform felt design, structure, and conditioning. For CD moisture profile correction press rolls are in use which can vary the line force selectively across the width (in conventional press sections) and sectionalized steam boxes which heat the web to varying extents across the width.
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).