Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F7/00—Other details of machines for making continuous webs of paper
  • D21F7/08—Felts
  • D21F7/083—Multi-layer felts

Definitions

• the subject invention concerns a needled press felt which is designed for use in the press section of a papermaking machine.
• the batt part of the felt comprises at least two different kinds of fibers, of which a first kind of fiber is of a considerably finer-diameter size than the other kinds of fibers.
• the batt part may be needled into a base fabric.
• a papermaking machine comprises three different sections, viz. the forming section, the press section and the dryer section.
• the forming section the stock suspension is discharged onto a forming fabric or between two forming fabrics. Most of the water contained in the stock suspension is removed by gravity drainage or by suction, passing through the forming fabric, whereby a continuous paper sheet is formed.
• the formed sheet is carried to the press section and in this section more water is removed from the fibrous web when the latter is squeezed between press rollers. Finally, the sheet is dried in the dryer section by being applied against heated cylinders, the moisture being removed through evaporation.
• the press section From a dewatering point of view the most important section is the press section. Economically, it is considerably more advantageous to remove the water by using squeezing methods than to drive it off by using heat. For this reason, one aims at achieving a fibrous web having maximum dry contents when leaving the press section in order to reduce the energy consumption in the drier section.
• the paper web In the press section of the papermaking machine the paper web is made to travel together with one or several press felts through a number of press nips. In the press nips, water is squeezed from the paper web and into or through the press felt.
• the press felt structure contains one soft and comparatively compressible surface layer. Under­neath the surface layer is positioned another layer of a more incompressible nature, which layer is designed to retain most of its volume, also when the felt is exposed to compressive pressure.
• This part of the felt serves the purpose of absorbing an optimum amount of water from the paper web when the web and the felt are compressed in the press nip, of retaining an optimum amount of the squeezed-out water following the press nip and of carrying the squeezed-out water away from the press nip and discharging it before the felt is reintroduced into the press.
• the surface layer of the felt preferably should contain as fine-dimension fibers as possible, whereby the pore size of the layer is minimized.
• the higher adhesive properties of the finer-dimension capillaries improve the water-­retainment ability of the felt.
• the surface structure of the paper is considered largely to depend on the nature of the pressing operation and therefore dependent on the surface evenness of the press felt.
• the pressure distribution felt/paper web is considered to be decisive in determining the paper quality and the dewatering results.
• the side of the press felt which faces the paper web therefore must be as even as possible and contain as fine-dimension pores as possible. This aspect, too, speaks in favour of using fine-dimension fibers in the surface layer.
• US 3 392 079 proposes the possibility of needling a finer-fiber layer onto the surface which is intended to abut against the paper web.
• US 3 928 699 describes a felt for a papermaking machine, which felt comprises two fibrous layers of which the layer intended to face the paper web contains fibers which predominantly have a diameter size of 0.027 mm or less, whereas an underlying layer contains fibers having a diameter which is at least 1.75 times larger than that of the fibers of the surface layer.
• fibers of a fineness of 0.012 mm have been used for the surface layer.
• a press felt comprising at least two layers of fibers having different fineness of which the layer facing the paper web contains finer-dimension fibers than the layer below.
• the running-in period is the period immediately following the mounting of a new press felt on the papermaking machine and this period may last from a couple of hours of operation up to several weeks. During this period it often becomes necessary to lower the speed of production and several other types of operational problems may occur.
• the felt when passing through the press nip, is permanently compressed, and this permanent compression is at its highest when the machine is started up but it is reduced succcessively, as the felt is being compressed.
• the pores of the felts become clogged by fibers and fillers emanating from the paper sheet.
• US 4 482 601 describes a method of mixing a temporary material into a textile batt which is then incorporated into the felt structure. After strong pre-compression to reduce the volume of the felt and to increase the density thereof the temporary material is removed in order to form voids in the felt and give the felt a sufficient degree of openness. This technique has proved not to be useful in practice, probably because the voids which are formed during the running-in period are permanently compressed and clogged by material emanating from the paper suspension.
• prior-art solutions comprise designing felts having coarser surface fibers than what is desirable for the purpose of allowing permanent compression and clogging during the running-in period and still retain sufficient permeability for the continued operation of the felt.
• the subject invention has for its purpose to eliminate the drawbacks inherent in press felts during their running-in period and to provide a felt having optimum efficiency during its entire serviceable life.
• the invention is characterized therein that the fibers of the first kind of fiber which form or constitute at least part of the surface layer, have a diameter of 0.010 mm or less and are removed during the service of the felt on the papermaking machine whereas the rest of the fibers are essentially resistant during the removal of the first kind of fiber. The removal may be effected successively or periodically.
• the first kind of fiber thus may be fibers which are less abrasion resistent fibers and are successively worn off from the surface layer of the batt part or else the first kind of fiber may be fibers which could be dissolved in suitable chemicals, causing these fibers to degrade.
• the press felt travels in abutment against a hot cylinder.
• Machine constructions also exist wherein the felt passes across steam chests.
• the first kind of fiber preferably are fibers which are degraded by heat whereas the rest of the fibers essentially are thermal-resistant fibers.
• the surface layer of the felt consists at least partly of fibers having a diameter of 0.010 mm or less, this surface layer will form a system of very minute pores.
• the permeability thus is adjusted to ensure that optimum operation efficiency exists already at the start-up.
• the minute pores retain water inside the felt without causing rewetting in conjunction with the expansion which occurs following the press nip.
• the extremely fine fibers result in a felt surface which provides for optimum distribution of the pressure against the paper web and thus improved paper quality and improved dewatering effects already from the start.
• the first kind of fiber has been chosen to ensure that its fibers will disappear continuously or discontinuously through abrasion or degradation.
• the unevitable effects of permanent compression and clogging are compensated for by these fiber losses, and during its entire running-in period the felt will achieve a useful permeability balance.
• the felt may function with optimum results from the start and it reaches its normal degree of compression and clogging without reduction of the permeability because of the simultaneous removal of fibers from the batt with resulting opening-up of the felt.
• the fiber material in a press felt usually consist of polyamide fibers which are capable of resisting the abrasion occurring upon the repeated passages through the press nip. Tests have shown that if e.g. polyester fibers are used in the surface layer, these fibers will wear out within a very brief time. For this reason, these fibers are very suitable for use as the first kind of fiber in accordance with the invention.

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• Paper (AREA)
• Secondary Cells (AREA)
• Nonwoven Fabrics (AREA)

Priority Applications (6)

Applications Claiming Priority (1)

Publications (2)

Family

ID=8198494

Family Applications (1)

Country Status (6)

Cited By (1)

Citations (3)

Family Cites Families (4)

• 1987
  • 1987-12-18 ES ES198787850393T patent/ES2028133T3/es not_active Expired - Lifetime
  • 1987-12-18 EP EP87850393A patent/EP0320559B1/de not_active Expired - Lifetime
  • 1987-12-18 AT AT87850393T patent/ATE72464T1/de not_active IP Right Cessation
  • 1987-12-18 DE DE8787850393T patent/DE3776661D1/de not_active Expired - Fee Related
• 1988
  • 1988-12-16 FI FI885810A patent/FI88418C/fi not_active IP Right Cessation
  • 1988-12-16 CA CA000586264A patent/CA1317144C/en not_active Expired - Fee Related

Patent Citations (3)

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