The present invention relates to papermaking and especially to pressing a sheet, such as paper, by using at least four press rollers on substantially stationary horizontal axes and in rotating contact with one another as a closed train defining a chamber between the rolls and end seals, for exerting air pressure at a previous roller surface thereby to induce through drying of a sheet.
In papermaking, a pulp slurry firstly is formed into a sheet by drainage at a circulating endless mesh, secondly the sheet is pressed between rollers for more drainage and thirdly the sheet is heated for final dryness. As is well known in the art, moisture removal by pressing costs less than removal by heating, because pressing utilizes less apparatus and energy than heating does.
The present invention is directed toward improving moisture removal by pressing, for reducing the costly heat requirement, and toward multiple pressing for improving bond of fibers together to yield improved paper strength. Multi-nip pressing is known, including pressure-chamber means for extending pressure beyond a nip, liquid-pressure means urging impervious-belt means against a sheet and press roll for a small arc at a nip. Pressure-pad means also are known for pressing a sheet and felt against a small arc at a nip.
These prior art means have problems, such as difficulty of drainage escape and abrasion of apparatus, that the present invention is aimed to overcome.
Document US-A-4.124.942 refers to a conditioner type of system disposed after a dryer for the purpose of increasing, decreasing or equalizing the moisture content in a web. On figure 4, a cental zone is created between rolls 2,2' and 4, but there is no pressure differential between said zone and the interior of the upper roll 4, where the web passes. As a result, gas cannot flow across that part of web: drying could not be carried out. On figure 5, a central zone is created between porous rolls 32 and 33, and gas flows from the central zone to rolls 33, through the web W. However, a press felt-which is of importance in removing moisture by pressing-could not be satisfactorily utilized in conjunction with these rolls.
An object of the present invention is to overcome problems of prior art and to improve pressing a sheet by providing a method and apparatus with the feature of:
According to the above features, the invention provides a four nip press for drying a web of material comprising four rollers, each mounted for rotation about respective axes, said four rollers being positioned relative to each other such that each roller is in contact with two other rollers forming nips therebetween to thereby provide a closed train of rollers, means defining with said four rollers an enclosed central zone into which said web is received through a first of said nips and passing circumferentially over at least one of said rollers in the central zone and discharged therefrom through a second of said nips; said at least one roller being a porous cylinder and having a hollow interior; and means for providing a circulation of gaz in said enclosed central zone and in the interior of said at least one press roller, characterized in that said four rollers are press rollers so adapted that water is squeezed out of the web by said nips, in that said second nip is formed between said porous cylinder being at least one press roller and a non-porous second press roller, means resiliently urging together said porous cylinder being at least one press roller and said non-porous second press roller, and in that said means for providing a circulation of gas comprise either gas inlet means or outlet means connected to the central zone and either gas exhaust means or inlet means, respectively connected to the interior of said press roller, such that said gas will flow from said central zone to the interior of said press roller through said web, or vice versa, in order to remove water from said web, said press including a papermaking press felt supporting said web and said papermaking press felt being porous allowing the gas to pass therethrough between the central zone and the interior of said at least one press roller, said papermaking press felt with respect to said at least one press roller being downstream of the web with respect to the direction of gas flow through said web, said web and said papermaking press felt passing circumferentially over said non-porous second press roller exteriorly of said central zone with said web being between said papermaking press felt and said non-porous second press roller.
In operation, air is urged through the sheet at roll pervious periphery wrapped by flet and sheet, whereby to extract moisture from the sheet and for convection of the moisture by conduit means.
Location of a press roller along the closed train helps to determine whether a roller should be pervious or impervious, hard or soft surfaced, bigger or smaller than mating rollers. These distinctive qualities of rollers along a closed train are somewhat analogous to the distinctive qualities of elements along a ring of an organic chemical molecule. Also, a closed train of press rollers according to the present invention may be joined to another closed train to form a chain press somewhat as organic chemical rings may be joined to form a chain molecule.
In choice of roller surface, it is well known for example, that having at least one press roller of a nip as soft surfaced will prolong felt life, while using a granite press roller in direct contact with a damp sheet will let a sheet release after the nip. Choice of suitable roller surface in this new combination for single and double felting would be apparent to those skilled in the art.
Some parts necessary for operation of the present combination are well known and therefore are not detailed or shown herein. They are (a) return mesh runs as to guide rolls, stretch rolls and felt-cleaning means, (b) felt carrying rolls not counted herein as press rollers, (c) framing, (d) resilient mounting of press rollers, (e) drive means. Also, nip contact between rollers is understood herein to mean through felting and during operation through a sheet as well.
A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, in which:
Referring now to the drawings and more particularly to Figure 1, there is shown the multi-nip suction press or drying apparatus of the present invention, as generally illustrated at 10. This apparatus is for utilization in a papermaking machine for pressing and drying a sheet of pulp stock 11. The press comprises at least four press rollers, herein stationary solid rollers 12 and 12', plus rollers 13 and 15. Roller 12 may be resiliently biased by biasing means schematically illustrated at 14. In this embodiment roller 12 touches the sheet directly and would preferably be of granite for sheet release. Rollers 13 and 15 could be soft surfaces while roller 12' could be hard surfaced. At least one of the rollers, herein roller 15 is a hollow roller having a pervious outer wall 16 in registry with endless belt 17 which is a pervious carrier of the paper stock of sheet 11 to be dried.
The four press rollers 12, 12', 13 and 15 are in close contact with one another in a succession as a closed train whereby to define nips at the area of contact therebetween and these are indicated by reference numeral 18. Thus, the four rollers will define four nips which will apply pressure on the endless felt and paper stock passing therebetween.
The endless felt 17 advances as indicated by arrow 19 and is a pervious carrier for the paper sheet whereby air may pass therethrough to convect water out of the pulp stock. One of the rollers has a drive connected thereto, by means well known in the art, for imparting rotation to all of the four rollers.
A substantially sealed chamber 20 is defined in the area between the rollers 12,12', 13 and 15 and between chamber end walls 21, (here shown in phantom line). In order to achieve air movement between the pervious hollow roller 15 and the chamber 20, there is provided suitable conduit means which may be constructed in several ways. As shown in Figure 1, an air box 22 is mounted in an external sealed relationship with another surface of the roll 15, and a conduit 23 is connected to the box 22 for the convection of air with moisture herein. Similarly, the roll 13 may be a pervious roller and may have a conduit 24 connected thereto for the convention of air and moisture. Rotation of roll 13 is accommodated at stationary conduit 24 by well known rotary joint means 32. A further conduit 25 is connected to one of the end walls 21 for the ingress of air under pressure. Thus, the chamber 20 will force air through the pulp stock and felt 17 passing through the pervious outer wall of rolls 13 and 15 thereby forcing moisture out of the pulp stock, through to the endless felt 17 and into the hollow rolls 13 and 15 for convention of air and moisture by conduits 23 and 24. It is pointed out that the roller 13 is suitably sealed in its opened area by means such as a solid shoe 26. By shoe 26 being external to roller 13, the rim 13 remains continuously shielded from atmospheric pressure and the shoe is more accessible than an internal seal 31 would be.
In its operation, as the pulp stock enters the multi-nip press, water is squeezed out at a first nip. The pulp stock is then subjected to a suction by the hollow roller 15 in its travel area within the chamber 20. Thereafter the pulp stock is again squeezed at a second and third nip and enters into the chamber 20 for a second travel while subjected to a suction along a surface area of the second press roller 13. Thus, the paper stock exits the multi-nip press in a drier condition than possible with known prior art multi-nip press devices.
Referring now to Figure 2 there is shown another example of the conduit means. As herein shown, a single hollow roller is provided with a pervious surface and resiliently biased against three other rollers. For example, roller 28 may be a granite solid roller and rollers 29 and 30 may be rubber rollers. Air is exhausted from the chamber 20 via the exhaust conduit 25 and this causes an ingress of air, as shown by arrows 30 through the felt 17 and the paper stock which is located between the felt and a large outer surface portion of the pervious roller 27 causing moisture to be drawn within the roller 27 and exhausted through conduit 25 via the central chamber 20. The invention should not be limited to the combinations of the air convection means disclosed herein, as other arrangements of air convection are possible.
Referring now to Figure 3, there is shown the invention as utilized in a double felted press. As herein shown, a second endless felt 31 is driven in an endless manner about the rollers 12, 12', 13 and 15 and in registry with the other endless felt 17. The pulp stock 11 is trapped between the felts. Both endless felts are of a porous construction. The operation of the double felted press, for the removal of moisture of the pulp stock, is the same as that disclosed for Figure 1.
It is within the ambit of the present invention to cover any obvious modifications of the examples of the preferred embodiment described herein. For example, the multi-nip suction press may comprise six rollers defining a chamber therebetween with air being injected into the chamber and out of four pervious rollers. In a still further foreseeable arrangement there may be provided seven rollers, three axially aligned with two pairs of granite rollers in nip contact between these whereby to define two sealed chambers with the exhaust being made through three previous rollers and air fed to both of the chambers.
In another example of seven rollers in accordance with the invention there may be one roller fixed centrally, three plain rollers at substantially 120 degree locations in external nip contact therewith, two pervious rollers each in nip contact with two of the three plain rollers and one sealing roller in nip contact with two of the three plain rollers whereby to define chamber means at substantially 120 degree directions outside the roller fixed centrally.