EP0064933B1

EP0064933B1 - Extended nip press

Info

Publication number: EP0064933B1
Application number: EP82630035A
Authority: EP
Inventors: Edgar J. Justus
Original assignee: Beloit Corp
Current assignee: Beloit Corp
Priority date: 1981-04-30
Filing date: 1982-04-30
Publication date: 1986-10-15
Also published as: DE3273815D1; JPS57183491A; CA1181975A; BR8202582A; IN156378B; KR860001618B1; ES8304243A1; JPS5930836B2; FI820642L; EP0064933A1; ES511807A0; AR226644A1; KR830010247A

Description

The invention relates to a press mechanism for removing water from a traveling fibrous web according to the pre-characterizing portion of claim 1.
The residence time of the web in the nip of such a press mechanism is increased over that of a roll couple. The extended press nip provides for more efficient extraction of moisture from the web. Other structures have been provided heretofore which have attempted to increase the time over which a web is subjected to a pressure, and yet permit the web to continue movement at a speed necessary in a high speed papermaking machine. Such structures have met with degrees of success and are exemplified by the disclosures of US-A-3 748 225, US-A-3 783 097 (Re. 30 268), US-A-3 797 384, US-A-3 798 121, US-A-3 804 707, US-A-3 808 092, US-A-3 808 096, US-A-3 840 429, US-A-3 853 698 and US-A-4 201 624. The devices discussed by these previous patents have taken advantage of the knowledge that the static application of mechanical pressure to a wet paper mat can reduce the moisture content in the mat to below 40%. Under the dynamic short-term mechanical pressing which occurs in the usual paper machine where the web is run between a series of nips formed between press roll couples, it is often difficult to maintain moisture levels below 60%. Attempts to obtain increased dryness in the conventional roll couples are usually made by increasing the press nip pressure, but a plateau is soon reached where major increases in roll loading result in relatively small decreases in moisture.
As is known, it is far more efficient to remove water in the press section of a paper machine than in the thermal dryer section and significant reduction in energy costs and significant reduction in the space needed for the dryer drum section of the machine are achieved for every fraction of a percent of moisture that can be additionally removed in the press section. The difficulty of removing moisture in the press section is increased with increase in machine speed because limiting factors are reached in press nip pressures in that compacting and crushing of the web results with higher nip pressures and resultant higher hydraulic pressures within the paper mat. The most feasible way that has been discovered to increase water removal at high speeds has been to increase the residence of pressure time to allow more time for flow to occur within the paper mat and for the hydraulic pressure to dissipate and for water to be pressed out of the web into the felt.
A press mechanism for removing water from a traveling fibrous web according to the pre-characterizing portion of claim 1 is known from US-A-4 201 624. The looped belt of the press mechanism disclosed therein is wrapped over belt guide rolls which are located inside the loop defined by the belt and can be driven. If a driving force is applied to the belt guide rolls, the inner surface of the looped belt, which slides over the respective pressing shoe in operation of the press mechanism, may be subject to abrasion which tends to increase friction between the belt and the respective pressing shoe. Increased friction between the belt and the respective pressing shoe, however, is undesirable.
Furthermore, the known press mechanism utilizes a doctor for removing moisture from the outer surface of the looped belt upstream of the nip. Use of a doctor is disadvantageous in that it may cause abrasion of the looped belt.
A press mechanism of the kind defined in the pre-characterizing portion of claim 1 is also known from US-A-3 783 097 (Re. 30 268). A separate drive for the belt may be employed in that press mechanism to drive it at substantially the speed of the outer surface of a backing roll. However, there is no indication in this prior patent publication as to the location of the belt drive means in relation to the belt.
A press mechanism similar to that defined in the pre-characterizing portion of claim 1 but without a drive means for the belt is disclosed in each of US-A-3 748 225 and GB-A-2 057 027. In each press mechanism the belt will travel only as a result of its contact with a moving felt used for receiving water pressed from the web.
An object of the invention is to provide a press mechanism of the kind defined in the pre-characterizing portion of claim 1, in which the drive means are so arranged that the sliding surface of the belt is not subject to any abrasion caused by the drive, and in which traction and guidance of the belt as well as the removal of moisture from the outer surface of the belt are improved.
This object is achieved by the features stated in the characterizing portion of claim 1.
The main advantage of the press mechanism according to the invention is that it provides for an improved treatment of the web in the press nip.
Further features of the invention are stated in the subsidiary claims.
The press mechanism according to the invention accommodates relatively high speed travel of a paper web and the application of a pressing force which is controlled to optimum nip pressure for the type of paper being manufactured.
Furthermore, the press mechanism according to the invention is relatively uncomplicated and capable of continued operation over a long period of time without requiring servicing or adjustment or significant attention that would require stopping the machine.
Moreover, uniform pressing, pressure across the width of the travelling web _Tor more uniform water removal can be obtained by the press mechanism according to the invention.
An advantage of the press mechanism according to the invention also lies in that it improves the quality of the sheet, improves moisture removal, performs a better pressing operation and reduces rewetting.
Other advantages and features, as well as equivalent structures which are intended to be covered herein, will become more apparent with the teaching of the invention in connection with the disclosure of a preferred embodiment thereof in the specification and drawings, in which:
The single figure of the drawings, labelled figure 1 is a somewhat schematic side elevational view of a press section of a papermaking machine embodying the structure and principles of the present invention.
A nip N for pressing and dewatering a web is formed between first and second shoes 10 and 11. The shoes have a relieved leading edge shown at 12 and 13 leading to a pressing face 14 and 15. The pressing faces each face the extended nip and are smooth and essentially straight so that they apply a pressing pressure to the web W during the time it is passing through the nip.
As will be recognized by those versed in the art, the shoes extend laterally across the web for slightly longer than the web width, and can be referred to as being elongate in the direction transverse of the web travel as indicated by the directional arrow on the web.
The web is sandwiched between felts 18 and 19 which provide means for receiving water pressed from the web, and the felts and web are carried through the nip sandwiched between looped endless impervious belts 16 and 17. The webs guided into the nip and guided away from the nip by suitable guide members, not shown, and the felts are similarly guided into and out of the nip by rolls such as 18a and 18bforthe upperfelt 18, and rolls 19a and 19b for the lower felt 19.
The looped belts 16 and 17 are made of extremely strong reinforced rubber or similar material, and are driven at the speed of travel of the web so that they carry the web and felts through the nip, and the belts are tensioned on guide rolls shown for the belt 16 at 16a, 16b, 16c and 16d. Roll 16c may be a tension roll movable in the direction of the arrowed lines by suitable tensioning mechanism. The lower belt 17 is guided by similar tensioning rolls 17a, 17b, 17c and 17d, and suitable mechanism is provided for the tension roll 17c to move it in the direction of the arrowed lines for maintaining the desired tension in the belt 17.
An important feature for proper treatment of the web is the means of driving the belts, each of which is wrapped over a grooved traction and guide rolls 20 and 22 for the belts 16 and 17 respectively. The belt is wrapped over these rolls for an arc preferably in excess of 90°, and it has been found that an improved tracking and driving relationship is attained by providing uniform circumferential grooves in the surface of the roll with the grooves shown schematically by the broken lines 21 and 23 for the rolls 20 and 22 respectively. These grooves are annular recesses cut at uniform intervals along the surface of the roll leaving flat land areas therebetween. These grooves apparently permit the escape of any moisture which may be entrapped between the belt and roll, and permit the belt to depress slightly into the grooves due to its being tensioned to thereby increase the traction between the belt and rolls and also increase the uniformity of drive and guiding relationship. Uniform treatment of the belts is necessary so that the web and felts can be dragged uniformly through the nip.
The grooved traction rolls 20 and 22 engage the outer surface of the belt, which is the surface that is adjacent the felts in the nip. Thus, no driving or traction forces are applied to the inner surface of the belts which surfaces slide over the shoes 10 and 11. Any abrasion caused by the drive is not directed at the sliding surface of the belt. Further, it is to be noted that it is the outer surface of the belt which will be wet from its contact with the felt so that this wetness on the belt surface has been found to be accommodated by the unique grooved traction rolls 20 and 22.
A film of lubricating fluid is continuously developed between the flat surfaces 14 and 15 of the shoes and the belts due to the fact that their leading edges 12 and 13 are relieved and a continuous supply of lubricant is delivered at the location of these relieved edges by jets 24 and 25. The lubricant may be of various substances, such as oil, and is applied in a manner so that each of the shoes acts as a slipper bearing and actually while the shoes press toward each other, the film of lubricating liquid is continuous so that there is a hydraulic transmission of the force of the shoe to the belt through the lubricant. This, of course, reduces the friction between the belt and the shoe, but also provides for a more improved pressing action in that pressure uniformity is enhanced due to the fact that there is a body of liquid between the face of the shoe and the belt, and the liquid must adopt uniform pressure along the elongate length of the shoe.
Because of the hydraulic reaction against the shoe due to the lubricant layer between the shoe and belt, the shoes will tend to tilt to a neutral position. That is, the shoes are each shown as mounted on a central pivot 27 and 29. These pivots are located approximately midway between the front and back edges of the shoe, and the shoes will operate at a balanced position wherein the hydraulic forces of the lubricant ahead of the pivot are the same as the hydraulic forces behind the pivot.
Edge seals may be provided at the edges of the belt to prevent lubricant from migrating around the edge of the belt, but experience has shown that the amount of lubricant passing around the edge of the belt is minimal so that edge seals are not absolutely essential. The trailing edge of the shoes are rounded slightly at 10a and 11 a to reduce the concentration of pressure against the belt as the belts emerge from the trailing edges of the shoes. While each shoe is pivotally mounted by a pivot pin which extends laterally across the entire width of the machine so that the shoe is pivotally supported about its lateral axis, in some instances, it may be desired to pivotally mount only one of the shoes. This will still achieve the hydraulic balance in the nip inasmuch as the one pivoted shoe will pivot to a neutral position with hydraulic forces ahead of and behind the pivot being uniform.
The shoes are supported on opposed heavy beams 26 and 28 above and below the shoes 10 and 11. As referred to above, the pivot pin 27 may be omitted in some instances, and the shoe 10 mounted rigidly on the beam 26, and only the pivot pin 29 provided. This is an alternate structure, but the pivotal support for each of the shoes is preferred. The pivot pins can be so located relative to the leading and trailing edge of the shoe so that uniform pressure occurs throughout the length of the extended nip or may be set slightly downstream in the direction of the web movement so that a slightly lower unit nip pressure occurs ahead of the pin than after the pivot pin. The shoes will assume a position of balance wherein the total hydraulic forces of the lubricant in the nip ahead of the pivot pin equal those behind the pivot pin.
For applying the pressure to the shoes to attain the pressing force in the nip, the lower shoe is mounted on a piston 30 seated in a cylinder 31. Hydraulic liquid is delivered to the piston beneath the cylinder by suitable means indicated by the line 32. The cylinder or channel 31 beneath the piston is preferably continuous, or if separated, the same pressure is applied to each of the chambers so that the upward force on the shoe is uniform throughout its length across the machine. If the cylinder or chamber 31 is continuous, since the hydraulic fluid will be at the same pressure throughout the continuous cylinder 31, a uniform upward force will be applied to the shoe throughout its length insuring that the pressing force in the nip will be uniform across the machine. With high nip pressures, the upper beam 26 may bow upwardly slightly in its middle, but the shoes 10 and 11 are sufficiently limber so that they will bow with the beam without introducing any significant forces due to their resistance to bending, and thus the uniform pressure in the nip will not be altered. Similarly, the piston will bow slightly to conform to the bending of the upper beam 26, but the amount of curvature due to this bowing will be insignificant and will not affect the uniformity of pressure in the nip across the machine. If the lower beam 28 bows downwardly, this shape will not be transmitted to the shoes because the sole upward force on the piston is derived from the force of the hydraulic liquid in the cylinder 31 beneath the piston.
In some instances, it may be desirable to support the upper shoe on a piston and cylinder arrangement similar to the piston and cylinder 30 and 31 so that a force is applied both to the upper shoe and to the lower shoe. Or, of course, it will be understood that the lower shoe may be supported directly on a beam on a pivot pin without a piston and cylinder, and the piston and cylinder be provided for the upper beam and upper shoe. If a single piston and cylinder is used, it is preferred that it be used on the lower beam 28, because the weight of the upper beam 26 tends to counteract the upward reaction force from the shoe 10 applying pressure to the nip.
In operation the two belts 16 and 17 are driven at the speed of travel of the web due to their frictional tracking engagement with their driving rolls 20 and 22. The felts 18 and 19 are carried through the nip with the web between them, and as water is pressed from the web from the time it enters the nip N at the leading edge of the shoes 10 and 11 to where it leaves the shoes, it is received by the felts which are dried in the usual manner. Control of the pressure in the nip is obtained by the pressure of the hydraulic fluid delivered through the line 32 to the cylinder chamber 31. The shoes can be held in loose engagement with the belts at start-up and pressure increased as the machine reaches operating speed, and the pressure adjusted in accordance with the desired operation and the amount of water to be pressed from the web in the extended nip.
The driving rolls drive the belts at uniform speed maintaining constant and uniform traction despite any water which remains on the surface of the belts. It is to be understood that the advantages of the unique driving rolls may be employed in other forms of extended nip presses, such as, for example, structures such as shown in Re 30,268 by forcing a grooved driving roll into driving contact with the outer surface of the belt.
Other forms of structures and modifications of the method falling within the scope of the invention are intended to be covered herein, and applicant is not limited by the specific preferred embodiment of structure illustrated.

Claims

1. Press mechanism for removing water from a traveling fibrous web (W), comprising:

an elongate extended press nip (N) formed between a first stationary pressing shoe (10) and an opposed pressing element, both having elongate pressing faces facing said nip (N) for applying a pressing force to the traveling fibrous web (W) moving through said nip (N);

means in said nip (N) for receiving water pressed from said web (W);

a first traveling looped belt (16) passing through said nip (N) between said first stationary pressing shoe (10) and said water receiving means;

means (24) for supplying fluid to between said pressing shoe (10) and said belt (16) to thereby provide a hydraulic fluid pressure between the pressing shoe (10) and the belt (16);

guide rolls (16a-16d) arranged within the loop defined by said belt (16); and

drive means for said belt (16),

characterized in that said drive means comprises a drive roll (20) disposed outside the loop defined by said belt (16) to engage the outer surface of said belt, said drive roll (20) having circumferential grooves (21) for traction and guidance of said belt.

2. Press mechanism for removing water from a traveling fibrous web, according to claim 1, characterized in that the opposed pressing element comprises a second elongate pressing shoe (11), that a second traveling looped belt (17) passes through said nip (N), the two belts (16, 17) sandwiching the web and water receiving means therebetween, that means (25) are provided for supplying fluid to between said second elongate pressing shoe (11) and said second traveling looped belt (17) to thereby provide a hydraulic fluid pressure between said second pressing shoe (11) and said second looped belt (17), that guide rolls (17a-17d) are arranged within the loop defined by said second belt (17) and that drive means are provided for said second belt (17), said drive means comprising a second drive roll (22) disposed outside the loop defined by said second belt (17), said drive roll (22) having circumferential grooves (23) for traction and guidance of said second belt (17).

3. Press mechanism for removing water from a traveling fibrous web, according to claim 1 or 2, characterized in that the belt (16, 17) engages the associated traction drive roll (20, 22) along an arc in excess of 90°.

4. Press mechanism for removing water from a traveling fibrous web according to claim 2, characterized in that each of said first and second pressing shoes (10, 11) comprises a relieved leading edge (12, 13), that said means (24, 25) for supplying fluid to between said pressing shoes (10, 11) and the respective belts (16, 17) is formed for delivering said fluid to said relieved leading edge (12, 13) of each of said pressing shoes (10, 11 ).

5. Press mechanism for removing water from a traveling fibrous web, according to claim 2, characterized in that means are provided for supporting at least one of said pressing shoes (10, 11) for pivot motion about an axis transverse to the direction of web travel.

6. Press mechanism for removing water from a traveling fibrous web, according to claim 2, characterized in that both shoes (10, 11) are mounted for pivot motion about axes transverse to the direction of web travel.

7. Press mechanism for removing water from a traveling fibrous web, according to claim 2 or 4, characterized in that means are provided for applying pressing force to at least one of said shoes (10, 11).

8. Press mechanism for removing water from a traveling fibrous web, according to claim 2 or 4, characterized in that means are provided for applying pressing force to both of said shoes (10, 11 ).

9. Press mechanism for removing water from a traveling fibrous web, according to claim 7 or 8, characterized in that said means for applying the pressing force includes a transversely extending fluid support applying a force to the associated shoe (10, 11) which is uniform along the length of the shoe transverse of the direction of web travel so that a uniform force is applied to said web (W) across said nip (N).

10. Press mechanism for removing water from a traveling fibrous web, according to claim 9, characterized in that said fluid support is in the form of a piston (30) and cylinder (31) with the cylinder (31) carried on a relatively rigid support (28), and the piston (30) is supported in the cylinder by pressurized liquid and supports said shoe (11) on a pivot (29) extending transversely of the direction of web travel.

11. Press mechanism for removing water from a traveling fibrous web, according to claim 2, characterized in that said water receiving means is in the form of first and second felts (18, 19) positioned adjacent the traveling belts (16, 17) with the web (W) sandwiched between said felts.