Papermaking Press With Inflatable Rolls Having Thin Deformable Outer Shells

Abstract

An extended nip is created in a papermaking roll press assembly whereby the rolls are provided with thin, deformable outer shells and fixed, rigid seal plates in the ends of the shells for holding pressurized fluid within the shells. Separately inflatable annular seals provided between the fixed, end seal plates and the shells also help prevent escape of fluid from within the shells. Support beams extending the length of the roll shells opposite the nip have perforations therein for allowing fluid to pass between the beams and the rotating outer shells of the rolls for reducing sliding friction.

Description

BACKGROUND OF THE INVENTION

The invention relates to improvements in roll presses and more particularly to a press for use in a mechanism such as a paper making machine, wherein the press is capable of providing an extended or elongate nip to subject the traveling web to a longer pressing time to permit overcoming of the resistant hydraulic pressure within the web and obtain improved water removal from the web and the improved effects of an extended nip press.

In the copending application of Busker and Francik, Ser. No. 193,272, the principles and advantages of pressing a paper web for an extended period of time, and the advantages thereof, are discussed. In the present structure the principles of an extended time nip are utilized in a structure affording advantages over prior art arrangements.

As will be appreciated from the teachings of the disclosure, the features of the invention may be employed in the dewatering of other forms of webs than a paper web in a paper making machine. However, for convenience, a preferred embodiment of the invention will be described in the environment of a paper making machine which conventionally forms a web by depositing a slurry of pulp fibers on a traveling fabric or wire belt, transfers the web to a press section where the web passes through a number of press nips formed between roll couples, and the web then passes over a series of heated dryer drums and usually through a calender and this is wound on the roll. The present structure forms the entire press section and takes the place of other forms of press sections heretofore available. Many modifications can be made in this type of overall machine, as to the forming section, the press section, the dryer section, and the structure of the instant disclosure may be employed in pressing webs of various synthetic fibers.

The present invention relates to improvements for the press sections of a paper making machine. In such a machine the web usually arrives at the press section with about 80 per cent wet basis moisture (ratio of water to fiber plus water) and leaves the press section with approximately 60 per cent moisture, with the remaining moisture having to be removed by thermal evaporation in the dryer section as the web passes over a series of heated dryer drums. Because of various inherent limitations in the operation of roll couples forming press nips for the press section in a conventional paper making machine, only a given amount of water can be removed in each nip and, therefore, in a conventional paper making machine, a series of three press nips are usually employed. It has been found impractical to attempt to remove a significant amount of additional water by increasing the number of press nips, although the further removal of water by pressing can greatly reduce the expense and size of the dryer section. It is estimated that if the amount of water removed in the press section can be increased so that the wet basis moisture is decreased from 60 per cent to 50 per cent, the length of the dryer section can be reduced by one-third. This is significant in a typical 3,000 feet per minute newsprint machine which employs approximately 60 - 70 dryer drums. This significance can be appreciated in considering that the dryer drums are each expensive to construct and to operate and require the provision of steam fittings and a supply of steam for each drum. The relative importance of the removal of water in the press section is further highlighted by the fact that one of the most important economic considerations in justifying a satisfactory return on investment in the operation of a paper making machine is to obtain the highest speed possible consistent with good paper properties and the removal of more water through better pressing will shorten the necessary time in the dryer section and permit higher speeds and greater production rates.

It is accordingly an object of the present invention to provide an improvement in a paper machine which makes it possible to remove an increased amount of water in the pressing operation and makes it possible to provide a press section having fewer pressing nips of a unique elongated or extended nature which do not have the performance limitations of conventional roll couple presses and which require far less space in terms of requirements as to the overall length of the press section. By increasing the amount of water removed from the web in the press section, increased speeds are possible with existing equipment, i.e., a given length of dryer section can operate at higher speeds since it is required to remove less water. Also, new equipment can be constructed requiring less overall machine length and expense.

The present invention employs a principle which may be referred to as the extended nip concept wherein the time the web is subjected to a pressing action is greatly extended, i.e., a single pressing is provided having a residence time which exceeds that of the time of the web in a number of conventional roll couple press nips. With the reduction to a single pressing operation, the compound effects of rewetting the web as it leaves a plurality of nips are avoided.

A factor which presently limits water removal from paper by mechanical wet pressing is the flow property of water within the paper sheet. It has been found that other factors are not of dominant significance, for example, the effects of the moisture in the felt which travels with the web are small. It has been found further that the length of time that the web is in the nip, in other words the residence in the nip, can have a significant effect in overcoming the difficulties created by the flow properties of the water within the sheet. It has also been found that merely by increasing the residence time of the web in the nip, the water content of the sheet coming out of the press can be decreased so that the web will have 46 per cent dryness rather than 40 per cent dryness with other variables remaining constant. As is evident, the residence time of a web in a conventional roll couple press nip is limited and can only be increased by decreasing the speed of travel of the web, or can be increased slightly by increasing the diameter of the press rolls, but these factors are indeed limiting. It has been found, for example, that by applying a 1,300 pound per square inch pressure on a web for 5 minutes, a moisture level of less than 30 per cent can be attained. Yet, under the dynamic short term mechanical pressing of a paper machine press section using roll couples, even with a plurality of nips, a great deal of effort is required to maintain moisture levels below 60 per cent.

It has been found that significant decreases in moisture removal occur at higher speeds and that a loss in dryness of over 5 per cent is experienced in going from 300 feet per minute to 1,000 feet per minute with typical press loadings in a suction press. It has been found that a hydraulic pressure or wedge develops during the passage of the wet mat through the press nip. The hydraulic pressure that develops subtracts from the applied load and reduces the mechanical compacting pressure. The result is a loss in dryness. As the machine speed increases, the compacting rates are higher resulting in higher hydraulic pressures within the paper mat. These hydraulic pressures react against the pressure of the rolls and prevent the moisture from being squeezed from the web. The exact value of hydraulic pressure is difficult to determine either by direct measure or analysis because of the space and speeds involved. It is believed, however, that hydraulic pressure predominately determines press performance on machines operating at high speeds. Accordingly, the instant invention relates to avoiding disadvantages encountered with high speed press nips of the conventional type used in most commercial applications today, and provides a substantial increase in residence time within a press nip to allow time for flow to occur within the mat and for the hydraulic pressure to dissipate. The principles of extended nip or extended time pressing are further reviewed in the aforementioned copending application.

An object of the present invention is to provide an improved press which avoids disadvantages of the prior art and which obtains an extended nip pressing action capable of achieving the advantages provided by subjecting a web to a pressing action over an extended length of time.

A further object of the invention is to provide an improved pressing mechanism which is capable of performing the complete mechanical pressing action in a paper making machine, avoiding the necessity of having to provide a plurality of conventional press nips.

Other objects, advantages and features will become more apparent to those versed in the art as will other forms of the invention from the following teachings of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic end elevational view of a press constructed and operating in accordance with the principles of the present invention;

FIG. 2 is a fragmentary somewhat schematic side elevational view showing a press;

FIG. 3 is a fragmentary sectional view taken substantially along line III--III of FIG. 1;

FIG. 3a is a fragmentary sectional view illustrating another form of seal; and

FIG. 4 is a fragmentary sectional view taken substantially along line IV--IV of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in FIGS. 1 and 2, the press includes first and second opposed roll members 10 and 11. These roll members are elongate having a width somewhat wider than the web W passed between them. Each of the roll members is in the form of a thin, flexible roll shell which may be formed of metal, plastic or rubber, sufficiently flexible to deflect along the length of the extended nip N.

The roll shells 10 and 11 are pressurized in their interior to apply a pressure along the extended nip region N which pressure is substantially uniform along the axis of the roll shells. The roll shells are provided with means for sealing their ends, and may have integral end caps, although preferably are provided with stationary seals. Integral end caps will require a deflection or distortion of the material at the area of the nip N as the rolls rotate, whereas the stationary end seal will be shaped in accordance with the profile shown in FIG. 1. The form of stationary end seal is shown in FIG. 3 and will be described later herein.

For pressurizing the interior of the roll shells, fluid pressure inlet tubes 14 and 15 are connected to the shells at their axis. Fluid such as pressurized air is supplied from a suitable source controlled to obtain uniform pressure. The structure would operate with one of the opposing rolls being a rigid cylindrical metal roll and the other being formed of a thin, flexible shell. With one of the rolls being solid and the other having a flexible outer shell, the nip line would have to follow the arcuate contour of the rigid roll. A preferred form incorporates both opposing rolls being formed of a thin, flexible shell in order to obtain essentially a straight line N. In the form illustrated, both roll shells 10 and 11 are pressurized with the same interior fluid pressure.

To prevent the rolls from deflecting away from each other along their lengths, they are backed by the supporting means extending along the length of the rolls, preferably in the form of support beams 16 and 17 for the rolls 10 and 11, respectively. The support beams are constructed with substantial strength so that they exhibit minimum deflection while resisting the loads from the two opposed roll shells away from each other. In some instances where a long nip is employed and the nip forces are high, the support beams may be provided with reinforcing means to prevent their deflection. These reinforcing means for the beam may take the form of bending members cantileverly located at the ends of the beam to apply counteracting force and bend the beams in such a manner that they are bent downwardly at their centers to counteract the greater moment of deflection of the rolls at their center.

To permit rotation of the rolls at relatively high speed while providing means for resisting the pressure loads, a fluid bearing means is provided between the rolls and the beams. The beams are preferably formed with an arcuate concave facing surface 18 and 19 for the beams 16 and 17 respectively. These surfaces extend for approximately 150.degree. to 170.degree. of the arc of the rolls. A fluid bearing is provided between the surfaces 18 and 19 and the outer surfaces of the rolls. This fluid bearing is provided with air or other fluid introduced between the outer surface of the roll shell and the inner surface of the beam such as by openings as shown in FIG. 4.

By passing a felt 20 through the nip with the web, fluid pressed from the paper web will be received by the felt. In some operations, it may be desirable to provide two felts so that the paper web W is sandwiched therebetween. Means will be provided to remove moisture from the felts after separation from the paper web and on a return path to provide a desired condition of reduced felt moisture when again entering the nip.

FIG. 3 illustrates the form wherein stationary end seals 21 and 22 are provided within the flexible roll shells 10' and 11'. The roll shells are backed by beams such as 16'. The stationary end seals 21 and 22 will be shaped in accordance with the profile shown in FIG. 1. The end seals will be shaped so as to provide a nip length of optimum size, although if a longer or shorter nip is to be desired, the shape of the end seals 21 and 22 can be altered. For improved sealing, inflatable annular seals 23 and 24 may be provided between the stationary plates 21 and 22, and these seals provided with a suitable pressurizing means, not shown. Pressure supply lines such as 25 will be provided for pressurizing the interior of the roll shells 10' and 11'.

In FIG. 3a the flexible roll shell 31 has an end seal plate 30 carrying a hollow resilient inflatable seal 32 on its periphery. This seal is connected with a fluid pressure line 33 which inflates the seal to the pressure necessary to prevent the escape of fluid from within the roll shell.

In the arrangement of FIG. 4, the inner surface of a support beam 16" is shown. A plurality of perforations or openings 26 are provided in the concave inner surfaces of the beam 16', and pressurized air supply lines such as 27 lead to these perforations to generate a supporting film of air between the roll shell and the beam in order to provide a fluid bearing. This will permit rotation of the roll shells without wear between their surfaces and the bearing. The outer surface of the roll shell, of course, will be smooth so as to not create any marking on the paper web.

In most instances an annular end drive will be provided for driving the roll shells, but one roll shell might be driven through contact with the paper web and felt. The resistance to rotation will be relatively small in view of the substantially frictionless support afforded by the fluid bearings. Changes in nip pressure may be obtained during operation by changing the pressure of the fluid within the roll shells. Thus, different operating conditions such as different quantities of moisture within the web, and the handling of webs of different characteristics may be accommodated by the same mechanism.

Claims

1. A press mechanism for pressing liquid from a traveling web comprising in combination,

first and second opposed roll members mounted for rotation about parallel axes and forming an extended pressing nip therebetween for receiving a traveling web and expressing liquid therefrom in the pressing zone defined by the nip, at least one of said rolls having a thin deformable outer shell deforming at the nip to extend said pressing zone,

means for receiving water passing with said web through said nip,

fixed rigid seal plates in the ends of the shell for holding pressurized fluid within the shell,

means for pressurizing the interior of said roll shell for obtaining a predetermined pressure in the nip,

a supporting means extending along the length of the outer surface of said one roll opposite the nip preventing substantial deflection of the roll axis away from the nip,

and means for reducing sliding friction between said supporting means and

2. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 1, wherein said supporting means is in the form of an elongate beam extending parallel to the roll and in supporting

3. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 2, wherein said means for reducing sliding friction includes means for introducing a fluid between said beam and said roll so that a fluid bearing is provided supporting and lubricating the

4. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 3, wherein said beam has an arcuate inner surface extending over a substantial circumferential area of the roll and said surface is provided with a plurality of perforations with passages leading to the perforations and the fluid for the bearing is delivered to said passages under pressure for providing a fluid bearing between the roll and

5. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 3, wherein said means for introducing fluid is

6. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 1, and including stationary end seals fitted within the ends of said deformable outer shell so that the shell rotates relative to the seal and the escape of fluid from within the shell is

7. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 6, wherein said seal is hollow and resilient to be inflatable relative to the shell for controlling the operation of the

8. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 1, wherein each of said rolls has a thin deformable outer shell and each is provided with a supporting means extending along its length and preventing deflection of the rolls away

9. A press mechanism for pressing liquid from a traveling web constructed in accordance with claim 8, wherein said means for receiving water comprises a felt.