Busker, Leroy H. (Rockton, IL)
Daane, Robert A. (Rockton, IL)

05/278600

10/08/1974

08/07/1972

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Beloit Corporation (Beloit, WI)

162/205

162/900, 162/358.300, 100/120, 100/151

B30B9/24; D21F3/02; B30B9/02; D21F3/02

162/358,205,DIG.1 139/383A 34/243F 100/151,120

3214330	Duplex fabric paper press	October 1965	Wicker et al.
3293121	Pneumatically pressurized paper wet press assembly	December 1966	Martin
3748225	FIBROUS WEB PRESS NIP STRUCTURE INCLUDING NONPOROUS BELTS BACKED BY PISTONS SUPPORTED WITH FLUID PRESSURE	July 1973	Busker et al.

Bashore, Leon S.

Fisher, Richard V.

Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson

We claim as our invention

1. The method of pressing liquid from a traveling fibrous web while the web passes through an extended press nip which comprises,

2. A press mechanism for removing liquid from a traveling fibrous web which comprises,

3. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means is hydrophilic and said porous sheet has a hydrophobic characteristic.

4. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means has been treated with a wetting agent.

5. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is constructed of polytetrafluoroethylene.

6. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is coated with a hydrophobic material.

7. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said porous sheet is integral with the felt and forms a layer between the web and the more absorbent outer portion of the felt.

8. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a pair of traveling belts each having a fluid backing pressure chamber for transmitting fluid pressure to the web through the belts.

9. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said pressing surfaces are formed by a supporting rotating roll on one side of the web and a tensioned belt wrapping an arc of the roll on the other side.

10. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said felt means and said sheet are integral and form one of the surfaces for applying a pressing force to the web and the other surface is formed by a rotating roll with the felt wrapping an arc of the roll, and including means for tensioning the felt.

11. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a stainless steel.

12. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein said sheet is formed of a plastic material.

13. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein the porous sheet has permeability on the order of .5 × 10^-10.

14. A press mechanism for removing liquid from a traveling fibrous web constructed in accordance with claim 2 wherein the ratio of period of time of application of pressure to the release time is on the order of 20 to one.

BACKGROUND OF THE INVENTION

The invention relates to improvements in press mechanisms such as used in paper making machines wherein a pressure is applied to a traveling web to squeeze liquid therefrom and more particularly to such a structure wherein rewetting, i.e., the return of water from the felt to the web is appreciably reduced.

In a conventional two roll wet press, the paper web is passed through the nip of the press with a felt and water is pressed from the web into the felt during the ingoing or converging region of the press rolls. During the diverging or outgoing region, some of the expressed water transfers back from the felt to the paper web. This transfer of water from the felt to the paper in the expanding nip region is termed rewetting or rewet.

The driving force moving the water from the felt to the paper in rewetting is generally believed to be expanding voids which develop a vacuum in the expanding paper along with a capillary holding force favoring transfer to the paper.

In the two roll conventional wet press, the time of compression is nearly equal to the time of expansion inasmuch as the ingoing nip is equal in width to the outgoing nip. The present invention employs the principles of an extended nip press which offers the possibility of accomplishing a difference in the time period of compression relative to the time period for expansion. The time period of compression will be longer than the interval for expansion by a factor of 10 or more.

It is believed that rewetting is a function of time. That is, it is a flow phenomena and advantage can be taken of the difference of time of compression and the time of expansion to control the amount of rewet that occurs. This is accomplished by controlling or deterring the speed of travel of the liquid between the web and the felt so that by providing an adequate compression time even with a retarded flow of water from the web to the felt, adequate pressing action of the press and drying of the web will occur, yet the flow of water will be impeded sufficiently so that rewetting is substantially and appreciably reduced.

It is accordingly an object of the present invention to provide a press structure for pressing water out of a traveling web which is capable of providing a dryer web by reducing the effects of rewetting.

By utilizing the principles and advantages of an extended nip press, improved drying can be obtained in the press, and this improved drying effect can be wholly retained and improved by employing the principles of the present invention. In the copending application of Busker and Francik, Ser. No. 90,921, now U.S. Pat. No. 3,784,225, the principles and advantages of pressing a web for an extended period of time as discussed. Some of the features of an extended nip arrangement will be presented as follows.

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 fourdrinier wire, 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 then 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 percent wet basis moisture (ratio of water to fiber plus water) and leaves the press section with approximately 60 percent 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 water removed in the press section can be increased to decrease moisture from 60 percent to 50 percent, the drying load on the dryer section can be reduced by 1/3. This is significant in a typical 3,000 feet per minute newsprint machine which employs on the order of 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 stream 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 formation and better pressing will shorten the necessary time in the dryer section and permit higher speeds.

It is accordingly an object of the present invention to provide an improvement in the press section of a paper machine which makes it possible to remove an increased amount of water in this press section and makes it possible to provide a press section having only a single pressing nip of a unique elongated or extended nature which does not have the performance limitations of conventional roll couple presses and which requires 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 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 web 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 54 per cent moisture rather than 60 per cent moisture 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 losses in dryness 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 effect develops during the passage of the wet mat through the mat 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.

It is a further object of the present invention to provide a press of the type above described which substantially avoids rewetting and retains the achievements of an extended nip press by employing a thin sheet of a predetermined porosity between the web and felt to retard the rate of flow of water from the felt back to the web.

Other objects, advantages and features will become more apparent with the disclosure of the principles of the invention and it will become apparent that equivalent structures and methods may be employed in connection with the description of the preferred embodiment teaching of the invention in the specification, claims and drawings in which:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic showing of a structure embodying the principles of the present invention;

FIG. 2 is a graph illustrating the relationship of compression time to expansion time in an extended nip press in accordance with the invention;

FIGS. 3 through 5 show schematically three additional separate embodiments of the invention; and

FIG. 6 is a schematic side elevational view of another form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a liquid containing web such as paper coming from a fourdrinier wire is shown at W passing through the nip of an extended nip press. The web passes through the nip with a porous felt F which receives water pressed from the web. The felt is formed in the loop and normally will have felt dewatering means which reduces moisture although its moisture content has been found to not be critical.

In accordance with the principles of the invention, thin porous sheet S is passed between the web and the felt to retard or control the transfer of water between the felt and the paper. The sheet is in the form of a belt and is provided with suitable guide rolls. This sheet may be of various materials such as porous stainless steel or plastic. While the belt retards the flow of water, since the compression time is substantially longer than the release or expansion time, adequate water flows through the sheet to the felt for dewatering, but the retardation of the flow of water will greatly reduce wetting that occurs during the expansion time.

In the press nip which extends for the width of the bearings 16 and 17, hydraulic pressure is transmitted to the web through the belts 10 and 11. Belt 10 passes over guide rolls 12 and 13 and belt 11 passes over guide rolls 14 and 15. The bearings form a hydraulic pressure chamber 18 and 19 above and below the belts. Sealing walls are provided at the edges of the chambers 18 and 19 to prevent excessive escape of liquid, and the wall on the oncoming side of the chamber is shown at 20 for the top bearing and at 22 for the lower bearing. The wall at the offrunning side of the chamber is shown at 21 for the upper bearing and at 23 for the lower bearing. Suitable seals such as Teflon sliding members or lubricated members are provided, and these may be referred to as "sills" on the onrunning and offrunning side. Liquid pressure supply lines 24 and 25 for the upper or lower bearings are provided to maintain a predetermined desired pressure within the chambers 18 and 19.

As the belts are exposed to the pressure chambers, the paper web is subjected to a compression pressure. When the belts pass the trailing sills 21 and 23 of the pressure chamber, the pressure drops off and expansion occurs. This expansion time is preferably kept at a minimum and will be decreased as the width of the sills 21 and 23 is narrowed.

FIG. 2 shows the time pressure relationship as the web passes through the pressing zone. During the period indicated by the portion of the curve 26, the web is under pressure. As it leaves the pressure zone, pressure drops off as indicated by the expansion portion of the curve 27, with expansion beginning at the location of the broken line 28.

The period of time of compression is necessarily longer than the period of time of expansion, and preferably substantially longer having a ratio of 10 to one or greater.

The permeable belt could be a porous stainless steel belt of .010 inches thickness, for example. Other examples would be a low permeability surface covering on the felt or small pore size plastic belts. The permeability can be achieved by porosity of the belt material or by perforation or fabrication of a belt.

If the permeability of the permeable belt is made very high, then its presence has very little influence on the flow conditions in the compression-expansion cycle of wet pressing. Typical moistures might be as follows: Ingoing moisture 83% Moisture at end of compression cycle 43% Outgoing moisture 61%

On the other hand, if the permeability of the belt is made very low, then very little water penetrates the belt during the compression cycle and practically none flows back during the expansion cycle. Typical values might be:

Ingoing moisture 83% Moisture at end of compression cycle 80% Outgoing moisture 80%

There is indicated, therefore, a permeability of the belt which would achieve a maximum water removal in a wet press by allowing optimum water to be expressed during compression (at the long time interval) while minimizing the rewet water returned during the expansion cycle (at the short time interval). A typical set of moisture values for a twenty to one time interval (compression to expansion) are given below:

Permeability, Ingoing Minimum % Moisture Outgoing cm ² Moisture Before Expansion % Moisture ______________________________________ 50 × 10 ^{- 10} 83 41 83 5 × 10 ^{- 10} 83 41 53 1 × 10 ^{- 10} 83 42 45 .5 × 10 ^{- 10} 83 43 44 .3 × 10 ^{- 10} 83 43 44 .1 × 10 ^{- 10} 83 49 50 .05 × 10 ^{- 10} 83 61 61 ______________________________________

Thus a belt having a permeability of 5 × 10 ^{- 10} is too open and allows significant rewet to occur. On the other hand, a permeability of 0.5 × 10 ^{- 10} is too closed and the water is not expressed during the compression cycle. A permeability in the vicinity of .5 × 10 ^{- 10} permits maximum expression of water during compression coupled with minimum water of rewet during expansion and produces an optimum dryness from the wet press operation that cannot be achieved by conventional two roll presses with equal time intervals of compression and expansion.

FIG. 3 illustrates another structural arrangement which may be used wherein the extended nip is provided by passing a web through a nip formed between two surfaces provided by a backing roll 30 and a tensioned nonporous belt 32. The belt is provided with tensioning means not shown, and wraps an arc of the roll 30 so that the pressing zone extends from the time of contact of the belt 32 with the web and the roll until it separates from the web on the roll. The paper web is shown at W, and a felt F is passed over the surface of the roll 30. The roll may be provided with grooves, shown schematically at 34 to aid in the flow of water from the web into the felt and the grooves are necessarily made sufficiently small so as to avoid marking of the paper web W. The permeable barrier sheet S is positioned between the felt and the paper, and as in the structure of FIG. 1, inhibits the flow of water. However, the length of residence time in the nip is sufficiently long to permit adequate flow of water into the felt. When the belt 32 is separating from the roll and the web expands during the release or expansion time, the water flow deterrent or inhibition properties of the sheet prevent water from passing from the felt back to the web.

In the arrangement of FIG. 4, the felt is constructed of a nature so that it can withstand tension and is used for applying the surface pressure to the web. The felt F is provided with tensioning means shown by the arrowed lines 34a to pull up taut over an arc of wrap on the roll 30'. The paper web W is wrapped directly on the smooth outer surface of the roll 30'. A permeable barrier sheet S passes between the web and the felt. The barrier sheet has the same properties of porosity as above described in connection with the structures of FIGS. 1 and 3.

In some instances, it would be desirable to provide a felt having a fabric backing capable of exerting tensions that would be in the range useful for pressing water from the paper with the barrier sheet integral with the felt. This arrangement is shown in FIG. 5 wherein the controlled permeable barrier is shown at S as one layer of the felt with the other layer being shown at F. The web W is laid against the smooth outer surface of the roll 30" and by tensioning the integral felt, pressure is applied to the web over the extent of the pressing nip.

Another form of structure contemplated is illustrated in FIG. 6 wherein a roll 32a is wrapped by belt 34b to perform an extended nip pressing zone with the web W passing through the zone. Between the belt 34 b and the web is a felt 33 which is of a critical construction in that it has a "flow-resistant" composition. As used herein a flow-resistant shall mean a felt composition having a resistance to water flow, or in other words a low permeability of a degree such that the felt would not be feasible for use with a conventional twin roll press. As will be fully recognized by those versed in the paper making art, a felt for use in a conventional press section of the paper making machine must have a certain degree of permeability to be receptive of water being squeezed from the web during the short length of time the web and felt pass through the press nip. Felt makers provide felts which have this permeability. Felts which reach a degree of low permeability or substantial resistance to the flow of water are unusable in a conventional press nip. It is these felts which are used in the embodiment of FIG. 6 and are referred to herein and in the claims as "flow resistant felts."

A further structure which is contemplated is where the permeable sheet S, such as shown in FIGS, 1, 3 and 4 is hydrophobic. The felt will be of a hydrophilic nature. The hydrophobic characteristic of the sheet S will resist the flow of water therethrough but over the time afforded in the extended nip the water will be forced through the hydrophobic sheet. On the off-running side, however, the hydrophobic nature of the sheet will prevent the return of water from the felt to the sheet and the hydrophilic nature of the felt will retain the water.

A method which is contemplated for increasing the hydrophobic nature of the sheet is to coat it with a hydrophobic material. A preferred form of material is that which is known under the trademark Silanox, manufactured and sold by Cabot Corporation of Boston, Massachusetts. This coating may be placed on either or both sides of the sheet. It is also contemplated to use a hydrophobic plastic for the sheet. Polytetrafluoroethylene, (Teflon), is a preferred form of plastic. The sheet will be of relatively thin otherwise impermeable material such as Teflon. The small openings will have a diameter on the order of magnitude of the thickness of the sheet. The spacing between the openings will range from two times the sheet thickness to ten times the sheet thickness. As an example, with the use of a sheet having a thickness of .005 inches, the holes will have a diameter of .005 inches with the spacing between the holes ranging from .01 inches to .05 inches. Such a sheet would have the effect of introducing flow resistance into the system, and would further have the effect of minimizing the fluid film remaining next to the paper web after it is compressed, limiting that film approximately to the water which could reside in the small sparsely spaced holes.

The felt has a hydrophilic characteristic, and in certain circumstances, this should be enhanced by the treatment of the felt with surface active agents or wetting agents. A permeability for wet felt which is preferred is that having an air-permeability at 1/2 inch water pressure difference less than 10 cubic feet per minute per square foot.