Extended Fibrous Web Press Nip Structure With Contoured Leading And Trailing Sills

Abstract

A press structure and method for pressing a traveling fibrous web such as a web of paper received from the forming section of a paper machine including a pair of looped traveling impervious belts which come together in a pressing zone and pass the fibrous paper web through the pressing zone against a traveling felt. The belts are urged toward each other by fluid pressure chambers outwardly of the belts and the chambers have side walls which are sealed to the belt at their edges, and the side walls have a leading sill on the oncoming side and a trailing sill on the offrunning side with the leading sill contoured to provide a gradual increase in pressure over a period of time to avoid crushing of the fibers and with a trailing sill on the offrunning side which is short and flat to obtain a sudden drop of pressure at the end of the pressing zone.

Description

BACKGROUND OF THE INVENTION

The invention relates to improvements in presses for extracting water from a continuous traveling web such as a newly formed paper web in a paper machine, and particularly, the invention relates to a structure for providing an extended press nip which applies a pressing force to a web for a longer continuous time than structures heretofore available which merely pass the web through the nip of an opposed roll couple.

In the copending application of Busker and Francik, Ser. No. 193,272, the principles and advantages of pressing a paper web for an extending 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. More particularly, the copending application embodies a pair of opposed looped traveling belts which come together to press a web between them along an extended pressing zone wherein the pressure is applied by opposed fluid chambers exposed to the belts opposite the pressing zone.

In prior art structures generally in the conventional two roll wet presses, the rate at which pressure is applied to the web is chiefly determined by the geometry of the rolls and the web speed. Higher pressures are desirable, but if the pressure is applied at a very fast rate, especially to heavily beaten stock or heavy basis weight webs, the rapidly increasing fluid pressures within the web can cause disruption in the web formation. These rapidly increasing internal fluid pressures within the web result in high local fluid velocity of the water attempting to leave the web, and this disruption in web formation and structure is commonly called "crushing". It is impossible due to structural limitations to avoid this problem, and only minor changes in roll geometry can be practical. When crushing is encountered, the solution that is generally used is to run at lower speeds to decrease the rate at which the necessary pressing force is applied. An alternate method is to reduce press loading or to combine the two, i.e., run at lower speeds and reduce press loading. The solution applied depends upon the overall machine capabilities for water removal, but each of these solutions reduces the machine capacity.

The present invention relates to avoiding the disadvantages of crushing by utilizing an extended nip or hydrostatic bearing nip press of the type disclosed in my above copending application, Busker and Francik, Ser. No. 193,272 wherein the operation and structure are uniquely improved so as to control the rate of application of pressure to the wet web being pressed. This structure accomplishes and achieves the advantages of the extended nip pressure which will be discussed below as follows.

As will be appreciated from the teachings of this 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 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 reduce the remaining moisture from 60 per cent to 50 per cent, the length of the dryer section can be reduced by 1/3. This is significant in a typical 3000 feet per minute newsprint machine which employs approximately 100 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 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 utilize a hydrostatic bearing type of press 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 the 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 46 per cent dryness rather than 40 percent 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 1300 pound per square inch pressure on a web for five 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 percent.

It has been found that significant losses in dryness occur at higher speeds and that a loss in dryness of over 5 percent is experienced in going from 300 feet per minute to 1000 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 wet 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. The principles of extended nip or extended time pressing are further reviewed in the aforementioned copending application.

While the extended nip type of press overcomes the problems of these reacting hydraulic pressures, the present invention additionally permits the application of relatively high pressures in this type of press by avoiding the problem of the rapid application of pressure on the oncoming side of the nip which heretofore has caused web disruption or crushing. The present invention also avoids the disadvantage of gradual releases in pressure on the offrunning side of the nip which tend to cause rewetting, i.e., return of water to the web.

It is accordingly an object of the present invention to provide an improved press mechanism and method which applies a dewatering pressure to a web at a controlled rate which permits a gradual build-up in pressure and velocity of the water leaving the web as it is being pressed so that crushing is avoided.

A further object of the invention is to provide a method and mechanism which applies a gradual build-up of squeezing pressure to a web being dewatered with the increase in pressure being at a controlled rate that permits the water within the web to rush out of the web at an optimum velocity, but which does not attain a velocity so as to cause disruption of the fibers of the web.

A still further object of the invention is to provide a mechanism and method for applying a dewatering pressure to a traveling fibrous web over an extended period of time and suddenly decreasing the pressure to zero at a substantially instantaneous rate to decrease the time for the water to reenter the web.

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

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic side elevational view illustrating the structure of the type embodying the principles of the present invention;

FIG. 2 is an enlarged fragmentary view illustrating details of a portion of the structure;

FIG. 3 is a graph showing position -- pressure relationships as to the web in FIG. 2;

FIG. 4 is another fragmentary view of a portion of the mechanism; and

FIG. 5 is a graph of position -- pressure relationship for FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a hydrostatic extended nip press arrangement wherein a traveling fibrous web W is received such as from a forming section of a paper making machine. The web is carried on a traveling felt F.

The web and felt are carried between a pair of opposed endless traveling belts 10 and 11 which are impervious and may be made of rubber or nylon or similar flexible extremely tough material. The upper belt 10 is guided on rolls 12, 12a, 13 and 14. The lower belt 11 is guided on rolls 15, 16, 17 and 18.

As the belts come together, there is defined between them a pressing zone Z, and pressure is applied to the outer surfaces of the belts opposite the zone by bearing members 19 and 20 which have hydrostatic pressures 21 and 22. Suitable fluid pressure means are provided to direct fluid, preferably water, pressure to the chambers. The chambers have side walls with edges which seal against the belts and the edges are a polished steel or slippery substance which will hold pressure in the chambers. On the oncoming side of the pressing zone Z, the upper bearing 19 has a leading sill 23. On the oncoming side of the pressing zone Z, the lower bearing 20 has a leading seal 24. At the offrunning side of the pressing zone Z, the bearing members 19 and 20 have trailing sills 25 and 26 respectively. These sills extend in a cross-machine direction across the belt, and in accordance with the principles of the present invention, have a predetermined unique shape and length.

The bearings are shown in enlarged detail in FIG. 2.

In accordance with the principles of the present invention, the leading sills 23 and 24 have a configuration which applies a gradually increasing pressure to the outer surface of the belts. With longer sills such as 23 and 24, lubricating means may be provided such as shown at 26 and 27 in FIG. 1 which apply a spray of liquid to the inner surfaces of the belts as the belts approach the sills.

The sills are tapered gradually inwardly toward the belts as they approach the pressurized pockets 21 and 22. While the pockets may be generally referred to as providing the pressing zone, pressure is applied to the web at the beginning of the sill, and this pressure is continued throughout the pressing zone until the belts leave the trailing edge of the trailing sills 25 and 26, so that the entire length of the bearing defines the pressing zone. The pressure applied by sills depends on the space between them and this is adjusted by a mechanism which is capable of moving them toward or away from each other. A preferred arrangement is to support them by hydraulic fluid which is the same or a function of the pressure in the pockets 21 and 22. However, uniform and continuous pressure at a predetermined level is applied for the main portion of the pressing zone for the length of the pockets 21 and 22.

The trailing sills are very short so that after the web leaves the area of the pockets, the pressure drops off substantially instantaneously. This has advantages, and one of the advantages is that the possibility of rewetting with the moisture reentering the web is reduced.

The relationship of pressure to position of the web is shown in FIG. 3, and as indicated by the inclined portion 30 of the curve, the pressure increases gradually as the web enters into the pressing zone. The web is then subjected to a uniform pressure over an extended length of time indicated by the portion 31 of the curve, and at the end of the pressing zone, the pressure drops off abruptly to zero as indicated by the portion 32 of the curve.

In some instances it may be desirable to shape the leading sill to obtain a linear pressure increase as the web enters the pressing zone. This arrangement is shown in FIG. 4 wherein the sills 23' and 24' are shaped to obtain the linear increase shown by the portion of the curve 30' in FIG. 5. For the uniform portion of the pressing zone as the belts are exposed to the pockets 21' and 22', a uniform pressure is applied as indicated by the straight line portion 31' of the curve of FIG. 5. At the end of the pressing zone, short trailing sills 25' and 26' cause a very rapid, almost instantaneous drop off of pressure as indicated by the line 32' of FIG. 5.

With the use of contoured bearing sills, a substantial latitude in design is attained which permits applying optimum pressure within the uniform pressure portion of the pressing zone. Pressure can be built up to this uniform pressure at a maximum rate, yet a rate which will permit the water leaving the web to travel at a velocity which will not disturb the fibers in the web, and which will not result in consequent crushing.

Claims

1. A press structure for applying a dewatering pressure to a traveling fibrous web comprising in combination,

opposed looped flexible traveling pressing belts,

means guiding the belts to travel in opposed relationship for a portion of their length to define a pressing zone through which a web is pressed between them,

means in said zone between the belts for receiving moisture pressed from the web,

a fluid pressure chamber opposed to each of the belts at said pressing zone subjecting the belts to a force to press the web between them,

said chambers each having side walls confining fluid within the chamber with the walls having a leading sill facing the belt on the oncoming side of the chamber and a trailing sill facing the belt on the offrunning side of the chamber,

at least one of said sills on at least one of said chambers being shaped in the direction of belt travel so that a change in pressure applied to the belt along said sill is a function of the shape of said sill,

said sill having a uniform surface across the cross-machine width of the sill,

said shaped sill providing an area of increased pressure on the belts

2. A press structure for applying a dewatering pressure to a traveling fibrous web constructed in accordance with claim 1 wherein said leading sill on at least one of the chambers tapers toward the pressure zone for a

3. A pressure structure for applying a dewatering pressure to a traveling fibrous web constructed in accordance with claim 1 wherein at least one of the leading sills tapers toward the belt in the direction of the pressure zone and the sill is longer than the trailing sill so that the dewatering pressure on the oncoming side will increase over a longer period of time

4. A pressure structure for applying a dewatering pressure to a traveling fibrous web constructed in accordance with claim 1 wherein at least one of the leading sills tapers toward the belt in the direction of the pressing zone and the trailing sill is flat and parallel to the pressing zone and

5. A pressure structure for applying a dewatering pressure to a traveling fibrous web constructed in accordance with claim 1 wherein at least one of said leading sills is shaped so that a linear increase in pressure will be applied to the belt on the oncoming side from the leading edge of said

6. A pressure structure for applying a dewatering pressure to a traveling fibrous belt constructed in accordance with claim 1 wherein the trailing sills are short and flat so that a substantially instantaneous pressure

7. A press structure for applying a dewatering pressure to a traveling fibrous web constructed in accordance with claim 1 and including means for lubricating the outer surface of the belts exposed to said leading sills.

8. A method of pressing a traveling fibrous web for removing water therefrom which comprises,

applying a pressure to the surface of the web through an extended pressing zone of substantial length and increasing said pressure gradually and uniformly across the cross-machine width of the pressing zone for an extended time on the oncoming side of the zone until a predetermined optimum pressure is reached,

maintaining the pressure uniform throughout the pressing zone,

9. A method of pressing a traveling fibrous web for removing water therefrom which comprises,

applying a pressure to the surface of the web throughout an extended pressing zone of substantial length and increasing said pressure linearly and uniformly across the cross-machine width of the pressing zone for an extended time on the oncoming side of the zone until a predetermined optimum pressure is reached,

10. The method of pressing a traveling fibrous web for removing water therefrom which comprises,

applying a pressure to the surface of the web throughout an extended pressing zone of substantial length and increasing said pressure gradually for an extended time on the oncoming side of the zone until a predetermined optimum pressure is reached,

decreasing the pressure more rapidly on the offrunning side of the zone than the pressure is increased on the oncoming,

said pressure being applied uniformly across the width of the web

11. A method of pressing a traveling fibrous web for removing water therefrom in accordance with claim 10 wherein the pressure on the offrunning side is dropped suddenly and substantially instantaneously.