GB2062716A - Wet presses - Google Patents

Abstract

A press mechanism for dewatering a travelling fibrous web comprises: opposed first and second press rolls 10, 12 defining a nip N therebetween for receiving a travelling fibrous web W and applying pressure to the web for expressing water therefrom; an endless looped belt B passing through the nip formed of a thin noncompressible material, such as metal, with small perforations in the belt for receiving liquid expressed from the web W in the nip; and guide rolls 14-18 for guiding the belt through the press nip. The press roll 12 engaging the belt B is peripherally grooved, and a suction box 23 is provided for removing liquid from the perforations in the belt in advance of entry of the perforations into the nip N. <IMAGE>

Description

SPECIFICATION A press mechanism for dewatering a travelling fibrous web The present invention relates to improvements in methods and apparatus for removidng liquids from travelling fibrous webs, and particularly to an improved press mechanism for a paper making machine.

In the making of paper in a high-speed papermaking machine, the fibrous web in the press section is usually passed between a pair of press rolls to squeeze the water from the web and a wet felt is passed through the nip with the web to receive the water expressed from the web. In paper making this is known as the wet press and is designed to remove as much water as possible from the web before it passes to the dryer section, without damage to the fibre structure of the web and without rewetting the web as it passes through successive press nips, and further without marking the surface of the web. The wet press felt which passes through the nip with the web has traditionally been a necessary and important part of the wet press operation.The wet felt provides a vehicle for receiving and storing water pressed from the paper web in the nip and for transporting the water away from the nip as in the operation of a plain press.

In the more recent development of suction and grooved roll transverse flow presses, the felt serves as a permeable medium between the sheet and the water storage area such as grooves in the roll opposite the wet felt. The wet felt also provides a measure of compliance in the press nip to permit greater manufacturing tolerances in the press machinery. More importantly, the wet felt provides a degree of pressure uniformity for the sheet. Pressing directly against a grooved roll would not be acceptable using current commercial groove dimensions since the pressure on the sheet opposite the groove would be very low compared with the pressure on the sheet opposite the land area. Severe marking of the sheet would result. The wet felt serves to bridge the grooves of the roll so that at the sheet side a more less uniform pressure is applied to the sheet.

Another important function of the wet press felt is to support or convey the wet and relatively weak sheet up to and through the press section. In addition, the sheet will be released from the wet felt very readily when compared to releasing the sheet from the solid surface of the roll. The difference in release properties is used to control sheet direction and sheet transfer through the press section. In sdme constructions, the wet felt also serves as a power transmission belt to drive the undriven rolls in the press section.

While wet felts serve many necessary and useful purposes in the press section, they also suffer from many deficiencies and introduce problems in wet press operation. Perhaps one of the most pronounced deficiencies is a relatively short operating life. In some presses operating at 1000 to 1200 pounds per linear inch, a felt life may be only seven or eight days. Perhaps a life of thirty to sixty days is more typical in industry with some exceptional cases of 120 or more days being experienced. The cost of the felt may be approximately six to seven dollars per square foot. A 200 foot felt of 300 inch width would cost nearly $35,000, and this is a significant factor in operation costs.

Wet felts have short life due to compaction or plastic flow of the fibres, filling or contamination of the fibre network by fines and resinous matter expressed from the wet sheet, and by mechanical damage such as wad burns.

The pressure applied to the sheet by the wet felt is considered uniform when viewed on the scale of 1 inch or more. Recent measurements have shown that at the scale of .125", the uniformity is not very good, and a ratio of 5:1 between maximum and minimum pressure is non uncommon. Such large pressure non-uniformity can cause a reduced sheet water removal in the press. It can also relate to another deficiency of the wet felts, i.e. the propensity of the wet felt to leave a non-uniform impression on the sheet surface or a felt mark as it is commonly called. Such marking can impair printing and otherwise make the sheet lower in quality.

On some occasions, macro-scale basis weight differences in wet felts have been known to cause severe press vibration. The weight variation of the wet felt passing the press nip excites a vibration which gets progressively worse particularly at the speeds of operation of commercial machines. Weight variations of over 5% are not uncommon, and variations smaller than this are difficult to achieve with current wet felt manufacturing techniques.

The wet felt also contains a large reservoir of water, at least part of which is available to rewet the sheet if it is left in contact with the felt. A feít at 33 RO moisture (67% dry) will contain .5 pounds water/pound fibre. If half of this moisture, .25 pounds water/pound fibre, is returned to a newsprint sheet originally at 60% moisture (40% dry), it will be rewet to 86% moisture (14% dry). The same amount of water added to a 42 pound/1 000 ft2 sheet at 60% moisture (40% dry) will rewet the sheet to 70% moisture (28% dry). The following calculations indicate this phenomenon.

FELT: 3ozlft2 - .1875 Ib felt/ft2 33% moisture - .5&num; water/&num; felt .1875 &num; Felt &num; Water &num; Water x.5 -.09375 ft2 &num; Felt ft2 If one half this total water is transferred, then the water to the sheet is: .0468 &num; Water ft2 SHEET: A. 30 &num; F/3000 ft=.01 &num; F ft2 at 60% moisture (40% dry) or 1.5 &num; W/ &num;F &num;F &num;W &num;W .01 x1.5 =.015 ft &num;F ft Adding .0468 &num; W from the felt gives ft2 .0468 .015 .0618 &num; W/ft = 6.18 &num; W/ &num;F or 86% M(14% dry) &num;W .0618 .01 &num; F/ft ft2 B. 42&num; F/1000 ft2 - .042 &num; F/ft2 at 60% moisture (40% dry) or 1.5 &num; W/&num;F .042 &num; F/ft2 x 1.5 &num; W/&num;F = .063 &num; W/ft2 .063 + .0468 .1098 &num; W/ft2 - 2.61 &num; W/&num; F or 72% M (28% dry) .1098 &num; W/ft2 .042 &num; F/ft2 The actual amount of water that can be removed by capillary suction from the felt is unknown, but large rewets have been found to exist under circumstances where the wet felt remains in contact with the sheet for long time intervals after a press nip.

By its construction, the wet felt is also difficult to dewater and it is difficult to remove dirt or contamination therefrom. The fibres of the felt are woven (needled) together to form a mat of usually '' thickness uncompressed. This mat acts as a filter to remove any particulate matter passing to it during water removal, and contains a very large internal area where the dissolved materials can be deposited. The long tortuous flow path through the thickness of the felt prevents any effective flushing of the surfaces by flowing a liquid or gas through the felt when it is to be cleaned. While felt cleaning devices such as high pressure water showers are currently used, they are not highly efficient in cleaning the felt.

The aforementioned difficulties are considered in the solution presented by the present invention wherein an equivalent thickness of water film involved in water removal by wet pressing is removed by a storage means of novel construction.

According to the invention there is provided a press mechanism for dewatering a travelling fibrous web comprising: opposed first and second press members defining a nip therebetween for receiving a travelling fibrous web and applying pressure to the web for expressing water therefrom, an endless looped belt for passing through the nip formed of a thin noncompressible material with small perforations in the surface of the belt for receiving liquid expressed from the web in the nip, and guide means for guiding the belt through the press nip.

In a preferred embodiment of the invention, the belt is in the form of a thin metal endless band with perforations therethrough positioned and shaped to receive water expressed from a web. In considering the water storage capacity which is required, the following is a compilation of equivalent water film thickness for what might be considered as a first and third press nip water removal.

Equivalent thickness of water removed (inches) Newsprint Linerboard 1 st Press .0036 .015 3rd Press .0068 .0029 EQUIVALENT WATER FILM & STORAGE REQUIREMENT

MOISTURE CHANGE WATER REMOVED Belt Thkness For 100% Water Wet Basis Moisture Equiv. Water Storage At SHEET BASIS WEIGHT Dryness % Moisture % Ratio, W/F Lb.W/Lb.F Lb.W/ft Thickness Porosity Of g/m Lb/3000 ft Lb/ft From To From To From To In. 50% 20% 49 30 .01 20 32 80 68 4.0 2.12 1.88 .0188 .0036 .0072 .018 35 40 65 60 1.857 1.5 .357 .00357 .00068 .0014 .0034 205 126 .042 20 32 80 68 4.0 2.12 1.88 .079 .015 .030 .075 35 40 65 60 1.857 1.5 .357 .015 .0029 .0057 .0145 The belt thickness in the above chart refers to the thickness of the thin metal belt with perforations therethrough, the volume of the perforations being indicated by the figure indicating porosity.

In many wet presses, we are dealing with water films that are only .15" to less than 1 mil thick.

One attempt heretofore which has been made to replace wet felt with a porous roll is shown in U.S. Patent Specification No. 3,262,840. In that arrangement a porous roll surface having a porosity of at least 1 5% and a capillary size smaller than those found in paper was used. The water removal capability of the porous roll is theoretically effective, but it is believed that this porous roll is not commercially practical due to the compaction or creep of the porous nylon which reduces the size of openings and permits filling of the capillary structure with contaminants from the paper. The thick roll cover will prevent any effective cleaning of the pore structure.

The following is a detailed description of embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a schematic elevation of a press section arrangement in a paper making machine according to the invention, Figure 2 is a similar schematic view illustrating another form of the invention, Figure 3 is a greatly enlarged plan view showing a pattern of perforations which may be employed, Figure 4 is a view similar to Figure 3 illustrating an alternative pattern, Figure 5 is a greatly enlarged fragmentary section taken through a metal belt constructed in accordance with the invention, Figure 6 is a fragmentary sectional view similar to Figure 5 illustrating another form of construction, Figure 7 is a plan view with portions broken away illustrating in enlarged form details of construction, and Figure 8 is a fragmentary sectional view taken substantially along line VIlI-VIlI of Figure 7.

As shown in Figure 1, a travelling web W enters a nip N between rolls 10 and 11. The upper roll 10 has a smooth outer surface 1 3 to be contacted by the web. The lower roll 11 has a grooved outer surface with a thin porous belt B of incompressible material such as metal passing between the web and the roll 11. The belt B is guided through its looped travel by guide rolls 14, 15, 16 and 18, there also being provided a tension roll 1 7 which is adjustable to adjust the tension in the belt, and also provides a measure of guiding.

A shower 1 9 with a saveall 20 is provided for cleaning the grooves 12 of the roll 11.

The thin metal belt B has patterned perforations on its surface for receiving the water expressed from the web as it passes through the nip N. The grooves 12 in the roll 11 prevent the entrapment of air in the perforations so that the water can pass into the peforations. The water is carried in the perforations of the belt around to where the belt passes beneath cleaning showers 21 and a cleaning box 22 cleans the fibres and other foreign materials from the belt. Prior to the perforations of the belt again entering the nip, a dewatering box 23, which may be in the form of a suction box, is provided for removing the water from the perforations so that they can again receive water from the web in the nip.

In the structural arrangement shown in Figure 2, a pair of rolls 26 and 28 is provided to form a press nip N. While the structural arrangements of Figures 1 and 2 provide roll presses, the porous belt can be employed in what is known as an extended-nip press wherein the web passes through an elongate nip formed between two members such as a roll and a highly tensioned rubber belt or a rubber belt backed by sliding shoes, and in such structure the porous belt will perform the same fuction of receiving the water and carrying it from the nip.

In Figure 2, the upper press roll 26 has a smooth outer surface 27. The lower press roll 28 has grooves 29 on its outer surface and the lands of the grooves support a belt B' passing through the nip with the recesses of the grooves permitting the escape of air as the perforations in the belt fill with water.

The belt B' is trained over a circular path by a cleaning shoe 31 having a pocket 32 therein for cleaning the belt. Showers 33 and 35 are provided to aid in cleaning the belt. An additional shower 30 is provided for cleaning the grooves in the grooved roll 28.

Following the cleaning of the belt is a dewatering shoe 36 having a suction compartment 37 for drawing water out of the perforations prior to the belt re-entering the nip N.

Figure 3 shows a greatly enlarged plan view of the surface of the belt wherein the perforations are arranged in a rectangular or 900 layout pattern.

Figure 4 shows the perforations arranged in a 600 pattern which layout permits a 15% more open area than the 900 layout, for the same size of perforations.

The size of the perforations or holes is preferably in the range of .0001" (.0025mm) to .015" (.38mm).

The thickness of the metal belt is in the range of .002" to .125" (0.5 to 3.2 mm). The thickness will be associated with the water storage area needed. a light weight sheet of paper (50 g/m2) may require a metal belt thickness of only .010" (.25mm), whereas a pulp sheet may require .125" or greater thickness to provide adequate water storage. Thicker belts would not usually be run as a belt over rolls in the form shown in Figure 1, but would be preformed in a circular loop in the structure shown in Figure 2.

The holes may be of various shapes. For example, straight sided cylindrical holes such as shown in Figure 5 may be provided which have no bias in flow restriction or of water holding. In Figure 5, the belt material is shown at 40 with the perforations or holes being shown at 41.

Figure 6 illustrates another form of perforation with the vented side of the belt being shown at 46, and the sheet facing side at 45. The hole 47 is tapered with the smaller end shown at 43 and the larger end facing away from the sheet at 44. This shape of hole may introduce a desirable bias and flow restriction or water holding due to surface tension effects. Other shapes may be employed such as an hour-glass shape having a smaller entry size and a smaller size on the vented side of the belt.

Figures 7 and 8 illustrate the belt, with portions broken away, laid on the surface of a backing roll 50. The roll has ribs and grooves with the lands at the top of the ribs being shown at 52 and the grooves at 53. In this construction, the belt is formed with elongate oblong perforations 54. These perforations are such that they will be of sufficient length to overlap a groove when they are in a position so that a groove is beneath them. Thus, the possibility of a perforation being blocked and consequently causing an entrapment of air, is eliminated.

A loop or belt having a porosity of 50% and a thickness of .007" has adequate water storage for all lighweight sheets and press positions and is also adequate for heavy weight third press positions. Lower porosity requires greater thickness for the same storage capability and vice-versa. The belt must enter the nip completely dewatered or nearly so. A belt of .030" thickness at 50% porosity will cover almost all storage requirements. Venting of the back side of the metal belt to allow air to escape will be provided by grooving the roll or other methods used in transverse flow pressing. A rubber belt or fabric can be provided within the loop of the structure of Figure 1 during part of the run. The rubber belt would, of course, have openings in the surface to permit the escape of entrapped air and the rubber belt would be separated from the metal belt for cleaning and dewatering.

The scale of the surface porosity can be very small (of the order of .004" to .0035") so that for practical purposes, the paper web sees a flat surface of metal. The metal belt or loop can be manufactured to very close tolerance of thickness and uniformity. The sheet will, in essence, be pressed between a metal roll surface which is provided by the thin metal belt and a hard surface provided by the upper roll with said hard surface being provided either by a metal surface or by a hard rubber cover on the upper roll. Measurements have shown that a good pressure uniformity between the metal belt and a rubber covered roll achieves the uniformity necessary.

The paper web or sheet will be pressed against the flat uniform surface of the porous belt on one side and the flat uniform surface of the solid metal roll (or rubber covered roll) on the other, and the effects on the surfaces of the sheet should be identical so that two-sidedness or marking originating in the press nip is essentially eliminated. Porous belts running on both sides of the sheet are also possible for two-sided dewatering without the loss of sheet finish.

The compression pressures applied to the metal belt (e.g. 100 to 1 000 psi) are well within the elastic limits of the metal of the belt, and since the creep of metal is negligible when compared to plastics or other materials, the porous structure will retain its original form for extremely long periods of time. Life of the belt will be affected largely by the stress of bending around the guide rolls. Thus, a loop of metal free running through the nip will have minimal bending stress and will experience chiefly a cyclic compressive stress. The configuration of Figure 2 may be preferred over Figure 1 if a long belt life is being sought.

The relative thinness of the belt makes for greater potential ability to clean and maintain the porous structure away from the nip. Cleaning agents can be readily applied to the belt loop and any number of washing cycles (wetting and dewatering) can be applied.

The thinness of the material again makes for easy watering of the porous belt prior to entering the nip or after the cleaning operations. The pore size could be of the order of that in felts or larger, but the path length is straight and short in the porous belt as opposed to a long and tortuous path in the wet felt. The dewatering of the porous belt should be 100% for the most efficient use of the water storage area and is accomplishable with the perforations provided in the non-absorbent material. It should be possible to achieve this level of dewatering with conventional vacuum boxes, air showers, or other techniques.

The uniform thickness of metal loops or belts should minimise the tendency of press rolls to be set into vibration by non-uniformities in the nip.

While the potential of the metal loop or belt to rewet the sheet may approach that of a wet felt, the extent of rewet in the metal loop is limited to the water pressed from the sheet in the ingoing nip. By contrast, the wet felt in having its own moisture content, can cause a significantly greater rewet.

The adhesion between a paper web and a metal surface is significantly reduced by the surface porosity. Thus, the adhesion of the web to the metal surface approaches zero at high porosity. The surface porosity of the metal belt can be used to increase or decrease the adhesion of the sheet and to control guiding, transfer and release of the sheet as desired.

The cost per square foot for a metal belt should be significantly less, or at least not exceed, the cost of a wet felt. Since the overall square footage required for a belt is considerably less than a wet felt, the total cost will be less, and this decreases in cost is augmented by an increased operating life.

The porous metal belt performs all of the necessry functions of the wet felt, and in addition can improve the sheet finish, enhance the water removal, and has a potential for a very long life and easy cleaning. The metal belt may also not be as easily damaged by wads, wad burns, as conventional wet felts. The belt can be damaged by wrinkling or creasing, and care must be used during installation and handling.

Claims (14)

1. A press mechanism for dewatering a travelling fibrous web comprising: opposed first and second press members defining a nip therebetween for receiving a travelling fibrous web and applying pressure to the web for expressing water therefrom, an endless looped belt for passing through the nip formed of a thin noncompressible material with small perforations in the surface of the belt for receiving liquid expressed from the web in the nip, and guide means for guiding the belt through the press nip.

2. A press mechanism according to claim 1, wherein said first and second press members are in the form of opposed press rolls.

3. A press mechanism according to claim 1 or claim 2, wherein the endless looped belt is formed from metal.

4. A press mechanism according to any of claims 1 to 3, wherein said small perforations in the surface of the belt are uniformly patterned.

5. A press mechanism according to any of claims 1 to 4, further comprising liquid removal means for removing liquid from the perforations in the belt in advance of entry of the perforations into the nip.

6. A press mechanism according to any of claims 1 to 5, wherein said first press member has a smooth surface for contacting a web passing through the nip, and said second press member is an open roll for contacting said belt in the nip, with openings in the surface for aiding the flow of liquid from the web into the perforations in the belt.

7. A press mechanism according to claim 6 wherein said openings in the surface of the second press member comprise grooves in the surface of the roll.

8. A press mechanism according to any of claims 1 to 7, wherein said perforations are circular in shape.

9. A press mechanism according to claim 7, wherein the perforations in the belt are of such a size as to extend to bridge at least one groove for each perforation so that liquid will move in the perforation and air displaced from the perforation can escape into a groove.

10. A press mechanism according to claim 7 or claim 9, wherein said perforations are elongate in shape having a longer dimension transverse of the direction of belt movement than parallel to the direction of belt movement.

11. A press mechanism according to any of claims 1 to 10, including shower means for directing a cleaning shower of water onto the belt following the nip, cleaning means, for cleaning the belt, positioned downstream from the shower means, and means for removing water from the perforations positioned in advance of the entry of the belt into the nip.

12. A press mechanism according to any of claims 1 to 11, wherein the second press member is a roll positioned within the looped belt, and annular guide means are provided within the belt guiding the belt through a substantially circular path from the outgoing side of the nip to the entry side of the nip.

1 3. A press mechanism according to any of claims 1 to 1 2 wherein said perforations have a size in the range of .0001" (.0025mm) to .015" (.38mm).

14. A press mechanism according to any of claims 1 to 13, wherein said belt perforations are tapered so as to have a smaller diameter on the surface facing the web and a larger diameter on the surface facing away from the web.

1 5. A press mechanism according to any of claims 1 to 14, wherein the belt has a thickness in the range of .002" to .125" (.05 to 3.2mm).

1 6. A method of reducing the liquid content of a travelling fibrous web comprising the steps of: applying pressure to the opposite surfaces of a wet fibrous web for forcing liquid from between the fibres of the web, engaging the surface of the web with a thin smooth surface sheet of incompressible material having perforations therein and receiving the liquid expressed from the web into the perforations, and thereafter removing the liquid from the perforations.

1 7. The method according to claim 16, including supporting the perforate sheet against the web with a pressing member having grooves in the surface to allow the escape of entrapped air from the perforations in the sheet while supporting the sheet on the lands between the grooves.

1 8 A press mechansim for dewatering a travelling fibrous web substantially as hereinbefore described with reference to the accompanying drawings.