CA1321093C - Wet cellulosic web transfer system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A transfer system for transferring a moving wet cellulosic web between two moving elements of a paper mill without excessive sheet flutter or breakage, supports the web and permits higher web speeds than presently used.
The transfer system comprises a suction roll that forms a nip in contact with the web on the surface of a roll or web supporting belt, a doctor blade is positioned in contact with the surface of the roll or web supporting belt immediately after the nip to ensure the web separates from the surface, and an air jet adjacent the doctor blade blows air in a direction substantially opposite the moving web, between the moving web and the surface and the roll or web supporting belt, to guide and support the web towards the suction roll.

Description

1321~9~ .

WE:T CELLULOSIC WEB TRANSFSR SYSTEH
~ield and Backqround of the ~nvention The present inventlon relates to the transfer and support of wet cellulosic webs between two moving elements ln a paper machlne. More speclfically, the present inventlon relates to the detachment of moving wet cellulosic webs from a press roll or web supporting belt to another moving element in the press section or dryer section of a paper machine.
In the fabricatlon of paper, a suspenslon of cellulosic fibres, referred to as a furnish, is spread on one or more moving forming fabrics or carriers and the bulk of water drained away. Thls cellulosic web or sheet, which ls initially weak and wet, is transferred onto a press felt which carrles it into a press nip formed by two press rolls. The mechanlcal compression between the two press rolls compacts the web and eliminates part of the water form the wet web. The web usually leaves the press nip adhering to one of the press rolls, and must be peeled from the roll before it can be transferred to the next sectlon of the paper machine. Paper machines generally have one to four presses ln the press section followed by a dryer section with heated dryer rolls, to evaporate most of the water remaining ln the pressed web. In the fabrlcation of some paper grades, the dry web is moistened by the application of an aqueous suspension of sizing agents. This occurs in a size press after a first drying stage, and the moist sized paper is then again transferred to a second dryer sectlon where it is dried for a second tlme.
Whlle in the different sections of the paper machine, the wet celluloslc web is usually supported by a pervious belt such as formlng fabric, press felts, and drying fabric, or by other means such as a press roll. A
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mechanical support is often unavallable durlng web transfer between the lndividual moving elements of the machine. Thus during web transfer there is an lncreased danger of the web or sheet braking, especially if it is molst and the machlne operates at hlgh speed. To reduce the danger of sheet breaks it is sometlmes necessary to reduce the machine speed, even though thls leads to a decrease in productlon. The danger of sheet breaks is sometimes reduced by the additlon of chemlcals or be lncreasing the proportion of a stronger, but more excessive, component such as chemical pulp or long flbre pulp ln the furnlsh or initlal flbre mix.
The most crltlcal areas of sheet transfer are from the forming section to the press sectlon, between the consecutive presses in the press section, and between the last press in the press section and the first roll in the dryer section. In all of these transfer areas, the web or sheet ls still wet and thus ls comparatively weak.
Several methods have been used for transferrlng the sheet at these areas. In one method the sheet is pulled unsupported from one element to the next through a so-called ~open drawn. The wet sheet in the open draw is unstable at hlgh speeds and .eacts to small variatlons ln the process, sometimes havinq a tendency to oscillate or flutter. An excessive sheet flutter can cause deformations and wrinkling of the web and reduce the product quality or completely break the sheet and interrupt production. Thus, paper machines with an open draw between the former and the first press rolls usually operate at speed below 750 metres per minute.
All the machlnes operating at high speeds, that is to say, in excess of 1,000 metres per mlnute provide a continuous support of the web from the former at least the first nip in the press section. On machines with multiple ~ 3 ~ 1321~9~

roll press arrangement, the web ls continuously supported up to the second or third press nlp. However, on all present paper machines, the sheet passes through an open draw as lt is peeled from the roll of the last press.
In the open draw method o transfer, the reduction of excesslve sheet flutter and stabilization of the web ls sometimes achleved by increasing the tenslon ln the web.
The tenslon requlred to peel the web and to stabilize lt in the open draw transfer may, in some instance~, be sufflciently great to cause a brea~ in the web and even if it does not break, a high tenslon can permanently stretch the web and, therefore, make it more susceptible to breaks during the subsequent operations on the paper machlne.
This reduced extensibillty ls preserved even ln the flnlshed product and can lead to an increased number of paper sheet breaks during converting or printing operatlons.
Another method of transferring a web from a pervious carrler or belt such as a forming fabric to another pervlous carrier such as a press felt is 'with the assistance of a drilled roll equipped ~ith a vacuum chamber. Most high speed paper machines use such a vacuum pick-up system to transfer the web from the former to the first press roll. In a vacuum plck-up system, however, a suction roll can only efficiently transfer a web from a pervious carrier to another pervious carrier. Press rolls are generally solld rolls and thus a vacuum system such as a suction roll cannot by itself initiate peeling of a web from a solid press roll or even an impervious belt. In the case of a press roll, the web normally adheres better to the smoother and less pervious surface.
Since separation of the leading edge of a wet web from a press roll or web supporting belt is difficult to achieve, paper machine~ are commonly lnitlally threaded 1321 ~9~

wlth only a narrow band of the web whlch ls so~etlmes referred to as a ~talln. When thls narrow band has been successfully threaded through the length of the machine,lt is gradually wldened untll the full wldth of the paper machlne ls achleved. Thls narrow band of paper ls lnltially very weak because lt ls so narrow and alr currants in fast running machines frequently cause the narrow strip to break, thus prolonging the start-up procedure. All the paper produced during machine start-up ls unusable and must be recycled. If the machlnethreading tlme could be shortened and the machine threaded with the full wldth of the sheet or web, then productlon losses would be decreased and a higher efficiency achleved.
Undeslrable materlals, which generally represent fractions of celluloslc flbre~, often adhere to various paper machine rolls such as press rolls, dryers or calender rolls, and are commonly removed by so-called "doctor blades~ which have sharp edges posltloned in close proxlmity to the surfaces of the machlne rolls and peel off the web and fibres adhering to the roll. The web removed ln thls manner is generally densely crimped or creped and cannot be converted into a smooth paper.
Creping of a web by a doctor blade may be applied commerclally to produce soft and bulky tissue paper used primarily for hyglenlc products. For high bulk and softness, it is desirable that the tissue paper has regularly and densely spaced creped ridges. Good creplng requires a sharp doctor blade and an optimal contact angle between the blade and the impinging web. Canadian Patent no. 1,044,459 and Japanese Patent No. 43160 disclose methods of creping by using a hollow doctor blade from which a flat jet of compressed alr is blown from a location ad~acent the blade. Both of these patents have ~ 5 ~ 1321~93 as a prlmary objectlve,the reductlon of the wear of the roll through a reductlon or ellmlnation of blade contact with the roll. These hollow doctor blades were deslgned for productlon of creped paper rather than for inltlatlon of the transfer of a wet cellulosic web ln the press sectlon or lmmediately prior to the dryer section.
Because creplng occurs when a web ls removed from a smooth surface, such as a press roll by a blade, doctoring has not been used as a means of transfer for wet celluloslc webs to produce paper which requires a smooth surface.
Summarv of the Inventlon It is an alm of the present invention to provide an apparatus and method for the detachment of a wet celluloslc web from a press roll or web supportlng belt and contlnuously support thls web durlng lts transfer to a subsequent movlng element.
It ls a further alm of the present invention to provide a transfer system which permits safe transfer of a tall or a full width sheet or web durlng the start-up of a paper machine. It is a still further aim to provide a transfer system to transfer a wet fibrous web at web speeds greater than 1,000 meters per minute. Yet a further aim is to transfer webs which are weaker than those transferred on existlng paper machines without the necessity of having to increase wet web strength and to reduce the number of breaks that occur in conventional paper machines.
~ he present lnvention can be used to transfer a tail or a full width strip between a press roll or a web supporting belt to a following movlng element and comprises a doctor blade to initially separate the web from the roll or carrler, an air jet that blows air in a directlon opposite to the movement of the web, between the web and the roll or carrier, and a vacuum or suctlon roll 13?,1~9~

that may have a pervlous belt thereon to retain the web as lt is transferred from the press roll or carrler.
The presen~ lnventlon provides a system dedicated to transfer a fast movlng web of flexlble material from a surface of a flrst web supportlng movlng element to a second web supporting movlng element while contlnuously supportlng said web during the transfer thereof, comprlsing in combinatlon.
a first web supportlng moving element a suctlon roll in contact with sald web, deflnlng a nlp with sald flrst movlng element;
a doctor blade in contact with said surface immediately after said nip to cause separation of sald web from sald surface; and means for produclng an alr jet adjacent said doctor blade, between said web and sald surface and in a direction substantially opposlte the dlrection of movement of said web, said air jets constltuting ~eans for supportlng and guiding sald web toward said suction roll.
ln a preferred embodiment, a pervious belt moves through the nip, around the suction roll and the web ~s transferred to this pervious belt.
In another embodiment, the doctor blade and air jet comprise a unitary assembly with an air plenum connected to a tapered alr chamber culmlnatlng ln two llps with a gap between the lips forming an air jet. One of the two lips forming the doctor blade is positioned in contact with the surface of the roll or web supporting belt.
In yet another embodiment, the air plenum and air chamber form a unitary assembly, said assembly being movable between first and second posltlons, in said flrst posltion sald assembly contacts the surface of the roll or web supporting belt so that the alr jet therefrom ls directed towards the nip formed by the press roll and 1~2109~

suction roll, ln said second position sald assembly belng positioned so that it ls ln contact wlth said surface.
In a still further embodiment, there ls provlded ln a method of forming a web of flbrous sheet materlal, lncludlng the steps of forming a wet web of celluloslc fibres, movlng the web through a press section havlng a plurallty of press rolls to a dryer sectlon, the improvement of transferrlng the movlng web from a press roll to a followlng moving element, comprlslng the steps of: feedlng the movlng web around the press roll through a nip formed between the press roll and a suction roll, detaching the moving web from the press roll immedlately after the nlp by a combinatlon of a doctor blade and blowlng a jet of air ln a direction counter to the movlng web between the press roll and the web, and gulding and supportlng the moving web to the following moving element by a comblnatlon of the alr ~et and suction from the suction roll.
Brief Descriptlon of the Drawinqs FIG 1 is a schematic side elevational view of a solid roll with a suction roll forming a nip and a conventional doctor as provlded ln the prior art;
FIG 2 is a partial slde elevational vlew of a combined doctor blade and air jet according to one embodiment of the present invention;
FIGS 3 and 4 are partial slde views showing different shapes of alr chambers for the combined doctor blade and air jet;
FIGS 5, 6 and 7 are detailed side views showing different edges for doctor blades;
FIG 8 is a partial side elevational view showing a combined doctor blade and air jet positioned adjacent a solid roll forming a nip with a suction roll;
FIG 9 is a schematic side elevatlonal vlew of a paper - 8 - 13210~3 machine showlng the transfer system of the present lnvention positioned to transfer a web from the last nlp of the press section; and FIG 10 is a schematic side elevational vlew of yet a further embodlment of a transfer system according to the present invention wherein the transfer occurs between an impervious web supporting belt and a pervious dryer fabrlc.
DescriPtion of the Preferr~d Embodiment Referrlng now to the drawings, FIG 1 illustrates a solld roll 10, which is the last roll in a press section of a paper machine, with a web 12 of wet celiulosic fibres moving on the roll 10 from a nip 14 with the previous press roll 16. A suctlon roll 18 forms a nip 19 with the solid roll 10, and a pervious belt 20, in the form of fabrlc belt, moves around the suction roll 18 and through the nip 19. A second carrier belt 22, whlch is also pervious, ls conveyed about a further roll 24 positioned beneath the suctlon roll 18 to provide a passage for the web 12 between the first carrier belt 20 and the second carrier belt 22. ~he suction roll 18 is to provide suctlon to separate the web 12 from the surface of the solid roll 10 and direct lt between the carrier belts.
However, because the solid roll 10 does not allow air to pass therethrough, the suction roll 18 has little effect in separating the web 12 from the solid roll 10. As can be seen in FIG 1, the web 12 passes down to a conventional doctor blade 26 whlch separates the web 12 and crimps or crepes the web as it is separated from the roll 10. EIG
1 illustrates a prior art arrangement which is not considered satisfactory.
A doctor blade and alr ~et assembly 30 are shown in FIG 2 whlch comprises an alr plenum 32 ln the form of a plpe with a slot or a series of holes 34. Air passes into - 9 - 1321~93 an alr chamber 36 formed by two tapered walls 38 whlch taper down to a first llp 40 and a second llp 42. FIG 2 shows the flrst llp 40 representlng a doctor blade ln contact with the surface of a press roll 10 so that the web 12 ls shown to separate ad~acent the doctor blade lip 40. The second llp 42 is shown to extend not ~o far as the first lip 40 and a gap 44 or slit between the two llps 40 and 42 provides a longltudinal air ~et to e~ect a flat ~et of alr. Whereas the word "jet" ha been used throughout the speciflcation, this terminology includes a longitudinal stream of air as would be e~ected by a gap or slit 44.
~IG 3 shows one embodiment of an air chamber 36 wherein the tapered walls 38 ~oin to a first lip 40 and a second lip 42 which converge inwards to the gap 44 at the ends of the lips 40 and ~2. FIG 4 shows another embodiment wherein the two lips 40 and 42 are parallel to each other, thus the gap 44 represents a parallel gap and provides a flat jet of air therefrom.
FIGS 5, 6 and 7 represent different tips of the doctor blade lip 40. The contact angle alpha as shown in FIG 5 being similar so that used on conventlonal blades wherein the edge of the blade scrapes the roll surface.
One or both llps of the assembly may be replaced if the lip or lips are damaged or worn.
The optimum gap wldth depends on productlon parameters such as machine speed, product ~rade, web adhesive force etc. The gap wldth between the two lips may be between 0.1 and 3.0 mm wide, and preferably is in the approximate range of 0.3 to 0.8 mm.
FIG 8 shows a suctlon roll 18 positioned above the doctor blade and air jet assembly 30. In the lnitial phase of the start-up procedure, the assembly 30 is in position A and the leading edge of the tall or full machine width - 10 - 1321~9~

of web 12 ls detached from the roll 10 by lmpact wlth llp 40 of the doctor blade. The web 10 ls then forced by the air ~et towards the suction roll 18 and 1~ attracted to the roll surface by the vacuum within the suction roll 18.
The web 12 ls thus transferred to the felt 20. Once the movlng web 12 has been transferred to the felt 20, the assembly 30 is no longer required and ls switched to positlon B and the alr supply shut off. If a web break occurs, the web is rethreaded with the assembly 30 ln position A. During an operating period of anticipated web breaks, the assembly ls left in position A.
The suction roll 18 is shown having three zones. The first zone 50, located nearest to the nip 19, has a hlgh vacuum level to assist in establishlng the inltial contact between the web 12 and the felt covered suction roll 18.
The second zone 52 downstream from the first zone 50 is a larger zone and acts as a holding zone wlth a lower vacuum level. For example, the first zone 50 may have a vacuum level in the range of about 10 to 80 kPa and the second zone 52 has a vacuum level in the range of from O to 50 kPa. The second zone 52 is sufficient to maintain and support the web 12 on the felt 20. The third zone 54 provides a small positive air pressure to ensure that the web 12 on the felt 20 is easily parted from the suctlon roll 18 as the felt 20 separates from the suction roll 18.
The air pressure in the air plenum 32 depends on production variables such as doctor gap, width, machine speed, product grade and the web adhesion to the roll, but preferably ranges from about 14 kPa to 600 kPa. The most convenient air pressure for an air blade with a gap width of 0.5 mm was found to be between 34 kPa and 100 kPa.
FIG 9 illustrates the transfer system of the present invention ussd to transfer the web from the plain roll 10 of the last press which is the central roll of a three-'' '' 1321~93 roll inclined press. FIG 10 lllustrates two transfer systems, the second of which transfers from a pervlous or impermeable web supporting belt 60 onto a dryer felt 70.
The devlce could be used for the transfer of varlous flexlble thin materlals and ls partlcularly suitable for the transfer of weak and extensible sheets such as wet paper or paperboard, dry creped hyglenlc paper or non-woven products.

- 12 ~ 13 2 1 ~9'~

A pilot paper machine was used to make paper ln two different ways. Paper was flrst made wlth a dlrect transfer from the press sectlon using the alr doctor S transfer roll arrangement of the present lnventlon, and secondly, paper was transferred from the solld roll using the conventlonal open draw operation. The tenslle propertles of the paper so made were then compared.
The pllot machine consisted of a roll former wlth a suction pl~k-up, a three-roll incllned press and a sampler as lllustrated in FIG 9. The transfer system was installed between the second press nlp and the sampler as shown ln FIG 9. The paper machlne was producing a web 0.33 meters wide with a basis weight of about 50 grams per square meter at 800 meters per minute using a newsprint furnish. The first and second press nip loads were 60 and 120 kN/m respectively, and the solid contents of the paper after the second press was approximately 42%.
The wet paper used for the laboratory test was reeled with minimum draw (less than 1%) for the air doctor transfer roll experiments and at several draws between 2 and 4% for the open draw experiments. The experlmental results are shown in the following table.

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TABLE
P~rr_~___tch (Open draw ver~us air doctor - transfer roll arrangement) STRETCH %
Wet PaPer ~ DrY PaPer Draw Stretch Stretch % ~ %

2.1 3.1 1.4 2.1 3.6 1.4 2.2 3.0 1.2 2.4 3.9 1.2 3.2 3.7 1.2 4.0 2.7 1.2 0.0+ ,4.4 1.5 Sample size 1.5 x 10 cm ~* Sample size 2.5 x 10 cm Strain rate 100 mm/min + Using air doctor-transfer roll arrangement Draw ~ Speed of lSampler) - ~Second Press) x 100%
(Second Press) - 14 ~ 132109~

The wet stretch measurements were done on samples cut from the reel (2.5 x l0 centlmetres) sealed ln a bag, and handled in such a way to reduce moisture loss. Other samples were dried between blotters in a photographlc dryer and cut in strips (1.5 x 10 centimetres). Both wet and dry samples were strained at 100 mm per minute in a laboratory tensile strength tester. The measurements on both the wet and dry paper demonstrate that the paper produced with a transfer arrangement according to the present invention had more stretch than that produced using the open draw system. These results ind~cate that the paper ls less llkely to break in subsequent open draws on the paper machine and ln the converting or printing process.
ExamPle 2 Uslng the arrangement shown in FI~ 9, a 45 gram per square metre web was transferred to the sampler at 1,000 metres per mlnute ten times for ten attempts. After sheet transfer was inltiated by the transfer system of the present invention, lt was malntalned even when the air supply to the air jet was interrupted. WiShout the appllcation of the present transfer system, the transfer of a web having the full machine width could not be accomplished by experienced machine operators.
ExamPle 3 Uslng the transfer system described herein, a web of full machine width was transferred from the press to the sampler of a pilot paper machine at its maximum speed of 1,200 metres per mlnute. Without this transfer system, this could not be accomplished.
The transfer system described and claimed herein is capable of transferrlng wea~ wet webs, such as those made from 100% mechanlcal pulp~. In the past, thls has not been posslble without forfelting speed or having to add a - 15 ~ 1321~9~.

percentage of chemical pulps. Thus the present lnvention permlts the manufacture of paper from weaker and less expenslve starting materlals.
When practising the transfer system described herein, the number of web breaks is reduced and the speed of the paper machine can be increased above the highest speeds of conventional machines, in the order of about 1,400 metres per minute.
Whereas FIG 9 shows only a single transfer system, multiple transfer assemblies may be provided at different locations on the machine.
Various changes may be made to the embodlments described herein without departing from the scope of the present invention which is limited only by the following claims.

Claims (22)

1. A system dedicated to transfer a fast moving web of flexible material from a surface of a solid press roll to a following web supporting moving element while continuously supporting said web during the transfer thereof, comprising in combination:
a web supporting solid press roll;
a suction roll in contact with said web, defining a non-pressure nip means with the press roll to provide a closed draw;
a doctor blade in contact with said surface of the solid press roll positioned immediately after said nip means to cause separation of said web from said surface;
and means for producing an air jet adjacent said doctor blade, between said web and said surface and in a direction substantially opposite the direction of movement of said web, said air jet constituting means for supporting and guiding said web towards said suction roll.

2. The system according to Claim 1 wherein a pervious belt moves through the said non-pressure nip means and around said suction roll, said web transferring to said pervious belt.

3. The system according to Claim 1, including an air plenum coupled to a tapered air chamber culminating in two lips with a gap between said lips forming said air jet, one of said two lips forming said doctor blade.

4. The system according to Claim 3 wherein said two lips have the same length.

5. The system according to Claim 3 wherein one of said two lips forming said doctor blade is longer than the other lip.

6. The system according to claim 3 wherein adjacent walls of said two lips are converging inwards to said gap.

7. The system according to Claim 3 wherein adjacent walls of said two lips are parallel prior to said gap.

8. The system according to Claim 3 wherein the width of said gap (forming the air jet) is in the approximate range of from 0.1 to 3.0 mm.

9. The system as defined in Claim 8, wherein said means to produce an air jet constitute means to establish a pressure in said air plenum in the approximate range of from 14 to 600 kPa.

10. The system as defined in Claim 3, wherein said air plenum and said air chamber form a unitary assembly, said assembly being movable between first and second positions, in said first position said assembly contacts said surface, in said second position said assembly being positioned so that it is not in contact with said surface.

11. The system according to claim 1 wherein the non-pressure nip means is formed by a non-pervious web supporting belt and the suction roll.

12. In a method of forming a web of fibrous sheet material at high speed including the steps of forming a wet web of cellulosic fibers, moving the web through a press section of a paper machine having a plurality of press rolls to a dryer section of a paper machine, the improvement of transferring the moving web from a press roll to a following moving element in the paper machine, comprising the steps of:
providing a non-pressure nip formed between the press roll and a suction roll to provide a closed draw;
feeding the moving web through said non-pressure nip and around the press roll;
detaching the moving web from the press roll immediately after the non-pressure nip by a combination of a doctor blade and blowing a jet of air in a direction counter to the moving web between the press roll and the surface of the web; and guiding and supporting the moving web to the following moving element by a combined action of the air jet and suction from the suction roll.

13. The method of Claim 12, wherein the following moving element comprises a pervious belt moving around the suction roll and through the non-pressure nip, and the moving web being guided to the pervious belt.

14. The method according to Claim 13, wherein a pervious belt moves through the said nip and around said suction roll, said web transferring to said pervious belt.

15. The method according to Claim 12, including an air plenum coupled to a tapered air chamber culminating in two lips with a gap between said lips forming a passage for said jet of air, one of said two lips forming said doctor blade which cooperates with said jet of air for detaching the moving web.

16. The method according to Claim 15, wherein said two lips have the same length.

17. The method according to Claim 15, wherein one of said two lips forming said doctor blade is longer than the other lip.

18. The method according to Claim 15, wherein said jet of air is formed between adjacent walls of the two lips as the lips converge inward to said gap.

19. The method according to Claim 15, wherein adjacent walls of said two lips are parallel prior to said gap.

20. The method according to Claim 15, wherein the width of said gap forming the air jet is in the approximate range of from 0.1 to 3.0 mm.

21. The method as defined in Claim 20, wherein the pressure in said air plenum is in the approximate range of from 14 to 600 kPa.

22. A method as defined in Claim 15, wherein said air plenum and said air chamber form a unitary assembly, and further comprising the steps of moving the assembly between first and second positions, contacting said roll surface in said first position and not contacting said roll surface in said second position.