Note: Descriptions are shown in the official language in which they were submitted.
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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1321Q9~
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'~
EXAMPLE 1
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.
1321~9~
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.
