Field and Background of the Invention
[0001] The present invention relates to the transfer and support of wet cellulosic webs
between two moving elements in a paper machine. More specifically, the present invention
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.
[0002] In the fabrication of paper, a suspension 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. This cellulosic web or sheet, which is initially weak and wet,
is transferred onto a press felt which carries it into a press nip formed by two press
rolls. The mechanical compression between the two press rolls compacts the web and
eliminates part of the water from 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 section of the paper machine. Paper machines generally
have one to four presses in the press section followed by a dryer section with heated
dryer rolls, to evaporate most of the water remaining in the pressed web. In the fabrication
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 section where it is
dried for a second time.
[0003] While in the different sections of the paper machine, the wet cellulosic web is usually
supported by a pervious belt such as forming fabric, press felts, or dryer fabric,
or by other means such as a press roll. A mechanical support is often unavailable
during web transfer between the individual moving elements of the machine. Thus during
web transfer there is an increased danger of the web or sheet breaking, especially
if it is moist and the machine operates at high speed. To reduce the danger of sheet
breaks it is sometimes necessary to reduce the machine speed, even though this leads
to a decrease in production. The danger of sheet breaks is sometimes reduced by the
addition of chemicals or by increasing the proportion of a stronger, but more expensive,
component such as chemical pump or long fibre pulp in the furnish or initial fibre
mix.
[0004] The most critical areas of sheet transfer are from the forming section to the press
section, 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 is still wet and thus is comparatively weak. Several
methods have been used for transferring the sheet at these areas. In one method the
sheet is pulled unsupported from one element to the next through a so-called "open
draw". The wet sheet in the open draw is unstable at high speeds and reacts to small
variations in the process, sometimes having 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.
[0005] All the machines operating at high speeds, that is to say, in excess of 1,000 metres
per minute provide a continuous support of the web from the former to least the first
nip in the press section. On machines with multiple roll press arrangement, the web
is continuously supported up to the second or third press nip. However, on all present
paper machines, the sheet passes through an open draw as it is peeled from the roll
of the last press.
[0006] In the open draw method of transfer, the reduction of excessive sheet flutter and
stabilization of the web is sometimes achieved by increasing the tension in the web.
The tension required to peel the web and to stabilize it in the open draw transfer
may, in some instances, be sufficiently great to cause a break in the web and even
if it does not break, a high tension can permanently stretch the web and, therefore,
make it more susceptible to breaks during the subsequent operations on the paper machine.
This reduced extensibility is preserved even in the finished product and can lead
to an increased number of paper sheet breaks during converting or printing operations.
[0007] Another method of transferring a web from a pervious carrier or belt such as a forming
fabric to another pervious carrier such as a press felt is with the assistance of
a drilled roll equipped with 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 pick-up system, however, a suction roll can only efficiently transfer
a web from a pervious carrier to another pervious carrier. Press rolls are generally
solid 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.
[0008] Since separation of the leading edge of a wet web from a press roll or web supporting
belt is difficult to achieve, paper machines are commonly initially threaded with
only a narrow band of the web which is sometimes referred to as a "tail". When this
narrow band has been successfully threaded through the length of the machine, it is
gradually widened until the full width of the paper machine is achieved. This narrow
band of paper is initially very weak because it is so narrow and air currents 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 is unusable and must be
recycled. If the machine threading time could be shortened and the machine threaded
with the full width of the sheet or web, then production losses would be decreased
and a higher efficiency achieved.
[0009] Undesirable materials, which generally represent fractions of cellulosic fibres,
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
positioned in close proximity to the surfaces of the machine rolls and scrape off
the web and fibres adhering to the roll. The web removed in this 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 commercially to produce soft and bulky tissue
paper used primarily for hygienic products. For high bulk and softness, it is desirable
that the tissue paper has regularly and densely spaced creped ridges. Good creping
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
air is blown from a location adjacent the blade. Both of these patents have as a primary
objective, the reduction of the wear of the roll through a reduction or elimination
of blade contact with the roll. These hollow doctor blades were designed for production
of creped paper rather than for initiation of the transfer of a wet cellulosic web
in the press section or immediately prior to the dryer section. Because creping occurs
when a web is 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 cellulosic webs to produce paper
which requires a smooth surface.
[0010] There is disclosed in EP-A-0 254 665 web transfer apparatus for transferring a narrow
or tail portion of a web from a press section to a dryer section. It is mentioned
that there are devices in use for blowing the tail off the press roll and for directing
it towards the dryer section. Associated problems mentioned include the long open
spans between the press roll and the nip of the dryer section, and the need to blow
the tail off the press roll before it contacts a removing doctor blade, and the proposal
is made for reducing the length of the span between the press and dryer sections (which
may be in the range of 2.44 metres to 3.05 metres). For this purpose there is provided
a transfer fabric which cooperates with the dryer fabric during the majority of the
travel of the web between the press and the dryer section so that the web is supported
simultaneously from both sides. A lead-in roll 40 is disposed adjacent to a first
press roll 28. A doctor blade 38D is located after the press roll 28D and, in use,
the tail TD of the web is detached by the doctor blade and moved by curtains of air
96, 98 (see Fig 7), which are located at some distance from the doctor blade, into
a nip 104 of the dryer section, likewise at a relatively remote location from the
doctor blade.
[0011] Prior documents FR-A-1 572 200 and DE- C-234 713 and GB-A-2 158 047 disclose various
systems for employing air jets or air currents for transferring detached web ends
between successive portions of web treatment apparatus.
[0012] We have identified a requirement for :-
a) an apparatus and method for the detachment of a wet cellulosic web from a press
roll or web supporting belt and continuously support this web during its transfer
to a subsequent moving element; and/or
b) a transfer system which permits safe transfer of a tail or a full width sheet or
web during the start-up of a paper machine; and/or
c) a transfer system to transfer a wet fibrous web at web speeds greater than 1,000
meters per minute; and/or
d) to transfer webs which are weaker than those transferred on existing 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; and/or improvements generally.
[0013] According to the invention there is provided a method of forming a web of sheet material
as defined in the accompanying claims.
[0014] An embodiment can be used to transfer a tail or a full width strip between a press
roll or a web supporting belt to a following moving element and comprises a doctor
blade to initially separate the web from the roll or carrier, an air jet that blows
air in a direction opposite to the movement of the web, between the web and the roll
or carrier, and a vacuum or suction roll that may have a pervious belt thereon to
retain the web as it is transferred from the press roll or carrier.
[0015] The embodiment provides a method and apparatus to transfer a fast moving web of flexible
material from a surface of a first web supporting moving element to a second web supporting
moving element while continuously supporting said web during the transfer thereof.
[0016] The apparatus comprises
firstly a first web supporting moving element, and
secondly a suction roll in contact with said web, defining a nip with said first
moving element. The apparatus further comprises a doctor blade in contact with said
surface immediately after said nip to cause separation of said web from said surface.
Additionally, there is provided
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 jets constituting means for supporting and guiding sad web toward
said suction roll.
[0017] In another embodiment, a pervious belt moves through the nip, around the suction
roll and the web is transferred to this pervious belt.
[0018] In another embodiment, the doctor blade and air jet comprise a unitary assembly with
an air plenum connected to a tapered air chamber culminating in two lips 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.
[0019] In yet another embodiment, the air plenum and air chamber form a unitary assembly,
said assembly being movable between first and second positions, in said first position
said assembly contacts the surface of the roll or web supporting belt so that the
air jet therefrom is directed towards the nip formed by the press roll and suction
roll, in said second position said assembly being positioned so that it is out of
contact with said surface.
[0020] In a still further embodiment, there is provided in a method of forming a web of
fibrous sheet material, including the steps of forming a wet web of cellulosic fibres,
moving the web through a press section having a plurality of press rolls to a dryer
section, the improvement of transferring the moving web from a press roll to a following
moving element, comprising the steps of: feeding the moving 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 immediately after the 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 web, and guiding and supporting the moving web to the following moving
element by a combination of the air jet and suction from the suction roll.
Brief Description of the Drawings
[0021]
FIG 1 is a schematic side elevational view of a solid roll with a suction roll forming
a nip and a conventional doctor as provided in the prior art;
FIG 2 is a partial side elevational view of a combined doctor blade and air jet according
to one embodiment of the present invention;
FIGS 3 and 4 are partial side views showing different shapes of air 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 elevational view of a paper machine showing the transfer
system of the present invention positioned to transfer a web from the last nip of
the press section; and
FIG 10 is a schematic side elevational view of yet a further embodiment of a transfer
system according to the present invention wherein the transfer occurs between an impervious
web supporting belt and a pervious dryer fabric.
Description of the Preferred Embodiment
[0022] Referring now to the drawings, FIG 1 illustrates a solid roll 10, which is the last
roll in a press section of a paper machine, with a web 12 of wet cellulosic fibres
moving on the roll 10 from a nip 14 with the previous press roll 16. A suction roll
18 forms a nip 19 with the solid roll 10, and a pervious belt 20, in the form of fabric
belt, moves around the suction roll 18 and through the nip 19. A second carrier belt
22, which is also pervious, is conveyed about a further roll 24 positioned beneath
the suction roll 18 to provide a passage for the web 12 between the first carrier
belt 20 and the second carrier belt 22. The suction roll 18 is to provide suction
to separate the web 12 from the surface of the solid roll 10 and direct it 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 which separates the web 12 and crimps or crepes the web as it is separated from
the roll 10. FIG 1 illustrates a prior art arrangement which is not considered satisfactory.
[0023] A doctor blade and air jet assembly 30 are shown in FIG 2 which comprises an air
plenum 32 in the form of a pipe with a slot or a series of holes 34. Air passes into
an air chamber 36 formed by two tapered walls 38 which taper down to a first lip 40
and a second lip 42. FIG 2 shows the first lip 40 representing a doctor blade in contact
with the surface of a press roll 10 so that the web 12 is shown to separate adjacent
the doctor blade lip 40. The second lip 42 is shown to extend not so far as the first
lip 40 and a gap 44 or slit between the two lips 40 and 42 provides a longitudinal
air jet to eject a flat jet of air. Whereas the word "jet" has been used throughout
the specification, this terminology includes a longitudinal stream of air as would
be ejected by a gap or slit 44.
[0024] FIG 3 shows one embodiment of an air chamber 36 wherein the tapered walls 38 join
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 42. 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.
[0025] 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 to that used on conventional blades wherein
the edge of the blade scrapes the roll surface. One or both lips of the assembly may
be replaced if the lip or lips are damaged or worn.
[0026] The optimum gap width depends on production parameters such as machine speed, product
grade, web adhesive force etc. The gap width 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.
[0027] FIG 8 shows a suction roll 18 positioned above the doctor blade and air jet assembly
30. In the initial phase of the start-up procedure, the assembly 30 is in position
A and the leading edge of the tail or full machine width of web 12 is detached from
the roll 10 by impact with lip 40 of the doctor blade. The web 10 is then forced by
the air jet towards the suction roll 18 and is attracted to the roll surface by the
vacuum within the suction roll 18. The web 12 is thus transferred to the felt 20.
Once the moving web 12 has been transferred to the felt 20, the assembly 30 is no
longer required and is switched to position B and the air supply shut off. If a web
break occurs, the web is rethreaded with the assembly 30 in position A. During an
operating period of anticipated web breaks, the assembly is left in position A.
[0028] The suction roll 18 is shown having three zones. The first zone 50, located nearest
to the nip 19, has a high vacuum level to assist in establishing the initial 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 with 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 0 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 is easily parted from the felt 20 as the felt 20 separates from the suction roll
18.
[0029] 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.
[0030] FIG 9 illustrates the transfer system of the present invention used to transfer the
web from the plain roll 10 of the last press which is the central roll of a three-roll
inclined press. FIG 10 illustrates two transfer systems, the second of which transfers
from a pervious or impermeable web supporting belt 60 onto a dryer felt 70. The device
could be used for the transfer of various flexible thin materials and is particularly
suitable for the transfer of weak and extensible sheets such as wet paper or paperboard,
dry creped hygienic paper or non-woven products.
EXAMPLE 1
[0031] A pilot paper machine was used to make paper in two different ways. Paper was first
made with a direct transfer from the press section using the air doctor transfer roll
arrangement of the present invention, and secondly, paper was transferred from the
solid roll using the conventional open draw operation. The tensile properties of the
paper so made were then compared.
[0032] The pilot machine consisted of a roll former with a suction pick-up, a three-roll
inclined press and a sampler as illustrated in FIG 9. The transfer system was installed
between the second press nip and the sampler as shown in FIG 9. The paper machine
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 content of the paper after
the second press was approximately 42%.
[0033] 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 experimental results are shown in the following
table.

[0034] The wet stretch measurements were done on samples cut from the reel (2.5 x 10 centimetres)
sealed in a bag, and handled in such a way to reduce moisture loss. Other samples
were dried between blotters in a photographic 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 indicate
that the paper is less likely to break in subsequent open draws on the paper machine
and in the converting or printing process.
Example 2
[0035] Using the arrangement shown in FIG 9, a 45 gram per square metre web was transferred
to the sampler at 1,000 metres per minute ten times for ten attempts. After sheet
transfer was initiated by the transfer system of the present invention, it was maintained
even when the air supply to the air jet was interrupted. Without the application 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
[0036] Using 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 minute. Without this transfer system, this could not be accomplished.
[0037] The transfer system described and claimed herein is capable of transferring weak
wet webs, such as those made from 100% mechanical pulps. In the past, this has not
been possible without forfeiting speed or having to add a percentage of chemical pulps.
Thus the present invention permits the manufacture of paper from weaker and less expensive
starting materials.
[0038] 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.
[0039] Whereas FIG 9 shows only a single transfer system, multiple transfer assemblies may
be provided at different locations on the machine.
[0040] Various changes may be made to the embodiments described herein without departing
from the scope of the present invention which is limited only by the following claims.
1. A method of forming a web (12) of fibrous sheet material, including the steps of forming
a wet web of cellulosic fibres, moving the web through a press section having a plurality
of press rolls to a dryer section, the method comprising transferring the moving web
from a press roll in (10) to a following moving element, the method further comprising
the steps of :-
a) feeding the moving web (12) between said press roll (10) and a suction roll (18);
b) detaching the moving web from said press roll employing a doctor blade (40);
characterised by
c) said suction roll (18) cooperating with said press roll to define a nip (19), said
nip being defined on one side by a solid surface of said press roll (10) and on the
other side by said suction roll (18);
d) said doctor blade (40) being located close to said nip (19) and air jet means (30)
delivering air directly onto the leading edge of said blade and in a direction directly
into said nip (19) and substantially opposite to the direction of movement of said
web towards said leading edge from said nip, and said air jet serving in combination
with said suction roller to transfer said web within said nip (19) from said press
roll to said suction roll.
2. A method according to claim 1 characterised by causing said suction roll (18) to apply
maximum suction to said web (12) in a region (50) of said roll at the apex of said
nip (19).
3. Apparatus for transferring a wet web (12) of cellulosic fibres from a press roll (10)
to a suction roll (18), the apparatus comprising :-
a) said press roll (10) and said suction roll (18) being mounted so that said web
(12) can be fed therebetween;
b) a doctor blade (40, 42) for detaching said web (12) from said press roll (10)
characterised by
c) said suction roll (18) cooperating with said press roll (10) to define a nip (19)
having on one side a solid surface of said press roll and on the other side said suction
roll;
d) said doctor blade (40, 42) being located close to said nip (19) and air jet means
(30) being provided to deliver air directly onto the leading edge of said doctor blade
and in a direction directly into said nip (19) and substantially opposite to the direction
of movement of said web towards said leading edge, from said nip, and said air jet
means serving in combination with said suction roll to transfer said web within said
nip from said press roll to said suction roll.
4. Apparatus according to claim 3 characterised by said suction roll (18) having a region
50 located at the apex of said nip (19) at which maximum suction is applied, and a
region (52) spaced therefrom in which reduced suction is applied.
5. Apparatus according to claim 3 or 4 characterised by said air jet means (30) comprising
an air plenum (32) coupled to a tapered air chamber leading to two lips (40, 42) with
a gap (44) between said lips forming said air jet, one of said lips forming said doctor
blade.
6. Apparatus according to claim 5 characterised by said two lips (40, 42) having the
same length.
7. Apparatus according to claim 5 characterised by one of said two lips being longer
than the other lip.
8. Apparatus according to claim 5 characterised by adjacent walls of said two lips (40,
42) converging towards said gap (44).
9. Apparatus according to claim 5 characterised by adjacent walls of said two lips (40,
42) being parallel prior to said gap (44).
10. Apparatus according to claim 5 characterised by the width of said gap forming said
air jet is in the approximate range of from 0.1 to 3.0 millimetres.
11. Apparatus according to claim 10 characterised by said air jet means (30) comprising
means to establish a pressure in said air plenum in the approximate range from 14
to 600 kPa.
12. Apparatus according to any one of the preceding claims characterised in that said
air jet means (30) is movable between first and second positions in which it respectively
contacts and is spaced from the surface of said press roll (10).
1. Verfahren, eine Bahn (12) aus faserigem Papierbogenmaterial zu formen, welche die
Schritte einschließt, eine nasse Bahn von Zellulosefasern zu formen, die Bahn durch
einen Preßabschnitt mit einer Vielzahl von Preßwalzen zu einem Trocknerabschnitt zu
bewegen, wobei das Verfahren umfaßt die Überleitung der sich bewegenden Bahn von einer
Preßwalze (10) zu einem nachfolgenden sich bewegenden Element, das Verfahren weiterhin
die Schritte umfaßt:
a) Beschicken der sich bewegenden Bahn (12) zwischen die Preßwalze (10) und eine Saugwalze
(18);
b) Ablösen der sich bewegenden Bahn von der Preßwalze unter Einsatz einer Schaberklinge
(18);
dadurch gekennzeichnet, daß
c) die Saugwalze (18) mit der Preßwalze zusammenwirkt, um einen Spalt (19) zu umgrenzen,
wobei der Spalt auf einer Seite durch eine feste Oberfläche der Preßwalze (10) und
auf der anderen Seite durch die Saugwalze (18) begrenzt wird;
d) die Schaberklinge (40) dicht an den Spalt (19) gesetzt wird, und eine Luftstromvorrichtung
(30) Luft direkt an die Führungskante der Klinge leitet und in einer Richtung direkt
in den Spalt (19) und im wesentlichen entgegengesetzt zu der Bewegungsrichtung der
Bahn auf die Führungskante zu von dem Spalt her, und der Luftstrom dazu dient, in
Kombination mit der Saugwalze die Bahn in dem Spalt (19) von der Preßwalze zur Saugwalze
überzuleiten.
2. Verfahren nach Anspruch 1, gekennzeichnet durch Veranlassen der Saugwalze (18), ein Maximum an Saugkraft an die Bahn (12) anzulegen
in einem Gebiet (50) der Walze am Scheitel des Spaltes (19).
3. Vorrichtung für den Transfer einer nassen Bahn (12) von Zellulosefasern von einer
Preßwalze (10) zu einer Saugwalze (18), wobei die Vorrichtung umfaßt:
a) die Preßwalze (10) und die Saugwalze (18), die so montiert sind, daß die Bahn (12)
dazwischen beschickt werden kann;
b) eine Schaberklinge (40, 42), um die Bahn (12) von der Preßwalze (10) zu lösen
dadurch gekennzeichnet, daß
c) die Saugwalze (18) mit der Preßwalze (10) zusammenwirkt, um einen Spalt (19) zu
begrenzen, der an der einen Seite eine feste Oberfläche der Preßwalze hat und an der
anderen Seite die Saugwalze;
d) die Schaberklinge (40, 42) dicht an dem Spalt (19) angebracht ist, und eine Luftstromvorrichtung
vorgesehen ist, um Luft direkt auf die Führungskante der Schaberklinge zu liefern
und in einer Richtung direkt in den Spalt (19) und im wesentlichen entgegengesetzt
zu der Bewegungsrichtung der Bahn zu der Führungskante, von dem Spalt, und die Luftstromvorrichtung
in Verbindung mit der Saugwalze dazu dient, die Bahn in dem Spalt von der Preßwalze
zu der Saugwalze überzuleiten.
4. Vorrichtung nach Anspruch 3, gekennzeichnet durch die Saugwalze (18) mit einer Region (50), die am Scheitel des Spalts (19) liegt,
an dem ein Maximum an Saugkraft angelegt wird, und einer Region (52) in Abstand davon,
an die eine verminderte Saugkraft angelegt wird.
5. Vorrichtung nach Anspruch 3 oder 4, gekennzeichnet durch die Luftstromvorrichtung (30), welche ein Luftverteilergehäuse (32) enthält, das
mit einer sich verjüngenden Luftspeicherkammer verbunden ist, die zu zwei Lippen (40,
42) führt mit einer Öffnung (44) zwischen den Lippen, die die Luftdüse bilden, wobei
eine der Lippen die Schaberklinge bildet.
6. Vorrichtung nach Anspruch 5, dadurch gekennzeichnet, daß die zwei Lippen (40, 42) dieselbe Länge haben.
7. Vorrichung nach Anspruch 5, dadurch gekennzeichnet, daß eine der zwei Lippen länger ist als die andere Lippe.
8. Vorrichtung nach Anspruch 5, dadurch gekennzeichnet, daß benachbarte Wände der zwei Lippen (40, 42) zu der Öffnung (44) konvergieren.
9. Vorrichtung nach Anspruch 5, dadurch gekennzeichnet, daß benachbarte Wände der zwei Lippen (40, 42) vor der Öffnung (44) parallel sind.
10. Vorrichtung nach Anspruch 5, dadurch gekennzeichnet, daß die Breite der Öffnung, die die Luftdüse bildet, in dem annähernden Bereich von
0.1 bis 3.0 mm ist.
11. Vorrichtung nach Anspruch 10, dadurch gekennzeichnet, daß die Luftstromvorrichtung (30) ein Mittel umfaßt, um in dem Luftverteilergehäuse
einen Druck in dem annähernden Bereich von 14 bis 600 kPa aufzubauen.
12. Vorrichtung nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die Luftstromvorrichtung (30) zwischen einer ersten und zweiten Position beweglich
ist, in der sie die Oberfläche der Preßwalze berührt bzw. Abstand von ihr hat.
1. Procédé de formation d'une bande (12) de matière fibreuse en feuille, comprenant les
étapes de formation d'une bande humide de fibres cellulosiques, et de déplacement
de la bande à travers une zone de presse ayant une pluralité de rouleaux de presse
jusqu'à une zone de séchage, le procédé comprenant le transfert de la bande en déplacement
d'un rouleau de presse (10) à un élément mobile suivant, le procédé comprenant en
outre les étapes de :
(a) avance de la bande en déplacement (12) entre ledit rouleau de presse (10) et un
rouleau d'aspiration (18) ;
(b) détachement de la bande en déplacement dudit rouleau de presse, au moyen d'une
lame de raclage (40) ;
caractérisé en ce que :
(c) ledit rouleau d'aspiration (18) coopère avec ledit rouleau de presse pour définir
un pincement (19), ledit pincement étant défini d'un côté par une surface unie dudit
rouleau de presse (10) et de l'autre côté par ledit rouleau d'aspiration (18), et
(d) ladite lame de raclage (40) est placée près dudit pincement (19) et des moyens
à jet d'air (30) envoient de l'air directement sur le bord avant de ladite lame et
dans une direction d'admission directe dans ledit pincement (19) sensiblement opposée
à la direction de déplacement de ladite bande vers ledit bord avant à partir du dit
pincement, lesdits moyens à jet d'air servant en combinaison avec ledit rouleau d'aspiration
à transférer ladite bande passant dans ledit pincement (19), dudit rouleau de presse
audit rouleau d'aspiration.
2. Procédé suivant la revendication 1, caractérisé en ce qu'on agence ledit rouleau d'aspiration
(18) de manière à appliquer une aspiration maximale à la dite bande (12) dans une
région (50) dudit rouleau située au sommet dudit pincement (19).
3. Appareil pour transférer une' bande humide (12) de fibres cellulosiques d'un rouleau
de presse (10) à un rouleau d'aspiration (18), l'appareil comprenant en outre :
(a) ledit rouleau de presse (10) et ledit rouleau d'aspiration (18) montés de sorte
qu'on peut faire avancer ladite bande (12) entre eux ;
(b) une lame de raclage (40,42) pour détacher ladite bande (12) dudit rouleau de presse
(10) ;
caractérisé en ce que
(c) ledit rouleau d'aspiration (18) coopère avec ledit rouleau de presse (10) pour
définir un pincement ( 19) ayant d'uncôté une surface unie dudit rouleau de presse
et de l'autre côté ledit rouleau d'aspiration ;
(d) ladite lame de raclage (40,42) est située près dudit pincement (19) et des moyens
à jet d'air (30) sont prévus pour envoyer de l'air directement sur le bord avant de
ladite lame de raclage et dans une direction d'introduction directe dans ledit pincement
(19) sensiblement opposée à la direction de déplacement de ladite bande vers ledit
bord avant, à partir dudit pincement , lesdits moyens à jet d'air servant en combinaison
avec ledit rouleau d'aspiration à transférer ladite bande passant dans ledit pincement,dudit
rouleau de presse audit rouleau d'aspiration.
4. Appareil suivant la revendication 3, caractérisé en ce que ledit rouleau d'aspiration
(18) présente une région (50) située au sommet dudit pincement (19), à laquelle l'aspiration
maximale est appliquée, et une région (52) espacée de ce sommet, à laquelle une aspiration
réduite est appliquée .
5. Appareil suivant la revendication 3 ou 4, caractérisé en ce que lesdits moyens à jet
d'air (30) comprennent un collecteur d'air (32) en communication avec une chambre
d'air convergente aboutissant à deux lèvres (40, 52),un intervalle (44) entre lesdites
lèvres engendrant le dit jet d'air, une desdites lèvres constituant ladite lame de
raclage.
6. Appareil suivant la revendication 5, caractérisé en ce que lesdites deux lèvres (40,
42) ont la même longueur.
7. Appareil suivant la revendication 5, caractérisé en ce qu'une desdites deux lèvres
est plus longue que l'autre lèvre.
8. Appareil suivant la revendication 5, caractérisé en ce que les parois adjacentes desdites
deux lèvres (40,42) convergent vers ledit intervalle(44).
9. Appareil suivant la revendication 5, caractérisé en ce que les parois adjacentes desdites
deux lèvres (40,42) sont parallèles avant ledit intervalle (44).
10. Appareil suivant la revendication 5, caractérisé en ce que la largeur dudit intervalle
engendrant ledit jet d'air est comprise entre 0,1 et 3,0 mm environ
11. Appareil suivant la revendication 10, caractérisé en ce que lesdits moyens à jet d'air
(30) comprennent des moyens pour établir une pression, dans le dit collecteur d'air,
dans la plage de 14 à 600 kPa environ.
12. Appareil suivant l'une quelconque des revendications précédentes, caractérisé en ce
que lesdits moyens à jet d'air (30) sont déplaçables entre une première et une deuxième
positions dans lesquelles ils sont respectivement en contact avec la surface dudit
rouleau de presse (10) et espacés de cette surface.