Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Il BACKGROUND OF THE INVENTION
I . . _ . _
The invention relates to the dewaterin~ of fibrous
suspensions on a twin wire papermaking machine of the inverted
type, i.e. machines wherein water is removed predominately
upwardly by suction boxes located above the top forming wire
of a subst,antially,flat run of a continuously-moving sandwich
¦ formed by the top and bottom wires and a layer or web of
¦ fibrous suspension between them. This type of machine is
particularly applicable to the production of mult~i-ply webs
~o in which case subsequent plies are formed on a layer or web
of fibrous suspension carried by one or more previously
formed plies.
Known machines of this type include a large diameter
forming roll having an open surface. Some initial upward
drainage into the cellular surface of this roll may occur
¦ just behind the contact point between the roll and the top
l ¦ wire. The lowest point of the forming roll is level with or
¦ slightly below the running level of the bottom wire. This
l results in a slight "wrap" of the b,ottom wire around the
dc ¦ forming roll and creates a narrow arcuate section pressure
zone behind the perforated forming roll, which depends on
l the tension of the bottom wire. This pressure per unit
_............. . _
1057546
width of the roll, is indicated by the ratio of wire tension
to roll radius. This tension or pressure tends not only to
squeeze water from the suspension but also, unfortunately,
to restrict the depth of fibre suspension that can traver~e
the forming zone and hence force the incoming flow to reverse
direction forming a "puddle" in the nip between the forming
roll and the bottom wire. In practice, the size of this
puddle can be reduced by increasing the consistenc~ of the
fibre suspension thereby reducing its depth.
/o A second drainage may occur at the point where the
top wire "wraps" the so-called underwire or table roll.
Although the curvature of this zone is greater than the
forming zone, their respective pressures are comparable due
to a lower tension in the top wire. If the forming roll be
raised in an attempt to reduce the wrap, there is a tendency
for undesirable quantities of water to pass by the first
table roll and to form a reverse flow "puddle" at a high
curvature zone behind the table roll due to large~pressure
imposed by the so-called "autoslice" soon to be described.
~o This problem has usually made it necessary to use high
consistency suspension, i.e. to ensure that the fibre
suspension entering the system has a low water content.
The autoslice may be a suction box or suction
water scoop with an extended spout and a blade extending
toward the forming roll. Its function is to collect water
adhering to the upper surface squeezed out of the top wire,
the fibre suspension in the forming zone and the table roll
zone. The blade should just skim the top wire, but in
practice it is often depressed into the wire run to increase
~o the wrap of the table roll. This has the counter-effect of
. __ . . . _ . . . . _
1 1057546
increasing the pressure at the slice lip which is already
high due to its being substantially a "line" contact. Thus,
apart from problems due to wear, the sheet must be formed so
as to withstand high pressure at the slice lip. Hence all
effective forming must take place between the forming roll
and the table roll.
Further water removal and hence further consolidation
of the fibre sheet has been obtained by the inverted suction
boxes over the top wire, downstream from the forming zone.
o Raising the forming roll tends to increase back
flow at high pressure around the autoslice. If the autoslice
also is raised in an attempt to relieve this pressure, the
top and bottom wires tend to separate and the fibre sheet to
disrupt, and so the remedy may lead to problems more serious
than those it is designed to cure. Furthermore, suction
boxes tend to wear to a concave formation and this also
tends towards wire separation and sheet disruption.
SUMMARY OF THE INVENTION
. ~
According to the present invention, dewatering
apparatus is provided consisting o a suction box shaped,
such that the area contacting the wire is curved convexly in
the plane paraliel to the direction of motion of the wire.
The device is adjusted so that the open area between the
wires can be varied. The radius of curvature of the dewater-
ing device may be relatively large, in the range 50 to 400
inches so that pressure between the wires, due to bottom
wire tension, is only a few inches water-gauge and does not
substantially impede the passage of even a low consistency
fibre suspension into or through the region in the immediate
3o vicinity of the suction box.
- I 1057546
¦ In use, the suction box of the invention is so
¦ arranged as to depress the traveling wire sufficiently to
¦ cause the latter to follow the convex surface and prevent
¦ substantial separation of the wires and consequent disruption
¦ of the sheet, but not so low as to constrain the sandwich
¦ into high curvature which tends to impede the passage of
¦ fibre. A radius of 240 inches has proven to be an operable
figure. The variation of the open area of the curved surface
of the suction box contacting the wire may be obtained by ,
/o forming the surface from a plurality of bars or foils extending
transversely across the box, at least some of the bars or
foils be replaceable with others of different size.
The suction box must also be mounted so that it
may translate in the horizontal direction. This permits the
suction dewatering device to be placed at the optimum distance
from the headbox. The adjustable positioning feature of the
suction box structure in the downstream direction allows one
to control the extent of upward dewatering. Fine paper
stock requires two-sidedness and an even distribution of
~ol fillers throughout the web. To achieve this object, approximatel Y
40 to 60 percent of the water within the web should be
removed in the upward direction.
¦ The instant invention may also be used to add
¦ additional dewatering capacity to an existing Fourdrinier
forming table. The suction dewatering device would be
¦ located downstream of the primary headbox and can be used to
¦ assist dewatering at a rate ranging from 15 to 25 gallons
¦ per minute. This additional dewatering capacity could be
¦ used to shorten the overall length of the forming section or
.... _
- 11 1057546
could be used to augment the dewatering capacity of an
existing Fourdrinier. The improved formation which results
from upward dewatering enhances the range of webs which may
be made on a Fourdrinier machine.
An object of this invention is to provide for an
mproved dewatering of a fibre suspension in sandwich-
formation between two moving wires, and to overcome or at
least alleviate the above described difficulties that arise
in the forming process.
/o BRIEF DESCRIPTION OF TIIE DRAr~INGS
.
The invention will now be further described by way
of example with reference to the accompanying drawings in
which:
Fig. 1 is a schematic diagram of a dewatering
apparatus according to the invention; and
Fig. 2 is a practical embodiment of such an apparatus
shown in elevational view.
The schematic arrangement of Fig. 1 includes a
combination of autoslice and suction boxes in a unitary
~o compartmènt housing. This arrangement permits of a compact
forming zone and in combination with the mild curvature imposed
upon region 1 by the convexity of the suction housing 2,
allows the application of dewatering pressure to be distributed
as desired over an appropriate length of travel of the web.
1 1057546
The fibre suspension 3 is deposited on the bottom
wire 4 or on one or more previously formed plies carried on
wire 4. The top wire 5 is led towards the suspension 3
round a forming or turning roll 6. After passing between `
roll 6 and an underwire roll 7 sandwiched between wires 4
and 5, the suspension 3, with or without underlying previously-
formed ply or plies, is carried beneath suction housing 2
where it is dewatered upwardly by an autoslice 8 and inverted
multi-chambered suction box 9.
/0 If desired, the degree of vacuum may be the same
in the autoslice 8 and suction box 9, or may increase "down-
stream" from one compartment to be next.
The suction box is so constructed that the "open"
area of the suction surface may be varied by interchangeable
foil structure, according to the consistency of the suspension
¦ being treated and the dègree of formation required.
The suction box of the present invention may be
¦ advantageously arranged to contain a number of independent
adjacent vacuum chambers. An operable embodiment has been
¦ built which utilizes two chambers. However, closer control
¦ over web formation is more easily obtained with a larger
¦ number of chambers. It should also be noted that depressing
¦ the traveling wires into an arcuate path has a tendency to
~ insure that they will travel at the same speed. It has been
` ~ 1057546
noted that variations in speed of forming wires will cause
sheet disruption.
In practice it has been found, in forming heavy
sheets, that sheet quality may be enhanced if the height of
the forming roll 6 is adjusted to provide a very small wrap
at region 10. The forming roll 6 does not contact the fibre
and can be replaced by a small solid turning roll. Such is
less cumbersome and expensive than the open surface roll 6
and does not require the use of high pressure sprays to
clean its surface. Furthermore, its smaller size permits of
a useful reduction in the overall length of the station and
hence in the length and cost of the building required to
accommodate it.
The aforementioned convexity of the housing need
not be uniform. If desired, it may vary, e.g. from a larger
to a smaller radius in a downstream direction, and any curv-
ature variation may be related to the size, shape and degree
of vacuum in its several compartments, e.g. for the purpose
of achieving the desired distribution of ~he dewatering or
20 ! forming operation lengthwise of the web. This variation may
be in-built, or may be achieved by appropriate profiling of
removable bars or foils as hereafter described. The form
and effect of the convex surface may be satisfactorily
realized and attained by a series of individually replaceable
blades, bars or foils which, being preferably flat, can be
easily produced by conventional milling or planing machines
without the need to generate curved profiles. The body of
the box may form a series of flats or short chords along the
"arc" thus approximated. Additionally or alternatively, the
relevant part of the box could be machined to receive multi-
blade section preformed to the required curvature.
1057546
Advantageously the underwire roll 7 shown in
¦ Figure 1 may be replaced by a plate curved to a single
¦ radius larger than that of the roll it substitutes, or to
¦ radii progressively decreasing along the arc length of the
¦ plate. The function of such plate would be to keep the
¦ inter-wire pressure at a controlled low value during the
¦ initial dewatering phase. It may generally be found that
substitution of such a plate for the underwire roll may
allow deeper fibre suspensions to enter the zone immediately
upstream of suction apparatus 2. '
Referring to Fig. 2, a practical form of the
invention is illustrated, the direction of papermaking here
being from right to left. The bottom wire 11 carrying the
¦ fibrous suspension either directly on it or on a previously
formed ply carried by the wire is led below a solid turning
roll 12, which leads top wire 13 onto the top of the sus-
pension to form a sandwich. An optional lead-in bar 14 may
be provided at the point of formation of the sandwich if
required. The function of lead-in bar 14 may also be achieved
by using a curved shoe or rotating roll. The sandwich then
passes beneath the curved surface 15 of suction arrangement
16, where upward dewatering takes place as previously des-
cribed. Afterwards top wire 13 is led away around turning
roll 17 and the formed web is carried onwardly for further
treatment, or for the addition of another ply thereto, on
bottom wire 11. A convexly curved suction box 18, which may
in alternative constructions be a suction roll or foil,
creates a negative pressure below wire 11 to ensure retention
of the formed web on the wire. This negative pressure also
3D served to compact or consolidate the newly-formed web. Top
1057;'~6
wire 13 is then led round further turning and tensioning
roll 19 before returning to solid roll 12.
It will be evident from the foregoing, that the~
invention provides for simple and convenient improvement of
the dewatering of fibre, while avoiding wire separation and
consequent risk of disruption of the web. During the drainage
period, while the wires and pulp mass are sandwiched together,
there is a stability resulting from the fact that the
"sandwich", is trained around the convexly-curved surface 15
/o of suction arrangement 16, and this obviates the possibility
of stock disruption during the web formation period.
Additionally the replaceable blades, bars or foils
may be individually replaced or remachined to compensate for
localized wear.
It is to be appreciated that the prior art type of
"inverted forming machine", might easily embody six or seven
rolls of fairly large diameter, all requiring support from
cross machine members. ~he present invention provides a
greatly simplified construction, obviating many of the
~o supports prevlously requlred in the form of cross beams or
cross tles and, as may be seen from Figure 2, the greatly
reduced number of rolls can all be carrled from the main
suctlon arrangement 16.
Finally, it is to be understood that varlous
¦ alterations, modifications and/or additions may be intro-
¦ duced into the constructions and arrangements of parts
previously described without departing from the ambit of the
¦ invention.
