GB1582342A - Dewatering of fibrous suspensions - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO THE DEWATERING OF FIBROUS SUSPENSIONS (71) We, AUSTRALIAN PAPER MANU FACTURERS LiMITED, an Australian Company, of 4 South Gate, South Melbourne, Victoria, Australia, and BELOIT WALMSLEY LIMITED, a British Company, of Atlas Works, Bury, Lancashire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to the dewatering of fibrous suspensions on a paper machine of the Inverform (Registered Trade Mark) type, i.e. machines wherein water is removed predominantly upwardly by suction boxes located above the top wire of a substantially flat run of a continuously-moving sandwich formed by 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 multi-ply webs in which case subsequent plies are formed from a layer or web of fibrous suspension carried by one or more previously-formed plies.

A known machine of this type is shown diagrammatically in Figure 1 of the drawings accompanying the Provisional Specification. It includes a large diameter (usually of the order of 42 inches) forming roll having an open surface. Some initial upward drainage into the cellular surface of this roll may occur at point 10. The lowest points of the forming roll are level with or slightly below the running level of the bottom wire, and the slight "wrap" of the bottom wire around the forming roll creates a narrow arcuate-section pressure zone at 10 due to the tension in the bottom wire. This pressure increase, per unit width of the roll, is related to the ratio of wire tension to roll radius.But it tends not only to squeeze water from the suspension but also, unfortunately, to restrict the depth of fibre suspension that can traverse zone 10 and hence forces 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 consistency of the fibre suspension thereby reducing its depth.

A second drainage may occur at 12 where the top wire "wraps" the soWcalled underwire roll. Although the curvature of this zone is greater than zone 10, their rer spective pressures are comparable due to a lower tension in the top wire. If the form- ing roll be raised in an attempt to reduce the wrap at 10, there is a tendency for undesirable quantities of water to pass point 12 and to form a reverse flow "puddle" at a high-curvature zone 14 due to large pressure imposed by the sowcalled "autoslice" 16, soon to be described. This has usually made it necessary to use high-consistency suspension, i.e. to ensure that the fibre suspension entering the system from the left of Fig. 1 already has a low water content.

The autoslice 16 may be a suction box or suction water scoop with an extended spout and a blade extending into region 10. Its function is to collect water squeezed out of the fibre at regions 10 and 12. The blade should just skim the top wire, but in practice it is often depressed to increase the wrap at region 12. This clearly has the counter-effect of increasing the pressure at point 14 which at all events is high due to its being substantially a "line" contact.

Thus, apart from problems due to wear, the web must be formed so as to withstand high pressure at 14. Hence all effective forming must take place at regions 10 and 12. Further water removal (and hence further consolidation of the alreadyformed fibre web) has been obtained by inverted suction boxes 18 as illustrated in the diagram Fig. 1.

It has already been explained that raising the forming roll tends to increase back flow at high pressure zone 14. If the autoslice also is raised in an attempt to relieve this pressure, the top and bottom wires tend to separate and the fibre web may rupture, 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.

An object of this invention is to provide for an improved dewatering of a fibre suspension in sandwich-formation between two moving wires, and to overcome or at least alleviate the above-described and other difficulties that hitherto have arisen in the forming process.

According to the present invention a dewatering apparatus comprises a self-contained unit adapted for mounting and adjustment as such above a horizontal travelling forming wire, the unit including an encircling top wire entrained around a plurality of rolls defining the general overall shape of the unit, and a suction housing within the top wire and having an active surface which can urge that wire to wards the forming wire and which is generally curved convexly the axis of curvature being horizontal and perpendicular to the direction of travel of the wires.

Preferably the active surface of the suction housing is adjustable to enable the open area thereof to be varied. The radius of curvature of the active surface of the housing 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 will not substantially impede the passage of even a low consistency fibre suspension into or through the region in the immediate vicinity of the suction housing.

In use, the dewatering unit of the invention is so arranged as to depress the travelling forming wire and anything carried thereby sufficiently to cause them to follow its convex surface and prevent substantial separation of the two wires and consequent disruption of the web, but not so low as to constrain the sandwich into high curvature which might tend to impede the passage of fibre. Variation of the open area of the curved surface of the suction housing contacting the top wire may be obtained by forming the surface from a plurality of bars or foils extending transversely across the housing, at least some of the bars or foils being replaceable with others of different size.

The invention will now be further described by way of example with reference to the drawings accompanying the Provisional Specification, in which: Fig. 2 is a schematic diagram of a dewatering apparatus according to the invention; Fig. 3 is a practical embodiment of such an apparatus, and Figs. 4A and 4B and 4C show enlarged details of the curved suction area of the apparatus of Figure 3.

The schematic arrangement of Fig. 2 shows the lower portion only of the selfcontained unit, the top wire return roll and the supporting structure being omitted for clarity, the suction element of the unit is formed by an autoslice and suction boxes in a unitary compartment housing 22. This arrangement permits of a compact forming zone and in combination with the mild curvature imposed upon region 20 by the convexity of the suction housing 22 (the curved active surface not being shown in Figure 2) allows the application of dewatering pressure to be distributed as desired over an appropriate length of travel of the web.

Figure suspension 24 is deposited on one or more previously-formed plies carried on forming wire 26. The top wire 28 of the self-contained unit of the dewatering ap paratus is led towards the suspension 24 round a forming roll 30 or alternatively in a more compact unit, round a turning roll 32 (the alternative being shown in chain-dotted outline). After passing between roll 30 or 32 and an underwire roll 34 sandwiched between wires 26 and 28, the suspension 24, with the underlying previously-formed ply or plies, is carried beneath suction housing 22 where it is dewatered upwardly by an autoslice 36 and inverted suction boxes 38.

If desired the degree of vacuum may be the same in the autoslice 36 and suction boxes 38, or may increase "downstream" from one compartment to the next.

The suction boxes are so constructed that the "open" area of the suction surface may be varied according to the consistency of the suspension being treated and the degree of formation required. This is described hereafter with reference to Figs.

4A and B.

In practice it has been found, in forming heavy sheets from several webs or plies, that sheet quality may be enhanced if the height of the forming roll 30 is adjusted to provide a very small wrap at region 10. The leading forming roll 30 does not contact the fibre and can be replaced by a small solid turning roll 32, as indicated in chain-dotted outline. Such is less cumbersome and expensive than the open surface roll 30 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 unit 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 curvature 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 the dewatering or forming operation lengthwise of the web. This variation may be inbuilt, 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 realised 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.

Advantageously the underwire roll 34 shown in Fig. 2 may be replaced by a plate curved to a single radius larger than that of the roll it substitutes, or to radii pro gressively 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 22.

Referring to Fig. 3, a practical form of the invention is illustrated, the direction of paper-making here being from right to left.

The bottom forming wire 40 carrying the fibrous suspension on a previously-formed ply carried by the wire is led below a solid turning roll 42, which leads unit top wire 44 onto the top of the suspension to form a sandwich. An optional lead-in bar 46 may be provided at the point of formation of the sandwich if required. The function of lead-in bar 46 may also be achieved by using a curved shoe or rotating roll. The sandwich then passes beneath the curved surface 48 of suction arrangement 50, where upward dewatering takes place as previously described. Afterwards top wire 44 is led away around turning roll 52 and the formed web is carried onwardly for further treatment or for the addition of another ply thereto, on bottom wire 40.A convexly-curved suction box 53, which may in alternative constructions be a suction roll or foil, or an underwire press roll with trailing suction slot, creates a negative pressure below wire 40 to ensure retention of the formed web on the lower wire 40. This negative pressure also serves to compact or consolidate the newly-formed web. Top wire 44 is then led round further turning and tensioning roll 54 before returning to solid roll 42. Rolls 42, 52 and 54 with wire 44 and suction arrangement 50 form a self-contained unit, which can be adjusted vertically as a unit towards and away from the forming wire 40 and the webs carried thereon.

One way of obtaining the variability of the curved "open" area of suction arrangement 50 is shown in Figs. 4A and 4B, where removable and replaceable blades, bars or foils 56 constitute the suction area.

By suitable profiling and mounting of elements 56 on carriers 58 the open area can be readily changed. The arrangement of Fig. 4A provides a 30% open area, whilst Fig. 4B provides a 45% open area. Each provides a suction surface of 200 inch radus. Moreover the blades may be pro filed to permit of facilitate the use of cleaning showers between dewatering operations to prevent consolidation of fibrous matterial within the suction arrangement which could impair the suction applied therethrough. The detail shown in Fig. 4C illustrates towards the left hand side thereof, how the profiling of the removable blades 56 by the provision of the chamfered shoulders thereon enables a traversing spray 60 to be supported between adjacent blades.Alternatively or additionally, as shown towards the right of Fig. 4C, fore and aft spray nozzles 62 may be carried within the suction arrangement 50 above the blades 56 as upstanding members on a traversable spray pipe 64. Obviously other forms of cleaning equipment may be provided with arrangement 50 and/or between blades 56.

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 tor gether, there is a stability resulting from the fact that the "sandwich", is trained around the convexly-curved surface 48 of suction arrangement 50, 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 localised wear.

It is to be appreciated that the former type of "Inverform" (Registered Trade Mark), machine, which is shown diagrammatically in Figure 1 might easily embody six or seven rolls of fairly large diameter, all requiring support from cross machine members. The present invention provides a greatly simplified construction, obviating many of the supports previously required in the form of cross beams or cross ties and, as may be seen from Figure 3, the greatly reduced number of rolls and the wire 44 entrained therearound can all be carried from the main suction arrangement 50, forming a compact self-contained unit adjustable as such with respect to the underlying forming wire 40.

Finally, it is to be understood that various alterations modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the scope of the invention as defined in the

Claims (10)

appended claims. WHAT WE CLAIM IS:---

1. A dewatering apparatus complrising a self-contained unit adapted for mounting and adjustment as such above a horizontal travelling forming wire, the unit including an encircling top wire entrained around a plurality of rolls defining the general overall shape of the unit, and a suction housing within the top wire and having an active surface which can urge that wire towards the forming wire and which is generally curved convexly the axis of curvature being horizontal and perpendicular to the direction of travel of the wire.

2. A dewatering apparatus as claimed in claim 1 wherein the active surface of the suction housing is adjustable to enable the open area thereof to be varied.

3. A dewatering apparatus as claimed in claim 2 wherein the curved surface of the suction housing is formed from a plurality of blades, bars or foils extending transversely across the housing, at least some of the bars or foils being replaceable with others of different sizes.

4. A dewatering apparatus as claimed in claim 3 wherein the surface of each blade, bar or foil contacting the top wire is flat, the respective surfaces being angled relatively one to another to collectively form the convexly curved surface of the housing.

5. A dewatering apparatus as claimed in claim 1, 2, 3 or 4 wherein the vacuum applied through the active surface of the suction housing is adapted to be the same along that surface.

6. A dewatering apparatus as claimed in claim 1, 2, 3 or 4 wherein the vacuum applied through the active surface of the suction housing is adapted to increase downstream of that surface.

7. A dewatering apparatus as claimed in any one of claims 1 to 6 wherein the leading roll of the plurality of rolls within the top wire is a large open surface roll.

8. A dewatering apparatus as claimed in any one of claims 1 to 6 wherein the leading roll of the plurality of rolls within the top wire is a small solid turning roll.

9. A dewatering apparatus as claimed in any one of claims 1 to 7 wherein means are incorporated in the suction housing to keep the active surface thereof clean between dewatering operations.

10. A dewatering apparatus substantially as herein described with reference to and as illustrated in Figs. 2, 3 and 4A to 4C of the drawings accompanying the Provisional Specification.