GB2183685A - Paper web formation - Google Patents

Abstract

In a paper web formation process in which a wet web on a wire is dewatered by passage over drainage foils, pulse bans are located between some of the foils to subject the web to positive pressure pulses.

Description

SPECIFICATION Improvements in or relating to paper making machinery This invention relates to paper making machinery and has particular reference to that part of the machinery that is concerned with the removal of water from a very dilute suspension of fibre, inorganic filler, otherchemicals and dyes to form a continuous sheet of paper. Removal of water is effected by passing the suspension over a drainage or Fourdriniertable.

A typical example of drainage table is shown in diagrammatic form only in Figure 1.

The drainage table commonly comprises an endless or spliced forming fabricA supported by rolls B and driven by a main drivel, sometimes with a helperdriveC.Commonly,thedrainagetable commences with an aqueous suspension of between 0.1% and 1.5% filterable solids, and results in a matt of between 20% and 25% filterable solids, There are cases where the suspension may be outside these ranges.

The drainage table is commonly supplied with the liquid suspension from a flow box or head box D and through a projection orifice commonly called a slice E. The liquid is commonlydirected on tothe moving formingfabricunderwhich is commonly a forming board assembly F. This assembly has the purpose of assisting in directing the flow in the same direction as the forming fabric and in commencing the act of sheetformation. The forming board is commonly a solid assembly of between 100 and 400 mm wide in the paper machine running direction. This board is commonlyfollowed by between 2 and 6standard gravity foils suitably mounted and spaced apart.

These foils are commonly zero angled.

Following the forming board assembly is the rest ofthe section to which the present invention relates.

This section is where the majority of the water content is removed whils the sheet is being formed.

This section maybe followed by an upward dewatering unit G and will then be followed bythe next main section called the flat box section H. This section is in turn followed by a suction couch I which is also the main drive.

The main drainage area to which the present invention relates J is commonly fitted with table rolls, Figure 2, which provided alternating positive and negative pressure pulses, to activateth e suspension and then to remove some water, with minimal removal of the suspended elements of the paper mass. The action ofthe table roll is illustrated in Figure 2B. The roll rotates in the direction shown in Figure 2 and this creates a positive pressure beneath the forming fabric as the latter engages the surface of the roll and a negative pulse as the forming fabric separates from the roll. The positive and negative pulses are shown diagrammatically in Figure 2B.The positive pulse tends to lift the forming mass slightly and to loosen the suspension thereon and this action assits the subsequentwithdrawal of water by the action of the negative pulse.

As the paper machine speeds increased, the level of activity by the table rolls became unacceptable as it caused excessive sheet disruption. The gravity foil GF Figure 3 was then introduced. This considerably reduced the positve pulse thus preventing sheet disruption at higher speeds. This action is illustrated in Figure 3B from which it can be seen thatthe positive pressure pulse produced by the stationary gravity foil is much less than that produced by the rotating roll R of Figure 2. However, the dewatering capacity ofthe foil was thus reduced because of the smaller positive pressure pulse and more gravity foils were required than table rolls forthe same volumetric displacement of water.

As paper machine speeds increased still further, the capability of the gravity foil, or of a combination ofthe gravityfoil and table roll, became insufficient.

The concept was then introduced of considering the whole section as in two subdivisions, such that the first sUbdivision equal to approximate the first two-thirds of the section would be dewatered by gravity foils and that the second subdivision equaling about one third of the section would be dewatered by foils placed under vacuum. These are commonly called vacuum foils. This commonly resulted in having two completely different structures of assembly for each of the two above sub-sections.

The arrangement is shown diagrammatically in Figure 4.

However, as dewatering knowledge increased, it became obvious that the gravity foil and vacuum foil had a serious limitation in operation. This was that whilst the drainage principle prerequisite required an increasing positive activity level progressively along the section,the very opposite was occurring due to the reducing liquidity factor.

That isto say, as the suspension advanced along the drainage section, it became increasingly compact and it was found that less water could be extracted from it. This action is illustrated in Figure 5.

Attempts were made to overcome these problems with, for example, reinstatement ofthe occasione table roll or the compartmentising ofvacuum foil boxes wherein one section would operate at one vacuum level and the next section would operate at anotherhighervacuum level.

The first majoi im provement in principle and currentstateofartwasachieved by Cowan with his stepped foil. The stepped foil produced a positive and negative pulse of values that could be selected within a given range and applied to the most appropriate part ofthe whole section to optimise dewatering.

Figure 6 shows diagrammatically two stepped foils SF1 and SF2. Figure 5B shows the positive and negative pressure pulses produced between the two foil tips. A similar pulse pattern is produced after the next stepped foil SF2 but is not shown in Figure FB.

The stepped foil has, however, very serious limitations and drawbacks. The selection of a particular positive and negative pulse range is achieved from a particulardistance of bladeto blade distance. This obviously created a compromise because the physical dimensions of the box had to relate to the required blade spans, so different blades and different numbers of bladeswere used in an attempt to standardise box dimensions. This created a compromising principle of subsectionalising this part ofthe drainage table into three so that one of three blade sizes and spans could be applied so that a common box size could be utilised, which produces an undesirable restriction. This is illustrated in Figure 7.

Again, however, there is a decreasing activity as the suspension advances through the section with a consequent drop in water removai efficiency.

Laterthis principle was seen to be expanded by introducing gravityfoil boxes at the early part ofthe first subsection and also by adding different box sizes to increase blade numbers per box.

The present invention enables drainage principles to be properly followed without compromiseto satisfy design shortcomings of any of the current practices.

According to the present invention, a dewatering section for dewatering a suspension in a paper making machine comprises a sequence of pulse bars and drainage foils, the sequence being so arranged that a required degree of dewatering is achieved with increasing activity of the suspension as dewatering proceeds.

The ratio of the number of pulse bars to the numberofdrainage foils per unit length ofthe section increases as dewatering proceeds.

The invention aiso comprises a method of dewatering a suspension in a paper making process comprises the steps of passing the suspension over a series of drainage foils to extract water from the suspension and, between some of the foils exposing the suspension to positive pressure pulses induced by pulse bars.

The section may comprise, initially, one pulse bar between several foils, and,finally, one or more pulse barsforeach foil.

Some at least of the pulse bars may have an upper contour of rounded form.

Alternatively, some at least of the pulse bars may have an upper contour providing an upwardly inclined leading edge and a downwardly inclined trailing edge.

The leading and trailing edges may be separated by a level surface.

The principle of proper Fourdrinerdrainage isthat water must be progressively removed with minimal fibre orfilling losses from the suspension and without disruption to the sheet or its properties. It must also be possible to activate the sheet suitably to provide sheet properties.

Bywayofexample only, an embodimentofthe invention will now be described in greater detail with reference to the accompanying drawings of which:- Figure 9 shows the embodiment in diagrammatic form only, Figure 9B is an explanatory graph ofthe operation of the embodiment of Figure 9, Figure 10 is an other explanatory graph, and Figure 11 is a digrammaticcross section of an alternative form of a component.

A drainage section embodying the invention incorporates standard gravity foils. These foils comprise foil blades SGF1, SGF2 adapted to fit on to standard tee bars as is indicated by the slots S1,S2 in the blades.

Interspersed between the gravityfoils are pulse bars one ofwhich PB is shown in Figure 9 between the foil blades SGF1 and SGF2.

The pulse bar may have a rounded top section as indicated atTS in Figure 9 and will be adapted, as indicated bythe slot S3 to fit on to a standard tee bar.

Atthe beginning of the drainage section, there will be one or more gravity rolls with 'fall-off' angles of about 1 . Thefail-offangle is the angle of depression ofthe rear surface ofthe foil blade. Further along the section will be further gravityfoils whose blade angles will be about Tforthe first one or more ofthe additional foils and about30forthe remainder.

Where 3 blade angles are employed, the foil will be operated under a vacuum of up to 700 mm w.g..

Atadistance along the section of about 25% ofthe total length, a pulse barwill be introduced between the gravity foils to produce an arrangement where there may be one pulse bar between several foils.

The frequency of introduction ofthe pulse bars is increased along the length of the section until at the end thereof there will be one pulse bar between each foil or even more pulse bars than foils.

The particular arrangement adopted will depend inter alia upon the physical properties ofthe suspension. The aim is to increase the activity as the dewatering process proceeds and forthis reason the number of pulse bars increases along the length of the section. Figure9B shows,graphically,the positive and negative pulses produced by the arrangement of Figure 9. Pulse P1 produced by standard gravity foil SGF1 has a small positive pulse followed by a relatively large negative pulse. On the other hand, pulse P2 produced by the pulse bar PB has a large positive pulse and a smaller negative pulse.

It is the large positive pulse that produces the activity referred to above and counters the compaction ofthefibres.

The concept is illustrated in Figure 10. The rate of dewatering decreases along the length ofthe section as is illustrated by curve A. To counter that decrease, the activity is increased as illustrated by curve B.

Increase and activity arises from the increasing proportion of pulse bars which, as can be seen from Figure 9B produce a much greater positive pressure pulse.

The magnitude ofthe positive pulse and that ofthe negative pulse produced by the pulse bar is determined by contour of the rounded edge. An edge with a smaller radius of curvature produces pulses of greater magnitude. It is possibleto employ a rounded contourhaving different radii of curvature ontheleading and trailing faces.

Alternatively, the pulse bar may have a configuration as shown in Figure 11 .The pulse bar shown in Figure 11 is truncated conical cross section having an upwardly inclined leading edge LE, a flat top tans a downwardly inclined trailing edgeTE.

The angle of inclination of edge LE may be about 10 whilstthatoftheedgeTEabout5 .Smalierand greater values may be used if required.

The mounting ofthe pulse bars and drainagefoils is such that changes can readily be effected as required. This enables changes to be made to the dewatering potential to overcome operating problems that may arise and to assistwith major grade change needs.

The pulse bars 1 may be mounted in otherways than that described above. They may be dove-tailed, or half dove-tailed or mounted in someotherway that allows quick changes and replacements.

The pulse bar may be constructed from high density plastics material, for example high density polyethylene, or from ceramic materials or a combination offibreglass and ceramic material.

Thus, by a proper selection of pulse bars and drainage foils, a combination is achieved giving the most desirable activity level for a given liqudity situation withoutthe need to comprise and, in many cases, without the need to replace existing capital equipment.

Claims (11)

1. A dewatering section for dewatering suspension in a paper making machine comprising a sequence of pulse bars and drainagefoils,the sequence being so arranged that a required degree of dewatering is achieved with increasing activity of the suspension as dewatering proceeds.

2. A section as claimed in claim 1 in which the ratio of the number of pulse bars to the numberof drainagefoils perunitlength ofthesection increases as dewatering proceeds.

3. A method of dewatering a suspension in paper making process comprising the steps of passing the suspension over a series of drainage foils to extract water from the suspension and, between some ofthe foils exposing the suspension to positive pressure pulses induced by pulse bars.

4. A section as claimed in claim 2 in which initialling the section comprises one pulse bar between several foils, and finally, the section comprises one or more pulse bars for each foil.

5. A section as claimed in any one of claims 1,2 or4 in which some at least of the pulse bars have an uppercontourofroundedform.

6. Asection as claimed in anyone of claims 1,2 or4 in which some at least of the pulse bars have an upper contour providing an upwardly inclined leading edge and a downwardly inclined trailing edge.

7. Asection as claimed claim 6 inwhichthe leading and trailing edges are separated by a level surface.

8. A dewatering section as claimed in any one of claims 1,2 or 4-7 substantially as herein described with reference to and as illustrated by Figures 9, 9B and 10 or Figure 11 ofthe accompanying drawings.

9. A method of dewatering as claimed in claim 3 substantially as herein described.

10. Papermaking machineryincludinga dewatering section as claimed in any one of claims 1, 2, or4-8.

11. A method of making paper including a method of dewatering as claimed in claims 3 or 9.