CONTINUOUS CONTROLLED DRAINAGE

CONTINUOUS CONTROLLED DRAINAGE

This invention is concerned with paper making machines of the type having a "flat wire" or "open wire" forming section, which includes means to remove water from the stock by the use of suction.

In this type of machine, as opposed to "twin wire" machines, or "gap formers", an aqueous slurry known as the stock, which contains both fibers and other substances in an amount of from about 0.1% to 1.5% by weight, is fed from a head box slice onto a single moving forming fabric. Water is progressively removed from the stock through the forming fabric in what is known as the "forming section" of the paper making machine. In this forming section, a variety of drainage devices are used, until the stock contains from about 2% to about 4% by weight of solid material. At that point, the distribution and orientation of the fibers and other solids in the still very wet stock is largely determined, and will not change very much in the remaining paper forming steps unless other devices such as a dandy roll, or "top wire", is brought into contact with the stock. Thus at this point the formation of the paper is largely completed.

In outline, a conventional open wire forming section includes a forming fabric which is supported at the head box slice end by a breast roll, which is followed in sequence by a "forming board" and a series of drainage devices, which may be drainage foils or table rolls, and suction boxes. More recently, forming sections have included a forming board followed by suction boxes of the type described by Johnson, in U.S. Patent 4,140,573. These suction boxes heretofore have been distributed along the length of the forming section with gaps, or undrained spaces, in between them.

The one reported attempt to use vacuum assisted drainage for the full length of an open wire forming section appears to have been a failure. Such a paper making machine is described by E.J. Justus in U.S. 3,052,296 (issued in 1962, assigned to Beloit Iron Works). As described by Justus, the forming fabric is to be supported on a "continuous or substantially uninterrupted" series of suction boxes, starting as near to the head box slice as is practicable. These suction boxes are provided with a foraminous surface to support the forming fabric, for which several designs are proposed. Justus proffers several advantages for such a machine: an increase in fiber retention on the forming fabric of up to 70%, as compared to the usual figure of less than about 50%, reduced wire marking on the paper, and "better" paper. A further point made by Justus is that his essentially flat surfaced suction boxes do not cause the phenomenon known as "kick-up" in the stock associated with the table rolls then used as the primary dewatering devices. Kick-up results from the vertical deflection of the forming fabric caused by the suction produced by the roll as described in U.S. 2,928,465. When kick-up occurs, what is observed is an essentially vertical movement of both the forming fabric and the stock carried on it in the vicinity of a table roll: this movement can become so violent that it will literally lift the stock off the forming fabric. Such an occurrence is not conducive to the making of good paper. In a later communication originating from Beloit Iron Works (reported by P. Wrist in "The Formation and Structure of Paper", British Paper and Board Makers Association, London, England, 1962, at pages 863, 864) it is noted that although many of the benefits proffered by the all-vacuum assisted drainage technique proposed by Justus indeed are obtained, nevertheless "the formation of the [paper] sheet deteriorated to an unacceptable level." (Communication to P. Wrist, from Beloit Iron Works). In other words it proved to be impossible to make acceptable quality paper using the modified paper making machine proposed by Justus. Perhaps as a consequence of this failure, this approach to stock dewatering was not pursued further. Even Justus turned his attention to other methods (e.g. as in U.S. 3,102,066).

It has now been realized that the failure of the Justus attempts may be directly attributed to at least two seemingly unrelated causes. First, Justus in setting out to avoid the then known problems of heavy suction and kick-up becoming prevalent with table rolls (and which were becoming a handicap serving to limit paper making speed, since as the linear speed of the forming fabric increases the suction and kick-up effects become more violent) endeavoured to eliminate all stock agitation in the forming section.

It has now been known for some time that improved paper making operations can result if some deliberate and controlled agitation is introduced into the stock on the forming fabric whilst it is still in a highly fluid state.

It has now been discovered that the precise spacing of the devices used to generate stock agitation has a very important effect on paper sheet quality. When the devices are spaced apart in a uniform manner, they act in a periodic or harmonic relationship to each other, so that later devices (that is, ones further from the head box slice) can either reinforce and add to the stock agitation produced by earlier devices, or diminish and dampen that agitation. This provides a controlled and uniform stock agitation that is both easily generated and easily controlled, to benefit the paper sheet formation.

Second, Justus recommends to use a vacuum level ranging from a low level of effectively zero in a suction box adjacent the head box slice rising to a figure of 2 inches of mercury at the 3% point, that is a value of about 70 cms of water. It has been discovered that this is also a mistake, and that with dewatering devices somewhat similar to those advocated by Justus a far lower level of vacuum is often sufficient, rising from a very low level adjacent the head box slice to a value of no more than 50 cms of water at the end of the forming section. It has been discovered that much lower levels of vacuum than those suggested by Justus can be used with great benefit in retention and wire mark provided the above mentioned agitation or kick-up can be achieved. This can be achieved by the use of the static drainage unit known as the Isoflo (Trade Mark) which is described by Johnson in U.S. 4,140,573. Thus it has now been discovered that if first, the correct level of vacuum is used in the suction boxes and second, the path through which the forming fabric moves whilst the stock is still highly fluid is properly controlled to provide some agitation in the stock, then a forming section can be successfully operated with continuous vacuum assisted drainage. By this means, it becomes possible to increase the area of the forming fabric that is subjected to vacuum assisted drainage by an amount of often more than 30%, the actual figure varying from machine to machine. Additionally, fiber retention is also significantly improved.

Retention is fundamental in paper making. The commonly used definition in paper making for first pass retention (FPR) is

Head Box Consistency - White Water Consistency ,_Q0 Head Box Consistency

Values for FPR can range from 30 % in the case of papers with a high filler content to over 90% for some long fibered grades. Several factors affect the FPR including the type of stock, the kind of forming fabric, the use of chemical retention aids, the amount of stock agitation, the amount of suction used in forming the paper, and particularly the velocity induced in the stock by that suction while forming. Improving retention from 45% to 70% reduces the consistency of the recirculating white water considerably if the amount of slice opening is left unchanged. (By "consistency" in this context is meant the total suspended solids content in percent by weight in the stock or in the white water, as appropriate). This has beneficial effects on the entire paper mill and reduces the amount of fiber and filler loss. Alternatively, the paper maker may cut down on the slice opening and use less water to form the paper. Thus one benefit of this invention, which allows using low vacuums while still achieving good formation, is to reduce the velocity of drainage thereby improving retention and wire mark. Thus, in a first broad aspect this invention provides a process for improving paper formation in a paper making machine having an open surface forming section, including at least a travelling continuous forming fabric which passes over a breast-roll adjacent a head box having a head box slice through which aqueous stock is deposited onto the forming fabric, and a plurality of stock dewatering devices beneath the forming fabric which are provided with white water drainage means, in which forming section the solids content of the stock deposited from the head box through the head box slice onto the forming fabric rises from an initial low value to a value of from about 2% to about 4%, comprising the steps of:

(i) discharging onto the moving forming fabric an aqueous stock;

(ii) causing the forming fabric to move over a dewatering device comprising a continuous vacuum assisted suction box including a plurality of chambers to each which a controlled level of vacuum is applied, which includes a foraminous top support surface for the forming fabric, and which provides a vacuum tight seal between the forming fabric and the evacuated chambers of the suction box;

(iii) controlling the vacuum level applied to the chambers of the suction box so that the vacuum level rises from an initial low value of less than 5 cms water gauge adjacent the head box slice, to a value of no more than 50 cms water gauge at the end of the forming section; and

(iv) causing a desired level of uniformly spaced periodic harmonic agitation in the stock on the forming fabric by providing differences in level between different parts of the forming fabric along the length of the forming section.

In a preferred embodiment, the vacuum in the suction box is controlled in such a way that it rises in a stepwise fashion in the separate sections of the box along the length of the forming section, from the initial low value of below 5 cms water gauge to a maximum value of no more than 50 cms water gauge. Desirably there are as many vacuum levels as possible, preferably more than three, and most preferably at least five. In a second broad aspect this invention provides in a paper making machine having an open surface forming section, including at least a travelling continuous forming fabric which passes over a breast-roll adjacent a head box having a head box slice through which aqueous stock is deposited onto the forming fabric, and a plurality of stock dewatering devices beneath the forming fabric which are provided with white water drainage means, in which forming section the solids content of the stock deposited from the head box through the head box slice onto the forming fabric rises from an initial low value to a value of from about 2% to about 4%, an apparatus for stock dewatering consisting essentially of in combination:-

(a) a suction box comprising a plurality of evacuated chambers, each of which is provided with a vacuum tight drainage means, located in the forming section adjacent the head box slice and extending continuously for the length of the forming section;

(b) a foraminous surface on the suction box adapted to support the forming fabric, to provide apertures through which the forming fabric drains under the influence of the vacuum in the suction box, to provide a path through which the forming fabric moves which will cause a controlled level of uniformly spaced periodic harmonic agitation within the stock on the forming fabric; and to provide a vacuum tight seal between the forming fabric and the evacuated chambers of the suction box; and

(c) a vacuum control means whereby the vacuum in the evacuated chambers of the suction box is controlled to a value that rises progressively along the suction box for the length of the forming section from an initial low value of less than 5 cms water gauge adjacent the head box slice, to a maximum value of no more than 50 cms water gauge at the end of the forming section.

In one embodiment, the suction box comprises a plurality of contiguously adjacent suction boxes, each of which is the full width of the forming section. Alternatively, in a second embodiment, a series of vacuum-tight transverse divisions can be provided in a single large suction box. Thus the suction box comprises a sequence of separated drainage chambers, to each of which a controlled level of vacuum is applied, rising stepwise from a level of no more than 5 cms water gauge adjacent the head box slice to no more than 50 cms ^* water gauge at the other end of the suction box, that is at the end of the forming section. Each drainage chamber is also provided with a separate vacuum-tight drainage means.

In a preferred embodiment, the foraminous surface comprises a slotted type fabric-supporting cover comprising a series of spaced apart forming fabric-supporting blades having generally planar top surfaces transverse to the direction of travel of the fabric in a common essentially horizontal plane providing therebetween suction-accessible gaps in which the forming fabric is substantially unsupported and is drawn downward to form stock-agitating undulations in said gaps, the cover including water seal-forming blades disposed intermediately in the gaps between the fabric-supporting blades and having top surfaces transverse to the direction of travel of the fabric at a lower level than the top surfaces of the fabric-supporting blades, and at least forming water seals at the downward undulations of the forming fabric thereby interrupting the suction temporarily to limit drainage while causing vertical agitation of fibers on the fabric passing through the forming section. In a most preferred embodiment, the fabric supporting blades are spaced apart equally from each other for the length of the forming section. This provides the desired and required agitation in the stock, since the forming fabric and the paper stock thereon undulate in a periodic or harmonic manner for the length of the forming section.

In certain circumstances it is contemplated that it may not prove to be either practicable or desirable to utilize a full length open surface forming section using vacuum assisted drainage, for example when modifying an existing paper making machine. Furthermore, it has to be noted that obtaining and controlling the very small vacuum levels needed adjacent a head box slice itself is quite difficult. As noted above, a pressure difference of less than 5 cms water gauge, or approximately only 0.5% of an atmosphere, is being used. The basic concepts of this invention can still be used, nevertheless, in paper making machines having an open surface forming section wherein a vacuum assisted section is preceded by a short dewatering section using other static dewatering devices.

Thus in a third broad aspect this invention provides in a paper making machine having an open surface forming section, including at least a travelling continuous forming fabric which passes over a breast-roll adjacent a head box having a head box slice through which aqueous stock is deposited onto the forming fabric, and a plurality of stock dewatering devices beneath the forming fabric which are provided with white water drainage means, in which forming section the solids content of the stock deposited from the head box through the head box slice onto the forming fabric rises from an initial low value to a value of from about 2% to about 4%, an apparatus for stock dewatering consisting essentially of in combination:-

(a) a relatively short foraminous dewatering device adjacent the head box slice consisting essentially of a forming board section, including at least one forming board, situated adjacent the slice, and a foil section adjacent to the forming board section, which includes a plurality of foils, in which foil section each foil comprises a flat support surface, and a trailing portion (in the direction of forming fabric travel) diverging from the plane of the fabric at an angle greater than zero degrees and less than five degrees;

(b) a suction box adjacent the dewatering device extending for the remainder of the length of the forming section, which suction box consists essentially of either a single box provided with transverse vacuum tight divisions each provided with a vacuum supply means and a water drainage means or a plurality of drainage boxes each extending the full width of the forming fabric, which are placed contiguously with each other thereby effectively having a common dividing wall, and which are each provided with a vacuum supply means and a water drainage means;

(c) a foraminous surface on the suction box adapted to support the forming fabric consisting essentially of a slotted-type fabric cover comprising a series of uniformly spaced forming fabric-supporting blades having a generally planar top surface transverse to the direction of travel of the fabric in a common essentially horizontal plane providing therebetween suction-accessible gaps in which the forming fabric is substantially unsupported and is drawn downward to form stock-agitating undulations in said gaps, said cover including water seal forming blades disposed intermediately in said gaps between the fabric supporting blades and having top surfaces transverse to the direction of travel of the fabric at a lower level than the top surfaces of the fabric supporting blades and at least forming water seals at the downward undulations of the forming fabric, thereby interrupting the suction temporarily to limit drainage while causing vertical agitation of fibers on the fabric passing through the forming section; and wherein both the first, the last, and any intermediate support-blades placed over either an internal transverse vacuum tight division or a pair of contiguous transverse walls of two adjacent suction boxes, are all forming fabric-supporting blades, thereby inducing uniformly spaced periodic harmonic agitation in the stock; and

(d) a vacuum control means whereby the vacuum in the suction box is controlled to a value that rises progresively stepwise along the length of the suction box from a low value of less than 5 cms water gauge adjacent the foraminous dewatering device to a maximum value of no more than 50 cms water gauge at the end of the forming section.

Further, in a fourth broad aspect this invention provides a process for improving paper formation in a paper making machine having an open surface forming section, including at least a travelling continuous forming fabric which passes over a breast-roll adjacent a head box having a head box slice through which aqueous stock is deposited onto the forming fabric, and a plurality of stock dewatering devices beneath the forming fabric which are provided with white water drainage means, in which forming section the solids content of the stock deposited from the head box through the head box slice onto the forming fabric rises from an initial low value to a value of from about 2% to about 4%, comprising the steps of:

(a) discharging onto the moving forming fabric an aqueous paper making fiber stock across the width of the forming fabric;

(b) causing the forming fabric to move over a relatively short foraminous dewatering device adjacent the head box slice, which includes a plurality of stationary drainage elements disposed in a supporting relationship transversely of the forming fabric;

(c) thereafter causing the forming fabric to move over a continuous vacuum assisted suction box including a plurality of chambers to each of which a controlled level of vacuum is applied and which includes a foraminous top support for the forming fabric which extends for the remainder of the length of the forming section and which provides a vacuum tight seal between the forming fabric and the evacuated chambers of the suction box;

(d) controlling the vacuum in evacuated chambers of the suction box to a value that rises progressively along the length of the suction box from an initial value of less than 5 cms water gauge adjacent the short foraminous dewatering device to a maximum value of no more than 50 cms water gauge at the end of the forming section; and

(e) causing a desired level of uniformly spaced periodic harmonic agitation in the stock on the forming fabric by providing differences in level between different parts of the forming fabric along the length of the suction box.

In yet another refinement of these broad apparatus and process aspects of this invention it is also contemplated that the short foraminous dewatering device can also provide a path causing changes in level in the forming fabric thereby initiating a controlled level of agitation in the stock before it reaches the foraminous surface of the suction box. Thus it is preferred that the short foraminous dewatering device includes both a forming board adjacent to the head box slice, and a foil section (between the forming board and the suction box), wherein the foil section may comprise up to four fifths of the area drained by the short foraminous dewatering device. In the foil section, each foil comprises a flat support surface, and a trailing portion (in the direction of forming fabric travel) diverging from the plane of the fabric at an angle greater than 0° and less than 5°. The foils generally are grouped above drainage boxes of a suitable size.

Preferably the separate foils are also so placed as to contribute toward controlled agitation of the stock on the forming fabric. Preferably, the combination of forming boards and foil section comprises no more than about 20% of the length of the forming zone.

As was discussed above, it is preferred that the suction box either is a plurality of contiguously adjacent suction boxes (each of which is the full width of the forming section), or is provided with a series of vacuum-tight-transverse divisions to provide a series of separated drainage chambers to each of which a controlled level of vacuum is applied. The applied vacuum will thus rise in a stepwise fashion in the suction box from an initial low value of no more than 5 cms water gauge up to a maximum of no more than 50 cms water gauge at the end of the forming section. Such a mode of construction readily permits control of the applied level of vacuum.

Similarly, in a most preferred embodiment of these aspects of this invention, the same foraminous surface, comprising a slotted type fabric supporting cover, as described by Johnson, in U.S. 4,140,573, is used for this aspect of this invention. Thus in a more limited broad aspect this invention provides a process for improving stock formation on a paper making machine, including the steps of:

(a) discharging onto the moving forming fabric an aqueous paper making fiber stock across the width of the forming fabric;

(b) causing the forming fabric to move over a relatively short foraminous dewatering device comprising in combination a forming board section, adjacent the head box slice, and a foil unit section in which section each foil comprises a flat support surface, and a trailing portion (in the direction of forming fabric travel) diverging from the plane of the fabric at an angle greater than zero degrees and less than 5 degrees;

(c) supporting the forming fabric over a continuous suction box including a plurality of chambers to each of which a controlled level of vacuum is applied at uniformly spaced apart zones transverse to the direction of the travel of the fabric and permitting the fabric to sag in gaps between the supported zones and forming vertical fabric undulations in said gaps, thereby inducing uniformly spaced periodic harmonic agitation in the stock;

(d) providing water seal means intermediately of the gaps in a plane below where the fabric is supported to interrupt the suction;

(e) applying vacuum in said gaps to the underside of the forming fabric to draw the fabric downwardly between the gaps, the suction applied in each gap being interrupted by the water seal forming means as the aqueous paper-making suspension of fibers is dewatered; and

(f) controlling the vacuum applied in the gaps so that the applied vacuum rises progressively from an initial value of less than 5 cms water gauge adjacent the foil unit to a maximum value of no more than 50 cms water gauge at the end of the forming section.

The invention will now be described by way of reference to the attached drawings in which:

Figure 1 shows diagram atically the initial part of a conventional paper making machine;

Figure 2 shows a conventional foil blade;

Figure 3 shows a so-called Isoflo unit;

Figure 4 shows schematically harmonic stock agitation associated with a series of foils;

Figure 5 shows diagrammatically a paper making machine modified according to one aspect of this invention;

Figure 6 shows a modification to the machine of Figure 5; and

Figure 7 shows a detail of Figure 5.

In these Figures, relevant like parts have been given the same numbers. In Figure 1, the forming section of a conventional open surface paper making machine is shown, incorporating a forming fabric 1, which moves in the direction of the arrows shown at 1A and IB. The forming fabric moves over a breast roll 2, and various tensioning and idling rollers 3. The stock is deposited onto the forming fabric 1 from the head box shown diagrammatically at 4, through a slice 5, which extends across the forming fabric 1. Beneath the forming fabric in the dewatering zone are placed a sequence of drainage devices 6, 7, 8, 9, 10, 11 and 12, provided with white water drains

15, 16, 17, 18 and 19. The first of these drainage devices, 6, comprises a forming board, the second, 7, comprises an open foil unit, and the remainder are so-called Isoflo units (Trade Mark). Boxes 8 to 12 are also provided with a controlled vacuum, through the vacuum pipes 20, 21, 22, 23 and 24 respectively. The vacuum applied will typically range from zero to 5 cms water gauge in box 8, to no more than 50 cms water gauge in box 12; the white water drains 15,

16, 17, 18 and 19 contain suitable vacuum legs. A key feature, from the aspect of this invention, is that not all of the forming section is being actively drained. The drainage and suction boxes are separated by the spans marked a, b, c, d, e and f which represent undrained areas, apart from any water which may happen to drain through under gravity. In the machine shown, these spans represent nearly 30% of the total area of the forming section.

In this machine, which is typical of existing prior art machines, three different forms of drainage element are used, in sequence away from the head box slice 5. The first of these is a set of conventional flat forming board blades associated with box 6.

In box 7 the drainage elements 25 are conventional foil blades broadly conforming to the design shown in section in Figure 2. These foils comprise a supporting bar 28 with a tee-shaped head, onto which is slid the foil blade proper, 29. This includes a flat face 30 onto which the forming fabric 1 rests, and a divergent trailing face 31. In the figure the divergent angle Z is shown exaggerated for clarity. Generally it is far smaller than it is shown, ranging from about 1 degree to about 5 degrees, with angles of 2 to 2.5 degrees being commonly used. As the forming fabric moves over the foil in the direction of the arrow 32, as a consequence of hydraulic phenomena created in the nip provided by the trailing face 31, water is sucked from the stock through the forming fabric.

In boxes 8 through 12 a so-called Isoflo unit is used, which is described in detail in Johnson, U.S. 4,140,573. This is shown in Figure 3 (which corresponds broadly to Johnson's Figure 4), and can be seen to incorporate two groups of static devices 26 and 27. Devices 26 and 27 are each supported on a tee-bar 28; these tee bars 28 are supported across the width of the box by suitably placed supports 33. Although similar in appearance to the foil blades of Figure 2, the static devices 26 and 27 differ in two separate ways. The top faces of all of these devices which bear against the forming fabric 1 are generally planar and either in the plane of the forming fabric (devices 26) or a little below it (devices 27). As shown in Figure 3 the vertical lowering of the devices 27 is indicated at A, which is exaggerated for clarity. In practise, this distance generally will range from about 0.5 mm to about 5.0 mm. The forming fabric in moving over such a foraminous surface undulates between successive devices 26, and the intervening devices 27 are so placed vertically as to provide a water seal to the underside of the forming fabric. Sealing elements, not shown, are also provided along the sides of the boxes in between the drainage devices, parallel to the sides of the forming fabric. Water is drawn from the stock through the forming fabric by the application of vacuum to the box.

There is a further feature which is common to both of these forms of static drainage devices. Figure 4 shows diagrammatically the harmonic, or periodic, stock agitation that can be generated by a regular and uniform spacing of the vertical pulses generated by foil blades supporting a forming fabric. In Figure 4, a small section of the forming fabric 1 is shown moving in the direction of arrow 1A. The forming fabric passes over a series of foil blades all uniformly spaced apart by the distance Y, as indicated between foil blades 45 and 46 mounted on the tee bars 41 and 42. Because the stock agitation is generated by vertical movement of the forming fabric caused by the foil blades, which are each spaced apart by the constant distance Y, the area of vertical stock agitation shown by 48 is followed by another area 50. Similarly, the quiescent zone 49 is followed by another quiescent zone 51, following the area 50. As Figure 4 indicates, both the areas of vertical agitation 48 and 50 and the zones of quiescence 49 and 51 are each spaced apart at the same distance Y. As shown in Figure 4, with no foil blade on the tee bar 43, vertical agitation of the stock still occurs at the location 52 (which is differently shaded in Figure 4 to emphasize that there is no foil blade on tee bar 43), although the amplitude of the agitation at the location 52 is somewhat less than is obtained with a foil blade in place on tee bar 43. The occurrence of this activity in the vicinity of the tee bar 43 (which has no foil blade) is referred to as occurring at a "ghost blade". It is also important to note that these areas of agitation and quiescence in the stock do not move with the forming fabric, but rather remain in essentially the same place. A similar quiescent zone 53 follows the "ghost blade", and foil induced agitation 54 occurs in the vicinity of foil 47.

For the Johnson Isoflo device shown in Figure 3, the area of the stock vertical agitation is due to the downward deflection of the fabric as it moves from fabric support surfaces 26 to surfaces 27, and periodicity similar to that of Figure 4 is observed.

Thus it has been found that spacing of the various support surfaces for the forming fabric can be used to generate, to optimize and to control the agitation occurring in the stock on the forming fabric. The dewatering support surfaces can be placed to control the vertical movement which is initiated by earlier drainage devices.

This flexibility of control of the amount of vertical agitation permitted to occur in the stock on the forming fabric allows for better dewatering of the stock as it travels on the forming fabric through the forming section. Two possible ways of utilizing this invention are shown in Figures 5 and 6. Each of these figures shows essentially the same portion of a paper making machine as is shown in Figure 1, but with certain differences. In common with Figure 1, the forming fabric 1 passes over the rollers 3, around the breast roll 2, and then past the head box slice 5, at which point the stock is deposited onto it. Drainage is initiated by the forming board section on box 6, and continued by the foils associated with box 7; it is to be noted that boxes 6 and 7 are still separated by the gap a.

The remainder of the forming section comprises a single extended suction box 100, which is separated into the sequence of separate chambers 8, 9, 10, 11 and 12, either by using a single continuous suction box with dividers, or by using a plurality of smaller boxes, butting up closely to each other. These suction units also differ from the arrangement shown in Figure 3 in another way. In that figure, the first support surface 26 is an upper one, and is followed by a lower one, 27. As described by Johnson in U.S. 4,140,573, the last support surface in the box is also an upper one. When a sequence of boxes of this type are brought into the contiguous relationship of this invention, one of these surfaces becomes redundant, since the last support surface in any one box also becomes the first support surface for the next one. This is shown more clearly in Figure 7.

There are two different ways in which the continuous suction box of this invention can be utilized to improve the sort of machine shown in Figure 1. In Figure 5, the gaps c through f have been eliminated by lengthening the boxes 8 through 12, and therefore the overall length of the forming section has not been changed. Alternatively, the same effect could be achieved by adding another box to match the span of gaps c through f, and moving boxes 9, 10 and 11 to accommodate it. In either case, the length of the forming section is retained unchanged. The other option is simply to move the suction boxes 8 through 12 together to eliminate gaps c through f. Whilst this is effective, it has two disadvantages. The first is that it will still leave an equivalent length of forming fabric (corresponding substantially to the eliminated gap length) effectively unused, unless the somewhat drastic step of reducing overall machine length is also taken. The second is that although the same amount of water is being removed from the stock, the use of an overall shorter forming section means that water is being removed more rapidly, than is the case if the length of the forming section is retained unchanged. It appears to be advantageous to retain the forming section length, since removing the same amount of water over a longer length of forming fabric reduces the rate at which that water is removed. Decreasing the drainage rate generally improves the quality of the paper being made, since better paper mat formation occurs and wire marking is lessened. Further, the FPR figures also improve; it appears to be feasible to obtain an improvement of the order of 20% with the method of this invention.

In Figure 5 two of the noted gaps which are not actively drained are still present in the forming fabric: these are a and b. As is shown in Figure 6, it is also possible to eliminate gap b, and hence to lengthen the suction box 100 a little further. The same considerations will also apply concerning whether the gap b is removed by lengthening box 100, by adding another box, or by laterally displacing box 100 as are mentioned above in respect of Figure 5. It appears to be preferable to lengthen box 100, to remove gap b. It should also be noted that if gap b is to be eliminated, then the last foil in box 7, which will generally be over the wall of box b adjacent the gap, will become redundant, as the arrangement shown broadly in Figure 7 needs to be used, with a fabric support blade over the contiguous walls of chamber 7 and the suction box 100.