EP0978587B1 - Process for setting a uniform transverse profile for a paper web - Google Patents

Abstract

To set a consistent characteristic lateral profile in a paper web, the weight cross section profile is measured and adjusted by setting the sectional pulp density at the stock inlet. The lateral formation profile is then set by amending the specific pulp throughput at the delivery jet of the stock inlet. Simultaneously or subsequently, the dimensions of the delivery jet are increased or decreased at points where there is a raised pulp throughput. The change in the delivery jet is proportional to the change in the specific pulp throughput and, after setting, the flow is at the required lateral speed profile. At least one of the setting steps is effected by a control circuit. Additional chemicals are added pref. at the edge zone, to delay and/or increase the water extraction rate from the pulp. The variations in the specific pulp suspension flows are through sectional extraction or addition of a fluid in the Z-direction, together with filling materials where there are zones with a higher throughput. The technique is especially for a paper making machine with a double fourdrinier.

Description

The invention relates to a method for adjustment a uniform property cross profile one Paper web with respect to formation, basis weight and Fiber orientation in a paper machine with the help a section-wise material density-influenced Headbox with adjustable nozzle outlet gap.

Regarding the state of the art, reference is made to the referred to the following publications:

Voith Sulzer reprint p2919e, 3/94; Weekly for Papierfabrikation 122, 1994, No. 12, pages 485-491; "Das Papier", 49th year, edition 10A, 1995, pages V99-V105.

From the above publications, it is known, with a stock density-controlled headbox locally to reduce basis weight disturbances by appropriate mixing of stock suspension and a dilution liquid, preferably white water, adjust the amount of solids applied. It is also described that by setting the sectional throughput of stock suspension on Nozzle outlet gap the cross flows are affected can and thus the fiber orientation cross profile can be improved. Here an attempt is made as flat as possible the aperture on To reach the nozzle outlet gap. It is also suspected that there is a connection, especially with twin wire formers between the specific applied on the headbox Fabric suspension throughput (liters per meter Machine width) and the formation cross profile.

The problem with the prior art is therefore a method of operating a headbox describe that on the one hand the known good quality of the basis weight cross section and Fiber orientation cross profile obtained by a controlled headbox reached can be, but at the same time also positive Influence on the formation cross profile.

According to the invention, this problem is solved by the features of claim 1 solved.

The inventor recognized the following:

Becomes a in the well-known manner described above Fabric suspension layer using a sectional material suspension density and volume flow controlled Headbox made across the machine width seen a uniform mass distribution, so however, an error arises in the Basis weight cross profile by a different strong web shrinkage across the machine width. Particularly large deviations in web shrinkage arise in the edge area of the paper web, in which a particularly strong shrinkage occurs. Since at the Shrinkage is an originally evenly distributed mass moving into a smaller area is created in larger shrink areas Grammage. To compensate for this Shrinkage effect and to improve the Basis weight cross section is now in this area a decrease in stock density in the headbox brought about and at the same time the volume flow of the Stock suspension kept constant.

Because of the uneven across the width Fabric density also results in different levels Retention of the suspension during the dewatering process the sieve or between two sieves. This Unevenness is particularly noticeable Twin wire formers noticeable. Here is due to the Unevenness of retention in temporal The course varies in intensity Drainage. In the area of lower consistency the water drains faster, leading to an earlier one Immobilization of the fibers in this area leads. As The consequence of this thus arises transversely to the machine direction seen area where immobilization of the Fibers take place earlier and other areas where an immobilization of the fibers only later takes place. Because the formation-improving effect of Drainage elements only work if none Immobilization of the fibers has occurred the drainage elements don't cover the whole Machine width evenly, but areas occur on where the drainage elements are still effective because the fibers have not yet been immobilized has taken place and other areas where the Immobilization took place too early and as a result a formation improvement through drainage elements can no longer be done. Since the drainage in the Edge areas of the web particularly quickly this means that the formation-improved effect of the drainage elements on the fibers in the edge area can no longer work while the same Drainage elements in the middle of the web still work and in this way a difference the formation, at least from the middle of the track to the edge of the track, arises.

• Messen und Einstellen des gewünschten Flächengewichtsquerprofils durch Beeinflussung der sektionalen Stoffdichte im Stoffauflauf, anschließend
• Beeinflussung des Formationsquerprofils durch Beeinflussung des sektionalen, spezifischen Stoffsuspensionsdurchsatzes am Düsenauslaufspalt, gleichzeitig oder anschließend
• Vergrößerung des Düsenauslaufspaltes an Stellen mit erhöhtem Stoffsuspensionsdurchsatz und umgekehrt zur Einstellung des gewünschten Faserorientierungsquerprofils.

In order to standardize the point in time of immobilization of the fibers described above, the following method is proposed according to the invention for setting a uniform cross-property profile of a paper web with regard to the formation, basis weight and fiber orientation in a paper machine with the aid of a headbox which is influenced by the density of the material and which has an adjustable nozzle outlet gap, as follows:

• Measure and set the desired basis weight cross-section by influencing the sectional material density in the headbox, then
• Influencing the formation cross-profile by influencing the sectional, specific stock suspension throughput at the nozzle outlet gap, simultaneously or subsequently
• Enlargement of the nozzle outlet gap at points with increased material suspension throughput and vice versa to set the desired fiber orientation cross profile.

So it is through this process, contrary to So far prevailing opinion that with a stock density-controlled headbox one if possible uniform, specific material suspension throughput should be reached across the machine width walked off and consciously premature in places Immobilization of the fibers an excessive, specific Material suspension throughput causes, however, with it coupled to maintaining the desired, mostly uniform fiber orientation cross profile the aperture of the headbox open around that Cross speed profile of the exiting Keep stock suspension. That means that now in addition to the actuator for the Basis weight cross profile at the headbox Actuator for influencing the sectional Throughput is required. With such a The throughput, for example in the actuator Edge area, so far increased until the Formation cross profile is flat in the transverse direction. By this measure will be the immobility points in Cross machine direction viewed on a line arranged. With this increased throughput in Edge area of the aperture gap, as with Common density-controlled headboxes kept parallel, cross currents arise because the Greater throughput in the edge area through the nozzle gap a higher pressure loss than in the middle of the web generated. To avoid these cross currents, the Nozzle gap almost proportional to the change in throughput be adjusted. In the case of a increased throughput in the edge area that the gap can be opened further in the edge area.

In contrast to the usual procedure today, the consequently the headbox cover for Fiber orientation correction is used the invention is a method for Fiber orientation correction changes in local Volume flows used and the orifice for Formation optimization.

It is understood that the desired cross sections are not always over the entire machine width have to be even, but also one particularly advantageous, non-linear course can have.

According to a further embodiment of the The inventive method is proposed that Change in the nozzle outlet gap proportional to Change in specific material suspension throughput make. A particularly advantageous optimization of the outlet gap is when the change the nozzle outlet gap is made such that after setting a desired one, usually constant cross-speed profile on Nozzle outlet gap is present.

Furthermore, it can be advantageous if at least one of the above-mentioned process steps into one Control loop is integrated, so that an automatic Regulation of the actuators on the headbox is given.

Another advantageous training of The method according to the invention enters a combination with the use of the drainage rate influencing chemicals. This has the big one Advantage that not only influences the formation, but also on the leaf composition can be taken, since the binding forces of Wash-out affected fine and fillers can be influenced.

According to this idea, it is particularly advantageous in Areas with higher throughput a filler addition, preferably via sectionally supplied liquid, make. It is particularly advantageous if the supply or withdrawal of liquid to the Setting the specific Material suspension throughput seen in the z direction in Edge areas of the headbox takes place, whereby also essentially the top or the leaf Leaf underside is affected.

The procedure described above can be used by anyone known design of the wet area Paper machine can be used, however it is this method is particularly advantageous for Use twin wire former, as the one described Influencing the formation in the twin wire former is particularly pronounced.

Claims (9)

1. Method of establishing a uniform cross-machine property profile of a paper web with respect to formation, grammage and fibre orientation in a paper machine with the aid of a flowbox in which the consistency can be influenced section by section and which has an adjustable nozzle outlet gap, characterized by the following method steps:

   1.1 measuring and setting the desired cross-machine grammage profile by influencing the sectional consistency in the flowbox,

   1.2 then influencing the cross-machine formation profile by influencing the sectional, specific stock suspension throughput at the nozzle outlet gap,

   1.3 at the same time or subsequently, enlarging the nozzle outlet gap at points with an increased stock suspension throughput and conversely in order to achieve a desired cross-machine fibre orientation profile.
2. Method according to Claim 1, characterized in that the change in the nozzle outlet gap is made in proportion to the change in the specific stock suspension throughput.
3. Method according to Claim 1, characterized in that the change in the nozzle outlet gap is made in such a way that, following the setting, a desired cross-machine velocity profile is present at the nozzle outlet gap.
4. Method according to one of the preceding claims, characterized in that at least one of the abovementioned method steps is incorporated in a control loop.
5. Method according to one of the preceding claims, characterized in that in addition chemicals which delay dewatering and/or accelerate dewatering are added, preferably in the edge region.
6. Method according to one of the preceding claims, characterized in that the specific stock suspension throughputs are influenced by withdrawing or feeding liquid in sectionally.
7. Method according to one of the preceding claims, characterized in that in regions with a higher throughput, an addition of fillers is made, preferably via a liquid fed in sectionally.
8. Method according to one of Claims 6-7, characterized in that the feeding or the withdrawal of liquid is carried out in the edge regions, as viewed in the z direction.
9. Method according to one of the preceding claims, characterized in that a twin-wire former is used.