Method and device for varying the basis weight in a paper machine headbox

Abstract

 A method of varying the basis weight of the fibrous suspension coming out from a head box of a paper making machine is disclosed, wherein said variation of basis weight is obtained by extraction of controlled quantities of fibrous suspension from said head box. The head box (1) carrying out said variation of basis weight comprises a perforated plate (2) with a plurality of troughs (3) and a delivery chamber (6) ending with a slice (7) arranged above a movable filtering surface (F). One or more of said troughs (3) communicate with at least a suction pipe (8) connected to suction means (9) for extracting variable quantities of fibrous suspension.

Description

[0001] The present invention relates to a method of varying the basis weight of a fibrous suspension in a head box for paper making machines.

[0002] The invention relates also to a head box allowing to carry out said method of varying the basis weight.

[0003] It is well known that head boxes are devices applied to paper making plants, adapted to form a fluid flow with suspended cellulose fibers which is deposited on a traveling filtering surface. Water contained in the suspension is drained through the filtering surface on which a layer of wet cellulose fibers forming the paper sheet is left deposited.

[0004] The head box substantially acts as metering device depositing on the filtering surface a fluid film with suspended cellulose fibers having a constant thickness and producing continuous paper sheets.

[0005] In order that the paper being produced is of good quality it is necessary that the suspended fibers at the point of deposition on the filtering surface, are spread with a perfect uniformity because any difference of density would cause variations of paper thickness compromising its quality.

[0006] Indeed it is known that the increasing mechanical stresses on the paper due to the development of the printing processes and the need to avoid distortion and warping of sheets in the photocopying machines, require more and more stringent tolerance not only for the basis weight profile but also for the orientation of the fibers along the cross sectional line.

[0007] For this reason head boxes are provided with a slice generally consisting of a slit on the entire width of the filtering surface, said slit having a perfectly regular geometrical shape with smooth and burr free surface.

[0008] In order to obtain a distribution with uniform thickness of the fluid film on the filtering surface, it is however necessary that the basis weight of the film coming out from the slice of the head box is as constant as possible.

[0009] More particularly the constancy of the basis weight is indispensable to obtain a final product of good quality when suspensions with low concentration of cellulose fibers are treated, for instance with values of suspended fibers varying from 0.2% to 0.4% relative to the liquid in which the suspension is dispersed.

[0010] According to the prior art head boxes are provided with devices to correct the basis weight, consisting of water feeding ducts connected to troughs made in the head box, said ducts adding variable quantities of water to the suspension according to information processed by an electronic control board, so as to vary the basis weight keeping it as constant as possible within predetermined values.

[0011] This method of correcting the basis weight has however the drawback that inside the head box preferential water lines are often generated, causing lack of homogeneity in the suspension and consequently also in the final product.

[0012] The present invention aims at overcoming the above mentioned drawback.

[0013] More particularly the object of the invention is to provide a method of correcting the basis weight, avoiding formation of preferential water lines inside the head box.

[0014] Said object is attained by a method of varying the basis weight of a fibrous suspension coming out from a head box of a paper making machine that according to the main claim is characterized in that said basis weight variation is obtained by extracting from said head box controlled amounts of said fibrous suspension.

[0015] The head box carrying out the variation of basis weight according to the method of the invention, comprises a perforated plate with a plurality of troughs adapted to convey said fibrous suspension coming from a feeding header into a delivery chamber ending with a slice leading above a filtering movable surface adapted to receive the suspension, and is characterized in that one or more of said troughs are in communication with at least a suction pipe connected to suction means adapted to extract from said head box variable amounts of said fibrous suspension.

[0016] According to a preferred embodiment regulating means are arranged in the suction pipe, electrically connected to an electronic control board for varying the amount of fibrous suspension to be extracted.

[0017] Advantageously the method of the invention and the relevant head box remove the problem of possible formation of preferential water lines in the fibrous suspension coming out from the head box. Indeed the extraction of fibrous suspension does not involve variations of the concentration of said fibrous suspension as it happens now with the prior art using the addition of water, resulting only in the variation of the rate of flow from the head box.

[0018] The method and the head box both objects of the invention, will be better understood by reading the following description of a preferred embodiment making reference to the accompanying sheets of drawings in which:

• Fig. 1 is a longitudinal cross sectional view of the head box of the invention; and
• Fig. 2 is an enlarged view of a detail of Fig. 1.

[0019] The method of the invention is carried out by a head box generally indicated with reference numeral 1 shown in Fig. 1, which is applied at the starting end of a paper making machine. Only the filtering surface or Fourdrinier wire F of the machine can be seen in Fig. 1, the head box 1 depositing on said filtering surface a fluid film of suspended cellulose fibers.

[0020] As shown in Fig. 1, the head box 1 comprises a perforated plate 2 provided with a plurality of troughs 3 conveying the fibrous suspension coming from a header 4 in communication with a pulse damping tank 5, into a delivery chamber 6 ending with a slice 7 arranged above said filtering surface F. In some embodiments the pulse damping device may be omitted.

[0021] According to the invention one or more of said troughs 2 communicate with at least a suction pipe 8 connected to suction means 9 adapted to extract from said head box 1 variable amounts of said fibrous suspension.

[0022] More particularly it can be seen in Fig. 2 that the suction pipe 8 is provided with adjustable interception means consisting for instance of a cock 10.

[0023] The cock 10 is provided with an electric opening and closing actuator 10a electrically connected to an electronic control unit 11 which regulates opening of cock 10 and modifies the rate of flow through the suction pipe 8 as a function of the basis weight of the fibrous suspension coming out from the slice 7 of the head box.

[0024] In operation when the basis weight coming out from the slice 7 goes above a predetermined value detected by suitable measuring devices not shown in the drawings, the electronic control unit 11 opens the cock 10 so as to suck through the suction pipe 8 a portion of the fibrous suspension flowing through the troughs 2 into the delivery chamber 6.

[0025] Suction of the fibrous suspension from the head box 1 may occur in a continuous way or in intermittent mode as function of the control mode of the cock 10.

[0026] Depending on the circumstances, one or more suction pipes 8 may be provided and each pipe may be connected to one or more of said troughs 2.

[0027] When the measuring devices detect that the basis weight returned to the predetermined standard values, the cock 10 is closed so as to interrupt the suction of fibrous suspension by the suction pipe 8.

[0028] Therefore according to the method of the invention carried out by the head box 1 also being object of the invention, it is to be noted that the variation of the basis weight of the fibrous suspension coming out from the head box is obtained by extraction from said head box of controlled amounts of said suspension.

[0029] Therefore the adjustment of the basis weight occurs by reducing the rate of flow of the fibrous suspension in the head box without introducing water into the head box thus without diluting the fibrous suspension keeping its density at all its points.

[0030] In this way the risk of generating inside the head box preferential water lines causing lack of homogeneity in the suspension and consequently defects in the final product, is avoided.

[0031] The suction ducts connecting the suction means to the troughs may be in any suitable number and each duct may also be connected to any desired number of troughs.

[0032] In the constructional stage many modifications may be made to the head box of the invention, but it is to be understood that said modifications are all covered by the present patent when falling within the scope of the appended claims.

Claims

1. A method of varying the basis weight of a fibrous suspension coming out from a head box of a paper making machine, characterized in that said variation of the basis weight is obtained by extracting controlled quantities of said fibrous suspension from said head box.

2. The method according to claim 1) characterized in that said extraction occurs in a continuous way.

3. The method according to claim 1) characterized in that said extraction occurs in a discontinuous way.

4. A head box (1) adapted to carry out the variation of basis weight according to the method of claim 1) comprising a perforated plate (2) with a plurality of troughs (3) adapted to convey said fibrous suspension coming from a feeding header (4) into a delivery chamber (6) ending with a slice (7) leading above a movable filtering surface (F) adapted to receive said suspension, characterized in that one or more of said troughs (3) communicate with at least a suction pipe (8) connected to suction means (9) adapted to extract from said head box (1) variable quantities of said fibrous suspension.

5. The head box (1) according to claim 4) characterized in that adjustable intercepting means (10) are provided in said suction pipe (8).

6. The head box (1) according to claim 5) characterized in that said intercepting means (10) are electrically connected to an electronic control unit adapted to modify the rate of flow through said intercepting means (10) as a function of the basis weight of said fibrous suspension coming out from said slice (7) of said head box (1).

Drawing