CS273208B1 - Dewatering felt - Google Patents

Abstract

The design concerns the dewatering felt, which stops water transfer from the felt into the dewatered material on the output from the pressing zone of the dewatering device. The felt is formed by two layers, while the upper layer is made of material with lower hydrophility and larger pores than the inner layer, which is made of hydrophilic material.

Description

The invention relates to a dewatering felt which limits the transfer of water from the felt to the dewatering material at the exit of the press zone of the dewatering device.

The paper or board dewatering systems used up to now consist of trapping the water extruded from the pulp press in a porous material - usually in the felt and the subsequent removal of water from the felt outside the system. The primary porous material, typically felt, is preferably supplemented with another material capable of generating a sufficiently large volume in the press interface in which water can be displaced from the dewatered paper web in the direction of passage of the dewatering device. It is used with a sieve, punching, pitting, eventually grooving of the cylinder, which is in contact with 3 dewatering felt, or a combination of both. The backing screen may be separate, in which case it requires its own guidance in the dewatering machine, it may be part of the dewatering felt, in which case the longer life of the coarse fabric will not be utilized.

The conventional construction of a dewatering felt consists in a combination of a backing layer, which may be simple or combined in a coarse bottom and fine top layer to which the fibrous web is applied. In this case, the lower layer creates the aforementioned space for transporting water through the press contact line in the direction of movement of the dewatering device. The finer topsheet of the dewatering felt reduces local pressure differences in the press system and improves the surface uniformity of the dewatered material.

Fabrics are manufactured under different names and are designed for specific conditions with respect to production equipment - especially machine speed - and with respect to the product being manufactured - its basis weight, surface quality requirements and the like.

In the current construction of dewatering felt, it is assumed that on the side of the felt that comes into contact with the dewatered paper, the finest fibers forming the smallest pore system are used. These small pores, on the one hand, allow the formation of a relatively smooth paper surface and, on the other hand, have been shown to slow down the transfer of water from felt to paper (H.H. Heller, Ch.G. Tewksbur, TAPPI 55, 6, 1972, pp. 893-896). In addition, the fine felt surface of the felt limits the ingress of particles released from the paper stock into the felt. A second possibility - usually realized - is to use a web of uniform fibers for the actual working layer of the felt, the fiber fineness increasing from the first to the third and last press, respectively.

Treatments of felt surface in this sense affect the dewatering of paper pulp in the press and are the main component that has made progress in paper pressing, but do not significantly reduce backflow from felt to paper, which currently represents the largest margin in the paper and board production process and the latest paper machines. It has long been proven that we achieve high drainage (above 50 to 60% dry matter) in the press contact line, and the return of water from felt to paper at the exit of the press line depreciates this result (Andersson N. Gardh H. Svensk Papperstidning - 73, 13/14, 1970, pp. 425-429). Methods are known which seek to limit the transfer of water from felt to paper by reducing the contact time of the paper with the felt at the exit of the contact line (Ludnstrom K.,

USP 4,588,475) comprising the application of a resilient material interposed between dewatered paper and a press roll which is tensioned by a pair of additional rollers, thereby reducing the pressure drop area at the exit of the press nip line. The theoretical analysis suggests that a time equivalent to 1 ms is sufficient to transfer all the water available at the exit of the contact line at the paper-felt interface to the paper; this time corresponds to the design of all known systems even at the machines with the highest speeds. The results of this assay are confirmed experimentally (Jaaviduan J., Ceoler VI. H., Thompson E. V. TAPPI 71, 3, 1988, pp. 151-155).

CS 273 208 B1

The pores in the seam must be larger than the pores in the paper when the system is compressed in the press line. Otherwise, a hydraulic pressure would be generated at the paper-pl3tonec interface and this pressure would limit the efficiency of the molding process. Under these circumstances, the transfer of water from the large pores in the felt to the small pores in the paper necessarily occurs.

The driving force of this movement is given by the capillary pressure, which corresponds to the expression:

P S S ^{p =} r- ^{th (Pq) (1) K} 1 ⁿ 2 where je is the adhesive stress in the paper pore system, Ag canvas and R je is the hydraulic pore ratio of paper, Rg canvas.

The speed of water passing from the felt pores to the paper pores (v) can be described by:

dZ - ^R 1 ² p dt - K '(m / s) (2) where Z · is the path traveled by the water in the paper pores (m) over time t; K is the Kozeny constant and is the viscosity of water (Pa s). (Skalický G, Paper and Cellulose 42, 7/8, 1987, p. V 33-36).

The control of this adverse effect is solved by the invention, which is based on the creation of a double-layered linen by needling two layers of fibrous web of different quality. A hydrophilic thin-walled material (wool, polyamide, etc.) is introduced into the lower layer to form smaller pores. The coarser fiber and hydrophobic material is introduced into the upper layer to form a system with larger pores (polyester, polypropylene, etc.).

In this way, ae creates an additional interface between the lower layer (coarse fibers) and the upper layer (fine fibers) at which the capillary pressure p ₂ is generated, which causes water to pass behind the upper liner layer (coarse fibers) into the lower felt layer. (with fine fibers). The value of p ₂ corresponds to expression (3).

We keep the values with index 2 for the felt surface layer; values with an index of 3 for the inner felt layer.

A positive pressure value of p ₂ can be obtained by increasing the pores in the top layer of the felt (using coarser fibers) or by reducing the hydrophobicity of the material used for the top layer of the felt; or a combination of both.

To use the method, it is necessary that the thickness of the felt felt layer when compressed is such that the thickness of the water layer (Lv), discharged from the dewatered paper pulp, penetrates through the layer and enters the middle felt layer into an area capable of providing increased capillary pressure. would cause a desirable transfer of water from the paper felt interface to the middle felt layer.

Lv can be determined by an expression;

^Lv ~ o ^ ^(nun ) (4) on where P; is the basis weight of paper after drying (g / m) and c is the pulp concentration (g / 1 = kg / nr ³ ) before (index o) and after (index n) press contact line »

CS 273 208 B1

Achieving a sufficiently large Lv value at a high water content in the pulp is easy; With the increasing degree of dewatering, it fails because the pressed water is not sufficient to form a continuous layer of dewatered pulp through a felt layer with a lower Ag / Rg value up to a layer with a higher value of A ^ / R ^.

The principle was verified by dewatering a series of samples of standard fibrous material, modeled with layers of water-saturated filter paper with different basis weight, felt, composed of two layers:

1. Polyamide fibers (PA) with addition of wool (6.7 dtex thickness) - Polar layers with smaller pores PP - Area weight 394 g / m ² ;

P

2. polypropylene fibers 22 dtex basis weight 394 g / m.

Comparison of dry matter achieved by pressing during various treatment of dewatering felt:

system: paper-PP-PA verification system 1 (%) paper-commercial felt - verify system 2 (%) flat mass: 80 g / m ² 38.7 41.2 160 g / m ² 44.3 41.2 240 g / m ² 47.0 42.5 320 g / m ² 43.6 42.7 400 g / m ² 44.5 43.0

The table shows that at a basis weight of 80 g / m @ 2, when the Lv value was 0.041 mm, there was no increased capillary pressure that would lead to water transfer from the paper-felt interface to the inner layers of the felt.

At higher values, when the Lv value was 0.14 or more mm, 1c, the pulp was more drained than the control system. In all cases the drainage time was sufficient, the layer of dewatered material was drained evenly.

The canvases used for drainage were standardized before each experiment.

The solution according to the invention can be used for dewatering porous materials such as paper, cardboard, technical materials such as filter materials, fiber systems and the like.

Claims (1)

SUBJECT OF THE INVENTION A dewatering felt for dewatering fibrous materials, comprising two layers, characterized in that the surface layer in contact with the dewatering material consists of a material with coarser fibers of lower hydrophilicity than the lower layer which is composed of finer hydrophilic materials. fibers.