PULP DRYING MACHINE AND METHOD FOR DRYING A PULP WEB

The invention relates to a pulp drying machine comprising a press and a circulation of a continuous supporting wire that is permeable to water and is introduced together with the pulp web to be dried between the press surfaces that limit the press nip, for dewatehng the web in the direction of the supporting wire, wherein dewatehng means are provided on the opposite side of the pulp web, seen from the supporting web, for dewatehng the web in the opposite direction.

The invention also relates to a method for drying a pulp web, wherein the web is introduced together with a continuous supporting wire that is permeable to water through a press nip, wherein water is removed from the web through the supporting wire and in the opposite direction by the effect of dewatering means on the opposite side.

An example to be mentioned of a pulp drying machine is given in document WO 00/31336, comprising a former section for dewatering a pulp suspension, followed by a press section for dewatering by mechanical pressing of the pulp web to be formed of the suspension. In the pulp drying machine, these sections constitute the so-called wet end, which is also the subject of the present invention.

In said document, the supporting wire of the former section of the pulp drying machine is also introduced through the first press nip in the press section.

Thus, the wire section also comprises, in addition to the dewatering equipment based on a pressure difference in the former section, a press for the purpose of dewatering the web when dewatering by a mere pressure difference is no longer successful. Typically, the first press comprises an assembly in the following order (from below): a press roll, a wire, the pulp web, a press felt, and another press roll. There are also presses without a felt. In the press, the dewatering is effected in two directions from the pulp web. The water removed from the pulp web is absorbed by the felt in one direction, and the absorbed water is removed from the felt by a vacuum extractor in the felt circulation. In the other direction, water is removed

through the wire into grooves or blind holes in the press roll lying against the wire.

In the presses of this kind, the dewatehng into the felt has been found to be more efficient than through the wire into the press roll. On the other hand, if the press roll that is against the wire is a suction roll, then the dewatehng in the press is very efficient from the web through the wire.

At the terminal end of the press section, where the web has a high dry matter content and the pressing forces of the press rolls are high, smooth rolls are sometimes used. In most modern drying machines, the last press is normally a long nip press, that is, a so-called shoe press, and two shoe presses may be arranged one after the other. The last press is normally a felt press, in which a felt is provided on each side of the web, as in said publication.

After the former section, the dry matter content of the web is in the order of 20 to 25%, after the first shoe press it is about 40 to 45%, and after the second shoe press it is about 50 to 55%.

It is an aim of the invention to improve the drying of the pulp web in the press section and thus to reduce the need for energy in later drying that takes place by evaporation. For achieving this aim, the drying machine according to the invention is primarily characterized in that a circulation of a water-absorbent felt is placed within the circulation of the supporting wire and introduced between the supporting wire and the press surface that limits the press nip and further in that the press surfaces of the press constitute a long nip. The method according to the invention is primarily characterized in that a felt, in which the water passed through the wire is absorbed, is provided between the water-permeable supporting wire and the press surface limiting the press nip and further in that the press nip through which the pulp web, supporting wire and the felt are introduced is a long nip.

In the press nip, a supporting wire permeable to water is placed against the first side of the web. Water is removed from the web through this supporting wire into the felt. A felt known as such may be placed against the second

side, and water is absorbed from the web in this welt. Thus, water pressed out of the web can exit well in both directions.

It is a surprising observation that a felt behind the wire is effective in dewatering when compared with a grooving or bore holes in the roll that is in contact with the wire. The invention provides an increase in the dry matter content of the pulp web exiting the press when compared with conventional prior art. It has been found in tests carried out that the dry matter content of the pulp web after the shoe press increased from 42.6% to 48% when a felt circulation was introduced inside the wire circulation of the press. The effect is probably attributed to the fact that the wire lets water pass through from the pulp web into the felt and the felt receives the removed water better than a grooved roll. The felt retains the water even after the nip, whereas water from the grooves of the roll is easily reabsorbed by the web, in spite of the wire in between.

The invention provides savings in energy and costs, because dewatering by pressing will consume significantly less energy than dewatering the pulp web by evaporation in a dryer. Another advantage to be mentioned is the fact that by introducing a water-absorbing felt between the wire and the press surface that limits the press nip, the press surface can be selected more freely; that is, it may be a smooth-faced roll or even a static element.

In the following, the invention will be described in more detail with reference to the appended drawing that shows the press section of a drying machine according to the invention in a schematic side view.

The product of the pulp drying machine to be described hereinafter is used as raw material for making paper, usually as part of the fibrous papermaking stock supplied to the paermaking machine. For example if the paper is made in a location far away from the pulp production location, the pulp is dried in a pulp drying machine to avoid transport of water.

The press section shown in the figure is part of a pulp drying machine. In a pulp drying machine, pulp suspended in water is dried to such a dry matter content that it can be easily transported. As a result of the drying, a pulp web

W is obtained that can be used later as raw material in a papermaking process. For facilitating the handling and transportation, the web can be cut, for example into sheets.

In the cellulose drying machine, the pulp suspended in water is first fed to a former section where the dewatehng takes place through an advancing water-permeable wire, for example through two opposite moving wires, between which the suspension is fed. After the former section the pulp, already forming a fibrous web, is further dewatered in a press section my mechanical pressing, and final drying takes place in a dryer section through evaporation of water from the pulp web.

The press section comprises a first press P1 , into which the pulp web W that has started its formation in the former section is led on a supporting wire 2 moving in the same direction. The supporting wire 2 is a wet wire running all the way from the former section; in other words, the water has first been filtered from the pulp through this same wire by means of pressure. The press section comprises a press nip, in which the pulp web is subjected to a pressure for dewatering by mechanical pressing. The press nip is limited, in the direction perpendicular to the plane of the web W, by press surfaces which may be formed by the surfaces of the rotating rolls of the press. The press P1 shown in the figure is a shoe press in which the press surfaces constitute a long nip. In the embodiment of the figure, the press shoe creating the concave shape of the press surface being in the upper roll of the press P1. Carried between the supporting wire 2 underneath and an absorbent felt 1 on top, the pulp web W is introduced through the press nip, after which it is released from the wire and the felt and introduced into a second press P2 of the press section where it is carried between an upper and lower felt 1 through a corresponding press nip. The supporting wire 2 returns back to the former section of the pulp drying mach ine. Thus, the wire 2 forms a circulation where the wire moves in a closed loop.

Within the circulation of the supporting wire 2 in the first press P1 , a circulation of the water-absorbent felt 1 a is provided, running through the nip between the lower press surface (lower roll) and the supporting wire 2 . The felt 1a absorbs effectively water that has been pressed out of the web W and

passed through the wire 2. The lower roll does not necessarily require a grooving or bore holes, and its structure can be selected more freely. On the other hand, if the roll surface underneath the felt 1 a is provided with a grooving, bore holes or another structure for receiving water, the water possibly ending up in these will no longer wet the web but merely the felt. The water pressed out of the web on the other side will be absorbed, in a known way, by the upper water-absorbent felt 1 that runs through the nip between the web W and the upper press surface.

The invention will be illustrated by the following test example:

A test machine was used to test the installation of a felt between a press roll and a wire. In the test, a shoe press was applied, comprising a shoe at the top and a grooved roll at the bottom, below the wire. The function of the felt was to receive the water removed from the pulp web through the wire. The test was conducted first by making a reference run with a normal machine concept and then by installing a press felt in the inner circulation of the wire in the machine, and by running the same test series again using corresponding test run pulp. The test results were as follows:

reference run test run wire speed m/min 200 200 grammage (g/m ² ) 1000 1000 dry matter before press % 30.6 31.1 dry matter after press % 42.6 48.0

According to the test run, a press felt installed in the press of the wire section makes dewatering unexpectedly efficient. The dry matter content results are average values of test points.