In papermaking, a considerable amount of water, usually called tail water, is removed from the web at the wire section. Open channels are used to lead the tail water from the wire section to a tail water reservoir as gently as possible, in order to prevent air that would form foam from mixing with the tail water. The tail water reservoir is, as is known, a high, cylindrical vessel, the tail water being taken to the circula- tion from beneath its drain-like structure, so that the downward flow is slow and possible air bubbles can rise to the surface and exit from the tail water. From the tail water reservoir, the tail water is led through a mixing pump to the desired point of use in the paper machine. The tail water reservoir is also often called the wire drain.

The continual increase in the speeds of paper machines has made de-aeration capacity of present tail water reservoirs insuffi- cient. First of all, the increase in speeds has led to more air than previously becoming bound in the tail water. In addition, a high machine speed also affects the velocity of the tail water being led out of the wire section. This means that the tail water flow in the channels may become turbulent, causing even more air to become bound in the tail water. Correspond- ingly, the rapidly flowing tail water causes detrimental vortices in the tail water reservoir, disturbing the exit of the air. The use of present technology to increase the de-

aeration capacity would therefore require the size of the tail water reservoir to be increased. However, this is undesirable, as the tail water reservoir and channels already demand a great deal of installation space on the already cramped drive side of the paper machine. Indeed, there is a continual attempt to reduce the size of the tail water reservoir and similarly the volume of the entire short circulation, to allow faster changes of paper grade in the production process.

Too great an air content in the pulp causes many problems in the papermaking process. Pressure pulses, variations in stock consistency and the flocculation of the fibres, an increase in aerobic microbes and algae, and the dirtying of various components are some examples of the problems caused by too much air in the pulp. In addition, air bubbles can cause holes in the paper. Due to these and other factors, the significance of de-aeration in the short circulation of a paper machine cannot be underestimated.

The invention is intended to create a new type of method and tail water reservoir for implementing the method in the short circulation of a paper machine, in which the tail water's kinetic energy is exploited to improve the removal of air from the tail water. The characteristic features of the invention are stated in the accompanying Claim 1 while the characteristic features of the tail water reservoir are stated in Claim 4. In the tail water reservoir according to the invention, a new shape and a new tail water intake method are used. In addition, a de-aerating device, which improves the de-aeration of the tail water by the tail water reservoir according to the invention and the removal of air from the reservoir, is placed in the tail water reservoir.

According to one preferred embodiment of the invention, the overflow of the tail water reservoir can also be arranged to improve de-aeration. Thus, the overflow chamber for the aerated

tail water, arranged in connection with the tail water reser- voir, can be utilized, for example, as a circulating water reservoir in the paper machine's long circulation, thereby reducing the number of process structures required. The manner of discharging the tail water from the tail water reservoir can also be arranged in a new, advantageous manner, allowing the tail water reservoir to be arranged entirely on the machine level.

When using the tail water reservoir and de-aerating device according to the invention, most of the air and other gases in a bubble-like form are effectively removed from the tail water, thus simplifying the entire process. The new form of tail water intake prevents areas of dead flow from forming while creating a sufficient velocity in the tail water to prevent surfaces from dirtying.

In the following, the invention is examined in detail with reference to the accompanying drawings showing the invention, in which Figure 1 shows a schematic diagram of the location of a tail water reservoir according to the invention in a paper machine, Figure 2a shows a side view, in partial cross-section, of a tail water reservoir according to the invention, Figure 2b shows a top view, in partial cross-section, of a tail water reservoir according to the invention, Figure 3a shows a side view, in partial cross-section, of a second embodiment of a tail water reservoir according to the invention, and Figure 3b shows a top view, in partial cross-section, of a second embodiment of a tail water reservoir according to the invention.

Figure 1 shows schematically the location of a tail water reservoir according to the invention in a paper machine. The arrangement according to the invention concerns the wire section 10 of the paper machine and particularly the processing of the tail water in the tail water reservoir 13. In this connection, the term paper machine also refers to a board machine or similar.

During web formation, a head box 11 is used to feed pulp diluted to the desired consistency to the wire section 10. In the wire section 10 of Figure 1 there are two web-formation wires 12 and 12', between which the pulp is fed. In practice, most of the extremely watery pulp leaves as tail water at the wire section 10, with the fibres and fillers continuing to the press section that follows the wire section 10. As the amount of tail water nearly corresponds to the flow rate of the head box 11, the tail water must be collected. For this purpose tail water collection and guide devices (not shown) are arranged inside the loops formed by the web formation wires 12 and 12'.

The collection and guide devices are used to guide the tail water in a controlled manner away from the wire section 10 to the drive side of the paper machine and from there through channels 24 to the tail water reservoir 13, from which the tail water is used in the paper machine's short circulation.

Figure 2a shows a side view in partial cross-section of one embodiment of the tail water reservoir 13 according to the invention, shown in Figure 1. The tail water reservoir 13 is a cylinder with an essentially circular cross-section, from which a functional cyclone is formed, with a height that is essen- tially greater than its diameter. This creates the shortest possible exit path for the air bubbles, while nevertheless giving the vessel a sufficient capacity. The tail water is brought at high velocity along one or several entry channels 24 tangentially to the tail water intake connection 18 in the upper part of the cyclone. The aim is to maintain the surface

level 25 in the tail water reservoir 13 at the same level as the lower edge of the intake connection 18.

Above the tangential intake connection 18, the vessel has an overflow construction, formed by a cylindrical piece 19 that is higher up than the actual vessel. From the overflow vessel the tail water is then led to the circulating water reservoir 23.

The tail water outlet connection 16 is in the lower part 21 of the vessel, so that the tail water moves downwards by gravity in the vessel. The air removal connection 17 of the tail water reservoir 13 is located centrally in the cover of the tail water reservoir 13 and is preferably connected to the air- treatment equipment of the paper machine.

According to one preferred embodiment of the invention, a pole 14 is fitted to the middle of the tail water reservoir 13. The pole 14 is attached at its upper end, for example, to the air removal connection 17 made in the centre of the cover of the tail water reservoir 13 while the lower end of the pole 14 hangs freely. The length of the pole 14 is preferably essen- tially the length of the cylindrical vessel 22 of the tail water reservoir 13.

The pole 14 is shaped with angular protrusions according to Figure 2b. A flat surfaced piece 15 covering a segment 37 between two adjacent protrusions 14.1 and 14.2, or at least a substantial part of this, is attached beneath the pole 14.

The operation of the tail water reservoir according to the invention, shown in Figures 1,2a, and 2b, is as follows. In addition to gravity, the tail water is moved by its own kinetic energy, which is exploited by the intake connection 18 being placed essentially at a tangent to the vessel. As the tail water flows rapidly tangentially into the vessel, a vortex 26, in which the tail water is mixed in a controlled manner, is formed inside the vessel. The air separating from the tail

water, which would be detrimental to the entire process if it were to travel to the tail water mixing pump 20, collects in the centre of the vortex 26.

The centre of the tail water vortex 26 is slowed by means of the pole 14 with protrusions located in the middle of the tail water reservoir. This allows air bubbles in the tail water to rise upwards in the channel formed between two adjacent protrusions 14.1 and 14.2 of the pole 14. Once the air has risen to the cover of the tail water reservoir 13, it is sucked out mechanically through the air removal connection 17. The preferred shapes of the protrusions of the pole 14 are, for example, a star (Figure 2b), a spiral screw, or a sickle (not shown). However, the shape of the pole 14 is essentially such that it slows the vortex 26. A flat surfaced piece 15, covering the segment 37 formed by two adjacent protrusions 14.1 and 14.2 and preventing the tail water slowed by the pole 14 from entering the exit connection 16 connected to the downwardly narrowing conical part 21 of the tail water reservoir 13 and from there going to the mixing pump 20, is attached beneath the pole 14 fitted for de-aeration. The flat surfaced piece 15 can be a flat plate or can be shaped, for example, as a cone, and is attached to the pole 14 either on its flat side (Figure 2a) or at its point.

Figure 2b shows clearly the position of the intake connection 18 in the vessel. The intake connection 18 also narrows rapidly, so that the tail water flows into the vessel as an even jet over the full intake connection 18. Unlike in the known technology, an increase in the velocity of the tail water will improve the operation and effectiveness of the tail water reservoir 13. A sufficient vortex 26 in the vessel is achieved with tail water that has a velocity of at least 3 m/s when it exits from the wire section. Usually the tail water reservoir must be arranged at a height that will ensure a flow velocity in the cyclone sufficient to separate the air. In a vessel with

a circular cylindrical shape, the vortex 26 forms naturally in the centre of the vessel. Due to this, the air-treatment connection 17 and the de-aeration pole 14 are arranged essen- tially at the imagined vertical central axis of the vessel, so that the suction created by the air-treatment will be at the pole 14 located at the throat of the vortex 26.

In order to prevent the air mixing again, the bottom of the vessel is also arranged as a straight cone 21. There is sufficient tail water in the lower part of the vessel with only small air bubbles and a low rate of flow.

Figures 3a and 3b show a de-aerating tail water reservoir 13' according to a second preferred embodiment of the invention. In this case, the same reference numbers as in Figures 1,2a, and 2b are used for the components of the tail water reservoir 13' that are functionally similar. In this embodiment too, the upper part of the tail water reservoir 13'is also a cylindri- cal vessel 22', which in this case has a cylindrical inner wall 35 separating a coaxial cylindrical chamber 23'from the central part of the cylindrical vessel 22'. The upper edge of the outer wall of the cylindrical vessel 22'is at an essen- tially higher level than the upper edge of the inner wall 35.

The base of the cylindrical chamber 23'is, for example, flat and it is at the same level as the bottom of the cylindrical vessel 22'.

In this embodiment, the tail water overflows into the cylindri- cal chamber 23'arranged in the centre of the tail water reservoir 13'. This solution is advantageous to de-aeration, because the tail water enters the reservoir 13'in such a way that the air and gas bubbles in the tail water, as the lighter components in the tangential rotational movement, tend to move towards the central part of the tail water reservoir 13'. Once the tail water reservoir 13'is full up to the upper edge of the inner wall 35 of the cylindrical vessel 22', the excess

tail water, which contains air, flows into the said cylindrical chamber 23'in the manner shown by the arrows. The intention is to keep the surface level in the cylindrical chamber 23' essentially lower than the upper edge of the inner wall 35 of the cylindrical component 22'of the tail water reservoir 13'.

The cylindrical chamber 23'located in the centre of the tail water reservoir 13'according to the embodiment can be used advantageously as, for example, the circulating water reservoir of the paper machine. In that case, there is an outlet connec- tion 36 in the lower part of the cylindrical chamber 23', from which the tail water is led to the process's long circulation through a pump 34.

Further, the tail water outlet connection 16'of the tail water reservoir 13'can be arranged tangentially to the lower part of the vessel 22' (Figure 3b). The tangential outlet connection 16'is positioned so that it lies in the same rotational direction as the tail water intake connection 18. This allows the tail water reservoir 13'to be implemented entirely on the machine level 32.

Essential features of the invention are the cyclone tail water reservoir are the tangential intake connection 18 that creates a rotating movement in the tail water, and the special de- aerating device located in the reservoir, in which the kinetic energy of the tail water is used to effectively remove the air.

The arrangement's structures are both simple and operationally reliable. In addition, the rapidly flowing tail water cannot precipitate in the de-aerating device, which has therefore little need of cleaning. The arrangement also requires less space than previously while its structures can be positioned more freely. In addition, the size of the water reservoir 13 can be substantially reduced, which is advantageous to the entire paper machine and production process.