The invention also relates to a method for controlling the pressure in the short circulation of a closed former, in which method wire water is passed from a wire section through a dilution duct to the dilution of thick stock and excess wire water is passed out of the short circulation through an outlet duct, which dilution duct and outlet duct are in flow communication with each other.

In this connection the closed former refers to a twin-wire former in which the slice jet from a headbox is fed into the forming gap of the former in a closed volume and the dewatering in the wire section is also accomplished in a closed fashion such that the system does not comprise any free liquid surfaces at all and air is not able to mix with wire water at any stage.

US pate7çt 3, 823, 062 describes one closed former of this kind in which fibre suspension is passed from a headbox into a forming gap of the twin-wire former between two walls such that it does not come into contact with the ambient air. In the forming section, water is removed from the fibre suspension through wires into dewatering boxes filled with water. The wire water is passed from them into an open wire water tank, from which it is passed further to the dilution of the stock to be introduced into the headbox. As the wire water passes through the

open tank, the system cannot make use of the overpressure which would be still present in the wire water after a combination of the closed headbox and the former.

Figure 4 of FI patent applicatiora 981327 illustrates a process arrangement for the short circulation of a paper machine, which arrangement comprises a headbox and a wire section that are closed and which arrangement does not have any deaeration tank. The wire water collected from the wire section is fed by a circulation water pump to the first and second dilution stages of the main line of the process. Part of the wire water is pumped by means of a dilution water pump through a screen into the dilution headbox. The process is closed with respect to the ambient air, since only the overflow of wire water, centrifugal cleaning reject and second screen reject are in contact with the ambient air. The arrangement is, however, complicated and a large number of pumps and a large amount of pumping power are required for its accomplishment. Also, the pressure left in the wire water cannot be fully utilized in it.

An object of the invention is to make it possible to maintain a higher back pressure than heretofore in the short circulation of a closed former, which enables a substantial reduction in the energy needed for pumping in the short circulation.

With a view to achieving these objects as well as those which will come out later, the process arrangement according to the invention is characterized in what is stated in the characterizing part of claim 1. Correspondingly, the method according to the invention is characterized by what is stated in the characterizing part of claim 9.

The stable operation of the system requires that the pressure in the short circulation at the suction side of the feed pump may be kept constant. In the arrangement in accordance with the invention, the excess water from the short circulation is passed through a rather large pipe to a tank placed at a suitable level

above the level of the headbox and provided with control of the surface level, which may be, for example, an overflow. This excess water to be passed to the overflow tank is taken most advantageously from dewatering elements at the end of the forming section, which means that it is usually the purest water fraction of the forming section. Because of the overflow or some other control of the surface level arranged in the tank, the pressure in the pipe line leading from the wire section to stock dilution remains constant. When needed, the pressure level can be adjusted so as to be suitable by means of valves.

No vacuum need be arranged for the dewatering elements of the closed former because the pressure of the pulp suspension fed from the headbox is sufficient for removal of water through the dewatering elements. As there is no direct connection with the ambient pressure, the overpressure of the wire water can be utilized effectively in the short circulation, which reduces the energy needed in pumping. An outlet connection from the short circulation to the atmospheric pressure is arranged through an outlet duct and a tank provided with surface level control and placed at a level that is substantially higher than the level of the former. Alternatively, the difference in levels can be replaced partly or totally, for example, with valves or the like.

As no substantial amount of air is allowed to enter the wire water circulation, a wire pit or a separate deaeration stage is not necessarily needed in the short circulation. At elevated pressure, air dissolves more readily in the circulation water. However, thick stock may bring air with it to the short circulation, which air is apt to remain circulating in the process, so that the short circulation may, in the course of time, become saturated with air. The amount of air in the short circulation can be reduced by passing a small part, advantageously less than half of the wire water circulating in it to deaeration. Advantageously, the portion of the wire water used for regulating the dilution profile of the headbox is circulated through deaeration. k some cases, passive deaeration by means of a device of the wire pit type is sufficient for deaeration of dilution water. In that connection,

dilution water can be taken either from the overflow tank or from the water stream going to the long circulation after the overflow.

The arrangement in accordance with the invention reduces considerably the number of devices needed in the short circulation, which means that the arrangement in question is very economical in its investment and operation costs.

From the point of view of stock cleaning, the situation is also excellent since the stock can be screened at high consistency before it enters the short circulation, thus allowing low flow rates and a low pressure to be used in screening. In production efficiency, advantages are achieved because of very quick grade change.

In the following, the invention will be described with reference to Figs. 1-3 of the appended drawings, but the invention is not meant to be strictly limited to them.

Figure 1 is a schematic view of a short circulation of a paper machine in accordance with the invention.

Figure 2 is a schematic view of a short circulation that also includes a centrifugal cleaning stage.

Figure 3 is a schematic view of a short circulation in which part of the wire water is circulated through a deaeration stage.

Fig. 1 shows a short circulation process arrangement for a closed former in which pulp suspension or wire water is not in contact with the ambient air at any stage.

The parts of the process outlined with broken lines are not necessary in the process arrangement in accordance with the invention.

Thick stock M to be passed to a paper machine is conducted, possibly through a screening stage 20, so as to be mixed with wire water Fo flowing in a dilution duct

17 on the suction side of a feed pump 10. The dilution of the stock is accomplished as pipe dilution without a separate wire pit. The feed pump 10 pumps a diluted pulp suspension Fi into a headbox 11. A pulp suspension jet J is passed from the headbox 11 between walls 12 provided as continuations of a slice opening into a forming gap defined by two wires 13 and 14 such that between the headbox 11 and first dewatering boxes 15,16 of a forming section the pulp suspension jet does not come into any contact with the ambient air. In a twin-wire zone situated after the forming gap, water is removed from the pulp suspension by means of the dewatering boxes 15 and 16 placed on both sides of a web and sealed against the wires 13,14. Both dewatering boxes 15 and 16 are divided in the running direction of the web into compartments which are filled with water.

The ducts for removing water from the compartments can be provided with control valves 21 to control dewatering pressure. No vacuum is needed in the dewatering boxes 15,16 because the wires are pressed against the web, whereby water drains from the web under pressure through the wires 13,14 into the dewatering boxes 15,16.

Part of the wire water recovered through the dewatering boxes 13,14 is passed through the dilution duct 17 back to the feed pump 10 and, before the feed pump, the thick stock flow M is introduced into this wire water flow Fo. In a manner known per se, excess wire water must be continuously removed from the short circulation through an outlet duct 18 to the long circulation of the paper machine.

The dilution duct 17 and the outlet duct 18 are in direct flow communication with each other, so that substantially the same pressure prevails in them. In order that this pressure should remain even irrespective of any pressure variations possibly occurring in the removal of water, the outlet duct 18 is connected with a tank 19, which is placed at a suitable level above the wire section and provided with an overflow or other control of the liquid level. A wire water flow FOUT is passed from the tank 19 further to the long circulation of the paper machine. In that case, the pressure prevailing in the dilution duct 17 is directly dependent on the location of the tank 19 and on its liquid level which is kept constant. The controllability of

pressure can be additionally improved by arranging a control valve 22 in the outlet duct 18 for throttling the wire water flow in the outlet duct 18.

The arrangement in accordance with the invention makes it possible to keep the pressure on the suction side of the feed pump constant, which is an important factor when the aim is to achieve uniform dilution and uniform feed of the stock to the headbox. The process arrangement in accordance with the invention also makes it possible to reduce the number of devices needed in the short circulation.

At its minimum, only one feed pump 10, a pipe duct for passing the stock flow Fi from the feed pump 10 to the headbox 11, and the dilution and outlet ducts 17 and 18 for circulating wire water and for passing it away from the short circulation are needed, in addition to a closed former, for providing the short circulation. The energy needed for pumping can be reduced in the short circulation because of a higher back pressure and a simpler process. The thick stock M passed to the paper machine can be screened as undiluted before it is conducted to the short circulation. The machine screen 20 intended for this is placed outside the short circulation.

Fig. 2 illustrates an alternative arrangement in which diluted stock is cleaned by means of centrifugal cleaners 23 before the feed pump 10. As part of the pressure transferred by the feed pump 10 to the pulp suspension flow Fi is still left in the wire water flow Fo coming from the dewatering elements 15,16, a second pump is not necessarily needed in the short circulation before the centrifugal cleaners 23.

The examples of Figs. 1 and 2 do not contain any deaeration tank or other open tank in which air could escape from the wire water. In the course of time and in certain conditions there may be a risk that air accumulates in the short circulation, with the result that part of the wire water may have to be circulated through deaeration. The arrangement of Fig. 3 includes a deaeration tank 25, into which part of the wire water coming from the dewatering elements 15,16 is passed along a pipe line 24. The amount of water circulated through deaeration is at

maximum 50 % of the amount of the wire water circulating in the short circulation. From the deaeration tank 25 the wire water is passed through a pump 26 to the headbox 11 for use as dilution water in regulating the cross-direction profile of the headbox.

In this connection, by the paper machine is meant paper, board or soft tissue paper machines and pulp drying machines.