The wire pit of the paper machine functions as a coarse air separator before pumping operations. The water drained from the wire section is passed as gently as possible to the upper part of the wire pit so that air is not able to mix with the wire water and form foam and, when needed, additional dilution water is added to the wire pit at the same time. In the pit-like construction, the wire water is taken to circulation from below, with the result that the flow downwards is slow. The aim would be to enable possible air bubbles to rise to the surface, but they must struggle against the flow. The conventional wire pit may, in fact, mainly only dissolve the air in the bubbles. The surface level of the wire pit is kept constant by means of an overflow. Thus, variations in the total volume entering the wire pit cannot have any effect on the operation of the fan pump nor on the individual partial flows conducted to the wire pit because the back pressure remains constant.

In the device construction of the invention, which replaces the conventional wire pit, the general principle has been to avoid any dead areas and to use a sufficient, yet not excessively high flow velocity. For the purpose of removing air, the area of the wire pit must be so large that the velocity of the downward flow in the wire pit is lower than the speed at which the air bubbles rise in the wire water, i. e. a large area enhances removal of possible air from the liquid.

A wire pit that is correctly dimensioned with respect to the production of the paper machine attenuates consistency variations, remains clean, removes air efficiently, and keeps the surface of the wire water constant, whereby a constant pressure is provided on the suction side of the fan pump. A wire pit that is too

large or has too slow a flow rate adds to the problems with keeping it clean and increases the volume of the entire short circulation, with the result that reacting to process changes, such as a grade change, is slow.

The pit-like wire pit arrangement used today on printing paper and board machines has proved, in its geometry, to be easily contaminated and the use of a rather large volume brings with it the above-noted"slowness"problems in response times. Moreover, air removal in the pit-like wire pit is not accomplished in an ideal manner.

In the prior art, wire pit constructions are also known which are placed directly under the fourdrinier wire section. In these constructions, the flow is passed from the fourdrinier wire section through a pipe system to a wire pit part which comprises horizontal wire water flows for the wire water.

In the wire pit arrangement in accordance with the invention, the flow enters the wire pit from its side and flows in a horizontal direction and exits to pumps along at least one, preferably two downwardly extending pipes. An overflow is located at the end of the duct. A duct-like wire pit arrangement has been in use on tissue machines. In that connection, however, the volume of the duct has been equal to that of conventional wire pits.

In the new wire pit arrangement, wire water is collected making use of a small volume. By this means, the response delays caused by a large liquid volume are avoided, and thus this can be called"a fast wire pit arrangement". The water is collected after a planar part to pumps, while pressure loss evens out the flow downwards in accordance with the invention. The volume required for collecting water is substantially smaller than that of prior art techniques.

With respect to the wire pit duct arrangement of tissue machines, the length and the depth of the duct have been reduced, but its width has been increased in order

to obtain a liquid-surface area that is sufficiently large with respect to the removal of air. Because of the greater width it is important that the water to be passed out is collected as evenly as possible across the entire width of the duct.

The wire pit arrangement in accordance with the invention comprises an outlet duct, to which the flow is collected evenly across the entire width of the duct. The delays on the different sides of the duct are thus evened out and the result of air removal is more uniform. The shape of the outlet duct also serves to prevent formation of an air-sucking vortex caused by the Coriolis force.

By means of the large free liquid-surface of the wire pit in accordance with the invention-which wire pit is also called a flume in this patent application-the mass transfer surface in the construction will be maximized, which enhances removal of air from the liquid.

In the printing paper and board processes, the consistency of wire water is higher than in the tissue process. For this reason, attention must be paid to maintenance of cleanliness.

No problem with maintenance of cleanliness arises in connection with the improved flume since attempts are made to avoid dead areas (flow velocity is zero) in the flow field, as seen from the illustration of Fig. 1 representing the state of the art. In the new wire pit in accordance with the invention, the geometry of the horizontal flow is arranged such that the flow velocities at the bottom of the planar part of the flume are clearly higher than those of the surface because of the shape of the feed part used, wherefore fillers etc. will not cause any contamination problem at the bottom. The bottom flow velocity can also be raised by placing a flow obstruction, for example, an outlet tube, close to the surface.

The wire pit in accordance with the invention is suitable for use in paper machines or equivalent, such as, for example, in board machines.

The wire pit construction in accordance with the invention in a paper machine, board machine or equivalent is characterized by what is stated in the claims.

In the following, the invention will be described with reference to some preferred embodiments of the invention illustrated in the figures of the appended drawings, but the invention is not meant to be exclusively limited to them.

Figure 1 shows a prior art wire pit.

Figure 2A is an axonometric view of a wire pit in accordance with the invention.

Figure 2B shows a section taken along the line I-I in Fig. 2A.

Figure 2C shows a section taken along the line II-II in Fig. 2B.

Figure 2D shows, as an illustration of principle, another embodiment of the invention (corresponding to the area Y, in Fig. 2A) associated with the discharge of wire water from the wire pit shown in Fig. 2A.

Figure 2E shows a section taken along the line III-III in Fig. 2A.

Figure 3 shows one arrangement for supply of wire water into the wire pit.

Figure 4A shows another embodiment which corresponds to the section II-II and in which wire water is passed away from the side of the wire pit.

Figure 4B shows a section taken along the line IV-IV in Fig. 4A.

Figure 4C shows a section taken along the line V-V in Fig. 4A.

Figure 5 shows an embodiment of the device arrangement in accordance with the invention in which an elongated slot ends in two outlet ducts.

Fig. 1 shows a prior art wire pit, and the problems associated with the prior art are described on the basis of this figure. Fig. 1 is a cross-sectional view of a flow pit in accordance with the prior art. The figure illustrates with flow arrows the mutually different progress of different flows in the wire pit. As shown in the figure, dead areas are easily created in the flow pit, and air in said dead areas tends to mix detrimentally further with and dissolve in the wire water. Thus, the desired removal of air from the wire water cannot take place efficiently. In addition, single dead areas whose flow velocity is zero, are easily created in the prior art wire pit. Vortices and backflow areas causing air content variations are also readily formed in the wire pit. This is, of course, not efficient from the viewpoint of operation of the wire pit.

Fig. 2A shows a first preferred embodiment of a wire pit 10 in accordance with the invention. The wire pit 10 comprises a housing 11 that constitutes the wire pit and which in the section III-III is a rectangular section whose corners are, however, rounded. A flow duct D is formed in the space defined by side walls 11 a, 11 a2 and a bottom 11 a3 and end walls 11al4, 11a5 and a top wall 11 a6 of the elongated housing frame of the wire pit. The above-mentioned structure provides a large area confined to air for the duct D. Thus, air is removed efficiently from the wire water when the wire water is introduced through an inlet duct 12 into an inlet end El of the wire pit 10 and when the wire water is discharged from an outlet end E2 of the wire pit. Between the inlet end El and the outlet end E2, the wire water flows in a horizontal direction. At the outlet end E2 there is a first elongated slot-shaped outlet connection lib) and a second elongated slot-shaped outlet connection llb2. The outlet connections llb, and llb2 become narrower towards outlet ducts or outlet pipes 13a, and 13a2 located in the middle plane of the wire construction. The flow distance from sides T, and T2 of the slot-shaped outletconnection llb, and llb2tothecentraloutletductoroutletpipe 13al, 13a2

is substantially longer than it is from a middle area T3 of the flow slot 1 lbl, 1 lb2.

In order that the flow should thus remain even and unchanged in velocity into the outlet ducts 13a"13a2, the flow is throttled in the middle area T3, for example, by forming the flow duct narrower in said portion than at the sides T, and T2. It is also possible to use a separate flow resistance means KI, such as a perforated plate. In the figure, the wire water flow in the elongated wire duct D is shown by the arrow Li and the flows to the outlet duct 13al are shown by the arrow L2 and those to the outlet duct 13a2 by the arrow L3.

In Fig. 2A, the direction of the field of gravity is denoted with the arrow g. The inlet duct 12 comprises flow guides Jl, J2, J3..., which are plate-like curved parts, by which the wire water flow is guided smoothly into the space defined by the walls 11 al, 11 a2, 11 a3. The flow is guided towards its end from a narrowing duct I, i. e. an inlet header, to the vicinity of the bottom wall lla3 and, preferably, below the surface of the wire water in the flow duct D.

The wire water is passed to so-called headbox dilution through the elongated slot- shaped outlet connection llb, situated first with respect to the flow L, and extending across the bottom 1 la3 of the wire pit 10. The wire water with which fresh pulp is mixed, is passed through the elongated slot-shaped outlet connection 1 lb2 situated second with respect to the flow L and extending across the bottom 11 a3 of the wire pit 10 further to a fan pump and to the short circulation of the headbox of the paper machine or equivalent. In Fig. 2A, an overflow located after the flow slot llb2 is designated by the reference numeral 14. In Fig. 2A, a flow obstruction for directing the flow to the bottom of the duct D is designated by the reference numeral 15. The flow obstruction 15 in the embodiment of the figure is a tubular structure which extends transversely in the duct D perpendicularly to the flow Ll in the duct D. The flow velocity in the surface is retarded and the flow velocity at the bottom is increased. The bottom of the wire pit is kept clean in this manner.

Fig. 2B shows the section I-I of Fig. 2A and Fig. 2C shows the section II-II of Fig.

2B. In the construction shown in Figs. 2B and 2C, dilution water, for example, for dilution of the stock in the headbox, for so-called headbox dilution, can be taken into dilution tubes 16al, 16a2 from the area of the throttling Kl, i. e. from the middle area of the elongated slot 1 lb2. In the middle area of the elongated slot llbz, the slot llbi and llb2, respectively, are narrower than at the edges, which edge areas comprise a tubular expansion having a circular cross-section. In the other parts, the flow duct is slot-shaped. The elongated slots lib) and llb2 become narrower towards the outlet pipe 1 3al and 13 a2. In the embodiment of the figure, the outlet pipe 13a, and 13a2 is located centrally with respect to the wire pit construction. In accordance with the invention, the throttling K, is selected to be greater in the middle area of the construction than at the edges of the construction from which the distance is longer to the outlet pipe 13a, and 13a2.

Fig. 2D illustrates the narrowing of the flow slot 1 lbo to the outlet pipe 13al. The outlet pipe 13a, is located in the middle area of the flow slot with respect to its length. Thus, as illustrated in the figure, the distance dl from the middle area of the elongated slot 1 lbl to the outlet pipe 13au is shorter than the distance from the edge area of the elongated slot 1 lbl, wherein the distance has the sign d2. In order that the flow shall enter the outlet pipe 13a, at the same velocity from all parts of the slot, the flow is throttled more in the middle area than in the edge area of the flow slot 1 lbl. This is accomplished such that the throttling Kl is made greater in the middle area of the flow slot than in the end area thereof. This is accomplished such that the width of the flow slot lib) in its middle area AS, is made smaller than the width AS2 of the flow slot at the edges of the flow slot, i. e. AS, < AS2.

Fig. 2E shows the section Ill-Ill of Fig. 2A. As shown in Fig. 2E, the wire pit 10 has a rectangular cross-section, however, so that the corners of said rectangular cross-section are rounded. By this structure the wire pit is prevented from being contaminated and the flow is kept even over the entire length of the wire pit.

Fig. 3 shows an embodiment of the invention in which the inlet duct 12 to the wire pit 10 is formed of two parallel duct portions 12au, 12a2 which become narrower in the direction of the longitudinal axis (Xi axis) of the housing structure and which become narrower such that a flow Lo is directed from both inlet ducts 12al, 12a2 to the bottom 1 la3 of the wire pit 10. Said arrangement serves to keep the flow velocity at the bottom of the duct D of the wire pit 10 higher than that of the liquid in the surface of the duct D. By this means the bottom of the duct D remains clean and additives/fillers/dirt is/are not precipitated and accumulated on said bottom lia3.

Fig. 4A shows an embodiment of the invention in which the flow slot lib) narrows towards the outlet duct 13a,, which outlet duct 13al is located at the edge of the wire pit. Thus, the flow space inside the flow slot becomes triangular. In that case, the flow is throttled more in that part of the flow slot 1 lbl in which the distance to the outlet pipe 1 lbl is shorter than in that part of the flow slot in which the distance to the outlet pipe 1 lbl is longer.

Fig. 4B shows the section IV-IV of Fig. 4A. As shown in Fig. 4B, the width AS3 of the flow slot is greater than that of the section V-V shown in Fig. 4C. By this means, the velocities of flow into the flow pipe 13a, will be the same from the entire area of the slot l lob,. The flow can be collected evenly into the flow pipe 1 3al over the entire length of the flow slot lib I b 1.

Fig. 5 shows an embodiment of the invention in which there are two parallel outlet pipes 13a,', 13as", which open into the same slot-shaped outlet connection 1 lob). In those parts of the outlet connection 1 b I in which the flow distance to the outlet pipe 13a,'or 13al"is shorter, the flow is throttled more than on both sides of said location. The maximum throttling is denoted with Kl in Fig. 5. The structure is symmetrical with respect to the central axis XI. The areas of smaller throttling are denoted with K2. In this embodiment shown in the figure, the desired throttling Kl and K2 is also achieved by making the slot-shaped outlet

connection lib) narrower at the throttling K, than on the sides of it at the throttling K2. In the figure there are two outlet pipes 13a,', 13apside by side.

There may also be several outlet pipes 13a', 13a", 13az"side by side.