The procedure and apparatus of the invention, which are here- inafter referred to by the common designation solution of the invention', are applicable for use for dry formation of a fibre layer in web material, e. g. in conjunction with paper manufacture. In this type of processes, a fibrous raw mate- rial, such as wood pulp, is generally defibrillated before the formation of a fibre layer e. g. in a hammer mill to pro- duce separate fibres, which are passed with a conveying air flow via various devices associated with the process into a distributor unit called a former. The production line may contain several formers like this placed one after the other e. g. to produce several fibre layers. A typical former is e. g. a drum former, which may have one or more cylindrical drums or screen tubes side by side, with apertures or through holes in their cylindrical surface for spreading the fibre flow onto a formation surface on a moving forming wire below the former.

Placed inside the former drum, close to the inner surface of the drum, is generally a consecutive series of bladed wheels or a single elongated bladed wheel, which breaks up any lumps in the fibre flow and distributes the fibre flow as evenly as possible to the through holes in the drum. In a dual-drum former, the fibre flow is conveyed by a conveying air flow blown into the drums to produce a fibre layer as even as possible e. g. from the ends of the drums so that the fibre flow enters the drums from their opposite ends.

Below the former is a moving wire called forming wire, whose upper surface acts as a fibre layer forming surface. Below the wire, aligned with the former is a suction box, which

produces a suction that draws fibres from the former to the forming surface.

The function of the former is to distribute the fibres as a layer as even as possible on the forming surface of the wire in order that that the product to be manufactured should be as homogeneous as possible. However, a problem with prior-art dry formation solutions is that the fibre flow cannot be distributed so as to form a sufficiently even layer over the entire width of the forming surface.

To solve the above-mentioned problem, improved versions of the above solutions have been developed. Finnish patent no.

FI58804 discloses a solution, which includes an arrangement for circulation of the fibre flow conveying air. In the basic solution according to this patent, the fibre flow is spread onto the forming surface by means of a downwards widening discharge conduit. Similarly, figures 5-7 present a second embodiment in which the supply conduit is divided into four separate discharge conduits. As the device has no flow ad- justment in the supply or discharge conduit, the device re- quires the use of complex mixers to mix the fibre mass to produce a layer as even as possible on the forming surface of the wire. The drawback is a complicated and failure-prone construction that requires frequent maintenance.

Finnish patent no. FI73270 and corresponding US patent no.

US4761858 also disclose an improved structure for producing from the fibre flow a layer as even as possible on the form- ing wire. The apparatus is provided with air circulation and an intermediate chamber placed above the drum former and having in its upper and lower parts mutually perpendicular fins for adjustment of the passage of the fibre flow. The adjustment enables a more even formation of the fibre layer, but the problem here is that the adjustment can not be made during operation. Therefore it is not possible to achieve an ideal adjustment and the time required for the adjustment

reduces the operating time of the apparatus. Moreover, the adjustment made from the fins is a complicated and difficult operation. An additional disadvantage is that the lattices formed by the fins are tight and are easily blocked, necessi- tating an interruption of production and cleaning of the lattices.

The object of the present invention is to overcome the above- mentioned drawbacks and to achieve an effective and reliable former structure that produces a fibre layer as even as pos- sible and makes it possible to attain a good final result and capacity in conjunction with the manufacture of paper or corresponding material. The procedure of the invention is characterized by what is disclosed in the characterization part of claim 1. Correspondingly, the apparatus of the inven- tion is characterized by what is disclosed in the characteri- zation part of claim 5. Other embodiments of the invention are characterized by what is disclosed in the other claims.

The solution of the invention has the advantage that the invention enables good, fast and flexible adjustments of fibre flow during operation, thus allowing an even fibre layer to be formed on the forming surface. In addition, the adjustment is effective and operation-time adjustment enables a high capacity as the apparatus need not be stopped to allow adjustments. Moreover, the construction of the apparatus is simple and reliable.

In the following, the invention will be described in detail with reference to an embodiment example and the attached drawings, wherein Fig. 1 presents a diagrammatic and simplified view of a part of a production line, showing one drum for- mer partially sectioned and in side view, and

Fig. 2 presents a simplified cross-sectional front view of the part of a production line presented in Fig. 1.

Fig. 1 presents a side view of an apparatus 1 implementing the concept of the invention, which in this case comprises a drum former 2 as a distributor unit having two parallel screen drums 3 transverse to the direction of motion of the web to be formed, said drums 3 being placed substantially above the forming surface of a moving forming wire 7. Inside each screen drum 3 is an elongated bladed wheel 4 which is mounted in the axial direction of the screen drum and which breaks up any lumps in the fibre flow and distributes the fibre flow as evenly as possible to through holes provided in the drum shell. The fibre flow is supplied into the screen drums 3 along with a conveying air flow via inlet openings 6 and 6 at opposite ends of the screen drums 3, and in the solution according to Fig. 1 inlet opening 5 is in the front end of a first screen drum and inlet opening 6 is in the back end of a second screen drum.

Placed oppositely to the screen drums 3 below the forming wire 7 is a suction box 8, through which the fibre flow con- veying air is drawn by suction through the forming surface of the forming wire 7 to form the fibre layer of the web. Corre- spondingly, placed above the former part is a set of channels 9 transverse to the direction of motion of the forming wire 7, via which channels the air circulated in the system is passed again to the former 2.

In Fig. 2, the apparatus of the invention is presented in front view, i. e. as seen against the direction of motion of the web to be formed. For the sake of clarity, the apparatus is shown in a diagrammatic and simplified form. Features essential to the solution of the invention are the channel system 9 arranged above the former 2 in the circulation of conveying air and the channels provided in the suction box 8.

The channels 9 above the former 2 are divided by partitions 16 into flow channels 17, a suitable number of which is two or more, preferably e. g. four. The flow channels 17 start with equal cross-sectional areas from a regulating element 18 placed in the circulation air channel 15 and extend with a curvature in the transverse direction of the forming wire 7 while at the same time expanding towards the upper part of the former 2 above the screen drums 3 so that, as they join the upper part of former, each flow channel 17 has an equal cross-sectional area and each flow channel 17 is equally wide in the transverse direction of the forming wire 7 and their total width covers substantially the entire transverse width of the forming wire.

The suction box 8 is correspondingly divided by partitions 10 into channels 11, the number of which is substantially the same as the number of channels 17 in channel system 9. The cross-sectional area of channels 11 and their width in the transverse direction of the forming wire at the upper surface of the suction box correspond to the corresponding dimensions of channels 17 at the upper edge of the drum part of the former 2. Channels 11 are bent to guide the circulating con- veying air to the side of the suction box 8 and the channels become smoothly narrower so that the channels 11 have sub- stantially the same cross-section in their outlet openings in the lateral surface of the suction box 8. In addition, the outlet openings are provided with a regulating element 20 corresponding to regulating element 18, including regulating plates 21. By turning these regulating plates 21, the free cross-sectional area of each one of the channels 11 can be separately adjusted independently of the other channels ei- ther during operation or when operation has been stopped.

Attached to the lateral surface of the suction box 8 is a exhaust duct 12 for discharge of circulated conveying air, the mouth of which duct covers the outlet openings of all channels 11. The exhaust duct 12 leads to a fan 13, on the

delivery side of which is further a circulation air channel 15 with an exhaust air duct 14 branching out from it. The circulation air channel 15 coming from the fan 13 continues in a curved form from the side of the former 2 towards the channel system 9 above the former and joins the channel sys- tem 9 substantially in the region of the regulating element 18, completely covering the beginnings, i. e. mouths of the channels 17. One of the reasons why an exhaust air duct 14 is necessary is that more air enters into the fibre drying cir- culation through the inlet openings 5 and 6 in the ends of the screen drums 3.

The regulating element 18 has e. g. regulating plates 19 at the beginning of each channel 17, and the free cross- sectional area of the mouth of each channel 17 can be ad- justed separately and independently of the other channels by turning these regulating plates. Via adjustment of the mouth, the flow of circulation air and fibres carried in it to the former can be regulated during operation, i. e. during forma- tion of the web, and thus the regulation makes it possible to achieve a fibre layer thickness as uniform as possible on the forming surface of the forming wire 7.

According to the procedure of the invention, dry formation of the fibre layer takes place e. g. as follows. Let the starting point be the supply of fibre flow via the inlet openings 5 and 6, the fibre flow being fed into the screen drums 3 of the former 2 along with the conveying air. From the screen drums 3, the fibre flow is passed via the through holes in the screen drums onto the forming surface of the moving form- ing wire 7 to the upper part of the suction box 8. By means of the fan 13, an air circulation is generated in the suction box 8 to suck, i. e. to draw the conveying air flowing through the forming wire 7 through the suction box via channels 11 and regulating element 20, and this air circulation is passed further via the fan 13 mainly into the circulation air chan- nel 15 and into the exhaust air duct 14 as far as necessary.

From the circulation air channel 15, the conveying air con- taining fibres and to be circulated is passed through the regulating element 18 into the channels 17 in the channel system 9 provided in the upper part of the former 2 in such manner that, before being passed into the channels 17, the circulation air flow is adjusted if necessary to a desired level by means of the regulating device 18 separately for each channel. The channel-specific adjustment is preferably made while the apparatus is running, so that the effect of the adjustment on the fibre layer is seen at once and the adjustment can be corrected if necessary. The adjustment is effected at or near the mouths of the channels 17 by decreas- ing and increasing the cross-sectional flow area of the mouths of the channels 17 by means of the regulating plates 19. The adjustment can also be made while the plant is stand- ing still. Thus, it is possible to make a preliminary adjust- ment e. g. after maintenance or other interruption of opera- tion before the apparatus is started again and a final and more accurate adjustment is then made during operation.

It is obvious to the person skilled in the art that the in- vention is not limited to the embodiment example described above but that it can be varied within the scope of the claims presented below. Thus, the former may be of some other type than a dual-drum drum former as described above. The essential feature is circulation of conveying air and opera- tion-time adjustment of the circulation flow.

It is also obvious to the person skilled in the art that the construction of the apparatus may differ in other ways too from the above description. The placement of the channel systems and the numbers of channels may vary. For example, a solution implemented with two, three or more channels may give an equally good final result as the above-described embodiment implemented with four channels. Thus, the number of channels may quite well be even higher than four. The appropriate number depends, among other things, on the width

of the web to be formed. It is likewise obvious that the numbers of channels in the channel system 9 and in the suc- tion box 8 need not necessarily be the same. Thus, for exam- ple, the suction box 8 may also contain only two or three channels while the channel system 9 contains e. g. four chan- nels. The suction box may also be implemented completely without said channels.

In addition it is obvious to the person skilled in the art that the circulation may be so controlled that the flow takes place along some other route than directly from the suction box to the upper part of the same former. In this case the circulation may be passed to the upper part of a second for- mer in the apparatus or before the former e. g. to a fibre separator or to some other desired apparatus.