US3792943A

US3792943A - Dry fiber distribution

Info

Publication number: US3792943A
Application number: US00183120A
Authority: US
Inventors: J Helgesson
Original assignee: INGENJORSFA AB
Current assignee: INGENJORSFA AB
Priority date: 1970-10-14
Filing date: 1971-09-23
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19
Also published as: SE343243B; FI49991B; FI49991C; CA957141A; DE2149892A1

Abstract

Fibers are suspended in a rising stream of air which is deflected into a distribution chamber having a bottom formed by a belt-like, gas-permeable screen advanced over a suction box, whereby a dry fiber web is produced on said screen. At the deflection of the fiber-carrying air stream a second stream of air is added to control the inflow of suspended fibers into the distribution chamber and to cause the fibers to follow a trajectory through said chamber. The pattern of fiber deposition on the moving screen is further controlled by admitting additional air from above through an adjustable opening in the enclosure of the distribution chamber.

Description

[ Feb. 19, 1974 3,635,627 1/1972 Palmer................................425/141 2,219,346

[ DRY FIBER DISTRIBUTION 10/1940 Thomas et al. 425/83 X Inventor: Jonas Anders Giite Helgesson, Taby,

Sweden Primary ExaminerRobert L. Spicer, Jr.

Attorney, Agent, or Firm-Fred C. Philpitt [73] Assignee: Ingenjorsfirma B-projekt Aktiebolag, Enebyberg, Sweden Sept. 23, 1971 [57] ABSTRACT Fibers are suspended in a rising stream of air which is 22 Filed:

[21] Appl' deflected into a distribution chamber having a bottom formed by a belt-like, gas-permeable screen advanced over a suction box, whereby a dry fi her web is pro- [30] Foreign Application Priority Data Oct. 14, 1970 Sweden.............................

3 2 70 duced on said screen. At the deflection of the fibercarrying air stream a second st ream of air is added to .l an ma w hO O mf no .I SS rr Ce bb an d [52] US. 425/83, 425/141, 264/91, r l he inflow of suspends 264/109 bution chamber and to cause the F1 jectory through said chamber. Th

position on the moving screen is admitting additional air able opening in the enclosure 0 chamber.

09 00 l 5 7 04 2 B4. ,6 02 0... /l 4 c i 9% d m C mm mfi m 4 e m H Tm a Mr R a "e "S l m cm e .mF HM N 55 5 UNITED STATES PATENTS 3,671,210 6/1972 Richardson....................... 425/83 X 13 Claims, 2 Drawing Figures 1 DRY FIBER DISTRIBUTION This invention relates to the manufacture of more or less felted fiber products from natural or artificial fibers, such as wood fibers, textile fibers, glass and mineral wool fibers, asbestos fibers and the like, and more particularly the invention is concerned with the production of webs of such fibers on a gas-permeable screen, commonly made of wire fabric, while applying a so-called dry process in which flowing gas is used as a medium to sort, transport and distribute the fibers. The dry process referred to comprises two main steps, viz. first suspending the fibers in a vertically rising gas stream and subsequently distributing the suspended fibers over a gas-permeable screen that is advanced at the bottom of a so-called distribution chamber and has its underside exposed to a negative pressure, the rising fiber-carrying gas stream being admitted into the distribution chamber in a substantially horizontal direction.

As compared to the wet processes hitherto most commonly applied to making paper or similar web-type fiber products, the use ofa gas instead of water or other liquid as a suspension medium for the fibers to be distributed over a wire offers many advantages. This is true not only fromm the viewpoint of safe-guarding the environment but in particular in those cases where the fibers themselves contain liqud-soluble matters which would contaminate the water or where the fibrous material for one reason or the other isintermixed with soluble impurities that are difficult to remove. In addition, the wet process commonly requires preparatory drying of the produced web, before the latter can be subjected to the pressing, with or without heating, which is usually required for finishing the product. Also, the wet process is objectionable, when it is desirable e.g. to impregnate the fibers or to cover them with adhesive before the web is being formed, because in such a case large quantities of impregnating agent or adhesive will be lost in the carrier liquid.

Although the dry process for the production of webs of more or less felted fibers as referred to hereinbefore has been known for more than half a century, it has only rarely, if at all, been applied in practice. Instead great efforts have been made to develop rather complicated mechanical means for strewing dry fibers over a moving wire without letting them become sorted and literally transported by a gas stream. So far the practical results of these efforts have been unsatisfactory for several reaons. Among other things, it has been found very difficult'with the strewing method to meet the requirements as to uniformity in thickness of the produced web, and it is almost impossible to obtain that controlled distribution of finerand coarser fibers in the thickness direction of the web, which is frequently important in order to give the product the desired properties.

Alreadywhen the first attempts were once made to substitute a gaseous medium, such as air, for water as a fiber carrier in the distribution of loose fibers over a movable wire, a vertical shaft or duct was used, in which a rising gas stream was-created and caused to carry along fibers that in one way or the other were spread or thrown into the gas stream. Almost from the beginning, it was also suggested that the rising fibercarrying gas stream should be deflected in a substantially horizontal direction and then be admitted into a distribution chamber, where the fibers were permitted to fall down towards an endless wire or screen that was advanced over a suction box at the bottom of the distribution chamber. Unfortunately, great difficulties came up when this basic idea should be put into practice. Firstly, it was found that the fibers were swept away rather irregularly by the gas stream and had a great tendency to get stuck and deposit in the duct connecting the vertical shaft to the distribution chamber. Secondly, it was difficult to obtain a uniform distribution of the fibers on the wire or screen, before all in the transverse direction thereof, and also to adjust the thickness of the fiber web formed on the wire in a sufficiently reliable manner.

Accordingly, it is the object of this invention to offer a simple and practical solution of these specific problems. With these and other objects in view the invention consists, primarily, in a method of distributing fibers suspended in a vertically rising gas stream over a gas-permeable screen being advanced at the bottom of a distribution chamber and having its underside exposed to a negative pressure, the rising fiber-carrying gas stream being admitted into the distribution chamher in a substantially horizontal direction. The invention also consists in an improved apparatus for producing fiber webs in a dry process by carrying out the beforementioned method. Basically, the invention overcomes the inconveniences referred to hereinbefore by suggesting that additional gas streams should be used for controlling the fiber-carrying gas stream as it enters the distribution chamber from the vertical shaft or duct, and also for controlling the pattern according to which the fibers are distributed and deposited over and on the wire or screen within said distribution chamher.

For a more complete elucidation of the invention an embodiment thereof will be described in the following and with reference to the accompanying drawing. In the drawing FIG. 1 shows a slightly diagrammatic sectional elevation of an apparatus embodying the invention and useful for distributing fibers suspended in a gas stream over a movable, endless wire or screen, while FIG. 2 is a fragmentary front view of the apparatus as seen from the line IIII in FIG. 1.

The section in FIG. 1 is taken in the running direction of the wire, and it shold be understood that the wire can, of course, have any desired width, whereas the width of the fiber distributing means forming part of the apparatus can be adapted either to the entire width of the wire or to the width of a selected part thereof, if it is desirable to form a fiber web that is narrower than the wire itself. In practice, this means that the apparatus shown in FIG. 1 usually has a considerably larger extension perpendicularly to the plane of the drawing than in this plane.

Referring to FIG. 1, an endless, flexible and gas permeable screen 1, suitably of close-meshed wire cloth, runs over conducting and supporting rollers 2, 3 and 4 and passes over an upwardly open suction box 5, the interior of which is connected to a suction source, not shown, e.g. in the form of a suction ventilator having a relatively large capacity, so that a negative pressure can be maintained in the suction box under the wire 1. Suitable means, not shown, are provided for driving the wire at a preferably adjustable speed. Above the section of the wire that passes over the suction box there is arranged a distribution chamber, genhorizontal, movable bottom of a generally rectangular shape in the distribution chamber. In FIG. 1 the wire 1 is supposed to move from the left to the right as indicated by an arrow but, as it will appear from the following, it is also possible to reverse the moving direction thereof in order to achieve special productional results. The required seal between the wire 1 and the wall portions of the distribution chamber 6 extending across the wire is obtained by means of rollers 7 which are movable towards and away from the wire and against which lean sealing flaps 8 hinged to the outer sides of the respective distribution chamber walls. One of the rollers 7 will then come to roll in direct engagement with the wire 1, as shown to the left in FIG. 1, whereas the other one will roll on top of the fiber web 9 formed on the wire, as shown to the right in FIG. 1.

The distribution chamber 6 has a vertical crosssection as seen in the running direction of the wire, FIG. 1, that generally takes the form ofa sector ofa circle, the one bounding radius of said sector being represented by the section of the wire 1 extending over the suction box 5, the other bounding radius being represented by a vertical wall member 10 extending across the wire, and the bounding are being represented by a curved cap or hood 11. The latter forms the top and the right hand wall of the distribution chamber 6 and has approximately at the middle of its are, a slot-shaped opening 12 extending in the entire width of the distribution chamber, i.e., perpendicularly to the plane of the drawing. This elongate opening 12 is provided with an upper adjustable damper plate 13 extending along the entire length thereof and also with a horizontal series of lower damper plates 14 mounted adjacent each other in successive order and being individually adjustable in the direction towards and away from the upper damper plate 13 for varying, section by section, the width and, hence, to a certain extent also of the configuration of the opening 12, as can be seen from FIG. 2. By adjusting the damper plate 13, the general size of the opening 12 and to a certain extent also the angular position thereof in the hood 11 can obviously be varied. The opening 12 puts the interior of the distribution chamber 6 in communication either with the external air as shown, or with a duct for closed delivery of gas, not shown.

The upper part or top of the hood 11 is extended towards the left beyond the wall member 10 and is separated from the upper edge of the latter by an opening 15 that in the plane of the wall member 10 is rectangular and has a length corresponding to the width of the distribution chamber 6 across the running direction of the wire 1. The portion or extension 11 of the hood ll projecting outside the wall member 10 forms an upper, substantially flat and horizontal top cover for a shaft or vertical duct 16 of substantially rectangular horizontal cross section, said shaft having also a length corresponding to the width of the distribution chamber 6. The uppermost portion of the wall member 10 is common to both the shaft 16 and the distribution chamber 6, i.e., the wall member 10 forms a partition between the shaft and the distribution chamber. The shaft 16 and the distribution chamber 6 also have a common end walls 17 which in the illustrated form of apparatus extend downwardly on each one side of the wire 1 and, in fact, represent the supporting end walls of the entire apparatus. The vertical shaft 16 is, on the side facing away from the distribution chamberr 6, bounded by a vertical wall member 18, the upper edge of which is spaced from the hood extension 11' by a slot-shaped opening 19 extending horizontally between the end walls 17. The size of this opening is variable by means of a vertically adjustable damper plate 20. lf desired, this single damper plate may be replaced by a series of smaller damper plates resembling the damper plates 14 previously described, and in such a case not only the size but also the configuration of the opening 19 may be varied, which sometimes can be advantageous. It is also possible to make and attach the damper plate 20 in such a manner that the configuration of the opening 19 can be varied by tilting the damper plate in the plane of the wall member 18.

The vertical shaft 16 has its lower end connected to a chamber 21, at the bottom of which there is arranged a conveyor screw 22 for the removal of any such fiber material that is not carried upwards by the gas stream passing through the chamber 21 and the shaft 16. The chamber 21 has a gas inlet opening 23 through which the main flow of air or gas enters the apparatus and the size of which is variable by means of an adjustable damper device 24. Like the opening 19, referred to above, the gas inlet opening 23 can be connected to the external air, as shown, or both openings can communicate with a closed gas delivery duct e.g. from the suction source, if a closed gas circuit is desired.

Outside the wall member 18 is a feed chute 25 into which more or less matted together fiber bundles are supplied at a suitable, preferably adjustable rate such as by means of a belt conveyor indicated at 26. At the lower end of the feed chute 25 there is mounted a rotary disintegrator 27 in the form of a roller having a plurality of radially projecting pins thereon. This roller is rotated in the clockwise direction at a rather high speed for disintegrating the fiber bundles received in the chute 25 and for throwing out the separated, free fibers into the lower part of the shaft 16 and into the chamber 21 from which the shaft proceeds upwards. Usually, a small amount of air also enters the apparatus through the feed chute 25, but this can be avoided, if desired, by delivering the fiber bundles into the chute through an air lock arrangement well known in itself but not shown.

The fiber bundles received in the feed chute 25 are broken up by the rotating disintegrator 27 and the free fibers are caught and carried along upwards by the gas stream passing through the chamber 21 and rising through the shaft 16. Such fiber material that, for one reason or another, is not carried away by the rising gas stream, gathers at the bottom of the chamber 21 and is removed by means of the conveyor screw 22 which, of course, can be replaced by any other suitable feedingout device. The removed residue of the fiber material may, if desired, be returned to the feeding-in conveyor 26. The feed chute 25 and the disintegrator 27 have a length corresponding to the length of the chamber 21 and that of the horizontal cross-section of the shaft 16, whereby a good distribution of the fibers in the entire shaft is achieved. In the chamber 21 an important fiber sorting will take place, whereby heavy fibers as well as unresolved fiber bundles and at least certain heavy im purities become separated, because the rising gas stream is unable to carry them upwards through the shaft 16.

As a result of the fact that air is admitted into the upper end of the shaft 16 immediately below the hood extension 11 through the opening 19, the deflection of the fibre-carrying gas stream through the opening 15 into the distribution chamber 6 is greatly improved. In addition, the fact that the wall member forms only a thin partition between the distribution chamber and the shaft 16 assists in preventing uncontrolable fiberdeposits. The second gas stream admitted through the opening 19 causes the fiber-carrying vertical gas stream to follow a path resembling a trajectory when entering the interior of the distribution chamber and continuing towards the wire 1. By adjusting the size of the opening 19, the thickness of the fiber web 9 formed on the wire 1 can be controlled within rather wide limits. Itt should be noted, however, that also the amount of fiber material supplied to the apparatus per time unit, as well as the running speed of the wire are factors involved in determining the average thickness of the fiber web formed. By varying the configuration of the opening 19, the thickness of the fiber web formed on the wire 1 may be controlled in the transverse direction of the latter to a certain degree. Such transversal thickness control is, however, accomplished with better accuracy by means of the additional gas stream entering the upper part of the distribution chamber 6 through the opening 12. This will result from the fact that the lastmentioned gas stream, although having a main direction from above and downwards in the distribution chamber, will also have a certain component of movement in a direction which more or less crosses the fibercarrying gas stream and thereby disturbs its trajectory.

By sensing the thickness of the fiber web 9 in a plurality of points spaced in the transverse direction of the wire 1 by means of sensing means indicated at 28, which is suitably done slightly before the fiber web is leaving the distribution chamber 6, and by letting these sensing means 28 govern the operation of small actuators 29 adjusting the positions of the lower damper plates 14, possible tendencies of thickness variations in the fiber web can be inhibited, so that, from a practical point of view, a uniform thickness of the fiber web is achieved over the entire width of the wire 1. If, on the other hand, it is desirable to produce a fiber web having a thickness that varies in the transverse direction of the wire 1, a similar control may be used, of course, in order to obtain a desired corrugation of the upper side of the fiber web. In fact, the possibility of varying andd correcting the thickness of the fiber web in the way described is very striking and surprising.

The gas-suspended fibers entering the distribution chamber 6 through the opening will follow a trajectory starting in a substantially horizontal direction. As known per se, this will result in a certain and frequently very desirable separation or sorting of the fibers according to size and weight. More particularly, the coarser and heavier fibers will be thrown farther away from the entrance opening 15, while the finer and lighter fibers will be sucked down the shortest way towards the wire 1, i.e., be deposited on the wire closer to the wall member 10. This effect can, if desired, be utilized for achieving a certain Stratification of differently sized fibers in the fiber web. Thus, if the wire 1 is driven as illustrated in FIG. 1 from the left to the right, the lighter and finer fibers will collect at the bottom of the fiber web, closest to the wire, while the heavier and coarser fibers will be deposited on top of the lighter ones. However, by reversing the running direction of the wire 1 approximately the opposite result may be achieved. On the other hand, the stratification effect may to a certain degree be controlled by more or less disturbing the flow of gas inside the distribution chamber 6 which is also effected by means of the gas streams entering through the opening 12. If a very marked disturbance of this kind is desirable, additional gas-admitting, slot-shaped openings may be provided in the hood 11, preferably above the opening 12. It is also possible to change the position of the opening 12 in the hood in order to vary the distribution pattern of the fibers.

As should be apparent from the foregoing description the invention offers surprising possibilities for the control and variation of the operation of the apparatus, whereby the advantages of the basic operating principle may be fully utilized. The apparatus itself is nevertheless relatively simple from a structural point of view and, before all, it is reliable in operation and capable of producing fiber webs of a high quality and of a great accuracy in thickness at a high speed from most types of fibers which at all lend themselves to transportation by means of gas streams.

I claim:

1. In the known apparatus for producing a web of dry fibers and comprising a suction box structure having a substantially rectangular top opening, a movable beltlike and gas-permeable screen extending over said top opening of the suction box structure, a hood-like cover extending over said top opening of the suction box structure and above the screen to form a fiber distribution chamber in which said screen forms a substantially horizontal bottom, said distribution chamber having a fiber inlet opening vertically spaced from said movable screen, means for moving said screen longitudinally over said top opening of the suction box structure, a vertical shaft communicating at its upper end with said fiber inlet opening of the distribution chamber, means for creating a flow of gas upwardly through said vertical shaft, into said distribution chamber, downwardly through said screen and out through said suction box structure, means for suspending fibers in the gas stream rising through said vertical shaft, the improvement which comprises:

said vertical shaft being generally rectangular in horizontal cross section and extending across the full width of said distribution chamber and being separated from said distribution chamber by a substantially upright partition wall member extending transversely of the running direction of said screen, said partition wall member forming the rear wall of the distribution chamber, said distribution chamber having a section extending in the running direction of said screen that resembles a sector of a circle, of which one bounding radius is represented by said screen, the other bounding radius is represented by said partition wall member and the arc is represented by curved wall member forming part of said hood-like cover, said curved wall member being extended beyond the upper edge of said partition wall member to form a substantially horizontal top cover for said vertical shaft and being vertically spaced from said partition wall member so as to define with the latter and above the same an opening extending in a vertical plane and representing said fiber inlet opening of the distribution chamber, a slot-like gas inlet opening being provided at the upper end of said vertical shaft and horizontally opposite to said fiber inlet opening for the introduction of a second stream of gas in a substantially horizontal direction across the upper portion of said vertical shaft and immediately beneath said horizontal top cover thereof and into said distribution chamber.

2. An apparatus according to claim 1, wherein said slotlike gas inlet opening for the introduction of the second gas stream is variable as to its size.

3. An apparatus according to claim 2, wherein said slotlike gas inlet opening is variable as to its configuration.

4. In the known apparatus for producing a web of dry fibers and comprising a suction box structure having a substantially rectangular top opening, a movable beltlike and gas-permeable screen extending over said top opening of the suction box structure, a hood-like cover extending over said top opening of the suction box structure and above the screen to form a fiber distribution chamber in which said screen forms a substantially horizontal bottom, said distribution chamber having a fiber inlet opening vertically spaced from said movable screen, means for moving said screen longitudinally over said top opening of the box structure, a vertical shaft communicating at its upper end with said fiber inlet opening of the distribution chamber, means for creating a flow of gas upwardly through said vertical shaft, into said distribution chamber, downwardly through said screen and out through said suction box structure, means for suspending fibers in the gas stream rising through said vertical shaft, the improvement which comprises:

said vertical shaft being generally rectangular in horizontal cross section and extending across the full width of said distribution chamber and being separated from said distribution chamber by a substantially upright partition wall member extending transversely of the running direction of said screen, said partition wall member forming the rear wall of the distribution chamber, said distribution chamber having a section extending in the running directionn of said screen that resembles a sector ofa circle, of which one bounding radius is represented by said screen, the other bounding radius is represented by said partition wall member and the arc is represented by a curved wall member forming part of said hood-like cover, said curved wall member being extended beyond the upper edge of said partition wall member to form a substantially horizontal top cover for said vertical shaft and being vertically spaced from said partition wall member so as to define with the latter and above the same an opening extending in a vertical plane and representing said fiber inlet opening of the distribution chamber, an adjustable slot-like gas inlet opening being provided at the upper end of said vertical shaft and horizontally opposite to said fiber inlet opening for the introduction of a second stream of gas in a substantially horizontal direction across the upper portion of said vertical shaft and immediately beneath said horizontal top cover thereof and into said distribution chamber, said arc-forming, curved wall member having at least one inlet opening located approximately midway between the ends of the are formed thereby, said at least one gas inlet opening being provided for the introduction of an additional stream of gas in a mainly downward direction through said distribution chamber towards said movable screen.

5. An apparatus according to claim 4 wherein each of said at least one gas inlet opening of the distribution chamber is variable as to its size.

6. An apparatus according to claim 4 wherein each of said at least one gas inlet opening of the distribution chamber is adjustable as to its position.

7. An apparatus according to claim 4 wherein each of said at least one gas inlet opening of the distribution chamber is variable as to its configuration.

8. An apparatus according to claim 7 wherein individually adjustable damper means are provided adjacent each of said at least one gas inlet opening of the distribution chamber to vary the size and configuration thereof.

9. An apparatus according to claim 8 wherein means for sensing the thickness of the fiber web formed on said movable screen are provided at points distributed transversely of said screen and actuating means are provided for adjusting the setting of said damper means in accordance with the input to said thickness sensing means.

10. An apparatus according to claim 4 wherein the running direction of said movable screen is reversible.

11. An apparatus according to claim 4 wherein said distribution chamber is provided with a roller that is movable towards and away from said screen and adapted to ride on said fiber web, and a hinged flap connected in sealing engagement with a transverse wall member of said distribution chamber rests on said roller.

12. An apparatus according to claim 4 wherein said open box under said movable screen is a suction box and said means for creating a flow of gas is a suction source connected to said box.

13. An apparatus according to claim 4 wherein said means for suspending fibers in the gas stream rising through said shaft comprises a rotary disintegrator for breaking up collections of tangled fibers and for throwing the freed fiber into said vertical shaft and wherein the lower end of said vertical shaft is connected to a chamber, at the bottom of which conveyor means are provided for removing residues of fiber material which the rising stream of gas in said shaft is unable to carry upwardly into said distribution chamber.

Claims

1. In the known apparatus for producing a web of dry fibers and comprising a suction box structure having a substantially rectangular top opening, a movable belt-like and gas-permeable screen extending over said top opening of the suction box structure, a hood-like cover extending over said top opening of the suction box structure and above the screen to form a fiber distribution chamber in which said screen forms a substantially horizontal bottom, said distribution chamber having a fiber inlet opening vertically spaced from said movable screen, means for moving said screen longitudinally over said top opening of the suction box structure, a vertical shaft communicating at its upper end with said fiber inlet opening of the distribution chamber, means for creating a flow of gas upwardly through said vertical shaft, into said distribution chamber, downwardly through said screen and out through said suction box structure, means for suspending fibers in the gas stream rising through said vertical shaft, the improvement which comprises: said vertical shaft being generally rectangular in horizontal cross section and extending across the full width of said distribution chamber and being separated from said distribution chamber by a substantially upright partition wall member extending transversely of the running direction of said screen, said partition wall member forming the rear wall of the distribution chamber, said distribution chamber having a section extending in the running direction of said screen that resembles a sector of a circle, of which one bounding radius is represented by said screen, the other bounding radius is represented by said partition wall member and the arc is represented by a curved wall member forming part of said hoodlike cover, said curved wall member being extended beyond the upper edge of said partition wall member to form a substantially horizontal top cover for said vertical shaft and being vertically spaced from said partition wall member so as to define with the latter and above the same an opening extending in a vertical plane and representing said fiber inlet opening of the distribution chamber, a slot-like gas inlet opening being provided at the upper end of said vertical shaft and horizontally opposite to said fiber inlet opening for the introduction of a second stream of gas in a substantially horizontal direction across the upper portion of said vertical shaft and immediately beneath said horizontal top cover thereof and into said distribution chamber.

4. In the known apparatus for producing a web of dry fibers and comprising a suction box structure having a substantially rectangular top opening, a movable belt-like and gas-permeable screen extending over said top opening of the suction box structure, a hood-like cover extending over said top opening of the suction box structure and above the screen to form a fiber distribution chamber in which said screen forms a substantially horizontal bottom, said distribution chamber having a fiber inlet opening vertically spaced from said movable screen, means for moving said screen longitudinally over said top opening of the box structure, a vertical shaft communicating at its upper end with said fiber inlet opening of the distribution chamber, means for creating a flow of gas upwardly through said vertical shaft, into said distribution chamber, downwardly through said screen and out through said suction box structure, means for suspending fibers in the gas stream rising through said vertical shaft, the improvement which comprises: said vertical shaft being generally rectAngular in horizontal cross section and extending across the full width of said distribution chamber and being separated from said distribution chamber by a substantially upright partition wall member extending transversely of the running direction of said screen, said partition wall member forming the rear wall of the distribution chamber, said distribution chamber having a section extending in the running direction of said screen that resembles a sector of a circle, of which one bounding radius is represented by said screen, the other bounding radius is represented by said partition wall member and the arc is represented by a curved wall member forming part of said hood-like cover, said curved wall member being extended beyond the upper edge of said partition wall member to form a substantially horizontal top cover for said vertical shaft and being vertically spaced from said partition wall member so as to define with the latter and above the same an opening extending in a vertical plane and representing said fiber inlet opening of the distribution chamber, an adjustable slot-like gas inlet opening being provided at the upper end of said vertical shaft and horizontally opposite to said fiber inlet opening for the introduction of a second stream of gas in a substantially horizontal direction across the upper portion of said vertical shaft and immediately beneath said horizontal top cover thereof and into said distribution chamber, said arc-forming, curved wall member having at least one inlet opening located approximately midway between the ends of the arc formed thereby, said at least one gas inlet opening being provided for the introduction of an additional stream of gas in a mainly downward direction through said distribution chamber towards said movable screen.