EP0118369A1 - Methods of forming fibre felts - Google Patents

Abstract

The invention relates to improvements in the techniques for forming fiber felts.

According to the invention, for the formation of felts by means of a gaseous current carrying the fibers, said current passing through an oscillating guide duct (8) to distribute the fibers uniformly on a receiving conveyor (3), the guide (8) is animated by a movement the characteristics of which: frequency, shape, amplitude, direction, or at least some of these characteristics are adjusted as a function of fiber mass measurements per unit area.

The invention makes it possible to significantly improve the distribution of fibers.

Description

The invention relates to improvements made to the techniques for forming felts, and in particular thick felts such as those intended for thermal and acoustic insulation.

Traditionally, the formation of felts from fibers carried by a gas stream is carried out by passing this gas stream through a perforated receiving conveyor which retains the fibers. To fix the fibers together, a binder is sprayed on the fibers during their trajectory towards the receiving conveyor. This binder is then hardened, for example by a heat treatment.

This technique is used in particular for the production of mineral fiber felts. Due to the importance of this type of production, we will refer in the following to the formation of fiber felts of glass materials. Nevertheless, the improvements according to the invention are applicable to all the processes for preparing felts, whether the fibers are mineral or organic.

One of the difficulties encountered in the preparation of these felts is linked to the uniform distribution of the fibers throughout the felt. The gas stream carrying the fibers ordinarily has a section of a limited magnitude which is a function, in particular, of the device for producing the fibers. Also the gas stream usually does not cover the entire width of the conveyor and the fibers do not distribute evenly.

Various means have been proposed to improve the distribution of the fibers on the conveyor. Among these means, one of the most useful in practice is of the type described in US Pat. No. 3,134,145. This means consists in passing the gas flow carrying the fibers through a guide duct. This duct is mobile and driven by an oscillating movement which alternately directs the gas flow from one edge to the other of the fiber receiving conveyor.

By this means, if the conditions of use are suitably chosen, the fibers are deposited over the entire width of the conveyor.

From experience, however, it appears that a strictly uniform distribution is very difficult to obtain. Deviations in the mass of fibers per unit area of 15% or more from the average value are not uncommon on samples taken at different points of the width of the felt - Reasons for the existence of such irregularities are indicated in following the description - It is therefore important to improve the implementation of this distribution technique in order to reduce as much as possible the variations which are observed in the distribution of the fibers.

The invention aims to provide an improved technique for the distribution of fibers in the felts formed.

The object of the invention in particular is to enable the correction of variations in distribution which appear during operation.

Another object of the invention is to ensure that the correction of the variations in the distribution of the fibers is carried out automatically.

These aims are achieved thanks to the invention. According to this, the parameters determining the oscillating movement of the guide duct are variable during operation. Permanent measurements of the distribution of the fibers in the felt formed also make it possible, according to predetermined corrections as a function of the deviations observed with respect to the desired distribution, to restore the conditions of the best possible distribution at all times.

The invention also proposes a set of means making it possible to implement the regulation of the distribution according to the method indicated above.

• - la figure 1 est une vue schématique d'une installation de formation de feutres de fibres, vue transversale par rapport au sens de progression du convoyeur de réception,
• - la figure 2 est une vue partielle agrandie de la figure 1 montrant de façon plus précise la constitution du dispositif de distribution des fibres,
• - la figure 3 est un schéma montrant un ensemble de mesure de la masse de fibre par unité de surface,
• - la figure 4 est un schéma synoptique du mode de régulation du système de distribution des fibres,
• - les figures 5a, 5b, 5c et 5d illustrent de façon schématique quatre configurations types de distribution des fibres dans le travers du feutre,
• - la figure 6 montre un mode de combinaison des mesures réalisées pour mettre en évidence les caractéristiques fondamentales de la répartition mesurée,
• - la figure 7 est un exemple de l'évolution de la répartition des fibres lors de la mise en oeuvre de la régulation selon l'invention,
• - la figure 8 est un autre exemple, analogue à celui de la figure 7.

The invention is described in detail below with reference to the accompanying drawing plates:

• FIG. 1 is a schematic view of an installation for forming fiber felts, a transverse view relative to the direction of progression of the receiving conveyor,
• FIG. 2 is an enlarged partial view of FIG. 1 showing more precisely the constitution of the fiber distribution device,
• - Figure 3 is a diagram showing an assembly for measuring the mass of fiber per unit area,
• FIG. 4 is a block diagram of the mode of regulation of the fiber distribution system,
• FIGS. 5a, 5b, 5c and 5d schematically illustrate four typical configurations for the distribution of fibers across the felt,
• FIG. 6 shows a method of combining the measurements made to highlight the fundamental characteristics of the measured distribution,
• FIG. 7 is an example of the evolution of the distribution of the fibers during the implementation of the regulation according to the invention,
• FIG. 8 is another example, similar to that of FIG. 7.

The felt-forming installation of FIG. 1 comprises a device for forming fibers, a receiving assembly and distribution means.

In this figure, the forming device is of the type in which the material to be fiberized is projected in the form of fine filaments out of a centrifuge having a multitude of orifices. The filaments are still entrained and stretched by a gas stream directed vertically from top to bottom. Ordinarily, the gas stream is at high temperature, which allows the filaments to be kept in the conditions suitable for drawing.

The fibers entrained by the gas stream form a kind of veil 2 around and above the centrifuge 1.

This mode of fiber formation has been the subject of numerous publications. For a detailed description of the conditions of implementation and of the device, reference may be made in particular to French patent n ° 78 34616.

Of course, the invention is not limited to a particular mode of fiber formation. On the contrary, it encompasses all the techniques in which a fiber felt is made from fibers conveyed by a gas stream. The example of fiber formation by this centrifugation technique was chosen because it is of great industrial importance.

In this type of formation, the veil of fibers tightens under the centrifuge for reasons which relate to the geometry of the fiberizing device. Then, in contact with the ambient atmosphere, the gas stream carrying the fibers flourishes.

The development of the gas stream, we can note it, is a completely general phenomenon which is independent of the shape of the current at the origin and therefore of the mode of formation of fibers used.

The gas stream carrying the fibers is directed into an enclosure 4, the base of which is constituted by a conveyor 3. This enclosure is closed laterally so that the gaseous stream cannot be removed other than by passing through the perforated conveyor 3.

Laterally, walls 5 channel the gas flow. As shown in FIG. 1, these can be mobile walls. These walls have the advantage of being able to be continuously rid of fibers which could become undesirably attached to them, and this all the more easily if the fibers have received a binder composition by spraying on their path towards the conveyor. The spray assembly is not shown.

The observation of the gaseous current carrying the fibers shows that its development is relatively slow. In the case considered, the gas stream takes a conical shape whose opening angle A is of the order of about twenty degrees. The prepared felts very often have a width of more than two meters, and the current at the origin being relatively narrow, it is conceivable that it is not possible to obtain a flux large enough to cover the entire surface of the conveyor. This is shown in Figure 1.

Under the conveyor belt 3 the gases pass into the box 6 maintained in depression relative to the enclosure 4, by suction means not shown.

The box 6 is arranged so that the suction takes place over the entire width of the conveyor 3. This avoids the formation of undesirable turbulence in the enclosure 4. To a certain extent, the uniform suction also promotes a regular distribution of the fibers, the areas of the conveyor already loaded with fibers having a higher resistance to the passage of gases which prevents the accumulation of additional fibers.

However, the balance which tends to be established on the conveyor by the presence of the fibers themselves is insufficient to obtain a suitable distribution on a conveyor whose width is much greater than that of the gas stream. The accumulation of fibers is greater in the center of the conveyor, that is to say on the direct path of the gas stream.

To improve the distribution of the fibers, an oscillating guide duct 8 is arranged on the path of the gas stream. The current is channeled through the conduit 8 whose dimensions are such that its swing deflects the current forcing it to sweep the entire width of the conveyor 3.

The guide duct 8 is placed at the top of the enclosure 4, as far as possible from the conveyor so that the changes in direction to be imparted to the gas stream are as small as possible. In addition, it is preferable to channel the gas stream while its geometry is well defined, that is to say as close as possible to the fiber-forming device.

Figure 2 shows in more detail the guide duct 8 and the mechanism which drives it in an arrangement according to the invention.

In the prior techniques, and in particular in US Pat. No. 3,134,145, the movement of the gas flow guide duct is ensured by a motor and a mechanical transmission comprising a cam and a set of connecting rods.

Improvements have been proposed which involve a mechanism formed by a series of gears, the assembly having the effect of producing a more complex movement of the duct. This movement includes, for example, a greater speed of movement in the extreme positions than in the middle position.

The adjustment of the fiber distribution devices must be very precise. We will see in the examples of implementation of the invention that a very slight change in the parameters defining the movement of the guide duct causes a very significant change in the distribution. On known devices, these adjustments are made by the operators before the start of production. Interventions during operation are not entirely excluded, but are difficult and temporarily disrupt production. In practice, these interventions are only undertaken when the distributional defects are very significant.

The device used according to the invention allows on the contrary modifications of the operating conditions without requiring an interruption of the production or even without disturbing it. For this reason, these modifications can be as frequent as desired. It is also possible to envisage the correction of even relatively small distribution defects and to end up with products of appreciably increased quality.

In Figure 2, the guide duct has at its upper part a slightly frustoconical shape flared towards the fiber forming device. This flared shape facilitates the channeling of the drawing gases emitted by an annular drawing member 10 at the periphery of the centrifuge 1.

The conduit 8 is supported by means of two pivots 11 engaged on bearings fixed on uprights not shown. The axis of rotation is placed high enough on the duct so that the arrangement of the opening thereof vis-à-vis the gas stream is little modified by the oscillation movement.

The movement is generated by an engine assembly which in the example shown is constituted by hydraulic cylinder 9. This drive mode is obviously not the only one usable. It is possible to provide for example an electrical or electromechanical assembly making it possible to ensure both the oscillation movement of the conduit 8 and the modification of the parameters defining this movement.

The movement is communicated to the conduit 8 via an articulated mechanical transmission comprising the rod 16 of the jack 9, an arm 14, a connecting rod 13 and another arm 12 integral with the conduit 8.

The arm 14 pivots on an axis 15 carried by bearings arranged on a fixed frame, not shown. The rod 16 of the jack 9 is attached to the arm 14 by an articulation 22.

The jack 9 is held on a frame 26 by means of pivots 27 which allow it a certain rotation travel in a vertical plane.

The connecting rod 13 articulated on the arms 12 and 14, in the form shown, constitutes with these arms a deformable parallelogram. The movement of the two arms is therefore identical. Other similar arrangements are obviously possible in the context of the invention. This arrangement has the advantage of simplifying the determination of the position of the conduit 8, a determination which, as we will see later, is involved in the regulation according to the invention.

The movement transmission assembly has a whole series of adjustment means making it possible to fix its geometry with precision. These traditional means for this type of assembly are not shown.

The jack 9 is double acting. It can therefore be animated by an alternating movement back and forth. Such a movement can also be obtained using two simple counter cylinders, but for the convenience of implementation a double cylinder is preferable.

The operation of the jack 9 is controlled by a proportional distributor shown diagrammatically at 17. The latter regulates the flow rate of the fluid admitted into the jack. It is associated with a hydraulic power station supplying the pressurized fluid, shown diagrammatically by block 28.

The stroke of the jack 9 and the construction of the mechanical transmission are chosen so that the swinging of the guide duct 8 can meet all practical needs. In other words, the limits of the movement, materialized for example in FIG. 1 by the angle B formed by the axis of the conduit in the extreme positions, are such that the gas stream would exceed the width of the conveyor if it did not collide at the side walls 5.

The use of a hydraulic cylinder offers great facilities for adjusting the movement. It is of course possible to modify the amplitude. It is also possible, by maintaining the same amplitude, to modify the extreme positions. It is still possible to vary the speed.

In general, the movement that can be made to the cylinder 9, and therefore communicate to the guide duct 8, can follow any instruction. It is possible for example to make the cylinder follow a walking program in which the speed would vary during an oscillation according to a complex law. It is also possible, of course, to combine variations of several of the parameters determining movement, speed, frequency, amplitude, extreme positions.

All changes are made without interrupting the movement by an appropriate adjustment of the proportional valve.

The hydraulic cylinder constitutes a preferred means according to the invention because of its robustness and its flexibility of use. Other means can also be used to produce this type of variable movement as we indicated above.

The dispensing device used according to the invention therefore lends itself to frequent corrections to the dispensing method such that these may appear necessary in the production of felts.

Indeed, the dispersion of the fibers on the conveyor, whatever the precautions taken, is subject to many hazards. We understand that it is very difficult to maintain perfectly stable the gas flows inside the enclosure 4. In addition to the current carrying the fibers, large induced currents develop. In addition, in the same enclosure are usually assembled several fiber-forming devices, the gas streams of which do not fail to influence one another. As a result and despite the suction established under the conveyor, the enclosure 4 is the seat of significant turbulence. To these causes of irregularities is added, if necessary, an accidental lack of uniformity in the aspiration.

Whatever the reasons, experience shows that during operation irregularities in the transverse distribution of the fibers appear which are maintained for relatively long periods, so that it is desirable to modify the operating conditions of the guide duct in an attempt to restore better uniformity.

Another advantage of the use, according to the invention, of hydraulic means for actuating the guide duct is to allow automated control. Indeed, the variations discussed above occur by chance. It is therefore very desirable that corrections can take place as soon as a distribution fault is detected.

The fiber distribution measurements in the felt formed can be established by different methods. In the perspective of automatic regulation, the methods that can be used must operate continuously and not disturb production.

A preferred method is constituted by a measurement of absorption of radiations, in particular of X-rays, but other methods are also possible.

The measurement of X-ray absorption is preferred when the felt is thick, in other words when the absorption is relatively strong. For thinner, and therefore less absorbent, layers of fibers, such as those of products of the type designated by the name of "veil", a measurement carried out with beta radiation, for example, may be preferred.

The measurement of the mass of fibers per unit area on the felt by absorption of X-rays is carried out according to the invention according to very specific methods.

Thus the measuring device must be located at a point in the production chain which lends itself to significant measurement.

Leaving the receiving enclosure 4, the felt formed is often loaded with moisture. This comes in particular from the binder solution sprayed on the fibers. Optionally, water is also sprayed on the path of the fibers to cool the drawing gases and the fibers which they transport. Water that strongly absorbs X-rays can significantly modify the results of the measurements, if its distribution is not homogeneous. It is therefore advantageous to operate at a point in the production chain where the felt is rid of its moisture.

For this reason, the measurement of fiber mass per unit area is preferably located at the outlet of the binder treatment chamber.

However, if the fibers collected cause little humidity or if the humidity is well distributed, the measurement can be made before treatment, right out of the fiber reception enclosure.

When the measurement is made after treatment of the binder, it takes place relatively far from where the fiber distribution takes place. Between the deposition of the fibers on the conveyor belt and the passage to the measuring point, it may take several minutes, even ten minutes. This delay, which is systematically introduced into the implementation of the regulation of the distribution as a function of the measured homogeneity defects, is however not very troublesome. As we will see in the examples of implementation, the regulation according to the invention makes it possible to correct distribution defects which manifest themselves over relatively long periods with respect to the delay in question. In addition, during production, irregularities usually appear gradually. If they are corrected as they appear, the deviations found are usually relatively small and do not compromise production.

The measurements must also be made over the entire width of the felt, a mobile measuring device is used for this purpose which moves transversely to the felt.

Figure 3 shows schematically a measuring device used according to the invention.

In this figure, the felt 7 passes through a frame 29. The frame 29 supports in the upper transverse a source 30 emitting radiation in the direction of the felt 7.

The emitting source 30 disposed on bearings is mobile. Its transverse movements are ensured by a system of chains arranged in the frame but not shown.

In the lower transverse part a mobile receiver 31 is arranged opposite the source. The receiver is driven in a movement identical to that of the source, also by a chain system.

A single motor assembly housed in the housing 32 ensures perfectly synchronized movement of the source 30 and the receiver 31.

The emitted radiation is partially absorbed by the felt and the fraction of the radiation reaching the receiver is measured.

The measurements are taken while the device is moving and each corresponds to the scanning of a fraction of the width of the felt.

The duration of each of the measurements, and therefore the width of the fraction analyzed, can be chosen according to the use made of these measurements.

Furthermore, the measurements must be carried out on fractions of the width of the felt such that the discontinuous structure of the fibrous material does not constitute an obstacle to obtaining significant values. The minimum width of the "sample" on which the measurement is made is a function of the mass per unit area of the felt. It is all the smaller the denser the felt.

For felts whose mass per unit area is of the order of 1 to 3 kg / m ^2, an analysis width of a few millimeters to a few centimeters is sufficient.

In practice, as we will see below, the regulation of the fiber distribution device can only be carried out on a limited number of parameters. A large number of measures are therefore of interest only by the additional possibilities which result from it with regard to the treatment of these measures.

The regulation mode of the felt-forming installation, for the part relating to the distribution of the fibers, is shown diagrammatically in FIG. 4.

In this figure only one fiber forming device is shown. In this type of installation, these devices are usually six to twelve aligned along the conveyor 3 in the same enclosure 4.

In the case of installations comprising several fiber-forming devices, each of them is advantageously equipped with a distribution system of the type used according to the invention. Depending on the case, the movement of these devices may or may not be identical. In general, they have a movement of the same frequency but this is not necessary, the movements may not be synchronized.

Likewise, the amplitude and middle direction settings may vary from device to device.

When an automated regulation according to the invention is carried out, this can relate to one or more devices of the same installation.

The felt 7 leaving the enclosure 4 is taken up by the conveyor 20 running at the same speed as the conveyor 3. It passes through an oven 19 where it is subjected to a circulation of hot air to polymerize the binder.

On leaving the oven 19, the dry felt passes through the measuring device by absorption of the X-rays 21.

The regulatory loop implemented is as follows.

The measuring device 21 transmits the quantities corresponding to the absorption for the "sample" analyzed as well as the position of this sample on the felt to a computer shown diagrammatically at 23.

Furthermore, the computer 23 also receives information on the operation of the distribution device via the regulation assembly represented by the block 24. In particular the computer receives the signals concerning the position of the guide duct 8. This position is identified for example by means of a potentiometric detector 18 (FIG. 2) which follows the rotational movement of the arm 14 around the axis 15.

Optionally, the computer 23 also receives the information relating to the speed of movement of the felt 7, by means of a system for regulating the speed of the conveyors shown diagrammatically by the block 25.

The computer compares this information with a set of data in memory and, according to the deviations observed, develops instructions which are sent to the regulating assemblies 24 and 25. These assemblies consequently modify the operation of the distribution device and the speed of the conveyors.

As we have indicated previously, the parameters available for controlling the distribution of fibers are few.

The running speed of the conveyors makes it possible to modify the mass per unit area of the fibers in general but not the transverse distribution. Ordinarily, the overall quantity of fibers is controlled at the time when these fibers are formed, for example by regulating the quantity of material to be fiberized. In this hypothesis, the running speed remains constant.

However, the presence of a mass measurement unit per unit area of the felt allows, if necessary, an automatic speed adjustment as indicated above. To this end, the computer 23 is led to integrate the local measurements in order to determine the mass per unit area of the entire felt. The comparison of the result with an imposed value controls the acceleration or the deceleration of the conveyors according to whether this mass appears higher or lower than the imposed value.

The parameters which determine the running of the distribution duct 8 and therefore the transverse distribution of the fibers are the frequency of the oscillations, the amplitude of the oscillating movement and the middle direction.

The frequency is an important element to obtain a good distribution of the fibers on the conveyor. When it comes to forming felts with a high mass of fibers per unit of surface area, several successive deposits are usually superimposed, each corresponding to a device from a series of aligned devices as mentioned above. In this case the influence of the frequency, above a relatively low minimum threshold, is less sensitive. For lighter felts, the precise setting of the frequency is much more important for the final result.

In general, the frequency must be sufficient for the entire surface of the moving conveyor to be effectively covered by the flow carrying the fibers. When several fiber forming devices are used to produce the same felt, complete covering by each of the flows is not always essential. It suffices that the overall effect of these devices effectively corresponds to complete recovery.

Conversely, it is not advantageous to increase the frequency too much. The improvement which one can obtain from it is not appreciable and one comes up against the inertia of the veil of fibers. Beyond a certain frequency, it can be seen that the movement of the gas stream no longer succeeds in following that which is imposed on the guide duct. Regulation efficient fiber distribution becomes impossible.

It is possible to provide a frequency regulation for example as a function of a previously determined optimum for each areal mass. Frequency regulation can then be carried out in combination with the adjustment of the conveyor running speed as a function of the average surface mass measured over the entire width of the felt.

The amplitude and the middle direction of movement of the guide duct directly determine the transverse distribution of the fibers. The use of guide conduits in traditional modes has made it possible to obtain simple results on the way in which these parameters act on the distribution. The modification of the middle direction, the amplitude remaining constant, causes a displacement of the deposit of the fibers in the same direction as this modification. Given the presence of the side walls, this displacement results in fact in an increase in the mass of fibers per unit area on the side towards which the displacement takes place. Similarly, it can be seen that an increase in the amplitude of the movement favors the deposition of the fibers on the edges of the conveyor to the detriment of the center and vice versa.

The fiber mass measurements per unit area and their processing by the computer are in particular aimed at achieving the best possible adjustment of these two parameters. For this, distribution models have been established, to which correspond responses, the whole being stored in the calculator.

Four basic distributions are distinguished. These four distributions are shown diagrammatically in FIGS. 5a, 5b, 5c and 5d. In these figures the difference in mass per unit area is indicated relative to the average value on a cross section of the felt. For the average value, the difference is zero. These four shapes correspond respectively: to the gas current shifted to the left (Figure 5a), shifted to the right (Figure 5b), to an amplitude of oscillation that is too large (Figure 5c) or too small (Figure 5d).

The comparison of the measurements, processed and weighted as we will see, with these four models determines the correction imposed on the running of the guide duct.

The processing of the measurement initially comprises the accumulation of several measurements corresponding to successive passages at the same location in the width of the felt. The average value which is deduced from it is thus a more complete and more precise image. the actual distribution in the area considered. The measures are also grouped by sectors, which are weighted. The choice of sectors and their respective weighting is determined by tests to ensure that the values obtained are well representative of the distribution and that the resulting corrections result in an effective improvement.

These value treatments are also chosen as far as possible to adapt to all the configurations or dimensions of the installations which are equipped with these regulation systems.

In FIG. 6, a preferred grouping mode for the measurements of the masses of fibers per unit of area is indicated. In this mode, for example, the width of the felt L is cut into four sectors which partially overlap. The weighted measures grouped in these four sectors make it possible to avoid giving too great importance to the measures corresponding to the sides of the felt relative to the central part.

Other methods of treatment are of course possible. The tests in each case show the advantage of the mode studied to solve the problems actually encountered.

By way of example, tests have been carried out on a pilot installation for the formation of glass wool felt. This installation comprises only one fiber-forming device.

The fiber-forming device, as well as the assembly of the guide duct and the motor system, is of the type shown in FIG. 2.

In this installation, the felt formed has a width of 2.40 m. It has a mass per unit area of 1 kg / m2.

Due to the fact that only one fiber forming device is used, the speed of the receiving conveyor is relatively slow. It is 5.25 m / min.

The felt coming out of the reception room goes into an oven.

At the exit of the oven, the felt scrolls through an X-ray absorption measurement assembly, the source of which is americium 241. This mobile source traverses the entire width of the felt in 32 s. During each movement across the width of the felt 64 measurements are made. The values are saved with their location.

A rolling average is established over the last eight passes of the X-ray probe.

The values are grouped into four bands I, II, III, IV as shown in Figure 6.

The regulation is based on the average values for these four bands according to the mode described above.

Between two successive corrections, it is necessary to take into account the time between the formation of the felt and the measurement. In the present case, this period is 10 minutes. It is also necessary to consider the time corresponding to at least eight successive passages of the probe on the felt formed after the previous correction to have all of the eight measurements that we have set.

In these tests, corrections are systematically made at 18-minute intervals.

Figure 7 shows the evolution of the distribution of fibers on a side strip of felt of a width of 30 cm. The corresponding value is therefore the average of eight measurements for each of the eight successive passages, for a total of 64 measurements.

The graph represents the relative difference in density of the strip considered in relation to the average basis weight over the entire width of the felt. The timing of corrections is indicated by a vertical bar.

The initial movement of the guide duct corresponds to an amplitude defined by the half-angle B of 8.7 ° and a median direction making an angle of + 0.8 ° relative to the vertical. The oscillation frequency which remains unchanged during the tests is 60 round trips per minute.

Initially, that is to say before the first corrections, the deviation from the average varies between + 15 and + 7%. Quickly, after two corrections, this difference is reduced to less than 5%. It is then constantly below 5% in relative value and after the fifth correction, even drops below 3%.

The improvement obtained is therefore quite remarkable.

It should also be emphasized that if the surface mass of the selected side band has been corrected, similar measurements made on the other fractions of the felt show that for the whole of the felt, the differences are maintained at a value less than 5% of the average value. In other words, the corrections made which made it possible to bring better distribution to the outer strip were not made to the detriment of the distribution of the rest of the felt.

The correction introduced according to the invention is an operation extremely precise as we indicated at the beginning of the description. At the end of the fifth correction applied, the amplitude of movement of the guide duct is 8.14 ° and the middle direction makes an angle of -0.5 ° relative to the vertical. The modifications imposed on the movement are therefore very slight.

These modifications show the degree of sensitivity of the distribution to the parameters of the movement of the distribution duct and what difficulty there could be in achieving an adjustment of the same quality if it were to be operated manually, assuming that the actuating device the guide duct is suitable for such corrections. We have seen that this has not been the case so far.

FIG. 8 also reproduces a regulation test on the same device as above.

These recorded measurements correspond to eight distinct bands across the width of the felt. As an indication, the measurements for bands 1, 2, 4, 7 and 8 are shown.

This example is interesting because it corresponds to a particularly irregular distribution at the origin. Thus the neighboring bands 1 and 2, or 7 and 8, show deviations for one positive for the other negative compared to the average.

In the present case, the average surface mass is 1.3 kg / _m 2.

Initially the half angle B defining the amplitude of the movement is 12.35 ° and the offset from the vertical is - 10.61 °.

Corrections are indicated on the time scale by a vertical bar.

It is remarkable to note that after two corrections the deviations for all the values, including the least good initially (+ 18% for band 2, - 12% for band 8) are brought back in an interval varying from + 5 at 5 %. The values then remain within this range.

At the fourth correction, the half angle B is 12.72 ° and the middle direction - 10.25 °. As in the example in FIG. 6, the variations leading to the improvement of the distribution of the fibers are therefore extremely small.

Claims (12)

1. Method for the formation of a fiber felt in which the fibers carried by a gas stream are directed on a perforated conveyor which retains the fibers and lets the gases pass, and in which the gas stream is caused to sweep the conveyor d '' an oscillating movement across the width of the conveyor, the characteristics of the oscillating movement, frequency, shape, amplitude, middle direction, or at least one of these characteristics being automatically regulated during operation according to the result of measurements of mass of fibers per unit area on the felt formed. 2. The method of claim 1, wherein the mass of fibers per unit area in the felt is measured by a radiation absorption method. 3. Method according to claim 1 or claim 2, wherein the amplitude of the oscillations and the middle direction are automatically regulated according to local variations in mass per unit area measured in the width of the felt. 4. The method of claim 3, wherein the frequency of a movement, and the speed of the conveyors, are controlled by a control loop, according to the measurement of the mass of fibers per unit area for the entire width of the felt. 5. Installation for the formation of a fiber felt comprising a fiber production assembly generating a gas stream carrying the fibers in a receiving enclosure (4), a gas permeable conveyor (3) forming a wall of this enclosure ( 4), the conveyor (3) allowing the gases to pass through and retaining the fibers which constitute the felt (7), a device giving the gas current an oscillating movement in the width direction of the conveyor, an assembly (19} for processing the felt leaving the receiving enclosure (4), the device giving the oscillating movement to the gas current being constituted by a guide duct (8) movable in rotation and the movement of which can be modified at any time in frequency, shape, amplitude and direction according to instructions developed by a regulation assembly comprising a set of measurements (21) of the mass of fibers per unit area on the felt formed, a computer (23) for processing the measurements and comparing sound of the result of this processing with setpoint sizes stored, and developing signals controlling means (9) setting in motion the guide duct (8). 6. Installation according to claim 5 in which the fibers being produced by a centrifugation device and entrained by an annular gas stream running along the peripheral wall of the centrifuge (1), pass through a guide duct ge (8) of circular section disposed at near the centrifuge. 7. Installation according to claim 5 or claim 6, characterized in that the guide duct (8) is moved by a double-acting hydraulic cylinder (9), which is controlled by a proportional distributor (17). 8. Installation according to one of claims 5 to 7, in which the assembly for measuring the mass of fibers per unit area of the felt is a set for measuring radiation absorption. 9. Installation according to claim 8, characterized in that the measuring assembly is constituted by a device analyzing the absorption of X-rays (21), the device being movable in the direction of the width of the felt. 10. Installation according to claim 7 and claim 9 wherein the mass measurements of fibers per unit area in the width of the felt feed a regulation loop which controls the amplitude and the average position of the stroke of the cylinder rod hydraulic (9). 11. Installation according to claim 9 in which the mass measurements of fibers per unit area for the entire width of the felt feed a regulation loop which controls the speed of the conveyor (3) receiving the fibers and, where appropriate, the frequency. of the oscillating movement of the guide duct (8). 12. Installation according to one of claims 6 to 11 in which several fiber centrifuge devices are aligned along the same conveyor - (3) for receiving fibers, each centrifuge device being associated with a guide duct ( 8), at least one of these conduits being regulated.

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