Process and device for the distribution, on a receiving surface, of fibres conveyed by a gas flow.

A method and apparatus for distributing fibers carried by gas streams to a receiving member

The invention relates to a method for better distributing the fibers carried by a gas flow directed towards the surface of a receiving member to the receiving member, which gas flow may consist of two or more fractions, one of which forms a layer surrounding the others. The invention also relates to a device for improving the distribution of fibers carried by a gas flow directed towards the receiving surface to the receiving member.

The fibers are, in particular, mineral fibers, such as glass fibers or the like, and are carried by the gas streams used to form the fibers and are collected on a collecting member to form non-fibrous mats or the like.

Due to its importance, the invention has been developed especially with a view to the production of mineral fiber mats. However, it is self-evident that the invention can be applied to all kinds of fibers conveyed to a gas flow receiving member, which are desired to be well distributed on the surface of this receiving member. Even distribution is necessary to provide good mechanical and insulating properties to nonwovens or similar products.

Usually, the formation of the fibers takes place immediately before the formation of the mat, and the gas streams used to pull the fibers also act as fiber carriers, so the properties of the gas streams used are mainly determined by the conditions required in the fiber formation step.

The distribution of the fibers to the receiving member is closely related to the geometrical characteristics of the location and structure of the gas flow. In the following explanation, the gas flows are presented as if their structure, and in particular their limits, were precisely defined.

This is, of course, only a simplification dictated by practice, so that the phenomena that occur can be described more clearly. There is a strong change in the free-flowing gas flow in the atmosphere. It draws in the surrounding air with which it is in contact, and its volume tends to increase. At the same time, some of its original energy is transferred to the entrained air and the flow rate decreases. Both changes are more pronounced at the edge of the flow than at its center.

Experimentally, it can also be stated that the fibers carried by the flow are no longer evenly distributed in the flow after the starting point. In an expanding flow, there are more of them in the middle of the flow than at its edge. In addition, the flow rate is higher in the middle. Therefore, more fibers enter the part of the receiving member located at the center of the flow.

In addition, the device from which the fibers leave is usually relatively small in size compared to the dimensions of the spaces to which the fibers are directed and, above all, compared to the dimensions of the receiving member to which the fibers are collected. The same often applies to the gas flow carrying the fibers at the level of the receiving member, which makes it even more difficult to distribute the fibers evenly.

Namely, the flow of gas passes through a receiving member which stops the fibers - it is usually a perforated belt conveyor - and the passage of gases through this receiving member specifically determines the distribution of the fibers thereon. If the flow covers only a part of the area of ​​the receiving member, the fibers are mainly concentrated in the flow path, leaving the rest of the area either poorly covered or practically uncovered.

Since the dimensions of the receiving member are primarily determined by the dimensions of the finished product and the production rate of the equipment, it has been necessary to find ways to obtain a satisfactory distribution of the fibers even over relatively large areas.

One solution to solve this distribution problem has been proposed by means of a reciprocating tube of a gas flow carrying fibers. The gas flow is then directed to sweep the entire surface area of ​​the receiving member. In this solution, complex mechanisms change the direction and velocity of the moving tube.

Due to the complexity of the mechanical equipment and the unreliability of the result obtained, it seems desirable to avoid recourse to this type of equipment as far as possible.

In another proposed solution, separate jets are directed across the path of the gas flow carrying the fibers to correct some of the systemic irregularities observed when, for example, more fibers enter one side of the receiving member than the other. The use of jets has also been proposed to allow the gas flow carrying the fibers to reciprocate, which has previously been achieved by mechanical means. This avoids the use of mechanical means, but such solutions are not without problems. First, although it is somewhat easy to change the gas flow, it is more difficult to achieve a uniform distribution corresponding to this change. Second, changing the flow by the impact of intermittently transmitted jets generates undesirable eddy currents. Such vortices can, for example, cause bundles detrimental to the quality of the final product.

It is an object of the invention to improve the distribution of fibers to the receiving member and thus the properties of the finished product.

Another object of the invention is to provide devices which facilitate the improvement of the distribution, and in particular such devices with which the flow of the gas flow carrying the fibers can be changed without the disadvantages of the prior art.

The studies which led to the invention have shown that the normal flow of a gas flow carrying fibers, and in particular the cross-sectional dimensions of this flow, can be altered by surrounding it with a gas layer with suitable properties. This gas layer is made such that it at least partially in its path bypasses the variable gas flow. If the flow direction of the bed is directed to a plane lateral to the gas flow, one component of this direction differs from the flow direction of the gas flow. That is, in the lateral plane, a certain angle (hereinafter referred to as Bridge) is formed between the imaginary lines of the gas flow and the corresponding directions of the gas layer.

More illustratively, the gas layer surrounding the gas flow carrying the fibers revolves around the gas flow. It makes a motion that sort of corresponds to rotation around the gas flow.

The method according to the invention can be applied to different gas streams. In practice, gas streams with a somewhat circular cross-section are most commonly used.

In the following description, such flows are considered in particular, but it is clear that it does not in any way exclude flows with a different cross-section, but which are nevertheless suitable for forming the above-mentioned lateral gas jacket. The shape of the gas flow considered here is as it is in the plane where it comes into contact with the surrounding gas layer.

This shape is not necessarily the same as it was originally, i.e. in the fiber formation zone. Thus, in some fiber formation methods, the gas flow comes from an elongated rectangular orifice in cross section. When the gas stream comes into contact with the ambient air, its original shape is observed to change rapidly. After a relatively short distance, a flow of approximately circular cross-section is formed again. This type of deformation does not depend on the original form of the free-flowing gas flow in the atmosphere. It can be exploited when the original shape of the gas flow is such that it is difficult to contact the gas bed under the conditions required by the invention.

Moving to a plane where the flow is somewhat circular in cross-section facilitates the use of the gas bed.

As already stated, the properties of flows with a circular cross-section also change as the flow progresses. The flow tends to expand conically upon contact with the surrounding atmosphere. Under normal conditions, this expansion occurs relatively slowly. The opening angle of the cone is small (about 20 °).

When the flow is circular in cross-section, the side layer according to the invention is initially hyperbolic and its axis of rotation is congruent with the axis of gas flow.

It is clear that as it progresses, the gas layer also changes upon contact, on the one hand, with the gas flow in which it causes the change and, on the other hand, with the surrounding atmosphere.

By operating in accordance with the invention as described above, a combined gas flow is obtained which expands faster than the original flow. In other words, the opening angle of the cone is larger, so that without changing the position of the fiber forming device and the receiving member, it is possible to cause the gas flow to flow evenly through the entire surface of the fiber receiving device.

The complex interaction mechanism between gas flow and gas bed cannot be fully analyzed. Two phenomena can be used as models to explain the effect of the gas layer:

The movement of the gases into the bed creates a kind of vortex. The area inside the layer is under vacuum compared to the layer itself. The gas flow directed to this area tends to fill this vacuum and is as if absorbed towards the layer forming the jacket.

In addition, the gas layer promotes the generation of its own direction of motion in the gas flow with which it is in contact. The side component of the movement of the gas layer is thus partially transferred to the gas flow. Inside, the outward movement of the gases is formed by the effect of centrifugal force.

Both of these phenomena explain only in part the mechanism of gas motion. In practice, only a combination of different effects can be identified, from which the different components cannot be distinguished. Whatever the mechanism, the result is an increase in the angle of expansion of the gas flow, as will be apparent from the embodiments described below.

The choice of the conditions for carrying out the invention is strictly related to the gas flow carrying the gases as well as to the characteristics of the equipment. The most favorable conditions are

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Ίΰ69Ί must therefore be determined in each case in the light of the following findings.

The extent of the change in gas flow according to the invention depends on many factors.

It is clear that the first important factor in changing the gas flow is the geometry of the surrounding gas layer. We have seen that in the lateral plane, there was a component in the flow direction of the bed that was not congruent with the flow direction component of the gas flow — In this plane, the angle between these directions may vary within relatively wide limits.

For obvious reasons related to the proper flow of the gas flow, the flow direction of the bed cannot even be partially reversed. That is, the angle between the flow and the directions of the bed, still in the lateral plane, cannot be greater than 90 °. An angle too close to 90 ° could have a very local effect, with the gas jacket gas moving very quickly away from the gas flow having very little effect on it.

Likewise, a very small angle between the flows would have a rather limited effect. The gas layer would surround the gas flow well, but would not substantially change its trajectory.

In practice, in order for the layer to surround the gas flow well and at the same time give it a certain movement, the angle between the directions should preferably be 5-60 ° (on both sides) and especially 10-45 °.

Looking above at the geometry of the gas flows in the region where the bed comes into contact with the flow, we talked about the flow direction component of the bed in the plane where it bypasses the gas flow carrying the fibers. However, this direction does not necessarily have to be in a completely lateral plane. In certain cases, it may be advantageous to also give it a radial component in a plane perpendicular to the flow direction of the gas flow.

For the inwardly directed radial component of the gas flow, the gas layer first shrinks and then expands again at an opening angle which, as above, depends on the angle of the layer flow direction in a plane lateral to the gas flow direction. The initial contraction of the gas flow by the bed does not prevent the flow angle from increasing. The growth only moves further downstream.

The presence of this radial component may be useful in particular when, regardless of the issue of fiber distribution to the receiving device, it is considered desirable that the cross-section of the gas flow does not increase too early. For example, it may be a question of wanting to prevent certain parts of the equipment from being in the path of the gas flow. It may also be desirable to reduce the gas flow to perform some treatment step, such as atomization.

The achievement of a contraction in the gas flow path must not impair the general characteristics of this flow. In other words, the effect must be relatively limited and correspondingly the radial component in the direction of the gas layer must be weak compared to the other components, because if this is not the case, the gas layer guided too sharply into the gas flow would create vortices that would interfere with normal flow.

The relative magnitude of the radial component in the plane, which includes the point of contact of the bed with the flow and the axis of the flow, is expressed as the angle between this axis and the projection of the flow direction of the bed. This angle, denoted S: 11a, is most preferably not greater than 20 ° when the radial component is directed into the flow.

In most cases, as mentioned above, the radial component is directed into the flow. However, it can also be directed outwards. In this case, the 45 ° angle is a limit that is not desirable to exceed.

The shape of the gas layer surrounding the flow is not the only factor influencing the change in the shape of the flow carrying the fibers. The extent of this change is also related to the motions, i.e. impulses, of the flow and the gas layer

In order for the effect of the gas layer on the flow to be appreciable, the momentums must be comparable in magnitude to the plane at which the effect occurs. If the momentum of the bed is too weak, there will be virtually no change in flow. On the other hand, it is neither necessary nor desirable for the volume of the layer to be too large, because an increase in the production cost of this gas layer does not necessarily lead to a further improvement in the result.

The momentum is expressed as follows:

I = / repeat.v² -S where is the mass density of the gas flow, v is its velocity, and S is its direct section. From the point of view of carrying out the invention, it seems advantageous to keep the momentum I of the gas jacket

O ratio to flow momentum Ι_β within:

0,5 ^IS ^ 2.

These values ​​relate to the most preferred case, i.e. one in which the gas layer leaves in the immediate vicinity of the gas flow carrying the fibers. If what is possible within certain limits - the gas layer starts far from the flow, the ratio can increase significantly.

^TB

The effect of the ambient air on the gas layer must be taken into account. The farther the starting point of the gas layer is from the flow, the greater the part of the momentum already lost in contact with the surrounding air. In order to achieve the same effect on the flow, it has been considered appropriate to increase the initial momentum of the bed.

One advantage of the method of changing the distribution of fibers of the present invention is that it can momentarily change the parameters used to account for irregularities in the conditions of nonwoven formation. By varying the pressure of the gas bed in the etching device, the speed of the bed and thus its momentum can be changed.

Preliminary experiments can be used to determine the change in gas flow corresponding to each pressure, as we will see in the embodiments of the invention.

The location of the gas layer, i.e. more specifically the location where it affects the gas flow, depends essentially on the structure of the flow and the effect of this structure on the fibers.

It has already been mentioned that it is advantageous to apply this effect to the point where the cross-section of the flow is circular. It should be added that it is desirable to use a flow whose flow is precisely determined, which requires that the effect of the bed be located at a moderate distance from the starting point of the gas flow. Likewise, since the change in gas flow provided according to the invention is precisely an increase in the opening angle of the flow, it is necessary that the change takes place far enough away from the receiving device for this increase to occur as a significant expansion of the flow at the receiving element.

In general, if the fibers are initially evenly distributed in the gas flow, the strong deceleration in the gas flow prior to its arrival at the receiving member and the resulting turbulence will tend to break this uniformity.

One example that is particularly important for industry may shed light on the importance of gas flow structure relative to fibers and thus demonstrate the criteria for locating a gas layer. These are methods in which the fibers are formed by a centrifugal drum which at the same time acts as a defibering device. In this type of process, the drawing of the fibers is supplemented by blowing a gas flow transverse to the direction of entry of the fibers from the drum. The blowing is carried out annularly at the edge of the centrifugal drum in its immediate vicinity.

The gas flow proceeds from its starting point as follows. Immediately below the centrifugal drum, a slight vacuum is created because the annular gas flow carries with it the gases of this zone. As a result of the vacuum, the gas flow contracts (and compresses) below the drum and then expands again in the same way as the conical flows discussed above. In addition to these deformations, there is, of course, a strong deceleration in contact with the surrounding air.

The fibers form a kind of continuous curtain carried by a gas flow. The deceleration and compression under the drum causes folds and breaks in this curtain. Arbitrary breaks cause the distribution of fibers or groups of fibers in the gas flow to be not completely uniform at a certain distance from the fiber-forming member.

In this case, it has been found desirable to bring about a change in the flow according to the invention before the irregularities in the distribution of the fibers in the gas flow have become significant. In practice, this roughly means that the effect of the gas flow is adapted to take place preferably before the area where the gas flow is compressed below the centrifugal drum.

In all occurring cases, the gas layer causes a change in the flow of the gas stream carrying the fibers without altering the fibers in any way. Its effect only occurs when the fibers are no longer pullable. Because very thin fibers cool rapidly, the blowing of the layer-forming gases can occur relatively close to the fiber-forming zone.

For the same reasons, the blown gas is at a temperature lower than the melting temperature of the fibrous material. Preferably, the gas is at about room temperature.

It is clear that the blown gas can be of various qualities. It is usually air or water vapor for practical reasons, but other types of gases or gas mixtures may be used.

The invention also relates to devices for carrying out the method described above. In conventional equipment having a receiving member through which a gas flow passes and which retains the fibers carried by said flow, they are members arranged above the receiving member in the gas flow path and which provide the gas layer surrounding the flow.

These devices are equipped with openings from which the gas layer is sent. The openings are arranged around the gas flow and are oriented so that the bed has the above-mentioned properties. In particular, the direction of the orifices in a plane adjacent the edge of the gas flow at the point where it comes into contact with it differs from the flow direction of the flow.

In the most favorable case, i.e. when the cross-section of the flow is circular, it is advantageous to send a gas layer from the annular fan surrounding the flow.

The gas layer and thus also the fan are preferably as close as possible to the gas flow without, however, forming an obstacle to its path. Under these conditions, it can be seen that the fan is in a position lateral to the gas flow. The angle between the direction of the transmission openings of the gas layer and the axis of the fan is then preferably 10-60 °.

As mentioned earlier, when talking about the direction of flow of the layer, the orientation of the openings may also have a radial component, a radial component. The presence of a radial component is particularly advantageous when the annular fan is slightly separated from the edge of the gas flow or when the fan is located at a point where the flow tends to compress for the reasons described above.

If the effect of the gas layer can be changed by changing the gas pressure in the fan, it is also possible to change the flow direction of the gas layer momentarily to a certain extent. In theory, a possible mechanical modification of the fan is hardly desirable due to the complexity of the procedure and the unreliability of the result.

In order to change this direction, it is advantageous to arrange two annular fans in parallel, the blowing directions of which differ from each other. The openings of the two fans are close enough to each other for the gas jets they blow to fuse quickly. This forms a combined gas layer, the properties and in particular the direction of which depend on the properties of each fan.

For example, the second fan may have openings whose blowing direction is the same as the direction of gas flow, while the direction of the openings of the second fan is clearly oblique to the direction of gas flow. The momentum of each fan (controlled by pressure) determines the flow direction of the combined gas bed.

By means of the devices according to the invention, which have been described above, the flow direction of the bed can be changed momentarily without changing the structure. As with the pressure variations used in the case of a single fan, the distribution of the fibers according to the continuous measurement of the finished product can be corrected in this way. The operation of the device may also be subject to an automatic control element connected to the product monitoring devices.

The invention is described in more detail in the following description with reference to the drawings, in which:

Figures 1a and 1b schematically show the distribution of fibers without the method and apparatus according to the invention;

Figures 2a and 2b show how the direction of the gas layer is relative to the gas flow;

Figures 3a and 3b schematically show the shape of a modified gas flow according to the invention;

Figure 4 is a partial section of a blowing device with which a gas layer is formed according to the invention;

Figure 5a is a sectional view of an application of a blowing device according to the invention to a centrifugal fiber forming unit;

Figure 5b is a partial section of Figure 5a showing the approximate progression of the gas flows;

Fig. 5d is another partial sectional view of the device of the type of Fig. 5a, in which the position of the blowing device is different;

Figures 5c and 5e are also some partial views of devices such as the device of the type of Figure 5a with a double blowing device;

Fig. 6 is a schematic perspective view of a centrifuge unit used for forming fibers, equipped with a fan according to the invention, showing the effect of the fan on the gas flow path;

Fig. 7 is a schematic diagram showing the deformation of an initially rectangular gas flow upon contact with ambient air and the effect of a fan according to the invention.

Figures 1a and 1b show how the fibers are distributed to the receiving member when the latter is relatively wide compared to the gas flow carrying the fibers.

The gas flow 1 is shown schematically according to what can be considered as its assumed limits. In fact, even if these limits are initially relatively precise, they are much less clear as we move towards the receiving body 2. The approximate limits are those where the gas flow in question carries 95% of the fiber produced. Be that as it may, it is stated that the distribution takes place without changing the flow approximately in accordance with the clockwise section.

This distribution changes more or less according to the larger or smaller cross-section of the gas flow with respect to the dimensions of the receiving surface. In the cases shown in the figures, the gas flow 1 remains the same and the dimensions of the receiving space and the receiving member change.

The side walls 3 of the receiving space prevent the full expansion of the gas flow in the case shown in Fig. 1a. As a result, the amount of fibers accumulating at the edges of the receiving member increases. This effect of the side walls or the effect of a wide gas flow compared to the dimensions of the receiving surface, respectively, shows very simply that the expansion of the flow according to the invention improves the distribution of the fibers.

The structure of the gas-modifying gas flow layer carrying the fibers is shown in detail in Figures 2a and 2b.

These figures show in perspective a conical gas flow 1. The gas layer at point A, where it comes into contact with the gas flow, is determined by its flow direction D.

In the case shown in Figure 2a, the direction D is completely in the plane P, which is lateral to the gas flow at A. The characteristics of the direction D are determined by the angle β it forms with the flow direction C of the gas flow at A.

In Figure 2b, in the direction of flow of the layer, a radial component is additionally included at point A. This component occurs in the direction R, which is perpendicular to the plane P and passes through the axis of flow. The magnitude of the projection angle g of D to the plane R, together with the flow direction C, determines the radial component of the direction D.

7

These images show the direction D at point A at the edge of the flow. The direction at any point is that obtained by rotating the image about the axis of the flow.

In these Figures 2a and 2b, the flow direction C of the flow is shown along the base line of the cone forming the extreme limits of the gas flow. In practice, certain changes can be noted in this direction. In particular, in methods using a rotating member, the direction of flow may form an angle (usually not a few degrees greater) with the base lines of the cone. The information given below regarding the direction of flow is equally applicable.

The flow shape 4 resulting from the action of the gas layer according to the invention is shown schematically in Figures 3a and 3b, which correspond to the cases of Figures 2a and 2b, i.e. without the radial component and with the radial component directed into the flow.

In both cases, the flow of the gas flow 1 without the gas bed is shown in thin lines and the flow 4 formed by the bed and the flow together is shown in strong lines.

Angle a indicates the expansion of the gas flow. As already mentioned, without the change according to the invention, this angle is small (about 20 °). In both cases shown, the angle α is considerably larger in the co-flow, despite the fact that in the case of Fig. 2b a contraction first occurs in the gas flow.

The formation of a gas layer according to the invention can be effected by a device of the type shown in Figure 4. This device, which is intended to change the gas flow of circular cross-section, has an annular shape. It surrounds the gas flow 1 as close as possible without mixing it.

The device consists of a blowing ring with an annular space 5 conducting a gas under pressure. The gas supply means are not shown in the figure. The gas comes out of the lower part of the annular space, from a series of openings 6 arranged around its edge. The openings 6 are at a constant angle of inclination with respect to the axis of the device. Their direction does not include a radial component in this figure.

The openings 6 are sufficiently close to each other that the individual jets, as they expand, flow in a practically continuous layer along the gas flow 1.

The blow ring shown in the figure has only one series of openings 6; it is, of course, possible to fit several sets of concentric openings, either at the same level or at different levels.

In this figure, the openings are slits made in the body forming the bottom of the space 5. The slits are closed laterally by the body forming the lid of said space.

Other types of arrangements can also be used to achieve the desired direction. As an alternative, the gas layer can be sent from a continuous gap, in which case the flow direction is provided, for example, by means of inclined flanges arranged at regular intervals in this gap on the path of the gas layer.

Several devices of the type shown in Figure 4 can be used simultaneously to combine their effects. In this case, the devices, and more specifically their supply openings, are placed close enough to each other to allow the different gas layers formed to fuse.

Figure 5a shows an embodiment of the invention in connection with a fiber forming apparatus in which the fibers are formed by centrifugation.

The apparatus shown in the figure is described in detail in published French Patent Application No. 2,459,783.

This apparatus comprises a centrifuge, which is indicated in its entirety by the reference number 7. At the edge 8 of this centrifuge there are openings 12 from which the material to be defibered comes.

The material to be defibered 9 comes through the hollow shaft 10. It also falls into a rotating distribution basin 11, from where it is sprayed through the openings 12 into the inner wall inside of the centrifuge.

Threads, not shown, which separate from the wall 8 by the action of centrifugal force, are subjected to the transverse gas flow from the annular space 13 provided with the annular conduit 14. The space 13 is fed from one or more combustion chambers 15.

In this device, the combustion gases form a gas flow which follows the circumference 8 of the centrifuge and entrains the fibers, completing their drawing.

To heat the lower part of the centrifuge, a high-frequency induction ring 16 is arranged in the device concentrically with the centrifuge and far enough away from it so that the gas flow carrying the fibers can flow freely.

In some previously known embodiments, an additional blowing device may be added. This annular blowing device is concentric with the centrifuge 7 and the burner line 14. It blows gas jets parallel to the axis of the centrifuge, that is, with the gas flow from the burner. The main function of this blower is to channel any fibers that may pass through the gas flow from the burner so that they do not spread to the surrounding atmosphere. The additional blowing also limits the expansion of the gas flow from the burner and thus facilitates the flow in the space delimited by the inductor 16.

According to an embodiment of the invention, the blowing device according to the above-mentioned patent application is replaced by a blower 17, the openings of which are chamfered as described above.

In Figures 5a and 5b and in accordance with the invention, the openings 18 of the fan 17 are located substantially flush with and spaced from the line 14. This distance is several times the width of the line 14, so that the point of interaction of the gas layer S from the annular fan 17 and the gas flow B from the line 14 is located at the lower end of the centrifuge 7 or even on the other side thereof as shown in Fig. 5b.

The fibers carried by the gas flow B are thus already formed at the point where the layer S changes the flow shape of the flow B.

The ensemble formed by the layer S, the flow B and also the conducted gases I (indicated by an arrow indicating their general direction) goes to the induction ring 16. To prevent the formation of vortices, a modifier 19 is arranged in this ring to channel different gas flows.

In Fig. 5c, the fan 17 is arranged below the induction ring 16 this time. Layer S and flow B then travel together practically as soon as they enter the fan.

Figure 5d shows another variant in which the fan 17 is placed above the induction ring. The part of the fan 17 that gets in the way of the gas flow is shaped and acts as an anti-vortex modifier. Of course, it is possible to position the fan in this way and connect to it a modifier adapted as described above.

Figure 5e shows an assembly similar to Figure 5a or 5b, in which two concentric fans are used simultaneously. The fans are arranged so that their respective openings are quite close to each other so that the jets from both fans mix very quickly with each other, forming a uniform layer before coming into contact with the flow B.

As in the above, the lateral component of the gas layer S surrounding the flow B is obtained by a fan 17, the openings of which are inclined with respect to the axis of the centrifuge, while the openings of the fan 20 are oriented parallel to the axis of the device.

As has been observed, the position of the fan forming the gas layer relative to the gas flow can vary within quite wide limits. However, for the reasons described above, it is desirable that it be located prior to the flow contraction that occurs systematically in gas flows used in devices of the type shown in Figures 5a-5e.

Figure 6 is a perspective view of a general form of gas flow in this type of device. The typical shape of the flow without a gas layer is marked in part by a dashed line.

There are three distinct levels in this flow. Near the centrifuge, the flow forms a kind of shuttle 21.

It then tapers below the centrifuge 7 at 22 and then expands again into a conical flow 23.

The fan 17 providing the gas layer is preferably arranged above the area 22 where the flow is at its narrowest.

The modified flow 24, which combines the gas bed and the gas flow, is indicated by solid lines.

Figure 7 schematically shows the change that takes place in the shape of an initially rectangular gas flow when it comes into contact with the ambient air. The gases from the rectangular orifice 25 rapidly assemble into a shape in which distribution to the surrounding atmosphere occurs most easily, i.e. into a circular stream. After contraction at 26, the flow becomes conical as before. In connection with this type of flow, it is advantageous to place the fan forming the side layer on the gas flow path close to said contraction point 26.

The case shown in Figure 7 is an extreme case. With less elongate orifices, the return of the flow back to a somewhat circular cross-section occurs very quickly and the annular fan can be fitted very early to the gas flow path.

The following examples are intended to show what kind of results can be obtained according to the invention. These examples do not, of course, limit the scope of the invention in any way.

Example 1

These experiments were performed to determine the effect of the gas jacket according to the invention on the shape of the gas flow carrying the fibers.

The deformed gas flow is a flow formed in a fiber forming apparatus such as Fig. 5a. In the plane of the blow ring, the gas flow is approximately 340 mm in diameter.

Initially, the total gas flow is about 70 N.

Without the fan, the opening angle of the gas flow is about a = 20 °.

The blowing ring is arranged as shown in Figure 5a.

It has a row of openings matched in diameter

380 mm in a circle. The total dimension of the opening of these openings-4 2 is 5 x 10 m.

The axis of each opening of the fan forms an angle of 8 = 30 ° with the general axis of the device.

The opening angle of the flow carrying the fibers is determined using a series of different atmospheric pressures in the fan.

The results are as follows:

<td>Pressure $ 10 Pa</td><td> 0</td><td> 0,45</td><td colspan="2"> 1,05 1,4</td><td> 1 ,8</td>

<td>Momentum N</td><td> 0</td><td> 29</td><td> 58</td><td> 74,5</td><td> 94</td>

<td>Opening angle</td><td> 20°</td><td> 25°</td><td> 33°</td><td> 38°</td><td> 42°</td>

<td>These experiments show</td><td>that</td><td colspan="2">by enlarging</td><td>pressure</td><td>or</td>

the momentum of the fan gases, the shape of the flow carrying the fibers changes substantially.

Example 2

These experiments were performed as in Example 1, but this time by varying the angle of the blowing direction with respect to the flow direction of the gas flow.

The device and the operating conditions of the burner are the same. The gas flow is thus similar.

The dimensions of the fan are also the same as in Example 1. The pressure 5 in the fan is 1.4 x 10 Pa in all experiments.

The deformation of the gas flow expressed by the magnitude of the angle at the different inclinations of the fan jets is shown in the following table.

β = 10 20 30 45 α = 28 34 38 45

It is noted that the opening angle α increases in line with the increase in the angle between the gas jets and the gas flow directions over the whole considered area.

As mentioned in the description, the angle 8 is preferably kept within certain limits. Although it is possible to make even more significant changes in the shape of the gas flow by increasing the angle β, the final effect on the flow direction of the gas flow and the distribution of the fibers may not be advantageous. Increasing the angle β too much may interfere with the normal flow of the gas flow and, in addition, a very large angle α is not always necessary to achieve a good distribution.

In this context, it should be recalled that the purpose of increasing the angle of the gas flow is to improve the distribution of fibers to the receiving member, especially when this member is relatively wide. For each device and for each shape of the gas flow to be modified (in particular the distance of the starting point of the gas flow from the receiving member and the dimensions of this receiving member), the required extent of changing the gas flow according to the invention can be determined in advance.

Example 3

The method according to the invention is carried out in an apparatus for manufacturing nonwovens.

The fiber forming device is a centrifugal device as above. It is fitted at a distance of 3.75 m from the belt conveyor on which the nonwoven mat is formed. The width of the conveyor is 2 m.

The 300 mm diameter centrifuge produces 14 tons of material per day.

The burner supplies gas flow under the conditions of Example 1.

The test is performed without a fan. The distribution of fibers on the carpet is very uneven. The deposition profile is bell-shaped as can be seen in Figure 1b.

Variations in fiber weight in the carpet, depending on the point of reception, result in deviations of ί of 80% from the mean, depending on whether it is the middle part or the edge of the carpet.

However, a similar test is performed this time using a blower. The characteristics of the fan are the same as in Example 1 and the pressure is 1.4 x 10 ^ Pa.

The distribution on the belt conveyor is significantly improved. The deviations of the weight of the fibers from the average in the transverse direction of the conveyor are not greater than 7.5%.

These additional variations are not repeated systematically. Their location varies. according to the test batch. A conventional production line has several fiber forming devices in a row above the same belt conveyor. The variations corresponding to each fiber layer tend to statistically compensate for each other, even so well that the deviations observed in the final product are still relatively small.

Thus, in this way, nonwoven mats are obtained which are very uniform in all respects in terms of both mechanical and thermal properties. These results are achieved using only the devices according to the invention. These devices can, of course, be combined with previously known devices, such as those mentioned at the beginning of the description.