US3887428A - Manufacture of continuous material webs of fibrous particles at high consistencies by passing particles through a series of constrictions - Google Patents

Abstract

Sheets of fibrous particles such as paper sheets are formed from high consistency suspensions of fibrous particles. The consistency of the fibrous particles in the suspension is not less than twice the sediment concentration, i.e., 1-6 percent by weight and preferably 4-5 percent by weight, the fibres having lengths of 1-5 mm or longer. The fibres are first dispersed by forcing the fibre suspension through a series of constrictions and deflected after each such constriction to produce a turbulent flow insuring uniform fibre distribution and the turbulent flow is then decayed to cause the fibres to consolidate as a three dimensional network structure with entrained liquid. The network is then deposited on a paper machine wire or felt. Apparatus for the high consistency formation process is described.

Description

United States Patent Wahren et al.

[ MANUFACTURE OF CONTINUOUS MATERIAL WEBS OF FIBROUS PARTICLES AT HIGH CONSISTENCIES BY PASSING PARTICLES THROUGH A SERIES OF CONSTRICTIONS [75] Inventors: Douglas Wahren, Taby; Lennart Reiner, Gustavsberg, both of Sweden [73] Assignee: A. Ahlstrom Osakeyhtio,

Noorrnarkku, Finland [22] Filed: Dec. 20, 1973 [21] Appl. No.: 427,024

Related U.S. Application Data [63] Continuation-impart of Ser. No. 157,120, June 28,

1971, abandoned.

[52] U.S. Cl. 162/216; 162/293; 162/336;

[51] Int. Cl. D2lf l/06 [58] Field of Search 162/216, 336, 338, 343,

[56] References Cited UNITED STATES PATENTS 2,205,693 6/1940 Milne l62/343 June 3, 1975 3,216,892 11/1965 Wahlstrbm 162/343 3,661,702 5/1972 Means 162/216 3,769,155 10/1973 Schiel 162/343 Primary ExaminerS. Leon Bashore Assistant Examiner-Richard V. Fisher Attorney, Agent, or FirmBrooks, Haidt, Haffner & Delahunty [5 7] ABSTRACT Sheets of fibrous particles such as paper sheets are formed from high consistency suspensions of fibrous particles. The consistency of the fibrous particles in the suspension is not less than twice the sediment concentration, i.e., 1-6 percent by weight and preferably 4-5 percent by weight, the fibres having lengths of l-5 mm or longer. The fibres are first dispersed by forcing the fibre suspension through a series of constrictions and deflected after each such constriction to produce a turbulent flow insuring uniform fibre distribution and the turbulent flow is then decayed to cause the fibres to consolidate as a three dimensional network structure with entrained liquid. The network is then deposited on a paper machine wire or felt. Apparatus for the high consistency formation process is described.

13 Claims, 10 Drawing Figures SHEET PATENTEU 3 I975 fiwm l I 3 /////4 r 4 I 7///// THEMED JUH 3 I975 SHEET MANUFACTURE OF CONTINUOUS MATERIAL WEBS OF FIBROUS PARTICLES AT HIGH CONSISTENCIES BY PASSING PARTICLES THROUGH A SERIES OF CONSTRICTIONS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of applicants prior copending application, Ser. No. 157,120 filed June 28, 1971, entitled MANUFACTURE OF CONTINUOUS MATERIAL WEBS OF FIBROUS PARTICLES, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the making of continuous material webs from fibre suspensions and particularly to a process and apparatus at the wet end of a web forming machine.

2. Description of the Prior Art The method of manufacturing paper on an industrial scale has been substantially the same since the beginning of the 19th century. The only changes made relate to machine dimensions and web speed, which both have been increased. Owing to the greater machine widths and speeds, the machine parts have to be manufactured with an increasingly higher degree of precision. This again has resulted in considerable cost increases. As an example it is estimated that the invest ment cost for a modern newsprint machine with peripheral equipment and buildings comes close to 150 million Swedish Crowns. A large part of the machine investment cost is required for its wet part, i.e., the distribution system for the suspension, the headbox and wire section.

At the wet end of a paper machine, the procedures for conventional sheet formation are basically as follows: A fibre suspension of more or less freely moving cellulose fibres in water is distributed coarsely across the width of the machine by a distribution system, such as a cross-machine distributor. The headbox of the machine is also intended to uniformly distribute the fibres on a small scale by means of irregular movements (turbulence) of the transporting medium. In order to eliminate certain deficiencies in the distribution system (involving, for example, an oblique velocity profile in the headbox which in addition to a non-uniform velocity profile across the paper web also indirectly gives rise to an unstable flow with a large scale turbulence, which becomes apparent in the wire section and disturbs the sheet formation), in most cases a number (two to five) perforated rolls are placed in the flow path. The fibres in the suspension, for mechanical-geometrical reasons, have a tendency to fiocculate. A further object of the perforated rolls is to produce turbulent shear fields for breaking up the fibre flocks developed. Owing to the tendency of the fibres to form flocs, particularly at increased concentrations, the fibre concentration in conventional processes must not exceed about 0.5 percent if acceptable paper is to be produced. By 0.5 percent concentration is meant a concentration of g of fibres per kg of water.

From the headbox the suspension, optimally with uniformly distributed fibres, is made to flow out through a narrow gap Slice as a nearly horizontal jet on to the wire (a more or less open metal or plastic wire cloth) advancing with approximately the same speed as the web. The thickness of the jet can vary from a few mm up to and above 50 mm. On the wire, most of the water is to be removed. Prior to the fixing of the fibres in a fibre bed, the concentration is to be increased from 0.5 percent to about 10 percent. At a jet height of 40 mm and with an original fibre concentration of 0.5 percent, thus, about 40 litres of water per square meter of the wire have to be removed. At the machine speeds now employed in high-speed machines, this water removal has to take place within a period of about 1 second. The water is removed by means of different types of dewatering units which according to the circumstances may improve or worsen the sheet formation. In any case this conventional process is very difficult to control.

At the moment of contact between the jet and the wire, the wire has approximately the same speed as the jet. The sheet formation proper, therefore, can be compared to a filtration process with sedimentation forced by the dewatering units. The sheet is built up from below in such a way that the last water remaining must be drained through substantially the entire sheet. The fibres included in the sheet have a certain size distribution always containing a greater or smaller amount of fines, i.e., fibre fragments, that are so small that they follow along with the water being drained. The retention, that is, the fraction of the fibrous material remaining on the wire, often amounts to only 50 percent or even less. This kind of water removal process imparts a certain two-sidedness to the sheet. In other words, the bottom or wire side of the sheet is lacking in fines content, while at the same time there is an excess of such material toward the upper surface of the sheet. This two-sidedness is particularly distinct when fillers such as clay are added to the suspension as is the case with certain grades of printing paper. This two-sidedness is also a characteristic of some ligno-cellulose-containing papers, for example, newsprint, where a high percentage of fines is contained in the furnish. This gives the two surfaces of the sheet different printing properties. Owing to the aforesaid analogy between the mechanism of sheet formation and filtration with sedimentation, the sheet is formed with a special two-dimensional structure. All fibres, because of their geometric shape (length l-5 mm, diameter 30-50 pm), are oriented in such a way that their main extension is essentially parallel to the plane of the sheet. The sheet, thus, can be said to be built up of a number of essentially parallel layers. This structure, of course, affects such paper properties as strength, stiffness, etc.

The above simplified description basically sets forth the usual process of sheet formation in present paper manufacturing technology. Other processes do exist, but they do not differ very much in principle from what has been described. The cellulose fibres are deposited on a wire cloth or between two wire cloths in one way or another. When the dewatering operation has proceeded far enough that the strength of the formed sheet allows it, the sheet is transferred to the press section for a further dewatering. The final dry content of the paper is obtained after drying of the paper against a number of heated cylinders.

In the Finnish Pat. No. 40,263, hereby incorporated by reference, there is disclosed a paper machine head box wherein a suspension flow is distributed by allowing it to pass through a fixed apertured plate extending crosswise in relation to the paper machine. Downstream in close proximity to this apertured plate there is a second apertured plate having the same number of apertures as the first mentioned plate but staggered with respect thereto, whereby the total cross-sectional area of the apertures in the second plate is greater than that of the first mentioned plate.

Such apparatus operates only with suspensions having a low concentration wherein the tendency of flocculation is relatively low and it should be noted that the fibres are loose when discharged from this head box as a suspension flow.

An attempt to secure uniform distribution of flow across the width of a web is illustrated by Milnes British patent specification No. 469,203 of 1937, relating to a headbox for conventional fibre concentrations. According to the Milne specification a suspension is allowed to flow through a series of baffles, which would incidentally generate low intensity turbulence, but there is no suggestion of generating intense turbulence by passing a suspension through very small openings and Milnes drawing shows large passages. Nor is flow consolidated after passage through Milnes headbox.

US Pat. No. 3,652,392 to Appel represents an attempt to distribute a suspension evenly without using distributor rolls. Appel sought to control turbulence through his inlet arrangement so that small scale turbulence would be preserved as the suspension moves on to a wire. As in other prior art headbox and inlet arrangements this Appel device relied on the belief that it is necessary to avoid excessive flocculation to form a good sheet from a very low fibre concentration suspension.

U.S. Pat. Nos. 3,514,372 and 3,216,892 relate to headbox arrangements wherein suspensions pass through passages and apertures of successively increasing areas.

These and other prior art attempts to improve web formation using conventional fibre concentrations have proposed various ways of controlling either flow distribution or turbulence or both, always basing their methods and devices on the idea that a very low fibre concentration must be employed in order to avoid excessive flocculation in the suspension and poor sheet formation.

The disadvantages of prior art techniques are eliminated by the method and apparatus of the present invention, which employ unconventionally high fibre concentrations and, instead of avoiding flocculation, put the tendency of such high concentrations to form flocs to use to form a uniform sheet.

The present invention relates to a novel method wherein the web is consolidated in the head box to a three-dimensional network structure with entrained liquid.

SUMMARY OF THE INVENTION Fibres suspended in water have, as mentioned above, the tendency of agglomerating and flocculating into local networks. This tendency is due to several factors. A factor of decisive influence is the high length-toradius ratio of the fibres, combined with the fact that the fibres have a certain stiffness under certain circumstances. It is then possible for the fibres to become fixed mechanically relative to one another and to remain in these fixed positions. This circumstance, which has caused much trouble in conventional paper manufacturing processes, is utilized in the present invention for the sheet formation. Instead of trying to prevent flocculation, flocculation is facilitated to the highest possible degree and in such a way that a continuous network develops. To develop such a network requires a sufficiently great number of fibres per unit of volume, i.e., the concentration must be high. To obtain a continuous network, all local networks with a high density must be dispersed so as to render it possible for the individual fibres to assume new locations in the continuous network. This implies that relatively high shearing forces must be generated in order to form the new network. Once the new network has been formed, i.e., when the fibres have occupied their locations, all disrupting forces have to cease as quickly as possible. It is this new network, that in a more compressed state is to form the sheet of paper. The thickness of the network depends on the desired basis weight of the paper to be made and on the fibre concentration used. With a basis weight of, for instance, 50 g/m and a fibre concentration of 5 percent, the thickness will be 1 mm.

Shear forces occur in a turbulent flow. Turbulence generally is a measure of the intensity of random velocity, the function variations occurring in a flow under certain circumstances. In turbulent flow, eddies with a certain distribution of sizes occur simultaneously in the flow field. The eddy size distribution largely depends on the geometry of the flow conduit. If the flow space is restricted the size of eddies occurring there will also be limited. The intensity of the eddies is directly proportional to the power per unit volume of the turbulence produced by the creating section of the flow, i.e., in many cases against the pressure loss within the section. The rate of damping of the turbulence depends primarily on the size of the eddies and of the viscosity and of possible strength of a fibre network in the flow (the flow energy is transformed to heat via viscous dissipation).

The term sediment concentration, abbreviated C can be defined as the final concentration in the sediment formed when fibres are allowed to settle out from a very dilute suspension. This concentration corresponds to that at which the fibres, after agitation, start to form coherent networks. Such network formation causes the formation of flocs which disturb sheet formation according to prior art techniques of paper manufacture. Accordingly paper has traditionally been formed from suspensions at consistencies ranging from approximately 0.1 to 1 percent by weight, although the consistencies can be higher when the major part of the solids content is made up of filler materials. Paper has normally been formed from suspensions wherein the concentration of fibres is below, or slightly above the sediment concentration of the fibres. Throughout this application and the appended claims the term sediment concentration shall be defined as follows:

where C, is the sediment concentration, r is the fibre radius and L is the fibre length. Thus C, for pine sulphate suspensions is 0.2 to 0.4 percent and for groundwood, 0.6 to 0.9 percent. In contrast to prior art processes operating with concentrations of about C the process and apparatus of the present invention employ suspensions having concentrations not less than twice the sediment concentration of the suspended fibres as defined. This minimum concentration of suspensions formed into webs in accordance with the present invention i.e., twice the sediment concentration is called the double sediment concentration Thus the present invention deals with concentration ranges of l.0 to 6 percent, and preferably 4 to 5 percent which is about times the concentration normally used in the art.

The length of fibres in the suspension is preferably in the range of from about l mm to about 5 mm and the width of the fibres is preferably in the range of from about 30 am to about 50 ,urn. The kinds of fibres used may vary within very wide limits from natural fibres to synthetic fibres and the like. The range of fibre lengths in suspensions treated in accordance with the invention is such that it is convenient to discuss fibre length in terms of the average length, i.e., the mean fibrous particle length.

Because of the use of suspensions having concentrations much higher than those of the prior art, considerably less water has to be removed from the web. Accordingly the loss of fines with the water removed is also reduced.

Compared with sheets manufactured by prior methods and arrangements the three-dimentional sheet manufactured according to the invention provides improved strength properties in a direction perpendicular to the plane of the sheet. The size of the apparatus according to the invention is drastically reduced compared with prior constructions and hence the expenses are remarkably reduced.

It is apparent that the intensity and scale of the turbulent flow required to dissipate all fibre flocs depends on the concentration of the suspension but the injection rate required for producing consolidated network structure is readily determinable by an operator controlling the process.

Following the apertures in which the intensity of the turbulent flow is increased and the scale decreased there is a chamber in which the flow is allowed to decay in order to form the consolidated three-dimensional network structure. This network structure is then discharged through an outlet channel which has walls diverging outwardly toward the outlet of the head box so as to prevent friction between the channel walls and the formed web from destroying the three-dimensional network structure of the web. In effect the web is formed before deposition on the wire. The purpose of this diverging inside channel is thus entirely different from that of similar channels in prior head boxes in which the fibres are still separate and not consolidated into a network as in the present invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows the forming part for the web in a sectional view from the side.

FIG. 2 is a view in section taken along the lines 11-11 of FIG. 1.

FIGS. 1a and 2a are views similar to FIGS. 1 and 2, showing another embodiment of the forming part for the web in a sectional view from the side and along the line Ila-Ila in FIG. la; looking in the direction of the arrows.

FIGS. 3 and 4 respectively show a distribution unit in a sectional view from the side and along the line IV-IV in FIG. 3.

FIG. 5 shows in a schematic way the entire sheet forming arrangement with the forming part and groups of distribution units assembled.

FIG. 6 shows an elevational view of a preferred embodiment of the invention, in which the distributing and web forming parts have been combined to a single compact unit. 7

FIG. 7 shows a section taken along line c c in FIG. 6.

FIG. 8 shows two sections taken along lines aa and b-b in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the sheet forming method according to the invention the fibre suspension is injected through closely spaced nozzles 1, see FIGS. 1 and 2, into a narrow channel 2 having a front end portion or chamber 3 the sheet forming zone of a special conformation, as appears from FIG. 1, for bringing about certain desired effects. The object to be achieved here is a turbulent flow with a certain eddy size distribution. The intensity of the eddies depends on the injection rate of the fibre suspension. The eddy size and intensity are to be balanced so that the turbulence has decayed substantially by the time the fibre suspension passes the narrow gap 4 constituting the beginning of the channel 2 proper.

The sheet forming process may be described in greater detail as follows. On injection, a large part of the kinetic energy of the fibre suspension is converted to turbulent energy appearing in the form of small eddies. Owing to the small size and high intensity of these eddies, every individual fibre is affected, thereby rendering it possible for the fibre to arrange itself into a network, the strength of which soon exceeds the shear forces of the decaying turbulent field. During the initial phase of the forming process all of the material flows in such a way that the fibres are caused to selectively arrange themselves into a network that is as homogeneous as possible. This condition obtains at high fibre concentrations. At a concentration of 5 percent there are from 100,000 to 500,000 fibres per cubic centimeter of suspension.

Having thus been formed, the network must not be subjected to any disrupting forces exceeding its strength. The network is compressed at the passage through the narrow section 4 and, owing to the stresses produced there and inherent in the forming process, the network will expand in the course of time. During passage of the network through the channel 2, the fibres may gradually start to press against the channel walls. The boundary layers often observed in the plug flow regime and which are more or less free of fibres at low concentrations, may not be capable of withstanding the mechanical pressure of the expanding network. If no special measures were taken, the frictional forces between the fibres and the walls of the channel could reach such a magnitude that the network would be partially disrupted. By giving the channel 2 a somewhat diverging conformation in the flow direction, there is some space available for expansion of the network. Owing to the decreasing flow rate in the widening channel, a further positive effect is obtained in that the network is compressed in the direction of motion. This appears to have a balancing effect on the microstructure of the final sheet. Due to the mechanical strength of the network, the flow does not become unstable. On flowing out of the channel the fibre network may be placed on the wire at an arbitrary angle to the horizontal plane according to conventional sheet forming techniques, but because of the high dry content of the network, which can be considered a web or sheet even at this stage, it is also possible in some cases to direct the jet right on to a press felt, between two felts or some such other arrangement for handling a web, for further transport of the web into a press or some other means for further dewatering.

FIGS. la and 2a show another embodiment of the forming part. By breaking down the fibre jet at least one more time after passage through the nozzles 1 and chamber 3, but before the entry of the fibres into the channel 2, an improved uniformity of the final product can be obtained. In connection with said latter breaking down step, it has proved expedient also to let the suspension change its direction of motion before, at or after it has entered the channel 2.

As in FIGS. 1 and 2, the designation 1 in FIGS. 1a and 2a refers to closely spaced nozzles feeding into the chamber 3 extending along the row of nozzles. Instead of providing a channel extending in the main direction of motion of the suspension out of the chamber 3, i.e., substantially perpendicularly to the direction of motion in the nozzles, the channel 2 according to FIGS. 1a and 2a is arranged so as to extend from the chamber 3 at an angle of substantially 90 to the general direction of motion which the suspension has when leaving the chamber 3. Between the chamber 3 and the channel 2 an enlargement a extending along the width of the channel 2 is arranged as shown in FIGS. la and 2a. The connections between the chamber 3 and the space 5a, and between said space 5a and the channel 2 are narrow sections 4 and 6a respectively.

In FIGS. la and 2a the channel 2 is shown to extend substantially in parallel with the direction of motion of the suspension in the nozzles, i.e., about 90 to the general direction of motion of the suspension in the narrow section 4. This provides a compact and, accordingly, advantageous embodiment of the forming device. The channel 2, however, may also be given another suitable direction in relation to the flow in the narrow section 4. It is possible as well, depending on the result desired with respect to the final product, to arrange additional enlargements 5a in series upstream of the channel 2.

The distribution across the machine of a suspension with a high fibre concentration poses a special problem. It is not suitable to utilize a conventional distribution system such as a tapered cross-machine distributor. The highly unstable flow conditions that prevail when the network strength largely controls the flow, as is the case when the concentration of elongated particles is high, renderes it difficult to obtain a uniform distribution. A distribution unit solving these problems is shown in FIGS. 3 and 4. The main flow is accelerated through a converging section 5 in a pipe 6 and, at a high speed, meets a wall 7 that forces the flow to assume a radial direction. Radial outlets 8, through which the suspension flows, are arranged in the wall 7. The function of this structure is, as in the forming zone 1, 3, to produce highly turbulent shear fields in order to make the material flow, so that the fibres can follow the pressure gradient as well as possible. The reference numeral 9 designates an optional inlet for fresh water that may be used when starting up.

The number of outlets 8 as well as the necessary distance between them depends primarily on the size and shape of the fibres. The fibre length affects the choice of the distance between the outlets inasmuch as the fibres should not be able to bridge the distance between two holes, thereby initiating clogging of the outlets 8. The clogging action, however, is not entirely due to one factor, but depends also on the intensity of the turbulent shear fields at the outlets. The damping process of the turbulence, thus, will determine the distance between the center and circumference of the pipe. Thus, if the distance between the outlets 8 and the radius of the pipe 6 are given, the number of outlets is also determined. The damping process can be affected by varying the flow space as mentioned before, but also here it is desirable to have a certain scale of the turbulence (eddy size distribution).

The distribution system for a large machine can be as shown in FIG. 5. In the Figure the sheet forming section is indicated uppermost by the reference numeral 10, and the distribution units shown individually in FIGS. 3 and 4 are arranged in groups indicated as group 1, group 2 and group 3. The operating conditions for the distributing units in groups 1 and 2 are not as critical as those for the final group 3. The outlet diameters are relatively large and, consequently, no tendency to clog will occur. However, the scale of the turbulence should be small to promote a uniform fibre and flow distribution even if it is not nearly as small here as in the forming section 10. The main flow enters group 1 comprising one single distribution unit with a number n, of outlets, is there distributed to group 2 (comprising n units each with n outlets) and from group 2 to group 3 (comprising 11 units each with n outlets), and so on for further distribution until the flow reaches the forming part 10. The number of distributing units are dependent, as mentioned before, on the appearance and concentration of the fibres.

For the transport of the suspension between the different groups 1-3 and finally to the forming zone 10, pieces of plastic tubing, preferably all of the same length, may be used. The important factors are that the pressure drops along the connections should be equal and an energy absorption capacity should be embodied to eliminate most of the pressure pulses that may emanate from previous components, such as screens, pumps, etc. 1 Some pertinent points concerning sheet formation at high concentration according to the method described above may be stated as follows. The sheet forming unit proper will be very small and requires little space. Its manufacture will be simple even at machine widths of about 10 m. The choice of material is not a critical factor, because of the small size of the surfaces subjected to pressure, which implies a relatively small total load. Plastics, such as, for instance, plexiglass, can be suitable. In addition to certain advantages with respect to the final product the paper probably the greatest advantage is that the method offers the possibility of radically reducing or even entirely abandoning the wire section. Since web or sheet structure is already formed, and only a small fraction of the usual water content is still present, there is nothing to prevent feeding the sheet directly into a press. By abandoning the headbox and wire section, the length of the paper machine is reduced by almost 25 percent. The cost of a paper machine is reduced even more, since the headbox and wire sections are above average in cost. The paper will have properties superior in certain respects to those of conventional paper.

One of the advantageous paper properties obtained in a sheet formed according to the invention is the existence of a more three-dimensional structure, evolved as the fibres assume their locations in the network by chance and, thus, will not necessarily be located in the plane of the web. When the network is compressed in the presses and thereafter subjected to drying, the chemical bonding strength in the paper will be assisted by a purely mechanical strength, because of the entanglement of the fibres. A strength property which is undoubtedly improved is the z-strength, i.e. the strength perpendicular to the plane of the sheet. Furthermore, an improvement of the porosity and bulk values can be expected, which also may be advantageous for certain grades of paper. In the introduction poor retention of fines in conventional formation of some grades of sheet was mentioned. In the sheet forming method according to the invention only a relatively small quantity of water has to be removed. Thus, the flow through the sheet is small and, consequently, the washing out of fines is much reduced. Thereby the extent of material transport will be reduced, which will result in improved conditions both with respect to two-sidedness and from an environmental point of view, because less fines will be discharged with the waste water. The sheet forming method according to the invention further offers possibilities of better control of the sheet formation for obtaining 21 better basis weight distribution than is possible with conventional methods.

An especially preferred embodiment of apparatus for practicing the invention is depicted in FIGS. 6-8, in which a distribution part 20 and a web forming part 21 are combined in a single compound unit head box having an inlet at one side thereof and an outlet slice 23 at the rear end thereof.

The distributing part 20 includes an elongated tubular duct 19 extending over the entire width of the head box and provided with a side having constrictions or outlets in which each outlet is offset with respect to each adjacent outlet in relation to a horizontal middle plane. Each outlet 15 opens into a co-axial circular disclike cavity 16 having a wall 17 opposite the outlet 15 and positioned at regular intervals on the periphery of the cavity 16. The apertures 18 connect the distributing part 20 with the web forming part 21 which includes a common chamber 13 divided into a lower and an upper section by the baffle 14 extending horizontally from a wall opposite the inlet to the outlet channel 12.

It is seen from FIG. 8 that the apertures 18 are arranged in two horizontal superposed rows at regular intervals from each other.

The diameter of the outlets 15 is in this special case v"? times the diameter of the apertures 18, i.e., the cross-sectional area of the outlets 15 is equal to the total cross-sectional area of the aperture 18. The diameter of the apertures 18 is at most three times the mean fibrous particle length or smaller.

The aim of the forming operation is to obtain as good a dispersion of the fibres as possible and then to let this dispersion be transformed into a continuous fibre network. The most important factor that determines the degree of dispersion that can be attained is the dispersing power (turbulent energy) per unit volume. This is analogous to the power dissipated by, for instance, a stirrer operated in the suspension.

The power dissipated per unit volume is determined by the pressure drop across and average retention time in the forming zone. This can be understood in the following way: suppose the fiow rate is q m /s, the volume of the forming is Vm and the pressure drop across this volume is p N/m". The power dissipated in the forming zone is equal to the power necessary to force the suspension through the zone. This power is pq (N/m") (m /s) (Nm/s) watt. During the time t the total energy expended is pqt. This energy is dissipated into the volume of suspension qt. Hence the average energy dis sipated per unit volume of suspension is p (=Nm/m (=ws/m It has been found, however, that this energy must be expended at a certain rate, i.e., that the power (not the energy) per unit volume is the determining factor. By similar reasoning one then obtains:

Power per unit volume P/t pq/ V.

Thus it is equivalent if the volume of the forming zone, V, is made small, the pressure drop, p, or the flow rate q is made high. This leaves considerable freedom in the design of forming zones for different purposes.

If only a minimum of dispersion is needed, as, for instance, in a forming zone for a pulp drying machine, a pressure drop, P, in the range /2 1 atm has been found adequate in connection with a total power per unit volume of 2 to 6 X 10 w/m If on the other hand very good dispersion is desired, as in a former for sack paper, the dissipated power per unit volume has been found to have to be in the range 5 to 15 X 10 w/m.

The pressure drop in the forming zone can be varied without adverse effects on the sheet characteristics. A good operating range has been found to be 3 to 12 X lO Nm As has been pointed out above, however, the dimensioning of the apparatus can be varied considerably as long as the above parameters are kept within the desired ranges.

As already mentioned, the invention has been described here only by way of one example. The distribution units, for example, may be designed in a different way, they may be directly assembled with the forming zone, and so on, and the nozzles to the forming zone may be arranged in several rows alongside each other. The channel 2, furthermore, may have different lengths, depending on the desired effect, and the nozzles 1 may have different dimentions.

Numerous modifications, substitutions and variations of the method and apparatus of the invention will suggest themselves to those skilled in the art, within the spirit and scope of the invention. What is disclosed is a method and apparatus for high consistency formation of webs of fibrous particles by effecting a high pressure drop across a very small volume and then causing a consolidated fibre network to form.

What is claimed is:

1. An improved method of manufacturing a continuous material web of elongated fibrous particles including preparing a suspension of the particles in a liquid with the concentration of the fibrous particles in suspension being about 1.5 percent to 6.0 percent of the total weight of the liquid plus the fibrous particles; then forcing the suspension through a series of constrictions and deflecting the flow of the suspension after each constriction to create a turbulent flow ensuring uniform distribution over the web; and finally decaying the turbulent flow before deposition of the material and comprising the steps of:

gradually decreasing the scale and increasing the intensity of the turbulent flow in the series of constrictions by distributing the flow from one constriction over a plurality of subsequent constrictions of smaller diameter and said one constriction, the smallest constrictions of said series having a diameter not exceeding three times a mean particle length of said fibrous particles; and

eliminating all disrupting forces after the flow has passed through the smallest constrictions to transform the turbulent flow into a flow of a consolidated three-dimensional network structure of fibrous particles with entrained liquid before deposition thereof.

2. The improved method of claim 1, wherein the flow rate of the network structure is successively decreased to the speed of the deposited web.

3. The improved method of claim 1, comprising preparing the suspension with a fibre concentration of 4-5 percent by weight.

4. The improved method of claim 1 wherein said smallest constrictions have a diameter not exceeding about 15 mm.

5. An improved apparatus for the manufacture of a continuous material web of fibrous particles having a length of about 1 to 5 mm and of the type having a base; a support on the base; a headbox attached to the support and comprising a transversal apertured front wall and an inside chamber for decaying a turbulent flow before deposition thereof; means connected to the headbox for distributing a suspension of the particles in a liquid over the entire width of the headbox; means connected to the distributing means for preparing the suspension; and means for transporting the suspension to the distributing means, the improvement comprising: inside the headbox at least one other aperture wall in close proximity to the apertured front wall providing points of impingement for the turbulent flow forced through apertures to deflect the flow, the apertures in said other wall being smaller than and displaced with respect to the apertures of said front wall so that each point of impingement is surrounded by a plurality of apertures of less diameter than the next preceding aperture so as to successively decrease the scale and increase the intensity of the turbulent flow and the apertures of the last of said walls having a diameter not exceeding the threefold mean particle length; and baffles behind the last apertured wall defining a chamber wherein the turbulent flow is deflected before it is passed into an outlet channel.

6. The improved apparatus of claim 5, wherein the total area of the apertures in the front wall is equal to the total area of the smaller diameter apertures in said other apertured wall.

7. The improved apparatus of claim 5, in which the cross-sectional area of the outlet channel is increased in the flow direction to decrease the velocity of the network structure flow to the speed of the deposited web.

8. The improved apparatus of claim 5, further comprising means in the distributing means for creating turbulence therein and transforming the suspension flow into a turbulent flow before it is passed into the headbox through the apertured front wall.

9. The improved apparatus of claim 5, in which the smallest dimension of the outlet channel perpendicular to the flow direction does not exceed twice the mean particle length of the fibrous particles.

10. In an apparatus for the manufacture of a continuous material web of fibrous particles the combination of:

a series of units for dividing a suspension flow of the particles in a liquid into a plurality of turbulent flows with successively reduced scale and increased intensity, each unit comprising a tubular chamber with an inlet at one end thereof; a striking surface at the opposite end; a plurality of smaller diameter peripheral outlets around the striking surface; and inside the tubular chamber between the inlet and the outlets a constricted portion to increase the velocity of the turbulent flow before it impinges onto the striking surface;

a headbox comprising a chamber having at least one transversal row of inlets for introducing the turbulent flow from the series of units into the chamber over the entire width thereof; a striking surface for deflecting the incoming flow; an enlarged portion for decaying the flow to produce a flow of a threedimensional network structure of the particles with entrained liquid; and an outlet channel, the chamber having such dimensions as to prevent any body having a diameter exceeding the threefold mean particle length from passing therethrough;

means connecting the outlets of the units to the inlet of one next unit; and

means connecting each outlet of the last units to one inlet of the transversal row of inlets.

11. The apparatus of claim 10 in which the headbox chamber comprises two superposed rows of inlets and a baffle extending therebetween from the wall of the chamber opposite the outlet channel to provide two opposite striking surfaces for particles passing through the inlets, one striking surface for each row of inlets.

12. The apparatus of claim 10, in which the inlets of one row are displaced in relation to the inlets in the other row.

13. The apparatus of claim 10, wherein the total cross-sectional area of the outlets of each unit is equal to the cross-sectional area of the inlet of the same unit.

Claims (13)

1. AN IMPROVED METHOD OF MANUFACTUREING A CONTINUOUS MATERIAL WEB OF ELONGATED FIBROUS PARTICLES INCLUDING PREPARING A SUSPENSION OF THE PARTICLES IN A LIQUID WITH THE CONCENTRATION OF THE FIBROUS PARTICLES IN SUSPENSION BEING ABOUT 1.5 PERCENT TO 6.0 PERCENT OF THE TOTAL WEIGHT OF THE LIQUID PLUS THE FIBROUS PARTICLES; THEN FORCING THE SUSPENSION THROUGH A SERIES OF CONSTRICTIONS AND DEFLECTING THE FLOW OF THE SUSPENSION AFTER EACH CONSTRICTION TO CREATE A TURBULENT FLOW ENSURING UNIFORM DISTRIBUTION OVER THE WEB; AND FINALLY DECAYING THE TURBULENT FLOW BEFORE DEPOSITION OF THE MATERIAL AND COMPRISING THE STEPS OF: GRADUALLY DECREASING THE SCALE AND INCREASING THE INTENSITY OF THE TURBULENT FLOW IN THE SERIES OF CONSTRICTIONS BY DISTRIBUTING THE FLOW FROM ONE CONSTRICTION OVER A PLURALITY OF SUBSEQUENT CONSTRICTIONS OF SMALLER DIAMETER AND SAID ONE CONSTRICTION, THE SMALLEST CO*STRICTIONS OF SAID SERIES HAVING A DIAMETER NOT EXCEEDING THREE TIMES A MEAN PARTICLE LENGTH OF SAID FIBROUS PARTICLES; AND ELIMINATING ALL DISRUPTING FORCES AFTER THE FLOW HAS PASSED THROUGH THE SMALLEST CONSTRICTIONS TO TRANSFORM THE TURBULENT FLOW INTO A FLOW OF A CONSOLIDATED THREE-DIMENSIONAL NETWORK STRUCTURE OF FIBROUS PARTICLES WITH ENTRAINED LIQUID BEFORE DEPOSITION THEREOF.

1. An improved method of manufacturing a continuous material web of elongated fibrous particles including preparing a suspension of the particles in a liquid with the concentration of the fibrous particles in suspension beiNg about 1.5 percent to 6.0 percent of the total weight of the liquid plus the fibrous particles; then forcing the suspension through a series of constrictions and deflecting the flow of the suspension after each constriction to create a turbulent flow ensuring uniform distribution over the web; and finally decaying the turbulent flow before deposition of the material and comprising the steps of: gradually decreasing the scale and increasing the intensity of the turbulent flow in the series of constrictions by distributing the flow from one constriction over a plurality of subsequent constrictions of smaller diameter and said one constriction, the smallest constrictions of said series having a diameter not exceeding three times a mean particle length of said fibrous particles; and eliminating all disrupting forces after the flow has passed through the smallest constrictions to transform the turbulent flow into a flow of a consolidated three-dimensional network structure of fibrous particles with entrained liquid before deposition thereof.

2. The improved method of claim 1, wherein the flow rate of the network structure is successively decreased to the speed of the deposited web.

3. The improved method of claim 1, comprising preparing the suspension with a fibre concentration of 4-5 percent by weight.

4. The improved method of claim 1 wherein said smallest constrictions have a diameter not exceeding about 15 mm.

5. An improved apparatus for the manufacture of a continuous material web of fibrous particles having a length of about 1 to 5 mm and of the type having a base; a support on the base; a headbox attached to the support and comprising a transversal apertured front wall and an inside chamber for decaying a turbulent flow before deposition thereof; means connected to the headbox for distributing a suspension of the particles in a liquid over the entire width of the headbox; means connected to the distributing means for preparing the suspension; and means for transporting the suspension to the distributing means, the improvement comprising: inside the headbox at least one other aperture wall in close proximity to the apertured front wall providing points of impingement for the turbulent flow forced through apertures to deflect the flow, the apertures in said other wall being smaller than and displaced with respect to the apertures of said front wall so that each point of impingement is surrounded by a plurality of apertures of less diameter than the next preceding aperture so as to successively decrease the scale and increase the intensity of the turbulent flow and the apertures of the last of said walls having a diameter not exceeding the threefold mean particle length; and baffles behind the last apertured wall defining a chamber wherein the turbulent flow is deflected before it is passed into an outlet channel.

6. The improved apparatus of claim 5, wherein the total area of the apertures in the front wall is equal to the total area of the smaller diameter apertures in said other apertured wall.

7. The improved apparatus of claim 5, in which the cross-sectional area of the outlet channel is increased in the flow direction to decrease the velocity of the network structure flow to the speed of the deposited web.

8. The improved apparatus of claim 5, further comprising means in the distributing means for creating turbulence therein and transforming the suspension flow into a turbulent flow before it is passed into the headbox through the apertured front wall.

9. The improved apparatus of claim 5, in which the smallest dimension of the outlet channel perpendicular to the flow direction does not exceed twice the mean particle length of the fibrous particles.

10. In an apparatus for the manufacture of a continuous material web of fibrous particles the combination of: a series of units for dividing a suspension flow of the particles in a liquid into a plurality of turbulent flows with succeSsively reduced scale and increased intensity, each unit comprising a tubular chamber with an inlet at one end thereof; a striking surface at the opposite end; a plurality of smaller diameter peripheral outlets around the striking surface; and inside the tubular chamber between the inlet and the outlets a constricted portion to increase the velocity of the turbulent flow before it impinges onto the striking surface; a headbox comprising a chamber having at least one transversal row of inlets for introducing the turbulent flow from the series of units into the chamber over the entire width thereof; a striking surface for deflecting the incoming flow; an enlarged portion for decaying the flow to produce a flow of a three-dimensional network structure of the particles with entrained liquid; and an outlet channel, the chamber having such dimensions as to prevent any body having a diameter exceeding the threefold mean particle length from passing therethrough; means connecting the outlets of the units to the inlet of one next unit; and means connecting each outlet of the last units to one inlet of the transversal row of inlets.

11. The apparatus of claim 10 in which the headbox chamber comprises two superposed rows of inlets and a baffle extending therebetween from the wall of the chamber opposite the outlet channel to provide two opposite striking surfaces for particles passing through the inlets, one striking surface for each row of inlets.

12. The apparatus of claim 10, in which the inlets of one row are displaced in relation to the inlets in the other row.

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