The present invention relates to paper-manufacturing machines and to methods for operating such machines.
In particular, the present invention relates to twin-wire paper machines and methods for operating the same.
Machines of this latter type conventionally include a pair of endless wires which respectively extend along closed loops, the wires having common run portions between which the web is compressed and formed while travelling from an inlet end to an outlet end of the common run portions.
The most commonly used type of paper-manufacturing machine is the fourdrinier machine which has a single wire guided so as to have an upper horizontal planar portion on which the web is formed. The forming of a continuous paper web in such a machine takes place on the substantially horizontal wire while the pulp slurry, or paper-fiber stock, is fed onto the wire from a headbox in the form of a dilute suspension in which water is present in a quantity of approximately 100-400 times the quantity of fibers. The water which is present in the fiber suspension is removed, or filtered off, through the wire in one direction, namely downwardly, partly due to gravity and partly by means of suction created by units which support the wire. The pulp stock is supplied to the fourdrinier from the headbox which has at a lower part adjacent the wire a narrow slice the length of which equals the width of the wire. From the slice the fiber suspension is discharged onto the wire in the form of a relatively coherent jet. The headbox is located at the starting end of the upper horizontal run of the wire and for the most part above the level of the wire where the web is formed.
The slice of the headbox is formed between upper and lower lips, and generally the upper lip is adjustable with respect to the lower lip so that it is possible in this way to regulate the angle at which the stock jet is directed toward the wire. The velocity of the stock jet with respect to the speed of the wire is also adjustable. Also a forming board which is customarily located beneath the wire between the breast roll and the first table roll is adjusted to determine the manner in which dewatering takes place during the initial stage of web formation, i.e. through a length of 50-150 cm subsequent to the slice.
In recent years there have been developments in connection with twin-wire paper machines which differ from fourdriniers in that the web is formed entirely between a pair of wires with dewatering taking place simultaneously in two opposed directions through both of the wires. This type of machine and principle of operation adds considerably to the dewatering rate. With such machines the area in which formation of the web takes place is generally curved with its direction deviating considerably from a horizontal plane. As a result of its efficient dewatering action, the twin-wire type of machine is considerably shorter than fourdrinier machines.
With known twin-wire machines the pulp stock is supplied by directing the stock jet from the headbox to the junction of the two wires at the inlet end of the common run portions thereof so that the stock jet enters at a converging entrance region where the wires converge toward each other to form a throat leading to the inlet end of the common run portions of the wires. The direction of the jet is maintained symmetrical with respect to the converging portions of the wires which travel toward the inlet end of the common run portions thereof, with the result that dewatering takes place rapidly in two opposed directions through both wires.
While the above mode of feeding of the stock is without doubt efficient with respect to dewatering, nevertheless it involves, as compared with a fourdrinier, certain drawbacks as pointed out below.
In order to better understand the present invention and the problems solved thereby, certain theoretical factors in connection with formation of a paper web on a fourdrinier wire section are reviewed.
The paper produced on a paper machine always has a certain lack of homogeneity either with respect to properties which are visually observed or, for example, with respect to the strength characteristics of the paper. This lack of homogeneity partly results from the fact that the paper fibers are relatively long as compared with their thickness. As a result the fibers suspended in water become entangled, forming aggregates known as flocs which are visible as relatively dark spots in the finished paper when the latter is viewed against light. The presence of the flocs in paper is undesirable. Also as a result of the fact that the fibers are relatively long as compared to their thickness, the pulp suspension flows through the slice in such a way that the fibers tend to become oriented in the direction of flow and thus these fibers tend to remain oriented in the formed web longitudinally with respect to the long axis of the paper machine, which is to say the fibers tend to extend in the direction of web travel. This particular orientation of the fibers has a large influence on the properties of the strength of the paper as well as on the behavior of the paper with regard to water and water vapor.
Paper which has its fibers oriented in a pronounced manner in the direction of web travel has a much higher tensile strength longitudinally, or in the machine direction, than in the cross-machine or transverse direction. In some cases such a high strength in the longitudinal direction is desirable, as when manufacturing spinning paper from which paper string is made. However, in most cases it is of advantage to maintain the tensile ratio of the paper, which is the ratio of the strengths thereof longitudinally and transversely, as low as possible.
The fiber orientation becomes clearly apparent from the behavior of the paper with respect to water and water vapor. When dry paper is transported, for example, into a humid storage area, it begins to curl as a result of the hygroscopic nature of the fibers as well as to the fact that the fibers swell to a greater extent transversely, as compared to any longitudinal swelling. Thus, where the paper has been manufactured in such a way that the fibers are primarily oriented in the direction of web travel, expansion due to humid conditions takes place in a much more pronounced manner transversely of the web, when considered with respect to the direction of machine travel, so that curling undesirably occurs.
It is therefore apparent that paper may be nonhomogeneous in two different ways. Flocs, which are created in the suspension and present in the paper formation, are immediately apparent to the eye. Anisotropy, or the difference in the characteristics of the paper longitudinally as compared with its characteristics transversely, is a phenomenon which may become visible under certain circumstances as when the paper curls in the manner described above. For the most part, however, this anisotropy is demonstrable by laboratory tests such as tests for determining strength properties. It is possible to influence both the presence of flocs as well as web formation and anisotropy by controlling the interaction between the headbox and the wire which receives the stock jet therefrom. The most important variables which should be controlled in this connection are:
1. the relationship between the velocity of discharge of the stock jet and the speed of the wire, PA1 2. the angle with which the stock jet impinges on the wire, and PA1 3. the rate at which water is removed from the suspension which is deposited on the wire.
These factors affect not only the orientation of the fibers but also the web formation. In addition it is to be noted that if it is desired to reduce the presence of flocs and improve web formation, it is necessary to operate under conditions which will result in relatively pronounced fiber orientation. The paper-maker must choose operating conditions at the paper machine which will provide a compromise between those factors which will reduce fiber orientation and those factors which will improve web formation.
By way of example it should be noted that as a rule fiber orientation is least pronounced when the wire speed approximately equals the velocity with which the stock jet is discharged onto the wire. On the other hand, the best possible web formation requires the wire to travel at a speed somewhat higher than the jet velocity.
In addition, it is usually advisable, in order to prevent pronounced fiber orientation, to provide for removal of water from the deposited fiber suspension at a comparatively slow rate during the first few meters of web formation immediately subsequent to issue of the jet from the headbox. The dewatering process is influenced, for example, by regulating the angle at which the stock jet impinges on the wire. The smaller this angle the slower the passage of water through the wire. The magnitude of this angle can be changed most simply by adjusting the position with respect to each other of the lips of the headbox which define the slice thereof.
However, slow dewatering impairs web formation in many cases.
Thus, there are no generally valid rules with respect to the influence of the above process variables on the characteristics of the paper. Optimization of paper machine operations depends, for example, on the fiber material which is utilized, the consistency of stock, and the machine speed, as well as, naturally, on the type and basis weight of the paper which is manufactured.
The achievement of the best results requires extensive experimentation and a paper machine which has the greatest possible versatility in the adjustment of the components thereof.
While it is indeed possible with a fourdrinier to influence the fiber orientation during the initial 50-150 cm region of web formation on the wire beyond the slice of the headbox in the direction of web travel, such control of the fiber arrangement is not possible with twin-wire machines where it has been noted from observation and experience that when the pulp jet is directed into the throat to the inlet end of the common wire portions the arrangement and orientation of the fibers in the web which is produced is the same as in the jet itself. Further influence on fiber orientation is no longer possible with a twin-wire machine.
Of course, it is recognized that many headbox designs presently used in twin wire machines deliver the pulp suspension into the gap between the wires in such condition that there is no particular fiber orientation in the jet and as a result the web which is formed has no pronounced fiber orientation. Nevertheless, as has been indicated above, there are certain types of paper which in view of their intended use should have a predetermined fiber orientation. For example orientation of the fibers longitudinally, in the direction of web travel, is highly favorable for certain intended uses of paper. In principle, orientation of the fibers may be achieved by discharging the stock from the slice at a speed smaller than the speed of travel of the wire on which the web is formed, in which case the combing effect of the wire which occurs at the start of the web formation results in fiber orientation. However with twin-wire machines no such combing effect can be produced because the arrangement of the fibers in the stock jet is "frozen" between the wires.
Furthermore, with conventional twin-wire machines it is not possible to adjust to a desired extent the angle at which the stock jet impinges on the wire or wires, so that as a result of this latter deficiency one possible way of controlling web structure is lost.