Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
1999-01-14
2001-01-23
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S272000, C162S336000, C162S347000
Reexamination Certificate
active
06176975
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a headbox in a paper machine, and more particularly relates to headboxes which are positionally adjustable.
BACKGROUND OF THE INVENTION
If a paper web is to be well formed a good grammage distribution must be achieved, by which is meant a uniform distribution of fibers in the paper. Good formation improves drainage, pressing and drying and makes creping less prone to problems, while at the same time the paper becomes stronger and softer. The function of a headbox is thus to distribute the fiber suspension, the stock, in such a manner that said good formation is obtained. This is achieved by a jet of stock being delivered from the headbox through a stock outlet, the discharge opening, formed by upper and lower lips, so that the jet of stock encounters a forming wire in a forming zone. The paper web is then formed by water being drained out through the wire so that the layer of stock is gradually thickened to a continuous fiber network. To obtain a correct sheet structure with varying jet thickness, web speed or type of stock it is therefore of great significance that the stock is delivered in a manner carefully determined for a particular production since the drainage, and thus the formation, is greatly affected by where and how the stock jet encounters the wire, i.e., the angle of impact, the distance from the discharge outlet and the speed at which the stock jet encounters the forming wire.
To enable desired alteration of the point of encounter of the stock jet, its length and angle of impact, the headbox and its stock outlet must be adjustable. The thickness of the stock jet is regulated by means of setting devices which set the geometry of the lip opening. When the headbox is installed after exchanging, service, etc., it must be set in a position and aligned in relation to the forming zone so that said three parameters are correct.
The headboxes of soft paper machines do not differ in basic principle from other boxes, the box type depending on the speed range. Even at a low speed, i.e., above 300 m/min., closed hydraulic headboxes are used nowadays, which give a more stable profile and better formation. The speed into the hydraulic headbox is chosen so that good fiber distribution is obtained, and suitable turbulence intensity in the stock without unnecessary pressure losses. However, this requires the hydraulic headbox to be run with the flow for which it was dimensioned since a lower flow rate will give poorer fiber distribution and a poorer grammage profile. If the hydraulic headbox must be changed because a new optimal flow requires it, it must be possible to adjust the stock jet quickly and simply to a new position for the new hydraulic headbox.
Twin wire machines in which dewatering occurs between the twin wires have been known since the 70s. In essence this type of wire machine comprises a breast roll and a forming roll which rolls, together with said wires, form an inlet nip through which both wires then run together along almost 180° of the circumference of the forming roll, forming outer and inner wires. Only hydraulic headboxes are used as headboxes for twin wire machines and these hydraulic headboxes must be constructed so that they can be inserted into the nip between the wires since the free stock jet should not exceed a maximum length of about 250 mm, preferably about 100 mm. This is important so that the jet, which may have a speed of 1800-2000 m/min., is not broken up by turbulence before it encounters the forming wire. Even after deaeration, the stock jet contains undesired pressurized air bubbles which increase the risk of said turbulence arising, particularly at high machine speeds, since the air bubbles in the stock jet quickly expand when they emerge from the hydraulic headbox, thus disintegrating and deforming said jet. The lips are of approximately equal length, which contributes to facilitating adjustment of the stock outlet when the position of the headbox must be adjusted or re-set. Even so, said adjustment constitutes extremely complicated and time-consuming work due to the lack of space in said inlet nip and the hydraulic headbox must therefore first be adjusted roughly before any fine adjustment of the discharge opening can be carried out with the aid of setting devices.
The hydraulic headbox is placed close to the forming roll, just before the inlet nip, with its angle of impact and alignment carefully set. Since a very large part of the dewatering process occurs in the inlet nip, what happens there is extremely important to the forming of the paper.
The best formation is obtained if about 30-40% of the stock jet lies below the tangent to the forming roll in the direction of the jet so that part of the stock jet encounters the forming roll. This results in a pressure surge at the impact point which can, however, be regulated by lifting the jet so that its lower surface only touches the forming roll at a tangent. The more of the stock jet lying below the tangent to the forming roll the greater will be the pressure surge because more of the jet encounters said forming roll. The best position for the stock jet, and thus for the stock jet at different web speeds, stock concentrations and grades of paper with different requirements as to what constitutes an acceptable formation, must therefore be determined by trial and error. The setting device for the hydraulic headbox must also be so stable that the relation of the stock jet to the forming zone is not altered.
Several designs of twin wire machines are currently available, which differ from each other with regard to location of the hydraulic headbox, inclination of the nip, etc.
Depending on the design of the wet section, the stock jet can be directed downwardly, horizontally or upwardly. The wire mesh contains air which must be removed. An upwardly directed jet gives the best deaeration and the least risk of liquid splashes, but also has the drawback of more complicated dewatering. If, instead, the stock jet encounters the forming wire in an inclined, downwardly directed nip, while the wires are running upwardly over the forming roll, the dewatering takes place substantially downwards. All three alternatives are used with success.
If the stock jet is directed towards a desired point of impact it must be re-set or adjusted when trimming or regrinding the roll, after a roll exchange or wire replacement, etc. When replacing the wire, for instance, the headbox must be moved and with the bulky adjustment devices available today, this takes far too long. The hydraulic headbox must also be accessible for inspection and cleaning and must therefore be easily accessible so that it can be opened. A simple adjustment device, allowing all the above-mentioned configurations to be set, as well as efficient and quicker resetting after roll or wire replacement would be extremely profitable.
The machine manufacturers also at present build hydraulic headboxes capable of spraying two or three stock jets simultaneously into the nip between two wires, the individual jets being kept apart with the aid of wedges of air right up to the point of impact, and the length of the stock jets, i.e., the setting of the distance between the hydraulic headbox and the inlet nip, acquires increased significance.
The hydrodynamics become extremely important at the high speeds applicable for modem soft paper machines. Considerable demands are placed, for instance, on the hydraulic headbox being able to supply a stock jet having such well suppressed turbulence that it stays together until it reaches the nip between the wires. The direction and point of impact in this nip are then critical. Due to the cramped space between the inlet nip and the hydraulic headbox, adjustment of a new angle of impact while retaining the length of the jet may be extremely complicated. With the link system used today these small changes constitute a great problem since such small calibrations are not easy to perform without considerable risk of the hydraulic headbox coming into contact with and damagi
Andersson Tommy Goran
Haraldsson Nils Ulf
Svensson Ernst Tomas
Alston & Bird LLP
Hastings Karen M.
Valmet-Karlstad AB
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