The goal of the forming section is to produce a continuous wet paper web of a certain basis weight and of the required uniform quality parameters in both cross machine (CD) and machine directions (MD). This is accomplished by
. • feeding a constant volume rate of suspension of constant consistency and fur¬nish ratio to the headbox (approach flow system)
. • equally distributing the suspension in the cross direction of the paper machine, accelerating it and transferring a suspension jet of high uniformity to the forma¬tion section (headbox)
. • dewatering the suspension and hence forming an endless web (wire section).
Approach Flow System
The approach flow system is the connection between stock preparation and the headbox of the paper machine. It is responsible for metering and mixing the dif¬ferent stock components, diluting and blending them with other components like fillers, chemicals and additives and after possibly cleaning, deaeration and screen¬ing, finally feeding them to the headbox
6.4 Forming Section
. • constant metering/proportioning of the components, particularly of thick stock and white water
. • uniform, effective mixing of all suspension components, particularly of thick stock and white water
. • constant feeding of the suspension to the headbox.
Final cleaning and screening of the suspension are applied in many cases to pre¬vent wear or damages in the paper machine, for instance clothings, foils, rolls etc., and to improve the final paper quality with respect to cleanliness. In some installa¬tions, for instance for board and packaging production, part of the stock prepara¬tion tasks are, for economic reasons, taken over by the approach flow system. Deaeration devices are also often installed for runnability and quality purposes. Special engineering and automation measures support the necessary high con¬stancy of the suspension fed to the headbox. The main process stages of an ap¬proach flow system for modern high speed paper machines are shown in Fig. 6.27. In the following the individual stages of an approach flow system and its equip¬ment are described in detail.
184.108.40.206 Metering/Proportioning and Mixing
First the various furnish components (e. g. fiber stock and fillers) including stock from fiber recovery and broke treatment systems are metered and mixed in the desired proportions. Apart from the required solids ratio of the individual compo¬nents, constant total stock consistency which usually lies in the range of 3 to 4 % must be ensured. Therefore a constant consistency of each stock component is an issue. Subsequently the mixed stock is diluted with white water I to a consistency of 0.1 to 1.5 % (depending on the paper grade and machinery). Ideally the suspen¬sion will now have a constant consistency and composition. All variations in con¬sistency will result in basis weight deviations in the paper web of the same order of magnitude.
In the past, big buffers, i. e. stock chests and water silos, with agitating devices were used in order to fulfill the demands on constancy. In modern machines the
volumetric throughput to the headbox can be about 1.5 m3 sor more. This means that the necessary buffer sizes would require huge investments and ex¬cellent mixing. Deaeration in the tank is also more difficult as the air bubbles have to rise upwards against the suspension flow direction. As the demands on paper machine efficiency are steadily increasing, grade changes consequently should be as quick as possible. This cannot be achieved with big buffer volumes resulting in long deadtimes. Therefore the metering and mixing of different stocks as well as the metering and mixing of stock and water is crucial in order to avoid large tanks and to ensure constant suspension feeding at the same time [2–5].
220.127.116.11.1 Mixing of Stock Components
In modern approach flow systems the conventional sequence of mixing chest and machine chest are either minimized in volume or partly replaced by special mixing devices. These may consist of a hydraulic mixer followed by a chest to ensure the complete mixing of all stock components (Fig. 6.28). Here the different furnish components are fed tangentially into the mixing pipe in the order of dewatering behavior and filler content: the component with the best dewatering characteristics being fed first, so it can be also used as sweetener stock for the saveall unit in the
6.4 Forming Section
case of a disc filter installation (see Section 5.4.1.). The remaining small con¬sistency variations may be further reduced in the following mixing chest [2–5].
18.104.22.168.2 Mixing of Stock and White Water
The huge volumes of the white water silos can lead to microbiological pollution and also result in long process deadtimes. The silos therefore have been re-engi-neered and replaced by stock-water mixers, as shown in Fig. 6.28. One task of these mixers is to re-combine backflows e. g. from deaeration, cleaning and screen¬ing, by adding them to the white water I before the thick stock injection in the vertical collector. The kinetic energy of the back flows is used to pre-mix the streams. The thick stock is injected in the center at the lower part of the mixer. Good mixing of the thick stock is provided by optimum velocity difference between thick stock and white water at injection [2–6].
22.214.171.124 Final Cleaning and Screening
Depending on the process design of the preceding fiber preparation plant, the final cleaning and screening in the approach flow system may only have a “police func¬tion”. In this case the stock preparation system includes sufficient cleaning and screening capacity. On the other hand, especially in packaging paper mills, for cost reasons screening and cleaning are often partly or completely done in the ap¬proach flow system.
126.96.36.199.1 Final Cleaning
Cleaning is done after thick stock dilution with hydrocyclones (see Section 4.2.4) in a multistage cascade system in order to remove small dense debris. The debris can be sand, grit, shives, pitch or other dense particles. In the case of coated paper production, the end-stage of the cleaner cascade is very rich in pigments or filler. Due to the separation principle of the cleaners, especially coarse filler particles and agglomerated pigments from coated broke are rejected. The amount of rejects can be reduced by recovering these minerals .
188.8.131.52.2 Final Screening
Even if all stock components including broke were previously fine-screened, bun¬dles and lumps can be created thereafter by deposition. Secondary stickies and pitch particles may also form in the paper machine system. Final screening is therefore done directly before the headbox with pressure screens (see Section 4.2.3). In most cases the screen baskets are slotted. Due to their position in the process, the screens exhibit special characteristics, i. e. very low pulsation genera¬tion by using special rotary elements, polished surfaces to avoid deposits, high availability by simple design and special design to prevent air pockets .
Air in the system may cause problems during paper production. It can lead to reduced machine runnability by reducing the dewatering capacity and sheet breaks, to decreased pumping and screening efficiencies, to foam problems and subsequently accumulation of hydrophobic and sticky material as well as to in¬creased microbiological activity. Paper quality can also be negatively affected by poor formation, by pinholes and by dirt inclusions. As air may lead to cavitation and pressure and flow velocity variations, poor basis weight profiles can result. Air intake in water and stock happens through splashing in the forming unit of the paper machine, in the white water tray and in the chests.
Deaeration usually refers to physical (vacuum) treatment of the stock suspen¬sion after dilution and/or of the dilution water (white water I). It is mainly applied in high speed paper machines as well as for fine paper production. The vacuum treatment takes place in deaeration tanks, like the one shown in Fig. 6.29. Here air is desorbed when operating at the boiling point, and is effectively driven out by creating a large surface while impinging the suspension onto the interior surface of the tank. A sufficiently high vacuum is used so that boiling can take place without the need to heat the suspension. The minimum required vacuum there¬fore depends on the feed temperature. For a feed temperature of 50 °C, the vac¬uum in the deaeration tank is about 0.87 bar. The overflow of the deaeration tank enables hydaulic decoupling of the system, so that any pump pulsations from the first (i. e. cleaner feeding) fan pump do not affect the down-stream system [1, 2, 4, 5]. Another possibility for physical deaeration is to use special centrifugal degass¬ing pumps where, in a rotating chamber, the gas bubbles are removed from the stock suspension under the action of centrifugal forces and are eliminated from the system . Chemical deaeration with defoaming agents is also possible (see Section 3.7.3).
6.4 Forming Section
A good performance of the approach flow system is not only due to its components but also to a large extent due to the system design as a whole. For instance the layout of piping and chests has great influence on the stability of the system. This means for instance that all pipes should have a positive gradient in the flow direc¬tion, all flows should have a defined flow direction, and suspension velocities in pipes should be high enough in order to prevent de-mixing, build-up of bacterial slime or fiber stringing. In many cases polished surfaces are chosen in order to prevent deposits. Dosage positions (e. g. when shear sensitive and/or different chemicals are used) and dosing techniques (for instance multiple injectors) for chemicals have to ensure good and fast mixing and high efficiency of the chem¬icals. [1, 2] 184.108.40.206 Automation
Automation by fast, stable and accurate control loops for consistency, flow, pres¬sure and level is elementary for providing the necessary constancy in the approach flow system. Variations in pressure for example will mainly directly influence the MD profile whilst stock consistency deviations will affect both CD and MD basis weight profiles. In addition the retention on the paper machine and the chemical conditions of the water systems must be kept constant. Here a retention control loop is standard in many applications keeping the white water consistency con¬stant by adapting the quantity of retention agent added. Controls for filler, color, air content, cationic demand or zeta potential are also available today. Combining the different controls to a total control concept will lead from purely functional con¬trols to control systems which also address quality and production issues.
The goal of the headbox is to equally distribute the suspension in the cross direc¬tion of the paper machine and to supply a suspension jet of high uniformity and about machine speed to the wire section. Here dewatering of the suspension takes place. The high uniformity of the jet exiting the headbox has to be ensured by adequate distributor and nozzle layout, design and manufacture. The required high uniformity relates to equal velocity and thickness as well as to equal direction of the jet, both over the whole machine width (CD) and over time. Thus the head-box is a key element in the paper machine defining many important quality param¬eters of the finished paper.
The suspension is delivered from the approach flow system through piping that has a circular cross section. The flow must be turned and distributed uniformly and with great accuracy across the full width of the headbox. It must then flow out of a narrow slit (normally called a slice, 6–25 mm in height, and more in certain applications) that can be more than 10 m wide. In modern headboxes, this uni¬form distribution across the width is achieved by passing the suspension through a distributor on the backside of the headbox to at least one perforated plate. The many individual streams thus produced are reunited to form a single uniform stream in the nozzle, which then provides a constant jet of suspension. Essential requirements for good sheet formation are the breaking of fiber flocs that have already formed, and the prevention of flocculation, at least for a short time until the suspension is transferred to the wire. For this purpose, turbulent shear forces are generated in the suspension. The turbulence intensity should be adjusted to the stock type, and the wavelength (i. e., effective length) should be short. In mod¬ern headboxes (high-turbulence headboxes) turbulence is produced by friction in tubes and channels, or by changes in cross section as carried out in step diffu¬sors.
In the headbox nozzle, the suspension is accelerated to approximately machine speed. The thickness of the jet and, therefore, the amount of suspension is usually adjusted by swinging the upper wall of the nozzle. The slice at the nozzle outlet is often limited by a bar, which can be adjusted to an accuracy of ca. 1/1000 mm by means of spindles. This bar can be adjusted locally across the width; this has been used or is still used in older headboxes to compensate for deviations in the cross machine weight profile of the web. The direction of the jet in the machine direc¬tion can be influenced by horizontally shifting the upper wall of the nozzle. In this way, the point and angle of impingement of the jet on the wire can be adjusted (Fig. 6.30).
A wide variety of headbox designs is found in the paper industry. This is due to the many different paper machine designs and forming sections resulting from individual production requirements of the various grades. Figure 6.31 shows some examples of earlier as well as more recent headbox designs for different applica¬tions. The pond slice headbox (A) is open and no longer meets the modern de¬
6.4 Forming Section
Fig. 6.31 Schematics of various headbox types:
A Principle of a pond slice headbox,
B principle of a rectifier roll headbox with large air
C rectifier roll headbox (modern design),
D Fourdrinier headbox,
E Headbox with a radial central distributor
(sources: A–D Voith, E GL & V).
mands on throughput and speed. The essential functional elements for distribu¬tion and dispersion in evener roll headboxes (B) are the perforated rectifier rolls that rotate in the suspension. The suspension passes through these rolls, thereby generating shear forces. The air cushion above the suspension is usually under pressure (air cushion headboxes). This headbox principle has been improved and is still used in special cases for lower throughputs and machine speeds (C). The state-of-the-art high-turbulence headboxes (also called hydraulic headboxes) have a closed suspension guidance without a free surface. Here a headbox for Fourdrinier machines (D) can have a separate pulsation damper (Fig. 6.32) or the dampening system may be integrated in the headbox. A single-layer headbox and a two-layer headbox, both for gapformers, will be described later in detail. Another headbox design (E) makes use of a centrally positioned tank reducing pulsations and dis¬tributing the suspension to a plurality of flexible pipes of equal pressure loss end¬ing at the backside of the headbox equally distributed across the width.
The mean basis weight (g m–2) to be produced is related to the mean volumetric flow through the nozzle and the consistency of the suspension. If the consistency has to be lowered (or increased) for operational reasons at a given basis weight, a higher (lower) volumetric flow is required to obtain the same basis weight. This is reached by opening (closing) the nozzle. The headbox control device keeps the pre¬set jet velocity constant by adjusting the headbox pump motor speed.
Process machines installed upstream of the headbox, for instance the fan pump or the screen, generate pulsations which may be higher than allowed (for instance less than 1 % deviation). In such a case a pulsation damper reducing pulsations over a wide frequency range should be installed ahead of the distributor of the headbox or integrated into the headbox. Figure 6.32 shows an example of such a
6.4 Forming Section
damper with a perforated plate to reflect incoming pulsations, the upper part of the tank being partly filled with air thus acting as a hydro-pneumatic damping system. This is to absorb and dissipate most of the remaining pulsation energy passing the perforated plate.
The delivery of a constant jet velocity timewise and over the machine width is a basic paper technological requirement. The velocity ratio of suspension on the wire and the wire itself affects web structure by the degree of fiber orientation. Assume that the fibers in the suspension jet are randomly orientated in the three dimensions, the suspension flow on the wire is parallel to the machine direction and no velocity difference is given to the wire itself. The fibers then remain ran¬domly oriented on the wire. When there is a certain velocity difference between the suspension and the wire, the amount of fibers laid down in the machine direction during web formation is greater than in the cross machine direction. The fibers tend to align mainly in the direction of the velocity difference between the suspen¬sion and the wire, be it drag or rush. This results, for instance, in differences in the web characteristics, such as tensile strength or stiffness, in MD versus CD. It also influences shrinking behavior during web drying and expansion behavior when the finished paper sheet is exposed to heat or moisture. This might be of interest for instance in a copy machine or during printing in the press room. Figure 6.33 gives an example of how higher (rush) or lower (drag) suspension velocity on the wire in relation to the wire itself affects formation quality and the MD/CD tensile strength ratio.
As today’s machine speeds exceed 2000 m min–1 the pressure in the nozzle chamber is about 5 bar and above. For operational reasons, especially for improved dewatering characteristics in the wire and press sections the suspension is fed to the headbox at elevated temperatures, for instance at 40 to 75 °C, and in special applications up to more than 90 °C. Furthermore a headbox of more than 10 m width has a high dead weight. All these parameters induce a strong deflection of the originally straight structure of the headbox, and on the most sensitive parts which are the nozzle chamber and the slice blade. These deflections would neg¬atively affect the uniformity of the suspension jet exiting the headbox. To avoid these problems the headbox design has to overcome these influences.
Figure 6.34 shows a typical design of a state-of-the-art headbox for twin wires meeting the high demands of fast running paper machines. It consists of
. • the header redirecting the suspension flow into the machine direction and equally distributing the suspension across the machine width
. • a stilling chamber for coarse turbulence reduction and further equalizing of the CD suspension distribution
. • a turbulence generator built up in one or several rows which has to break up fiber flocs and feed the suspension equally to the nozzle,
. • the nozzle to accelerate the suspension up to the required velocity
. • lamellas for optimum jet surface quality and random fiber orientation in the suspension
. • the slice blade at one of the nozzle lips to finally form the jet
. • two heated chambers, one for the top lip and one for the turbulence generator, to eliminate negative thermal influences on the CD structure uniformity of the nozzle chamber
. • two supports across the width carrying the dead weight and ensuring that the headbox structure stability is independent of machine width
. • a controlled pressurized chamber at one lip (or both lips) of the nozzle chamber to counteract the bending impact of the nozzle chamber pressure on the CD structure uniformity.
6.4 Forming Section
The deviation from the desired CD basis weight has to be a minimum, for instance
0.5 %. In order to obtain such a high accuracy a control device is required. Fur¬thermore, for operational reasons, the ideal CD basis weight profile should often have somewhat lower values at the edges. As a standard for a long time in the past
– and still today – the CD basis weight profile has been and is controlled by a slice blade. This is equipped with a lot of spindles across the width with a standard spacing between these spindles of about 75 mm. Adjusting one or more spindles the nozzle opening is locally reduced or increased according to the requirements. The effect on the basis weight profile is shown in Fig. 6.35. It demonstrates that at the position of the lowered spindle the basis weight is reduced to a certain degree, whereas in the neighborhood at both sides of the adjusted spindle the local basis weight is increased. Furthermore a larger width in the basis weight profile is af¬fected compared with the width of slice lip adjustment. This kind of response is induced by local cross flow of the suspension in the nozzle when adjusting the local slice opening. The cross flow in the exiting jet also has a disadvantageous impact on other quality parameters of the formed web such as surface markings and out-of-plane defects.
In the last two decades a different kind of CD basis weight control principle, the dilution principle, has been developed and has become the state-of-the-art control device. Here a constant “high consistency” volumetric flow is fed to the headbox where it is mixed with a “low consistency” stream. At positions across the width where a lower basis weight is required, the “low consistency” stream is increased at a constant local overall flow rate. If the local basis weight should be increased the local “low consistency” stream is reduced at a constant local overall flow rate. The minimum spacing of the control modules can be as small as the modules of the construction, for graphic paper grades modules of about 60 mm are standard. Thus a very narrow area can be influenced.
A major advantage of the dilution principle is that CD profiles of main fiber orientation can be rectified (Fig. 6.36 ). As mentioned before local slice bar adjust¬ment causes cross flow in the nozzle chamber and in the exiting jet. Even a small angle of the jet velocity vector against the machine direction results in a large main fiber orientation angle. The main fiber orientation angle describes the direction of the plurality of the fibers in the paper and can be measured by laser or by ultra¬sonic devices. It has an impact on other important paper properties.
The example in Fig. 6.36 shows that a deviation of 1° in the jet at 1000 m min–1 means a CD velocity component of 10 m min–1. Together with an MD velocity difference between the suspension on the wire and the wire itself of for instance 40mmin–1, an angle of 14° in the resulting velocity vector on the wire occurs. This angle of the velocity factor defines the orientation of the plurality of the fibers which are laid down during dewatering of the suspension in the forming process. At a jet velocity of 2000 m min–1 and again 1° deviation in the jet the CD compo¬nent is 20 m min–1. With the same MD velocity difference between the suspension and the wire it adds up to an angle of about 27° of the main fiber orientation. This demonstrates the necessity for the application of high level fluid dynamics knowl¬edge in the design and construction of a headbox. On line control of the quality parameter main fiber orientation is currently being developed, with a special chal¬lenge being the on line measurements.
In recent years some multi-layer headboxes (Fig. 6.37) have been installed in tissue, fluting and graphic paper production. Here two separate suspension lines with different furnishes are fed to the headbox, which in principle consists of two headboxes in one housing. The two suspension lines are kept separate up to the nozzle end. Only in the jet itself and during dewatering can mixing of the two suspensions occur. A multi-layer headbox is advantageous
. • for multi-ply production by saving one of the forming sections
. • in single-ply production by hiding inferior furnish under the more expensive cover furnish.
6.4 Forming Section
In the wire section, a fiber web is formed out of the suspension supplied by the headbox. The kind and quality of suspension delivery from the headbox to the wire has a strong impact on the quality of the paper web formed. Therefore headbox and wire section – together with the approach flow system – have always to be regarded as one unit. The main objectives of the wire section are:
1. 1. Extensive separation of fibers from water (drainage)
2. 2. Well-ordered deposition of the fibers on the wire (oriented shear)
3. 3. Prevention of too much fiber flocculation (turbulence).
The separation of the fibers from water is a combined filtration and thickening process. During pure filtration a filter cake is built up above the auxiliary filter layer whereas the consistency of the suspension above the filter cake remains the same as before. Pure thickening means that the consistency of the suspension as a whole is increased. In paper web forming filtration prevails. The water extracted from the suspension contains fines, fillers and fibers and is called white water.
The driving forces for dewatering the fiber suspension can be hydrostatic, vac¬uum or mechanical:
. • The height of the suspension above the wire including additional pressure ap¬plication.
. • A vacuum behind the wire, produced by direct vacuum application or by the hydrodynamic effect of dewatering elements.
. • The pressure generated by the tension of the outer wire covering the suspension when running in a sandwich over a curved surface which may be a roll or a curved shoe. The pressure p exerted on the suspension is p = S/R with S repre¬senting the wire tension in N cm–1 and R the radius in cm. p has to be larger than the centrifugal forces acting on the suspension and on the wire. Due to the centrifugal acceleration c = v2/R a suspension thickness of t represents a suspen¬sion height of c*t/g in the gravity field. The pressure acting against the inner wire is reduced by this amount, which is remarkable at high machine speeds.
Since the days of Robert, Donkin and Fourdrinier a wide range of web forming principles has been developed and used for different purposes (Fig. 6.38):
. • Fourdrinier wire section, the most common forming principle in the past and which has been continuously improved since its early invention, is a horizontal forming wire supported by different kinds of dewatering elements.
. • Mold former where the wire covers a water-permeable cylinder rotating in a vat filled with suspension.
. • Suction former, forming the web within a short section of the circumference of an open cylinder covered with a wire.
. • Inclined wire, forming the web on a straight inclined wire supported by dewater¬ing boxes with a controlled height and pressure of the suspension in the form¬ing zone.
. • Twin wire hybrid former where a rotating second wire is mounted on top of the fourdrinier wire, dewatering part of the suspension through the top wire.
• Twin wire gap former, which is the state-of-the-art wire section in high speed web forming, mostly dewatering the suspension to both sides.
The different elements used in the wire section for wire support, dewatering and formation improvement are:
. • The forming board, positioned at the beginning of the fourdrinier wire where the stock jet impinges. It consists of several blades or bars arranged closely together. Thus, it performs gentle, initial drainage of water from the suspension. Too intensive drainage at this position would increase drainage resistance in the following drainage elements because of excessive compaction of the formed fiber mat.
. • Table rolls (Fig. 6.39), used in the fourdrinier section for drainage and to gen¬erate turbulence. Pressure is developed in the upstream wedge between the wire and roll, and a vacuum is induced in the downstream nip. With increasing machine speeds the pressure and vacuum pulses increase over-proportionally and thus limit the application of table rolls to machine speeds of about 500 m min–1.
. • Dewatering foils (Fig. 6.40), used on both the fourdrinier wire and twin wire formers. They have an acute-angled leading edge to doctor off the water hanging under the wire and a slope on the downstream side (foil angle of 0–3°) which induces a vacuum for drainage. Apart from wedge-shaped foils, step foils are also in use.
. • Foil boxes which combine several foils in one unit. In addition, the foil box can operate under controlled vacuum (vacuum foil box).
. • Blades being “foils” with zero foil angle, whereas counter blades are blades which are not fix mounted but are pressed with adjustable forces perpendicu¬larly to the wire. Their main target is to doctor off the water and to improve formation quality.
. • Wet suction boxes, which are dewatering elements that are located in front of the water line. They operate under vacuum and, in contrast to suction boxes, they mainly remove white water from the suspension. The water line is a line beyond which no free water is present on the surface of the freshly formed web and that is discernible on the fourdrinier wire by a change in light reflection.
6.4 Forming Section
(b) uni-flow former, (c) nonimmersed mold
former, (d) suction former,
C (a) suction former with rotating wire on a
fourdrinier, (b) Suction breast roll former in
D inclined wire,
E Fourdrinier with hybrid former,
F twin wire gap former (source: Voith).
• Suction boxes, which are dewatering elements that are generally located behind the water line. They operate under vacuum and, in contrast to wet suction boxes, also suck air through the paper web.
6.4 Forming Section
. • Suction rolls, which have an open shell of different design. Vacuum is applied through the interior. They are used in sheet formation as suction breast rolls, suction formers, suction forming rolls, or suction rolls at the end of the wire section. Suction rolls accelerate drainage and increase the dry content in the web.
. • Dandy rolls, which are highly open, wire covered rolls used on fourdrinier ma¬chines ahead of the water line, for improvement of formation quality and for watermark application (Fig. 6.41).
Drainage is opposed by the resistance to filtration, which depends on the degree of beating, chemical treatment, and type of stock, as well as on the amount of fines and fillers present. The dry content after the wire section in most cases is about 18–20 %. The water removed in the filtration process (white water) carries away fibers, fines and fillers. The percentage of solids of the suspension retained on the wire, also called retention, can be increased by the addition of retention aids. The white water is reused to dilute the thick stock in the stock approach flow system. Figure 6.42 shows the filler distribution in the z-direction (across the web thick¬ness) for dewatering the stock to only one side and symmetrically to both sides.
The short-wave turbulence (micro-turbulence), generated in the suspension in the headbox to maintain fiber deflocculation, dissipates rapidly. For this reason, good formation requires either the fiber web to be fixed very quickly or additional turbulence to be generated in the suspension to be dewatered. This can be achieved by means of pressure and vacuum impulses from table rolls, foils and blades. However, impulses that are too strong are harmful, for example by table rolls at machine speeds above approx. 500 m min–1 or by foils with too high a foil angle at elevated machine speeds. In special cases, on fourdrinier wires, formation is improved by agitating the wire. A shaker vibrates the breast roll and thus the fourdrinier wire horizontally in the cross machine direction with a frequency of up to 10 Hz and an amplitude of up to 25 mm. It is used at low machine speeds and where consistencies are still low, i. e. at the front end of the wire section of four¬driniers and hybrid formers.
The difference between the velocity of the jet and the wire is decisive for the controlled deposition of the fibers on the wire. If the jet and the wire have the same velocity, the fibers are deposited with random orientation or due to a possible pre-orientation in the headbox nozzle. If the jet is slower or faster than the wire, more fibers are aligned in the machine direction. The highest value for the tensile strength of paper is observed in the direction of the main fiber orientation. The relationship between the properties in the longitudinal and cross directions is often important in the processing and use of paper. So, depending on the various paper grades, a range in jet to wire velocity difference of 15 to 70 m min–1 is run. If the jet is not directed exactly in the machine direction, this angular deflection is magnified on the wire many times over (Section 6.4.2). The main fiber orientation is then no longer in the machine direction, which can lead to problems (diagonal sheet stress) in certain types of paper (e. g., copying paper).
The properties of the wire, which acts as a filtering auxiliary layer, influence the surface properties of the web (wire mark), fiber orientation, retention, dewatering velocity, and machine operation. Important parameters are the topography of the wire surface, resistance to fluid flow, free volume, cross stability (so that the wire remains level), and the wear characteristics of the wire. Therefore high require¬ments are put on the design and the maufacture of the wires as well as on their uniformity. An example of dimensions will give a good idea of the process during initial dewatering: The wire, as the auxiliary filter layer, has a weft yarn diameter of about 120 mm, the distance between neighboring weft yarn centers is about 150 mm, the “hole” in between the yarns is then 60 mm deep, converging to about 30 V 30 mm. A fiber may have a length of 2000 mm and 30 mm thickness, a clay particle a size of about 2 mm.
Today, mainly multi-layer wires are used. The material now employed is plastic but in some cases bronze or steel are still used (Section 6.3.1). A good wire should have over its whole area and for its whole lifetime
. • low resistance to z-direction water flow
. • low misting (white water entrainment)
6.4 Forming Section
. • flat topography of the paper touching surface
. • high wear resistance against wear at the reverse side in contact with the dewater¬ing elements and rolls and against wire cleaning devices.
Thorough wire cleaning is of the utmost importance to ensure uniform dewater¬ing and to prevent or reduce formation interference and sheet breaks as well as fines deposits in the machine. An example of a state-of-the-art cleaning device is shown in Fig. 6.43. It consists of a traversing head with one or more rotating nozzles, operating at a water pressure of up to 250 bar to generate high-impact drops for cleaning the wire. Dirt and water are sucked away.
During the forming process, the suspension is guided between either the wire and air (fourdrinier wire), between the wire and a solid wall (former), or between two wires (twin wire formers). Drainage can occur on one or both sides of the web (Fig. 6.38). Two-sided drainage produces a more symmetrical paper and reduces drainage time and the length of the dewatering zone. Typical figures for four¬drinier wire sections are web forming lengths of about 20 m, at 800 m min–1 this means a drainage time of about 1.5 s. A hybrid former 20 m in length and operat¬ing at 1200 m min–1 has a drainage time of about 1 s, whereas the drainage time for a twinwire gap former of 5 m length at 2000 m min–1 operating speed is about
0.15 s. The different principles involved in sheet formation shown schematically in Fig. 6.38 are explained below in more detail.
220.127.116.11 The Fourdrinier Wire Section
The fourdrinier wire (Fig. 6.38A) is the classical method of sheet formation. Speeds up to about 1200 m min–1 are achieved with the fourdrinier. This is a reasonable limit due to excessive turbulence on the free suspension surface and dewatering capacity. Normally, the fourdrinier is equipped with a forming board, foils and/or table rolls, suction boxes, wet suction boxes, and suction rolls. Drain¬age proceeds in the direction of gravity. A dandy roll is often used just in front of the water line to improve formation. This is a wire-covered open roll, with a honey¬comb structure, which dips into the suspension and distributes the fibers in the web more uniformly. The dandy roll is also used to produce watermarks in the web through displacement and compression of fibers. These marks are visible in the finished paper in transmitted light. They are generated by a raised pattern soldered onto the roll.
18.104.22.168 Cylinder Former
In the cylinder former family (Fig. 6.38B), web formation occurs on a wire-cov-ered, water-permeable cylinder. The uni-flow and contra-flow former with an im¬mersed mold represents the oldest design. These were later replaced by the non-immersed mold former and finally by the suction former. In suction formers, drainage is further increased by vacuum in the interior of the forming cylinder. The suspension is led between the wire cylinder and a solid wall (lip, former box). Suction formers are employed in the manufacture of single layers of board at speeds up to more than 400 m min–1. Higher speeds (up to 1500 m min–1) are only achieved with the suction breast roll former in the production of tissue (Fig. 6.38C). A similar forming unit can also be placed e. g. on a fourdrinier wire in multi-ply packaging paper production.
22.214.171.124 Inclined Wire
Sheet formation on an inclined wire section is performed in the area where the wire is covered by a box filled with the suspension and is usually under pressure (Fig. 6.38D). Here the wire is supported by some type of forming board. The pres¬sure under the forming board can be controlled. This type of forming unit is for low machine speeds and operates with low consistencies of down to about 0.01 %. It is used for the production of special papers and nonwovens.
126.96.36.199 Hybrid Former
The first section of the hybrid former consists of a fourdrinier, which is followed by a twin wire section in which dewatering also occurs through the top wire (Fig. 6.38E). This increases the drainage capacity of the base forming unit and improves symmetry in the z-direction of the paper web. For good formation results the free suspension height entering to the top wire/fourdrinier wedge has to be optimized by adjusting dewatering conditions ahead. In the past a large variety of configurations of hybrid formers has been developed with applying rolls, foils, blades and suctionboxes in different sequences.
188.8.131.52 Gap Former
In a twin wire gap former (Fig. 6.38F), the suspension from the beginning is led between two wires operating at the same speed, and is drained through one side, or mostly both sides. One of the driving forces in gap former dewatering is the drainage force due to wire tension, which counteracts the centrifugal force of the suspension. Open rolls, suction rolls, forming blades, and vacuum shoes are used to increase drainage capacity and improve formation. In the gap former, the jet is injected directly into the gap between the two wires.
Also in gap former develop¬ment a large variety of configurations have been and still are on the market. Todays standard of a high-speed gap former is a roll-blade-roll configuration (Fig. 6.44). This means that the forming roll is followed by a curved forming shoe, usually with counteracting forming blades (counter blades), and by the couch roll. The dewatering zone is S-shaped with the forming roll in contact with the outer wire and the couch roll contacting the transfer wire. This helps to adjust the z-direction symmetry of the sheet. The main direction of the wires in the dewatering zone is vertical. This eases white water handling as well as maintenance and changing the rolls and wires. With machines of this kind or of similar configuration, speeds of more than 1900 m min–1 are achieved for newsprint, 1800 m min–1 with SC and LWC papers, and 1550 m min–1 for wood-free writing and printing grades production.