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Publication: Pulp & Paper International
Issue: August 1, 1994
Author: AmandaMarcus

WET-END TECHNOLOGY

Wet-end technology: On form and ahead

Suppliers of headboxes and formers continue to focus their research on product quality and productivity. PPI rounds up the news from the major players.

By Amanda Marcus

Formation, sidedness, profile uniformity: some of the key concerns that suppliers of headboxes and formers are addressing in the 1990s. As papermakers become more oriented towards tailor-made products, advances in formers are focusing on specific grade requirements, from tensile ratio control of fine papers to engineered sheets for board. Multi-layer headboxes and gap formers, discerned as having higher speed potential and better curl control and formation, are becoming increasingly popular. So too is using dilution water to control basis weight profile.

Investment in research and development on the part of the suppliers to the sector is continuing. Beloit Corporation, for example, started up a new pilot machine at its Rockton Research Center in the USA this year. The largest pilot machine in the world, it has a 1-m pondside with a design speed of 2,500 m/min.

A fourth Concept in headboxes

The latest fruit of Beloit's research into headbox development is the recently-launched Concept IV-MH, a hydraulic headbox patterned after the company's Concept III model. Developed jointly with Mitsubishi Heavy Industries, Concept IV-MH boasts several new or enhanced features compared with its predecessor.

The inlet header, for example, has a parabolic tapered contour. Beloit says that this more closely matches the theoretical taper than conventional linear taper headers and that the design ensures a more uniform pressure drop across the tube bank for more uniform stock delivery to the wire.

Its second component is the tube bank which consists of a series of flow tubes that turn the header flow 90 degrees and direct it towards the nozzle. According to Beloit, the flow tube is unique, providing uniform flow from pondside to pondside, in terms of velocity, consistency and stability. It uses a circular-to-circular sudden expansion to create pressure drop. This pressure drop puts energy into the stock to break up fiber flocks and also insures uniform flow from the tapered header. The flow thern passes through a contour section, converting the flow from a circular tube to a rectangular flow. The flow accelerates continuously as it goes through this transition portion of the flow tube (which Beloit has patented). The flow tubes are combined to form a tube bank in which the rectangular outlets are stacked to cover completely the exit, from pondside to pondside.

After the tube bank, the flow passes through a converging nozzle which is completely in-line with the flow tubes, but has a low convergence to maintain stable flow.

The Concept IV-MH can operate with flexible divider sheets in the nozzle which dampen large-scale turbulence while developing a high level of micro-turbulence on each side of the divider sheets.

The new headbox also has special flow tubes at its edges for fine adjustment of edge-flow velocities. The edge flows can be adjusted manually, for single-grade operation, or automatically.

Basis-weight profile is controlled by bending a conventional slice lip or by adjusting the headbox consistency in the cross-machine direction. The Concept IV-MH uses either Measurex-Devron ThermoTrol rods or AutoSlice actuators for bending the slice lip. For finer basis weight control, the headbox uses Measurex-Devron AutoFlow actuators to adjust the flow of low-consistency white water. Low-consistency injection ports are located on 35-mm centers which Beloit claims are the closest of any profile control system.

The first mill to implement the technology on a commercial scale is Catawba, part of Bowater's Carolina Division in the USA. PM 1, which started up with the new headbox last May, makes LWC papers. According to Beloit, the unit is now "producing the lowest CD variability in the industry." The first European mill to follow Bowater's example is expected to start up at the end of the third quarter.

Papermaking on good form

For PMs that require lower sheet tensile ratios than are achievable with gap formers, Beloit has developed a range of opposed-blade formers. The first is the Bel-Bond CB former which can be fitted to existing Bel-Bond and Bel-Form formers, as well as on new machines. A matt is first formed on a conventional pre-fourdrinier table, which is then sandwiched between two wires over a large radius shoe before entering the CB (counter-blade) portion of the Bel-Bond CB.

Following the CB unit, the wires wrap around a large radius Curved Inverted Vacuum Box (CIVB) which consists of a lead-in auto slice and a series of blades. The blades impart pulses to the matt, enhancing formation and drainage. The wires separate over a vacuum transfer box following the CIVB.

A second member of Beloit's opposed-blade range is the High Capacity Bel-Bond CB. It differs from the standard Bel-Bond CB in that the lead-in shoe is a large-radius, low vacuum-forming shoe to establish initial sheet surfaces and increase the drainage capacity of the former. The CB unit is located opposite the CIVB where the fibers are still mobile and formation can be enhanced.

Multi-layer forming: Many uses

One of the major applications that suppliers are concentrating on is multi-layer forming, which can mean both layered sheets formed with one headbox, as well as sheets formed by combining separate plies. For the latter, Beloit has developed the Bel-Liner top-ply former (figure 3) which is used in place of a mini-fourdrinier to form a middle or top ply.

The MH-B former is used for high-speed, high-capacity, multi-ply applications. It uses a two-channel, high-consistency headbox to form a ply on a wire that wraps around a curved, bladed forming shoe. This ply is then transferred to the forming felt with subsequent plies added progressively. Developed by Mitsubishi Heavy Industries, the former is now in operation on four machines in Japan.

Flexibility is key to good form

Valmet is currently highlighting the success of its Sym-Former MB, a hybrid former where MB stands for "multi-basis" weight to reflect its versatility. It has a series of fixed blades inside the top wire run opposed by a series of loadable flexible blades inside the bottom wire run. The drainage pressures and pulsation profile through the former can be adjusted easily, Valmet says, by adjusting the individual loading pressures. Originally developed for the board industry, the Sym Former MB was then adapted for paper grades and 14 have been sold for fine paper production since 1990.

Valmet's recent development work has been concentrating on controlling misalignment angle profile. A new control system called Jetprofile (see figure 4) now automates adjustment of its Sym-Flo headbox edge flows and basis weight profile. The misalignment angle profile is measured periodically off-line in the laboratory and the results fed to the Jetprofile program which interacts with the Optiprofile CD basis weight control program to determine the best compromise between misalignment angle and basis weight profiles. The tightest misalignment angle tolerances are normally selected for sheeted grades since they are better able to tolerate the greater basis weight profile corrections needed than products sold in rolls. The latter grades have tighter tolerances on basis weight profile but less on misalignment angle profile, so the operator can set a weighting factor to control the magnitude of corrections allowed to the basis weight profile, to adjust the misalignment angle profile according to the grade being produced.

Jetprofile corrects the edge flow rates directly. The control of "edges at zero", Valmet explains, or minimization of overall misalignment angle profile can be set by the operator according to the needs of the grade. The company predicts that the system will mainly be applied to hybrid formers. The first Jetprofile was installed last April on a Finnish machine using Valmet's Damatic XD control system. According to the supplier, the results have been very successful so far.

Making the Flo flow smoothly

Valmet's latest work on headbox development has been the modification of its Sym-Flo headbox for dilution control of the CD basis weight profile (see figure 5). A new dilution block has been located ahead of the turbulence generator to introduce dilution water into its central row(s) of tubes. The chamber in the dilution block is designed to keep the total flow through the tubes constant while the consistency changes.

Valmet explains that the advantage of a dilution headbox is that the CD basis weight control can be on a finer scale (30-60 mm) compared to conventional slice spindle spacing (75-120 mm). It also means that basis weight profile can be adjusted without affecting the misalignment angle profile. However, points out the supplier, there are no means for actively controlling the cross direction flows in the nozzle, so the edge flow system of the Sym-Flo is still needed in dilution headboxes.

Valmet says that it is too early to say to what extent dilution headboxes will replace conventional models - it is not yet clear whether the papermaking compromises will be beneficial in every case.

The ultimate fine paper former

Figure 6 is Valmet's idea of what the fine paper forming section of the foreseeable future will look like. A roll and blade gap former will be used for most applications, it says, but there will always be some market for hybrid formers, for specialty grades, wide basis weight ranges and low-cost rebuilds. Dilution headboxes will become a popular choice for grades made with high slice openings on fourdriniers and hybrid formers. However, whether they are used with roll and blade gap formers will come down to cost rather than performance differences (without the slice adjusting system, Valmet explains, a dilution headbox is actually cheaper, but it is offset by the additional cost of the dilution water supply system).

Variety is not the spice of life

Sulzer Papertec is also continuing its efforts to improve basis weight profiles in a variety of applications. Its Jetco System works by adding dilution water to the Step diffuser turbulence generator in the headbox, enabling stock consistency to be altered locally across the machine, while local throughput through the headbox remains unchanged.

To minimize residual variations, the company has developed turbulence generators of various geometries to suit different machine speeds. For grades with particularly demanding formation and minimum residual variation requirements, Sulzer's Step Diffusor headboxes have an intermediate chamber and two turbulence blocks. The first block redirects the stock flow into the machine direction and the intermediate chamber dampens the turbulences generated. The second has a rectifier effect, reducing turbulence to the required level for optimum sheet formation by the time the stock reaches the slice.

Packing in performance

For linerboard machines running at up to 915 m/min, Sulzer Papertec has developed the Twin Former GH gap former which incorporates a two-layer Step Diffusor headbox (figure 7). The forming roll in the bottom wire has an open wire grill design on a closed cylinder surface while pressure impulses generated by the first forming shoe in the bottom wire are said to improve sheet formation.

Sulzer Papertec says that the Twin Former GH has several advantages:

A uniform MD/CD tensile ratio is achieved over a wide range and the operator can adjust the ratio over a wide range.

Compared with a fourdrinier, stock consistency can be increased, for example, from 0.6% to 1.2%.

The speed potential of the roll/blade former can be up to 1,500 m/min for linerboard and packaging papers.

Figure 8 shows the complete forming section of a linerboard machine which incorporates the Twin Former GH and which is now on order for installation in a US mill towards the end of this year. The machine will make a three-layer grade using a secondary Step Diffusor headbox to produce the brown topliner, followed by the Twin Former SP, a hybrid blade former. Overall basis weights of up to 300 g/m2 can be achieved with this configuration, depending on the furnish.

The three-layer Step Diffusor headbox can also be used to produce printing and writing grades, where the two outer layers use similar furnishes and the middle layer is made of cheaper stock. Basis weight can be corrected, both by using local dilution water and by the conventional method of adjusting the lip slice. The Jetco system allows local addition of dilution water over a width of 50 mm, resulting in an area of influence at the nozzle of around 90 mm. Figure 9 shows Sulzer Papertec's multi-layer headbox, here for three-layer production, incorporated into the Twin Former GH for printings and writings production.

The Twin GH forming roll is operated under vacuum, which the supplier says ensures an effective distribution of fines and fillers, even at higher machine speeds. The angle of wire wrap around the forming roll and the length of the forming shoes in top and bottom wire positions are fine tuned together for an optimum sheet in terms of cross-machine profile and formation.

To achieve good formation of the layers for multi-layer printings and writings, Sulzer Papertec says that layer intermixing should only be very slight. Several factors, it explains, contribute to achieve this:

Optimized low turbulence exiting from the headbox;

Gentle jet impingement on the forming roll. This is important, says the company, particularly at high speeds, to avoid turbulent mixing of the layers at jet impingement.

Dewatering the suspension in the forming-roll area takes place without pressure impulses, also helping to minimize layer mixing.

Mixing time is kept to a minimum where the outer layers are formed in the forming roll area.

Optimum use is made of the turbulence in the headbox to form the outer layers of the sheet on the forming roll while turbulence generated by the subsequent forming shoe blades is used to influence the middle layer.

Forming takes new approach

At Black Clawson Kennedy, USA, research emphasis has been on developing the Newport/Attwood System, a newly-developed pressure former which operates in conjunction with an endless forming fabric and a curved forming box. Because the forming wire is pulled under tension over the curved surface, the forming fabric is always level and free of undulations in the forming zone.

Black Clawson (BC) reports that satisfactory results have been obtained with furnishes ranging from wastepaper to virgin pulps, cotton linters and synthetic fibers. Freeness has ranged from 80-600 CSF, operating consistencies from 0.2-2.5% and basis weights from 20-800 g/m2. The pilot unit has run at 80-450 m/min and BC says that current trials indicate a potential for higher speeds.

The process allows the operator to control a number of web characteristics such as tensile ratios by adjusting the throat opening at the point at which the fibrous suspension flows from the explosion chamber on to the moving forming fabric.

BC claims that the former has several advantages: It is less costly than other systems, relatively small in size and operates over a wide range of conditions, allowing for easy control of draining and forming.

The company also comments that a compartmentalised forming box and associated circulating system mean that it is possible to have a closed system at the point where preliminary forming and high rates of drainage take place. The system can be used as an "on top" former, as well as in the underfelt position, as a twin-wire forming system and as a headbox on a conventional fourdrinier wire part. The Attwood could also be used as a method for producing multi-layer webs, either from individual plies couched together, or by simultaneous forming. Previous studies have indicated the unit's potential for operating as a "foam" forming unit too.

Bristol looking Super

From its existing Bristol former, the company has moved on to develop the Bristol Super Former, which is designed to produce high-quality board grades at speeds of up to around 350 m/min.

BC explains that the problem of "throw-out" water associated with rotary formers has been solved on the Super Former by developing a cylinder-mould evacuation system, incorporating an air-assisted doctor which prevents throw-out water hitting the fragile web on the underside of the making felt.

The unit also has step diffusers in the inlet pipes to improve the dispersion of the fibrous suspension before it reaches the explosion chamber.

A recent development of the Bristol Former system is the Bristol HC (High Capacity) Former which consists of a cylinder mould in association with a relatively short wire run.

The pressure former applies the fibrous suspension to a forming wire draped around the cylinder. After forming, the web is brought into contact with the under side of a making felt for at least one meter while on the forming fabric. While in this position, the web is protected from any throw-out water associated with the forming roll. It can be dewatered further by using a doctor, vacuum boxes, and/or pressing against a plain/ grooved roll or an extractor roll.

BC describes the arrangement as a kind of twin-wire forming where dewatering takes place through the forming screen in a downward direction, assisted by gravity, as opposed to upward dewatering, as is the case in conventional multi-ply twin-wire forming. In addition, pre-forming takes place against a forming fabric and not on a previously-formed layer.

The system can operate at speeds of up to 400 m//min and over, and will operate at medium consistency (1.5-2.2%) when producing middle plies of 150-180 g/m2.

Module Jet headbox takes off in Scandinavia

PM 5 at Munkedal in Sweden made history this year, according to Voith, by starting up a headbox in which the CD basis weight profile is controlled by varying the stock consistency across the headbox width. The machine is a printings and writings Fourdrinier making 45-250-g/m2 grades and is now fitted with Voith's Module Jet headbox.

The supplier explains that the difference between the Module Jet and conventional headboxes is that fiber orientation in the Module Jet is not influenced by the slice blade profile.

Munkedal's machine has a pond width of 3.6 m and reel-up speed of up to 750 m/min.

How it works: Stock and whitewater are supplied to the headbox through two headers. The whitewater is added to 50 adjacent sections through 50 metering valves. The amount of whitewater added to each section can be adjusted individually, allowing independent adjustment of the stock consistency of each section of the headbox. As the total throughput from each section always remains constant, the hydraulic conditions are not changed, and consequently, the fiber orientation profile is not influenced by changes in the CD basis weight profile.

Voith has also developed The Module Jet's own CD profile control system, the Profilmatic M which includes a new control algorithm and on-line process identification and mapping adaptation facilities.


 

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