Monday, March 10, 2008



Headbox, is a very important part in a paper machine, it located before the forming part, so this part of machine is to deliver the stock inside it into an even and rectangular discharge, which equal in width to the paper machine at a uniform velocity in machine direction by a fan pump. Individual fibers tend to cling to each other and form flocs; the headbox breaks up the floes and prevents them from reforming by creating a slight turbulence. If, however, there is too much turbulence, cross currents are created that lead to uneven stock delivery to the sheet forming section. A holey roll - 8'' to 24'' in diameter and perforated with many holes .75'' to l.5'' in diameter - controls the turbulence and evens out the velocity of the flow.

Perforated Rolls (Rectifier Rolls)
This hollow perforated rolls (rectifier rolls or holey rolls) are generally used in the air - cushioned headbox . This is very important to create turbulence to keep the fibers deflocculated. The major design and operational variables for the perforated rolls are hole diameter, % of open area, wall thickness, direction of rotation, and rotational speed .The perforated rolls must be installed in order to close tolerances within the head box.
Headbox Slice
The headbox slice is a small sharp- like slice situated onto the contact between the headbox and the forming wire. It is full-width orifice or nozzle with a completely adjustable opening to give the desired rate of flow, the stock will deliver to the forming wire through the slice. The thickness of the slice jet is determined by the slice geometry and the opening. While the velocity is determined by the headbox pressure. Each slice has a top lip and an bottom lip (apron). The apron is usually slightly sloped toward the opening. The top lip is adjustable up or down. The function of the slice is to control the thickness of the stream of stock, to deliver a jet of stock at even velocity across the machine width, and to maintain constant fiber distribution ensuring an even caliper across the sheet. There are many different slice designs, all variations on a general theme.

Slice Edge Bleeds
These slice edge bleeds are usually found in the pressure headbox to give more control of edge sheet formation. The natural tendency for the jet to "fan out" can be offset by removing a small flow at the edge of the box.

Operation of the Headbox
The main operating variables for headbox include:
a) stock consistency
b) temperature
c) jet-to-wire speed ratio
The consistency is set low to achieve good sheet formation , without compromising first - pass retention or exceeding the drainage capability of the forming section . Consistency is varied by raising or lowering the slice opening. This change in slice opening will mainly affect the amount of white water circulated from the wire pit. If the jet velocity exceeds the wire speed, the sheet is said to be "rushed". If the jet velocity lags the wire, the sheet is said to be "dragged".

The operation of the headbox is vital for a quality paper making because the headbox decide the even dispersion of fibers and fillers of the final product. A headbox which is not functioning properly can cause many defects in the finished sheet. The worst of these is poor formation caused when fibers are not evenly dispersed. The sheet has light and dark areas, caused by variable concentrations of fibers, when viewed by transmitted light. These lumpy spots have a detrimental effect on printing, particularly by letterpress.
The objectives of the headbox are stated as follows:
" To spread the stock evenly across the width of the machine.
" Level- up the cross- currents and consistency variations.
" Level p machine direction velocity gradients
" Create controlled turbulence to eliminate fibre flocking.
" T discharge evenly from the slice opening and impinge on the forming fabric at the correct location and angle.

Headbox can be characterize into open type and pressurize types, pressurize type can also be divided into hydraulic and air- cushion type.

In the air- cushion type, the discharge energy is derived from the feeding pump pressure, but a pond level is maintain and the discharge head is attenuated by air pressure in the space above the pond.
Opened headbox is mainly use in slow speed pulp machine. Flow spreader is sometimes considering as a separate component because its function is so critical on high speed machines, and its performance can be evaluate apart from the rest of the headbox.

Headbox Performance
Property variations in the paper sheet often originate in the headbox. Controlling basis weight variations in the cross-machine direction and reducing the MD/CD ratio can significantly improve profit margins. Traditionally, headbox designs and modifications have been based on experience and simple physical modeling. Because of the complexity of the geometry and the three dimensional nature of the flow field, such an approach is often inadequate to properly identify problems and improve headbox performance. PSL simulates three dimensional flows throughout the entire headbox and predicts in advance the performance of proposed headbox modifications. Management decisions can then be supported by reviewing the best available information.

Headbox for papermaking machine with more uniform flow
A headbox for a papermaking machine with an outlet slot that distributes pulp suspension over the, working width of the papermaking machine. For controlling operating parameters of throughput, pulp density and fiber quality of the suspension over the width of the machine, the headbox has a plurality of individual sections across the width of the machine. Each section has respective channels there through for passing pulp suspension. At least one connection at each section is to a controllable supply of pulp suspension where the operating parameters of that supply are controllable. Only separate operating parameter controlled streams pass through the sections of the headbox. Operating parameter control devices may deliver adjusted streams to a mixer upstream of the headbox channels. The mixer may also have individual sections across the width of the machine. The headbox has a common outlet nozzle downstream of the individual channels and the individual sections, where the pulp suspension stream from channels with controlled suspension parameters and from any channels without controlled suspension parameters are reconstituted to have the desired suspension operating parameters.

Paper machine headbox and method of controlling pulp material parameters
A paper machine headbox having divided sections for feeding pulp suspensions with different properties there through includes a single inlet line and a plurality of feed lines connected thereto. In at least one of the feed lines, there is a separation device which separates certain pulp suspension components from the pulp suspension flowing there through. A discharge line connected to the separation device conveys separated pulp suspension components there from. A continuing feed line connected to the separation device conveys the remaining pulp suspension to the headbox.
Paper machine headbox with adjustable lower lip
A headbox for a machine for the manufacture of paper from a web of paper pulp has an upper lip and a lower lip disposed below the upper lip and operatively associated with the upper lip to form an interior region between the lower lip and the upper lip and a discharge region. The shape of the lower lip, adjacent the discharge region, over which the web of paper pulp flows is adjustable. The shape may be adjusted by the rotation of a round, rotatable member disposed flush with the lower lip of the headbox or by the deformation of a deformable member provided at the discharge region.

Apparatus for damping pulp stock pressure fluctuations in a headbox
A headbox of a paper machine includes a header chamber for receiving pulp stock, a distribution pipe system for receiving pulp stock from the header chamber, a flow-equalizing chamber for receiving pulp stock from the distribution pipe system, a turbulence passage system for receiving pulp stock from the flow-equalizing chamber, and a lip slice for receiving pulp stock from the turbulence passage system and discharging the pulp stock from the headbox. At least one of the above chambers is defined in part by a movable wall structure which has an inner surface contacting the pulp stock so that this movable wall structure can move in response to pressure fluctuations in the pulp stock. This wall structure has an outer surface which defines part of the hollow interior of an enclosure in which air under pressure is situated, so that through the movable wall structure the air under pressure can act on the pulp stock to damp pressure fluctuations thereof.

Advanced Headbox Modeling
PSL provides detailed analysis of headboxes that can be used to identify potential sheet-forming problems. Working with the mill, we find ways to improve headbox performance using modeling tools which predict flow and turbulence characteristics throughout the headbox. Results from the analyses are used to:
" Decrease property variations in the sheet
" Increase capacity of the headbox without adversely affecting paper quality
" Increase profit margins through improved paper quality
Flow Variation in Headbox Plenum Flow Variations in the Contraction Region
New development of the Headbox
The headbox and the former are the key paper machine components affecting the product quality. A combination of the Concept IV-MH Headbox and Bel Baie Former is optimum in order to produce a paper with excellent texture that offers high printability and minimized two-sidedness.
Mitsubishi-Beloit's Concept IV-MH Headbox is a new generation of headbox, designed to deliver far higher speed, better paper quality, improved profile and uniform paper density.
Employing the uniquely shaped Concept IV-MH tube permits the paper to flow uniformly and stably from the edge to the center. In addition, an edge flow rate regulator and a new weight control unit (CP system) greatly improve the fiber orientation and weight CD profile performance. This headbox also allows various shapes of sheets to be combined, thus improving the flow stability and fiber distribution. The Concept IV-MH Headbox can be used for a wide diversity of paper types and at various operating speeds.

The technological development of the headbox has contributed to higher machine speeds and efficiencies. New innovative headbox solutions and today's precise dimensioning know-how have raised the end product quality to an entirely new level. Metso Paper's SymFlo headbox family is adapted to all board grades. SymFlo headboxes have been very successful in achieving excellent stability, fiber orientation control, minimized streaking tendency and good profile.

The new ValFlo headbox is aimed at middle-sized paper and board machines where quality, well-proven solutions and cost effectiveness are the key. ValFlo is especially designed for rebuilds requiring a compact headbox structure and
OptiFlo headbox has many superb features that give higher quality for the end products and higher operation efficiency for the board machine line. Its embedded automation and the clever industrial design assist the operator to maintain a continuously high level of performance.

New machine designs balance sheet quality, production goals
Two critical requirements of a headbox are uniformity of basis weight profile and uniformity of fiber orientation profile. These quality parameters must be met while running at the high production rates of today's machines. Two headbox designs that meet these criteria are the Beloit Concept IV-MH headbox and the Voith Module Jet.
The Concept IV-MH Headbox, jointly developed by Beloit and Mitsubishi Heavy Industries, uses a tapered header and tube bank for cross-directional distribution. The primary objective of these two elements is to provide uniform CD flow distribution.
The parabolic-shaped tapered header replaces the conventional style linear header back, allowing for a uniform pressure distribution across the machine. The tube hank creates even distribution by providing a pressure drop for the tapered header and the energy needed for fiber dispersion. The tube bank also establishes a uniform velocity profile into the nozzle section, without the tendency for cross-flows.
The nozzle section of the headbox is a converging region that is divided into multiple channels, each separated by a flexible sheet. The tube bank and nozzle are inline. This design maintains velocity without a change in fluid direction, which is required for a stable flow delivery and clean headbox operation. The inline arrangement with high nozzle velocities also produces hydraulic stability for very low residual basis weight variations.
Hydraulic stability and a unique consistency profile control system provide an automated profile control method that eliminates the adjustable slice lip. Instead of adjusting the slice lip, the basis weight profile is controlled by adjusting local consistency in the headbox, with injection tubes that meter low-consistency whitewater into the header. The slice lip opening can be kept uniform, eliminating headbox cross-flows, complex control algorithms, and bending limitations of the conventional slice lip.
Voith also manufactures a consistency control headbox that replaces the slice lip control design. The new headbox is called the Module Jet. This design is especially suited to paper grades sensitive to fiber orientation, and for machines producing a high range of basis weights.
The area of the headbox upstream of the turbulence tube bank is divided into separate flow modules across the entire machine width. Into each of these flow modules, two volumetric flows having different stock consistencies are mixed. By altering the ratio of the two flows, stock consistency and, therefore, basis weight can be adjusted separately in each region. By altering the total flow volume from each module, lateral flows can be introduced in the nozzle, making it possible to influence the fiber orientation in each region separately.
Vaahto headbox
Vaahto headbox family is developed to meet today's requirements of high quality paper, board and pulp manufacturing. HQ family of headboxes guarantees an excellent formation and top quality MD/CD basis weight profiles.
Vaahto?s headboxes are all designed with unique structure which binds the upper and lower parts together through the whole width of the headbox. This solution has dramatically decreased the needed cross machine size and total weight of the headbox. The weight difference compared to traditionally constructed headboxes is remarkable and yet the structure stiffness is higher.
To obtain good formation level, high enough velocity variation inside the headbox is required. High turbulence must also be maintained during the whole time fibres are in the headbox. Vaahto has solved this requirement by two separate turbulence elements (distributor and turbulence block). Both elements have 3-stage turbulence tubing with adjusting pieces designed individually for each headbox. Replacing the traditional standard size distributor and turbulence tube philosophy with a customized system, Vaahto can now design the best possible flow dimensioning for all our customers. Our adjustment parts can easily be changed and so the headbox max/min flow levels can be altered cost effectively according to the future production and machine development needs.
" HQ - HEADBOXES hydraulic headbox with air pad -type pulsation absorption
" HQD - HEADBOXES hydraulic headbox with dilution control and air pad -type pulsation absorption
" HQR - HEADBOXES rectifier roll headbox
" HQRD - HEADBOXES rectifier roll headbox with dilution control
" HQS - HEADBOXES hydraulic secondary headbox with dilution control and elastic inlet pipe -type pulsation absorption
" TH - HEADBOXES total hydraulic headbox with elastic inlet pipe-type pulsation absorption
" THD - HEADBOXES total hydraulic headbox with dilution control and elastic inlet pipe-type pulsation absorption
TH family of headboxes have the same rigid structure as HQ, but offers a more economical choice for low speed range machines.
All Vaahto headboxes are delivered with build-in dilution control design. This means that our headbox can first be purchased just with lip adjustment control and dilution control can be installed later without internal changes to the headbox. This design prevents unnecessary investment costs and enables further headbox development.
Vaahto headbox family includes also secondary headboxes. Secondary headboxes can be installed on base wires with cantilevering system and therefore there is no need to dismount the headbox connections for the wire change.

Vaahto developed its own dilution water valve for the HQD dilution control headbox because there were not sufficiently linear, accurate valves available on the market. All the available valves were based on standard ball valve structure, which typically has a strong S-shaped capacity curve. Vaahto S-valve is unique and represents the state of art in this technology. It has an almost flat capacity curve, is compact and simple in its construction, has exceptionally low actuation power consumption, has non-clogging structure and high maximum flow rate, so that the dilution flow rate can be raised up to 30-percent of the total flow. The structure of the valve is designed for open connectivity and it is compatible with the profile control systems supplied by all automation manufactures.

Vaahto?s headboxes are designed with easy self cleaning properties. The sidewalls do not carry big loads due to the stiff structure of the headbox. This is why Vaahto?s headboxes have large sidewall cleaning openings in critical positions. All the surfaces in contact with fibres inside the headbox are polished by electrolytic. The manifold blocks can be opened to ease the cleaning of the distributor piping. The lip part of the headbox can be lift up to the maintenance position.
In addition to new headbox manufacturing Vaahto does modernizations to the existing headboxes. In many cases an old headbox can be updated to a dilution control headbox. This is less costing way to improve machines productivity by improving profiles, formation and fibre orientation. Rebuilds can be done both for hydraulic and rectifier roll headboxes. The installation of new dilution control manifold to an old headbox can be done in reasonable short machine shut time.

Latest headboxes: Separate basis weight, fiber orientation control
In the Voith unit, basis weight adjustment valves are arranged in the supply as an integral part of the closed-loop control system in much the same way that conventional headboxes employ closed loop control to slice lip adjuster motors. Depending on a customer's requirements, the flow regulator can be designed as a fixed throttle with interchangeable inserts, or, in the future, with a remote-controlled, variable-orifice element that will enable closed loop control of fiber orientation. The Module Jet design includes an array of adjacent flow mixing modules, each isolated from one another and positioned at 50-mm wide intervals across the entire machine width. It is this arrangement that makes high-resolution control of CD basis weight and fiber orientation profiles possible.
Apart from the modular mixing valves, other components employed in the headbox, such as the tapered header, turbulence tube bank, and nozzle, have been tried and tested in conventional Voith headboxes for many years. The adjustment devices for basis weight and fiber orientation are located within easy reach, being positioned away from the jet and thus protected against contamination. For production conditions requiring large slice openings, basis weight can be adjusted more effectively than is possible with the slice lip, which allows only slight relative gap adjustments. Elimination of the slice lip adjustment spindles permits shortening of the jet length on gap former machines.
When applied to multi-ply headboxes, basis weight and fiber orientation can be adjusted not only independently of each other but also separately in each layer. Application of Module Jet to multilayer headboxes permits the production of diagonal curl-free papers having good formation and purity of layers.
The first Module Jet installation was in 1994 at the Ahlstrom-Kauttua woodfree specialty papers mill in Finland. Since then there have been almost 50 installations, including eight in North America, with several more on the drawing board for the next couple of years.
The Valmet Sym-Flo D headbox also controls basis weight by varying stock consistency in narrow bands across the slice, while keeping the slice opening constant. Changing local consistency in this manner ensures that while basis weight changes, the direction of stock flow on the wire, and therefore fiber orientation, remains constant. Thus, basis weight and fiber orientation become independent variables that can be separately controlled.
Changing consistency to control basis weight is achieved by dilution. The dilutant can be pure water, clarified whitewater, or whitewater from the silo. Total flow must remain constant over the dilution ratio operating range, and stock and dilution flows must mix completely


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