Holik H. (ed.) Handbook of Paper and Board

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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

Fig. 6.32 Pulsation damper with

perforated plate and hydro-pneumatic

tank (source: Voith).

6 Paper and Board Manufacturing

262

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

Fig. 6.33 Formation quality and MD/CD tensile ratio vs.

velocity difference between suspension on the wire and the

wire itself.

6.4 Forming Section

263

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.

Fig. 6.34 State-of-the-art headbox with

dilution control for a gap former

(source: Voith).

6 Paper and Board Manufacturing

264

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.

Fig. 6.35 Effect of local slice adjustment and local dilution on

basis weight profile (source: Voith).

6.4 Forming Section

265

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.

Fig. 6.36 Influence of jet velocity, jet angle and wire speed on

main fiber orientation.

6 Paper and Board Manufacturing

266

6.4.3

Wire 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. Extensive separation of fibers from water (drainage)

2. Well-ordered deposition of the fibers on the wire (oriented shear)

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 = v

/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.

Fig. 6.37 Two-layer headbox (source:

Voith).

6.4 Forming Section

267

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 Paper and Board Manufacturing268

Fig. 6.38 Web forming principles:

A Fourdrinier wire section,

B cylinder formers: (a) contra-flow former,

(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

tissue production,

D inclined wire,

E Fourdrinier with hybrid former,

F twin wire gap former (source: Voith).

6.4 Forming Section 269

• 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.

Fig. 6.39 Schematic and operation mode of table rolls.

Fig. 6.40 Schematic and operation mode of foils.

6 Paper and Board Manufacturing

270

• 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

Fig. 6.41 Dandy roll.

6.4 Forming Section

271