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

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6.3.3

Dryer Fabrics

Antony Morton

6.3.3.1 Requirements

Dryer fabrics are utilised as the medium to transport the paper web through the

dryer section of a papermaking machine and to press the web to the hot cylinder

surface to increase the drying rate. The resulting requirements are :

• Paper contacting surface to have a high contact density for optimisation of heat

transfer from cylinder to paper web.

• Smooth paper contacting surface and in-line seam for non-marking of the pa-

per.

• Sufficient vapor permeability to allow evaporative drying to occur freely. The

permeability profile must be uniform across the full fabric width to ensure that

it does not influence the moisture profile of the paper web. Permeability must

not be too high or this will adversely affect web runnability. For example, for

high speed single tier type dryer sections web transport and control is critical.

For heavier weight packaging grades running at slower speeds heat transfer may

be of primary importance.

• Aerodynamic surfaces and low void volume for reduced air carrying to maintain

good web runnability and tail feeding.

• Assist in safe transfer of the web between one dryer section and another and

within a conventional dryer section.

• Uncomplicated structure to allow effective cleaning.

• High performance and quality running lifetime in hot, humid conditions.

• Dimensional stability and high performance and quality during running life-

time (typically 12–18 months) in hot, humid conditions.

• Safe, fast installation. All dryer fabrics are joined on the paper machine.

Fig. 6.24 SEM-Image of a transfer belt cross section A: Paper

side, B: Base weave, C: Roll side (source: Voith).

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6.3.3.2 Fabric Design and History

Before the invention of “man-made” fibers, dryer fabrics were made from natural

materials such as cotton.

Advances in chemistry saw the invention of polyesters and polyamides. These

materials could be spun and formed into monofilaments. Polyester quickly be-

came the preferred choice for dryer fabrics.

State of the art dryer fabrics are made from either polyester based polymer

derivatives or polyphenylene sulfide (PPS). PPS material is inert to the environ-

mental conditions encountered in a paper machine. This material would be used

for the construction of fabrics running on the hotter, more humid machines, e. g.

packaging machines.

Dryer fabrics comprise machine direction (warp) yarns and cross machine direc-

tion (weft). These yarns may comprise monofilament, multifilament, spun, plied

(twisted) etc. Figure 6.25 shows a dryer fabric suitable for high speed, state of the

art paper machines.

Dryer fabrics may also be produced by linking a series of alternating left- and

right-handed spiral monofilaments. The spirals are linked together by a connect-

ing monofilament. Figure 6.26 shows a spiral fabric design.

6.3.3.3 Dryer Fabric Manufacture

Dryer fabrics are woven as a continuous, flat fabric. The machine direction warp

yarns are held on canister spools at the back of the loom and passed through a

series of “reeds” and “sheds” (spacers and pattern makers). During the weaving

process sheds are raised and lowered according to the desired final weave pattern.

Fig. 6.25 Cross section of a dryer fabric for high speed

machines (source: Voith).

Fig. 6.26 Top view of a spiral fabric design (source: Voith).

6.3 Fabrics for Paper and Board Production

253

At each shed motion a weft system is fired through the so formed “warp tunnel”.

The reed then pushes the newly formed section tightly into line with the rest of the

fabric body.

The fabric is removed from the loom and relaxed and stabilised through the use

of heat and tension. A typical heat setting temperature is 180–200 °C for dryer

fabrics. The tension is set according to the running tension on the paper machine

section to which the fabric will be applied.

A seam is then created either from the machine direction yarns looped around

themselves, or by insertion of a spiral yarn around looped warp yarns.

In the case of a spiral link fabric there is no weaving to be done. The left- and

right-handed spirals are formed around a hot “mandril”. These are then meshed

together and joining wires used to link adjacent spirals. Spiral fabrics may or may

not be “stuffed” with filler material in order to control their permeability.

Dryer fabrics typically range between 1.2 and 2.5 mm in thickness, depending

on the fabric design required for the application. Their weight is typically

800–1500 g m

–2

Dryer fabrics are typically supplied through the application range 60 cfm to

around 100 cfm.

6.4

Forming Section

Herbert Holik

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).

6.4.1

Approach Flow System

Andrea Stetter

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 [1]. A constant flow rate of the suspension

at constant pressure, constant consistency and compound must be ensured in

order to obtain a uniform basis weight distribution in the paper. This is done by

6 Paper and Board Manufacturing254

• 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.

6.4.1.1 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

Fig. 6.27 Approach flow system for a modern high speed

paper machine (source: Voith).

6.4 Forming Section

255

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 m

–1

or 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].

6.4.1.1.1 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

Fig. 6.28 Metering and mixing in the approach flow

system [2] (source: Voith).

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256

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].

6.4.1.1.2 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].

6.4.1.2 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.

6.4.1.2.1 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 [1].

6.4.1.2.2 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 [1].

6.4 Forming Section 257

6.4.1.3 Deaeration

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 [3]. Chemical deaeration with defoaming agents is also possible (see

Section 3.7.3).

Fig. 6.29 Deaeration tank [2] (source: Voith).

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258

6.4.1.4 Engineering

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]

6.4.1.5 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.

6.4.2

Headbox

Herbert Holik

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-

6.4 Forming Section 259

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-

Fig. 6.30 Jet impingement parameters depending on jet exit

conditions (source: Voith).

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260

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

cushion,

C rectifier roll headbox (modern design),

D Fourdrinier headbox,

E Headbox with a radial central distributor

(sources: A–D Voith, E GL & V).

6.4 Forming Section

261