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

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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)

Fig. 6.42 Filler distribution in the z-direction for one- and

two-sided dewatering (TS: top side, BS: bottom side).

6 Paper and Board Manufacturing

272

• 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.

6.4.3.1 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-

Fig. 6.43 State-of-the-art wire cleaning

device (source: Voith).

6.4 Forming Section

273

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.

6.4.3.2 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.

6.4.3.3 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.

6.4.3.4 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.

6.4.3.5 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

6 Paper and Board Manufacturing274

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 pro-

duction.

6.5

Press Section

The purpose of the press section is to increase the dry content of the paper web as

much as possible by compression. This kind of mechanical dewatering reduces

steam consumption in the dryer section and increases the strength of the web in

order to avoid web breaks during production.

The wet paper web is picked up from the wire section and is transported by felts

through the press section which contains one or more press nips to be passed. The

so-called press nip is formed between two opposing rolls pressed together. The

paper web today is usually transported either on one felt in the top or bottom

position or between two felts as a sandwich. In some cases, when the paper web is

strong enough compared with the applied stress in operation, the web is trans-

Fig. 6.44 State-of-the-art gap former for high-speed paper

production (source: Voith).

6.5 Press Section

275

ported towards the next press nip or to the dryer section without any felt support.

This is called an open draw. The water removed from the web is stored and carried

away in the felt and, in the case of high water extraction, also in grooves or perfora-

tions in the surfaces of the rolls behind the felt. With suction press rolls, storage

and transportation of water are improved and rewetting of the paper web is re-

duced.

The felt has to be designed to meet high demands as regards large storage

capacity under pressure, good water retention behavior on leaving the nip which

reduces rewetting of the web, and the smoothest possible felt surface for uniform

pressure transfer to the web and to avoid felt marking. For the latter, e. g., the

staple fiber diameter at the web contacting felt surface goes down to 20 mm which

is even less than the paper fiber thickness (about 30 to 40 mm). Different types of

felts are selected for the top or bottom positions in the individual nip as well as for

the different press nips, taking into account the press water flow and the target of

sheet transfer. The dewatering of the felts is performed with suction pipes. Con-

stant or intermittent treatment with a fine, high-pressure water jet and/or wetting

facilities (felt conditioning) keeps the felt clean and maximizes its working life.

A conventional press section usually consists of three to four successive press

nips (Fig. 6.45). The rolls are pressed against each other with linear forces of

20–150 N mm

–1

and up to more than 300 N mm

–1

in some special designs. For

conventional press rolls the magnitudes of the resulting pressure and nip length

mainly depend on the diameter of the roll, the elastic characteristics of the material

and the geometry of the shell and of the coating (steel, or bronze, rubber or plastic;

plain, grooved or perforated), as well as on the felts; for high basis weights the

visco-elastic properties of the paper web are also important.

Generally the applied nip pressure must first overcome the fiber structure re-

sistance of the web before it can generate the hydraulic pressure required for web

dewatering (see Fig. 6.7 in Section 6.2.4). Fiber structure resistance at the begin-

ning is low and increases with higher dry content which is shown in Fig. 6.46 for

static conditions. This means that the dewatering pressure must be elevated with

increasing dry content to overcome the higher structure resistance. However,

Fig. 6.45 Conventional four nip press section (source: Voith).

6 Paper and Board Manufacturing

276

when the hydraulic pressure is too high the paper web may be damaged (“crush-

ing”), especially at higher moisture content. The higher the hydraulic pressure and

the longer the time of action (both factors resulting in the “press impulse”), the

better the dewatering performance. The flow resistance of the water in the web is

determined by the type of stock, the fiber treatment, and the amount of fines and

fillers. Increasing the web temperature by steam or infrared heating facilitates

dewatering by lowering the viscosity of the water and the structural strength of the

web. The actual nip pressure is increased from nip to nip, corresponding to the

decreasing water content. For symmetry in both densification and fines and ash

content of the top and bottom web surface symmetrical dewatering has to be

aimed for.

The time during which the pressure acts on the web can be prolonged up to

about tenfold compared to conventional presses by using shoe presses. In this

press type, a concave shoe presses a flexible revolving plastic sleeve or belt against

the counteracting press roll (Fig. 6.47). The shoe works on the hydrostatic/hydro-

dynamic lubrication principle. The increase in pressure along the press nip is by

far not as steep as in conventional presses, the maximum nip pressure is lower (up

to about 90 bar). Furthermore the shape of the pressure curve along the press nip

can be adjusted in a certain range. Figure 6.48 shows the total pressure distribu-

tion in the machine direction in a conventional two-roll press nip and in a shoe

press nip.

Nowadays, the dry content after a shoe press section reaches about 50 to 55%,

depending on the product and the raw material used. Because of these high dry

contents, less thermal energy is required in the dryer section, and the resulting

increased web strength results in fewer breaks.

Fig. 6.46 Fiber structure resistance as a function of web dry

content under static loading conditions for two different

stocks (source: V. Lobosco, see references).

6.5 Press Section

277

Shoe presses have been standard in press sections for board and packaging

grades since the 1980s. Later, they have also become state-of-the-art presses for

graphic paper machines. Today modern press sections of high-speed paper ma-

chines consist of only two nips. They have for instance two double-felted nips or

one double-felted nip followed by a second nip with a felt and a transfer belt

Fig. 6.47 Shoe press consisting of a top roll with sleeve and

concave shoe and a bottom roll with iron shell supported by

hydrostatic bearings (source: Voith).

Fig. 6.48 Pressure distribution in the machine direction in a

conventional two-roll press nip and a shoe press (source:

Voith).

6 Paper and Board Manufacturing

278

(Fig. 6.49). A press system with two single-felted nips is mainly applied for graphic

paper production. These types of press operate at line loads of up to 1250 N mm

–1

A steam box ahead of the second press nip for enhanced dewatering effect and CD

moisture control is optional. The sandwich felt-paper-felt in the press nip assists

symmetrical dewatering providing a good z-direction symmetry of the sheet. Dur-

ing press dewatering the web layers towards the felt are more densified. So one-

sided dewatering means for instance nonsymmetrical surface characteristics such

as printability of top and bottom side.

The newest development is a single nip press section where the web is dewa-

tered in just one nip (Fig. 6.50). With a shoe of more than 300 mm length a dry-

ness of about 52% is reached in woodfree paper production at more than

Fig. 6.49 State-of-the-art press section with a double-felted

first press nip and a second nip with felt and transfer belt

(source: Voith).

Fig. 6.50 Newly developed press section with a single double-

felted nip (source: Voith).

6.5 Press Section

279

1300 m min

–1

. The requirements placed on the felt quality and uniformity are

correspondingly high. Due to the nonexisting second nip investment costs, energy

requirement (vacuum and drive), press sleeve and felt costs as well as machine

shutdown time for their changing are lower.

Shoe presses are also used in tissue production. Here the advantages are higher

bulk and sheet dryness after the press. In this application the shoe press replaces

the suction press roll of a conventional machine acting against the Yankee dryer

(Fig. 6.51).

A uniform moisture profile in the cross machine direction on the reel is very

important for reasons of quality and economy. This requires a uniform nip pres-

sure as well as a uniform dewatering effect of the felts in CD. Preconditions for

that are uniform felt design, structure, and conditioning. For CD moisture profile

correction press rolls are in use which can vary the line force selectively across the

width (in conventional press sections) and sectionalized steam boxes which heat

the web to varying extents across the width.

A reliable web guidance system is important to prevent web breaks, especially at

high machine speeds, low basis weights, and at low dry contents of the web, as

found after the first press nip. In modern paper machines, especially in the press

section, the web is not conveyed freely, but is nearly always supported by a felt, belt

or wire, or by the surface of a roll (closed draw).

6.6

Dryer Section

6.6.1

Overview

The purpose of the dryer section is to increase the dry content of the paper web,

usually to 90 to 98%, by evaporation. During drying the fibers develop hydrogen

bonds which provide the natural strength of the paper. Drying is a coupled heat

Fig. 6.51 Shoe press in tissue produc-

tion (source: Voith).

6 Paper and Board Manufacturing

280

and mass transfer process so heat has to be transferred from a heat source to the

paper and the evaporated water has to be carried off. During drying, the paper web

which has been picked up from the press section has to be guided safely through-

out the dryer section to the reel where it is wound up. Some dryer sections include

a size press for paper strength improvement and/or a breaker stack for pre-cal-

endering. Depending on the type of paper machine different heat transfer and

drying principles as well as their combinations are applied.

6.6.2

Drying Principles

6.6.2.1 Contact Drying by Steam Heated Cylinders

With this most common principle in paper drying steam condenses at the inner

surface of the cylinder wall, the heat is transferred through the wall to the paper

web, the web is heated and water is evaporated. Air flow takes up the evaporated

water. The heat transfer rate from the steam to the cylinder shell depends on the

flow pattern of the condensate motion. This flow pattern is mainly dependent on

the machine speed, and to a lower degree on the amount of condensate volume in

the cylinder and on the cylinder diameter. At low speeds of up to about 300 to

500 m min

–1

a pond of condensate is found in the cylinder. At higher speeds –

above the “rimming speed” – the condensate builds up a ring. Acceleration during

“ascending” of the condensate ring is against the rotating direction, and in the

rotation direction when descending. This results in a swinging condensate motion

Fig. 6.52 View inside a drying cylinder equipped with spoiler

bars (source: Voith).

6.6 Dryer Section

281