Biermann Ch. Handbook of Pulping and Papermaking

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234 9, PAPER MANUFACTURE

9.6 THE CYLINDER MACHINE

Cylinder

The cylinder paper machine (Fig. 9-38) with

a cylindrical forming wet end was invented in

1809.

The web is formed on a rotating cylindrical

screen 36 to 60 in. in diameter and immediately

picked off. Typically, 5 to 10 of these operate in

series to make a multi-layered sheet. This method

is usually used to make heavyweight board from

secondary fiber, which is not de-inked, for folding

boxboard, chipboard and gypsum board. A high

quality fiber surface may be added for printing

upon. Heavy weight paper and centrifugal force

limit speeds to less than 7.5 m/s (1500 ft/min).

The cylinder is a large hollow roll covered

with a wire screen (similar to a fourdrinier wire)

which revolves in a bath or tub of stock; as the

wire comes out of the vat, a mat of fibers collects

on the wire. The water passes through the wire

and is carried away by a vacuum from the inside

of the rotating cylinder. A horizontal felt is

pressed against the top side of the cylinder with a

press roll, which further dewaters the mat, to pick

up the web. Several other webs are added as the

felt picks up other webs from additional cylinders

in tandem. Normally there are no more than six

or seven cylinders in a single machine.

FELT AND MULTI-LAYER

SHEET TO PRESSES ETC

TO NEXT CYCLINDER

NEWLY FORMED

SHEET ON UNDER-

SIDE OF FELT

TO WHITEWATER CHEST

Fig. 9-38. Diagram of a cylinder machine.

Courtesy of W. Bufolitz.

Many new designs of cylinder machines are

available (Anon., 1978). Modern versions of

cylinder machines, such as the roto former, con-

trol the stock flow to the cylinder with channels

and baffles to give improved control over forma-

tion of the web. The Ultraformer (Fig. 9-39) is

one of the few machines that applies the stock to

Fig, 9-39. The Ultraformer. Courtesy of Kobayashi Engineering Works, Ltd.

THE CYLINDER MACHINE 235

the cylinder and the plies are then picked up by

the topside of the felt.

9.7 THE PRESS SECTION

Press, press section

A press is a pair of squeeze or wringer rolls

designed to remove water mechanically and

smooth and compress the sheet (Fig. 9-40). A

suction press is shown in Fig. 9-41. A variety of

press configurations are possible depending on the

desired goals. Fig. 9-42 shows two configura-

tions.

Some of the methods include:

The use of a pickup felt while the web is still

in contact with the forming material so that

there is no unsupported web during the

transfer. An open draw is delayed until

several press nips have been passed. An

unsupported web is called an open draw.

Double felts (one felt on each side of the

web) in the first press because of the large

amount of water being removed. This avoids

sheet crushing (described below) and allows

more water to be removed.

Shoe presses or other special presses to

promote good fiber bonding and decreased

web moisture contents.

Plain Top Roll

-Felt Conditioner

Tension Roll

Fig. 9-41. Suction press of Fig. 9-43.

A design that gives the ability to have high

machine speeds with operation stability and

low vibration.

Allowing both sides of the web to be pressed

against a solid roll (at different times).

6. Designs that minimize space and cost.

Fig. 9-40. Diagram of

press. Reprinted from

Zellerbach Corp., with permission.

Fig. 9-42. Two press designs. Courtesy Black

Clawson-Kennedy .Guarding omitted for clarity.

236 9. PAPER MANUFACTURE

Fig. 9-43. The press section of the machine shown in Fig. 9-33.

Often three presses (Fig. 9-43) are used in a

press section. Water from the sheet transfers to

the press felt which is then cleaned with water jets

and dewatered by vacuum boxes (Fig. 9-26).

In a plain press, two smooth rolls are used

and the capacity to remove water depends on the

void volume of the press felt(s) to receive the

water (Fig. 9-44). The first press is often a plain

press and is often double-felted to receive more

water. A grooved roll press uses helical grooves

cut into the bottom press roll that is in contact

with the press felt (Fig. 9-45). These grooves

provide additional volume and a path for water

transfer from the sheet through the press nip. A

grooved roll press is much less expensive to install

and operate than a suction press.

In a suction press, one of the press rolls is

equipped with a suction box (Fig. 9-46 and Plate

34) to remove water and hold it into the felt

against centrifiigal force until the sheet has sepa-

rated from the felt. The material of the shell is

important. A variety of suction shell materials is

available to tolerate the high cyclic pressures.

Pressing can cause some compaction of the

sheet, but the level of refining is the principal

factor in final sheet density. Press rolls are

crowned

to give uniform pressure across the press;

that is, the middle of the roll might be on the

order of 0.05 in. thicker so that pressure will be

even across the width of the paper machine.

BOTTOM PRESS ROLL

Fig. 9-44. A plain press nip showing limitation

of water removal. Courtesy of W. Bublitz.

THE PRESS SECTION 237

Pressures of 400-8500 N/cm (50-1000 pounds

per linear inch, PLI) are used depending on the

press location and the paper grade; fine papers use

3000-5000 N/cm (350-600 PLI), kraft bags use

5000-7000 N/cm (600-800 PLI), and linerboard

uses up to 13,500 N/cm (1600 PLI) in the final

press.

If too much pressure is used, particularly

at the first press where the consistency is low,

crushing marks appear (on the dried sheet) where

water escapes too quickly, creasing the web; con-

sequently, later press sections are normally operat-

ed at higher pressures.

Several techniques can be applied to increase

water removal. Often the web is heated prior to

the press section to reduce the viscosity of water

and increase pressing effectiveness. [The viscosity

of water in centipoise is 1.79 at 0°C (32°F), 1.00

at 20°C (68°F), 0.65 at 40°C (104°F), 0.47 at

60°C (140°F), 0.35 at 80°C (176°F), and 0.28 at

100°C (212°F).] Double felting or extended nip

presses are used on heavy grades of paper. Well

designed felts that run without distortion and are

effectively cleaned improve dewatering. It is

important to minimize rewetting of the sheet just

beyond the press nip by quickly separating the

web and felt from each other. The more water

removed in the press section, the less water that

must be removed in the dryers, meaning faster

paper machine speeds and lower energy costs.

Fibers may stick to the top press roll or even

to the felt. Sticking, or picking, may be caused by

weak internal bonding of the web and by tacky

contaminants (called stickies) arising from wood

pitch, rosin or synthetic sizes, paper machine

additives, polymers from secondary fiber, etc.

Various additives such as clay or talc can be used

to counteract the tackiness of the contaminant.

Shoe

presses, Beloit extended

nip

press

Shoe presses use a long nip that is formed

against a stationary shoe. The extended nip press

of Beloit, the first and best known of the shoe

presses (Fig. 9-47), is a special press used in the

last position of the press section on paper ma-

chines producing heavy paper grades requiring

high strength and stifftiess, such as linerboard or

Fig. 9-45. A grooved press roll press from a linerboard machine. The paper and felt are quickly

separated after the nip. Notice the water being thrown off the bottom press roll.

238

9. PAPER MANUFACTURE

1) side seals, 2) end deckle seals, 3) pneumatic tubing for strips, 4)coil springs for deckles, 5) neoprene

seal ring, 6) head shims, 7) studs and nuts, 8) drive end housing, 9) pilot bearing housing seals, 10) front

and rear bearing retaining plates, 11) two main bearings, 12) one suction box pilot bearing.

Fig. 9-46. A suction roll (top) and diagram of a suction roll of the type in Plate 34 (bottom).

Courtesy of Black Clawson-Kennedy.

corrugating medium. A plastic, impermeable belt

and a special shoe increase the nip to about 25 cm

(10 in.) wide, allowing better compaction of the

web and increased water removal. The first

extended nip press was installed at the Weyer-

haeuser mill in Springfield, Oregon in the late

1970s to make linerboard. The increase in web

consolidation gives papers with high stiffness,

which is especially important in linerboard. Dryer

requirements are reduced as the consistency from

THE PRESS SECTION 239

Fig. 9-47. Beloit extended nip press. The bottom is a close-up of the shoe where the plastic belt,

felt, and sheet are visible. Note the sheet is quickly separated from the felt at the nip exit.

extended nip presses may be above 50%, allowing

faster machine speeds and decreased energy

requirement per unit production. The pressure

applied to paper is approximately 4000 kPa (600

psi) or 40,000 N/cm (5000 PLI). Some machines

now use two extended nip presses in a row.

The extended nip press uses a plastic,

impermeable belt next to the shoe. Oil lubricant

is added between the belt and the shoe to decrease

friction. The web is carried by a felt that sepa-

rates it from the belt and picks up the water

discharged from the web during pressing.

Other suppliers provide shoe presses as well.

Fig. 9-48 is a diagram comparing various shoe and

conventional presses in terms of nip pressures and

nip lengths.

240

9. PAPER MANUFACTURE

KG/CM2 PSI.

1201

110-]

100H

90 H

80-j

70 H

50 H

40 H

30-

20-

10^



1 

1500 J

1350 J

1200 J

1050

900 J

750 J

600 J

450 J

300 J

150 J

Linear Pressure

3456789 10

Inches

10 2D X 40 50 60 70

90 100 110 120 130 140 150 leo 170 180 190 200 210 220 230 240 250

Nip Width

Fig. 9-48. Comparison of various types of press nips. Courtesy Black Clawson-Kennedy.

Press felt

The press felt normally runs underneath the

paper web going through a press section. It is

designed to absorb water squeezed out of the sheet

and to support the sheet through the presses. As

with forming fabrics, a coarse felt allows for more

water removal but gives a coarser sheet. The felt

is separated as quickly as possible after the press

nip in order to avoid rewetting of the sheet from

the felt.

Reverse press

A reverse press is mounted in such a way

that the paper web goes through in a direction that

is opposite to its original travel direction so that

both sides on the web ultimately contact a smooth

roll, thereby providing smoothing action on the

opposite side of the paper web. This can also be

accomplished by using two smooth press rolls, one

on the bottom of the web for one press and one on

the top for another press.

9.8 THE DRYER SECTION

General aspects

Removing water from the web is accom-

plished by adding heat to the web and circulating

the air. Heat is applied by the pressurized, steam

filled circular steel [up to 1000 kPa (150 psi)] or

cast iron [up to 500 kPa (75 psi), older designs]

dryer drums (or cans). Circulation of air over the

web is very important since the air quickly be-

comes saturated with water vapor that prevents

further removal of water from the web's surface.

Most dryer sections are completely enclosed, with

side doors that open for dryer access, to allow

efficient air circulation. Fig. 9-49 shows a dryer

section where the doors are open in order to

thread the paper through the section after a web

break. Plates 35 and 36 show close-ups of several

dryer cans with the area requiring pocket ventila-

tion.

The rate of water removal depends on many

factors: 1) the temperature and amount of steam

entering the dryer can. 2) adequate removal of the

steam condensate and air from the interior of the

drying can. 3) the amount of sheet-dryer surface

contact, contact time, and contact pressure. 4)

Cleanliness of the drum's exterior and interior

surfaces (mainly in regards to the amount of scale

on the surfaces). 5) Type of felt and condition of

felt. 6) Circulation of hot, dry air.

Dryers

Dryers are hollow, revolving, steam filled

drums (about 2 m or 6 ft in diameter) designed to

heat the web by direct contact and remove water

by evaporation. Fig. 9-50 is a diagram of a dryer

drum with a stationary (does not rotate) siphon.

About

1.2-1.5

kg of steam are used to remove 1

THE DRYER SECTION

241

Fig. 9-49. A dryer section on a machine making white printing paper. The doors are open to allow

the paper to be threaded after a web break.

Front Side

-Shell

Back Side

Drive Gear

iVlanhole

Journal

Head

Steam

Hollow Journal

Stationary Sypbon

Fig. 9-50. Diagram of a dryer can. Reprinted from Making Pulp and

Paper^

Zellerbach Corp., with permission.

kg of water from the web in the dryer section.

The web is dried on one side than the other side,

alternating between drums, making numerous felt

transfers necessary. The initial drums cannot be

too hot, otherwise large amoimts of liberated

steam will cause the paper to cockle.

The paper often stretches a few percent in the

machine direction while contracting in the cross

machine direction through the course of drying,

causing the machine direction to be stiffer and to

have a higher tensile strength than the cross

machine direction. Some of this is due to some

242

PAPER MANUFACTURE

preferential fiber alignment in the machine direc-

tion, but mostly it is due to differential shrinkage

in the dryer section. Sophisticated control of

individual dryer motors controls the tension

throughout the dryer section and reduces web

breaks.

Pocket ventilation

The ventilation system is designed to keep

hot, dry air over the free paper surface, especially

in the pockets between dryer drums where water

quickly evaporates. Pocket ventilation systems

blow hot, dry air into the pockets through air

ducts and nozzles at high velocity. Hot, dry air

may also be used to dry the dryer felts to improve

web drying. Exhaust fans over the dryer hoods

draw the moisture-laden air out.

Original felts were made of wool and were

impermeable to air and made ventilation very

difficult. With the advent of synthetic felts of

much higher permeability in the 1960s, ventilation

has improved significantly.

The last top dryer can be run with cold water

to condense water vapor on its exterior surface to

add some water to the sheet prior to calendering.

Carrier

ropes

carry the paper tail through the

dryer section when this section is threaded with

paper during startup or after paper breaks. These

ropes run in grooves on each side of the dryer

section.

Return

Run

Top Felt Dryer

Bottom Dryers

Feeney Dryer

Return

Run

^r-Bottom Felt

Bottom Felt Dryer

Fig. 9-51. An arrangement of dryer felts. Re-

printed from Making Pulp and

Paper^

Crown Zellerbach Corp., with permission.

Dryerfelts

Dryer felts hold the paper web against the

drums to improve heat transfer between the drum

and web and absorb a portion of the water evapo-

rated from the web. They help keep the sheet flat

to minimize cockle formation on the web and

improve drying. Even tension on the felt across

the machine is important. Ideally, the felt is non-

absorbent, strong, flexible, and (as always) inex-

pensive. Monofilament fabrics tend to be less

absorbent; tend to avoid pick-up of fillers, fines,

and pitch; and tend to deliver a more uniform

cross machine tension than multifilament fabrics.

Fig. 9-51 is a diagram of dryer felt runs.

Most dryer felts are constructed of polyester

monofilament. However, in hot, moist areas of

paper machines, polyester is not suitable as it is

subject to hydrolysis of the ester. This is particu-

larly true on machines making heavy grades of

paper, such

linerboard, where temperatures tend

to be high. In these areas multifilament fabrics

containing nylon, fiberglass, acrylic polymers, and

other materials resistant to hydrolysis are used.

Research is necessary in the development of

monofilament fabrics of reasonable cost that can

be used in adverse conditions.

Steam control systems

The first step in controlling the heating of

dryer drums is to remove the steam condensate

from the drum. Fig. 9-52 (top, left) shows the

possible positions of the condensate. The conden-

sate tends to form pools or puddles at low machine

speeds and rims at speeds above about 6 m/s

(1200 ft/min). Larger amounts of condensate

increase the speed required for rimming. At

intermediate speeds, the condensate will splash or

cascade. Therefore, older, slow machines used

stationary siphons that continuously collected

condensate from the bottom of the dryer drum.

Modern, faster machines use rotating siphons that

continuously removes rimming water. Rotating

siphons are mechanically more reliable because

they do not impact against stationary pools of

water. Too much condensate, especially when

rimming, decreases the heat transfer.

A pressure gradient across the inlet and outlet

of the dryer, known as the differential pressure or

DP,

is used to control the rate of condensate

THE DRYER SECTION

243

Dryer Shell

Cross Section

Pressure

Control

Splashing Rimming

Condensate

Dryers

Steam Supply Header

Shut

Off

Valve

f ^;

To Steam

Plant

Relief

Valve'

I Pressure

1^^^»

Regulator

Cascade Steam System

Steam Header

Dryer Section

Vent

Level Control

Return

Steam Plant

Condensate Tank

{compressor^

Pressure

Control

•-1

,_.-J

U-x /t / # —y —jf —I

Dryers

Differential

Pressure Control

-*"Vent

Condensate Pump

Basic Steam

and

Condensate Control System

Level Control

To Steam

^ X

Plant ^^T>-

Steam System with Thermo-compressor

Fig. 9-52. Dryer condensate behavior and steam control systems. Reprinted from Making pulp and

removal. A differential pressure of

3-5

psi is used

at machine speeds of 1000-1500 ft/min and is well

over 25 psi at speeds above 4000 ft/min. Rotating

siphons generally operate with a higher differential

pressure than stationary siphons due to the

centrif-

ugal force the condensate must overcome on its

way out. TAPPITIS 0404-31 gives recommended

differential pressures for given dryer speeds and

condensing rates for rotating and stationary si-

phons. The differential pressure must be high

enough so that some steam blows through without

condensing; otherwise condensate will collect in

the system and the dryer fills with water (becomes

flooded), which decreases its efficiency and in-

creases the drive load.

After the drying rate has been selected by the

operator, it is important to maintain it at the

desired level using a steam control system. Fig.

9-52 shows three control systems used to maintain

the proper amount of steam in the dryer section.