Biermann Ch. Handbook of Pulping and Papermaking

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224 9- PAPER MANUFACTURE

Table 9-2. Wasted vacuum requirements (ftVmin) for various fabrics. Courtesy of

V.E.

Hansen.

Void volume, %

1 Coefficient V |

1 W

vol.

10"

Hg IC

1 300 in @ 3600

Single wire

58.6

7.5

324

729

Double

56.1

9.9

71.3

428

962

Double X

57.0

10.3

74.2

445

1001

Triple

63.0

19.3

139

834

1876

Kraft D |

67.0

23.2

167

1002

2255

V is standard atmosphere cftn/(100 in. width)/(100 fpm)/box

IC is 1 chamber @ 2400 fpm/(200 in. wide) @ 10 in.Hg; 6C is 6 chambers (units) @ 10 in.Hg

them into multi-chambered units that have to be

evacuated only once.

In summary, free draining grades require

three or four hivacs, depending on speed. Me-

chanical pulps may require as many as eight,

which is conveniently carried out with four two-

chambered

units.

Linerboard is a special case that

requires careful design to control the dragload.

Before the secondary delivery, one hivac chamber

is necessary if the primary sheet requires 15%

dryness because whitetop is being manufactured.

The secondary layer is accepted on a chamber at

a low AP. Then four chambers are required

where the AP is slowly increased at a linear rate

to raise the sheet to the dryline at about 9%

consistency. After the dryline is achieved, four of

five hivacs are used in the usual fashion. [Suction

boxes with showers are also used for cleaning felts

in the press section (Fig. 9-26).]

A note about ceramic materials

Slow paper machines still use polyethylene

surfaces to resist wear on moving parts. Fast

paper machines are using more ceramic materials

for surfaces that contact their clothing. Neverthe-

less,

there are no established ceramic standards for

the paper industry and few for the ceramic indus-

try, and those few are applied differently by each

supplier of ceramics, so technical specifications

supplied by the manufacturers of ceramics must be

supplemented by specific tests relevant to paper

machine operation to be useftil to those fabricating

ceramics into devices. This area must be ad-

dressed by the pulp and paper industry.

Dandy roll

The dandy roll (Fig. 9-27) is a hollow, light,

wire covered roll that rides on top of the four-

drinier wire just ahead of the suction

boxes.

This

roll has four purposes:

to impart a water mark to the sheet. Water

marks are visible in high quality bond papers

when held to the light and can be used to

show a company's logo, for instance.

to improve the top surface for printing.

to improve formation by mechanical shear.

to increase the drainage capacity of the flat

wire.

The dandy must be axially mounted, like any

roll, or end ring mounted so that it will rotate

without vibration. During the 1980s, dandy diam-

eters have increased, and the mounting mechanism

19.0%

o.d., InHg kPa

4- 10 33.8

+ 5 16.9

25 mm I 19 mm I 16 mm I

r Slots I 0.7S" Slots 10.63" Slots •

Multi-Chambered Hivac

17.0%

0.(1.

InHg

3 Single Hivacs

Fig. 9-25. Multi- vs. single-chamber hivac effi-

ciency. ®1991 TAPPI. Courtesy of

V.E.

Hansen.

THE FOURDRINIER WET END 225

Fig. 9-26. Profile of yacuum box and showers for cleaning a press felt.

Fig. 9-27. The dandy roll of a machine operating in 1976.

226 9- PAPER MANUFACTURE

has improved. A minimum diameter for purposes

(3) and (4) is 1.09 m (42 in.) but 1.2 to 1.5 m (4

to 5 ft) will impart more shear. The roll must be

driven by a timing belt and monitored for speed to

impart shear. It is most effective when mounted

between lovacs, with the lovac separation equal to

the diameter of the dandy. If the objective is to

improve formation, the consistency should be 2.5

to 3%, controlled by a ganmia gauge. For drain-

age improvement, about 1.3 to 1.4% consistency

will cause "dandy throw". This stock is collected

and returned to the stock tank. This "throw"

sometimes succeeds in evening out the two sides

of the sheet.

Couch roll

The couch roll (Fig. 9-28) is the guide or

turning roll for the fourdrinier wire, where the

paper web leaves the wire and the wire returns to

the breast roll. It has two purposes: to transmit

power to the fabric and to increase the dryness of

the sheet. In an open draw machine the fabric

wraps the couch at least 180°. In a machine that

uses a pickup felt, the fabric wraps the couch for

about 40° and then passes on to the wire turning

roll. The fabric wraps this roll for about 135°.

These two rolls are responsible for driving the

fabric, with the wire turning roll providing 55-

80%

of the drive load. The couch roll is really a

drilled shell with countersunk holes 5.5 nmi (7/16

in.) in diameter for 28-65% of its surface. Inside

the shell is a close fitting "box" containing seal

strips on the ingoing and outgoing edges. This

box is connected to a vacuum pump capable of

generating as high as 85 kPa (25 in. Hg) at sea

level, depending on the grade of paper (Fig. 9-

29).

For linerboard the box often has two cham-

bers.

The first chamber may offer 34-41 kPa (10-

12 in. Hg) starting at top dead center, and the

second chamber offers up to 85 kPa (25 in. Hg).

The ratio of the size of the first to second chamber

is usually

2:1.

Airflow density in the couch varies

from 440-1460 cmVs/cm^ (6 to 20 ftVmin/irf).

The limitation of the couch's ability to dry

the sheet is its ability to "throw off" the water it

draws into the shell in the suction zone. As each

Fig. 9-28. A couch roll (bottom left) and lump breaker roll of a linerboard machine. Note the

water being thrown off the couch roll (through the drilled holes) after the felt separates from it.

THE FOURDRINIER WET END

227

POWER FOR DE-WATERING TO A PARTICULAR DRYNESS

CALCUUTED AS FABRIC TENSION INCREASE (AT)

SI Units

At 3000

n/min

(914.6 m/min)

5 ft

diameter (1524 mm)

couch rotates 3.2 times per

second.

13% entering

and 18% leaving,

the couch cannot throw

all this water in

1/3

second l>ecause of friction.

'Haafi

Atmospheric

Pressure

Wasted Volume

D.A.

Drilled Area

50%

Drilled Width

286 In (7.26 m)

Shell Thickness

2.5 In (6.35 cm)

Machine Speed

3000 ft/min (914.6 m/mIn)

A P

24 In Hg (81.3 kPa)

D.A. (W) U (t)

Example

*•

•s(f)(f)'««'(§)•=«»

«''-

•"=a(W)(1i)<»"«(TO)=^»'"'"''

Fig. 9-29. The couch assembly (top); courtesy

of Beloit. Operation

the drilled couch roU

and wasted volume

the

couch.

TAPPI. Courtesy of V. E. Hansen.

roll of holes enters the suction zone the water and

air must be reduced to make room for the water to

be removed from the sheet and the water

from

the

seal shower. This water

pulled into the shell

until the row of holes passes the seal strip on the

exit edge of the box. Now the pressure is equal-

Fabric Speed

m/s

To Convert

English Units: (5.71)

Fig. 9-30. Dragload.

of V. E. Hansen.

ized at atmospheric pressure at each end of these

holes.

During

the

remainder

the revolution

out

centrifugal force.

As the

diameter

the

holes has been reduced

an attempt to increase

the open area, the friction

the holes has been

increased. (Consider the flow through

pipe

proportional

r*.) Consequently,

machines

run faster,

the

couch runs wetter and does less

drying of the sheet.

This repeated pumping out of the couch leads

its

mechanical inefficiency (Fig. 9-29).

speeds increase,

necessary

improve

the

hivac operation to make up for the loss

couch

efficiency. This makes

necessary

monitor

dragloads, the power required

keep the paper

machine moving per unit of velocity.

Dragload

The term dragload resulted from the necessity

of fabric manufacturers to monitor the power used

to drive the fabric. Dragload is reported in units

of lb/in.

N/m (Fig. 9-30).

It is a

measure of

the increase

tension (AT)

the fabric

result

the

suction forces pulling

the

fabric

against the foil surfaces, the lovac surfaces, and

the hivac surfaces.

calculated number,

although

can be measured with an instrument.

Dragload is equivalent to drive power divided by

machine speed and can

calculated from

the

product of the volts and amps supplied to the drive

motor divided by the machine speed. An on line

readout

dragload

recommended

useful

information for the paper machine operator.

Fig. 9-31 shows typical equipment and the

228 9' PAPER MANUFACTURE

FORMING

BOARD

mOHJOf

pmammm

OSCILLATJNG

CLEANING SHOWER

TRJVS^CWITHStOTTEO SUCTION BOX WITH StOrTEO MAHOMETER

DURAMIC COVER DURAMIC COVER

AUT0\»^C2

INDEXiNG

SELF CLEANING FABRUSH

Fig. 9-31. Equipment and controls for dewatering. Courtesy of JWI Group, Johnson Foils.

THE FOURDRINIER WET END 229

FOIL UNIT

DURAFOJL INSERT BLADE DURAFOIL OUTSEm- BLADE SHOWER AND

OSCILLATOR

FELT

SUCTION BOX

t Felt Suction Box &

Lubricating Shower

High Pressure Needle

Cleaning Shower

3. Flooding Shower

ChemJcal Cleaning

Shower

Fig. 9-31. Continued.

230 9. PAPER MANUFACTURE

layout of that equipment used in dewatering at the

wet end. This figure summarizes many of the

concepts discussed in this section.

Lump breaker roll

The lump breaker roll (Fig. 9-28) is a solid

roll with a suitable soft cover that is mounted over

the couch roll to assist in drying the sheet by

pressure. If the couch has two chambers, the

lump breaker roll is mounted over the separating

wall between them. As the sheet passes under the

lump breaker, it is squeezed by the lump breaker.

Actually, this arrangement is a press. The soft

cover causes a relatively wide nip to be formed.

As the sheet is squeezed, water flows in reverse to

enter the holes of the first chamber. Since some

of the holes of the second chamber are covered by

the nip formed by the soft cover of the lump

breaker, the air velocity in the second chamber is

increased. This combination of actions will re-

move an additional 5% of the water.

Lump breaker rolls are always present on

linerboard machines and machines using mechani-

cal pulps. As the speeds increase, the lump

breaker efficiency will decrease unless the couch

can be improved as a result of

hivac

improvement.

Usually a misting shower, which must be absorbed

by the couch, keeps the cover wet so that fibers

will not adhere to it.

Pickup

(transfer)

felt

The pickup felt is a traveling felt (blanket)

designed to pick the wet paper web off the wire

and transfer it to press section. This means that

the web is supported at all times by either the wire

or felt. In the past, paper machines that operated

at slow speeds with heavy paper grades used to

have an open draw, where the web was actually

unsupported during this transfer. A supported

transfer is known as a closed

draw.

Some exam-

ples of felt transfers are given in Section 9.7.

9.5 TWIN WIRE FORMERS

Twin wire formers

Twin wire formers are machines that use a jet

of stock imparted on two convergmg wires to

accelerate water removal and maintain better web

uniformity. These are particularly useful for high

speed machines, where the fourdrinier wet end

would tend to give a two-sided sheet, since both

sides are wire sides and the sheet is formed sym-

metrically on the two sides. D. Webster is credit-

ed with the invention of the twin wire former.

The first twin wire machine for board, the

inverformer, was commercially available in 1958

and for paper in 1965.

Some examples (Fig. 9-32) are the Black

Clawson Verti-Former®, the Papriforma, the

Beloit Bel Baie H, the Inverformer, and the

Duoformer. Fig. 9-33 shows a Bel Baie II making

computer printing paper. Fig. 9-34 shows the

Verti-former. In the early verti-formers, the stock

flowed downward from the headbox mounted

above and between the two wires traveling verti-

cally downward. In the Bel Baie former, the stock

is sprayed as a high speed jet in an upward direc-

tion from the headbox that is mounted below and

between the two wires that move upward.

Many flat wire machines use a second wire

part way down the table to help with dewatering

and formation. The Top Flyte'^'^ "C" former is

shown in Fig. 9-35 and Plate 33. Note that this

type of machine has two flat wire parts: one

leading to the nip and one after the former. These

STOCK EXITS FROM

CLOSED HEAD

BOX THROUGH

JET ORIFIC

VERTIFORMER

(BLACK CLAWSON)

BEL-BAIE FORMER

(BELOIT)

Fig. 9-32. Diagrams of several types of twin-

wire formers. Courtesy of W. Bublitz.

TWIN WIRE FORMERS 231

Fig. 9-33. Beloit BelBaie II twin wire fonner (forming section) for making writing paper. The

insert is a close-up of the slice where the dilute pulp slurry is applied upward between the wires.

two areas must obey flat wire operation. In the

flat section after the headbox, the table consists of

a forming board, foils, and one or two lovacs. If

this section is very short, the danger of sheet

sealing is present. If the sheet enters the nip in a

sealed condition, the formation is unsatisfactory

and the retention is low. The flat wire must be

managed carefully if foils are used as they tend to

seal the sheet between 0.8 and 1.4% consistency.

Fig. 9-36 is an audit of a news retrofit with a

bladed shoe. Note that the sheet is 7.25% consis-

tency upon entering the hivacs. The foil unit

Fig. 9-34. Verti-former V twin wire former and headbox. Courtesy Black Clawson-Kennedy.

232 9. PAPER MANUFACTURE

sLARGE

FtOCS

CERAMIC BLADE

Fig. 9-35. Top Flyte^^

"C"

Former. Courtesy of Black Clawson-Kennedy.

shown is for the fabric support only. With four

hivacs (two drilled and two slotted) the sheet

enters the couch at 11.8% and leaves at 14.0%

consistency. This leaves much work for the first

press to perform. Here, the first press vibrated.

Fig. 9-37 shows this machine after some modifica-

tion. The main change occurred at the hivac area:

the two drilled covers were changed to two slot-

ted; the piping was enlarged; automatic valves

were added; the airflow density was increased

from 60 to 130 cmVs/cm^ (0.8 to 1.8

ftVmin/irf;

and the maximum AP raised to 60 kPa (18 in.

Hg).

The remarkable increase in dryness to and

from the couch eliminated press section vibration

and resulted in 15% savings in steam to the dry-

ers.

The foils were changed to lovacs that pulsate

the fabric. As a result, more water was added and

formation improved. Note that the consistency to

the nip was reduced below the 1.4% danger level

for sealing, and so extra shear resulted in the two

wire portion.

Twin wire formers can exert high pressures

between the two sides of the web, whereas single

wire machines are limited to a pressure differential

of one atmosphere. Shear forces occur that are

not possible on fourdrinier machines. These shear

forces greatly help the formation of the sheet and

improve the overall quality of paper.

TWIN WIRE FORMERS 233

RECYCLE NEWSPRINT

30 lb/3000

ft’ (48.8 g/m’)

RETROFIT WITH FOIL TABLE

FABRIC

3341

ft/mln

(1019

m/min)

65%

RECYCLE

^S 8SaS!!«9«

Ck \3

57^««.®«aaaaa v^ QU

8 3

DD DD @

W JZL

+20

+ 10

ja_

-EL

Fig. 9-36. Retrofit of a newsprint machine using 65% recycled fiber. The fabric speed is 17.0 m/s

(3341 ft/mm) and the basis weight is 48.4 g/m^ (30 lb/3000 ft^). Courtesy of V. E. Hansen.

55 +

5o|

ji,-

0) O>30 +

01-

2 WIRE RETROFIT

Type:

Blades & PDM

Product: News

B.W.: 30 lb/3000 ft* (48.8 g/m')

-f 150

125

100

Fabric Speed:

3375 ft/min (102

Dragload: 31 lb/in (5.42 kN)

HIvacs: 1.8 ft'(mln)'ln^@18 in Hg

(131.6

cm's'cm*

60.9 kPa)

/min)

SI »

"1—I r

3 2

75 _-

-4

. 1 i i

d)"^

ta s

B B

-J

1 1

a D a •

' '

as-r

^•=

I ,.

mMlimmm^mmmMKLmmmmmJMLmt

+ 50

+ 25

Fig. 9-37. Blade-type retrofit of news machine with a remarkable

19%

before the couch. Suitable

risers and header msure air velocity < 25.4 m/s (5000 fpm). ®1991 TAPPI. Courtesy V.E. Hansen.