Biermann Ch. Handbook of Pulping and Papermaking

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214 9. PAPER MANUFACTURE

protrudes beyond the headbox to direct the slice

outward. Wi\h pressure formation, the upper lip

protrudes beyond the apron and the jet is directed

toward the wire, which may cause poor formation,

two sidedness, poor retention, and wire mark. As

a result of the development of water removal

equipment, it is no longer necessary, or desirable,

to use pressure forming except for tissue grades.

The equation, F = h sin

jS,

for the forming

force, F, shows that the force will increase with

the speed (since h, the head, increases with speed)

unless the approach angle,

/3,

can be reduced (Fig.

9-6. If the forming pressure reaches 34 kPa (5 psi

or 10 in. Hg) then the sheet is "welded" to the

wire,

and cannot be removed. When

= 8°, this

will occur at a speed of 23 m/s (4500 ft/min).

The energy of the jet is used to displace the

air accompanying the return fabric into the pans.

As speeds increase, the energy of this air film

increases, so the angle cannot be reduced. There-

fore,

the forming force increases with advancing

speed. To circimivent this contradiction that

occurs at high speeds, the breast roll can be

lowered to decrease jS by allowing a flat delivery

as depicted in Fig. 9-7.

9.4 THE FOURDRINIER WET END

Fourdrinier

The fourdrinier, or flat wire machine, is a

paper machine with a horizontal, moving, fine

mesh, woven wire cloth or plastic fabric upon

which the pulp slurry is deposited, forming the

web (Fig. 9-8). The front side or tending side is

the side from which the paper machine is con-

trolled, whereas the other side is the backside or

drive side. The wire forms a continuous belt that

picks up fiber at the breast roll from the headbox,

runs over the table rolls, foils, suction boxes, and

then over a couch roll, where the web of fibers

leaves the fourdrinier table. The wire, however,

continues around the couch roll, under the ma-

chine, to the breast roll where more fiber is

received. A wire might make several hundred

thousand trips in its life. The position and tension

of the wire are controlled by special rolls. Fig. 9-

9 shows the guide roll with wire showers and Fig.

9-10 shows the tensioning roll from a linerboard

machine.

Forming Force

h sin p

P = Jet Angie

h = Total Head

Example

8°

sin 3

0.035

0.087

0.139

inWG

300

400

672

kPa

74.8

99.6

167.3

Forming Force |

inWG

10.5

34.8

93.4

icPa 1

2.62

8.67 i

23.2

Fig. 9-6. The forming force.

Courtesy of V. E. Hansen.

F = h sinp = 500 sin 0.5°

= 500 (0.008) = 4 In WG

= 124.5

VPa

(0.008) = 1 kPa

Fig. 9-7. Reducing pressure formation by

tesy of V. E. Hansen.

Clothing

Paper machine clothing consists of forming

fabrics (wires), press felts, and dryer felts.

Wire,

forming fabric

The wire, or now more precisely called the

forming fabric, is a continuous loop or belt of

finely woven screen made from wire or plastic;

the mesh size varies from 40 to 100 mesh (open-

ings per inch). A coarse wire allows faster drain-

age but gives a coarser paper; as in most aspects

of pulp and paper, there is always a tradeoff in

goals.

Before 1960, wires were made from metals

such as bronze, but they are now almost invariably

made from plastic such as polyester which lasts

much longer and is corrosion resistant, although it

stretches more and cannot handle highly abrasive

furnishes. The forming media has three functions:

to transport the fiber.

to permit draining the sheet.

to transmit power.

THEFOURDRINIERWETEND 215

Headbox

Dandy Roll

Fourdrinier Wire

Forming Board

Guide

Rot!

Tension

Roll

Saveall Tray

Suction Box

Suction Couch Roll

Wire Return

Roll

Fig. 9-8. A fourdrinier wet end. Reprinted from

Making

Pulp and

Paper^

Corp.,

with permission.

The power input is usually through the couch and

wire turning roll.

The geometry, weaves, mesh, wear patterns,

void volume, and numerous other aspects of

forming fabric can be quite complex. The three

references listed at the end of the chapter for this

area are a good introduction to the topic.

Wire showers

Wire showers (Fig. 9-9) are high pressure

showers on the underside of the wire used to

remove fillers and other material which may plug

the wire. Usually, they move back and forth

slowly to clean all parts of the wire and prevent

wire wearing in one spot.

Fig. 9-9. Sensor paddle Oeft, on the machine backside) and guide roll (right, on the machine

frontside) for linerboard machine. Wire showers are visible to the top left of the guide roll.

216 9- PAPER MANUFACTURE

Fig. 9-10. The tensioning roll for linerboard.

The continuous mat of fibers that is in the

process of forming or which has already formed

the final paper is known as the web.

Breast roll

The breast roll is located under the headbox

and serves to return the fabric to the forming area

to receive the stock once again. It must be rigid

enough to resist deflection. Fig. 9-11 shows the

breast roll from a linerboard machine. During the

era of pressure forming, it served as a water

removal device acting like a table roll. Pressure

forming is now not advised except in tissue form-

ing.

Forming board

The forming board (Fig. 9-12) consists of a

large leading blade to reach in close to the slice

followed by several smaller blades, usually with

gaps between

them.

The blades must be located at

intervals such that the harmonic (pulse frequency)

developed agrees with the harmonic of the rest of

the drainage equipment. Several designs of

surfaces give configurations that serve to retard

drainage as well as to provide agitation that pre-

vents flocculation. Retarding early drainage

facilitates uniform water removal and improves

Fig. 9-11. Breast roll from a linerboard machine also showing the forming board and headbox slice

adjustments.

THE FOURDRINIER WET END 217

formation.

The

blade material

can be

plastic,

usually polyethylene,

or a

ceramic (such as alumi-

num oxide, zirconium oxide, silicon nitride,

or a

combination

these)

minimize wear.

The

blades are usually attached

the heavy structure

Tee

bars

simplify maintenance. Forming

boards often cause problems and will likely not be

used on newer designs machines.

Deckle board

Deckle boards are used

prevent the stock

from flowing

off the two

sides

of the

forming

fabric when

thick layer

stock

delivered

the fabric by

large slice opening. Often the two

edges

the fabric are curled upward by means of

plastic edge curlers. Stationary deckle boards

cause

deckle wave

due to the

friction between

the moving stock and the stationary board. Curl-

ing

the

edge

of the

fabric helps

alleviate this

problem. Often water jets

are

used

place

boards

curlers

provide

hydraulic "wall"

contain the stock

for

small slice openings.

Trim squirts, edge squirts

The width

the sheet delivered to the press

section

controlled by trim squirts. The narrow

bands

stock left

the fabric

are

knocked

off

by edge squirts. On newer designs

fabric there

may have

to be

two trim squirts

line

each

side

to get a

clean "cut". Economics dictate that

a minimum

wasted

trinuning, about 5-7

(2-3 in.), although this material

returned

to the

paper machine.

Table roll

Table rolls (Fig. 9-13, top) are freely revolv-

ing rolls under

the

fourdrinier wire that support

the weight

of the

wire

and wet

web. Water

removed from the bottom

the web

by a

partial

vacuum [45 kPa below ambient pressure

in.

of mercury

machine speeds

of 10

m/sec

(2000 ft/min) over

distance of about

cm] at the

meniscus that forms

at the nip

where

the

roll

leaves

the

wire. Burkhard

and

Wrist (1956)

showed

the

maximum suction

about VipU^,

where

p is the

density

water

and U is the

machine speed. Table rolls

are not

useful

speeds much higher than this, because

higher

speeds, water

on the

incoming side

the table

X—h-X-*i—X—t*-X—h-X-

Fig. 9-12. Fonning board with variable open

E. Hansen.

roll goes back into the sheet; this pumping action

causes stock jump and tends

remove fines

and

fillers leaving

the

paper's wire side deficient

these items. Also, since

the

white water may

fairly warm,

it may

actually boil

at the

reduced

pressure

the nip

the table roll. Thus,

two

sided sheet forms, one with different properties on

the

two

sides. Foils have, therefore, replaced

table rolls on all paper machines.

Foils

A foil (Fig. 9-13, bottom photo), introduced

by Wrist

and

Burkhard (1956),

is a

stationary

blade

5-10 cm (2-4 in.)

wide with

divergent

surface

that

angle forms between

the

sta-

tionary fabric

and the

foil surface.

The

angle

usually between

0.5 to 3°.

With stock

on the

fabric,

and the

fabric

motion,

the

suction that

develops causes the fabric

draw down towards

the foil surface.

As the

fabric leaves

the

suction

area,

must rise

to the

height

of the

next foil

blade.

order

develop fine scale turbulence,

the frequency should be

40 to

90 foils

per

second

with

maximum

120

per

second. (This does

not mean there

are

that many foils.) The move-

ment of the fabric in the vertical plane must not be

so rapid as

cause the stock to separate from the

fabric

and be

"thrown" down

the

machine,

be-

cause,

due to air

resistance,

will land

on a

different area

of the

fabric from which

left.

Foils have two functions apart from supporting the

fabric. These are:

218 9. PAPER MANUFACTURE

Fig. 9-13. Comparison of table rolls (top) and foils on fourdrinier machines.

to provide hydraulic shear (i.e., activity, as

shown in Fig. 9-14).

to give uniform, controlled water removal.

In his classic work on paper formation from

dilute fiber slurries, J. D. Parker (1972) main-

tained that the diameter of the "spouts" should

equal the fiber length to prevent flocculation.

Fig. 9-15 shows a foil removing much water

shortly after the headbox. Foils are effective at

relatively high speeds and form a smaller vacuum

force, but over a longer distance than table rolls

(about 10-13 kPa below ambient pressure or 3-4

in. of mercury over a distance of 5-10 cm or 2-4

in.).

The leading edge of a foil doctors water

pulled down from the previous foil.

A foil table must be designed carefully to

avoid the common problem of sheet sealing. This

condition arises from rapid drainage from 0.8 to

1.4% (overall) consistency (Hansen, 1991). A

high consistency (between 5 and 10%) layer of

fiber is "laid" on the fabric, but the stock above is

still at headbox consistency. Further controlled

drainage is not possible because of the short

duration of foil pulses. A high dragload and a

poor moisture profile results when drainage is

attempted by hivacs (suction boxes). Thus, the

table should be divided so that foils activate the

sheet and carefully advance the consistency

through 1.4% to about 1.6 or 1.8%. Then drain-

age is taken over by the lovacs and hivacs.

High speed designs must control the activity

so that stock jump does not occur. Activity must

THE FOURDRINIER WET END 219

Fig. 9-14. Activity on the forming fabric with proper operation of the

foils.

Courtesy of

V.E.

Hansen.

be fine scale to insure the best formation. In other

words, the headbox must deliver a well-formed

sheet, and the foil table must develop shear so the

formation is not destroyed. Uniform foil spacing

is necessary to maintain the frequency pulses until

1.6 to 1.8% consistency is reached.

Lovacs

A lovac (Hansen, 1991) develops suction

using water-filled drop legs (barometric legs like

those used in brown stock washers) to provide

siphoning action (Figs. 9-16 to 9-18). These are

sometimes called wet boxes. The pressure gradi-

ent) is proportional to the height of the water

column. Four distinct types of covers are used,

all of which are slotted. To cause drainage only,

a series of flat blades (7 to 13 is common) is used.

To impart activity as well as drainage, three

distinct cover shapes are in use: one uses foil

blades called a Vacufoil™; a second uses a

shaped blade, called an Unfoi^^; and a third uses

blades at different elevation, called an ISOFLO'^^

(Fig. 9-17). The Vacufoil allows imposing a AP

greater than the water leg permits as a result of

the suction from the foil. The Unfoil allows

activity to be generated using a AP generated from

the water leg. The disadvantage of these two is

that the white water consistency is greater than

Fig. 9-15. Foil action

a fourdrinier machine.

220 9. PAPER MANUFACTURE

= Mv

Atmospheric Pressure

AP Forces Water Through Fabric 5.5 to 9 %

Suction Line

^S^^^^^'^^^^'^

Max.

To HIVACS

Gage

W9P

TAPPI. Courtesy of V.E. Hansen.

when the flat blades are used. The ISOFLO uses

flat blades employing ceramic blades for fabric

support. The control blades limit the vertical

downward travel of the fabric, thereby control the

level of activity. Sheet sealing can be prevented

by both Unfoil and the ISOFLO. The unfoil uses

the return water to the bottom side of the sheet

(similar to the action of table rolls), and the

ISOFLO uses the pulse from the sudden stopping

of the downward travel of the fabric.

Fig. 9-18. A lovac on a paper machine.

Controls are essential for the successful

operation of lovacs. These entail proper size of

piping and using an exhauster to generate the

suction. Automatic valves are necessary to hold

the AP values selected (Fig. 9-19).

On free draining grades, it is usual to have a

series of lovacs, often four, operating at increas-

ing AP as the sheet advances towards the couch.

TAPPI. Courtesy of V. E. Hansen.

THE FOURDRINIER WET END 221

Fig. 9-19. Automatic valves to control the AP on lovacs. Courtesy of V. E. Hansen.

Both the wetline (5.5% consistency) and dryline

(10%

consistency) can be developed economically

on lovacs (Fig. 9-20). Until the dryline is

reached, water is removed in film form as with

foils.

After the dryline, air passes through the

sheet to transport the water in droplet form.

In slow draining stocks, such as mechanical

pulps,

a wetline cannot be developed economically

on lovacs. For some chemi-mechanical pulps with

freeness above 230 CSF, a wetline can develop on

a lovac. When the CSF is below 190, the maxi-

mum consistency obtained is about 4% by lovacs.

The lovac blades take on the shape of hivac blades

with doctoring edges (Fig. 9-21). This doctoring

edge is necessary because water is removed in

films until at least 9% consistency is reached.

Actually the unit is a special design of lovac. If

the usual blade spacing for lovacs is used at these

higher AP values on these grades, the fabric

"sags"

into the slots, and more power is required

to pull the fabric out of the slots.

Very good automatic control for AP is neces-

sary to minimize power consumption and maxi-

mize fabric life. The area of the waterlegs for

draining the box is not less than 20% of the open

area of the cover. Lovacs operate between 4 and

50 in. water gravity (WG) (1 to 12 kPa) generally.

However, for high speed machines where the

lovacs are used for activities (ISOFLO) the AP

may be as low as 1 in. WG (0.25 kPa).

Suction boxes, flat

boxes,

hivacs

In the progression along the forming table the

stock is subjected to increasing AP to drain the

sheet. After the lovacs, which rarely operate

above 12 kPa (50 in. WG), the stock encounters

the suction boxes or hivacs. The term "flat box"

is no longer appropriate since curved covers are

used regularly on two-wire machines.

Drilled ceramic became popular in the devel-

opment of suitable cover material for these boxes.

As machine speeds increased, however, these solid

ceramic covers were not strong enough for the

increasing

AP.

In addition, drilled holes presented

too much surface area inside the hole, creating

excessive friction for the water to pass through.

222 9. PAPER MANUFACTURE

Sheet Squeezed by Pressure Difference

from Atmosphere to Inside of Unit

/ / / / /

Fig. 9-20. Generation of wetline and dryline on

a lovac. Courtesy of V.E. Hansen.

A slotted technology developed wherein ceramic

pieces are bonded to stainless steel bars. Each of

these pieces are 15 and 20 mm wide in the ma-

chine direction, with the former being more

efficient. Slot sizes vary from 25 mm (1 in.) to

19 mm (3/4 in.) to 16 mm (5/8 in.) for maximum

AP of 17 kPa (5 in. Hg), 36 kPa (10.5 in. Hg),

and 61 kPa (18 in. Hg.), respectively. Eight is

the optimum number of slots, but as many as ten

are used for a particular AP (Fig. 9-22). The

bottom of the fabric must be kept wet to minimize

wear and power requirements; consequently, if

more slots are used at a given

AP,

fabric wear and

power requirements increase. To work within

these limits AP must increase exponentially. The

result is fewer hivacs are used (Fig. 9-23). Again,

the need for accurate, automatic control is obvi-

ous.

Adequate water removal at the hivacs, before

the drilled couch roll, is essential on high speed

machines since the couch has less tune to remove

Water Removed in Films by

Doctoring Edges

Fig. 9-21. Water removal between the wet- and

Courtesy of V.E. Hansen.

water; therefore, the hivacs must present a dryer

sheet to the couch in order to avoid pinholes.

The design of piping is critical to ensure that

water droplets can be transported from the sheet to

the box to the separator. After the separator,

when water flows down to the seal tank and the

air goes to the header and to the vacuum pump,

the air velocities can increase (Fig. 9-24). Pipe

diameters must be large enough to control the air

velocities to a maximum of 17.8 m/s (3500 ft/min)

from the hivac to the separator, 0.45 m/s (750

ft/min) through the separator, and 25.4 m/s (5000

ft/min) through the risers and headers.

To dry different grades of paper, different

flow densities are required, Kraft bags are very

"open", and require the highest at 290 cmVs/cm^

(4 ftVmin/irf). Most other grades require 110-145

cmVs/cm^ (1.5 to 2 ftVmin/irf). The objective is

to dry the sheet as much as possible with a suit-

able power consumption (drag-load).

Free draining grades can be "dried" to 10%

on lovacs at low dragload. Three or four hivacs

can be used to increase the consistency to 19%

before the couch. Slow draining grades can be

raised to 4% on lovacs, but then require two

special chambers to arrive at 10% (dryline). Four

more graduated chambers with proper airflow

density will normally raise the sheet consistency to

19%

before the couch. Some grades may require

six chambers.

Because of the development of multi-layer

fabrics that carry large volumes of air (Table 9-2),

multi-chambered hivacs are useful to minimize the

increase in airflow. As the fabric and sheet enter

THE FOURDRINIER WET END 223

Fig. 9-22. Multi-chambered (two, of eight bars each) hivac unit. Courtesy of V. E. Hansen.

the slotted area, the air must be removed from the

fabric before moisture can be removed from the

sheet. As the fabric leaves the hivac, air reenters

the fabric. The use of a multi-chambered unit

minimizes the number of times the fabric is evacu-

ated of air (Fig. 9-25). For example, a triple

layer fabric on a machine 5.1m (200 in.) wide at

HIVAC GRADUATION

Most Grades

Airflow: lj;-2ft*(inlnrin^

(109.7 • 146.3 cm's^citO

Kraft Bag

Airflow: 4ft*(mln)''ln^

(439 cmVcnO

Hg (kPa)

6 In Ha _

2 Ixx Ho ^^1

3/4 in (19fnin) 3/4 In (19mm) S/8ln(16mm) S/8ln(16mm)

Power

Fig. 9-23. Representative hivac graduation.

12.2 m/s (2400 ft/min) at 38 kPa (10 in. Hg)

requires 40.1 L/s (85 ftVmin) more airflow than a

single layer for each

hivac.

On a machine 7.62 m

(300 in.) wide running at 18.3 m/s (3600 ft/min)

can waste 0.755 m^/s (1600 ftVmin) pumping out

the fabric for three chambers. Thus, it is impor-

tant to minimize hivac chambers by combining

PiVi/Ti = RiVa/Ta

To Vacuum Sourc*

' Capablaof ISfntoiSlnHg

Small

Pip*

-»-

P s Pressure

V - Volume

T s Temperature

..«.

Largo Pipe

l \\\ "^'9®'

ITTV7

Umit Air Velocity

to 5000 ft/mln

Low Pressure Pt

Large Volume

Large Pipe -•'

Large Seal

Tank

I I

L.J

I I

L.J

^•^Plpe

t Extra

I Length

204 in

245 in

-«-> Small Seel Tank

Fig. 9-24. Pipmg for a hivac system.

TAPPI. Courtesy of V. E. Hansen.