Mechanisms of water removal during wet pressing

Phase-1: Compression of sheet and felt begins: air flows out structures until the sheet is saturated; no hydraulic pressure is built up (and therefore, no driving force for dewatering) Phase-2: The sheet is saturated and hydraulic pressure within the sheet structure causes water to move from the paper into the felt. Phase 2 continues up to the mid nip where total where total pressure reaches maximum. It has been shown that hydraulic pressure reaches maximum just prior to mid nip. Phase 3: The nip expends until the hydraulic pressure in the paper is zero, corresponding to the point of maximum paper dryness. Phase-4: Both paper and felt expend and the paper becomes unsaturated. Although a negative pressure is created in both structures a number of factors cause water to return from the felt to the paper.

Principles of pressing Total pressure = Mechanical + Hydraulic

Hydraulic pressure gradient causes water to flow in the path of least resistance

Mechanical pressure gradient equal and opposite hydraulic gradient

Types of presses

The most important requirement in press design is to provide the shortest path for the water to flow in escaping from the nip.

The shortest distance is equal to the felt thickness (vertical direction). The main water flow should be perpendicular to the

felt and lateral flow should be minimized

Plain Press

Top roll positioned so that sheet and fabric wrap this roll ahead of nip to remove air bubbles which would cause press

wrinkles in sheet.

Water pressed out of sheet and fabric flows back on surface of bottom roll against roll's rotation.

Plain press limited to 1,000 fpm (305 mpm) speed.

Double felted press (Above 130 g/m2)

Double felting a nip allows water removal in both directions.

Double felted presses are particularly beneficial in the first press where the greatest quantity of water is being handled and where

the greatest tendency for crushing exists.

In addition, double felted presses have more equal distribution of the fibre pressure on the top and bottom side of the sheet resulting

in less sheet two-sidedness as related to ink, size, and coating

absorption.

Suction Press

1/8 inch (3.2 mm) diameter holes drilled on about 5/16 inch (8 mm) centres over the entire roll surface

Air or spring-loaded seals are positioned between the inside shell surface and the box. Liquid ring type vacuum pumps

or centrifugal exhausters, located in the machine room

basement or outside the building, provide the vacuum.

Felt and sheet wrap suction zone to remove air bubbles and to seal suction zone

Water from sheet and felt drawn into holes to bottom roll by vacuum and hydraulic pressure.

Water must travel both vertically and horizontally through felt to holes in bottom roll.

Sheet is shown contacting felt after leaving press. This should be avoided because of rewetting of sheet.

Felt and sheet travel is arranged to wrap the suction area so that their surfaces at the in-going nip are under vacuum.

This provides a seal against air leakage and removes any air

trapped between the felt and sheet, preventing blowing and

felt wrinkles.

As the felt and sheet pass through the nip between the suction and plain rolls, water is squeezed out at the in-going

nip. Capillary action of the felt fibres and the hydraulic

pressure developed by the compression of the felt and sheet

in the nip cause water to flow in through the felt and into

the holes of the suction roll.

Grooved Press The grooves in the roll cover provide easily accessible receptacles for expelled water.

The helically cut grooves are typically 2.5mm (0.1) in depth, 0.5 mm (0.02) wide on 3.2 mm (0.125) centres i.e. 8 grooves per inch

The maximum lateral distance for water travel in the grooved press is only 1.3mm (0.05)

Since the grooved rolls are solid, higher pressure can be applied

The water caught in the grooves is thrown off by centrifugal force at high roll surface speeds and roll can be cleaned by the action of

spray or doctor blades

Blind drilled roll

Cover of the solid roll is drilled with small closely spaced

holes.

The wells tend to self clean by the action of centrifugal force

Fabric press

A multiple-weave, non-compressible fabric belt passes through the nip between the felt and the rubber-covered

roll to provide void volume to receive the expressed water.

Water is removed from the fabric by passing it over a suction box on the return run.

The shrink-sleeve press is a simplified modification of the fabric press, utilizing only a non-compressible fabric jacket

or sleeve which is shrunk over the press roll to provide void

volume.

Modern press felt designs with high void volumes have reduced the benefits of a separate non-compressible fabric

or sleeve.

Extended nip press

The "Extended- nip Press" features a very wide nip to give the sheet a long dwell time at high pressure.

When used as the last nip, this press provides not only a much drier sheet, but also a stronger sheet due to improved

consolidation of the web structure.

Key components are the stationary pressure shoe and the impervious elastomer belt, which form the bottom portion of the

double-felted nip.

The shoe is continuously lubricated by oil to act as a slip bearing for the belt.

The average loading of 4100 kPa (600psi) along the 25 cm (10 inch) length of shoe is equivalent to 1050 kN/m (6000 pli) by the

normal method of specifying press load.

45% or higher dryness on linerboard and other grades

Improved uniformity of dryness as a result of longer time in the nip

Increased physical properties such as Mullen burst, ring crush, and tensile

Use of lower strength pulps for a given sheet strength

Reduced refining with a given pulp

Increased production off dryer limited machines

Mechanism involved in the expressing of water from the web The Poiseulles equation describes between flow rate, pressure gradients etc. and the capillary dimensions for flow through capillary systems

8.

... 4

l

rnpQ

Where Q= flow rate, vol/time unit

p = Pressure drop across the capillary system r = Radius of the capillary

= Viscosity of the liquid n = number of capillaries

l = Length of capillary If we consider two capillary radii systems, having the same capillary fraction (same void fraction), and capillary radii r1 and r2 then the ratio between the volume flow rates Q1 and Q2 of the

two systems under a given pressure drop p wiil be Q1 = (r1) 2 Q2 (r2) Both the capillary radii and void fraction of the press felt are

much larger than those of paper.The capillary suction p of a wetted capillary is given by:

p = 2/r, where = surface tension of water/ air and r = capillary radius

The suction in a capillary has a reciprocal relationship to the capillary radius

The capillaries of paper are much larger than those of felt

The capillary suction of paper are much stronger than those of the felt

Sheet transfer

Suction pick-up arrangement

Straight-through press arrangement

Suction transfer press arrangement

Twinver press arrangement

Modern three-nip no draw press

FACTORS INFLUENCING WATER REMOVAL PERFORMANCE

PRIMARY (Influencing out going sheet dryness

by 4% or more)

SECONDARY

Post nip rewet Initial fabric flow resistance

In-going sheet dryness In-going fabric dryness

Furnish properties Rewet in the nip at speed

Double felting Shape of pressure profile

Sheet temperature Roll covers

Impulses Fabric design

Fabric pressure uniformity

Roll venting

Press loading:

Nip load (pounds per lineal inch, pli or kilonewtons per meter, kNm).

Higher nip loading decreases felt life and increases press drive load and increases press drive load

Losses are offset by appreciable gains in moisture removal and improved sheet consolidation

Nip load (kN/m) = Total roll load (kN)

Width of roll face (m)

Nip load (pli) = Total roll load (lb) Width of roll face (m)

Press action is a function of nip loading per unit area (pressure) in the nip

Average nip pressure (kPa) = Nip load (kN/m) Nip width (m) Average nip pressure (kPa) = Nip load (pli) Nip width (in)

Press load uniformity:

The largest factor in nip pressure uniformity is the fabric

Peak pressure variations

Bliesner and MacGregor prepared special baseless fabrics using a fiber diameter of 43 m in one fabric and 19 m in another. The smaller diameter fiber produced a dryness nearly 7% higher on 50 g/m2 sheet (31lb/3000 ft2)

Dryness vs fabric fineness

Machine speed:

As speed is increased water removal from the sheet will decrease with other conditions held constant

The more fundamental variable is nip residence time (NRT) NRT (msec) = Machine direction nip width Machine speed

Press impulse:

The effect of press loading and NRT has been considered as independent variable

Campbell showed that increase in out going sheet consistency is proportional to the product of pressure per unit area and duration of press application

Wahlstorm and Schiel used the pressure time product in relation to wet press water removal

Beck summarized the definition by giving the following definition:

t

Impulse = c P (t) dt = Pave X (tctb) tb = Press load Machine speed

Impulse is usually expressed as Mpa-sec or psi-sec Where: Pave = Average nip pressure, Mpa or psi P(t) = Pressure any time, Mpa or psi tb = Time profile begins (entering the nip), sec tc = Time profile ends (leaving the nip), sec

Two roll press sections is of the order of 0.013-0.02 MPa-sec

Pressure pulse parameters

Dryness vs impulse, kraft Dryness vs log impulse, kraft

Dryness vs impulse, newsprint Dryness vs impulse, linerboard

The dryness changes from near 30% at a press impulse of under 0.007 MPa-sec to near 60% dryness at a press impulse of 0.07 MPa-sec.

Dryness vs log impulse, roll and extended nip press

Sheet temperature:

Temp of the sheet as it is being pressed can have significant effects on water removal and thus qualifies as a primary variable.

As the temp in the sheet is raised both surface tension and viscosity of water decrease, which lowers resistance of water movement through the sheet and to ultimate removal in to a fabric at the press.

Dryness vs sheet temperature

At 33% ingoing moisture level the dryness has increased from 40% to near 45% with an increase in temp 27 0C to 82 0C or 1% drier for each 11 0C

Interrelationship of process parameters

In-going sheet dryness

At the 35 to 45% dryness level, a near linear relation of approximately 1% change in outgoing dryness was for each 2% change in in-going dryness.

Effect of in-going sheet dryness

Basis weight

A change in out going sheet dryness from about 43 to 39% was observed when basis weight changed from 45 to 120 g/m2

Effect of basis weight on dryness

Furnish properties

Fiber quality will depend on: 1. Tree species 2. Age of tree 3. Condition of the fiber in the chip entering the pulping

process 4. Chip thickness 5. Types of pulping process 6. Pulping variables 7. Bleaching operations 8. Beating 9. Sheet consolidation

A comparison of cross-sections and stress-strain behavior of

fiber models

When a sheet is pressure controlled, little resistance is offered to flow of water out of the mat and the mat elastically rebounds after the sheet emerges from the nip.

The rapid rebound of such furnishes probably influences their rewet gain.

The deformation of sheet will depend upon higher fiber surface area, higher basis weight or more swollen components, then pressing this sheet will be more flow controlled.

Flow controlled pulps acts as viscous elements, where sheet resists both deformation upon entering the nip and elastic expansion once out of the nip.

Deformation of Kelvin elements when either the spring or the

dapshot determines the compression response.

Dryness vs retention value

Felting

The wet fabric is the receptor that acts as an absorptive interface between the roll and the sheet

Fabric compressive properties are controlled by fabric condition and design.

The difference in void volume between the compressed and relaxed conditions is a first approximation of the fabrics absorptive capacity.

Double felting has been an effective method of increasing water removal in the press.

Rewet

The quantity of water going back in to the sheet (rewet) in the expending zone of the nip varies from 3-35 g/m2 (WAHLSTORM)