Nip Dewatering A Press Fabric Perspective · Felt Design Objectives • Steady State Pressing-Achieve some saturation point in the nip early in felt life • Enough water to achieve

Nip DewateringA Press Fabric PerspectiveA Press Fabric Perspective

May 4 2011May 4, 2011PaperCon 2011

John Neun – Albany InternationalJohn Neun Albany International

PaperCon 2011 Page 2006

Outline• Introduction• Introduction

- Press Dewatering Principles- Nip Dewatering

• The Role of the Fabric in Nip DewateringThe Role of the Fabric in Nip Dewatering• Fabric Design

- Parameters- Tools


Nip Width and Pressure

1000 0

1200.0

1400.0

1600.0

)

SS Newsprint Nip

400.0

600.0

800.0

1000.0

Stre

ss (p

si)

Tissue Nip

0.0

200.0

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

Position (inches)

• PNIP = Pmechanical + Phydraulic• Pmechanical

- Press loadPress load- Nip width (roll covers, felt)

• Phydraulic

Felt void volume- Felt void volume- Water content


Press Dewatering: Basic Physics

• Water movement occurs only with pressure differential• Pressure must be higher in sheet than feltPressure must be higher in sheet than felt• If hydraulic pressure exists, saturation must exist

- There must be a continuum of fluid• Therefore, to achieve fluid movement:

- Voids must exist in the felt, OR- Voids must exist “beyond” the felt (slots/grooves in roll) ORVoids must exist beyond the felt (slots/grooves in roll), OR- Water must flow in MD or CD (felt is permeable in every direction),

OR- The sheet explodes (crushing)- The sheet explodes (crushing)


Pressing and Felts

Sheet

Press fabric function- Accept water from sheet

Other stuff (impart finish convey sheet transmit power)- Other stuff (impart finish, convey sheet, transmit power)Nip dewatering

- Independent flow exits out of the nip- Water passes through felt out of nip

Pressure and flow depend on - void volume- Compressibility- permeabilitypermeability


Nip Dewatering vs. Uhle Dewatering• Water is expressed form the sheet into the felt at the press• Water is given up by the felt

- At the nip (water volume beyond saturation passes through the felt)p ( y p g )• Water goes through the felt• Permeability is very important: flow resistance• Compressibility and void volume determine saturation point

- At the uhle box (water carried from nip to uhle box)• Water enters sheet side of felt and exits from same side• Void volume determines capacity of felt• Permeability determines air flow uhle dewatering efficiency

• Nip dewatering benefits- Improved press solids (more efficient pressing)- Profile improvements (more even pressing)- Cleaner felts (better “flushing”)


Felt Water Balance Over LifeNip water

S ti B t

25

30Suction Box water

15

20

l/min

5

10

0

5

0 10 20 30 40 50 60 70Run TimeRun Time

• Felt compressibility, permeability, and void volume all change over lifeover life


Felt Design Objectives• Steady State Pressing

- Achieve some saturation point in the nip early in felt life• Enough water to achieve hydraulic pressureEnough water to achieve hydraulic pressure• Little enough water to avoid crushing

- Maintain some uhle flow• Help clean fabric and prevent filling• Help clean fabric and prevent filling• Control felt water balance


How Much Water?

• Optimal water content for nip dewatering depends onF lt- Felt

- Sheet- Belt/Roll venting- Nip Load- Nip Profile- Is unique to every pressIs unique to every press

• Uhle systems can moderate water to the nip- Too much nip rejection, crushing

T li l b i l i d i- Too little sub-optimal nip dewatering


Felt Design and Nip Dewatering• There are three options to enhance nip dewatering with felt design.

- Balance life and startup performance

11. Reduce permeability and void volume- Retard uhle dewatering- Enhance nip saturation with minimal void volume- Densified conventional styles- Densified conventional styles- Usually poor life

2. “Soften” compressibility of felt- Maintain permeability allow through flowp y g- Minimize mid-nip void volume achieve saturation easier- Unconventional structures in base to control mid nip performance

3. Maintain permeability and control void volume to reduce saturation volumevolume

- Conventional web structures to control permeability- “Crossless” base structures for void volume and compression control


1. Densified Fabric• Very dense web• Reduced perm, reduced void volume• Decent start p• Decent startup• Poor life


Enhanced Compression

• Viscoelastic layer in felt• Good permeability, p y,

controlled compressibility


Controlled Void Volume• Controlled base fabric void

volume• Good compression characteristicsGood compression characteristics


Cleaning

• Why do nip dewatering felts clean better?Uhl f lt t i d t f id• Uhle felts water in and out from same side

• Nip dewatering felts water flows through felt- Structure flushed better


Felt Design Summary • Objectives

- Optimize pressing efficiency• StartupStartup• Over felt life

- Maximize life• T l• Tools

- Felt structure• Batt• Base• Other

- Control• Permeability• Compressibility• Void volume (free and mid nip)( )


Thank You


Documents

Nip Dewatering A Press Fabric Perspective · Felt Design Objectives • Steady State Pressing-Achieve some saturation point in the nip early in felt life • Enough water to achieve