Nip Dewatering
A Press Fabric Perspective
May 4
4, 2011
PaperCon 2011
John Neun – Albany International
PaperCon 2011 Page 2006
Outline
• Introduction
- Press Dewatering Principles
- Nip Dewatering
• The Role of the Fabric in Nip Dewatering
• Fabric Design
- Parameters
- Tools
PaperCon 2011 Page 2007
Nip Width and Pressure
1600.0
1400.0
SS Newsprint Nip
1200.0
1000 0
1000.0
Stress (psi))
800.0
600.0
Tissue Nip
400.0
200.0
0.0
-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50
Position (inches)
• PNIP = Pmechanical + Phydraulic
• Pmechanical
- Press load
- Nip width (roll covers, felt)
• Phydraulic
- Felt void volume
- Water content
PaperCon 2011 Page 2008
Press Dewatering: Basic Physics
• Water movement occurs only with pressure differential
• Pressure 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”
beyond the felt (slots/grooves in roll)
roll), OR
- Water must flow in MD or CD (felt is permeable in every direction),
OR
- The sheet explodes (crushing)
PaperCon 2011 Page 2009
Pressing and Felts
Sheet
Press fabric function
- Accept water from sheet
- Other stuff (impart finish
finish, convey sheet
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
- permeability
PaperCon 2011 Page 2010
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
p ((water volume beyond
y saturation p
passes through
g the felt))
• 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”)
PaperCon 2011 Page 2011
Felt Water Balance Over Life
Nip water
S ti B
Suction Box water
t
30
25
20
l/min 15
10
0
0 10 20 30 40 50 60 70
Run Time
• Felt compressibility, permeability, and void volume all change
over life
PaperCon 2011 Page 2012
Felt Design Objectives
• Steady State Pressing
- Achieve some saturation point in the nip early in felt life
• Enough water to achieve hydraulic pressure
• Little enough water to avoid crushing
- Maintain some uhle flow
• Help clean fabric and prevent filling
• Control felt water balance
PaperCon 2011 Page 2013
How Much Water?
• Optimal water content for nip dewatering depends on
- Feltlt
F
- Sheet
- Belt/Roll venting
- Nip Load
- Nip Profile
- Is unique to every press
• Uhle systems can moderate water to the nip
- Too much nip rejection, crushing
- li l sub-optimal
T little
Too b i l nip
i ddewatering
i
PaperCon 2011 Page 2014
Felt Design and Nip Dewatering
• There are three options to enhance nip dewatering with felt design.
- Balance life and startup performance
1. Reduce permeability and void volume
- Retard uhle dewatering
- Enhance nip saturation with minimal void volume
- Densified conventional styles
- Usually poor life
2. “Soften” compressibility of felt
- Maintain ppermeabilityy allow through
g flow
- 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
volume
- Conventional web structures to control permeability
- “Crossless” base structures for void volume and compression control
PaperCon 2011 Page 2015
1. Densified Fabric
• Very dense web
• Reduced perm, reduced void volume
• Decent start
startup
p
• Poor life
PaperCon 2011 Page 2016
Enhanced Compression
• Viscoelastic layer in felt
• Good permeability,
p y,
controlled compressibility
PaperCon 2011 Page 2017
Controlled Void Volume
• Controlled base fabric void
volume
• Good compression characteristics
PaperCon 2011 Page 2018
Cleaning
• Why do nip dewatering felts clean better?
• f lt water
Uhl felts
Uhle t ini and
d outt ffrom same side
id
• Nip dewatering felts water flows through felt
- Structure flushed better
PaperCon 2011 Page 2019
Felt Design Summary
• Objectives
- Optimize pressing efficiency
• Startup
• Over felt life
- Maximize life
• T l
Tools
- Felt structure
• Batt
• Base
• Other
- Control
• Permeability
• Compressibility
• Void volume ((free and mid nip))
PaperCon 2011 Page 2020
Thank You
PaperCon 2011 Page 2021
