Press section General Flashcards
Is the press section dynamic?
Extremely, can change from week to week. It is very sensitive. It is crucial for PM runnability
What are the requirements of a press felt
Runnability
Paper quality
PErformance
Life (has shortest life of all clothing)
What are the tasks of a press felt
Remove water
Compress sheet (quality - roughness and strength)
Protect from crushing
Transfer the sheet
What is a press felt made of
Polyamide
For its water absorption capability and compression resistance. Not PET as this would split.
What is the base weave of a press felt?
The internal that forms the void
What is the outside of a press felt called
Batt
It is needled into the base weave layer
What is the task of the batt and what is the task of the base weave (press felt)
Base weave - less compactable than batt, it forms the void volume and gives stability and strength to the felt
Batt - forms the surface of the felt, determines the ability to transfer and absorb water from paper web
When is a press felt replaced?
Scheduled removal for expected end of life
Plugging (contamination or compaction) - can cause trim stealing edge napping etc
Wear - batt shaved off and therefor draining capacity.
Accidents
What is the raw material for press felt?
Polyamide
For its good compression & abrasion resistance, elasticity and high water absorption.
What parameter is usually used when talking about batt fibres (press felts)
dtex - this describes the coarse/fineness.
Fine = high contact - high dewatering
Coarse = good durability
Fine will be better for start up (nip dewatering) but coarse better for life - balance
Discuss the difference between coarse/heavy and fine/light batt
Fine batt means high contact with the paper and therefore high dewatering. It is good for start up and nip dewatering
Coarse is better for extending lifespan and uhle dewatering
Does a higher dtex mean coarser or finer batt?
Coarser - and therefore better wear resistance
What are the main dimensions when talking about felts?
Weight
Permeability (CFM)
Batt layer Dtex
With what kind of batt will you get better start up in the press?>
A fine batt to increase contact and thus dewatering. But coarser batt extends felt life so need a balance
What is the compromise when it comes to compressibility of base weave in press felt.
If the base weave is compressible it is good for start up and water removal.
But compressibility can increase crushing. There needs to be compromise
Why are press felts cleaned
To keep them open to absorb water + avoid crushing and avoid even more edge issues
What mechanical conditioning is there for press felts?
Showers + edge showers
Uhle boxes
How can you increase dwell time at the uhle box
Increasing open area of the slot.
This will increase dewatering but needs to be a balance, too big and we cause damage/wear
Should you use the showers the same way for the entire felt life
No - on start up you wont need all the showers, not so much to clean. As time passes increase # and also the pressure
How does both uhle box and nip dewatering progress with lifespan of felt
Uhle dewatering decreases as felt becomes more plugged
Nip increases as there will be higher hydraulic pressure
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