2. Why do we need Soft Models?
Mathematical models are so complex with so many assumptions
needed that practical application to operating refiners is limited.
Actual refining zone measurements of pressure, temperature,
plate bar forces along with high speed photography of steam &
fiber flows in plates & pulp samples extracted from multiple
radial locations in the plates give added insight into the refining
mechanisms, but math models based upon this data are still
extremely complex which limits practical use.
A less complex and more practical modelling method is needed
to allow a broader range of individuals like refiner operators and
non-mathematically inclined persons to visualize what is
happening in the refining zone.
4. Three Distinct Regions of the Single
Disc Conceptualization Model
Rotor
Stator
Plate Gap
5. Rotor Region
Fiber movement dominated by centrifugal force due to
rotation.
Rotor plate less filled & plate bars less covered with fiber
than stator where no rotation & centrifugal force exists.
Can result in more rapid rotor plate wear as bar tops more
exposed to fibers sliding across metal.
Wear differences more obvious at lower plate gaps associated
with lower energy plate concepts and refining processes.
Plate gap changes due to refining consistency changes
dominated by changes in rotor plate fiber filling, as fiber
mass variation changes centrifugal forces, varying fiber
retention time.
6. Pulp Displacement
Rotor
Stator
Rotor feeds forward due to centrifugal force.
The steam flowing back in the stator grooves will
carry the pulp, and make it move backwards
towards the plate inlet. This results in a large
accumulation of pulp in the inner part of the plate
(thick pulp pad).
In the outer zone, the forward flow of steam carries
the pulp forward and out of the refining zone.
7. Stator Region
Without rotation, fiber flow dominated by steam flow conveying
forces.
Fiber surface area affects ease of conveying.
Fiber mass(consistency) also affects ease of conveying but much
less than rotor centrifugal forces.
Temperature & pressure measurements indicate a steam pressure
peak exists at some radial location in the refining zone.
Outboard of peak, steam & fiber flow forward
Inboard of peak steam & fiber flow back to the inlet
Rotor must feed production rate + fiber returning in stator
Results in higher inlet zone filling; can completely fill inlet and feeding screw.
8. Temperature and pressure profile
110
120
130
140
150
160
170
180
190
Primary
Secondary
Temperature°C
Note : More coarse chips & fiber bundles absorb energy even at larger gaps @ plate inlet
9. Stator Region, cont’d
Publications have shown that the differential between
refiner inlet and outlet steam pressures impacts the
retention time of fiber in the refining zone and thus the
plate gap.
Higher outlet than inlet pressure(pressure boost) increase
fiber retention & plate gap.
Lower outlet than inlet pressures decrease fiber retention &
gap.
I would suggest that stator plate fill rate(fiber retention)
changes dominate the differential pressure gap changes, as
stator filling is dominated by steam flow conveying effects.
10. Plate Gap Region – Fiber trapped
between rotor & stator plate bar tops.
Conveying influenced by rotor and stator bar crossing
angle(sliding direction) & stator frictional forces.
Bi-directional plates evenly alternate inward and outward
sliding direction.
Directional feeding plates increase the portion of outward
vs inward sliding forces.
Directional holding plates do the opposite.
Typically need feeding inboard of pressure peak to reduce
inlet filling, and holding outboard of peak to retain fiber &
increase filling.
12. Plate Gap Region, cont’d
Higher stator bar frictional forces tend to hold fiber,
promoting sliding of fiber across the rotor bar tops,
increasing rotor bar edge rounding & wear.
Fiber tends to slide on bar surface with the lowest frictional
forces.
Bar angle and bar edge condition influence holding forces on
fibers.
Newly manufactured plates exhibit break-in period with
higher plate gaps and higher energy consumption.
I would suggest effect caused by the grinding burr on bar edge
giving increased roughness and holding = larger plate gap
Once burr worn away reach normal holding level until
excessive bar edge rounding begins to reduce plate gap.
14. Counter-rotating vs Single Disc
2 Rotors & no Stator
Must feed through holes in 1 disc(Feed End)
Feeding challenge w/chips, even more issues with fiber
Use added dilution water to help feed(lower refining consistency)
Use steam to help feed(discharge pressure lower than inlet)
Add steam to inlet to heat chips(not self pressurized)
Steam flow effects upon fiber flow more limited, centrifugal forces
dominate feeding
At 60 hz(1200 rpm) should be lower intensity than single disc as
higher retention time & more bar crossings(2400 vs 1800)
Lower refining consistencies & plate designs without surface dams,
both feeding requirements, actually make intensity higher
Non-feeding disc adjusts axially to control gap & load(Control End)
15. Counter-rotating vs Single Disc
Although a pressure peak & steam seal may still be formed
between the plates, backflowing steam must be limited & the
seal ring behind feed disc allows steam flow around disc.
Limited steam pressure boost possible for retention time
increase
Pressure boost would cause more steam flowing back to the inlet
across seal ring
16. Plate Gap
Fiber in the plate gap stops as it is not rotating with either
disc & dragging forces cancel out
Limits how large gap can be before feeding stops(Pinch-off)
Reason for sub-surface not surface dams @1200 rpm?
Keep fiber in Feed End disc in the inlet area to assure feeding.
No bars on Control End opposite feed openings to prevent stoppage
of Feed Disc rotational motion to maintain feeding
Cavitation(high pressure collapse) on any Control End bars
opposite the feed openings in the Feed disc
18. Model Similarities – Lo-Co vs Hi-Co
3 Zones – Rotor, Stator & Gap
Shear & Compression forces on fibers
Plate Gap impact upon energy transfer &
efficiency
Fiber retention time impacts probability of
treatment
Stator fiber recirculation
19. Model Differences – Lo-co vs Hi-co
Fiber retention function of flowrate & internal
recirculation, not centrifugal force
Fiber holding affect of surface dam only at dam, not
region before dam
Dams increase groove pressure drop significantly thus
reducing flow capacity
Inlet & outlet pressures function of flow, feed pump
operation, plate geometry, refiner rotational speed
and plate diameter, not independently adjustable
20. Lo-co Fundamentals
Rotor plate increases pressure from inlet to outlet similar to a pump impeller
Grooves in rotor and stator cause pressure drop as a series of small pipes in
parallel
Pressure increase from inlet to outlet is the net result of rotor plate pumping
less groove pressure drop
As plate gap closes, initially pumping efficiency increases as pump impeller
clearance decreases thus more pressure increase
Further gap closing can decrease pressure increase as less gap & more
groove flow increasing pressure drop
Larger refining gaps give a higher proportion of shear forces, promoting
more external fibrillation – potentially higher energy to a given pulp quality
Smaller refining gaps give a higher proportion of compressive forces,
promoting more internal fibrillation(fiber splitting) – potentially lower energy
Limit in reducing gap is fiber cutting(length reduction)
21. Lo-co Fundamentals, cont’d
Lower plate bar height with wear reduces rotor pressure
increase & increases groove pressure drop thus reducing
pressure increase across refiner & potentially causes a
pressure drop from inlet to outlet
As long as discharge flow control valve is < 100% open,
flow control is still possible
If valve 100% open, refiner plates are the flow control
valve
22. Rotor Region
Strongest groove vortex to load fibers onto bar edge
Potential for forward flow regime below vortex
Influenced by fiber type, consistency, groove geometry
Refiner plate geometry dominates pumping
characteristics:
Bar angle, & height
Plate diameter
Presence of dams, surface(full bar height) or sub-
surface(< full bar height)
23. Tracing of fluid particle at the top and
bottom of the rotor groove.
Reference: Numerical simulation of the flow in a disc refiner,
Gohar.M.Khokhar, Master’s Thesis 2011
Outlet
24. Stator Region
Weaker vortex rotationally but less vortex pitch
radially vs rotor
Internal recirculation due to higher discharge
pressure than inlet displaces main flow volume
Unrefined fiber velocity increases/retention time
decreases
Groove flow below vortex, if present, radially inward if
typical higher discharge than inlet pressure
Groove volume influences pressure drop not
pumping
25. Fluid particle tracing at the top and bottom of
stator groove going in the negative Y-direction
Reference: Numerical simulation of the flow in a disc refiner,
Gohar.M.Khokhar, Master’s Thesis 2011
Outlet
26. Plate Gap Region
Fiber flocs compressed & locally de-watered
Compressive and shear forces applied to flocs
Groove vortices bring flocs to gap region for possible
trapping
Flocs trapped then released
Plate bar geometry(width and crossing angle) impact
treatment probability & severity as well as closing force
Bar surface sliding on fiber wears while floc holding
surface is protected from wear
27. Lo-Co Refining Evaluation & Optimization
Energy consumption comparison must include feed pump energy
Less refiner no-load(pumping) but more pump energy reduces savings
Feed pump VFD needs to be part of any energy reduction plan
Replacing multiple refiners with a single refiner reduces no-load energy
even with the same refiner concept
Therefore the true energy reduction of any new refiner concept must be
single refiner vs single refiner
Refiner filling design determines actual forces on fibers, therefore filling
selection for optimization will impact long term peak performance.
Single source of fillings can limit long term optimization
Any increased filling cost per ton produced needs to be deducted from
energy cost savings
Filling supply competition can speed optimization & reduce filling cost
