Improving life of supercalendar Cotton Rolls

IMPROVING LIFE OF
SUPERCALENDAR COTTON ROLLS
deveshksinghal@gmail.com

Cotton Rolls
Cotton rolls are very important in a supercalendar
because of the following reasons-
1. After electricity, these are the major second cost1. After electricity, these are the major second cost
center.
2. Proper care ensures not only a good life of the
roll, but also better smoothness and gloss in
supercalendared paper.
"Improving Life of Supercalendar Cotton Rolls", May 2014

This Presentation
Unfortunately, much information is not available on
handling and care of supercalendar rolls.
Some experts and many operators have tried to
improve the performance of cotton rolls overimprove the performance of cotton rolls over
decades, but still a lot of information is passed on
verbally.

Why Rolls Fail
Let us first discuss different reasons for why the
cotton rolls fail during operation.
SlicingSlicing
Overheat

Slicing

Slicing
You may notice different slices in a cotton roll after
some time of operation.
In fact, the manufacturer make some slices of cotton
(mixed with the proprietary fiber of their choice) in(mixed with the proprietary fiber of their choice) in
circular shape with a hole punched in it.
They then put a lot of such rings on the roll shaft,
and compress the same at a very high pressure, so
that all the slices are mechanically joined to each
other thus forming a roll.

Slicing
During run, shear forces may result in minor
relocation of these slices, particularly if there had
been insufficient pressure applied while making the
roll.
If the roll is operated in one direction, this generally
creates no problem.
But, if the roll is taken for grinding and mounted
again in reverse direction, these slices loosen up
and make the roll unsuitable for use.

Slicing
That is why most suppliers want the rolls should be
rotated in one direction only.
They are right.
To ensure your operators may not accidently mountTo ensure your operators may not accidently mount
a roll in reversed direction, consider putting labels
on both side of shafts.
You may also add this point in startup checklist after
any roll has been replaced- “Ensure that the roll has
been mounted to run in specified direction only”.

Overheat
Cotton rolls get overheat during operation, and as
cotton itself is a bad conductor of heat, the locally
elevated temperature may result in burning of the
cotton, thus making the roll unsuitable.
Typical overheat behaviors are shown in following
slides-

Startup Overheat

Startup Overheat
In such case, the roll top surface looks almost
perfect, except a small black scar on it showing that
‘something is wrong’ inside.
Such defects appear typically within a couple ofSuch defects appear typically within a couple of
hours of operation if the supercalendar has been
started after a shut of one or more days.

Startup Overheat
Starting a cold supercalendar and increasing the
load much before the rolls have come to an
equilibrium is a major cause of such problem.
On load, due to friction between fibers a lot ofOn load, due to friction between fibers a lot of
heat is generated locally inside the roll.
Unfortunately, we measure temperature at the roll
surface only which looks cooler compared to our
normal operating condition.

Startup Overheat
Starting the supercalendar at reduce operating nip
load for a couple of hours with gradually increasing
load and speed is essential to avoid such problems.
Paper should be passed through the nips only afterPaper should be passed through the nips only after
sufficient time (say 90-120 minutes) after startup.
In plants where startup roll problems are common,
the above practice helps a lot.

Operational Overheat
The next greatest problem is operational
overheating of the roll.
In fact, while many operators and suppliers
recommend operation of supercalendar at reducedrecommend operation of supercalendar at reduced
temperature, many a times, temperature reduction
means reduction in load and speed, and hence
production. Alternatively, you need to stop
supercalendar and wait for the rolls to cool down,
and accept the production loss.

In an study conducted in a small plant, it was
observed that such problems were more in cold
weather.
In winter, where the temperature (outside) wasIn winter, where the temperature (outside) was
below 10-15˚C, problem was 3-4 times more than
in summer where the outside temperature reached
beyond 40-45˚C.

So, ambient temperature was not affecting the
performance of the roll.
Then what was it?

Relative humidity
In hot and humid summer the problems were much
less than in dry cold winters.
This information gave an idea that HUMIDITY has
something to do with performance of the cottonsomething to do with performance of the cotton
rolls.
To understand it further, let us have a look on cotton
properties.

Cotton
Cotton Fiber is made of cellulose.

Cellulose
Cellulose is made up of glucose molecules joined
together in a chain like structure.

Cellulose
These molecules are joined together with hydrogen
bonds.
Cotton absorbs moisture from the ambient under
humid conditions and releases the part of it whenhumid conditions and releases the part of it when
kept in dry area.
The water molecules are attached to the surface of
cotton (cellulose) by hydrogen bonding.

Hydrogen Bonding
The hydrogen bonds between water and cellulose
molecules are weak and break down even by small
energy.

Moisture Absorption
Cotton moisture absorption is a function of
environment humidity.

Back to Cotton Rolls
Now, if there is adequate moisture in cotton, due to
the heat generated during supercalendaring
process, some of the water molecules get detached
and evaporate.
This process consumes energy from the cotton rolls
thus saving the cellulose chain as well as individual
cellulose (glucose) molecules intact.
If cellulose chain is intact, the roll will run more time.

Temperature Increase
Furthermore, when the temperature is increased,
two different phenomena may occur.

Slow Heating
At relatively higher temperatures (ca. 150-200˚C),
cellulose bonds start breaking.
This results in darkening of roll surface slightly.

Rapid Heating
But, the first reaction is slightly exothermic, thus
producing more heat, and the resultant reaction
produces ‘Levoglucosan’, which is more exothermic.
The process becomes so fast that the roll startsThe process becomes so fast that the roll starts
‘burning’. At this stage, you may feel the smell of
‘tar’.

Instant Rapid Heating
In some cases, the heating
process is so fast that we do not
observe the darkening of the
roll surface, but the inside
temperature shoots up rapidlytemperature shoots up rapidly
and rapid tar generation starts
inside the roll.
Here, roll may seem perfect at
top, but at inside the damage
has already been done.

Simplification to Operator
In other words, we may say that we
need to check the first phase of
temperature increase.
There are operators saying that theThere are operators saying that the
roll temperature does not increase
beyond 80-90˚C, but that is the
temperature at roll surface.
Inside roll temperature may become
high enough to produce levoglucosan.

Effect of Moisture
In last slide, we have
seen that increase in
temperature leads to tar
generation or
spontaneous combustion.spontaneous combustion.
By adding Moisture to
roll, we can reduce rate
of temperature increase,
and thus problems like
this-

Increasing Roll Moisture
There are several methods of increasing moisture
content in a cotton roll.
Rub the roll with a wet sponge.Rub the roll with a wet sponge.
Directly spray water on the roll during reel change.
Pass impinging air on rolls through a wet scrubber.
Install humidifiers on supercalendars.

Monitoring Roll Condition
Continuous monitoring of roll condition plays an
important role in getting good life of cotton rolls.
But, the problem is that roll surface temperature
varies with time, speed and operating load.varies with time, speed and operating load.
Furthermore, as you apply water using sponge or
by spraying, the roll surface temperature goes
down, and start increasing slowly as the
supercalendar is running.

Regularly monitoring roll surface color is a good
tool in such cases.
If there is a lack of moisture, the generated heat
cracks hydrogen bonds between cellulose chains,cracks hydrogen bonds between cellulose chains,
rather than bonds between cellulose and water
molecules.
As a result, the color of roll becomes darker.

Roll in better condition
Lighter color
Roll in poor condition
Darker shade

Observing Grinder
Often, the dust generated at grinder gives good
information about the roll condition.
Uniform, white, granular dust is an indication of
good roll condition.good roll condition.

Observing Grinder
On the other hand, very fine, dark colored dust
indicates that the roll cotton had started degrading.

Observing Supercalendar
So, if cotton rolls remain in brighter, lighter color,
one can expect of getting better life compared to
the case where the rolls get dark too early.

Future Scope
As we can see the concept of applying moisture is
to provide additional low energy hydrogen bonds,
which can break at minor temperature increase and
protect the cellulose chains, there comes another
possibility to use something else with water whichpossibility to use something else with water which
may dissipate relatively more energy.
This should be (obviously) colorless and miscible in
water.

Glycerol
Glycerol can also be added with water and this
mixture can be uniformly applied on the cotton roll.
In case of any temperature increase, bound water
will evaporate first, leaving glycerol as a ‘fuse’ towill evaporate first, leaving glycerol as a ‘fuse’ to
protect cellulose.

Caution with Glycerol Application
I MUST indicate that I have never used glycerol for
such application. I just ‘FEEL’ that glycerol addition
may help.
However, I would really like to hear from someoneHowever, I would really like to hear from someone
who is willing to take such trial to get better cotton
roll performance.

With Glossy Paper Wishes…

Thank You.Thank You.

About the Author
Born in 1968, D K Singhal is B.E., M.E. (Pulp & Paper, 1993) from Deptt. of Paper Technology,
University of Roorkee (now IIT, Roorkee). He was certified as Certified Energy Auditor in 2004
with Bureau of Energy Efficiency.
With nearly 7 dozen publications, he has emphasized on development of low cost technologies
and management practices for quality and profitability improvement. With publications on
energymanagertraining.com and paperonweb.com, he has been constantly contributing to IPPTA.
He is also serving IPPTA as a member of Editorial Board. He has also been a member of IPPTAHe is also serving IPPTA as a member of Editorial Board. He has also been a member of IPPTA
Executive Committee.
An initiative by D K Singhal, a cyber campaign initiated against unjustified targeting of paper
industry by “Idea” mobile, in their “Sirjee” advertisement campaign, after which this advertisement
was taken off air.
He has also moderated a Yahoo group, “PaperTechnology” with nearly 365 members from India
and abroad to discuss problems related to pulp & paper making.
He can be contacted at deveshksinghal@gmail.com

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