IMPROVING THE LIFE OF SUPERCALENDAR COTTON (SOFT) ROLLS deveshksinghal@gmail.com
Cotton Rolls 2 Cotton rolls are very important in a supercalendar because of the following reasons- 1. 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.
This Presentation 3 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 over decades, but still a lot of information is passed on verbally.
Why Rolls Fail 4 Let us first discuss different reasons for why the cotton rolls fail during operation. Slicing Overheat
Slicing 5
Slicing 6 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 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 7 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 8 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 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 9 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 10
Startup Overheat 11 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 of hours of operation if the supercalendar has been started after a shut of one or more days.
Startup Overheat 12 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 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 13 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 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 14 The next greatest problem is operational overheating of the roll. In fact, while many operators and suppliers recommend 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.
Operational Overheat 15 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) was below 10-15 C, problem was 3-4 times more than in summer where the outside temperature reached beyond 40-45 C.
Operational Overheat 16 So, ambient temperature was not affecting the performance of the roll. Then what was it?
Relative humidity 17 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 cotton rolls. To understand it further, let us have a look on cotton properties.
Cotton 18 Cotton Fiber is made of cellulose.
Cellulose 19 Cellulose is made up of glucose molecules joined together in a chain like structure.
Cellulose 20 These molecules are joined together with hydrogen bonds. Cotton absorbs moisture from the ambient under humid 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 21 The hydrogen bonds between water and cellulose molecules are weak and break down even by small energy.
Moisture Absorption 22 Cotton moisture absorption is a function of environment humidity.
Back to Cotton Rolls 23 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 24 Furthermore, when the temperature is increased, two different phenomena may occur.
Slow Heating 25 At relatively higher temperatures (ca. 150-200 C), cellulose bonds start breaking. This results in darkening of roll surface slightly.
Rapid Heating 26 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 starts burning. At this stage, you may feel the smell of tar.
Instant Rapid Heating 27 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 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 28 In other words, we may say that we need to check the first phase of temperature increase. There 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 29 In last slide, we have seen that increase in temperature leads to tar generation or spontaneous combustion. By adding Moisture to roll, we can reduce rate of temperature increase, and thus problems like this-
Increasing Roll Moisture 30 There are several methods of increasing moisture content in a cotton roll. 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 31 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. 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.
Monitoring Roll Condition 32 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, rather than bonds between cellulose and water molecules. As a result, the color of roll becomes darker.
Monitoring Roll Condition 33 Roll in better condition Lighter color Roll in poor condition Darker shade
Observing Grinder 34 Often, the dust generated at grinder gives good information about the roll condition. Uniform, white, granular dust is an indication of good roll condition.
Observing Grinder 35 On the other hand, very fine, dark colored dust indicates that the roll cotton had started degrading.
Observing Supercalendar 36 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 37 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 which may dissipate relatively more energy. This should be (obviously) colorless and miscible in water.
Glycerol 38 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 to protect cellulose.
Caution with Glycerol Application 39 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 someone who is willing to take such trial to get better cotton roll performance.
With Glossy Paper Wishes 40
41 Thank You.
About the Author 42 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 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