What's new (and old) in press fabric conditioning: more attention to proper showering, vacuum application, direct fabric heating, and chemical cleaning can lead ultimately to better profitability.
There have been continuing trends toward more open fabrics, which has allowed better conditioning and extended fabric life, and to heavier total fabric weights with a higher percentage in the base fabric and less in the batt. A less fortunate trend, at least in terms of felt conditioning, has been the increased used of recycled fibers in almost all grades, increasing contaminant levels in fabrics.
In 1998, about 75% of the press fabrics sold in North America were continuous or endless loop, but pin-seamed fabrics are being used in more applications today. Pin-seamed felts offer many advantages, with the most important being safety. Another plus for pin-seam fabrics is the ability to use heavier felts on presses, since machine crews can handle the extra weight.
For the most part, the principles of showering of press fabrics have not changed, but good cleaning is even more important today than ever. The types of showers commonly used in the press section are flooding (chemical cleaning), high pressure inside and outside cleaning, suction or uhle box lubrication, doctor lubrication, suction roll cleaning, and grooved roll cleaning. A good reference to shower usage is the "Paper Machine Wet Press Manual." TAPPI TIP 0404-61 is also a good summary of shower applications. (For both publication, see "Additional Resources" at left.)
Showers add water to press fabrics, which can affect the dewatering capability of the fabric in the nip. As a result, some lighter weight publication grades have even eliminated showers and the uhle box on the last press.
With the increased use of recycled fiber in more mills, cleaning--especially stickies removal--becomes a real problem. High-pressure showers are being operated continuously at 200-300 psi, usually on the outside of the fabric, and intermittently at 350 to 650 psi on the inside of the fabric. High-pressure needle showers have to be oscillated, of course. Electro-mechanical oscillators are the most effective, but also the most expensive.
Many mills are also running low-pressure fan showers, for continuous application of felt cleaner.
There have been no major changes in vacuum requirements or piping standards over the last decade, but many mills still do not meet these standards.
Standards for uhle box dwell time are 3-4 milliseconds. Faster machines might have no more titan 2 milliseconds dwell, which may be acceptable. Single or double straight slots are the preferred configuration for modern uhle boxes. Some mills still use herringbone covers to reduce the wear on the fabric seam. However, older herringbone covers have much greater open area than slots and the airflow per inch of open area then becomes too low. Newer herringbone designs have much smaller open area. Remember, it is the air that strips the water from the felts. Also, modern seamed felts can stand the "slap" that occurs over a straight slot.
Target airflow is around 18 actual [ft.sup.3]/min per square inch of open area per uhle box, not per fabric. If there are two uhle boxes, then each box needs to meet these criteria. Depending on felt permeability, vacuum levels are normally around 15-18 in. Hg. Another important, though often overlooked factor, is the air velocity inside the uhle box itself and in the piping leading to the vacuum pump.
Uhle boxes must be large enough so that the velocity inside the box does not exceed 3500-4000 ft/min. If the uhle box is too small, and the corresponding air velocity too high, there can be a difference in airflow from one side of the box to the other and a difference in water removal across the felt.
TAPPI TIP 0404-27, Air Flow Requirements for Conditioning Press Felts at Suction Pipes, is still a good reference and is available online for a nominal fee.
STEAM BOXES FOR FABRIC HEATING
Virtually all paper machines making containerboard, pulp, or newsprint have one or more steam boxes on the fourdrinier or in the press section to increase sheet temperature and improve dewatering.
Typical experiences show that a 15-20 [degrees] F increase in sheet temperature in the presses results in a 1% improvement in sheet solids content. This translates to a 4%-5% reduction in drying load or possibly a 4%-5% increase in production. However, many machines do not use a steam box in the presses because of condensate dripping on the sheet or the lack of a suitable location where steam can be drawn into the sheet.
An interesting development in steam box application is the use of steam to heat the felt directly (see "Wells" in Additional Resources.) Steam boxes are normally located on the opposite side of the uhle box, allowing the differential pressure to push the steam into the felt. In the past, locating a steam box on the inside loop of the fabric run meant it had to be removed for clothing change, but seamed fabrics make that unnecessary.
Steam boxes normally use low-pressure steam (15-25 psi), so flash steam from the dryers, which is often condensed to make hot water, is a suitable source. Most applications use between 0.05 and 0.075 lb of steam per pound of production, so a paper machine producing 40 tons/h might use between 4000 and 6000 lb of low-pressure steam per hour. Increasing fabric temperature depends on many things including uhle box airflow, how well the steam box is "sealed" to the fabric, fabric permeability, and machine speed. However, a good target is a 20-30 [degrees] F. increase in fabric temperature.
What does this have to do with felt conditioning? Heating the water in the fabric decreases the viscosity of the water, which makes it easier to pull contaminants out of the fabric. This is especially true for machines running a high level of recycled fiber. Some machines have reported reduced batch cleaning as a result of direct felt heating.
Chemical suppliers also have new challenges. The old days of using strong solvents to clean press fabrics are over, and new chemical cleaning products must meet VOC standards while still being strong enough to provide real cleaning power. To make matters worse, with the dirtier furnish that most mills must work with, the job of keeping felts clean has become much harder.
Many mills have resorted to continuous chemical cleaning as well as batch cleaning to keep fabrics clean and open. Low dosage (200-500 ppm concentration) continuous applications of cleaning chemicals through a tan shower are augmented with stronger applications (5%-10% concentration) of caustic based products. Batch cleaning might be as often as once a shift or once a day, depending on the furnish and mill operating schedule.
IN THIS ARTICLE, YOU WILL LEARN:
* The importance of proper shower and vacuum application
* That the use of steam boxes can make press felt cleaning easier
* How chemical cleaning products are meeting new challenges.
Acknowledgements: Thanks to Jim Moloney, Albany International; John Neun, AES Kadant; Doug Sweet, Sweet Associates; Philip Wells, Wells Enterprises; and Greg Hood, Buckman Laboratories for their assistance.
* "Paper Machine Wet Press Manual," TAPPI Press. Go to www.tappi.org and click on TAPPI Press, On-Line Bookstore
* TAPPI TIP 0404-61, "Paper Machine Shower Recommendations." Go to www.tappi.org and enter TIP number in the search engine
* TAPPI TIP 0404-27, "Air Flow Requirements for Conditioning Press Felts at Suction Pipes"
* Wells, P., "Conditioning of Press Felts with Steam Showers," 2002 Lake States TAPPI Annual Fall Meeting.
About the author: Jim Atkins is president of Atkins Inc., and a member of the Solutions! Editorial Board. Contact him at 908-806-8689 or email@example.com.
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|Title Annotation:||Machine Clothing|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Feb 1, 2003|
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