OPTIMIZATION OF PM WET END VENTILATION, MACHINE CLOTHING CLEANING, POCKET VENTILATION AND WEB STABILIZING IMPROVE PM RUNNABILITY AND PRODUCTION EFFICIENCY
Paper making is more and more adjustment of small production details that all have a great impact on production efficiency and the quality of the end product. As competition on global markets puts more pressure on paper producers, the process efficiency, PM runnability and ability to adjust production parameters for the best possible quality of paper becomes the winning factor.
Machine runnability, efficiency and paper quality are a result of numerous production parameters that all should be adjusted to meet the same efficiency / quality requirements. In this paper we highlight what can be done in field of cleaning the machine clothing, pocket ventilation and sheet stabilizing to contribute to these parameters.
In forming section, the wire is cleaned with high pressure (hp) shower, but it needs special technology to support the cleaning operation, while simultaneously the mist from the shower must prevented from spreading into machine hall, thus worsening working conditions and causing contamination on machine frames etc. For example Burgo Ardennes mill in Belgium solved these problems with our Mist Removal System, resulting in reduced number of web breaks originated from the forming section by 25 % and 50 % less time need for forming section cleaning (wire section cleaning time was reduced from 60 minutes to 30 minutes a week).
In dryer section typical runnability bottleneck constitutes poor pocket ventilation. With pocket ventilation it is possible to improve evaporation and even the pocket/sheet moisture profile. It also affects steam consumption of the cylinder and therefore saves energy costs. For example in Myllykoski PM6, where we installed pocket ventilation units, the PM speed increased by 4 % and the production capacity by 5 %.
Sheet fluttering as runnability bottleneck can be prevented by blowing air with special web stabilizers. This helps to avoid many paper quality problems like wrinkles while simultaneously the stabilized sheet enables the PM speed up. With a proper analyze on what type of stabilizing technology should be used or if the machine geometry should be modified the paper mill can assure stable sheet run, thus meaning more speed and more production capacity in addition to improved paper quality. For example Burgo Sora, Italy, was able to solve runnability problems as our web stabilizers were installed to press and drying section. With new technology Burgo Sora was able to increase speed from 830 m/min to 930 m/min, while production capacity grew by 10 %.
Dryer fabric cleaning plays also significant role in process efficiency in several ways. It is not only important that the device is able to clean the fabric effectively, but also that it is able to work reliably in difficult conditions of the hood. In most paper grades there is no need to use chemicals or high pressure water or air thus meaning less costs and maintenance. Appropriate cleaning of dryer fabrics means fewer breaks, improved ventilation and paper quality.
To find the best solution it is important that the operational bottlenecks, breaks, paper quality problems etc. are deeply analyzed before any corrective action. This kind of an analyze requires experienced specialists who are able to figure out the situation clearly, looking at small production details as a part of the entire production.
For the past few decades there has been a remarkable increase of speed of new paper machines. For example, during the past 20 years the PM world record speeds have increased by 50% thus meaning also strong impact on production capacity.
The need to meet the increasing production volume as well as paper quality requirements puts significant pressure on fine tuning paper machinery. There are numerous 15-30 years old paper machines that still have years of mechanical life time ahead but which should be modified to meet the increasing speed and efficiency requirements. This is, however, quite complicated because of several production bottlenecks that limit the paper machinery speed up.
While new machines hit records on speed and production capacity, the paper producers with maturing machinery have to concentrate on process adjustments aiming at improvements on production efficiency. By investing in specific projects that eliminate production bottlenecks and bring better runnability at the machine it is possible to increase production capacity very cost-effectively.
Fig. 1.1 Development of world speed record for a 24-h period for printing paper machines 
2. FORMING SECTION CLEANING AND VENTILATION
2.1 Web breaks, paper defects and cleaning time as bottlenecks originated in wet end
Several significant bottlenecks and runnability problems are originated in forming section as a result of high pressure wire cleaning showers. The hp-showers are used to flush away small particles like fibres and fillers that gradually reduce the permeability of the wire, causing stripes, marking and reduced dewatering. A negative effect of the high pressure cleaning is, however, that a lot of mist is created. This mist follows the fast running wire and spreads to the surrounding area of the wire section. Because the mist consists not only of water, but also fibres and fillers, it gradually contaminates headbox, forming section frames, save alls, doctor frames and generally all devices installed cross the machine.
Especially in high speed paper machines the air is following the wire with a high speed and therefore the mist is transported very effectively to the whole area. In addition, the modern machines are using double or triple layer fabric construction, which means that the amount of water bouncing back from the wire is increased as the needle jets cannot easily penetrate the thick wire. In this case efficient fabric cleaning may need special devices to maintain the required cleanliness or desired permeability level.
As a result of the contamination on the head box, frames, save alls and hp shower pipe there is a risk that the fibre and filler lumps drop back to the wire or the paper web causing paper quality problems such as holes, paper faults, web breaks or even wire damage.
At the same time the mist has a negative effect on air state of machine hall. Due to the mist there is an increased demand on ventilation. The mist makes the air very humid worsening the working conditions for the operators. The moisture causes corrosion on frames, steel constructions and the crane as a result of condensation. It is also obvious that disturbance may be caused to the electrical systems and instruments because of high humidity level and condensation.
The volume of water created by the hp-shower in the hall air is amazingly high: in a forming section of big paper machine up to 90000 m3 of water (3 litres/second) plus fibres and fillers per year can be spread to the air if 30% of the hp-shower water is bounced back from the wire. This huge amount of water could be saved if all the mist could be removed with a modern mist removal device and returned to water circulation.
Fibres and fillers in the save alls also start to grow bacteria in high temperature and moist conditions. If these bacteria are returned to the process they can cause slime and additional paper quality and runnability problems. It is also possible that slimy lumps drop back to the wire creating sticky conditions, which clog the forming fabrics. As a result dewatering is reduced which in turn gives holes, spots, marking and profile problems.
2.2 Special ventilation technology contributes also to wire cleaning
Our special mist removal system has eliminated mist contamination by the hp shower.
It is essential that the technology does not only eliminate the mist, but also assists in wire cleaning thus meaning less need for forming section cleaning and reduced number of web breaks (time loss).
The main part of the technology is the mist suction box, which evacuates the mist caused by the hp-shower and simultaneously cleans the forming fabric full width. In addition to these main functions, a lot of attention has been paid to handling the evacuated mist in the most economical way. Therefore this system includes an exhaust duct leading from the suction box to the preseparator for primary separation of water, fibres and fillers. As a final part of the system there is a centrifugal fan.
The mist suction box can be adapted to different forming section types: fourdriniers, top formers and gap formers. The operation principle of our technology for different PM-positions is similar with only minor modifications.
2.3 Mist suction at top former position
The mist suction box is installed under the top wire at the point where the high pressure shower hits the wire. The suction box is equipped with ceramic foils to allow wire contact without wire damage. Additionally, the ceramic foils simultaneously clean the fabric. There are also lubrication showers on both sides of the suction box to keep the ceramics wet in all running conditions, prevent fibre contamination and re-wet the fabric to create full width foil shower for fabric cleaning.
An air doctor (knife) is mounted on the frame ahead of the vacuum slot of the suction box. The air doctor optimizes the operation of the suction box and the top frame is equipped with a perforated shower pipe to keep the doctor clean. The air doctor is easily removed for wire change.
Fig. 2.3. Mist suction box for top formers
2.4 Mist suction at the bottom wire position
On the suction box for the bottom wire there is a lubrication shower above the box. This shower is very essential because it keeps the box wet and prevents the contamination on the box located above the wire.
An exhaust duct is connected to the suction box and leads to a preseparator on the backside of the wire section. From the preseparator air is led through ducting to the separator fan. This fan is a special construction that combines a centrifugal fan and an efficient water drop separator. By means of the fan the residual water and fibre fluid is separated from the exhaust air so that the outgoing air does not include any water drops.
Fig. 2.4 Mist suction box for the bottom wire
2.5 Mist removal for gap formers
Gap formers need always a special construction for mist suction boxes. Installation and especially dimensioning are very important due to high PM speed and modern construction of the forming section itself.
Fig. 2.5 Mist Removal wire cleaning technology for gap formers
Anyway, according to the experience of EVG's installations at gap formers, this technology operates and also gives excellent performance up to speed of 1600 m/min. Single and twin ceramic boxes for the gap formers are used depending of age and type of the forming section. Applications for gap formers are adapted from top and bottom wire solutions.
2.6 Case Burgo Ardennes
For example in Burgo Ardennes Virton mill PM1, Belgium had 2 web breaks per 24 hours originated from the forming section. Furthermore, paper defects caused problems at mill's customers' printing machines. Because the forming fabric cleaning system with hp-showers caused a huge amount of mist, the personnel had to perform an additional cleaning of forming section once a week which took 60 minutes per week.
The biggest problem was the web breaks coming from wet end causing one hour production loss per day.
Fig. 2.6. Production bottlenecks at Burgo Ardennes
2.7 Results in Burgo Ardennes
The installation included two suction boxes for the bottom and top formers, the separator fan with exhaust ducts, and water preseparator.
Fig. 2.7.1 Mist removal wire cleaning technology at Burgo Ardennes PM1 bottom wire, speed after rebuild 1050 m/min
After the suction box the whole area is dry and nothing is allowed to drop to the wire. This means that the mist consisting of water drops, fibres and fillers is prevented from contaminating the save all, machine frames, platforms and head box. Elimination of humid air from the shower area also significantly improves machine hall air quality. There is less need for wire section cleaning and fewer web breaks do occur. After the suction box the wire is dryer which means improved dewatering and thus higher dry content after forming section; improved runnability and reduced energy consumption.
In Burgo Ardennes mill the improvement of wire section ventilation had following results:
It could be estimated that the investment pay-back time was about 154 days due to the increased production capacity.
Fig. 2.7.2 Results of mist removal wire cleaning technology at Burgo Ardennes PM1
3. ENERGY COMSUMPTION AT THE DRYING SECTION
3.1 Steam consumption of cylinders constitutes major part of drying section energy costs
Cylinders require steam to enable sheet drying. The steam consumption typically constitutes 60% of the total production heat energy costs.
This is why the drying section plays an important role on total PM heat energy costs. Steam consumption is a parameter that can be strongly affected by drying section ventilation solutions. An optimal drying process is a result of optimally operating cylinders, which in turn need well organized ventilation to remove hot, humid air economically while simultaneously the PM runnability must be paid attention to. This is why it can be concluded that cylinder pockets without proper ventilation create huge amount of energy waste in addition to PM runnability problems.
Firstly, the cylinder pockets remain more humid in the middle (CD profile) because the running sheet creates under pressure thus preventing air distribution effectively out from the pockets. This can be examined in the moisture profile: the middle parts of the sheet are more wet that the edges. Poor pocket profiles cause poor sheet moisture profile, which is usually compensated with overdrying the sheet before coater, sizer or moisturizing units.
Secondly, lack of cylinder pocket ventilation means that a runnability problem is created. This is a result of the under pressure in the pocket that tends to cause fluttering, edge defects and furthermore breaks and paper quality problems.
3.2 Pocket ventilation improves moisture profile and runnability
Pocket ventilators are developed to prevent humid air from staying in the cylinder pocket. They remove humid air from the pocket by blowing hot, dry air into the pocket, and together with suitable fabrics the pocket ventilators distribute humid air out from the pockets.
Fig. 3.2 Pocket ventilator
3.3 Case Myllykoski Paper
Before installation of pocket ventilators in Myllykoski Paper PM6, Finland, had production bottlenecks related to uneven moisture profile, runnability problems like sheet fluttering and especially limited evaporation capacity. Maximum steam pressure was limiting machine speed at the most of the line time.
The cylinder pockets were very humid and sheet moisture profile relatively poor.
Fig. 3.3.1 Uneven profile measured from several cylinder pockets at Myllykoski Paper PM6
The mill personnel had tried to correct the profile with steam box, moisturizing equipment and by adjusting press section loadings. Unfortunately, these corrective actions were not able to eliminate the problems.
Fig. 3.3.2 Huge amount of evaporation capacity was lost before cylinder pocket ventilation
3.4 Results in Myllykoski Paper
With pocket ventilation it is possible to improve the evaporation capacity of the drying section and improve sheet CD profile. As a result there is less need for steam, thus saving energy costs. Also the optimized and even drying improves sheet moisture profile turning to better paper quality.
In Myllykoski PM6 the mill constituted that after the installation of 23 of our pocket ventilator units the cylinder pockets were ventilated effectively, showing following results:
Fig. 3.4.1 Even moisture profile at Myllykoski Paper PM6, measured from several pockets
As a result of pocket ventilation the steam consumption decreased and the production capacity was increased. This means 93 days pay-back for the investment.
Fig. 3.4.2 Results of pocket ventilation at Myllykoski Paper PM6
4. WEB STABILIZING FOR IMPROVED RUNNABILITY
4.1 Sheet fluttering in different positions as runnability problem
Web stabilizers prevent the sheet fluttering problem, that is caused at press and drying section because machine geometry and air pressure changes tend to remove the sheet from the fabric. The stabilizers blow air thus creating under pressure, which secures that the sheet is constantly kept in connection with the fabric.
Web stabilizers can be used to support the sheet in single and double felted areas of drying section, at separate press and to transfer the sheet from press to drying section.
4.2 Case Burgo Sora
Burgo Sora PM2, Italy had runnability problems such as poor sheet runnability before free standing press causing web breaks and sheet defects. Firstly, high draw between the press and 1st drying section was caused by poorly working stabilizer installed as transfer box. This high draw caused high shrinkage and runnability problems at the 1st and 2nd drying sections.
Also old type of blow boxes at the 1st section were not able to create vacuum between the box and the fabric and caused very long and difficult tail threading time.
Secondly, after the ventilation rebuild of the press and the 1st drying section machine speed was increased up to 820 m/min (from 790 m/min), the 2nd drying section limited machine speed. Runnability was very poor due to wrong type of blow boxes and felt rolls installed in wrong position in a pocket.
4.3 Results in Burgo Sora, Italy
Web stabilizers (EVsp, EVp, EVsf and EVst) for PM2 free standing press, press transfer and slalom section were installed in summer 2002 (STEP 1).
Fig. 4.3.1 Situation before and after STEP1
This helped the mill to improve runnability and tail threading. The machine speed increased from 790m/min to 820 m/min thus meaning 3,8 % production capacity growth. The calculation shows that the pay-back for this investment was 377 days.
In May 2003 (STEP 2) stabilizers (EVdf) were installed also for the double felted section (2nd drying section).
Fig. 4.3.2 Situation after STEP2
After the installation the PM speed increased by over 10% to 920m/min thus meaning over 10% production capacity growth. The pay-back for the investment was 173 days.
Fig. 4.3.3 Results after Step 1 and Step 2
5. Optimal dryer fabric cleaning maintains high permeability and improves PM runnability
Cleaning of dryer fabrics has several objectives. Firstly, it should maintain high and even permeability to keep an even paper quality with an even profile and without any defects. Secondly, permeability should be at highest possible level to prolong the fabric life time to the longest possible period. Thirdly, cleaning should improve process efficiency: open fabrics help to maintain high runnability and dryer section performance because open fabrics improve heat transfer, pocket ventilation and decrease drying costs.
In virgin pulp, the main reason for cleaning is created because of permeability requirements. The fibers and dust / fillers penetrate in between the fabric filaments thus causing clogging and decrease of permeability. The permeability problems are biggest in the beginning of the drying section because there the fabric permeability is always much lower than in later drying groups. In paper machines with well operating web stabilizing technology this phenomenon is typically even worse because the vacuum tends to suck these free particles even deeper in the fabric.
VAC-roll technology with high machine speed requires proper fabric cleaning due to direct/continuous exhaust air flow through the drying fabric.
With recycled fibers for brown grades the main reason for fabric cleaning is the stickies. As sticky particles adhere on the fabric and start to accumulate fibers, sticky spots are created that cause marking the paper or holes in it.
During the recent years there has been strong efforts to decrease cleaning needs as fabric suppliers have been developing fabric construction and coatings for paper side of the drying fabrics. As a result there are modern fabrics that require less or only minor cleaning action.
5.1 Different methods of cleaning
The different methods for dryer fabric cleaning may be classified as follows:
5.2 Choosing the Optimal Cleaning Method
Below we highlight some factors that should be considered when selecting an optimal cleaning system for a dryer fabric:
Maintenance need and costs
The problem with oscillating high pressure water, compressed air or both using cleaning systems is the operational reliability in hot and humid conditions of the hood. These mechanically sensitive systems require massive maintenance and they get easily broken in the difficult conditions of the hood.
Some systems are operated constantly during machine run. This means high consumption of energy and / or water. Continuous operation means also more stress on device itself and increased need for maintenance. These super high pressure systems require ten times higher pressure thus lot of electricity.
Paper quality requirements
It should also be estimated if it is possible that the cleaning may leave stripes on paper. This problem is caused as traversing or oscillating showers tend to mark the fabric and thus the paper.
No fabric damages should be caused
The cleaning system should not create any threats of fabric damage like eg. some steam and chemical using systems have been causing as a result of hydrolysis.
High consumption of energy, water or chemicals mean more stress on the environments and unhealthier working conditions. The chemicals are also expensive and have to be rinsed after the cleaning.
Means of removing the cleaning water / air including fibers and other particles
It is essential that there is an effective transportation of cleaning water / air from drying section to prevent other contamination problems on drying section.
However, the most important factor affecting choice for an optimal cleaning method is the grade that is produced at the machine. Recycled brown fibre includes pitch, adhesives, waxes or glues that cause stickies which are very difficult to remove from the fabric. These grades normally require more effective / stronger cleaning method on continuous basis to prevent creation of sticky spots to the fabric.
On the contrary, printing papers from virgin pulp or from de-inked pulp include fibers that are not difficult to remove from the fabric. According to numerous references at mills, "shock impulse" cleaning technology with normal pressure water sprayed to closing nip gives excellent performance at paper machines producing newsprint, LWC and coated papers and with de-inked pulp. It constitutes a very cost-effective cleaning method because of minimum maintenance and water consumption need. This technology operates reliably and easily when the cleaning period and the device location are carefully adjusted according to PM speed, break frequency, fabric length, furnish and vacuum roll locations.
5.3 Technology based on "shock impulse principle" gives excellent performance on printing papers
Our system is based on this "shock impulse principle" and was developed in the 1990's to help our customers to clean their fabrics easily, with minimal maintenance to maintain high permeability evenly across the entire fabric width.
The device is located before the closing nip of the rotating roll. Cleaning is performed with normal warm water during wet end break. As the nip is closing it simultaneously press the water steadily towards the rotating roll and every fabric filament is cleaned. On the paper side beside the rotating roll there is a save all to collect both the water pressed through the fabric and the particles removed from the fabric so that nothing is allowed to drop back on the fabric or rotating rolls. It is essential to push the water from non-paper contacting fabric side towards paper contacting fabric side to prevent any contamination from penetrating even deeper between the fabric filaments as a result of water shower.
Fig. 5.3.1 Shower water penetrates the dryer fabric in the closing nip, and excess water including dirt particles exit to save-all
Fig. 5.3.2 UPM Stracel PM1, France, operates with 4 cleaning units at 1. - 4. slaloms. The most effective position for EV Cleaner is while installed next to existing felt roll in return loop of the fabric. Various machine constructions can be covered with this cleaning technology.
Normally the fabric cleaning device is controlled by machine DCS, which opens the cleaning valve during a break typically once a day for 15 to 20 seconds. When cleaning action happens during sheet breaks with relative short time, there will not exist corrosion problems of rolls or machine frames.
Cleaning may be performed on daily basis or less frequently, for example from about 20 seconds cleaning time to some longer cleaning period during web break at full speed.
This short time / full width cleaning uses water only a small part compared to high pressure or continuous showers. EV Cleaner for 6 meter wide machine uses only 80 litres of warm water during normal operation; 15 seconds with 3 bar(e).
5.4 High permeability and increased fabric life time as a result of efficient cleaning
For example SCA Ortviken mill in Sweden uses the dryer fabric cleaning technology (EV Cleaner) based on the shock impulse principle. On PM4 (cleaning time 20 seconds / day) it was noted that the dryer fabric permeability was still at about 90% level of the original after 105 days use. On PM5 the permeability was still at 85% of the original after 365 days use. Efficient cleaning technology increased the fabric life time 300% as before the mill had to replace fabric after 90 days.
Fig. 5.4.1 Efficient cleaning tripled the fabric life time in SCA Ortviken
UPM-Kymmene PM3 in Kajaani operates with 7 cleaning systems based on "shock impulse", installed during 2001-2002. The cleaning systems operate reliably in difficult conditions of the hood and maintain high permeability over the entire fabric life time. The high permeability helps to optimize drying section operation, PM runnability and speed.
Fig. 5.4.2 Cleaning device in Kajaani
In Burgo Mantova PM1, Italy, the fabric life time increased almost 300% after installation of our cleaning systems. Burgo Mantova operates with cleaning units in 1st and 2nd slalom sections. PM1 produces newsprint from 100% deinked pulp.
Total costs of EV Cleaner project, when machine width is from 6 to 9 meters, vary from EUR
39.000 to 60.000. Depending on length of the fabric, the pay back time for one fabric cleaning unit is 180-240days.
1. Karlsson Markku. Figure 9. Development of world speed record for a 24-hour period for printing paper machines. Papermaking Part 2, Drying. Fapet Oy, Gummerus, Jyväskylä, Finland 2000 p 32.