Increased paper machine efficiency through good runnability management

Abstract

My colleague Dr. Juha Leimu defended his doctoral thesis “The opening gap of the paper machine cylinder dryer, Theoretical and experimental investigation” at the Abo Academy, Turku, Finland, in 2008 (see reference). During the discussion after his presentation, I asked him a question; Which of the two principles that he presented in his dissertation is better for web stabilization and energy efficiency in the papermaking process: creating negative pressure with air blowing system (Coanda effect) or with air suction system (vacuum cleaner effect)? He simply answered,

“Depend on the situation!”.

This was the answer to avoid long explanations of the pros and cons of both systems. He did an excellent job of explaining the details of both systems in his Ph.D. thesis, but I have to admit I expected his conclusion and selection of the one principle. After many years of experience and work in the research and development of both systems, I think I have found an answer to this question. Today I would answer the same as my colleague answered, but from a different angle.

What means, “Depend on the situation!”?

There is some opinion that runnability devices are not that important in the papermaking process and that they are treated as machine parts of minor importance. I have noticed this behavior from the customer side but also from the supplier side and it sometimes takes so much strength and effort to convince both sides that they are wrong. I think that runnability devices are crucial for good and efficient production and for energy-saving potential. Sometimes runnability problems can lead to large production and energy losses and the reason can be found in the lack of maintenance and good management of the runnability systems.

“Prevention is better than cure!”

Technology in detail

The efficiency of a paper machine mainly depends on the efficiency of paper web transfer through the drying section. The bottleneck for the good runnability in single-tier drying section are forces that keep wet web on the cylinder and prevent the smooth transition of the paper from cylinder to cylinder.

To have good runnability on the paper machine it is necessary to understand forces that affect the web during the paper transfer in the drying section and try to resolve them. The wet paper after the press section is attached to the cylinder and pressed by fabric. When paper is leaving the top cylinder (top open nip), due to the forces between paper and cylinder, paper tends to stay attached to the cylinder instead to be attached to the fabric and transferred to the bottom roll (Figure 1). The two forces are dominant that influence web-behavior. Those are adhesion force at the cylinder and open nip force created with a two-component rotational air boundary layer between cylinder and fabric. To eliminate the negative effects of those forces, the runnability boxes are installed in the dryer pocket. The underpressure created with runnability boxes eliminates negative forces and makes the web stable on the fabric.

 

 

There are two main principles that can be used for creating underpressure in the drying group and keeping paper on fabric thus creating good runnability at PM.

 The first principle (Air blowing principle -Coanda effect)

 Many suppliers use runnability boxes with the air-blowing principle (Coanda effect) to create a vacuum in pockets and enable good runnability of the paper web. As always, everything has good and bad sides. The principle is good, does not require physical sealing to the fabric, and is reliable but uses more air and energy. For more information, see the literature or references in this article. Figure 2 shows the basic idea of creating negative pressure by air blowing on curved surfaces (Coanda effect).

 

 

The second principle (Direct air sucking – Vacuum cleaner effect)

 The other suppliers of the runnability boxes are using direct sucking or the “vacuum cleaner effect” to produce underpressure to keep the web on the fabric during the paper transfer. Direct air sucking is using less air and energy for creating underpressure and web stabilization but only under controlled conditions (good physical sealing to fabric, no air leakages). The damage to the sealing can cause uneven underpressure distribution, fabric bending, and web breaks. If the sealing is damaged during the uncontrolled fabric tension, the process is very difficult to control.

 

Despite runnability principles, maintenance is an important factor that controls the papermaking process and reduces costs. Runnability is very important to the papermaking process and should receive more attention.

 Running paper machines under those conditions can cause serious problems with web fluttering, extreme fabric bending, paper breaks, long shutdowns, and decreasing machine efficiency.

Those are good reasons to think about regular maintenance and service of the runnability equipment.

Service and periodical maintenance of runnability components enable their efficient performance, reducing paper breaks and time wasted due to tail threading and grade changes, thus avoiding unnecessarily high production costs.

Figure 6 shows an example where service on the runnability boxes is performed which results in improving underpressure of the runnability devices and better web control.

 

 

After the service and optimal adjustment of the runnability boxes, the boxes’ performance has almost doubled.

Regular maintenance and adjustment of runnability components can result in significant benefits in product efficiency and reduction of web breaks and time lost due to machine stops (Figure 7).

 

The example is showing lost time for breaks decreased on average by 46 % per month and the number of breaks decreased from 2,7 to 1,5 per day (44%). The tail threading was improved significantly, and paper machine efficiency increased.

Conclusion

The answer to the question from the beginning of the article is that RUNNABILITY IS IMPORTANT in the paper making process.

Both runnability systems have their weak and good points but a better system is one that is incorporated with the good service that will keep runnability devices optimal and close to the design performance values.

This is just an example of how good runnability management can improve paper machine efficiency, but of course, there are more factors that should be considered when addressing this topic.

Note

 In the next publication, we will try to reveal how the runnability devices can be kept clean while the paper machine continuously running (self-cleaning).

For runnability problems and solutions for the more efficient performance of the runnability device, you can contact the author of this article (nenad.milosavljevic@abo.fi). It will be very interesting to try to help you in your cases and to hear how you face the runnability problems on your machine.