The science of winding - more exciting than you think

By Kenneth Åkerlund

BRUSSELS, Dec. 20, 2010 (RISI) - The winder is not normally thought of as the most exciting part of a paper mill. That honor is usually reserved for the paper machine. But perhaps winders should get more notice.

After all, the winder is where the finished product, in which you invest considerable financial, natural and human resources, goes through its final converting stage before being shipped to the person paying the bill: the mill's customer. And it is also a potential bottleneck that can restrict production and, of course, cash flow, if it is not running at optimum throughput.

In mathematical terms, winder throughput can be thought of as a function of several variables:

Winder throughput = f (W, S, P, R, D) where

W = Winding speed

S = Set change time

P = Parent roll change time

R = Winder runnability

D = Drive control efficiency (acceleration/deceleration).

High speed is a key factor

Winding speed is often seen as a key requirement for getting more throughput from a winder. This is of course true and therefore, several years ago, the design speed for Metso's top winders was increased to 3,500 m/min as technological advances allowed this. But simply having the ability to run at high speed is a different matter from consistently running at that speed.

Experience in both the field and the pilot plant has shown that the other variables referred to in the winding equation can be more important in getting the most consistent throughput from your winder. And by optimizing those factors you decrease the risk of disturbances in your operation, and maximize the output of on-spec saleable tonnes, the aim of all paper mills.

However, the latest generation of winders addresses these issues through new, patented features that exploit the full potential of automation. An example is the WinDrum Pro winder technology.

In this text, the other variables in the equation where important advances have been made will be studied.

Set change time

Several years ago, the WinDrum Pro technology introduced the 15-second set change to the market, compared with 60 seconds or more for previous designs. The 15-second patented set change is based on the moving rear drum technology in a two-drum configuration.

This giant reduction in set change time has had a huge effect on throughput, allowing one winder to handle all of the output from large paper machines, and making it ideal for commodity grades such as liner, fluting and woodfree uncoated paper, Fig. 1.

Figure 1 - Set change time is the most effective capacity tool

Parent roll change time

Parent roll changes account for large chunks of lost time on the winder. Many paper mills say they can change the parent roll manually in five minutes, but in practice this is very rare. In most cases, manual parent roll changes take 6-10 minutes or even more. Figure 2 shows the effect of fast parent roll change times on capacity.

Figure 2 - One-minute paent roll change is a new possibility to increase capacity

To raise efficiency in this area, a groundbreaking, fully-automated fast splice system that can reduce the effective parent roll change time to one minute has just been introduced. This system has two unwind stands, with the second one used only for the last set. Running the last set from the second unwind makes it possible to load the main unwind while the winder is running. All parent roll movements are thus done simultaneously with set changes to save time. Moreover, the automated sequence sharply reduces operator interaction and forces the winder to keep moving, resulting in reduced lost time and a big gain in total capacity, Fig. 3.

Figure 3 - Parent roll change with two unwind stands

Winder runnability

Winder runnability should be understood as the maximum momentary running speed possible at any given time. In real life, however, the paper quality does not always allow higher winder speeds, due to temporary setbacks in web strength, defects in web profiles, etc. This is very familiar to papermakers. Does the winder itself play a role in this? Yes it might, as it may also restrict runnability, for instance due to different vibrations.

To dramatically reduce the possible harmful effects of vibrations, a technology called Smooth Ride was developed. This involves the use of numerous methods such as damping, structural design, control technology, hydraulics and material technology to improve the dynamic properties of the winder. Smooth Ride is based on detailed studies of the natural frequencies and modes in the winder, and on an understanding of the different grade-specific vibration phenomena that occur.

Practically, this involves optimizing the key components of the wind-up section such as the winding drums, rider roll, core chucks and frames. Figure 4 shows how typical fine paper vibration phenomenon has been removed, which in practice means eliminating speed reductions on the winder and increasing capacity.

Figure 4 - Smooth ride solutions reduce vibration

Drive control efficiency

The acceleration/deceleration strategy employed and advanced tension control are both critical for highest throughput. Individual optimization of winder acceleration/deceleration for each set, as shown in Fig. 5, can give a capacity increase of up to 15%. The optimized accel/decel ramp is achieved by controlling the speed reference constantly according to the maximum available drive capacity. This maximum available capacity changes constantly because, for instance, the inertia of the unwound parent roll and the wound up roll also changes constantly.

Figure 5 - Smarter drive control can give up to 15% more capacity

These improvements have been made possible by the new integrated Metso drive control. This means that there is now only one automatic control system instead of two, resulting in faster operation and no delays in communication. But the true breakthrough here is the more comprehensive drive control model, which also takes into account the web and winder characteristics as well as the winding process itself.

Tension control is a closely related factor, as all drive speed changes typically lead to tension variation, which in turn increases the risk of web breaks. The traditional way is to fine tune the control loop around an operating area. Our way is to bring more winding physics to the control loop, resulting in much better tension control.

A pilot winder, capable of 3,500 m/min and equipped with Metso drive control, has proven to be an excellent tool in developing this high efficiency accel/decel and tension control program. The benefits for papermakers are significant: better control efficiency, which is reflected directly in increased capacity.

Two sides to speed

Coming back to the speed question, it is clear that the more the capacity is achieved by speed, the greater will be the impact of the web properties on true capacity. An example of how speed affects the equation is given in Fig. 6. This shows the speed needed, when winding 90-g/m² liner grade on two different winders with different performance, to get the same throughput.

As Fig. 6 shows, almost double the speed is needed to compensate for quick set and parent change times and optimized acceleration/deceleration. Now, which concept is safer: one that has to run at 3,500 m/min, or one where 1,950 m/min will give the same throughput? And taking SmoothRide technology into account, what if we turn it around and give the Pro winder a clearly higher running speed?

High output at low risk

The point is that, although high running speed is of course an extremely important parameter in total winder efficiency, there is a lot more to it than that. WinDrum Pro focuses on optimizing all the variables in high-speed winding, and, taken together, they give papermakers very high-productivity winding. With SmoothRide technology, even higher speeds will be achievable in safe and consistent operation.

This philosophy of delivering high output at low risk, based on in-depth insights into winding mechanics and dynamics, has been critical in allowing WinDrum Pro to build an impressive reference list, including many of the largest, most productive paper and board machines in the world.

So whether you want to boost capacity from an existing winder, or rebuild it for a bigger jump in capacity, or perhaps even replace it totally, there are many options. Optimization of the variables outlined above can be applied wholly or in part to fit your specific mill situation. The only bad option is not doing anything. By applying the latest smart tools in winding science you can get more tons wound faster, while keeping your winder comfortably ahead of the PM output.

Kenneth Åkerlund is general manager of winders at Metso Paper and can be reached at kenneth.akerlund@metso.com.

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