A significant return on investment and more uniform packaged quality can be achieved with modern, high-efficiency steam profilers
Steam profilers serve a dual purpose in tissue, paper and board production by evening out CD profiles and raising the average sheet temperature so water is removed more easily and cost effectively in the pressing and drying sections. More uniform quality and energy cost savings are possible. For drying limited machines, some extra production can be squeezed out as well. As a result, numerous tissue machines have been equipped with modern high-efficiency steam profilers, often installed just before the pressure roll nip.
A matter of thermodynamics
This location is an ideal place to influence the thermodynamics of water removal. There are several factors which contribute to energy savings in tissue machines. First, mechanical dewatering is improved in the press nip since water viscosity is lower at higher temperatures. Then, there is less water to remove in Yankee section. Next, with higher web temperatures immediately before the Yankee, the energy required to heat the web to drying temperature is less. Furthermore, better moisture CD-profile provides more even contact and heat transfer from the Yankee surface. Also, the web is easier to doctor from the Yankee.
Figure 1 indicates a sheet dryness increase of 2.8% when steam was applied by a profiler. This means that less drying must be done in the Yankee/air cap system and this opens up the opportunity for some significant energy savings. Hood air cap temperatures can be reduced significantly and the fuel consumption can be reduced. Figure 2 shows a 33% saving in fuel oil consumption.
But you don't get something of value for free. The steam profiler consumes steam energy to save drying energy in the Yankee/air cap drying system. The tissue profiler application shown in Figure 2 consumed 0.32 tonnes of steam per tonne of tissue. When evaluating the net costs energy savings alone you have to look at the relative costs of providing steam for the profiler and the hot air to the air cap. An energy balance coupled with supplier guarantees will reveal the gains.
Gains in productivity, energy efficiency and quality
On the other hand, production increases on a drying limited machine are clear cut, as the steam profiler will free up extra drying capacity and allow a speed increase, if the machine drive and other machine parts can handle it. The returns from production increases can be quite valuable and the manufacturing costs per tonne will be trimmed.
In one European tissue mill the installation of the steam profiler was justified in the mill's evaluation by an expected improvement in cross-direction product quality, a production increase for the drying- limited towel grades and lower drying energy requirements for the toilet tissue grades. These interrelated benefits are possible with a steam profiler, since it provides an even CD sheet moisture profile and raises the overall sheet temperature for cost-effective water removal by pressing and more efficient use of drying energy.
A three-month mill-initiated study documented the lasting results of the profiler installation. These included:
- 8% increase in production on towel grades. This was achieved by a significant reduction in specific gas energy consumption. The production speed was previously limited by Yankee/air cap drying energy capacity.
- 5% reduction in electrical energy by driving hood fans at slower speed.
- 30% improvement in CD moisture profiles.
Although the steam consumption of the machine was raised 10 to 11% by implementing the steam profiler, this increase in energy was offset by the reduction in air cap natural gas energy consumption.
Quality improvement was also one of the major objectives of the project and that has been achieved conclusively. The improvement on the machine extends right through the production chain. There is less CD variation in parent reels, so that means the finished consumer roll weight is also more even. It is clearly better, the customer concludes. The total variation of weight was decreased by about 15% for towel grades whereas the total weight variation of toilet paper grades was decreased by a very significant 48%. A boost in productivity, lower costs per tonne and more uniform package quality satisfied this mill's objectives.
Maintaining performance for the long run
While the demonstrated benefits attractive may be attractive during the trial evaluation period, maintaining them over the long run is essential so the profiler remains a valuable tool for the operators. Otherwise, it may fall into a state of disrepair. Keeping a profiler clean and free from plugging in such a notoriously dirty environment is often very challenging. In many cases that concern can be alleviated by installing a spray shower that is activated when the steam profiler is retracted during a creping blade change. Ideally, no operator actions should be required. Ask suppliers for a viable and proven solution to the cleanliness problem as it is essential for continuing results.
Condensation and water dripping can cause all sorts of operational difficulties so suppliers' solutions to the potential game-altering issue must be evaluated carefully. Ideally, the steam should have the right energy level (superheat) and velocity so the steam condenses in the sheet and give up its latent heat without excessive sheet blowthrough or any condensation on machine surfaces. These problems have been solved so ask the suppliers how they did it.
Photos and drawings are courtesy of Metso.