By Jörg Bauböck and Andreas Anzel, Andritz AG
Drying costs are becoming a more and more important factor in the overall cost of tissue products due to today’s energy prices. Tissue mills everywhere are increasing efforts to reduce energy cost, decrease drying energy consumption and increase drying efficiency. The newly developed PrimeDry steel yankee and the X-Nip T shoe press are tools for the papermaker to address these concerns.
Steel yankees have been on the market for several years. Their main advantages compared to the cast iron yankees include a higher evaporation rate and increased operational safety. The lower wall thickness of steel yankees, which is responsible for their higher performance, is permitted by the properties of steel.
The sudden and potentially disastrous extension of cracks that can occur with cast iron yankees are not an issue with steel yankees. The wide safety margins, given by the pressure vessel codes for cast iron, are not necessary with steel, thanks to its greater strength.
The heat flow through the cylinder wall is about 20% higher with a steel yankee than with a cast-iron one. This results in a higher evaporation rate which can either be used to increase the production or, when the machine speed is kept constant, to reduce drying cost. Compared to a cast iron yankee of the same size, the steam consumption for a steel yankee is about 10% higher at the same machine speed and production; however, gas and electricity consumption is reduced.
With a steel yankee, impingement temperature and velocity in the hood can be lower. This increases the efficiency of drying slightly and conveys a higher portion of the drying with typically cheaper steam. Depending on the actual cost for steam, gas and electricity in a specific mill, the potential for about a 5% reduction in drying energy cost exists.
If the higher performance of the yankee is used to increase machine speed, up to 9% additional paper can be produced. The net result is a lower specific energy cost per ton of paper, increased production capacity and the potential for increased profit. In addition, the initial capital investment cost for the machine can be reduced in some cases, as a smaller yankee can be chosen to achieve a given production goal.
The higher steam consumption of a steel yankee can be compensated for by using a new concept for heat0 recovery in the yankee hood exhaust.
In the first stage, an air/condensate heat exchanger is placed in the exhaust flow. Condensate out of the yankee is re-evaporated and steam is produced by means of recovered energy. The steam consumption of the yankee can be reduced by up to 15% depending on the hood temperature level.
Additional drying energy cost savings of 3-6% are possible. When the subsequent heat recovery stages are sized properly, the same efficiency of these exchangers is possible as in the original version without the reevaporation heat-recovery stage.
Another method to reduce drying cost is to increase the amount of mechanical pressing and to increase the dryness before thermal drying. A new development in this concept is the X-Nip T shoe press. Gentle pressing using a longer nip and a lower maximum nip pressure are well known advantages of shoe presses for tissue production. The X-Nip T design offers even more flexibility in the choice of the production conditions.
The nip pressure profile can be changed on the run by simply tilting the shoe. This feature allows a simple switch from a high-quality, high-bulk-and-softness product, with the same dryness content post press as a conventional single suction press roll, to an economically produced standard product. The result is a much higher dryness content after the press with lower energy consumption or higher production
The X-Nip T is capable of exceeding even the post press dryness levels achieved with a two press configuration. Compared to a conventional single press roll arrangement, the X-Nip T is capable of up to 4% of increased dryness content.
Energy cost savings of up to 15% or machine speed increases up to 12% can be achieved using the full potential of the X-Nip T press on a conventional yankee. The XNip T is designed to provide special adjustments of the forces at the edges which allows an even nip pressure across the entire zone.
The increased dryness achieved with the X-Nip T using the tilting of the shoe is significant on a conventional yankee. Its full potential is achieved by using a reinforced yankee dryer, however. Higher line forces are achievable in this case compared to what a conventional yankee and a suction press can withstand. The specific design of the reinforced yankee dryer will determine the capability of the X-Nip T. The savings or production increases that can be achieved on these yankees still needs to be proven. However, it is clear that they will exceed the values of a conventional yankee.
In summary, the current energy cost situation is a major driving factor for new developments in tissue drying. Not only for today but for the future as well. TW
