by: Fabrizio Tonello - A.Celli Group
The Yankee Dryer is a key component in a tissue paper production plant. Being subjected to both the pressure and the heating of the steam from the inside and the load of the presses from the outside, the Yankee Cylinder undergoes high thermal and mechanical stresses. To cope with this, a very complex mechanical structure that must meet extremely high requirements in terms of safety and operational reliability is required.
The Yankee Dryer has a diameter that can vary from 2.5 to 6.7 meters and is usually built in cast iron or steel, although nowadays the trend sees the use of steel prevail for its many advantages, including a better heat transfer.
The extreme importance of the Yankee Dryer derives from the fact that in the production of tissue paper, since the latter is characterized by a low basis weight, the drying process of the paper sheet is performed using, as a drying cylinder, only the Yankee Dryer instead of various batteries of cylinders, as occurs in the production of normal paper. In summary, the main functions of the Yankee Dryer are:
1. The transport of the sheet of paper.
2. The creation of the nip with the suction press, to remove an additional amount of excess water in the sheet before the evaporation process.
3. The drying of the sheet by means of thermal transfer.
4. The creation of a base for the creping process.
The continuous increase in machine speed, steam pressure and production requirements, not to mention the intrinsic characteristics that a Yankee Dryer must possess to create a high quality final product, represents a major challenge for manufacturers.
For these reasons, and to overcome some of the technical limitations of the shell construction process that involves the use of welding, A.Celli has designed and built its steel Yankee Dryer with a shell completely forged in one piece: a solution, such as we will see, more secure, reliable and extremely efficient.
A.Celli’s objectives and the characteristics of the IDEAL® forged Yankee Dryer
Minimizing mechanical problems while ensuring greater solidity and better heat transmission: these were the objectives that A.Celli intended to achieve. All this has become possible thanks to the iDEAL® forged Yankee Dryer, the new production technology conceived and patented by A.Celli Paper to meet these growing needs. The distinctive feature of our YD is the construction process of the shell, made from a single piece of steel, which involves the following steps:
1. Heating of steel to reach a temperature of more than 1200 ° C
2. Forging
3. Hot rolling process aimed at achieving an optimal crystalline grain size
The result of this process is a shell of homogeneous material without welding: a highly innovative product that eliminates any chance of deformation, both during the processing phase and during its use. The internal processing of the grooves, also patented, is characterized by an innovative design that reduces the possibility of shell deformation, relieving the tension on the sides of the shell itself and ensuring better heat transmission, with a consequent increased drying capacity of the Yankee.
The homogeneous material, characterized by a high resistance to the propagation of cracks, and the unique structure, free from residual stress, allow a greater variation in operating pressure and definitively eliminate the risk of breakage. Therefore, unlike the traditional method that involves welding operations, both the possibility of finding mechanical defects in the welded area and the need to carry out the numerous periodic and maintenance quality checks that require the interruption of the production line are eliminated at the root.
The innovation offered by A.Celli is completed by the head insulation system, made by means of a patented insulating ceramic layer and a polycarbonate cover in order to minimize heat dispersion and consequently reduce standard steam consumption. Ceramics, guaranteeing better and more stable insulation than conventional materials such as rock wool, allows the use of thinner panels and maintains its insulating properties unaltered during production.
Finally, the use of bolts to fix the heads to the shell by means of bolts allows the elimination of welding and the consequent deformations and residual stresses in the coupling in another area subjected to high stress. The final result is an innovative and unique Yankee Dryer on the world market which, from the data collected, is able to reduce costs (due to maintenance activities and lost productivity during downtime) by 94% in the first 12 years of use.
TAPPI
http://www.tappi.org/