by: Nicola Cervelli - A.Celli Group
The Yankee Dryer has been used for the paper drying process since 1800 and, although its operating principle has remained substantially unchanged, numerous technical innovations have been made to improve the effectiveness and efficiency of the process, including the adoption of steel instead of cast iron as a material for its construction.
The traditional Tissue paper drying process is a combination of conduction drying, using the Yankee Dryer, and impingement drying. The energy needed to carry out this process is supplied both by the dry saturated steam condensed inside the Yankee cylinder, and by the hot air blown onto the sheet of paper through the hoods that cover the surface of the Yankee itself.
Over the years, some significant evolutionary steps have taken place in the technologies for the manufacture of a Yankee Dryer, among which we can highlight:
Ribbed shell
The introduction of the ribbed shell, by decreasing the root thickness of the Yankee on the one hand and increasing the heat exchange surface between steam and shell on the other, it has led to a significant increase in effectiveness and efficiency of the heat exchange. As a result, the drying capacity of the Yankee cylinder increased while the specific energy consumption of the drying process decreased.
Condensate extraction system
The ribbed shell solution has increased the complexity of the condensate extraction system with the introduction of transverse manifolds, to which a multitude of tubes are connected, organized in such a way as to each serve a circumferential portion of a single groove. This system, albeit complex, has made it possible to considerably improve the Yankee Dryer performance, thanks to the fact of ensuring the control of the condensate film in each groove, reaching a very reduced thickness. This resulted in a greater drying capacity and better sheet moisture profiles both in the longitudinal and transversal directions.
The implementation of the shell metallization process
A process that consists in spraying, by means of suitable equipment, a metal alloy on the surface of the shell to protect it against the effects of the doctor blade.
The design of Yankee Dryer adapted for the use of shoe presses
Over time, the Yankee Dryers have been improved for the specific use of shoe presses, a particular type of press with a concave shape capable of substantially widening the nip, thus allowing the application of a high linear load and a high pressure pulse to the paper. All this improves the squeezing process of the sheet without the need to increase the pressure exerted.
Subsequently, the technological evolution of the Yankee Dryer construction process led to a total change of the shell construction material, replacing cast iron with steel. Let’s see why this innovation is so important and the reasons why, to date, almost all of the Yankee Dryers built use steel.
From Cast Iron to Steel: the reasons for the evolution
The reasons for which we switched from cast iron to steel for the construction of the mantle are various:
o Low availability of specialized foundries and high operating costs of a foundry equipped to produce large castings.
o The uncertainty linked to the success of the casting of large monobloc pieces such as Yankee shells: very often defects in the homogeneity of the material, such as porosity, emerge only during processing; non-repairable shells must be discarded and this has a significant impact on costs and delivery times.
o The constructive complexity: the cast iron, not being weldable, forces all parts to be coupled by bolting, and this makes the geometries of the pieces more complex and expensive, also posing many more watertight problems for the assembly. All the bolt holes can, in fact, be a source of steam leaks and must therefore be suitably sealed.
Cast iron, however, had excellent and indispensable surface properties from the point of view of the papermaking process, such as microporosity, useful for the creping process, and self-lubricating properties against the very aggressive action of the creping and cleaning blades.
The same properties are not offered by steel and only the development of metal spray coatings in the last 25 years has been able to pave the way for the use of this material for the construction of Yankee shells. The use of steel has undoubted advantages:
o The weldability has allowed a simplification of the parts, in particular of the shell, and the elimination of watertight problems of some interfaces.
o Greater reliability from a metallurgical point of view, which made it possible to use less conservative sizing criteria, with direct benefits on the heat transmission capacity and consequently on the drying capacity.
It is clear that steel has a higher tensile strength with a thermal conductivity practically identical to cast iron: for this reason, the rules concerning pressure vessels allow the use of smaller thicknesses which give the fundamental advantage on the thermal side.
Furthermore, with steel, there will be no phenomena of porosity development during cooling, typical of cast iron castings. This allows to increase the depth of the shell grooves without the risk of leakage over the useful life of the Yankee. Adopting a steel shell instead of cast iron traditionally also has some disadvantages:
o As we have said, it is necessary to proceed with the metallization of the shell for the correct functioning of the steel Yankee Dryer, and this makes it impossible to verify the complex crown shape ground on the surface before the metallization process.
o Need to periodically check the welding of the shell obtained by joining calendered steel sheets.
These periodic non-destructive checks, i.e. visual inspections, checks with penetrating liquids or ultrasounds, have the major drawback that they can only be carried out with the machine stopped and the Yankee cylinder completely cooled, as most of the inspections are carried out from inside the cylinder itself. All this translates into a substantial increase in the hours of line unavailability.
Precisely to overcome these latter drawbacks, A.Celli has developed and adopted for its Yankee Dryer A.Celli iDEAL® Forged YD a monobloc forged shell without structural welds, which does not require ultrasonic inspections: a solution also inherited from the brand new A.Celli iDEAL® EvoLock®.
Conclusions
We have highlighted how the use of a steel shell compared to a cast iron shell brings advantages that far outweighing the disadvantages: it is for these reasons that most of the Yankee Dryers sold in recent years are built using this material. The design of a steel Yankee Dryer for a tissue company must have very specific objectives in terms of increasing safety, maximum drying capacity and reducing the number of integrity inspections required.
The new Yankee Dryer A.Celli iDEAL® EvoLock® was developed as an evolution of the iDEAL® Forged YD precisely to best meet these needs. By combining our characteristic forged shell with an innovative shell-to-head coupling system that does not involve the use of structural welds or screws, it will be possible to minimize the necessary inspections and consequent downtime without sacrificing drying capacity and operator safety.
TAPPI
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