Yankee dryer

A Yankee dryer is a pressure vessel used in the production of tissue paper. On the Yankee dryer, the paper goes from approximately 42–45% dryness to just over 89% dryness. In industry, MG cylinders or Yankee dryers are primarily used to remove excess moisture from pulp that is about to be converted into paper. The Yankee cylinder can be equipped with a doctor blade and sprayed with adhesives to make the paper stick. Creping is done by the Yankee's doctor blade that is scraping the dry paper off the cylinder surface thereby crêping the paper. The crinkle (crêping) is controlled by the strength of the adhesive, geometry of the doctor blade, speed difference between the yankee and final section of the paper machine and paper pulp characteristics.

Whereas in other paper productions a series of drying cylinders is used, in tissue production only one cylinder (the yankee cylinder) dries the paper. This is due to the necessity of creping and made possible by the low grammage (gsm = gram per square meter) of the paper sheet for tissue products, which is in the range of 14-45 gsm. For the production of the higher gsm in this range, some machines are nevertheless provided with some (4-10) drying cylinders after the yankee. In this case one speaks of "wet creping", as the creping of the paper done on the yankee is not made on the fully dried paper and the completement of drying is accomplished after the yankee cylinder.

Yankee cylinders are traditionally made of cast iron and have diamaters up to 6 m, therefore much higher than conventional drying cylinders. The width is a bit larger than that of the paper: typical for tissue are paper machine widths 1,74m 2,32m and 2,70m and their multiples (usually nowadays the doubles of these values are typical). Therefore yankees are very heavy (~100t) and difficult to cast. Since a couple of decades the production of yankees made of steel is gaining market and new machines are practically always nowadays equipped with a steel yankee, which is much lighter, easier to produce and to transport.

Due to the abrasive effect of the creping blade the surface of the yankee becomes irregular and rough. Therefore periodically (with very varying frequency: from every 6 months to some years)cast iron yankees have to be ground or polished. This decreases the thickness of the shell, and since the yankee is a pressure vessel, also the maximum pressure which can be used for paper production. A way to avoid this declassing of pressure is to make a metallisation, i.e. to spray onto the surface a special Chrom-Nickel alloy, similar to a high allyed stainless steel, which is then ground with the foreseen crown of the shell surface, leaving a coated thickness of ca. 0,7–1 mm. The metallised surface is then much more resistant to the abrasion and only a very mild polishing may be necessary ca. every 2 to 4 years. The thermal conductivity of the metallisation is a bit less than that of the original material, so that cast iron cylinders are metallised after some grindings of the shell, in order to gain thorugh thickness decrease some conductivity which is then again lost with the metallisation. Steel yankees are instead always metallised since the beginning.

...
Wikipedia