Yankee Dryer Crown Optimization
Due to change in process over time, the Yankee Dryer Crown may not be optimal under the current conditions leading to uneven CD profile, such as wet edges.
METHOD OF SOLUTION
For this optimization process it is not necessary to recalculate the Yankee Dryer crown. Using prior experience, and any data available from other machines within the company, develop the new crown, and apportion it to the Yankee and the Pressure Roll as explained below. Essentially, it is a tweaking process, and may require more than one try.
Current Yankee Dryer & Presssure Roll Crowns
CD Moisture & Bulk Profiles
Prior experience, or data from other similar machines.
Compute the new Total Crown.
1. Unless the Yankee is ready for a regrind, allot the total change to the pressure roll. Specify its new crown and degree of cosine curve.
2. If the Yankee is to be ground also, apportion this crown between Yankee and Pressure Roll for minimizing wear of felt and pressure roll cover, as discussed below.
Yankee Dryer Crown Sharing
On account of the crowns on the Yankee Dryer and the Pressure Roll(s), and the surface deformation of the Yankee Shell in the Pressure Roll nip, the felt undergoes a speed differential in the nip from the midlle to the edge. Outside the nip, the felt has a constant velocity at all points, but the Pressure Roll may not due to crowning.
As a result the felt experiences shear stress and slip on the surface, causing it to wear. Wear is computed as the differrential velocity multiplied by the contact force.
The pressure roll cover is also similarly affected.
For various crown sharing schemes, we compute the following 3 slips:
S1 = Slip between Yankee surface and felt inside the nip.
S2 = Slip between felt and Suction Pressure Roll inside the nip.
S3 = Slip between felt and Suction Pressure roll outside the nip.
From the results, the best case is picked, which will minimize slip and still lead to well defined crown curves.
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