Features Research & Innovation
CD shrinkage poses drying obstacles

PRESSING AND DRYING was the order of business at Thurday’s afternoon session, sponsored by the Paper Machine Technology Committee.

Peter Slater, Albany International Ltd, Bury UK, discussed the option of increasing shoe press capacity using process belts. “Shoe press technology has changed dramatically over the past few years,” Slater said. “There have been changes in the polyurethanes used to make the belts, and the average life expectancy of the belts has increased significantly.”

Slater pointed out that the average life expectancy of the belts is twice as high in Asia as in North America. This may be due to the relative number of each press type and grade per region, but clearly the maintenance methods used in each nation also have an influence.

Slater concluded that the best way to maintain shoe press quality and increase capacity is to employ grooved belts, which improve water removal and sheet control.

Greg Wadel, Beloit Corporation, presented Ten years of single-tier drying. Single-tier drying was developed in Canada to dry paper quickly, efficiently and without any adverse affects to sheet quality. However, problems with sheet curling have been an obstacle that Beloit has tried to overcome using various models of single and two-tier dryers.

Wadel feels that the challenges of the next generation dryer section are to reduce cross-directional sheet shrinkage and improve web quality from the centre outward. “We still have some homework ahead of us,” Wadel said.

Wadel’s second presentation explored the applications of impingement drying to future dryer sections. Wadel feels that impingement drying is the best way to augment conventional steam- heated cylinder drying.

Wadel said this system dries the sheet from both sides, resulting in reduced sheet shrinkage and more uniform paper properties in the cross machine direction. A shorter drying section and improved machine runnability are additional advantages.

Lars Kange, Lorentzen & Wettre USA, discussed a new method for the on-line measurement of strength and elastic properties of paper in the paper machine, using ultrasound. This method results in a continuous stochastic signal instead of the traditional pulse.

“One of the challenges of this system is to create a signal with enough energy to overcome the paper machine’s low signal to noise ratio,” Kange said. He concluded that the technology exists to measure and optimize strength properties on-line, to improve machine and converting profitability.

New developments in metering size technology was the subject of a presentation by Michael Trefz, Voith Sulzer Paper Technology Inc. Metered size press technology is no longer only used for surface sizing, Trefz said. It has become an interesting alternative to other coating or surface treatment methods, such as the sizing, coating and precoating of woodfree paper, or the impregnation and greaseproofing of specialty papers.