Sheet wrinkles ironed out at Kruger/Trois-Rivieres: this mill discovered that, with new HS blowbox technology, the price of speed need not include wrinkling.

Today's high-tech paper machinery can easily run at surface speeds over 2000 m/min. Yet despite this mechanical capability, the weakest, most sensitive link in the papermaking operation is the paper sheet itself. The leaps in web speeds over the last 20 years have been made possible by eliminating open draws and by supporting and stabilizing the sheet in the press and dryer sections. Blow box technology--one of the most important developments that has improved sheet runnability--has evolved and kept pace with the new web speed milestones.

LIMITED BY WRINKLING, ROPE THREADING

The Kruger Inc. newsprint mill in Trois-Rivieres, Quebec, Canada, was not achieving the lull speed potential of of its PM No. 10 because of sheet wrinkling in the third, double-felted section. The 305-in. (7747 mm) trim Beloit machine, which makes standard 45 to 48.8 g/[m.sup.2] newsprint, started up in January 1991. "The original design speed was 3500 ft./min, but the machine speed had been increased to about 4100 ft./min over the last 10 years.

Like many contemporary machines, the first two felt sections were Uni-Run designs. But the 4100 ft/min (1250 m/min) barrier proved to be a hard wall to climb. "The speed was limited by sheet wrinkling," said mechanical engineer Justin Paille. "it was very floppy and it was hard to thread through the ropes We felt that by solving these wrinkling problems we could increase speed and maintain machine efficiency."

With this justification, the mill invested in a dryer section runnability improvement program that included converting the third section to a Uni-Run configuration, adding six new high speed (HS) blow-boxes and felt transfer blow-boxes from Metso Paper, and replacing the ropes with a new vacuum-assisted sheet threading system. The turnkey project also included relocation of the existing twin felt run blow boxes from the third to the fourth section. The mill added four new dryers to close the long draw between the fourth and fifth sections formerly occupied by a breaker stack.

The machine had already been equipped with press felt transfer blow-boxes and previous-generation blow-boxes in the first two UniRun lelt sections. These were supplied by Valmet (Morse Paper).

SHEET CONTROL THROUGHOUT THE UNI-RUN SECTION

Maintaining a firm contact with the Uni-Run fabric is the key to avoiding runnability problems and wrinkling in at dryer pocket. First, the sheet tends to stick to the tippet; heated dryer cylinder, lifting it off the felt. Also, near the closing nip with the lower vacuum roll, pressure generated by time entrained boundary layer air builds up, thereby lifting the sheet. A blow-box generates a counter-balancing vacuum through the dryer fabric to ensure that that the sheet maintains close contact and avoids wrinkling.

The HS blow-box generates this underpressure by blowing high velocity air to create a vacuum. Air is ejected from the perimeter of the blow-box through specially designed blowing nozzles, which are essentially airfoils. The ejected air creates a lower pressure through the dryer fabric. This lower pressure holds the sheet. The air flow pattern generated by the nozzles provides the air cushion that separates the blow-box from the fabric, avoiding the need for mechanical seals that might prematurely wear and damage the felt. Paille said the mill preferred this non-contacting design.

This patented design is used in previous-generation blow-boxes on the "down run" sides of the first and second Uni-Run pockets, where stability problems are likely to occur. Above certain speeds, boundary layer air flows and centrifugal forces generated by the bottom Uni-Run rolls create even more sheet instability. At higher speeds, the newer HS blow-boxes control the sheet stability on both sides of the pocket. The sheet is held firmly against the fabric through all four diverging or converging nips. Grooved rolls and grooved Vacrolls complete the sheet stabilization requirements of the Uni-Run sections. With the HS boxes, Paille reported, "The sheet is totally supported. There are no more wrinkles."

VAC ROLLS DRILLED ONLY FOR TAIL

In a conventional Uni-Run conversion, the lower rolls or dryers are grooved and drilled across the entire roll width, while this is necessary for holding a wet sheet, it is not necessary in the third section where the sheet has more strength. To hold the tail during threading, Metso Paper recommended that the bottom rolls could be grooved across the width, but drilled only 27 in. (61 cm) from the front side. This is called a "Tailvac" threading system. The supplier recommends this solution up to 4600 ft./min. After that, full-width drilling is required.

When threading the sheet, the vacuum damper remains fully open. During normal operation it stays 90% dosed. A small air flow is needed to make sure there is no positive pressure around the roll that might cause the sheet to flutter.

Said Paille, "Originally, we were thinking of full width roll drilling, but that is a long procedure that requires a long shutdown. This proposal saved us a long shutdown time." The ropeless tail threading is working well. Mill statistics show threading time is reduced by about 35%, from 16.9 to 11.0 min.

'AS FAST AS WE CAN WIND PAPER'

The new blow-boxes, vacuum assisted tail threading, blow box relocations and related air system modifications were completed in a six-day shutdown in March 2001. According to Norman Helie, PM No. 10 superintendent, the machine was up to 1300 m/min (4265 ft./min) within two weeks after the startup--a gain of close to 160 ft./min. "Now, we can run as fast as we can wind paper," Helie said.

"The sheet threads easier than before," he added. Total draws have been reduced from 3.8% to 3.4% and that has given us an extra 2.5 in. (6.35 cm) in sheet width." Average machine production has increased from about 580 metric tons/day to 600 metric tons/day.

About the author: Mark Williamson is a freelance writer based in Thornhill, Ontario, Canada. Contact him by phone at [phone], or by email: [e-mail]