Paper Machine Runnability Issues and Solutions

With the advent of high machine speeds over the years, machine efficiencies have become an issue due to the increase of sheet breaks in the dryer section open draw.

To solve this problem, it is necessary to close up draws and support the sheet as much as possible. The areas to be addressed first are those where the sheet is most susceptible to breaks, closer to the wet end, where the sheet is the weakest. For the past decade, the first step was advancing from straight through open draw presses to the top, and bottom felted presses and eventually extended nip press (ENP), where the sheet has minimal open draws. (See Fig. 1 & 2)

The next step was the dryer section; 1st section, and transfer from the press to the 1st dryer section. Early machines had a long distance from the press center roll to the first dryer requiring a high draw resulting in sheet breaks and sheet stretching.

In addition to this, the 1st section was conventional felted top and bottom dryers with long open draws between the top and bottom dryers. To reduce this draw, squat dryer sections were installed, reducing the long open draw. The next step was adding the serpentine dryer section where the fabric follows the sheet through the area eliminating the draws. This worked well until machine speeds started to exceed 2500 fpm, and the centrifugal force applied to the sheet going around the bottom dryers resulted in a sheet bubble and wrinkling.

Sheet stabilizers were introduced on the down run to hold the sheet tight to the fabric/ dryers to eliminate this. Presently, at machine speeds above 3000 fpm, a double-sided sheet stabilizer is installed. At speeds beyond this, it is recommended to replace bottom dryers with drilled and grooved vacuum rolls, in addition to installing double-sided boxes to eliminate the draw in this section. Depending on the machine grade and speed, this will determine the number of dryer sections single-tiered.

Once the sheet leaves the single-tier section and enters the first top and bottom felt dryer section, the sheet is still susceptible to breakage, although drying has taken place in the previous sections. To minimize draws, the pocket felt rolls are offset to the wet end to reduce the open draw by more than half to obtain optimum runnability. Specially designed offset pocket ventilators are installed to hold the sheet to fabric and ventilate the pocket.

The following dryer section, which will be a standard top and bottom felted dryer section will be equipped with self-balancing pocket ventilators, introducing air to ventilate the pocket and eliminate high axial air flows that can cause edge flutter. All remaining sections will be installed with standard ventilator boxes due to higher sheet strength, and their sole function will be to reduce pocket humidities and increase the drying rate.

In summary, at present machine speeds, it is imperative to eliminate or at least reduce all open draws as feasible and install stability components to hold the sheet to the fabric throughout the dryer section.

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