Keys to optimize drying efficiency; start with the felt tension

As papermakers face increasing demands to elevate machine speeds and improve paper quality, they search for uncompromising methods to improve their operations. One such area worthy of closer attention is paper drying.

The dryer section plays a critical role in determining paper quality and consistency. However, because it is often housed in the hot and inhospitable confines of the hood, the dryer section and its components tend to receive less attention than other machine sections. Drying efficiency, as well as paper quality, suffer when drying equipment is not maintained. If dryers, consumers of an enormous amount of energy, do not operate at maximum efficiency, the mill’s bottom line is adversely affected.

Mills can maximize drying efficiency by controlling a critical force of the dryer section – the felt. Papermakers should increase felt tensions to practical maximum levels. Moreover, they should closely monitor these tension levels and keep felt rolls, guides and stretchers in peak operating condition.

Efficient drying

Increasing felt tension improves drying efficiency by squeezing air out between the web and dryer. Dryer felts force the paper into close and constant contact with the steam-heated cylinders and increase heat transfer. By maximizing the drying effect of the heat transfer, papermakers can either increase machine speed or decrease the amount of steam, thus reducing operating costs. Papermakers should note that there are practical limits beyond which no significant benefits in drying are obtained by increasing tension.

Paper quality can also improve with increased felt tension. Uniform tension in the cross dimension of the web results in more uniform drying with less shrinkage variations. Uniform drying produces fewer crepe wrinkles and cockles. Minimizing these types of imperfections reduces the risk of sheet tears and breaks.

Uncontrolled tension

Although increasing felt tension does have its benefits, it also has its limits. Felt tension that is not controlled and closely monitored can create numerous problems for both the papermaker and the end user:

• Excessive tension can shorten felt life.
• Bearing and drive loads are pushed to the limit, putting machinery – particularly older equipment – at high risk of failure.
• Tension variations result in non-uniform drying.
• Moisture variations result in varying paper thickness, which multiplies when wound to large diameters. Variations in wound roll diameter generate stress zones in the sheet. The end results are an increase in sheet breaks and lower paper quality.

Keys to optimizing efficiency

To reach maximum drying efficiency, mills should keep drying equipment in peak operating condition. However, in this age of retrofits where papermakers may use equipment 50-years-old or older, machinery doesn’t always cooperate. Complicating matters, the dryer section, with its components hidden under a hood that throws off air up to 150 degrees Celsius, doesn’t always receive regular maintenance checks.

To optimize drying efficiency and avoid costly breakdowns, follow these few steps:

1. Have the OEM manufacturer perform a thorough evaluation of the condition of the machinery prior to increasing tension.
2. Mills should perform complete maintenance checks at regularly scheduled shutdowns when the equipment has had time to cool.
3. Papermakers should keep guides and stretchers, which steer the felt through the dryer, free of lint, dust and paper fragments.
4. When upgrading to new felts, papermakers should thoroughly evaluate their equipment needs. (Tension levels recommended by felt suppliers may not be the proper tension levels for existing machinery.) The OEM manufacturer is best qualified to determine not only if the basic design is sufficient, but also if the individual units are in proper operating condition.
5. Mills should closely monitor tension levels, because proper tension levels will vary based on several factors: the amount of moisture remaining in the paper, the paper grade, machine speed and felt condition.

In summary, increased felt tension can optimize drying efficiency and improve paper quality; however, it has its limits. A thorough evaluation of machinery and close monitoring of felt tension, along with a complete maintenance program, are the keys to a headache-free dryer section.

For more information regarding optimizing your dryer section, contact your Valmet representative.