On-Site Dryer Services
Quality problems and web breaks in paper production are often the result of inadequate roll coatings in the dryer section. When recoating existing cylinders, Voith’s On-Site Dryer Services attach great importance to ensuring a homogeneous surface. In the long term, thorough preparation and attention to detail by an experienced team pays off for paper manufacturers through reduced wear, improved paper quality and higher productivity. Various practical examples show that the work of Voith’s service teams makes it possible to significantly extend the runtimes of components.
Longer runtimes reduce life cycle costs
Downtime on paper machines is always associated with costs and as such is kept as short as possible. The metalizing and grinding of dryer cylinders is a high investment item for the paper manufacturer. It requires downtime and if not done correctly can be very expensive in terms of further downtime and lost production. There are too many cases in the paper industry where service suppliers cut corners, resulting in further unnecessary lost production for the papermaker.
Voith shows how attention to detail on each process step when metalizing a dryer cylinder can provide long-term payback for the paper manufacturer.
Careful grinding reduces vibrations
The temptation can often be to grind off the cylinder surface quickly to save time, for example, in steps of 0.04 mm by using high grinding pressures. Voith’s experience proves that a gradual removal of thinner layers, for example 0.02 mm, reduces the risk of vibration, producing a more homogeneous cylinder surface. The subsequent blasting process removes all grease residues. This ensures that the new coating bonds better to the surface.
“Like most things in life, time spent to prepare the foundation pays off when building subsequent processes,” says Carl Taylor, Global Product Manager, Dryer Section & On-Site Service at Voith. “For example, time spent preparing the cylinder removes any evidence of chatter marks and ensures optimum particle bonding of the coating.” A case of unwanted chatter marks caused by too coarse grinding is shown in Figure 1.
Grinding is often required to remove damages or corrosion in the cylinder surface. It is critical to ensure the problem area is removed before metalizing the cylinder. Figure 2 shows how corrosion should be removed through grinding. If grinding is stopped too soon and some evidence of corrosion remains, this will not be apparent once metalized but will show through at a later stage. Voith ensures that all problem areas are fully removed before metalizing the cylinder, resulting in a longer service life.
Significant increase in production possible
Just how important this precise procedure is can be seen in the example of PT Indah Kiat Perawang, an Indonesian paper manufacturer. In August 2017, the company chose a service provider to metalize dryer cylinders on its No. 6 machine. After only nine months, the drying cylinders showed severe surface wear and corrosion, as seen in Figure 3. The result: a drop in paper quality and an increase in number of web breaks. In August 2015 and July 2017, Voith’s On-Site Dryer Services team applied TerraDry C Express coatings to six rolls on the No. 3 machine. In 2018, the team inspected the paper machine to find that the Teflon coating was still in near mint condition. This had a significant impact on paper production. “The customer had recorded significantly fewer paper breaks, and the paper quality had increased,” says Taylor.
Thicker hard metal coating with finer particles prevents corrosion
Voith has a TerraDry Express coating for all dryer cylinder applications. TerraDry C, CT, CN and CS are applied thermally through a HVOF (High Velocity Oxygen Fuel) process. The pressurized gas mixture flow accelerates through a nozzle at high speeds that lead to very dense particles with excellent bonding properties. The aim is to achieve a hard metal surface with the highest possible protection against corrosion. The temptation for some suppliers is to use large particle sizes, for example 75 microns. Voith uses small particle sizes, for example 45 microns, which create a denser structure and higher bonding strength, as shown in Figure 4. This does take a little longer to build up the coating thickness, but the payback is a higher resistance to corrosion.
Fewer web breaks thanks to homogeneous Teflon layers
The function (uppermost) layer can certainly improve the performance of the metalized coating; for example, it can be formulated to improve contaminant resistance and/or surface durability. The temptation for some service suppliers is to rush the metal coating and then make up for it with the Teflon function layer. “Our Voith On-Site Dryer Services experts know that a thin function layer improves heat transfer, leading to more efficient drying of the paper web,” explains Taylor. “The higher heat transfer can increase dryness, reduce sheet breaks and improve sheet quality.”
A homogeneous application of the Teflon coating is a prerequisite. An irregular coating leads to an unbalanced heat distribution and ultimately to uneven drying of the paper web with a corresponding loss of paper quality. Furthermore, an irregular coating of Teflon can result in different curing times across the length of the cylinder during its application. If not thoroughly cured, the Teflon will be easily removed in localized areas.
Eight years in operation
Other paper manufacturers have benefited from the detailed work of Voith's On-Site Dryer Services. At the Indonesian paper manufacturer PT Tjiwi Kimia, the team applied TerraDry CT Express coatings to two rolls on its No. 11 machine in January 2010. This contributed to a longer service life and running time of the dryer cylinders. Despite a running time of more than 10 years, both rolls continue to work so well, that no reconditioning has been necessary.
Furthermore, not only are the rolls more durable after metalizing by Voith; the service life of doctor blades has also extended by up to 100 percent.
Ultimately, the paper manufacturer needs to produce tons of paper. This can only be done efficiently when the paper machine is running. Voith On-Site Dryer Services is often called on to replace coatings applied by other service providers who have clearly tried to speed up the job. In the long term, the high quality of Voith TerraDry Express cylinder coatings and, most importantly, the attention to detail from the Voith On-Site Dryer Services teams more than makes up for the investment with longer component life, higher paper quality and a reduction in web breaks.
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Global Product Manager - Drying
Division Fabric & Roll Systems