Increasing felt tension overcomes dryer limitation, a case study

The problem - dryer limited

An engineer at a northern U.S. mill lamented, "We're dryer limited! We're dryer limited!" His remarks echoed around a room filled with key decision-makers, including the mill manager, the national sales manager, and the dryer equipment manufacturer. The purpose of the meeting between the mill representatives and OEM personnel was to determine whether it was feasible to rebuild the machine to further increase its operating speed.

Discussions among the parties concluded that the biggest obstacle in speeding up the paper machine was the lack of drying capacity. To increase the capacity of the dryer section, the mill had previously looked at various options, including the installation of spoiler bars and the addition of six more dryer cans. The mill had added spoiler bars, but found that there was not enough room to install additional dryers. What were they to do?

The mill was operating a 2,000-fpm corrugating medium machine. It had spent hundreds of thousands of dollars over the past decade rebuilding its paperboard machine, but management were at their wits end about what to do to further increase operating speed.

The solution - increase felt tension

Although most mills don't see increased felt tension as the solution to a dryer limited problem, this mill did. To increase the felt tension, the OEM said that the mill should install new felt rolls. "Would the dryer cylinders and framework be able to withstand this additional pressure?" the mill representatives asked the equipment supplier. "The simple answer is 'Yes.'" the supplier responded. The framework was sufficiently strong to handle the increased tension. The existing stretchers, which had replaced old stretchers in a previous rebuild, could also handle the additional felt tension.

Because replacement of felt rolls is generally required to increase the tension, most mills are reluctant to undertake this type of rebuild. On this particular machine, however, the mill did not have to increase the felt tensions on all dryer cylinders, but rather on 32 of 48 dryer cylinders, the last two dryer sections.

The mill decided to increase the felt tension to 15 pli from the current tension level, which meant that it had to replace all the existing felt rolls. The mill properly checked the stretchers to ensure that the pressure could be maintained without any danger to the dryer operation.

The results - increase speed up to 5%

As a result of increasing felt tension in the drying section, the mill was able to increase its machine operating speed by anywhere from two to five percent. In addition, the increased drying restraint could only improve the high quality of corrugating medium coming out of the machine.

Sometimes objections about dryer limitations can be resolved by simply increasing the felt tension. This corrugating medium mill increased the felt tension, thereby bringing the sheet into closer contact with the dryer cylinder and increasing machine speed without adding additional dryers.

NOTE: Valmet has studied many machines and we have seen that, often, fabrics are not as tight as the tensioning equipment indicates. This is reflected in the dryer can surface temperatures. This may have been a factor at this mill also, for at least some of the felts.

For more information about upgrading your machine to improve drying, contact your Valmet representative.