Beating the speed limit: improving dryer limited machines; here is a practical guide to new technology and processes that can improve throughput and speed up dryer limited machines.

Hitting the wall. Maxed out. At the limit. All of these catch phrases describe a common problem for older paper machines that have reached their production limit. Often, these machines are "dryer limited." The forming and pressing sections could operate at higher speeds, but the machine lacks the drying capacity to handle increased production. Short of replacing the dryer section, what can dryer limited mills do? There are several options, according to experts interviewed by Solutions! magazine.

The first step should be to step back and use diagnostic testing to evaluate all sections of the machine, said Ray Krumenacker, manager, products and technology for Metso Paper, Norcross, Georgia, USA. "With a dryer-limited machine, you should include the forming, pressing, and drying sections. There may be forming or pressing problems limiting water removal from the sheet. Also, if major work is needed, it may be more economical to rebuild the press section to improve sheet dryness. You can't focus just on the dryer--a shoe press, tandem shoes press, or some other emerging technology may be a better answer."

If the forming and pressing sections are delivering the best quality and dryness they are capable of, then mills should evaluate the dryer section more closely. "Evaluate the steam and condensate systems, air systems, surface temperature of cylinders compared to the steam supply, and heat transfer rates," said Krumenacker. "You also should look at pocket humidity and how the section is felted."

If this diagnostic work does not turn up any major operating problems and the existing drying equipment is performing as well as possible, the next step is to consider capital improvements, said Krumenacker.

"Examine the room constraints--can you add equipment in the machine track, above it, or below it? Mills can take several approaches, such as lengthening the dryer by adding more dryer cylinders or installing infrared (IR) air dryers or impingent dryers."

New technology may also be appropriate. Krumenacker pointed to the OptiDry concept--a combination of cylinders and high temperature impingement air dryers--and the Condebelt concept, in which the sheet is passed through two steel belts heated to different temperatures. The heat converts sheet moisture into steam; this water vapor condenses on the cooler belt and is carried away.

LOOK INSIDE

If you are looking to improve dryer section performance, what is inside the dryer cans is as important as what is outside them. Specifically, efficient removal of condensate that enters the dryer as steam--using turbulator bars and syphons--is critical to peak dryer performance. Ken Hill, president of Johnson Systems, Lenoir City, Tennessee, noted that mills should try to find the heat transfer "sweet spot."

"Optimizing heat transfer is critical," he said. "Often, the condensate layer inside the dryer is too thin, too thick, or sub-optimum. The dynamics of the condensate layer depends on the dryer bar spacing, height of the bars, condensing loads, syphon clearance, and dryer speed. Sometimes, a simple adjustment in syphon height can make a big difference in drying rates."

In other cases, mills should look at new technology, such as a hollow turbulator tube design, that can markedly increase drying rates, Hill said. "When combining the right syphon clearance with proper bar design and spacing, we have achieved a 5% to 6% increase in drying capacity on several machines compared to machines using conventional bars."

Drainage consistency is another improvement area. "System problems or syphon design can often impede consistent condensate removal," said Hill. "Modern stationary syphon systems provide better drainage consistency and improved condensate removal."

Mills using rotating syphons may want to consider retrofitting their dryer cans with stationary syphons--which operate with less differential pressure and less blow through. The reliability of stationary syphons has improved dramatically over the past decade. Hill noted, and about half of all paper machines in the United States are now equipped with them. Typically, retrofits can cost up to US$ 7000 per dryer can, but the investment produces increased dryer efficiency and operating speeds.

Steam joint reliability is another issue affecting dryer operations, said Hill. "Many older machines operate with outdated steam joints that have short seal life," he said. "It is not unusual to see 5% of dryers on a machine out of service due to bad steam joints. Keeping them all operating is the key. Newer steam joints are more reliable and with routine maintenance can achieve 100% reliability."

Dryer-limited mills should also consider thermocompressor pressure boosting. Some dryer sections operate at the makeup steam header pressure limitation, which may be well below the dryer can pressure rating, said Hill "It's not unusual to see dryer cans operating at a steam header of 60 psi when they are rated for 100 psi Employing new thermocompressor pressure boosting technology can make a big difference."

Finally, mills can use new automation strategies to optimize dryer performance. "New dryer management programs allow quick recovery from sheet breaks and grade changes," said Hill. "These software programs characterize the dryer temperature response and use that knowledge to adjust steam pressure to produce the correct surface temperature after breaks and grade changes."

These software programs measure drying conditions before the breaks, calculate the pressure turndown needed to keep the dryer temperature uniform during the break, and determine the time needed to resume steady state operations. "Without active control, it can take anywhere from 3-4 minutes to as long as half an hour to achieve stable operation, depending on hardware, condensing loads, and dryer speeds," said Hill. "Modern dryer control strategies use on-line analysis to produce the correct response and adjust the DCS set points accordingly."

MAXIMIZING STEAM

Retrofitting rotating syphons on dryer-limited machines with stationary syphons received an additional vote from Paul Masini, chief engineer, Deublin Co., Waukegan, Illinois. "This provides mills the ability to increase heat transfer from the steam entering the dryer can to the dryer shell, maximizing the steam. You want to be able to keep as thin a film of condensate as possible inside the dyer, and stationary syphons allow you to achieve that. Combined with turbulence bars, stationary syphons can maximize heat transfer through the dryer can."

Masini recommends retrofitting whole dryer sections at a time. "Typically, the steam system will be supporting the paper machine by sections. If you retrofit only half a section, then you need different equipment in place to maintain the rotating syphon system and the stationary system, since it takes a lot more differential pressure to run a rotating syphon system. Therefore, we recommend the 'section at a time' approach."

The cost of retrofitting varies and is based on the type of machine, size of steam joints used, and turbulence bars used, Masini noted. He said the cost can range from US$ 2000 to US$ 7000 per dryer can.

Pocket ventilation is another way of dealing with dryer limited machines. The greater the temperature difference between the steam inside the dryer and the outside air, the better the heat transfer through the dryer shell. "For example, if the temperature inside the dryer is 300[degrees]F and the temperature outside the dryer is 80[degrees]F, you will move heat quicker through the dryer," said Masini. "Pocket ventilation systems can make a major difference."

Mills should evaluate their steam and condensate systems annually to make sure that the installed steam joints and syphon systems are properly evacuating the condensate. A flooded system carrying excessive condensate will inhibit drying and slow the machine down, noted Masini, adding that an annual system evaluation can also identify where thermocompressor operations can be improved.

Finally, the use of balanced mechanical seals as a sealing technology has revolutionized traditional steam joints. Masini adds, "Typically, slow-to-medium speed machines are fitted with pressure type steam joints. They operate with high torque, wear out rapidly, and require frequent maintenance. With high-speed machines, the use of balanced mechanical seal is a must. Hydraulic balance allows the seal faces to rotate with less friction and torque reducing maintenance and downtime. For high-speed machines, this is critical."

Alan Rooks, Editorial Director