New syphons eliminate dryer section bottleneck at Atlantic Newsprint; AUXILIARY EQUIPMENT
By Pulp & Paper Canada
By Pulp & Paper Canada
Improvements in forming and press sections have allowed for dramatic increases in paper machine production speeds over the last few decades. But as speeds go up, the often overlooked dryer section has…
Improvements in forming and press sections have allowed for dramatic increases in paper machine production speeds over the last few decades. But as speeds go up, the often overlooked dryer section has, in many cases, become a bottleneck and a production headache. The Atlantic Newsprint mill in Whitby, ON, was a case in point. Its 6.6-m-trim Voith Duoformer machine, installed in 1991, was suffering from chronic dryer flooding problems. The machine produces standard newsprint with 100% deinked pulp.
After taking several smaller steps to improve the dryer section performance, the mill decided to replace the rotary syphons with stationary syphons supplied by Johnson Canada. Dryer bars from another supplier were also installed in all the heated dryers. The project was completed in September 1998. Since then the results have been dramatic. With no more flooding problems, the steam consumption has been reduced and, with better moisture profiles, the roll quality has been improved.
Chronic flooding problems
Sash Vidic, manager of engineering and maintenance, described the continuous problems with dryer flooding: “The condensate removal was poor. During start-ups and long breaks we would have flooding. You could feel the machine vibrate and frequently the drive would kick out. Also, with high blowthrough rates, the condenser would overload resulting in the loss of vacuum control in the wet end dryers.”
Greg Slater, paper machine superintendent, added: “The flooding contributed to extra downtime on a break. Also, when the dryers were flooded or semi-flooded, the heat transfer to the sheet was uneven. This caused problems with reel building, resulting in a higher cull rate.”
At speeds around 1050 m/min, the condensate was not being removed properly even with differential pressures of 55 to 60 kPa across the rotary syphons. To control the steam system, the main steam section pressure had to be at levels around 100 kPa and nine or 10 dryers out of 43 were turned off. The flooding problems remained.
In 1995, Johnson model 9800PT rotary joints and 4.5-in. cantilevered stationary syphons were installed on the first five wet end dryers. At the same time, the steam and condensate system was re-configured, changing from a 4-section thermocompressor loop to a 3-section cascade system with blowthrough control. The wet end dryer control was improved but flooding was still a problem in the rest of the dryer section.
Lower differential at higher speeds
To effectively evacuate condensate from a dryer at speeds over 1000 m/min, a small clearance between the syphon and the dryer shell and sufficient differential pressures are required. A rotary syphon has a small clearance but it is fighting against the centrifugal forces introduced by the rotating cylinder. At speeds over 1000 m/min, the differential pressures required to evacuate condensate rise exponentially. On the other hand stationary syphons require a very low differential pressure to evacuate the dryers since they do not rotate with the dryer shell and the centrifugal forces do not have to be overcome. Furthermore, the differential pressures are independent of machine speed. The mill decided to equip all of the remaining steam heated dryers with stationary syphons and dryer bars.
The dryer bars introduce turbulence into the condensate film, resulting in better heat transfer and lower steam pressures for the same production rate. Also, the cross direction surface temperature profile of the dryer shell becomes flatter, thereby improving moisture profiles.
To eliminate condensate flashing and problems pumping the condensate back to the boiler house, Johnson Systems Ltd. also converted an existing separator to a central condensate holding tank.
After the September rebuild, the dryer flooding problem was eliminated. Vidic reported that, “We now have no condensate problems and we are using at least 10 % less steam even at the higher machine speeds.”
Operational difficulties have also been solved. Slater added: “Downtime due to flooded dryers is now non-existent. We also have more consistent heat transfer across the sheet. It has improved reel quality immensely.”
With the dryer section improvements, Atlantic Newsprint may now take advantage of the full production potential of its machine.
Mark Williamson is a freelance writer based in Thornhill, ON.