Steam and condensate system rebuilds improve dryer control
Donohue mill improves steam and condensate systems, including stationary siphons and turbulence bars
WHEN PAPER MACHINE DRYER SECTIONS are designed for the best steam energy transfer to the sheet and efficient condensate removal, they are virtually set-and-forget operations. However, when they are reaching the limits of heat transfer or condensate removal capability, they can be difficult to control and can create many headaches for papermakers.
The Donohue newsprint mill in Thorold, Ont., faced such problems as the dryer sections on its No. 6 and No. 7 machines were limiting production and creating many operational difficulties.The 300 in. trim twin-wire Beloit machines, installed in 1981 and 1982, produce newsprint with basis weights from 45 gsm to 48.8 gsm (27.7 to 30 Ibs/3,000 ft). LIMITED CONTROL RANGE. Frank van Biesen, project manager at the Thorold mill, describes the situation before the steam and condensate systems underwent rebuilds in 1995 and 1996:
“Dryer pressures of 400 Kpa (58 psi) were just about at the maximum. On each machine, the existing thermocompressor was running wide open-out of control range-and we lacked the flexibility to control each dryer section individually. Operators found it difficult to change differential setpoints without dumping steam. To run the lighter weight grades (45 gsm) the operators had to shut down some dryers.’ The dryer sections were anything but set-and-forgetThe original steam and condensate systems for the 39 dryer sections on each machine were designed with single dry-end thermocompressor sections, which cascaded to the wet-end sections.A baby dryer was followed by three top uni-run dryers, which were individually pressure controlled.The bottom dryers of the two uni-run sections were not steam-heated, as the sheet is not in contact with the dryer surface. Both machines were equipped with dual rotating siphons up to dryer No. 23, with single rotating siphons in the remaining 16 dryers.
In addition to the lack of precise control, van Biesen reports the dryers were prone to frequent flooding, especially after repeated breaks. With the flooding problems and limited drying capacity, the mill looked for solutions to improve heat transfer to the sheet and to effectively remove condensate under all operating conditions, thereby improving machine efficiency.
PERFORMANCE STUDY, NEW SYSTEM DESIGN. To engineer an integrated solution to these problems, the mill chose Valmet-Enerdry of Thunder Bay, Ont. Valmet-Enerdry had previously conducted a dryer performance study from which it developed a longterm plan for improvements to the dryer sections. Based on this plan, the mill installed runability equipment such as blow boxes.
The study concluded that dryer surface temperatures were low for the steam pressure applied. The steam heat transfer to the dryer shell was limiting the drying rate. Also, the existing system design contributed to unreliable condensate removal, steam waste, and loss of production.
To achieve the desired goals of a steam and condensate rebuild, all components of the system-thermocompressors, steam separators, valves, siphons, turbulence bars-must be sized and their interdependent characteristics must be evaluated so that they work hand-in-hand. Valmet-Enerdry provided this detailed design engineering for the system. The new design incorporated a Valmet steam and condensate system for each machine, with stationary siphons and turbulence bars from the Deublin Co. of Waukegan, Ill. With machine speeds approaching 4,000 fpm, the condensate layer is certainly in a rimming condition. The Deublin turbulence bars, which were installed on all dryers except the non-heated uni-run dryers, disrupt the laminar condensate layer. This induced turbulence improves heat transfer to the shell.
To support the system design objectives of improved condensate removal, the mill selected Model FS100 Deublin DeltaSint stationary siphons. With stationary siphons, condensate is effectively evacuated at low differential pressures since the pressure difference does not have to overcome the centrifugal force introduced by high-speed rotation. Consistent with the original design of each machine, steam is supplied from the drive side, with condensate removed from the tending side.
The Deublin stationary siphons are relatively new to North American mills, but they are prevalent in Europe and Asia. To evaluate their performance, mill personnel visited sites in Finland, where the siphons are installed on similar machines.
To ensure reliability, the siphons have been designed to avoid metal-to-metal contact with the dryer shell. The siphon pickup shoe is designed so that it “water skis” on the condensate layer. Life of the carbon seals in the siphon joint is extended by an end-face balanced design, which ensures low loading on the face of the seal.An inspection gap is incorporated in the design so that seal wear can be visibly monitored by maintenance staff. Replacement can be planned systematically.