Dryer Bars and Paper Dryer Performance
Dryer bars are axial bars placed on the inside of steam-heated paper dryers to create turbulence in the rimming condensate layer. If the spacing of the bars is chosen correctly, resonant waves are created in the condensate film, between each pair of bars. This significantly increases the level of turbulence, even at high speeds. This increase in turbulence reduces the resistance to heat transfer from the steam, through the condensate film, to the dryer shell. The increase in heat transfer can be as large as 50%, even when compared to the performance of dryers with modern, close clearance rotating syphons.
The following are best practices for paper dryer operations:
- Dryer bars improve drying uniformity and increase drying capacity of dryer-limited machines and thereby either increase production or decrease operating steam pressures.
- Bars should be installed in dryers that operate above the condensate rimming speed. In typical dryer diameters, condensate rimming speed is above 1100 fpm (335 mpm).
- If bars are installed in only a few dryers, then they should be installed in those dryers that are most effective with respect to evaporating moisture from the sheet. This will typically be dryers in the main dryer section, in the “constant rate” zone.
- The number of rows of bars is selected by the supplier to produce the highest heat transfer with the syphon system that is installed in the dryer. Typically 18 rows of bars are used in a 5 ft (1.5 m) diameter dryer and 21 rows of bars are used in a 6 ft (1.8 m) diameter dryer.
- The bars should be rectangular in cross-section and be stiff enough to withstand the impact of tumbling condensate and be tall enough so that they are not covered by rimming condensate in dryers that have heavy condensing loads (over 3000 lb/hr (1360 kg/hr)). High-performance bars today are 0.75” (19 mm) wide and 0.5” tall (12 mm).
- In a dryer with a rotating syphon, the bars should extend across the dryer face, to about 1-2” (25-50 mm) from the dryer shell flange.
- In a dryer with a stationary syphon, the syphon shoe should either be located close to the dryer shell flange or a short segment of bars, such as Edge Control™ bars, should be placed just outboard of the syphon.