Guide to dryer section performance, key operating parameters

Here are some notes regarding key operation parameters of the dryer section:

Effective drying rate – 3 zones:

• Warm up zone – sheet is warmed up to evaporation temperature, depends on grade. Sheet temperature should be raised as quickly as possible without creating picking or cockling.
• Constant rate zone – water available for evaporation at surface is removed. Highest rate of evaporation in this zone. Sheet temperatures should be high and consistent in this zone.
• Falling rate zone – water inside sheet is removed, lower evaporation rates. Should be as short as possible.

Steam temperature to dryer can surface temperature difference – good range is 40°F. Use a contacting pyrometer to measure roll surface temperature. Can use these results to determine if there are any issues with steam or condensate system. Differential temperatures over 60°F indicate issues with condensate evacuation system. Steam needs to be supplied at its saturation temperature, minimal superheat for maximum heat transfer. High superheat reduces rotary joint life. Acceptable CD temperature range 3-5°F. Also there should not be any non-condensable gases in the dryer can.

Heat transfer coefficient – factors affecting heat transfer; condensate layer thickness, dryer shell thickness, air trapped between sheet and dryer, scale deposits on dryer can. Paper properties such as thickness, roughness, filler content, refining level, fiber type, and porosity affect heat transfer. Condensate forms a puddle at speeds less than 500 fpm, cascades from 700-900 fpm, and rims over 1000 fpm. Dryer bars are used to create turbulence in the condensate layer of fast machines which increases heat transfer from 5 – 15%.

Felted dryer sections improve heat transfer between can and sheet. Can temperatures can show the effect of un-felted sections or improper felt tension profiles. Felting is more important in early dryer sections and on lighter weight, fast machines. May not affect heavy weights at all.

Condensate removal systems:

• Rotary siphons – high heat transfer and low maintenance, but require high differential pressures and flood easily.
• Stationary siphons – low DP required, do not flood easily, siphon clearance critical, dryer bars help heat transfer rate.

Wet end sheet temperature rise – number of cans to raise sheet temperature to 180°F.

# of dryers in falling rate zone – try to keep as few as possible.

Average and peak pocket humidity – good range 0.2 to 0.3 lb water/lb dry air. High pocket humidity (above 0.8 lb water/lb D.A.) reduce drying capacity and can affect moisture profiles. Check pocket humidity at ends and towards the center of machine as far in as possible.

Average and peak hood exhaust humidity – good range 850-900 grains.

Pocket vent air flow – good range 130-190 cfm/ft.

Hood balance – good range 65-70% = dry air in / dry end exhausted x 100. Effects of hood balance on operation:

• Edge lifting and sheet instability in unorun section
• Excessive infiltration of basement air
• Edge sheet flutter
• CD drafts and humidity variation
• Pocket humidity changes with hood doors open
• Condensation in hood
• Low null point and spillage into room. Optimum null point about 6 ft above floor level.
• High energy use in the form of hot air supplied to dryers

Hood exhaust humidity:

• Open hood – 300-450 grains
• Closed hood, low to medium humidity – 750-900 grains
• Closed hood, high humidity – 900-1050 grains

Press moisture – 1% increase in sheet moisture = 4% increase in drying demand. Press moistures change with furnish and grade, causing wide swings in drying demand. Sometimes difficult to troubleshooting drying demand issues – press solids or drying performance?

For assistance with your drying section performance, contact your Valmet representative.