Paper Machine Knowledge

Paper Machine Three Sections: Structure and Working Principles

Paper machines are among the largest industrial equipment in the world, reaching over 100 meters in length with speeds up to 2200 m/min.

Section 1: The Forming Section — Where the Sheet is Born

The forming section receives stock at approximately 0.5-1.0% consistency and begins the process of water removal and sheet formation.

Fourdrinier (Long Wire)

• Traditional design: horizontal wire table
• Stock delivered from headbox onto moving forming fabric
• Dewatering elements: foils, table rolls, vacuum boxes, couch roll
• Typical speed range: 200-800 m/min
• 1% consistency in → 18-22% consistency out

Twin-Wire Former

• Stock injected between two forming fabrics
• Two-sided dewatering — higher capacity
• Shorter forming zone — less floor space
• Better formation symmetry
• Typical speed range: 600-1800 m/min

Hybrid / Gap Former

• Combination of fourdrinier and twin-wire elements
• Initial drainage on single wire, then twin-wire section
• Balances formation quality with drainage capacity
• Speed range: 400-1500 m/min

Forming Fabric Key Indicators:

• CFM (Air Permeability): 250-800 depending on grade
• FSI (Fiber Support Index): 50-200+ depending on structure
• Mesh Count: 25-95 threads/cm depending on basis weight
• Caliper (Thickness): 0.4-1.2 mm
• Open Area: 15-35%

Section 2: The Press Section — Mechanical Water Removal

The press section removes approximately 30-40% of the remaining water — the most cost-effective dewatering stage (mechanical vs. thermal).

Press Configurations:

Suction Press Rolls

• Vacuum-assisted water removal
• Shell drilled with 25-30% open area
• Typically first or second press position
• Nip pressure: 80-150 kN/m

Grooved Press Rolls

• Surface grooves channel water away from nip
• Higher nip pressures possible (150-250 kN/m)
• Requires doctoring to keep grooves clean

Blind-Drilled Press Rolls

• Holes don't penetrate shell completely
• Higher open area than grooved
• Better water storage capacity

Shoe Press

• Extended nip — 5-10x longer dwell time vs. roll press
• Nip pressure: 800-1500 kN/m
• Post-nip dryness: 48-55% (vs. 40-45% for roll press)
• Standard for modern high-speed machines
• Requires BOM press felts

Yankee Dryer / Tissue

• Large-diameter (3-6m) steam-heated cylinder
• Sheet pressed and dried simultaneously
• Creped off with doctor blade for tissue softness

Press Felt Key Indicators:

• Weight: 1100-1800 g/m²
• Void Volume: Important for water handling capacity
• Permeability: Water flow through felt structure
• Surface Fineness: Affects sheet surface quality
• Compaction Resistance: Determines service life

Section 3: The Dryer Section — Thermal Water Removal

The dryer section removes the remaining water (from ~50% to ~95% dryness) through steam-heated cylinders.

Dryer Cylinder Groups

• Typically 30-80 cylinders arranged in groups
• Steam pressure progressively increases through groups
• First group: lower temperature to prevent sheet picking
• Last groups: highest temperature for final drying

Dryer Fabrics:

• Hold sheet against dryer surface for conductive heat transfer
• Provide tension to prevent sheet shrinkage
• Remove moisture-laden air through permeability (CFM)
• 1% dryness improvement in press = 4% steam savings in dryer

Key Dryer Fabric Specifications:

• CFM: 300-1000 depending on paper grade and position
• Contact Area: 60-75% (flat yarn) or spiral design
• Temperature Resistance: PET up to 180°C (PAPTEX HR-PET standard)
• Seam Type: Pin seam (standard), spiral (wide machines >6m)

Section-by-Section Fabric Specification Reference

Machine Section	Fabric Type	Key Specs	Typical Life
Forming	Forming Fabric	CFM 250-800, FSI 50-200+	30-150 days
Press 1st	Press Felt (BOM/Lam)	1500-1800 g/m², high void volume	30-60 days
Press 2nd/3rd	Press Felt	1300-1600 g/m², balanced	45-90 days
Dryer 1st Group	Dryer Fabric	CFM 400-600, lower temp	60-150 days
Dryer Last Groups	Dryer Fabric	CFM 600-1000, higher temp	60-180 days