Humidity Control in Paper Mill Operations

Humidity control represents the most critical environmental parameter in paper manufacturing. Paper’s hygroscopic nature causes dimensional changes in response to ambient moisture variations, directly affecting product quality, machine runnability, and finished product performance. Precise humidity management throughout the mill prevents defects while optimizing production efficiency.

Paper Moisture Equilibrium

Paper continuously exchanges moisture with surrounding air until reaching equilibrium moisture content (EMC). This relationship follows established sorption isotherms specific to each paper grade and fiber composition.

Equilibrium Moisture Content:

The EMC relationship follows the modified Henderson equation:

$$\text{EMC} = \left[\frac{-\ln(1-\phi)}{A \cdot T}\right]^{1/B}$$

Where:

• $\text{EMC}$ = equilibrium moisture content (decimal)
• $\phi$ = relative humidity (decimal)
• $T$ = absolute temperature (K)
• $A, B$ = empirical constants (typical values: $A = 5.5 \times 10^{-5}$, $B = 1.8$)

For most printing and writing papers, target moisture content ranges from 4-8% by weight, corresponding to 40-55% RH at standard conditions (73°F). Premium grades require tighter tolerances of 5-6% moisture content with ±0.5% variation.

Dimensional Response:

Paper dimensional change in response to moisture variation follows:

$$\frac{\Delta L}{L_0} = k \cdot \Delta MC$$

Where:

• $\Delta L/L_0$ = fractional dimensional change
• $k$ = dimensional change coefficient (0.01-0.015 per 1% MC change in cross-direction)
• $\Delta MC$ = moisture content change (%)

Cross-machine direction exhibits 3-5 times greater dimensional change than machine direction due to fiber orientation. A 2% moisture content swing causes 0.3-0.5% dimensional change across the sheet width.

Machine Room Environmental Control

The paper machine environment divides into distinct zones, each requiring specific humidity and temperature conditions.

Forming Section (Wet End):

High humidity (65-75% RH) at the forming table prevents premature drying that would cause fiber migration and formation defects. Temperature control at 70-80°F maintains proper drainage characteristics without condensation on machine components. Air velocity below 50 FPM prevents web disturbance during formation.

Press Section:

The press section operates at intermediate humidity (55-65% RH) to facilitate water removal while preventing sheet shrinkage. Temperature stability within ±3°F ensures consistent pressing efficiency and prevents condensation on press rolls.

Dryer Section:

Active drying removes water through evaporation, creating a moisture-saturated environment within the dryer hood. Hood ventilation exhausts this moisture-laden air while supply systems introduce conditioned makeup air. Maintaining 50-60% RH in the dryer pocket area optimizes drying efficiency and prevents overdrying at the sheet surface.

Calender and Reel:

The dry end requires the tightest humidity control (45-55% RH, ±2%) to establish final paper moisture content. Temperature control within ±2°F prevents moisture gradients through the sheet thickness. This zone determines the moisture content delivered to the customer.

Zone Humidity Control Systems

Effective paper mill humidity control employs multiple HVAC zones with independent control capability.

Primary Air Handling Systems:

Central air handlers condition supply air to target dewpoint temperature using:

• Chilled water cooling coils for dehumidification
• Steam or hot water reheat coils for temperature control
• Direct steam injection for humidification
• MERV 11-13 filtration to remove fiber and dust

Design dewpoint control accuracy of ±1°F enables RH control within ±2-3% at constant space temperature.

Local Humidity Control:

Critical zones employ supplementary humidification or dehumidification:

Humidification Methods:

• Direct steam injection: Instantaneous response, 0.5-1.0 lb steam per 1,000 CFM
• Atomizing humidifiers: Evaporative cooling effect, 10-15 minute response time
• Ultrasonic humidifiers: Fine mist generation for uniform distribution

Dehumidification Approaches:

• Overcool and reheat: Precise control, higher energy consumption
• Desiccant systems: Independent temperature/humidity control for storage areas
• Heat pipe coils: Energy-efficient precool/reheat configuration

Control Algorithm:

Modern systems implement cascade control:

$$u(t) = K_p \left(e(t) + \frac{1}{T_i}\int e(t)dt + T_d\frac{de(t)}{dt}\right)$$

Where:

• $u(t)$ = control output (valve position, damper position)
• $e(t)$ = error signal (setpoint - measured RH)
• $K_p$ = proportional gain (typical: 2-5)
• $T_i$ = integral time constant (5-15 minutes)
• $T_d$ = derivative time constant (0-2 minutes)

Feedforward compensation adjusts for measured disturbances (outdoor air conditions, production rate changes) before humidity deviation occurs.

Air Distribution Design

Uniform air distribution throughout the machine room prevents local humidity variations that cause quality defects.

Supply Air Configuration:

• High-level supply: Diffusers at 20-30 ft elevation deliver conditioned air above the machine
• Low-velocity design: Terminal velocities below 100 FPM minimize fiber disturbance
• Multiple supply points: Spacing at 20-30 ft intervals ensures coverage across machine width
• Temperature stratification: Supply air temperature within 5°F of space temperature prevents thermal drafts

Return Air Optimization:

Return air intakes locate at low level to capture heat released from dryer section. Multiple return points prevent short-circuiting of supply air. Return air temperature typically measures 5-10°F above supply temperature due to internal heat gains.

Exhaust Systems:

Process exhaust (dryer hood exhaust) operates independently from HVAC systems. Building exhaust maintains slight negative pressure (-0.02 to -0.05 in. w.c.) relative to outdoors, preventing moisture migration to adjacent spaces.

Finishing and Storage Area Control

Converted products and finished rolls require strict environmental control to prevent dimensional changes after manufacturing.

Winder and Finishing Room:

Maintain 45-50% RH with ±2% control accuracy. Temperature setpoint at 73°F per TAPPI standards enables correlation with laboratory testing conditions. Supply air quantity of 6-8 air changes per hour provides adequate environmental control without excessive air motion.

Warehouse Storage:

Finished product storage operates at 40-50% RH to prevent moisture gain or loss during inventory. Vapor retarder wrapping protects rolls during shipment, but warehouse control maintains product condition before dispatch. Dehumidification during summer months and humidification during winter heating season stabilize conditions year-round.

Standards and Performance Criteria

TAPPI T 502 defines standard testing conditions (50% RH, 73°F) for paper physical properties. Most paper specifications reference these conditions, making mill environmental control critical for meeting customer requirements.

Typical Performance Targets:

Energy Considerations:

Humidity control represents 15-25% of total HVAC energy consumption in paper mills. Heat recovery from dryer section exhaust preheats outdoor ventilation air, reducing reheat energy. Economizer operation during intermediate seasons provides free cooling and dehumidification.

Proper humidity control prevents costly quality defects including curl, dimensional instability, poor printability, and static electricity problems. Investment in precision HVAC systems pays returns through reduced waste, improved runnability, and consistent product quality that meets demanding customer specifications.